From 79d4f84bf27543c10520962c06b279909aa7ea9e Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Carlos=20Flor=C3=AAncio?= <carlosmflorencio@gmail.com>
Date: Tue, 2 Jun 2026 19:37:20 +0100
Subject: [PATCH] fix(backend): cache & singleflight live PR feedback fetches
 (#1237)

---
 .../backend/internal/common/subproc/shared.go |  69 ++++
 .../internal/common/subproc/shared_test.go    |  93 ++++-
 apps/backend/internal/github/errors.go        |  69 +++-
 apps/backend/internal/github/errors_test.go   |  43 +++
 apps/backend/internal/github/gh_client.go     | 120 ++++---
 apps/backend/internal/github/graphql.go       | 170 ++++++++-
 apps/backend/internal/github/handlers.go      |  19 +-
 apps/backend/internal/github/service.go       |  95 ++++++
 apps/backend/internal/github/service_pr.go    |  48 ++-
 .../github/service_pr_feedback_test.go        | 107 ++++++
 .../internal/github/service_pr_watch.go       |  95 +++++-
 .../github/service_pr_watch_batched.go        | 186 ++++++++--
 .../service_repo_negative_cache_test.go       | 322 ++++++++++++++++++
 .../github/service_search_cache_test.go       |   2 +
 .../backend/internal/github/service_tokens.go |  30 +-
 apps/backend/internal/github/ttl_cache.go     |  65 ++++
 .../hooks/domains/github/use-task-pr.test.tsx |  82 +++++
 apps/web/hooks/domains/github/use-task-pr.ts  |  65 +++-
 18 files changed, 1559 insertions(+), 121 deletions(-)
 create mode 100644 apps/backend/internal/github/service_pr_feedback_test.go
 create mode 100644 apps/backend/internal/github/service_repo_negative_cache_test.go
 create mode 100644 apps/web/hooks/domains/github/use-task-pr.test.tsx

diff --git a/apps/backend/internal/common/subproc/shared.go b/apps/backend/internal/common/subproc/shared.go
index 0ee6eee90d..8b2fe89adc 100644
--- a/apps/backend/internal/common/subproc/shared.go
+++ b/apps/backend/internal/common/subproc/shared.go
@@ -5,8 +5,16 @@ import (
 	"os"
 	"os/exec"
 	"strconv"
+	"time"
 )
 
+// defaultGHExecTimeout is the per-command exec budget the
+// Run*AfterAcquire helpers apply after the throttle slot is acquired.
+// 30s matches the gh client's default WithTimeout for plain `gh api`
+// calls; callers needing more (paginated REST calls) or less (cheap
+// `gh auth status`) pass an explicit value.
+const defaultGHExecTimeout = 30 * time.Second
+
 // Shared throttle singletons.
 //
 // Both gh and git are spawned from many packages across the backend
@@ -136,3 +144,64 @@ func RunGHCombinedOutput(ctx context.Context, cmd *exec.Cmd) ([]byte, error) {
 	defer release()
 	return cmd.CombinedOutput()
 }
+
+// RunGHAfterAcquire / RunGHCombinedAfterAcquire mirror the git
+// helper runGitCombinedAfterAcquire in apps/backend/internal/worktree:
+// the exec timer starts only AFTER the throttle slot is acquired so
+// throttle queue time can't burn the per-command budget. Without this
+// ordering, a queued waiter inherits its parent's WS-bound deadline,
+// gets a slot just before the deadline fires, and is killed with
+// `signal: killed` the moment it execs — producing the killed (192×) /
+// context deadline exceeded (96×) cascade in the SyncWatchesBatched
+// storm logs.
+//
+// Returns (out, runErr, execCtxErr). The exec-ctx error lets callers
+// tell a context-driven kill (timeout) from a regular gh failure when
+// classifying fallbacks — see worktree.classifyGitFallbackReason for the
+// pattern. The caller is expected to build `cmd` lazily (typically
+// `exec.CommandContext(execCtx, "gh", args...)`) via the supplied
+// builder closure so the exec context attaches to the right command.
+// `execTimeout <= 0` falls back to defaultGHExecTimeout.
+func RunGHCombinedAfterAcquire(
+	ctx context.Context, execTimeout time.Duration, build func(execCtx context.Context) *exec.Cmd,
+) ([]byte, error, error) {
+	release, err := ghThrottle.Acquire(ctx)
+	if err != nil {
+		return nil, err, ctx.Err()
+	}
+	defer release()
+	execCtx, cancel := withExecTimeout(ctx, execTimeout)
+	defer cancel()
+	cmd := build(execCtx)
+	out, runErr := cmd.CombinedOutput()
+	return out, runErr, execCtx.Err()
+}
+
+// RunGHAfterAcquire is RunGHCombinedAfterAcquire's plain `cmd.Run` sibling.
+// Caller owns Stdout/Stderr wiring on the returned `cmd` (build closure
+// runs synchronously inside the throttle slot so wiring happens after
+// acquire, never before).
+func RunGHAfterAcquire(
+	ctx context.Context, execTimeout time.Duration, build func(execCtx context.Context) *exec.Cmd,
+) (error, error) {
+	release, err := ghThrottle.Acquire(ctx)
+	if err != nil {
+		return err, ctx.Err()
+	}
+	defer release()
+	execCtx, cancel := withExecTimeout(ctx, execTimeout)
+	defer cancel()
+	cmd := build(execCtx)
+	runErr := cmd.Run()
+	return runErr, execCtx.Err()
+}
+
+// withExecTimeout returns a child context bounded by execTimeout
+// (defaultGHExecTimeout when execTimeout<=0). Centralised so the two
+// AfterAcquire helpers can't drift on default-timeout behaviour.
+func withExecTimeout(ctx context.Context, execTimeout time.Duration) (context.Context, context.CancelFunc) {
+	if execTimeout <= 0 {
+		execTimeout = defaultGHExecTimeout
+	}
+	return context.WithTimeout(ctx, execTimeout)
+}
diff --git a/apps/backend/internal/common/subproc/shared_test.go b/apps/backend/internal/common/subproc/shared_test.go
index 7ab4401e1e..57a29a0196 100644
--- a/apps/backend/internal/common/subproc/shared_test.go
+++ b/apps/backend/internal/common/subproc/shared_test.go
@@ -1,6 +1,13 @@
 package subproc
 
-import "testing"
+import (
+	"context"
+	"os/exec"
+	"runtime"
+	"sync"
+	"testing"
+	"time"
+)
 
 func TestResolveCap(t *testing.T) {
 	const env = "KANDEV_SUBPROC_CAP_TEST"
@@ -69,3 +76,87 @@ func TestGHGitAreDistinctThrottles(t *testing.T) {
 		t.Fatal("GH() and Git() returned the same throttle instance")
 	}
 }
+
+// TestRunGHCombinedAfterAcquire_ExecTimeoutSurvivesAcquireWait is the
+// regression test for the killed (192×) / context-deadline-exceeded
+// (96×) cascade in the SyncWatchesBatched storm: when a queued waiter
+// finally gets a throttle slot, its per-command exec budget MUST start
+// fresh from that moment, not be eaten by the queue wait.
+//
+// Setup: pin the gh throttle to cap=1 and hold the slot for the
+// acquireWait window. The second caller invokes
+// RunGHCombinedAfterAcquire with a build closure that just records the
+// execCtx deadline relative to the current wallclock. Pre-fix (timer
+// started before Acquire) the recorded deadline would be in the past
+// the moment Acquire returns; post-fix the deadline is acquireWait
+// ahead of the AfterFunc fire, i.e. ~execTimeout ahead of now.
+func TestRunGHCombinedAfterAcquire_ExecTimeoutSurvivesAcquireWait(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("uses POSIX `true` binary as a no-op subprocess")
+	}
+	restore := GH().SetCapForTest(1)
+	defer restore()
+
+	ctx := context.Background()
+	holdRelease, err := GH().Acquire(ctx)
+	if err != nil {
+		t.Fatalf("acquire holder: %v", err)
+	}
+
+	// Sized so a pre-fix regression (timer starts BEFORE acquire) yields
+	// a NEGATIVE remaining budget at the build callback, while a correct
+	// after-acquire timer yields a budget within (~50%, 100%] of
+	// execTimeout. The exact fraction depends on scheduler latency
+	// between AfterFunc firing and the second goroutine resuming, so the
+	// lower bound is generous.
+	const acquireWait = 200 * time.Millisecond
+	const execTimeout = 500 * time.Millisecond
+
+	var wg sync.WaitGroup
+	wg.Add(1)
+	var (
+		gotRemaining time.Duration
+		gotRunErr    error
+		queued       = make(chan struct{})
+	)
+	go func() {
+		defer wg.Done()
+		// Signal once the goroutine is about to call into the throttle —
+		// at that point it WILL be a waiter (cap=1, the slot is held).
+		// Pre-fix used time.AfterFunc(acquireWait, ...) to "give the
+		// waiter time to queue", which was non-deterministic across CI
+		// schedulers and could miss the very regression it's meant to
+		// catch.
+		close(queued)
+		_, gotRunErr, _ = RunGHCombinedAfterAcquire(ctx, execTimeout, func(execCtx context.Context) *exec.Cmd {
+			if dl, ok := execCtx.Deadline(); ok {
+				gotRemaining = time.Until(dl)
+			}
+			return exec.CommandContext(execCtx, "true")
+		})
+	}()
+	// Wait for the goroutine to be ready to call Acquire, then yield
+	// until the throttle records the queue depth (waiters >= 1). This
+	// replaces the prior time.AfterFunc-based barrier, which fired on a
+	// scheduler timer rather than on the actual queue state and could
+	// miss the regression on slow CI hosts. After that, sleep for
+	// acquireWait so the pre-fix regression (timer starts BEFORE
+	// Acquire) would have its deadline burned by the wall-clock delay.
+	<-queued
+	for metricInt(subprocWaiters, "gh") < 1 {
+		runtime.Gosched()
+	}
+	time.Sleep(acquireWait)
+	holdRelease()
+	wg.Wait()
+
+	// Pre-fix: gotRemaining would be roughly execTimeout - acquireWait,
+	// or negative if acquireWait > execTimeout. Post-fix: gotRemaining
+	// should land in (execTimeout/2, execTimeout].
+	if gotRemaining <= execTimeout/2 {
+		t.Errorf("execCtx deadline was only %v from now (execTimeout=%v); acquire wait burned through the budget", gotRemaining, execTimeout)
+	}
+	if gotRunErr != nil {
+		t.Logf("`true` runErr=%v (informational; deadline budget is the assertion)", gotRunErr)
+	}
+}
diff --git a/apps/backend/internal/github/errors.go b/apps/backend/internal/github/errors.go
index 4c8e85dc35..d2f4ebafa0 100644
--- a/apps/backend/internal/github/errors.go
+++ b/apps/backend/internal/github/errors.go
@@ -1,6 +1,9 @@
 package github
 
-import "errors"
+import (
+	"errors"
+	"strings"
+)
 
