diff --git a/apps/backend/internal/agent/runtime/lifecycle/session.go b/apps/backend/internal/agent/runtime/lifecycle/session.go
index 65a94946f7..eb38ac6ba7 100644
--- a/apps/backend/internal/agent/runtime/lifecycle/session.go
+++ b/apps/backend/internal/agent/runtime/lifecycle/session.go
@@ -635,7 +635,17 @@ func (sm *SessionManager) applyRuntimeSessionLayers(
 				zap.String("execution_id", execution.ID), zap.String("mode", runtimeMode), zap.Error(err))
 		}
 	}
-	sanitizedOptions := profileconfig.SanitizeConfigOptions(runtimeConfigOptions)
+	// Fail safe: when the current agent's option catalog is not yet known, we
+	// cannot verify which persisted options it supports, so replay nothing
+	// rather than sending a prior agent's keys (spec failure mode: unknown
+	// catalog must not send unverified options). This covers flat-model-list
+	// agents whose startup wait exits on the model list before any config
+	// options settle.
+	modelState := execution.GetModelState()
+	var sanitizedOptions map[string]string
+	if _, catalogKnown := capturedRuntimeConfigOptionCatalog(modelState); catalogKnown {
+		sanitizedOptions = sanitizeRuntimeConfigOptionsWithCatalog(runtimeConfigOptions, modelState)
+	}
 	for _, configID := range sortedConfigOptionKeys(sanitizedOptions) {
 		value := sanitizedOptions[configID]
 		if err := execution.agentctl.SetConfigOption(ctx, configID, value); err != nil {
diff --git a/apps/backend/internal/agent/runtime/lifecycle/session_runtime_layers_test.go b/apps/backend/internal/agent/runtime/lifecycle/session_runtime_layers_test.go
new file mode 100644
index 0000000000..04993b97d4
--- /dev/null
+++ b/apps/backend/internal/agent/runtime/lifecycle/session_runtime_layers_test.go
@@ -0,0 +1,175 @@
+package lifecycle
+
+import (
+	"context"
+	"slices"
+	"sort"
+	"sync"
+	"testing"
+	"time"
+
+	agentctl "github.com/kandev/kandev/internal/agent/runtime/agentctl"
+	"github.com/kandev/kandev/internal/agentctl/types/streams"
+	ws "github.com/kandev/kandev/pkg/websocket"
+)
+
+// settledCatalogState builds a CachedModelState whose config-option catalog is
+// settled and advertises exactly the given option IDs (category left blank so
+// they are restorable), each with a current value.
+func settledCatalogState(currentModel string, advertised map[string]string) *CachedModelState {
+	options := make([]streams.ConfigOption, 0, len(advertised))
+	for id, value := range advertised {
+		options = append(options, streams.ConfigOption{
+			Type:         "select",
+			ID:           id,
+			CurrentValue: value,
+		})
+	}
+	return &CachedModelState{
+		CurrentModelID:       currentModel,
+		ConfigOptions:        options,
+		ConfigOptionsSettled: true,
+	}
+}
+
+// configOptionRecorder wraps the mock handler to capture the config_id values
+// that reach the agent via set_config_option, and optionally errors on one id.
+func configOptionRecorder(mock *mockAgentServer, failID string) *sync.Map {
+	seen := &sync.Map{}
+	base := mock.defaultHandler
+	mock.handler = func(msg ws.Message) *ws.Message {
+		if msg.Action == "agent.session.set_config_option" {
+			var payload struct {
+				ConfigID string `json:"config_id"`
+				Value    string `json:"value"`
+			}
+			if err := msg.ParsePayload(&payload); err == nil {
+				seen.Store(payload.ConfigID, payload.Value)
+				if failID != "" && payload.ConfigID == failID {
+					resp, _ := ws.NewError(msg.ID, msg.Action,
+						ws.ErrorCodeInternalError, "option not supported", nil)
+					return resp
+				}
+			}
+		}
+		return base(msg)
+	}
+	return seen
+}
+
+// connectAgentStream opens the agent updates WebSocket so SetConfigOption calls
+// can round-trip, then waits until the mock observes the connection.
+func connectAgentStream(t *testing.T, mock *mockAgentServer, client *agentctl.Client) {
+	t.Helper()
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	t.Cleanup(cancel)
+	if err := client.StreamUpdates(ctx, func(agentctl.AgentEvent) {}, nil, nil); err != nil {
+		t.Fatalf("connect agent stream: %v", err)
+	}
+	select {
+	case <-mock.wsConnected:
+	case <-time.After(5 * time.Second):
+		t.Fatal("agent stream never connected")
+	}
+}
+
+// TestApplyRuntimeSessionLayersFiltersUnadvertisedOptions verifies that the
+// startup/resume replay path drops persisted config options the current agent
+// does not advertise, so they are neither sent nor reported as startup
+// failures, while advertised options still apply and real failures surface.