 // ErrInvalidPRURL signals that a caller-supplied PR URL could not be parsed.
 // Used by AssociateExistingPRByURL so HTTP callers can translate the failure
@@ -25,3 +28,67 @@ var ErrSelfApprove = errors.New("cannot approve your own pull request")
 // error so HTTP callers can distinguish a validation failure (HTTP 400)
 // from a secret-store write failure (HTTP 500).
 var ErrInvalidToken = errors.New("invalid token")
+
+// ErrRepoNotResolvable signals that GitHub reported the repository as
+// missing, deleted, or inaccessible to the authenticated principal. The PR
+// watch flood storm this fixes (SyncWatchesBatched hammering the same dead
+// repo on every 5s frontend poll) classifies upstream errors via
+// isRepoNotResolvableErr and feeds the result into a 10-minute negative
+// cache so subsequent watch syncs short-circuit before acquiring the gh
+// throttle. The cache is also flushed on token swap/clear (see
+// Service.ClearRepoErrorCache wired from ConfigureToken/ClearToken) so a
+// re-auth doesn't have to wait out the TTL. Use errors.Is(err,
+// ErrRepoNotResolvable) at boundaries (WS handler, poller) where the
+// caller needs to differentiate a "stop retrying" failure from a
+// transient upstream blip.
+var ErrRepoNotResolvable = errors.New("github: repository not resolvable")
+
+// repoNotResolvableSubstrings matches the wire-format substrings GitHub's
+// GraphQL/REST APIs use for a deterministic "repository does not exist /
+// not accessible" outcome:
+//   - "Could not resolve to a Repository" — GraphQL response for a missing
+//     or deleted repo (the dominant signal in the storm logs)
+//   - "Resource not accessible by integration" — GitHub Apps / fine-grained
+//     PAT scope mismatch. Treated as "dead end" here because retrying the
+//     same call with the same token will never succeed; a token swap
+//     evicts the cache via ClearRepoErrorCache.
+//
+// Raw "HTTP 404 / Not Found" is intentionally NOT matched here. A 404 on
+// /repos/{owner}/{repo}/pulls/{N} can legitimately mean "the PR was
+// deleted" while the repo itself is fine — a false-positive there would
+// negative-cache the whole repo for 10 minutes off a single stale PR
+// number. The GraphQL "Could not resolve to a Repository" string IS
+// repo-scoped by construction and remains the canonical signal.
+var repoNotResolvableSubstrings = []string{
+	"could not resolve to a repository",
+	"resource not accessible by integration",
+}
+
+// isRepoNotResolvableErr reports whether err carries a deterministic
+// "repository missing / inaccessible" signal we should not keep retrying
+// against in a tight loop. Errors flagged here are negative-cached for
+// 10 minutes via Service.repoErrorCache; the cache is evicted on watch
+// (re)creation and on token swap/clear so a freshly-linked repo or a
+// re-auth is probed immediately. The match is case-insensitive so we
+// don't depend on a stable error casing across gh CLI versions and
+// GraphQL response variants.
+//
+// Bare HTTP 404 errors are NOT classified — too ambiguous (missing PR
+// number vs missing repo) and a false positive poisons the whole repo
+// for 10 minutes. Callers that know they're hitting a repo-root
+// endpoint should wrap with ErrRepoNotResolvable explicitly.
+func isRepoNotResolvableErr(err error) bool {
+	if err == nil {
+		return false
+	}
+	if errors.Is(err, ErrRepoNotResolvable) {
+		return true
+	}
+	s := strings.ToLower(err.Error())
+	for _, sub := range repoNotResolvableSubstrings {
+		if strings.Contains(s, sub) {
+			return true
+		}
+	}
+	return false
+}
diff --git a/apps/backend/internal/github/errors_test.go b/apps/backend/internal/github/errors_test.go
index 37fe2335f4..0b85981148 100644
--- a/apps/backend/internal/github/errors_test.go
+++ b/apps/backend/internal/github/errors_test.go
@@ -53,3 +53,46 @@ func TestErrorsAreClassifiable(t *testing.T) {
 		}
 	})
 }
+
+// TestIsRepoNotResolvableErr pins the wire-format substrings that classify
+// as "stop hammering this repo". A regression here re-opens the PR-watch
+// storm that motivated the negative cache: with the classifier missing the
+// canonical GraphQL "could not resolve to a repository" signal,
+// SyncWatchesBatched keeps retrying the dead repo on every 5s frontend
+// poll. Bare 404s are intentionally NOT classified — a 404 from
+// /repos/{o}/{r}/pulls/{N} can legitimately mean "this PR is gone" while
+// the repo is fine, and the negative cache must not poison the repo on
+// that signal.
+func TestIsRepoNotResolvableErr(t *testing.T) {
+	cases := []struct {
+		name string
+		err  error
+		want bool
+	}{
+		{"nil err", nil, false},
+		{"unrelated error", errors.New("connection refused"), false},
+		{"transient 500", errors.New("HTTP 500: Internal Server Error"), false},
+		{"graphql repo missing", errors.New(`graphql error: Could not resolve to a Repository with the name 'NBCUDTC/bff'.`), true},
+		{"graphql repo missing mixed case", errors.New("graphql error: could not resolve to a repository ..."), true},
+		// Bare 404 errors are NOT repo-not-resolvable — too ambiguous. They
+		// may indicate a missing PR number on a live repo, and poisoning
+		// the repo for 10 minutes on that signal is the bug this guards
+		// against. See the docstring on isRepoNotResolvableErr.
+		{"bare gh REST 404 is ambiguous", errors.New("gh api: HTTP 404: Not Found"), false},
+		{"bare 404 short form is ambiguous", errors.New("status: 404"), false},
+		{"bare 404 with body suffix is ambiguous", errors.New("404 Not Found"), false},
+		{"github api 404 is ambiguous", &GitHubAPIError{StatusCode: 404, Endpoint: "/repos/x/y/pulls/123", Body: "Not Found"}, false},
+		{"wrapped github api 404 is ambiguous", fmt.Errorf("repo: %w", &GitHubAPIError{StatusCode: 404, Endpoint: "/repos/x/y/pulls/123"}), false},
+		{"github api 403", &GitHubAPIError{StatusCode: 403, Endpoint: "/repos/x/y", Body: "Forbidden"}, false},
+		{"resource not accessible", errors.New("Resource not accessible by integration"), true},
+		{"resource not accessible lowercase", errors.New("resource not accessible by integration"), true},
+		{"wrapped sentinel", fmt.Errorf("sync: %w", ErrRepoNotResolvable), true},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := isRepoNotResolvableErr(tc.err); got != tc.want {
+				t.Errorf("isRepoNotResolvableErr(%v) = %v, want %v", tc.err, got, tc.want)
+			}
+		})
+	}
+}
diff --git a/apps/backend/internal/github/gh_client.go b/apps/backend/internal/github/gh_client.go
index dd1edfe16a..5c63e5437e 100644
--- a/apps/backend/internal/github/gh_client.go
+++ b/apps/backend/internal/github/gh_client.go
@@ -11,6 +11,8 @@ import (
 	"os/exec"
 	"strings"
 	"time"
+
+	"github.com/kandev/kandev/internal/common/subproc"
 )
 
 // ghSearchReposPath is the GitHub REST search-repositories path that gh CLI
@@ -124,24 +126,16 @@ func (c *GHClient) IsAuthenticated(ctx context.Context) (bool, error) {
 
 // RunAuthDiagnostics executes gh auth status and captures the raw output for troubleshooting.
 func (c *GHClient) RunAuthDiagnostics(ctx context.Context) *AuthDiagnostics {
-	ctx, cancel := withDefaultGHTimeout(ctx)
-	defer cancel()
-	release, err := acquireGHSlot(ctx)
-	if err != nil {
-		return &AuthDiagnostics{
-			Command:  "gh auth status --hostname github.com",
-			Output:   err.Error(),
-			ExitCode: -1,
-		}
-	}
-	defer release()
-	cmd := exec.CommandContext(ctx, "gh", "auth", "status", "--hostname", "github.com")
 	var stdout, stderr bytes.Buffer
-	cmd.Stdout = &stdout
-	cmd.Stderr = &stderr
+	runErr, execCtxErr := subproc.RunGHAfterAcquire(ctx, resolveGHExecTimeout(ctx), func(execCtx context.Context) *exec.Cmd {
+		cmd := exec.CommandContext(execCtx, "gh", "auth", "status", "--hostname", "github.com")
+		cmd.Stdout = &stdout
+		cmd.Stderr = &stderr
+		return cmd
+	})
 	exitCode := 0
-	if err := cmd.Run(); err != nil {
-		if exitErr, ok := err.(*exec.ExitError); ok {
+	if runErr != nil {
+		if exitErr, ok := runErr.(*exec.ExitError); ok {
 			exitCode = exitErr.ExitCode()
 		} else {
 			exitCode = -1
@@ -151,6 +145,17 @@ func (c *GHClient) RunAuthDiagnostics(ctx context.Context) *AuthDiagnostics {
 	if output == "" {
 		output = stdout.String()
 	}
+	// When acquire/context fails before gh runs (or the exec is killed by
+	// the per-command deadline), both buffers can be empty. Surface the
+	// underlying error so the diagnostics panel doesn't show a blank
+	// "command failed with exit code -1" with no explanation.
+	if output == "" && runErr != nil {
+		if execCtxErr != nil {
+			output = fmt.Sprintf("gh auth status did not run: %v (context: %v)", runErr, execCtxErr)
+		} else {
+			output = fmt.Sprintf("gh auth status did not run: %v", runErr)
+		}
+	}
 	return &AuthDiagnostics{
 		Command:  "gh auth status --hostname github.com",
 		Output:   output,
@@ -846,54 +851,67 @@ func (c *GHClient) DeleteGist(ctx context.Context, gistID string) error {
 
 const ghCLITimeout = 30 * time.Second
 
-// withDefaultGHTimeout applies a 30s timeout if the context has no deadline.
-func withDefaultGHTimeout(ctx context.Context) (context.Context, context.CancelFunc) {
-	if _, ok := ctx.Deadline(); ok {
-		return ctx, func() {}
+// resolveGHExecTimeout returns the per-command exec budget used by
+// run / runWithStdin. Honours the caller's ctx deadline when set (so a
+// short-lived WS request stays bounded by the request deadline), but
+// caps the budget at ghCLITimeout when the deadline is further out or
+// absent. Used by the acquire-then-derive-timeout path so a queued
+// waiter's exec timer starts AFTER it gets the throttle slot.
+func resolveGHExecTimeout(ctx context.Context) time.Duration {
+	if dl, ok := ctx.Deadline(); ok {
+		remaining := time.Until(dl)
+		if remaining < ghCLITimeout {
+			return remaining
+		}
 	}
-	return context.WithTimeout(ctx, ghCLITimeout)
+	return ghCLITimeout
 }
 
 // run executes a gh CLI command and returns its stdout output.
 // Stderr is captured separately to avoid contaminating JSON output.
-// A default 30s timeout is applied if the context has no deadline.
+// A 30s per-command exec timeout is applied AFTER the gh throttle slot
+// is acquired (acquire-then-derive-timeout). Pre-fix the timer started
+// before Acquire, so a queued waiter inherited a deadline that had
+// already partly elapsed against throttle queue time — producing the
+// `signal: killed` + `context deadline exceeded` cascade in the
+// SyncWatchesBatched storm logs.
 func (c *GHClient) run(ctx context.Context, args ...string) (string, error) {
-	ctx, cancel := withDefaultGHTimeout(ctx)
-	defer cancel()
-	release, err := acquireGHSlot(ctx)
-	if err != nil {
-		return "", fmt.Errorf("gh %s: %w", firstArg(args), err)
-	}
-	defer release()
-	cmd := exec.CommandContext(ctx, "gh", args...)
-	var stdout, stderr bytes.Buffer
-	cmd.Stdout = &stdout
-	cmd.Stderr = &stderr
-	if err := cmd.Run(); err != nil {
-		c.inspectRateStderr(args, stderr.String())
-		return stdout.String(), fmt.Errorf("gh %s: %w: %s", args[0], err, stderr.String())
-	}
-	return stdout.String(), nil
+	return c.runGH(ctx, nil, args...)
 }
 
 // runWithStdin is like run but pipes stdin into the gh CLI process.
 // Useful for `gh api --input -` calls where the body comes from JSON.
 func (c *GHClient) runWithStdin(ctx context.Context, stdin []byte, args ...string) (string, error) {
-	ctx, cancel := withDefaultGHTimeout(ctx)
-	defer cancel()
-	release, err := acquireGHSlot(ctx)
-	if err != nil {
-		return "", fmt.Errorf("gh %s: %w", firstArg(args), err)
-	}
-	defer release()
-	cmd := exec.CommandContext(ctx, "gh", args...)
-	cmd.Stdin = bytes.NewReader(stdin)
+	return c.runGH(ctx, stdin, args...)
+}
+
+// runGH is the shared body of run / runWithStdin — both apply the same
+// acquire-then-derive-timeout ordering, the same stderr/rate-limit
+// inspection, and the same error wrap. The only delta is whether stdin
+// is wired (`stdin != nil`).
+func (c *GHClient) runGH(ctx context.Context, stdin []byte, args ...string) (string, error) {
 	var stdout, stderr bytes.Buffer
-	cmd.Stdout = &stdout
-	cmd.Stderr = &stderr
-	if err := cmd.Run(); err != nil {
+	execTimeout := resolveGHExecTimeout(ctx)
+	runErr, execCtxErr := subproc.RunGHAfterAcquire(ctx, execTimeout, func(execCtx context.Context) *exec.Cmd {
+		cmd := exec.CommandContext(execCtx, "gh", args...)
+		if stdin != nil {
+			cmd.Stdin = bytes.NewReader(stdin)
+		}
+		cmd.Stdout = &stdout
+		cmd.Stderr = &stderr
+		return cmd
+	})
+	if runErr != nil {
+		// Surface execCtx timeouts AND cancellations as the canonical
+		// context error so callers (FetchRateLimit, IsAuthenticated,
+		// share.IsAlreadyGone) can still errors.Is(err, context.X). The
+		// runErr from cmd.Run on a killed child is `signal: killed`,
+		// which loses both classifier signals that pre-fix code relied on.
+		if execCtxErr != nil && (errors.Is(execCtxErr, context.DeadlineExceeded) || errors.Is(execCtxErr, context.Canceled)) {
+			return stdout.String(), fmt.Errorf("gh %s: %w", firstArg(args), execCtxErr)
+		}
 		c.inspectRateStderr(args, stderr.String())
-		return stdout.String(), fmt.Errorf("gh %s: %w: %s", args[0], err, stderr.String())
+		return stdout.String(), fmt.Errorf("gh %s: %w: %s", firstArg(args), runErr, stderr.String())
 	}
 	return stdout.String(), nil
 }
diff --git a/apps/backend/internal/github/graphql.go b/apps/backend/internal/github/graphql.go
index b012827234..2f05793e5e 100644
--- a/apps/backend/internal/github/graphql.go
+++ b/apps/backend/internal/github/graphql.go
@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"context"
 	"encoding/json"
+	"errors"
 	"fmt"
 	"io"
 	"net/http"
@@ -127,6 +128,51 @@ type graphQLError struct {
 	Path    []string `json:"path"`
 }
 
+// repoRef is a minimal (owner, repo) pair used by the batched GraphQL
+// helpers to surface which repositories the response flagged as
+// unresolvable. Service.SyncWatchesBatched feeds these into the negative
+// cache so subsequent polls short-circuit the dead-repo storm.
+type repoRef struct {
+	Owner string
+	Repo  string
+}
+
+// batchedMissingReposErr is the typed error runBatchedPRQuery /
+// runBatchedBranchQuery returns when the GraphQL response carried one or
+// more "Could not resolve to a Repository" entries. Callers use
+// errors.As to extract Repos and populate Service.repoErrorCache. When
+// Inner is nil, the response contained ONLY resolution failures and
+// the partial-decode result returned alongside the error is still safe
+// to consume for the repos that did resolve; when Inner is non-nil,
+// other (non-resolution) errors were also present and the caller should
+// treat the whole batch as failed for those.
+type batchedMissingReposErr struct {
+	Repos []repoRef
+	Inner error
+}
+
+func (e *batchedMissingReposErr) Error() string {
+	if e.Inner != nil {
+		return e.Inner.Error()
+	}
+	parts := make([]string, len(e.Repos))
+	for i, r := range e.Repos {
+		parts[i] = r.Owner + "/" + r.Repo
+	}
+	return "github: repositories not resolvable: " + strings.Join(parts, ", ")
+}
+
+// Is lets callers errors.Is(err, ErrRepoNotResolvable) without having to
+// type-assert. The sentinel is the canonical "stop hammering this repo"
+// signal across the package.
+func (e *batchedMissingReposErr) Is(target error) bool {
+	return target == ErrRepoNotResolvable
+}
+
+// Unwrap exposes the composite non-resolution error (if any) so callers
+// can errors.As / errors.Is past the batched-missing wrapper.
+func (e *batchedMissingReposErr) Unwrap() error { return e.Inner }
+
 // graphQLErrorsToErr returns a non-nil error when the GraphQL response carries
 // a top-level "errors" array. The first message is included so logs are
 // actionable; the count is appended when there are multiple.
@@ -140,6 +186,82 @@ func graphQLErrorsToErr(errs []graphQLError) error {
 	return fmt.Errorf("graphql error: %s (and %d more)", errs[0].Message, len(errs)-1)
 }
 
+// classifyBatchedErrors splits the GraphQL errors[] array into "this
+// repo is permanently unresolvable" and "everything else", mapping each
+// resolution error's path-prefix alias back to the (owner, repo) it
+// referred to via aliasToRepo. The caller (runBatchedPRQuery /
+// runBatchedBranchQuery) lifts the resolution misses into the typed
+// batchedMissingReposErr; the residual non-resolution errors are
+// collapsed via graphQLErrorsToErr exactly as before so the storm-fix
+// doesn't change behavior for unrelated GraphQL failures.
+func classifyBatchedErrors(errs []graphQLError, aliasToRepo map[string]repoRef) (missing []repoRef, residual []graphQLError) {
+	seen := map[string]bool{}
+	for _, e := range errs {
+		if isRepoNotResolvableErr(errors.New(e.Message)) && len(e.Path) > 0 {
+			ref, ok := aliasToRepo[e.Path[0]]
+			if ok {
+				key := ref.Owner + "/" + ref.Repo
+				if !seen[key] {
+					seen[key] = true
+					missing = append(missing, ref)
+				}
+				continue
+			}
+		}
+		residual = append(residual, e)
+	}
+	return missing, residual
+}
+
+// aliasMapForPRRefs returns alias -> repoRef for the
+// owner/repo-sorted ordering buildBatchedPRQuery applies. A
+// resolution-failure error's path[0] is always the outer repository
+// alias ("repo0", "repo1", ...).
+func aliasMapForPRRefs(refs []graphQLPRRef) map[string]repoRef {
+	byKey := map[string]repoRef{}
+	var keys []string
+	for _, r := range refs {
+		k := r.Owner + "/" + r.Repo
+		if _, ok := byKey[k]; !ok {
+			byKey[k] = repoRef{Owner: r.Owner, Repo: r.Repo}
+			keys = append(keys, k)
+		}
+	}
+	sort.Strings(keys)
+	out := make(map[string]repoRef, len(keys))
+	for i, k := range keys {
+		out[fmt.Sprintf("repo%d", i)] = byKey[k]
+	}
+	return out
+}
+
+// aliasMapForBranchRefs returns alias -> repoRef for the input-order
+// b0, b1, ... aliases buildBatchedBranchQuery applies.
+func aliasMapForBranchRefs(refs []graphQLBranchRef) map[string]repoRef {
+	out := make(map[string]repoRef, len(refs))
+	for i, r := range refs {
+		out[fmt.Sprintf("b%d", i)] = repoRef{Owner: r.Owner, Repo: r.Repo}
+	}
+	return out
+}
+
+// wrapBatchedErrors composes a classifyBatchedErrors result into the
+// (result, error) shape runBatchedPRQuery / runBatchedBranchQuery
+// already return. Behaviour:
+//   - no resolution misses, no residual errors → (nil): caller continues
+//   - no resolution misses, residual errors only → return composite err
+//     unchanged (pre-storm-fix semantics)
+//   - resolution misses present → return *batchedMissingReposErr; Inner
+//     is nil iff there were no residuals so callers can keep the
+//     partial decode, otherwise Inner carries the composite
+func wrapBatchedErrors(missing []repoRef, residual []graphQLError) error {
+	residualErr := graphQLErrorsToErr(residual)
+	if len(missing) == 0 {
+		return residualErr
+	}
+	return &batchedMissingReposErr{Repos: missing, Inner: residualErr}
+}
+
 // graphQLRateLimit mirrors GitHub's top-level rateLimit field, the most
 // accurate source of GraphQL quota since limits are point-cost based.
 type graphQLRateLimit struct {
@@ -411,6 +533,14 @@ func runBatchedPRQuery(ctx context.Context, exec GraphQLExecutor, refs []graphQL
 		return nil, nil
 	}
 	result := make(map[string]*PRStatus, len(refs))
+	// Accumulate missing / residual errors across all chunks so a chunk
+	// of purely-dead repos doesn't short-circuit the loop and drop the
+	// good data in later chunks. Pre-fix, a workspace with >50 watches
+	// where the first 50 happened to be dead would silently skip the
+	// remaining watches on every poll cycle until the first batch's
+	// repos landed in the negative cache.
+	var allMissing []repoRef
+	var allResidual []graphQLError
 	for _, chunk := range chunkedRefs(refs) {
 		query, vars := buildBatchedPRQuery(chunk)
 		var resp struct {
@@ -421,15 +551,27 @@ func runBatchedPRQuery(ctx context.Context, exec GraphQLExecutor, refs []graphQL
 			return nil, err
 		}
 		// GitHub returns HTTP 200 with a top-level "errors" array for partial
-		// auth failures, schema mismatches, or per-alias errors. Surface them
-		// as an error so the caller falls back to per-watch checks instead of
-		// silently absorbing the failure.
-		if err := graphQLErrorsToErr(resp.Errors); err != nil {
-			return nil, err
-		}
+		// auth failures, schema mismatches, or per-alias errors. Split out
+		// "Could not resolve to a Repository" entries via classifyBatchedErrors
+		// so SyncWatchesBatched can negative-cache the dead repos and keep
+		// partial results for the ones that resolved; non-resolution errors
+		// still bubble up so the caller falls back to per-watch checks.
+		missing, residual := classifyBatchedErrors(resp.Errors, aliasMapForPRRefs(chunk))
 		if err := decodeBatchedPRChunk(chunk, resp.Data, result); err != nil {
 			return nil, err
 		}
+		allMissing = append(allMissing, missing...)
+		allResidual = append(allResidual, residual...)
+	}
+	if err := wrapBatchedErrors(allMissing, allResidual); err != nil {
+		// When the error is purely "missing repos" across all chunks,
+		// partial Statuses for the other refs are still in `result`;
+		// surface them to the caller alongside the typed error so it
+		// can populate the negative cache without dropping the good data.
+		if len(allMissing) > 0 && len(allResidual) == 0 {
+			return result, err
+		}
+		return nil, err
 	}
 	return result, nil
 }
@@ -499,6 +641,10 @@ func runBatchedBranchQuery(ctx context.Context, exec GraphQLExecutor, refs []gra
 		return nil, nil
 	}
 	result := make(map[string]*PRStatus, len(refs))
+	// Same accumulation pattern as runBatchedPRQuery — see that function
+	// for the rationale (one dead-repo chunk must not drop later chunks).
+	var allMissing []repoRef
+	var allResidual []graphQLError
 	for _, chunk := range chunkedRefs(refs) {
 		query, vars := buildBatchedBranchQuery(chunk)
 		var resp struct {
@@ -508,12 +654,18 @@ func runBatchedBranchQuery(ctx context.Context, exec GraphQLExecutor, refs []gra
 		if err := exec.ExecuteGraphQL(ctx, query, vars, &resp); err != nil {
 			return nil, err
 		}
-		if err := graphQLErrorsToErr(resp.Errors); err != nil {
-			return nil, err
-		}
+		missing, residual := classifyBatchedErrors(resp.Errors, aliasMapForBranchRefs(chunk))
 		if err := decodeBatchedBranchChunk(chunk, resp.Data, result); err != nil {
 			return nil, err
 		}
+		allMissing = append(allMissing, missing...)
+		allResidual = append(allResidual, residual...)
+	}
+	if err := wrapBatchedErrors(allMissing, allResidual); err != nil {
+		if len(allMissing) > 0 && len(allResidual) == 0 {
+			return result, err
+		}
+		return nil, err
 	}
 	return result, nil
 }
diff --git a/apps/backend/internal/github/handlers.go b/apps/backend/internal/github/handlers.go
index d233d449d5..5572b03b30 100644
--- a/apps/backend/internal/github/handlers.go
+++ b/apps/backend/internal/github/handlers.go
@@ -250,16 +250,27 @@ func wsGetTaskPR(svc *Service, _ *logger.Logger) func(ctx context.Context, msg *
 // PR yet"); multi-repo callers iterate and call setTaskPR for each so the
 // per-repo PR icon stays in sync. The legacy single-PR shape would have
 // silently dropped every repo's PR except the most-recently-updated one.
+//
+// permanent=true on the response signals the frontend's 5s retry loop to
+// stop. It's set when every watch on the task points at a repository
+// classified as unresolvable (missing, deleted, or inaccessible to the
+// authenticated principal) — without this, the frontend keeps re-polling
+// dead repos every 5s for the lifetime of the task, which was the
+// dominant signal in the production SyncWatchesBatched storm.
 func wsSyncTaskPR(svc *Service, _ *logger.Logger) func(ctx context.Context, msg *ws.Message) (*ws.Message, error) {
 	return wsWithField("task_id", func(ctx context.Context, taskID string) (interface{}, error) {
-		prs, err := svc.TriggerPRSyncAll(ctx, taskID)
+		prs, permanent, err := svc.TriggerPRSyncAllPermanent(ctx, taskID)
 		if err != nil {
 			return nil, err
 		}
 		// Return an envelope so the frontend always gets a deterministic
-		// shape even on empty results (`{prs: []}`); a bare `nil` would
-		// confuse the WS handler's success/error branching.
-		return map[string]interface{}{"prs": prs}, nil
+		// shape even on empty results (`{prs: []}`); a bare `nil` slice
+		// would serialize to `prs: null` and confuse the WS handler's
+		// success/error branching, so normalize to an empty slice here.
+		if prs == nil {
+			prs = []*TaskPR{}
+		}
+		return map[string]interface{}{"prs": prs, "permanent": permanent}, nil
 	})
 }
 
diff --git a/apps/backend/internal/github/service.go b/apps/backend/internal/github/service.go
index 8a3662a62e..e3e6c297d7 100644
--- a/apps/backend/internal/github/service.go
+++ b/apps/backend/internal/github/service.go
@@ -84,8 +84,10 @@ type Service struct {
 	taskEventSubs        []bus.Subscription
 	searchCache          *ttlCache
 	prStatusCache        *ttlCache
+	prFeedbackCache      *ttlCache
 	mergeMethodsCache    *ttlCache
 	accessibleReposCache *ttlCache
+	repoErrorCache       *ttlCache
 	protectionCache      *branchProtectionCache
 	rateTracker          *RateTracker
 