+func TestApplyRuntimeSessionLayersFiltersUnadvertisedOptions(t *testing.T) {
+	sentKeys := func(seen *sync.Map) []string {
+		keys := make([]string, 0)
+		seen.Range(func(k, _ any) bool {
+			keys = append(keys, k.(string))
+			return true
+		})
+		sort.Strings(keys)
+		return keys
+	}
+
+	t.Run("drops unadvertised prior-agent keys", func(t *testing.T) {
+		mock := newMockAgentServer(t)
+		defer mock.Close()
+		seen := configOptionRecorder(mock, "")
+
+		sm := NewSessionManager(newSessionTestLogger(), newTestStopCh(t))
+		client := createTestClient(t, mock.server.URL)
+		defer client.Close()
+		connectAgentStream(t, mock, client)
+		execution := &AgentExecution{ID: "e1", SessionID: "s1", agentctl: client}
+		execution.SetModelState(settledCatalogState("cur", map[string]string{
+			"reasoning": "high", "fast": "on", "context": "repo",
+		}))
+
+		ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+		defer cancel()
+		_, failed := sm.applyRuntimeSessionLayers(
+			ctx, execution, "acp-1", "cfg", "", "", "", "",
+			map[string]string{"reasoning": "high", "fast": "on", "effort": "x", "thinking": "y"},
+		)
+
+		if len(failed) != 0 {
+			t.Fatalf("expected no failed options, got %v", failed)
+		}
+		got := sentKeys(seen)
+		want := []string{"fast", "reasoning"}
+		if !slices.Equal(got, want) {
+			t.Fatalf("sent config options = %v, want %v (effort/thinking must be dropped)", got, want)
+		}
+	})
+
+	t.Run("unknown catalog (nil model state) sends no options and reports no failures", func(t *testing.T) {
+		mock := newMockAgentServer(t)
+		defer mock.Close()
+		seen := configOptionRecorder(mock, "")
+
+		sm := NewSessionManager(newSessionTestLogger(), newTestStopCh(t))
+		client := createTestClient(t, mock.server.URL)
+		defer client.Close()
+		connectAgentStream(t, mock, client)
+		// Leave model state nil: the current agent's option catalog is unknown,
+		// so replay must fail safe and send nothing (spec failure mode).
+		execution := &AgentExecution{ID: "e3", SessionID: "s3", agentctl: client}
+
+		ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+		defer cancel()
+		_, failed := sm.applyRuntimeSessionLayers(
+			ctx, execution, "acp-3", "cfg", "", "", "", "",
+			map[string]string{"reasoning": "high", "effort": "x", "thinking": "y"},
+		)
+
+		if len(failed) != 0 {
+			t.Fatalf("expected no failed options for unknown catalog, got %v", failed)
+		}
+		if got := sentKeys(seen); len(got) != 0 {
+			t.Fatalf("expected no options sent for unknown catalog, got %v", got)
+		}
+	})
+
+	t.Run("advertised option value rejection still surfaces", func(t *testing.T) {
+		mock := newMockAgentServer(t)
+		defer mock.Close()
+		configOptionRecorder(mock, "reasoning")
+
+		sm := NewSessionManager(newSessionTestLogger(), newTestStopCh(t))
+		client := createTestClient(t, mock.server.URL)
+		defer client.Close()
+		connectAgentStream(t, mock, client)
+		execution := &AgentExecution{ID: "e2", SessionID: "s2", agentctl: client}
+		execution.SetModelState(settledCatalogState("cur", map[string]string{
+			"reasoning": "high", "fast": "on",
+		}))
+
+		ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+		defer cancel()
+		_, failed := sm.applyRuntimeSessionLayers(
+			ctx, execution, "acp-2", "cfg", "", "", "", "",
+			map[string]string{"reasoning": "high", "effort": "x"},
+		)
+
+		if len(failed) != 1 || failed[0] != "reasoning" {
+			t.Fatalf("expected [reasoning] in failed, got %v", failed)
+		}
+	})
+}
diff --git a/apps/web/components/task/model-selector.test.ts b/apps/web/components/task/model-selector.test.ts
index 45a3c48f81..75fd74ed8e 100644
--- a/apps/web/components/task/model-selector.test.ts
+++ b/apps/web/components/task/model-selector.test.ts
@@ -382,6 +382,81 @@ describe("legacy agent config hydration", () => {
   });
 });
 
+const modelConfigOption: ConfigOptionEntry = {
+  type: "select",
+  id: "model",
+  name: "Model",
+  currentValue: providerModelId,
+  category: "model",
+  options: [{ value: providerModelId, name: providerModelName }],
+};
+
+function selectOption(id: string, currentValue: string): ConfigOptionEntry {
+  return {
+    type: "select",
+    id,
+    name: id,
+    currentValue,
+    options: [{ value: currentValue, name: currentValue }],
+  };
+}
+
+function catalogEntry(configOptions: ConfigOptionEntry[], settled: boolean) {
+  return {
+    currentModelId: providerModelId,
+    models: [],
+    configOptions,
+    configOptionsSettled: settled,
+  } as Parameters<typeof hasCompleteDynamicConfig>[1];
+}
+
+describe("cross-agent persisted config reconciliation", () => {
+  it("hydrates when persisted-only keys are unadvertised and the catalog is settled", () => {
+    const session = makeSession({
+      metadata: {
+        runtime_config: {
+          // effort/thinking were written by a prior agent that no longer runs.
+          config_options: {
+            model: providerModelId,
+            reasoning: "high",
+            fast: "on",
+            effort: "x",
+            thinking: "y",
+          },
+        },
+      },
+    });
+    const advertised = [
+      modelConfigOption,
+      selectOption("reasoning", "high"),
+      selectOption("fast", "on"),
+    ];
+
+    expect(hasCompleteDynamicConfig(session, catalogEntry(advertised, true), noAgents)).toBe(true);
+  });
+
+  it("keeps a profile-snapshot key required even when the agent has not advertised it", () => {
+    const session = makeSession({
+      metadata: { runtime_config: { config_options: { model: providerModelId } } },
+      agent_profile_snapshot: { config_options: { verbosity: "terse" } },
+    });
+
+    expect(
+      hasCompleteDynamicConfig(session, catalogEntry([modelConfigOption], true), noAgents),
+    ).toBe(false);
+  });
+
+  it("still requires persisted-only unadvertised keys while the catalog is unsettled", () => {
+    const session = makeSession({
+      metadata: { runtime_config: { config_options: { model: providerModelId, effort: "x" } } },
+    });
+
+    expect(
+      hasCompleteDynamicConfig(session, catalogEntry([modelConfigOption], false), noAgents),
+    ).toBe(false);
+  });
+});
+
 it("is hydrated when there are no required config keys", () => {
   expect(hasCompleteDynamicConfig(makeSession(), undefined, noAgents)).toBe(true);
 });
diff --git a/apps/web/components/task/model-selector.tsx b/apps/web/components/task/model-selector.tsx
index 2fbd121d1f..c0c5e26540 100644
--- a/apps/web/components/task/model-selector.tsx
+++ b/apps/web/components/task/model-selector.tsx
@@ -87,13 +87,23 @@ function isLegacyAgentConfig(session: TaskSession | null, agents: Agent[]): bool
   });
 }
 
-export function requiredConfigKeys(session: TaskSession | null, agents: Agent[]): string[] {
-  if (!session) return [];
+// profileRequiredConfigKeys returns keys the current agent profile itself
+// declares (snapshot + matched profile). These stay required even when the
+// running agent has not advertised them, because the profile chose them.
+function profileRequiredConfigKeys(session: TaskSession, agents: Agent[]): Set<string> {
   const keys = new Set(configValueKeys(session.agent_profile_snapshot?.config_options));
   for (const agent of agents) {
     const profile = agent.profiles.find((item) => item.id === session.agent_profile_id);
     for (const key of Object.keys(profile?.configOptions ?? {})) keys.add(key);
   }
+  return keys;
+}
+
+// persistedRuntimeConfigKeys returns keys recorded only in the session's
+// persisted runtime metadata. A prior agent type may have written keys the
+// current agent never advertises; these must not block the selector.
+function persistedRuntimeConfigKeys(session: TaskSession): Set<string> {
+  const keys = new Set<string>();
   for (const value of [
     session.metadata?.runtime_config,
     session.metadata?.runtime_config_overrides,
@@ -103,6 +113,13 @@ export function requiredConfigKeys(session: TaskSession | null, agents: Agent[])
       keys.add(key);
     }
   }
+  return keys;
+}
+
+export function requiredConfigKeys(session: TaskSession | null, agents: Agent[]): string[] {
+  if (!session) return [];
+  const keys = profileRequiredConfigKeys(session, agents);
+  for (const key of persistedRuntimeConfigKeys(session)) keys.add(key);
   return [...keys];
 }
 
@@ -122,13 +139,21 @@ export function hasCompleteDynamicConfig(
   // configOptions. Treat that key as satisfied when the session has a flat model
   // list — the selector renders fine from it.
   const hasFlatModelList = !!sessionModelsData.models.length;
-  const hasLegacyAgentConfig =
-    sessionModelsData.configOptionsSettled === true && isLegacyAgentConfig(session, agents);
+  const catalogSettled = sessionModelsData.configOptionsSettled === true;
+  const hasLegacyAgentConfig = catalogSettled && isLegacyAgentConfig(session, agents);
+  // Keys written only by a prior agent type into persisted runtime metadata are
+  // not required for the selector to render: once the current agent's catalog
+  // has settled without advertising them, they are stale cross-agent leftovers
+  // that the backend replay also drops.