@@ -129,8 +131,10 @@ func NewService(client Client, authMethod string, secrets SecretProvider, store
 		logger:               log,
 		searchCache:          newTTLCache(),
 		prStatusCache:        newTTLCache(),
+		prFeedbackCache:      newPRFeedbackCache(),
 		mergeMethodsCache:    newMergeMethodsCache(),
 		accessibleReposCache: newAccessibleReposCache(),
+		repoErrorCache:       newRepoErrorCache(),
 		protectionCache:      newBranchProtectionCache(),
 		rateTracker:          NewRateTracker(eventBus, log),
 		cleanupFailureCounts: make(map[string]int),
@@ -215,3 +219,94 @@ func (s *Service) IsAuthenticated() bool {
 func (s *Service) AuthMethod() string {
 	return s.authMethod
 }
+
+// isRepoCachedAsMissing reports whether the (owner, repo) tuple is in the
+// 10-min negative cache. Called from SyncWatchesBatched and per-watch
+// probes before acquiring the gh throttle, so the storm against
+// missing/unauthorized repos short-circuits without burning a slot.
+func (s *Service) isRepoCachedAsMissing(owner, repo string) bool {
+	if s == nil || s.repoErrorCache == nil {
+		return false
+	}
+	v, ok := s.repoErrorCache.get(repoErrorCacheKey(owner, repo))
+	if !ok {
+		return false
+	}
+	_, isErr := v.(cachedErr)
+	return isErr
+}
+
+// repoErrorGenSnapshot returns the current cache generation. Callers
+// snapshot this BEFORE issuing the fetch that may classify a repo as
+// missing, and pass the snapshot to markRepoAsMissing — that way an
+// evictRepoNegative / ClearRepoErrorCache that fires DURING the fetch
+// (bumping gen) causes the post-fetch write to be dropped, and the
+// negative cache reflects the user's latest intent rather than the
+// stale classifier result.
+func (s *Service) repoErrorGenSnapshot() uint64 {
+	if s == nil || s.repoErrorCache == nil {
+		return 0
+	}
+	return s.repoErrorCache.generation()
+}
+
+// markRepoAsMissing stores ErrRepoNotResolvable for (owner, repo) in the
+// negative cache. Called after a per-watch or batched probe deterministically
+// classifies the repo as missing/unauthorized (see isRepoNotResolvableErr).
+// `gen` must be a snapshot taken via repoErrorGenSnapshot BEFORE the fetch
+// — passing the current generation at write time would defeat the guard:
+// a concurrent del() / clear() that bumped gen between the fetch start and
+// the write would still be present at write time, so a write-time snapshot
+// would land in the new generation and the just-evicted entry would be
+// reinserted. The fetch-start snapshot turns the write into a no-op when
+// any eviction has fired since the fetch began.
+func (s *Service) markRepoAsMissing(owner, repo string, gen uint64) {
+	if s == nil || s.repoErrorCache == nil {
+		return
+	}
+	s.repoErrorCache.setIfCurrentGeneration(
+		repoErrorCacheKey(owner, repo), cachedErr{err: ErrRepoNotResolvable}, gen)
+}
+
+// evictRepoNegative drops any negative-cache entry for (owner, repo). Called
+// from the PR watch (re)create paths and AssociatePRByURL so a freshly
+// linked or re-linked repository is probed immediately on the next sync,
+// without waiting out the 10-min TTL.
+func (s *Service) evictRepoNegative(owner, repo string) {
+	if s == nil || s.repoErrorCache == nil {
+		return
+	}
+	s.repoErrorCache.del(repoErrorCacheKey(owner, repo))
+}
+
+// ClearRepoErrorCache drops every entry from the negative repo-resolution
+// cache. Called from ConfigureToken / ClearToken so a token swap or clear
+// invalidates classifications that were made under the prior identity —
+// without this, a repo that the old token couldn't see would stay
+// "permanent: true" for up to 10 minutes after the user fixes auth, and
+// the frontend retry loop would stop early on stale data. Nil-guards the
+// cache because tests construct Service literals without going through
+// NewService.
+func (s *Service) ClearRepoErrorCache() {
+	if s == nil || s.repoErrorCache == nil {
+		return
+	}
+	s.repoErrorCache.clear()
+}
+
+// ClearPRCaches drops the PR status and PR feedback caches. Called from
+// the token-swap path because review visibility differs across identities
+// (a PR readable to the old token may not be readable to the new one, and
+// vice versa); without this, the new identity could see stale review /
+// check / comment data for up to one TTL after the swap.
+func (s *Service) ClearPRCaches() {
+	if s == nil {
+		return
+	}
+	if s.prStatusCache != nil {
+		s.prStatusCache.clear()
+	}
+	if s.prFeedbackCache != nil {
+		s.prFeedbackCache.clear()
+	}
+}
diff --git a/apps/backend/internal/github/service_pr.go b/apps/backend/internal/github/service_pr.go
index 4c81195bdb..3ef40e02d0 100644
--- a/apps/backend/internal/github/service_pr.go
+++ b/apps/backend/internal/github/service_pr.go
@@ -107,12 +107,34 @@ func (s *Service) GetPR(ctx context.Context, owner, repo string, number int) (*P
 	return s.client.GetPR(ctx, owner, repo, number)
 }
 
-// GetPRFeedback fetches live PR feedback from GitHub.
+// GetPRFeedback fetches live PR feedback from GitHub. Cached briefly with
+// singleflight coalescing: the task page fires this for the same PR many times
+// in quick succession (chat-bar CI chip, detail panel, hover popover — each on
+// its own render/mount triggers), and each miss is 4 sequential GitHub REST
+// calls. Without this, a burst fanned out into dozens of slow, rate-limited
+// duplicate requests. The returned pointer is shared — callers must not mutate it.
 func (s *Service) GetPRFeedback(ctx context.Context, owner, repo string, number int) (*PRFeedback, error) {
 	if s.client == nil {
 		return nil, fmt.Errorf("github client not available")
 	}
-	return s.client.GetPRFeedback(ctx, owner, repo, number)
+	// Detach the upstream call's context from the singleflight leader: a
+	// cancelled leader (user closes the tab) would otherwise return its
+	// context error to all co-waiters, cascading a single client disconnect
+	// into a burst of transient failures across concurrent callers.
+	// context.WithoutCancel strips the deadline too, so re-attach the
+	// parent deadline explicitly — without this, a still-active caller's
+	// timeout wouldn't bound the shared fetch and quota could keep being
+	// consumed past the deadline.
+	fetchCtx, cancelFetch := derivedFetchContext(ctx)
+	defer cancelFetch()
+	key := prStatusCacheKey(owner, repo, number)
+	v, err := s.prFeedbackCache.doOrFetch(key, func() (any, error) {
+		return s.client.GetPRFeedback(fetchCtx, owner, repo, number)
+	})
+	if err != nil {
+		return nil, err
+	}
+	return v.(*PRFeedback), nil
 }
 
 // GetPRStatus fetches lightweight PR status (review + checks + mergeable).
@@ -123,9 +145,13 @@ func (s *Service) GetPRStatus(ctx context.Context, owner, repo string, number in
 	if s.client == nil {
 		return nil, fmt.Errorf("github client not available")
 	}
+	// Detach the upstream call from the singleflight leader's context; see
+	// GetPRFeedback for the cascading-cancel + deadline-preserve rationale.
+	fetchCtx, cancelFetch := derivedFetchContext(ctx)
+	defer cancelFetch()
 	key := prStatusCacheKey(owner, repo, number)
 	v, err := s.prStatusCache.doOrFetch(key, func() (any, error) {
-		return s.client.GetPRStatus(ctx, owner, repo, number)
+		return s.client.GetPRStatus(fetchCtx, owner, repo, number)
 	})
 	if err != nil {
 		return nil, err
@@ -133,6 +159,22 @@ func (s *Service) GetPRStatus(ctx context.Context, owner, repo string, number in
 	return v.(*PRStatus), nil
 }
 
+// derivedFetchContext builds the upstream-fetch context used by the
+// singleflight-cached GetPRFeedback / GetPRStatus paths. It strips the
+// caller's cancellation via context.WithoutCancel so a single client
+// disconnect doesn't cascade the same error to all co-waiters, but
+// preserves the caller's deadline (when present) so the fetch can't
+// outlive the request budget and keep consuming GitHub quota past it.
+// Returns the derived context and a cancel func the caller must defer
+// to release resources promptly.
+func derivedFetchContext(ctx context.Context) (context.Context, context.CancelFunc) {
+	fetchCtx := context.WithoutCancel(ctx)
+	if deadline, ok := ctx.Deadline(); ok {
+		return context.WithDeadline(fetchCtx, deadline)
+	}
+	return fetchCtx, func() {}
+}
+
 // PRRef identifies a pull request by owner/repo/number.
 type PRRef struct {
 	Owner  string `json:"owner"`
diff --git a/apps/backend/internal/github/service_pr_feedback_test.go b/apps/backend/internal/github/service_pr_feedback_test.go
new file mode 100644
index 0000000000..142e199321
--- /dev/null
+++ b/apps/backend/internal/github/service_pr_feedback_test.go
@@ -0,0 +1,107 @@
+package github
+
+import (
+	"context"
+	"sync"
+	"sync/atomic"
+	"testing"
+)
+
+// countingFeedbackClient counts top-level GetPRFeedback calls so the cache /
+// singleflight behavior of Service.GetPRFeedback can be asserted without
+// reaching real GitHub.
+type countingFeedbackClient struct {
+	*MockClient
+	calls atomic.Int32
+}
+
+func (c *countingFeedbackClient) GetPRFeedback(_ context.Context, owner, repo string, number int) (*PRFeedback, error) {
+	c.calls.Add(1)
+	return &PRFeedback{PR: &PR{RepoOwner: owner, RepoName: repo, Number: number}}, nil
+}
+
+// Repeated feedback fetches for the same PR within the TTL window must collapse
+// to a single upstream call. Before the cache was added, every call hit GitHub
+// (4 sequential REST calls each), so a render/mount burst on the task page
+// fanned out into dozens of slow, rate-limited duplicate requests.
+func TestService_GetPRFeedback_UsesCache(t *testing.T) {
+	client := &countingFeedbackClient{MockClient: NewMockClient()}
+	svc := newTestService(client)
+	ctx := context.Background()
+
+	for i := 0; i < 3; i++ {
+		if _, err := svc.GetPRFeedback(ctx, "o", "r", 1); err != nil {
+			t.Fatalf("call %d: %v", i, err)
+		}
+	}
+	if got := client.calls.Load(); got != 1 {
+		t.Fatalf("expected 1 upstream call, got %d", got)
+	}
+
+	if _, err := svc.GetPRFeedback(ctx, "o", "r", 2); err != nil {
+		t.Fatal(err)
+	}
+	if got := client.calls.Load(); got != 2 {
+		t.Fatalf("expected new fetch for different number, got %d", got)
+	}
+}
+
+// blockingFeedbackClient blocks inside GetPRFeedback until released, so the
+// test can force many concurrent in-flight fetches for the same PR and assert
+// they coalesce into one upstream call (singleflight). `started` is signaled
+// the moment a goroutine enters the upstream call so the test can be certain
+// at least one fetch is in flight before releasing siblings.
+type blockingFeedbackClient struct {
+	*MockClient
+	calls   atomic.Int32
+	release chan struct{}
+	started chan struct{}
+}
+
+func (c *blockingFeedbackClient) GetPRFeedback(_ context.Context, owner, repo string, number int) (*PRFeedback, error) {
+	c.calls.Add(1)
+	// Non-blocking signal so only the FIRST entrant fires `started`; later
+	// arrivals queue behind the singleflight without re-signaling.
+	select {
+	case c.started <- struct{}{}:
+	default:
+	}
+	<-c.release
+	return &PRFeedback{PR: &PR{RepoOwner: owner, RepoName: repo, Number: number}}, nil
+}
+
+// Concurrent feedback fetches for the same PR must coalesce to a single
+// upstream call. This reproduces the production pile-up: the task page fired ~8
+// identical GetPRFeedback requests concurrently, and without singleflight each
+// independently hammered GitHub, ballooning durations to 40-73s.
+func TestService_GetPRFeedback_CoalescesConcurrentCalls(t *testing.T) {
+	client := &blockingFeedbackClient{
+		MockClient: NewMockClient(),
+		release:    make(chan struct{}),
+		started:    make(chan struct{}, 1),
+	}
+	svc := newTestService(client)
+
+	const n = 16
+	var done sync.WaitGroup
+	done.Add(n)
+	for i := 0; i < n; i++ {
+		go func() {
+			defer done.Done()
+			if _, err := svc.GetPRFeedback(context.Background(), "o", "r", 1); err != nil {
+				t.Errorf("concurrent fetch: %v", err)
+			}
+		}()
+	}
+	// Wait for the leader to actually enter the upstream call (not just be
+	// scheduled). Pre-fix this used launched.Wait, which only guaranteed the
+	// goroutines had STARTED — late arrivals after release could have seen
+	// a warm cache and skipped the singleflight, masking regressions.
+	<-client.started
+	close(client.release)
+	done.Wait()
+
+	if got := client.calls.Load(); got != 1 {
+		t.Fatalf("expected concurrent calls to coalesce to 1 upstream call, got %d", got)
+	}
+}
diff --git a/apps/backend/internal/github/service_pr_watch.go b/apps/backend/internal/github/service_pr_watch.go
index b6c4e4e6c1..d581f3f672 100644
--- a/apps/backend/internal/github/service_pr_watch.go
+++ b/apps/backend/internal/github/service_pr_watch.go
@@ -22,6 +22,12 @@ import (
 // its own watch row. Multi-branch tasks store one watch per branch so a
 // secondary branch's push isn't lost behind the primary's existing watch.
 func (s *Service) CreatePRWatch(ctx context.Context, sessionID, taskID, repositoryID, owner, repo string, prNumber int, branch string) (*PRWatch, error) {
+	// Evict any negative-cache entry up front. Caller intent on either
+	// branch (creating a new watch or re-finding an existing one) is "I
+	// want this repo watched", which means a stale "missing" verdict
+	// from a prior auth failure should be re-probed immediately rather
+	// than held for the rest of the 10-min TTL.
+	s.evictRepoNegative(owner, repo)
 	existing, err := s.store.GetPRWatchBySessionRepoAndBranch(ctx, sessionID, repositoryID, branch)
 	if err != nil {
 		return nil, err
@@ -134,6 +140,10 @@ func (s *Service) CheckPRWatch(ctx context.Context, watch *PRWatch) (*PRStatus,
 // so each branch gets its own watch — keying on (session, repo) alone
 // drops secondary branches' watches behind the primary's existing row.
 func (s *Service) EnsurePRWatch(ctx context.Context, sessionID, taskID, repositoryID, owner, repo, branch string) (*PRWatch, error) {
+	// Same up-front eviction as CreatePRWatch — see that function for the
+	// rationale. The existing-watch early-return path must not inherit a
+	// stale "missing" verdict from a prior incarnation.
+	s.evictRepoNegative(owner, repo)
 	existing, err := s.store.GetPRWatchBySessionRepoAndBranch(ctx, sessionID, repositoryID, branch)
 	if err != nil {
 		return nil, err
@@ -619,20 +629,46 @@ func (s *Service) TriggerPRSync(ctx context.Context, taskID string) (*TaskPR, er
 // when the batched fetch itself fails; in those cases we fan out one
 // subprocess per watch as before.
 func (s *Service) TriggerPRSyncAll(ctx context.Context, taskID string) ([]*TaskPR, error) {
+	prs, _, err := s.triggerPRSyncAllPermanent(ctx, taskID)
+	return prs, err
+}
+
+// TriggerPRSyncAllPermanent extends TriggerPRSyncAll with a "permanent"
+// flag: true iff the task has at least one PR watch and every one of
+// those watches is currently in the 10-min negative cache (or its repo
+// is otherwise classified as unresolvable). The WS handler uses this to
+// tell the frontend to stop the 5s sync-retry interval, so a task
+// pointing at a deleted repo doesn't keep hammering the gh throttle for
+// the lifetime of the task.
+func (s *Service) TriggerPRSyncAllPermanent(ctx context.Context, taskID string) ([]*TaskPR, bool, error) {
+	return s.triggerPRSyncAllPermanent(ctx, taskID)
+}
+
+func (s *Service) triggerPRSyncAllPermanent(ctx context.Context, taskID string) ([]*TaskPR, bool, error) {
 	watches, err := s.store.ListPRWatchesByTask(ctx, taskID)
 	if err != nil {
-		return nil, fmt.Errorf("list PR watches: %w", err)
+		return nil, false, fmt.Errorf("list PR watches: %w", err)
 	}
 	if len(watches) == 0 {
 		// No watches — fall back to whatever TaskPRs already exist (e.g.
 		// PRs imported via task-create-from-PR-URL where the watch is
-		// optional). Empty slice if none.
+		// optional). Empty slice if none. permanent=false (the task can
+		// still acquire a watch later via push detection).
 		existing, listErr := s.store.ListTaskPRsByTask(ctx, taskID)
 		if listErr != nil {
-			return nil, fmt.Errorf("list task PRs: %w", listErr)
+			return nil, false, fmt.Errorf("list task PRs: %w", listErr)
 		}
-		return existing, nil
+		return existing, false, nil
 	}
+	prs, syncErr := s.runBatchedOrPerWatchSync(ctx, taskID, watches)
+	return prs, s.areAllWatchesPermanentlyMissing(watches), syncErr
+}
+
+// runBatchedOrPerWatchSync is the shared "try batched, fall back to
+// per-watch" body of triggerPRSyncAllPermanent. Split out so the
+// permanent-flag computation can wrap the result without duplicating
+// the batched / fallback branches inline.
+func (s *Service) runBatchedOrPerWatchSync(ctx context.Context, taskID string, watches []*PRWatch) ([]*TaskPR, error) {
 	if _, batchErr := s.SyncWatchesBatched(ctx, watches); batchErr != nil {
 		s.logger.Debug("batched PR sync failed; falling back to per-watch",
 			zap.String("task_id", taskID), zap.Error(batchErr))
@@ -643,6 +679,24 @@ func (s *Service) TriggerPRSyncAll(ctx context.Context, taskID string) ([]*TaskP
 	return s.store.ListTaskPRsByTask(ctx, taskID)
 }
 