+  const profileRequired = session ? profileRequiredConfigKeys(session, agents) : new Set<string>();
+  const isPersistedOnlyStaleKey = (key: string): boolean =>
+    key !== AGENT_CONFIG_KEY && catalogSettled && !available.has(key) && !profileRequired.has(key);
   return required.every(
     (key) =>
       available.has(key) ||
       (key === AGENT_CONFIG_KEY && hasLegacyAgentConfig) ||
-      (key === MODEL_CONFIG_KEY && hasFlatModelList),
+      (key === MODEL_CONFIG_KEY && hasFlatModelList) ||
+      isPersistedOnlyStaleKey(key),
   );
 }
 
diff --git a/docs/plans/session-config-cross-agent-reconcile/plan.md b/docs/plans/session-config-cross-agent-reconcile/plan.md
new file mode 100644
index 0000000000..814dabdb47
--- /dev/null
+++ b/docs/plans/session-config-cross-agent-reconcile/plan.md
@@ -0,0 +1,167 @@
+---
+spec: docs/specs/session-config-cross-agent-reconcile/spec.md
+created: 2026-08-19
+status: draft
+---
+
+# Implementation Plan: Session Config Reconciliation Across Agent Types
+
+## Overview
+
+Two coordinated, independently verifiable changes fix a cross-agent config
+contamination bug. Backend first: the startup/resume replay path filters
+persisted runtime config options against the current agent's advertised catalog
+before sending them, so options a prior agent wrote (e.g. `effort`, `thinking`)
+are neither applied nor reported as startup failures. Frontend second: the chat
+model-selector render gate stops treating persisted-but-unadvertised option keys
+as required, so the selector renders for the resumed session. The two tasks are
+disjoint (Go vs TS, no shared file) and each carries its own regression test.
+
+## Confirmed root cause
+
+- Backend replay `applyRuntimeSessionLayers`
+  (`apps/backend/internal/agent/runtime/lifecycle/session.go:602-651`) sanitizes
+  persisted options only with `profileconfig.SanitizeConfigOptions`, which keeps
+  every non-model/mode key. `SetConfigOption("effort"/"thinking", …)` fails on an
+  agent that does not advertise them; each key is added to `failed`.
+- The orchestrator turns `failed` into the user warning
+  (`apps/backend/internal/orchestrator/event_handlers_streaming.go` around the
+  `warnWorkflowSessionConfig` call).
+- The frontend gate `hasCompleteDynamicConfig` requires every key from
+  `requiredConfigKeys` (which reads `session.metadata.runtime_config(_overrides)
+  .config_options`); un-advertised keys keep `configHydrated=false` and hide the
+  selector (`apps/web/components/task/model-selector.tsx:90-133`).
+
+The catalog-aware helper `sanitizeRuntimeConfigOptionsWithCatalog(options,
+execution.GetModelState())` already exists and is used on the reset/restore path
+(`apps/backend/internal/agent/runtime/lifecycle/manager_interaction.go:627-648,
+696`). The startup replay path simply does not use it.
+
+---
+
+## Backend
+
+### Runtime replay layer (`internal/agent/runtime/lifecycle/session.go`)
+
+Change `applyRuntimeSessionLayers` so that, after the runtime model is applied
+(the `SetModel` block at lines 617-630), the persisted `runtimeConfigOptions`
+are filtered against the current agent's advertised catalog before the
+`SetConfigOption` loop.
+
+- Replace the `sanitizedOptions := profileconfig.SanitizeConfigOptions(
+  runtimeConfigOptions)` call at line 638 with a catalog-aware filter that reuses
+  the existing `sanitizeRuntimeConfigOptionsWithCatalog(runtimeConfigOptions,
+  execution.GetModelState())` from `manager_interaction.go`. That helper already
+  drops model/mode/blank keys (via `isRestorableRuntimeConfigOption`) and, when
+  the catalog is known, drops any key not present in the agent's advertised
+  option set. When the catalog is not yet known it returns the restorable set
+  unfiltered — acceptable and unchanged from today for same-agent resumes.
+- Only keys surviving the filter reach `SetConfigOption`, so unsupported prior-
+  agent keys never enter the `failed` slice and never reach the warning.
+- Fail-safe note from spec: because the model is applied first in this same
+  function and `waitForFreshSessionModelState` runs before the layers
+  (`session.go:371-374`), the catalog is normally settled by replay time for the
+  resuming agent.
+
+Do not change `applyProfileSessionLayers`; profile options come from the current
+agent's own profile and are not the cross-agent contamination source. Keep
+`profileconfig.SanitizeConfigOptions` there.
+
+If reusing `sanitizeRuntimeConfigOptionsWithCatalog` across files is awkward
+(it lives in `manager_interaction.go`, same package `lifecycle`), call it
+directly — both files are in package `lifecycle`, so no export change is needed.
+
+---
+
+## Frontend
+
+> User-facing: the chat model/mode selector visibility.
+
+### `apps/web/components/task/model-selector.tsx`
+
+Make the render gate resilient to persisted-but-unadvertised keys. Two options;
+plan picks the narrowest:
+
+- Keep `requiredConfigKeys` as the record of "what the session has configured",
+  but change `hasCompleteDynamicConfig` so a required key sourced only from
+  persisted `runtime_config(_overrides)` is not blocking when the current agent
+  has settled its config options and does not advertise that key. Concretely:
+  the `required.every(...)` check (lines 127-132) treats a key as satisfied when
+  it is advertised (`available.has(key)`) OR it is a persisted-metadata-only key
+  absent from the settled agent catalog. The agent catalog is considered settled
+  using the existing `sessionModelsData.configOptionsSettled === true` signal
+  already read in this file (line 126) and/or a non-empty `configOptions`.
+- Keys that come from the current agent profile snapshot / matched profile
+  (`session.agent_profile_snapshot.config_options`, profile `configOptions`)
+  remain required as today; only the persisted-runtime-only keys become
+  non-blocking. This preserves the existing legacy-agent and flat-model-list
+  exceptions (lines 124-131).
+
+Net effect: for the resumed Cursor session, `effort`/`thinking` (persisted-only,
+not advertised, catalog settled) no longer block, `configHydrated` becomes
+`true`, and the selector renders from the agent's live advertised options.
+
+### State / API client
+
+No store, hook, or API-client change. `sessionModelsData` and its
+`configOptions` / `configOptionsSettled` fields already exist.
+
+---
+
+## Tests
+
+- **Backend unit (replay filter):** a table-driven test on
+  `applyRuntimeSessionLayers` (or a focused test on the shared
+  `sanitizeRuntimeConfigOptionsWithCatalog` usage from the runtime path) proving
+  that given persisted options `{reasoning, fast, effort, thinking}` and an agent
+  catalog advertising `{reasoning, fast, context}`, only `reasoning` and `fast`
+  are sent and `effort`/`thinking` are not in the returned `failed` slice. Real
+  failure (advertised key, `SetConfigOption` returns error) still lands in
+  `failed`. File: `apps/backend/internal/agent/runtime/lifecycle/session_test.go`
+  (new test in a new file if the existing one is near the 800-line limit).
+- **Frontend unit (gate):** extend the existing `requiredConfigKeys` /
+  `hasCompleteDynamicConfig` tests to assert that a persisted-only, unadvertised
+  key with a settled agent catalog yields `configHydrated=true`, while a key that
+  the agent profile snapshot requires and the agent has not advertised still
+  yields `false`. File: the existing model-selector test
+  (`apps/web/components/task/model-selector.test.ts` or `.test.tsx` if present;
+  otherwise add beside the component per repo test conventions).
+
+No integration or E2E is planned: the observable outcome is unit-testable on both
+sides, and the spec adds no new contract. (If a reviewer wants end-to-end proof,
+an E2E resuming a session with stale metadata could be added later; not required
+for this repair.)
+
+---
+
+## Verification Results
+
+Both tasks complete.
+
+- **task-01 (backend):** `applyRuntimeSessionLayers` now filters via
+  `sanitizeRuntimeConfigOptionsWithCatalog(runtimeConfigOptions, execution.GetModelState())`.
+  New regression test `TestApplyRuntimeSessionLayersFiltersUnadvertisedOptions`
+  (`session_runtime_layers_test.go`). `go test ./internal/agent/runtime/lifecycle/`
+  → ok; changed-file `golangci-lint` → 0 issues; `gofmt` clean.
+- **task-02 (frontend):** `hasCompleteDynamicConfig` no longer requires
+  persisted-runtime-only keys the current agent does not advertise once the
+  catalog is settled. Extended `model-selector.test.ts` (26 passed);
+  `typecheck` clean; `lint` 0 warnings; `i18n:ratchet` clean.
+
+---
+
+## Implementation Waves And Parallel Candidates
+
+```
+Wave 1 (parallel candidates — user authorization required; disjoint Go vs TS):
+- [x] [task-01-backend-replay-catalog-filter](task-01-backend-replay-catalog-filter.md)
+- [x] [task-02-frontend-selector-gate](task-02-frontend-selector-gate.md)
+```
+
+Default is sequential execution in the primary conversation. The two tasks touch
+no shared file, schema, migration, generated contract, or lockfile, so they are
+parallel-safe if the user explicitly authorizes parallel work.
+
+## Open Questions
+
+- None.
diff --git a/docs/plans/session-config-cross-agent-reconcile/task-01-backend-replay-catalog-filter.md b/docs/plans/session-config-cross-agent-reconcile/task-01-backend-replay-catalog-filter.md
new file mode 100644
index 0000000000..5ff6658264
--- /dev/null
+++ b/docs/plans/session-config-cross-agent-reconcile/task-01-backend-replay-catalog-filter.md
@@ -0,0 +1,114 @@
+---
+id: "01-backend-replay-catalog-filter"
+title: "Backend: filter runtime config replay against current agent catalog"
+status: done
+wave: 1
+depends_on: []
+plan: "plan.md"
+spec: "../../specs/session-config-cross-agent-reconcile/spec.md"
+parallelism: sequential
+---
+
+# Task 01: Backend replay catalog filter
+
+Filter persisted runtime config options against the currently running agent's
+advertised option catalog on the startup/resume replay path, so options a prior
+agent wrote are neither applied nor reported as startup failures.
+
+## Context / root cause
+
+`applyRuntimeSessionLayers`
+(`apps/backend/internal/agent/runtime/lifecycle/session.go:602-651`) sanitizes
+persisted options with `profileconfig.SanitizeConfigOptions`, which only strips
+`model`/`mode`/blank keys. Prior-agent keys (`effort`, `thinking`) survive, are
+sent via `SetConfigOption`, fail, and land in `failed` -> user warning
+"Some session settings could not be applied at startup: effort, thinking."
+
+The catalog-aware helper already exists and is used on the reset/restore path:
+`sanitizeRuntimeConfigOptionsWithCatalog(options, execution.GetModelState())`
+(`apps/backend/internal/agent/runtime/lifecycle/manager_interaction.go:627-648`,
+called at line 696). Both files are in package `lifecycle`.