+// areAllWatchesPermanentlyMissing reports whether every supplied watch's
+// (owner, repo) is currently in the 10-min negative cache. Used to set
+// the permanent flag on wsSyncTaskPR's response so the frontend stops
+// retrying when a task points only at deleted/inaccessible repositories.
+// Returns false when the input is empty so an empty task doesn't get
+// classified as permanently missing.
+func (s *Service) areAllWatchesPermanentlyMissing(watches []*PRWatch) bool {
+	if len(watches) == 0 {
+		return false
+	}
+	for _, w := range watches {
+		if !s.isRepoCachedAsMissing(w.Owner, w.Repo) {
+			return false
+		}
+	}
+	return true
+}
+
 // triggerPRSyncAllPerWatch is the legacy fan-out path: one gh subprocess
 // per watch. Kept as a fallback for the NoopClient and for the rare case
 // where the batched GraphQL call fails (auth glitch, network blip) so a
@@ -706,6 +760,13 @@ func (s *Service) triggerPRDetection(ctx context.Context, watch *PRWatch, taskID
 // searching (pr_number=0) watch. It runs inside triggerPRDetection's
 // singleflight so only one probe per watch is in flight at a time.
 func (s *Service) detectPRForWatchOnce(ctx context.Context, watch *PRWatch, taskID string) (*TaskPR, error) {
+	// Short-circuit when this repo is already in the 10-min negative
+	// cache. Without this, an unresolvable repo gets a fresh per-watch
+	// probe every 5s (the frontend retry cadence) until the freshness
+	// timestamp below stamps in — multiplied by every watch the task has.
+	if s.isRepoCachedAsMissing(watch.Owner, watch.Repo) {
+		return nil, ErrRepoNotResolvable
+	}
 	// Throttle branch-detection probes. A watch still searching for its PR
 	// (pr_number=0) has no TaskPR row yet, so triggerPRStatusSync's freshness
 	// check can't gate it — we gate on the watch's own last_checked_at instead.
@@ -716,7 +777,19 @@ func (s *Service) detectPRForWatchOnce(ctx context.Context, watch *PRWatch, task
 		return s.store.GetTaskPRByRepository(ctx, taskID, watch.RepositoryID)
 	}
 
+	// Snapshot the negative-cache generation BEFORE the fetch so an
+	// eviction (CreatePRWatch / EnsurePRWatch / explicit clear) that
+	// fires WHILE FindPRByBranch is running wins over our post-fetch
+	// classification — see Service.markRepoAsMissing for the rationale.
+	repoErrGen := s.repoErrorGenSnapshot()
 	pr, err := s.client.FindPRByBranch(ctx, watch.Owner, watch.Repo, watch.Branch)
+	if err != nil && isRepoNotResolvableErr(err) {
+		s.markRepoAsMissing(watch.Owner, watch.Repo, repoErrGen)
+		// Wrap so wsSyncTaskPR can errors.Is(err, ErrRepoNotResolvable)
+		// to flag the WS response as permanent without re-running the
+		// classifier on the raw upstream error string.
+		err = fmt.Errorf("%w: %w", ErrRepoNotResolvable, err)
+	}
 	// Stamp last_checked_at regardless of outcome — including on error. The
 	// flood this fixes IS the error path (an unresolvable repo makes `gh` exit
 	// non-zero), so stamping only on success would leave the bug unfixed. The
@@ -775,13 +848,27 @@ func (s *Service) triggerPRStatusSync(ctx context.Context, watch *PRWatch, taskI
 		}
 	}
 
+	// Short-circuit on negative cache so a 5s frontend retry against a
+	// dead repo doesn't burn the gh throttle. Eviction happens on watch
+	// (re)create paths, so a freshly linked repo is probed immediately.
+	if s.isRepoCachedAsMissing(watch.Owner, watch.Repo) {
+		return nil, ErrRepoNotResolvable
+	}
+
 	// Coalesce concurrent syncs for the same PR
 	key := fmt.Sprintf("%s/%s/%d", watch.Owner, watch.Repo, watch.PRNumber)
 	v, err, _ := s.syncGroup.Do(key, func() (interface{}, error) {
 		bgCtx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
 		defer cancel()
+		// Snapshot cache generation BEFORE the per-watch probe so a
+		// concurrent eviction wins; see Service.markRepoAsMissing.
+		repoErrGen := s.repoErrorGenSnapshot()
 		status, _, checkErr := s.CheckPRWatch(bgCtx, watch)
 		if checkErr != nil {
+			if isRepoNotResolvableErr(checkErr) {
+				s.markRepoAsMissing(watch.Owner, watch.Repo, repoErrGen)
+				return nil, fmt.Errorf("%w: %w", ErrRepoNotResolvable, checkErr)
+			}
 			return nil, checkErr
 		}
 		if status == nil {
diff --git a/apps/backend/internal/github/service_pr_watch_batched.go b/apps/backend/internal/github/service_pr_watch_batched.go
index d3b099134f..2c3e975c27 100644
--- a/apps/backend/internal/github/service_pr_watch_batched.go
+++ b/apps/backend/internal/github/service_pr_watch_batched.go
@@ -2,7 +2,10 @@ package github
 
 import (
 	"context"
+	"errors"
 	"fmt"
+	"sort"
+	"strings"
 	"time"
 
 	"go.uber.org/zap"
@@ -59,38 +62,160 @@ func (s *Service) SyncWatchesBatched(ctx context.Context, watches []*PRWatch) ([
 
 // fetchBatchedWatchStatuses runs the numbered- and branch-keyed GraphQL
 // queries and merges their results. Returns an error when either query
-// fails so the caller can fall back to per-watch checks.
+// fails so the caller can fall back to per-watch checks. Watches whose
+// (owner, repo) is already cached as missing are filtered out before
+// hitting GraphQL, and any newly-discovered missing repos in the
+// response are fed into the negative cache so subsequent polls
+// short-circuit before acquiring the gh throttle.
+//
+// The full fetch is gated by a service-level singleflight keyed by the
+// sorted ref set. Without it, a burst of concurrent SyncWatchesBatched
+// calls (poller racing the WS sync racing the workspace background
+// refresh) all hit GraphQL during the window before the first caller
+// seeds the negative cache — exactly the storm this change is trying to
+// calm. The shared-result map is read-only at the call sites (only
+// map lookups in applyBatched*), so it's safe to share.
+//
+// The upstream fetch detaches from the leader's cancellation via
+// derivedFetchContext so one caller (WS) disconnecting mid-flight
+// doesn't cascade context.Canceled to all co-waiters — same pattern as
+// GetPRFeedback / GetPRStatus. The leader's deadline is preserved so
+// the fetch can't outlive the request budget.
 func (s *Service) fetchBatchedWatchStatuses(
 	ctx context.Context, exec GraphQLExecutor, numbered, searching []*PRWatch,
 ) (map[string]*PRStatus, error) {
-	combined := make(map[string]*PRStatus, len(numbered)+len(searching))
-	if len(numbered) > 0 {
-		refs := make([]graphQLPRRef, 0, len(numbered))
-		for _, w := range numbered {
-			refs = append(refs, graphQLPRRef{Owner: w.Owner, Repo: w.Repo, Number: w.PRNumber})
+	key := batchedFetchSingleflightKey(numbered, searching)
+	fetchCtx, cancelFetch := derivedFetchContext(ctx)
+	defer cancelFetch()
+	v, err, _ := s.syncGroup.Do(key, func() (interface{}, error) {
+		// Snapshot the negative-cache generation BEFORE the batched
+		// fetch so an eviction (relink / clear) firing while the
+		// GraphQL call is in flight wins over the post-fetch
+		// markRepoAsMissing writes — see Service.markRepoAsMissing.
+		repoErrGen := s.repoErrorGenSnapshot()
+		combined := make(map[string]*PRStatus, len(numbered)+len(searching))
+		if err := s.fetchBatchedPRStatuses(fetchCtx, exec, numbered, combined, repoErrGen); err != nil {
+			return nil, err
 		}
-		out, err := runBatchedPRQuery(ctx, exec, refs)
-		if err != nil {
-			return nil, fmt.Errorf("batched PR query: %w", err)
-		}
-		for k, v := range out {
-			combined[k] = v
+		if err := s.fetchBatchedBranchStatuses(fetchCtx, exec, searching, combined, repoErrGen); err != nil {
+			return nil, err
 		}
+		return combined, nil
+	})
+	if err != nil {
+		return nil, err
 	}
-	if len(searching) > 0 {
-		refs := make([]graphQLBranchRef, 0, len(searching))
-		for _, w := range searching {
-			refs = append(refs, graphQLBranchRef{Owner: w.Owner, Repo: w.Repo, Branch: w.Branch})
-		}
-		out, err := runBatchedBranchQuery(ctx, exec, refs)
-		if err != nil {
-			return nil, fmt.Errorf("batched branch query: %w", err)
-		}
-		for k, v := range out {
-			combined[k] = v
-		}
+	if v == nil {
+		return nil, nil
 	}
-	return combined, nil
+	return v.(map[string]*PRStatus), nil
+}
+
+// batchedFetchSingleflightKey builds a deterministic key from the sorted
+// ref tuples in (numbered, searching). Sorting ensures ordering doesn't
+// split equivalent calls into separate slots, and lowercasing matches
+// repoErrorCacheKey so the in-flight key is stable regardless of caller
+// casing (watches from the DB always have consistent casing in practice,
+// but it costs nothing to make the key insensitive). Distinct "n:"/"b:"
+// prefixes prevent a numbered ref colliding with a branch ref that
+// happens to render the same string.
+func batchedFetchSingleflightKey(numbered, searching []*PRWatch) string {
+	parts := make([]string, 0, len(numbered)+len(searching))
+	for _, w := range numbered {
+		parts = append(parts, fmt.Sprintf("n:%s/%s#%d",
+			strings.ToLower(w.Owner), strings.ToLower(w.Repo), w.PRNumber))
+	}
+	for _, w := range searching {
+		parts = append(parts, fmt.Sprintf("b:%s/%s@%s",
+			strings.ToLower(w.Owner), strings.ToLower(w.Repo), w.Branch))
+	}
+	sort.Strings(parts)
+	return "batched-fetch:" + strings.Join(parts, "|")
+}
+
+// fetchBatchedPRStatuses runs the numbered-watch (PR-number-keyed) batch.
+// Watches whose repo is in the negative cache are dropped before building
+// the GraphQL refs so the dead-repo storm doesn't burn gh throttle slots.
+// Missing repos surfaced by the response are negative-cached for the next
+// 10 minutes; partial results for the repos that did resolve are merged
+// into `combined`.
+func (s *Service) fetchBatchedPRStatuses(
+	ctx context.Context, exec GraphQLExecutor, numbered []*PRWatch, combined map[string]*PRStatus,
+	repoErrGen uint64,
+) error {
+	refs := make([]graphQLPRRef, 0, len(numbered))
+	for _, w := range numbered {
+		if s.isRepoCachedAsMissing(w.Owner, w.Repo) {
+			continue
+		}
+		refs = append(refs, graphQLPRRef{Owner: w.Owner, Repo: w.Repo, Number: w.PRNumber})
+	}
+	if len(refs) == 0 {
+		return nil
+	}
+	out, err := runBatchedPRQuery(ctx, exec, refs)
+	out, err = s.absorbMissingReposErr(out, err, repoErrGen)
+	if err != nil {
+		return fmt.Errorf("batched PR query: %w", err)
+	}
+	for k, v := range out {
+		combined[k] = v
+	}
+	return nil
+}
+
+// fetchBatchedBranchStatuses runs the branch-keyed batch with the same
+// negative-cache filter and missing-repo absorption as the numbered path.
+func (s *Service) fetchBatchedBranchStatuses(
+	ctx context.Context, exec GraphQLExecutor, searching []*PRWatch, combined map[string]*PRStatus,
+	repoErrGen uint64,
+) error {
+	refs := make([]graphQLBranchRef, 0, len(searching))
+	for _, w := range searching {
+		if s.isRepoCachedAsMissing(w.Owner, w.Repo) {
+			continue
+		}
+		refs = append(refs, graphQLBranchRef{Owner: w.Owner, Repo: w.Repo, Branch: w.Branch})
+	}
+	if len(refs) == 0 {
+		return nil
+	}
+	out, err := runBatchedBranchQuery(ctx, exec, refs)
+	out, err = s.absorbMissingReposErr(out, err, repoErrGen)
+	if err != nil {
+		return fmt.Errorf("batched branch query: %w", err)
+	}
+	for k, v := range out {
+		combined[k] = v
+	}
+	return nil
+}
+
+// absorbMissingReposErr extracts the negative-cacheable repos from a
+// runBatched* result and seeds the cache, then returns (partial,
+// remaining-err). When the error was PURELY missing-repo errors, the
+// partial decode is preserved and a nil error is returned so the caller
+// can use the resolved repos' data. When other errors were also
+// present, the inner error is returned unchanged so the caller falls
+// back to per-watch checks. `repoErrGen` is the negative-cache generation
+// snapshot taken BEFORE the fetch so a concurrent eviction wins.
+func (s *Service) absorbMissingReposErr(out map[string]*PRStatus, err error, repoErrGen uint64) (map[string]*PRStatus, error) {
+	if err == nil {
+		return out, nil
+	}
+	var missingErr *batchedMissingReposErr
+	if !errors.As(err, &missingErr) {
+		return nil, err
+	}
+	for _, r := range missingErr.Repos {
+		s.markRepoAsMissing(r.Owner, r.Repo, repoErrGen)
+		s.logger.Debug("repo not resolvable; negative-cached",
+			zap.String("owner", r.Owner), zap.String("repo", r.Repo))
+	}
+	if missingErr.Inner != nil {
+		return nil, missingErr.Inner
+	}
+	return out, nil
 }
 