+
+## Steps (TDD)
+
+1. Set `status: in_progress`.
+2. Write the regression test FIRST and confirm it fails for the expected reason
+   (stale keys reach `SetConfigOption` / appear in `failed`).
+3. In `applyRuntimeSessionLayers`, replace the line
+   `sanitizedOptions := profileconfig.SanitizeConfigOptions(runtimeConfigOptions)`
+   (session.go:638) with
+   `sanitizedOptions := sanitizeRuntimeConfigOptionsWithCatalog(runtimeConfigOptions, execution.GetModelState())`.
+   Leave the `SetConfigOption` loop, `failed` handling, and `applyProfileSessionLayers`
+   unchanged.
+4. Run the targeted test; confirm it passes.
+5. Reconcile files-touched, set `status: done`, update `plan.md` checkbox and
+   `## Verification Results`.
+
+## Acceptance
+
+- Given persisted options `{reasoning, fast, effort, thinking}` and a current
+  agent catalog advertising `{reasoning, fast, context}`, replay sends only
+  `reasoning` and `fast`; `effort`/`thinking` are not sent and not in `failed`.
+- Given an advertised key whose value the agent rejects, that key still appears
+  in `failed` (real failure preserved).
+- Given a catalog that is not yet known (unsettled), behavior matches today's
+  restorable-set behavior (no regression for same-agent resume).
+
+## Verification
+
+```bash
+cd apps/backend && go test -run TestApplyRuntimeSessionLayers ./internal/agent/runtime/lifecycle/...
+```
+
+If the new test uses a different name, substitute it in the `-run` filter. Also
+run the changed-file linter before pushing (see backend AGENTS.md):
+
+```bash
+cd apps/backend && golangci-lint run ./internal/agent/runtime/lifecycle/... --new-from-rev="<base-sha>" --timeout=5m
+```
+
+## Files likely touched
+
+- `apps/backend/internal/agent/runtime/lifecycle/session.go`
+- `apps/backend/internal/agent/runtime/lifecycle/session_test.go` (new test;
+  put in a new `*_test.go` file if the existing one is near the 800-line limit)
+
+## Dependencies
+
+None.
+
+## Parallelism
+
+`parallel-safe` with task-02 (disjoint Go vs TS files). Default sequential.
+
+## Inputs
+
+- Spec: "Desired behavior" (Backend), "Failure modes", first and fourth scenarios.
+- Plan: Backend section.
+- Existing helper: `sanitizeRuntimeConfigOptionsWithCatalog`,
+  `capturedRuntimeConfigOptionCatalog`, `isRestorableRuntimeConfigOption`
+  (`manager_interaction.go:627-679`).
+
+## Output contract
+
+Summary, files changed, exact test command + result, blockers, risks, and
+task/plan status update in the same conversation.
+
+## Results
+
+Done.
+
+- Change: `applyRuntimeSessionLayers` (`session.go:638`) now calls
+  `sanitizeRuntimeConfigOptionsWithCatalog(runtimeConfigOptions, execution.GetModelState())`
+  in place of `profileconfig.SanitizeConfigOptions(runtimeConfigOptions)`. The
+  `profileconfig` import is retained (still used at `session.go:585` and `:794`).
+- Test (TDD): added `session_runtime_layers_test.go` with
+  `TestApplyRuntimeSessionLayersFiltersUnadvertisedOptions`. Red first: stale
+  `effort`/`thinking` reached `SetConfigOption` and landed in `failed`
+  (`got [effort fast reasoning thinking]`). Green after fix.
+- Commands:
+  - `go test -run TestApplyRuntimeSessionLayersFiltersUnadvertisedOptions ./internal/agent/runtime/lifecycle/` → ok
+  - `go test ./internal/agent/runtime/lifecycle/` → ok (69.9s, full package)
+  - `gofmt -l` on changed files → clean
+  - `golangci-lint run ./internal/agent/runtime/lifecycle/... --new-from-rev=1648cb8011b10cec0085c5fc3c2aea314404272e --timeout=5m` → 0 issues
+- External side-effect boundaries: None (in-process WS mock only).
diff --git a/docs/plans/session-config-cross-agent-reconcile/task-02-frontend-selector-gate.md b/docs/plans/session-config-cross-agent-reconcile/task-02-frontend-selector-gate.md
new file mode 100644
index 0000000000..c3ed433b23
--- /dev/null
+++ b/docs/plans/session-config-cross-agent-reconcile/task-02-frontend-selector-gate.md
@@ -0,0 +1,121 @@
+---
+id: "02-frontend-selector-gate"
+title: "Frontend: don't require persisted-only unadvertised keys in selector gate"
+status: done
+wave: 1
+depends_on: []
+plan: "plan.md"
+spec: "../../specs/session-config-cross-agent-reconcile/spec.md"
+parallelism: sequential
+---
+
+# Task 02: Frontend model-selector render gate
+
+Make the chat model/mode selector render for a resumed session even when the
+session's persisted metadata records option keys the current agent does not
+advertise, so `configHydrated` no longer stays `false` forever.