 // applyBatchedNumberedWatch mirrors Poller.applyPRStatus on the service
@@ -98,6 +223,14 @@ func (s *Service) fetchBatchedWatchStatuses(
 func (s *Service) applyBatchedNumberedWatch(
 	ctx context.Context, w *PRWatch, statusByKey map[string]*PRStatus, now time.Time,
 ) PRWatchSyncResult {
+	// Repo is in the 10-min negative cache — the fetch path didn't include
+	// it in the GraphQL batch, so the apply path can't probe upstream either.
+	// Bump last_checked_at to suppress retry storms and surface SyncFailed
+	// so the WS handler can flag the result as permanent for the frontend.
+	if s.isRepoCachedAsMissing(w.Owner, w.Repo) {
+		_ = s.store.UpdatePRWatchTimestamps(ctx, w.ID, now, nil, "", "")
+		return PRWatchSyncResult{Watch: w, SyncFailed: true}
+	}
 	status, ok := statusByKey[prStatusCacheKey(w.Owner, w.Repo, w.PRNumber)]
 	if !ok || status == nil {
 		// Alias missing — best-effort liveness bump so we don't immediately re-probe.
@@ -135,6 +268,9 @@ func (s *Service) applyBatchedSearchingWatch(
 	// paths, so hoist the single call above the branch to make that
 	// invariant obvious.
 	_ = s.store.UpdatePRWatchTimestamps(ctx, w.ID, now, nil, "", "")
+	if s.isRepoCachedAsMissing(w.Owner, w.Repo) {
+		return PRWatchSyncResult{Watch: w, SyncFailed: true}
+	}
 	status, ok := statusByKey[graphqlBranchKey(w.Owner, w.Repo, w.Branch)]
 	if !ok || status == nil || status.PR == nil {
 		return PRWatchSyncResult{Watch: w}
diff --git a/apps/backend/internal/github/service_repo_negative_cache_test.go b/apps/backend/internal/github/service_repo_negative_cache_test.go
new file mode 100644
index 0000000000..ca5146fc15
--- /dev/null
+++ b/apps/backend/internal/github/service_repo_negative_cache_test.go
@@ -0,0 +1,322 @@
+package github
+
+import (
+	"context"
+	"runtime"
+	"strings"
+	"sync"
+	"sync/atomic"
+	"testing"
+)
+
+// TestSyncWatchesBatched_NegativeCache_ShortCircuits is the regression
+// test for the SyncWatchesBatched storm: once a repo is classified as
+// unresolvable, subsequent syncs for watches on that repo must NOT issue
+// a fresh GraphQL call. Before the negative cache, every 5s frontend
+// retry against a dead repo (NBCUDTC/bff was the production smoking
+// gun) burned a gh throttle slot and produced a fresh "Could not resolve
+// to a Repository" log line.
+func TestSyncWatchesBatched_NegativeCache_ShortCircuits(t *testing.T) {
+	_, svc, gh, store := setupBatchedPollerTest(t)
+	ctx := context.Background()
+
+	w := &PRWatch{SessionID: "s1", TaskID: "t1", Owner: "Dead", Repo: "Repo", PRNumber: 7, Branch: "br"}
+	if err := store.CreatePRWatch(ctx, w); err != nil {
+		t.Fatalf("create watch: %v", err)
+	}
+	// CreatePRWatch evicts the negative cache, so prime it AFTER the
+	// watch is in place.
+	svc.markRepoAsMissing(w.Owner, w.Repo, svc.repoErrorGenSnapshot())
+
+	// No canned responses — if anything reaches gh the test fails fast
+	// with "no canned PR response" / "no canned branch response".
+	results, err := svc.SyncWatchesBatched(ctx, []*PRWatch{w})
+	if err != nil {
+		t.Fatalf("SyncWatchesBatched: %v", err)
+	}
+	if len(gh.prQueries) != 0 {
+		t.Errorf("expected zero PR GraphQL calls when repo is cached as missing, got %d", len(gh.prQueries))
+	}
+	if len(gh.branchQueries) != 0 {
+		t.Errorf("expected zero branch GraphQL calls when repo is cached as missing, got %d", len(gh.branchQueries))
+	}
+	if len(results) != 1 || !results[0].SyncFailed {
+		t.Errorf("expected one SyncFailed result for cached-missing watch, got %+v", results)
+	}
+}
+
+// TestSyncWatchesBatched_LearnsAndCachesMissingRepo verifies the
+// "first call discovers the dead repo, the second call short-circuits"
+// flow that collapses the storm down to one upstream call per 10
+// minutes per repo. Mirrors TestService_GetPRFeedback_UsesCache's
+// repeated-call assertion but on the SyncWatchesBatched seam.
+func TestSyncWatchesBatched_LearnsAndCachesMissingRepo(t *testing.T) {
+	_, svc, gh, store := setupBatchedPollerTest(t)
+	ctx := context.Background()
+
+	w := &PRWatch{SessionID: "s1", TaskID: "t1", Owner: "Dead", Repo: "Repo", PRNumber: 7, Branch: "br"}
+	if err := store.CreatePRWatch(ctx, w); err != nil {
+		t.Fatalf("create watch: %v", err)
+	}
+
+	// Canned response: data has "repo0": null, errors[].path = ["repo0"]
+	// with the deterministic "Could not resolve to a Repository" message.
+	// This mirrors GitHub's actual response shape for a dead repo alias.
+	gh.prResponses = []string{`{
+		"data": {"repo0": null},
+		"errors": [{"message": "Could not resolve to a Repository with the name 'Dead/Repo'.", "type": "NOT_FOUND", "path": ["repo0"]}]
+	}`}
+
+	if _, err := svc.SyncWatchesBatched(ctx, []*PRWatch{w}); err != nil {
+		t.Fatalf("first SyncWatchesBatched: %v", err)
+	}
+	if !svc.isRepoCachedAsMissing(w.Owner, w.Repo) {
+		t.Fatalf("expected repo to be negative-cached after first sync; cache=%v", svc.repoErrorCache.entries)
+	}
+
+	// Second call must NOT hit gh — the cache short-circuits everything.
+	if _, err := svc.SyncWatchesBatched(ctx, []*PRWatch{w}); err != nil {
+		t.Fatalf("second SyncWatchesBatched: %v", err)
+	}
+	if got := len(gh.prQueries); got != 1 {
+		t.Errorf("expected 1 upstream PR call across two syncs (first discovers, second short-circuits), got %d", got)
+	}
+}
+
+// blockingExec wraps the existing graphQLMockClient with a controllable
+// blocker. The test releases it after asserting that concurrent
+// SyncWatchesBatched calls have all coalesced onto the same in-flight
+// fetch — mirroring TestService_GetPRFeedback_CoalescesConcurrentCalls.
+type blockingExec struct {
+	*graphQLMockClient
+	calls   atomic.Int32
+	release chan struct{}
+}
+
+func (b *blockingExec) ExecuteGraphQL(ctx context.Context, query string, vars map[string]any, out any) error {
+	b.calls.Add(1)
+	<-b.release
+	return b.graphQLMockClient.ExecuteGraphQL(ctx, query, vars, out)
+}
+
+// TestSyncWatchesBatched_NegativeCache_CoalescesConcurrentCalls verifies
+// that N concurrent SyncWatchesBatched calls for the same dead repo
+// don't fan out into N gh subprocesses while waiting for the first
+// classification to land in the negative cache. The throttle protects
+// us at process level, but the cache check + classifier is the seam
+// that has to coalesce here — pre-fix, every concurrent caller would
+// race the empty cache and queue behind the gh throttle.
+func TestSyncWatchesBatched_NegativeCache_CoalescesConcurrentCalls(t *testing.T) {
+	_, svc, gh, store := setupBatchedPollerTest(t)
+
+	w := &PRWatch{SessionID: "s1", TaskID: "t1", Owner: "Dead", Repo: "Repo", PRNumber: 7, Branch: "br"}
+	if err := store.CreatePRWatch(context.Background(), w); err != nil {
+		t.Fatalf("create watch: %v", err)
+	}
+
+	// Prime the negative cache directly — the test then asserts that
+	// every concurrent caller short-circuits without queueing on
+	// ExecuteGraphQL. (Coalescing-via-singleflight inside the cache is
+	// covered by lower-level cache tests; here we just need to confirm
+	// SyncWatchesBatched honors the cache under concurrency.)
+	svc.markRepoAsMissing(w.Owner, w.Repo, svc.repoErrorGenSnapshot())
+
+	blocker := &blockingExec{graphQLMockClient: gh, release: make(chan struct{})}
+	svc.client = blocker
+	close(blocker.release) // never block — if any call reaches gh the test will catch it via blocker.calls
+
+	const n = 16
+	var wg sync.WaitGroup
+	wg.Add(n)
+	for i := 0; i < n; i++ {
+		go func() {
+			defer wg.Done()
+			if _, err := svc.SyncWatchesBatched(context.Background(), []*PRWatch{w}); err != nil {
+				t.Errorf("concurrent SyncWatchesBatched: %v", err)
+			}
+		}()
+	}
+	wg.Wait()
+
+	if got := blocker.calls.Load(); got != 0 {
+		t.Fatalf("expected 0 upstream calls (every caller should short-circuit on cache), got %d", got)
+	}
+}
+
+// TestSyncWatchesBatched_FirstMissCoalesces is the regression test for
+// the burst-before-first-cache-hit window: when N concurrent
+// SyncWatchesBatched calls race against an EMPTY negative cache for the
+// same dead repo, the singleflight wrap around fetchBatchedWatchStatuses
+// must collapse them to a single upstream GraphQL call. Without it, the
+// pre-filter's isRepoCachedAsMissing check would all return false (cache
+// is empty) and every caller would issue its own batched query, stacking
+// upstream requests during the exact window the negative cache is trying
+// to calm down.
+func TestSyncWatchesBatched_FirstMissCoalesces(t *testing.T) {
+	_, svc, gh, store := setupBatchedPollerTest(t)
+
+	w := &PRWatch{SessionID: "s1", TaskID: "t1", Owner: "Dead", Repo: "Repo", PRNumber: 7, Branch: "br"}
+	if err := store.CreatePRWatch(context.Background(), w); err != nil {
+		t.Fatalf("create watch: %v", err)
+	}
+
+	// Block the first in-flight ExecuteGraphQL so concurrent callers
+	// have time to join the singleflight before any response lands. The
+	// `joined` counter is the deterministic barrier: every caller that
+	// joins the singleflight bumps it before the leader's fetch returns,
+	// so we can wait for "all n callers are inside the singleflight"
+	// without a timing-sensitive sleep.
+	release := make(chan struct{})
+	var gotFirstInFlight sync.WaitGroup
+	gotFirstInFlight.Add(1)
+	var firstSignaled atomic.Bool
+	var joined atomic.Int32
+	gh.onExecute = func() {
+		if firstSignaled.CompareAndSwap(false, true) {
+			gotFirstInFlight.Done()
+		}
+		<-release
+	}
+	gh.prResponses = []string{`{
+		"data": {"repo0": null},
+		"errors": [{"message": "Could not resolve to a Repository with the name 'Dead/Repo'.", "type": "NOT_FOUND", "path": ["repo0"]}]
+	}`}
+
+	const n = 16
+	var wg sync.WaitGroup
+	wg.Add(n)
+	errs := make(chan error, n)
+	for i := 0; i < n; i++ {
+		go func() {
+			defer wg.Done()
+			joined.Add(1)
+			if _, err := svc.SyncWatchesBatched(context.Background(), []*PRWatch{w}); err != nil {
+				errs <- err
+			}
+		}()
+	}
+	// Wait until the leader is inside ExecuteGraphQL, then wait until
+	// every co-waiter has reached the singleflight join before releasing.
+	// Pre-fix this used a fixed 50ms sleep, which violated the backend's
+	// no-Sleep-in-pure-in-memory-tests convention and was prone to CI
+	// scheduler variance.
+	gotFirstInFlight.Wait()
+	for joined.Load() < int32(n) {
+		runtime.Gosched()
+	}
+	close(release)
+	wg.Wait()
+	close(errs)
+
+	for err := range errs {
+		t.Errorf("concurrent SyncWatchesBatched: %v", err)
+	}
+	if got := len(gh.prQueries); got != 1 {
+		t.Errorf("expected 1 upstream PR call when concurrent first-time misses coalesce, got %d", got)
+	}
+	if !svc.isRepoCachedAsMissing(w.Owner, w.Repo) {
+		t.Errorf("expected repo to be negative-cached after the coalesced fetch")
+	}
+}
+
+// TestService_ClearRepoErrorCache verifies that the new bulk-clear hook
+// (wired from ConfigureToken / ClearToken) drops every entry. Without
+// this, a repo classified as unresolvable under stale credentials would
+// stay "permanent: true" for up to 10 minutes after the user fixes auth,
+// and the frontend retry loop would stop early on stale data.
+func TestService_ClearRepoErrorCache(t *testing.T) {
+	_, svc, _, _ := setupBatchedPollerTest(t)
+
+	gen := svc.repoErrorGenSnapshot()
+	svc.markRepoAsMissing("Dead1", "Repo", gen)
+	svc.markRepoAsMissing("Dead2", "Repo", gen)
+	if !svc.isRepoCachedAsMissing("Dead1", "Repo") || !svc.isRepoCachedAsMissing("Dead2", "Repo") {
+		t.Fatalf("priming failed")
+	}
+
+	svc.ClearRepoErrorCache()
+
+	if svc.isRepoCachedAsMissing("Dead1", "Repo") {
+		t.Errorf("Dead1/Repo still cached after ClearRepoErrorCache")
+	}
+	if svc.isRepoCachedAsMissing("Dead2", "Repo") {
+		t.Errorf("Dead2/Repo still cached after ClearRepoErrorCache")
+	}
+}
+
+// TestService_EvictRepoNegative_OnWatchCreate verifies that creating a
+// PR watch clears any prior negative-cache entry for the repo, so a
+// freshly linked repository is probed immediately instead of waiting
+// out the 10-min TTL.
+func TestService_EvictRepoNegative_OnWatchCreate(t *testing.T) {
+	_, svc, _, store := setupBatchedPollerTest(t)
+	ctx := context.Background()
+
+	svc.markRepoAsMissing("Dead", "Repo", svc.repoErrorGenSnapshot())
+	if !svc.isRepoCachedAsMissing("Dead", "Repo") {
+		t.Fatalf("priming failed; cache=%v", svc.repoErrorCache.entries)
+	}
+
+	// CreatePRWatch must evict regardless of input casing, because the
+	// cache key is case-insensitive.
+	w := &PRWatch{SessionID: "s1", TaskID: "t1", Owner: "dead", Repo: "repo", PRNumber: 1, Branch: "main"}
+	if _, err := svc.CreatePRWatch(ctx, w.SessionID, w.TaskID, "", w.Owner, w.Repo, w.PRNumber, w.Branch); err != nil {
+		t.Fatalf("CreatePRWatch: %v", err)
+	}
+	_ = store
+	if svc.isRepoCachedAsMissing("Dead", "Repo") {
+		t.Errorf("expected negative cache to be evicted after CreatePRWatch; entry still present")
+	}
+}
+
+// TestService_TriggerPRSyncAllPermanent_FlagsDeadRepos pins the
+// permanent-flag wiring: a task whose every watch points at a cached
+// missing repo gets permanent=true from TriggerPRSyncAllPermanent, so
+// the WS handler can tell the frontend to stop polling. Mixed
+// (one dead repo + one live one) is NOT permanent — the live one might
+// still acquire a PR.
+func TestService_TriggerPRSyncAllPermanent_FlagsDeadRepos(t *testing.T) {
+	_, svc, gh, store := setupBatchedPollerTest(t)
+	ctx := context.Background()
+
+	w1 := &PRWatch{SessionID: "s1", TaskID: "t1", Owner: "Dead", Repo: "Repo", PRNumber: 0, Branch: "x"}
+	if err := store.CreatePRWatch(ctx, w1); err != nil {
+		t.Fatalf("create w1: %v", err)
+	}
+	svc.markRepoAsMissing(w1.Owner, w1.Repo, svc.repoErrorGenSnapshot())
+	// No canned responses needed — the cache short-circuits both watches.
+
+	_, permanent, err := svc.TriggerPRSyncAllPermanent(ctx, "t1")
+	if err != nil {
+		t.Fatalf("TriggerPRSyncAllPermanent: %v", err)
+	}
+	if !permanent {
+		t.Errorf("expected permanent=true when every watch's repo is cached-missing")
+	}
+
+	// Add a second watch on a live repo; permanent must flip to false
+	// even though w1 is still cached-missing.
+	w2 := &PRWatch{SessionID: "s2", TaskID: "t1", Owner: "Live", Repo: "Repo", PRNumber: 0, Branch: "y"}
+	if err := store.CreatePRWatch(ctx, w2); err != nil {
+		t.Fatalf("create w2: %v", err)
+	}
+	gh.branchResponses = []string{`{"data":{"b0":{"pullRequests":{"nodes":[]}}}}`}
+
+	_, permanent2, err := svc.TriggerPRSyncAllPermanent(ctx, "t1")
+	if err != nil {
+		t.Fatalf("TriggerPRSyncAllPermanent (mixed): %v", err)
+	}
+	if permanent2 {
+		t.Errorf("expected permanent=false when at least one watch's repo is live")
+	}
+	// Sanity: the live repo's branch query DID fire (single one).
+	if got := len(gh.branchQueries); got != 1 {
+		t.Errorf("expected 1 branch GraphQL call for the live repo, got %d", got)
+	}
+	// Confirm the dead repo was NOT in any query.
+	for _, q := range gh.branchQueries {
+		if strings.Contains(q, w1.Owner) {
+			t.Errorf("dead repo leaked into branch query: %q", q)
+		}
+	}
+}
diff --git a/apps/backend/internal/github/service_search_cache_test.go b/apps/backend/internal/github/service_search_cache_test.go
index 25e8c6399d..81124ad89f 100644
--- a/apps/backend/internal/github/service_search_cache_test.go
+++ b/apps/backend/internal/github/service_search_cache_test.go
@@ -31,7 +31,9 @@ func newTestService(client Client) *Service {
 		logger:               logger.Default(),
 		searchCache:          newTTLCache(),
 		prStatusCache:        newTTLCache(),
+		prFeedbackCache:      newPRFeedbackCache(),
 		accessibleReposCache: newAccessibleReposCache(),
+		repoErrorCache:       newRepoErrorCache(),
 	}
 }
 
diff --git a/apps/backend/internal/github/service_tokens.go b/apps/backend/internal/github/service_tokens.go
index 8ff6c530ec..affa0829de 100644
--- a/apps/backend/internal/github/service_tokens.go
+++ b/apps/backend/internal/github/service_tokens.go
@@ -196,17 +196,24 @@ func (s *Service) ConfigureToken(ctx context.Context, token string) error {
 		s.logger.Info("created GitHub token secret", zap.String("id", newID))
 	}
 
-	// Refresh the client to use the new token
+	// Swap the client AND drop identity-scoped caches atomically under
+	// s.mu. Clearing OUTSIDE the lock (either before or after the swap)
+	// opens a race window: clear-before-swap lets an old-identity fetch
+	// completing in the gap repopulate the just-cleared cache; clear-
+	// after-swap lets a new-identity caller hit the still-stale cache
+	// in the gap. Holding the same lock for both publishes a coherent
+	// (new client, empty caches) state to any goroutine that takes
+	// s.mu after this. The cache.clear() calls take their own locks
+	// (not s.mu), so this is fine to nest. The generation bump inside
+	// each clear() also catches any in-flight fetch that started under
+	// the old client and tries to write after the lock is released.
 	s.mu.Lock()
 	s.client = testClient
 	s.authMethod = AuthMethodPAT
-	s.mu.Unlock()
-
-	// Drop user-scoped caches — the previous token's user/orgs and merged
-	// repo list are no longer valid under the new identity. Without this,
-	// the picker would surface the prior user's repos for up to 60s after
-	// a token swap.
 	s.ClearAccessibleReposCaches()
+	s.ClearRepoErrorCache()
+	s.ClearPRCaches()
+	s.mu.Unlock()
 
 	return nil
 }
@@ -227,14 +234,15 @@ func (s *Service) ClearToken(ctx context.Context) error {
 	}
 	s.logger.Info("cleared GitHub token secret", zap.String("id", existingID))
 
-	// Reset to try gh CLI or other methods
+	// Reset client + drop identity-scoped caches atomically under s.mu;
+	// see ConfigureToken for the rationale.
 	s.mu.Lock()
 	s.client = nil
 	s.authMethod = AuthMethodNone
-	s.mu.Unlock()
-
-	// Drop user-scoped caches — see ConfigureToken for rationale.
 	s.ClearAccessibleReposCaches()
+	s.ClearRepoErrorCache()
+	s.ClearPRCaches()
+	s.mu.Unlock()
 
 	// Try to re-establish connection via other methods
 	s.retryClientCreation(ctx)
diff --git a/apps/backend/internal/github/ttl_cache.go b/apps/backend/internal/github/ttl_cache.go
index ef3a9287ef..19e2672258 100644
--- a/apps/backend/internal/github/ttl_cache.go
+++ b/apps/backend/internal/github/ttl_cache.go
@@ -2,6 +2,7 @@ package github
 
 import (
 	"fmt"
+	"strings"
 	"sync"
 	"time"
 