+
+## Context / root cause
+
+`hasCompleteDynamicConfig`
+(`apps/web/components/task/model-selector.tsx:109-133`) requires every key from
+`requiredConfigKeys` (lines 90-107), which includes keys read from
+`session.metadata.runtime_config(_overrides).config_options`. When the current
+agent never advertises a persisted key (e.g. `effort`, `thinking`), the
+`required.every(...)` check never passes, `configHydrated=false`, and the render
+gate at line 533 hides the selector permanently. Observed logs:
+`requiredKeys=[context, reasoning, effort, fast, mode, model, thinking]`,
+`rawConfigOptionIds=[mode, model, context, reasoning, fast]`, `willHide=true`.
+
+## Steps (TDD)
+
+1. Set `status: in_progress`.
+2. Write/extend the regression test FIRST and confirm it fails: a session with a
+   persisted-only, unadvertised key plus a settled agent catalog currently gives
+   `hasCompleteDynamicConfig === false`.
+3. Change the gate so a required key that is sourced ONLY from persisted
+   `runtime_config` / `runtime_config_overrides` (i.e. not from the agent profile
+   snapshot / matched profile) is treated as satisfied when the agent catalog is
+   settled (`sessionModelsData.configOptionsSettled === true` and/or non-empty
+   `configOptions`) and does not advertise that key. Preserve the existing
+   `AGENT_CONFIG_KEY` legacy exception and the `MODEL_CONFIG_KEY` flat-model-list
+   exception (lines 124-131). Keys required by the profile snapshot/matched
+   profile remain blocking.
+   - Suggested shape: split `requiredConfigKeys` conceptually into
+     profile-required keys (snapshot + matched profile) and persisted-only keys,
+     then in `hasCompleteDynamicConfig` require all profile-required keys as
+     today but only require a persisted-only key when the catalog is unsettled.
+     Keep the change minimal and within this file.
+4. Run the targeted test; confirm it passes. Run typecheck and lint.
+5. Reconcile files-touched, set `status: done`, update `plan.md` checkbox and
+   `## Verification Results`.
+
+## Acceptance
+
+- Given a settled agent catalog advertising `[mode, model, context, reasoning,
+  fast]` and persisted-only keys `effort`, `thinking`, then
+  `hasCompleteDynamicConfig` returns `true` (selector renders).
+- Given the agent profile snapshot requires a key the agent has NOT advertised
+  and the catalog is settled, `hasCompleteDynamicConfig` still returns `false`.
+- Given an unsettled catalog, behavior is unchanged from today.
+
+## Verification
+
+```bash
+cd apps && pnpm install --frozen-lockfile && pnpm --filter @kandev/web test -- components/task/model-selector.test.ts
+cd apps/web && pnpm run typecheck
+cd apps && pnpm --filter @kandev/web lint
+```
+
+If the test file is `.test.tsx` or lives elsewhere, substitute the correct path.
+
+## Files likely touched
+
+- `apps/web/components/task/model-selector.tsx`
+- The existing model-selector unit test
+  (`apps/web/components/task/model-selector.test.ts(x)`); create beside the
+  component if none exists, following repo frontend test conventions.
+
+## Dependencies
+
+None.
+
+## Parallelism
+
+`parallel-safe` with task-01 (disjoint TS vs Go files). Default sequential.
+
+## Inputs
+
+- Spec: "What" (selector rendering bullet), "Desired behavior" (Frontend),
+  second and third scenarios.
+- Plan: Frontend section.
+- Existing gate/exception logic: `model-selector.tsx:56-133`.
+
+## Output contract
+
+Summary, files changed, exact test/typecheck/lint commands + results, blockers,
+risks, and task/plan status update in the same conversation.
+
+## Results
+
+Done.
+
+- Change (`model-selector.tsx`): split `requiredConfigKeys` into two helpers,
+  `profileRequiredConfigKeys` (snapshot + matched profile) and
+  `persistedRuntimeConfigKeys` (runtime_config/_overrides). `hasCompleteDynamicConfig`
+  now treats a required key as satisfied when it is a persisted-runtime-only key
+  the current agent does not advertise once the catalog is settled
+  (`isPersistedOnlyStaleKey`). The `AGENT_CONFIG_KEY` is explicitly excluded from
+  the new exception so the existing legacy-agent semantics keep owning that key;
+  the flat-model-list and legacy-agent exceptions are otherwise unchanged.
+- Test (TDD): added `describe("cross-agent persisted config reconciliation")` in
+  `model-selector.test.ts` (3 cases). Red first: the persisted-only-unadvertised
+  settled-catalog case returned `false` (`expected false to be true`). Green
+  after fix. Extracted `modelConfigOption` / `selectOption` / `catalogEntry`
+  helpers to keep the block under the 100-line function limit.