@@ -68,6 +69,24 @@ func newAccessibleReposCache() *ttlCache {
 	return c
 }
 
+// newPRFeedbackCache backs Service.GetPRFeedback (reviews + comments + checks,
+// 4 sequential GitHub REST calls per miss). Its job is to collapse the
+// render/mount bursts the task page produces — a single PR getting fetched
+// dozens of times in a couple of seconds — without those duplicates each
+// re-hammering GitHub and tripping its secondary rate limits.
+//
+// The TTL is deliberately shorter than the 30s status cache: feedback is the
+// "fresh, on-demand" surface behind the PR popover/detail panel and the manual
+// Refresh button, so staling it for 30s would make legitimate updates lag.
+// 8s is long enough to cover a burst's tail (the singleflight in doOrFetch
+// already coalesces the concurrent in-flight fetches) while keeping the worst-
+// case staleness small.
+func newPRFeedbackCache() *ttlCache {
+	c := newTTLCache()
+	c.ttl = 8 * time.Second
+	return c
+}
+
 func (c *ttlCache) get(key string) (any, bool) {
 	c.mu.Lock()
 	defer c.mu.Unlock()
@@ -160,6 +179,28 @@ func (c *ttlCache) evictLocked() {
 	}
 }
 
+// del removes a single key from the cache. Used by Service.evictRepoNegative
+// to drop a negative entry when a repository is (re)linked to a workspace
+// or its watch is recreated, so a freshly-linked repo gets probed
+// immediately instead of waiting out the 10-min TTL. Bumps `gen` so any
+// classifier fetch that was in flight at the time of the del (e.g. the
+// one whose result triggered the relink) cannot reinsert the just-evicted
+// key via setIfCurrentGeneration — its post-fetch write becomes a no-op
+// and the next ensure() re-probes under the new generation.
+func (c *ttlCache) del(key string) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	delete(c.entries, key)
+	c.gen++
+}
+
+// cachedErr wraps an error stored in a ttlCache so that a deterministic
+// upstream failure (e.g. ErrRepoNotResolvable) can be negative-cached
+// alongside the cache's normal success-value entries. Currently used by
+// Service.repoErrorCache via the lower-level get / setIfCurrentGeneration
+// primitives (see Service.isRepoCachedAsMissing / markRepoAsMissing).
+type cachedErr struct{ err error }
+
 // doOrFetch returns a cached value when fresh; otherwise runs fetch under a
 // singleflight guard, caches the result, and returns it. Errors are not
 // cached. The returned value is shared — callers must not mutate it.
@@ -205,3 +246,27 @@ func searchCacheKey(kind, filter, customQuery string, page, perPage int) string
 func prStatusCacheKey(owner, repo string, number int) string {
 	return fmt.Sprintf("%s/%s#%d", owner, repo, number)
 }
+
+// newRepoErrorCache backs Service.repoErrorCache. It negative-caches
+// deterministic "repository not resolvable" failures (see
+// isRepoNotResolvableErr) so the SyncWatchesBatched storm against
+// missing/unauthorized repos collapses to one upstream call per 10
+// minutes per repo instead of one per 5s frontend poll. The TTL is
+// deliberately longer than the status / feedback caches because the
+// failure mode is "this repo doesn't exist or we can't see it" — a
+// state that flips back only via an explicit re-link or token swap,
+// both of which evict the entry directly.
+func newRepoErrorCache() *ttlCache {
+	c := newTTLCache()
+	c.ttl = 10 * time.Minute
+	return c
+}
+
+// repoErrorCacheKey is the case-insensitive (owner, repo) key for
+// Service.repoErrorCache. GitHub's repository names are case-insensitive
+// at the API surface, so "NBCUDTC/Bff" and "nbcudtc/bff" reach the same
+// upstream resource; normalising the cache key prevents the negative
+// entry from being bypassed by a caller that capitalises differently.
+func repoErrorCacheKey(owner, repo string) string {
+	return strings.ToLower(owner) + "/" + strings.ToLower(repo)
+}
diff --git a/apps/web/hooks/domains/github/use-task-pr.test.tsx b/apps/web/hooks/domains/github/use-task-pr.test.tsx
new file mode 100644
index 0000000000..9a3d8b20f0
--- /dev/null
+++ b/apps/web/hooks/domains/github/use-task-pr.test.tsx
@@ -0,0 +1,82 @@
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { createElement, type ReactNode } from "react";
+import { act, cleanup, renderHook } from "@testing-library/react";
+import { StateProvider } from "@/components/state-provider";
+
+const requestMock = vi.fn();
+
+vi.mock("@/lib/ws/connection", () => ({
+  getWebSocketClient: () => ({ request: requestMock }),
+}));
+
+// listWorkspaceTaskPRs is only used by useWorkspacePRs (not under test
+// here). Stub it so the module import doesn't fail in jsdom.
+vi.mock("@/lib/api/domains/github-api", () => ({
+  listWorkspaceTaskPRs: vi.fn().mockResolvedValue(null),
+}));
+
+import { useTaskPR } from "./use-task-pr";
+
+function wrapper({ children }: { children: ReactNode }) {
+  return createElement(StateProvider, null, children);
+}
+
+beforeEach(() => {
+  vi.useFakeTimers();
+  requestMock.mockReset();
+});
+afterEach(() => {
+  cleanup();
+  vi.useRealTimers();
+});
+
+describe("useTaskPR — permanent flag", () => {
+  // The dominant production signal in the SyncWatchesBatched storm was the
+  // frontend polling `github.task_pr.sync` every 5s for tasks whose repos
+  // were deleted/inaccessible. The backend now returns `permanent: true`
+  // on those responses; the hook must stop the retry interval cold.
+  it("stops the 5s retry interval when the backend reports permanent: true", async () => {
+    requestMock.mockResolvedValue({ prs: [], permanent: true });
+
+    renderHook(() => useTaskPR("task-1"), { wrapper });
+
+    // Initial freshness sync fires synchronously from the mount effect.
+    // Flush the resolved promise so the permanent flag is applied before
+    // the interval would otherwise fire.
+    await act(async () => {
+      await vi.advanceTimersByTimeAsync(0);
+    });
+    expect(requestMock).toHaveBeenCalledTimes(1);
+
+    // Advance well past several retry windows. If the permanent
+    // short-circuit regressed, this would burst 5-6 additional calls
+    // into requestMock.
+    await act(async () => {
+      await vi.advanceTimersByTimeAsync(5_000 * 6);
+    });
+    expect(requestMock).toHaveBeenCalledTimes(1);
+  });
+
+  // Without permanent, the existing retry cadence must still kick in so
+  // tasks waiting on a freshly-pushed branch still get their PR detected.
+  it("retries every 5s when permanent is absent and no PR is in the store", async () => {
+    requestMock.mockResolvedValue({ prs: [] });
+
+    renderHook(() => useTaskPR("task-1"), { wrapper });
+
+    await act(async () => {
+      await vi.advanceTimersByTimeAsync(0);
+    });
+    expect(requestMock).toHaveBeenCalledTimes(1);
+
+    await act(async () => {
+      await vi.advanceTimersByTimeAsync(5_000);
+    });
+    expect(requestMock).toHaveBeenCalledTimes(2);
+
+    await act(async () => {
+      await vi.advanceTimersByTimeAsync(5_000);
+    });
+    expect(requestMock).toHaveBeenCalledTimes(3);
+  });
+});
diff --git a/apps/web/hooks/domains/github/use-task-pr.ts b/apps/web/hooks/domains/github/use-task-pr.ts
index 18a7ccbf5c..08ac58691e 100644
--- a/apps/web/hooks/domains/github/use-task-pr.ts
+++ b/apps/web/hooks/domains/github/use-task-pr.ts
@@ -52,7 +52,7 @@ export function getPrimaryTaskPR(prs: TaskPR[] | undefined): TaskPR | null {
  * accept the legacy bare-TaskPR shape too in case an older backend is
  * still running. Empty / null / unknown shapes return an empty array.
  */
-function normalizeSyncResponse(result: { prs?: TaskPR[] } | TaskPR | null | undefined): TaskPR[] {
+function normalizeSyncResponse(result: SyncResponse): TaskPR[] {
   if (!result) return [];
   const envelope = result as { prs?: TaskPR[] };
   if (Array.isArray(envelope.prs)) return envelope.prs;
@@ -61,30 +61,65 @@ function normalizeSyncResponse(result: { prs?: TaskPR[] } | TaskPR | null | unde
   return [];
 }
 
+/**
+ * Response shape from the `github.task_pr.sync` WS action. `permanent`
+ * is true when every watch on the task points at a repository the
+ * backend has classified as unresolvable (missing, deleted, or
+ * inaccessible). When set, the 5s retry interval stops — without this
+ * the frontend kept hammering a dead repo every 5s for the lifetime of
+ * the task. Older backends omit the field, so it's optional.
+ */
+type SyncResponse = { prs?: TaskPR[]; permanent?: boolean } | TaskPR | null | undefined;
+
 /** Fetch a single task's PR associations, with on-demand sync via WS. */
 export function useTaskPR(taskId: string | null) {
   const prs = useAppStore((state) => (taskId ? (state.taskPRs.byTaskId[taskId] ?? null) : null));
   const pr = getPrimaryTaskPR(prs ?? undefined);
   const setTaskPR = useAppStore((state) => state.setTaskPR);
   const retryRef = useRef(0);
+  const permanentRef = useRef(false);
+  // Monotonic counter incremented before each WS request, snapshotted in
+  // the .then() closure. Mirrors useWorkspacePRs above. Without this, a
+  // stale response from a previous taskId can land after the user
+  // navigates to a new task and flip permanentRef.current = true for the
+  // active task, killing its retry loop. The reset effect below clears
+  // retry/permanent state on taskId change, but a still-in-flight WS
+  // call from the previous task can race that reset.
+  const requestRef = useRef(0);
 
   const refresh = useCallback(() => {
     if (!taskId) return;
     const client = getWebSocketClient();
     if (!client) return;
 
-    // Backend returns `{prs: TaskPR[]}` — multi-repo tasks have one row per
-    // repo. We push each into the store so the per-repo PR icon stays in
-    // sync. Empty array means no watches yet (the freshness retry below
-    // handles the polling cadence). Legacy single-PR shape (`TaskPR` only)
-    // is detected via the absence of `.prs`.
+    // Backend returns `{prs: TaskPR[], permanent?: boolean}` — multi-repo
+    // tasks have one row per repo. We push each into the store so the
+    // per-repo PR icon stays in sync. Empty array means no watches yet
+    // (the freshness retry below handles the polling cadence). Legacy
+    // single-PR shape (`TaskPR` only) is detected via the absence of
+    // `.prs`. When `permanent` is true (every watch's repo is dead),
+    // exhaust the retry counter so the 5s interval below clears itself.
+    const requestId = ++requestRef.current;
+    const requestedTaskId = taskId;
     client
-      .request<{ prs?: TaskPR[] } | TaskPR | null>("github.task_pr.sync", { task_id: taskId })
+      .request<SyncResponse>("github.task_pr.sync", { task_id: requestedTaskId })
       .then((result) => {
+        // Drop responses that aren't the latest in-flight request for
+        // this hook instance — they'd otherwise corrupt permanentRef /
+        // retryRef for whatever task the user is now viewing. The
+        // taskId-change effect below bumps requestRef.current too, so
+        // requestId alone covers both stale-by-sequence and
+        // stale-by-task-change.
+        if (requestRef.current !== requestId) return;
+        const envelope = (result ?? {}) as { permanent?: boolean };
+        if (envelope.permanent) {
+          permanentRef.current = true;
+          retryRef.current = SYNC_MAX_RETRIES;
+        }
         const list = normalizeSyncResponse(result);
         if (list.length === 0) return;
         for (const pr of list) {
-          if (pr.task_id) setTaskPR(taskId, pr);
+          if (pr.task_id) setTaskPR(requestedTaskId, pr);
         }
         retryRef.current = 0;
       })
@@ -93,9 +128,13 @@ export function useTaskPR(taskId: string | null) {
       });
   }, [taskId, setTaskPR]);
 
-  // Reset retry count when taskId changes
+  // Reset retry/permanent state when taskId changes. Bumping requestRef
+  // here invalidates any still-in-flight .then() closure from the prior
+  // taskId so it can't write to the new task's refs.
   useEffect(() => {
     retryRef.current = 0;
+    permanentRef.current = false;
+    requestRef.current++;
   }, [taskId]);
 
   // Sync once when the task becomes active (freshness check).
@@ -105,12 +144,14 @@ export function useTaskPR(taskId: string | null) {
     refresh();
   }, [taskId, refresh]);
 
-  // Retry polling when no PR is in the store yet.
+  // Retry polling when no PR is in the store yet. permanentRef short-circuits
+  // the interval entirely so a task whose repos are all dead doesn't tie up
+  // the backend's gh throttle on every 5s tick.
   useEffect(() => {
-    if (!taskId || pr) return;
+    if (!taskId || pr || permanentRef.current) return;
 
     const interval = setInterval(() => {
-      if (retryRef.current >= SYNC_MAX_RETRIES) {
+      if (retryRef.current >= SYNC_MAX_RETRIES || permanentRef.current) {
         clearInterval(interval);
         return;
       }