+- Commands:
+  - `pnpm vitest run components/task/model-selector.test.ts` → 26 passed
+  - `pnpm run typecheck` → clean
+  - `pnpm --filter @kandev/web lint` → 0 warnings
+  - `pnpm run i18n:ratchet` → clean
+- External side-effect boundaries: None (pure function unit test).
diff --git a/docs/specs/session-config-cross-agent-reconcile/spec.md b/docs/specs/session-config-cross-agent-reconcile/spec.md
new file mode 100644
index 0000000000..0511bc3745
--- /dev/null
+++ b/docs/specs/session-config-cross-agent-reconcile/spec.md
@@ -0,0 +1,137 @@
+---
+status: implemented
+created: 2026-08-19
+owner: kandev
+---
+
+# Session Config Reconciliation Across Agent Types
+
+## Why
+
+A task session persists its runtime configuration options globally (model, mode,
+and provider-defined options like reasoning effort). When a session that ran
+earlier under one agent type is resumed under a different agent type, options
+that only the earlier agent understood are replayed onto the new agent. The new
+agent rejects them, the user sees a startup warning, and the model/mode selector
+disappears from the chat input, leaving the user unable to change model or mode
+for the resumed session.
+
+## What
+
+- On session startup and resume, Kandev SHALL only apply persisted runtime
+  config options that the currently running agent advertises. Options the
+  current agent does not advertise are dropped, not sent to the agent.
+- Dropping an unsupported option SHALL NOT be surfaced to the user as a failure
+  warning. The startup config warning is reserved for options the agent *does*
+  advertise but fails to apply.
+- Persisted options that the current agent still advertises SHALL continue to be
+  applied unchanged, including provider-defined options such as reasoning
+  effort, context, and fast.
+- The chat model/mode selector SHALL render for a resumed session whenever the
+  current agent has reported its model list and config options, even if the
+  session's persisted metadata still records option keys the current agent does
+  not advertise. Un-advertised persisted keys SHALL NOT be treated as required
+  for the selector to render.
+- Switching model within a single agent (where the agent re-advertises a
+  different option set for the new model) continues to work through the existing
+  live `session.models_updated` path and is unchanged by this repair.
+
+## Broken behavior (regression being fixed)
+
+1. Backend replay path `applyRuntimeSessionLayers`
+   (`apps/backend/internal/agent/runtime/lifecycle/session.go`) sanitizes
+   persisted options only with `profileconfig.SanitizeConfigOptions`, which
+   strips `model`/`mode`/blank keys but keeps every other key. Keys from a prior
+   agent (e.g. `effort`, `thinking`) are sent via `SetConfigOption`, the agent
+   errors, and each failed key is collected into the `failed` list.
+2. The orchestrator emits "Some session settings could not be applied at
+   startup: effort, thinking." from that `failed` list
+   (`apps/backend/internal/orchestrator/event_handlers_streaming.go`).
+3. The frontend gate `hasCompleteDynamicConfig`
+   (`apps/web/components/task/model-selector.tsx`) treats every key in
+   `session.metadata.runtime_config(_overrides).config_options` as required
+   (`requiredConfigKeys`). The current agent never advertises `effort`/`thinking`,
+   so the gate never passes, `configHydrated` stays `false`, and the selector is
+   hidden permanently.
+
+## Desired behavior
+
+- Backend: the startup/resume replay filters persisted runtime config options
+  against the current agent's advertised option catalog before sending them, so
+  unsupported keys are neither applied nor reported as failures. The catalog is
+  the same information used by the existing `sanitizeRuntimeConfigOptionsWithCatalog`
+  reconciliation on the reset/restore path.
+- Frontend: `requiredConfigKeys` / `hasCompleteDynamicConfig` do not require a
+  persisted key that the current agent's advertised config options
+  (`sessionModelsData.configOptions`) do not include. The selector renders from
+  the agent's live advertised options.
+
+## Data model
+
+No schema change. Existing session metadata keys are unchanged:
+
+- `runtime_config` -> `SessionRuntimeConfig{ model, mode, config_options }`
+- `runtime_config_overrides` -> same shape
+
+`config_options` remains a flat `map[string]string` of `{optionId: value}`.
+Stale keys already written to metadata are tolerated (filtered on read/apply),
+not required to be migrated away.
+
+## API surface
+
+No new HTTP/WS contract. The existing `session.models_updated` WS event and the
+existing startup config-warning message are reused. The warning message content
+changes only in that it no longer lists keys the current agent never advertised.
+
+## Failure modes
+
+- **Agent config catalog not yet settled at replay time:** if the current
+  agent's advertised option catalog is unknown when replay runs, the backend
+  MUST fail safe by not sending options it cannot verify are supported, rather
+  than sending all of them. It does not emit a spurious warning for options it
+  chose not to send.
+- **Agent advertises the option but rejects the value:** this remains a real
+  failure and is still reported in the startup warning (unchanged).
+- **Persisted metadata retains stale keys:** tolerated. The frontend gate and
+  backend replay both ignore keys the current agent does not advertise.
+
+## Persistence guarantees
+
+- No change to what survives a restart. Persisted `runtime_config` and
+  `runtime_config_overrides` are unchanged on disk.
+- This repair does not rewrite or delete persisted stale keys; it makes read and
+  apply paths resilient to them. Any future cleanup of persisted stale keys is
+  out of scope.
+
+## Scenarios
+
+- **GIVEN** a session whose persisted `runtime_config.config_options` contains
+  `effort` and `thinking` (written by a prior agent) plus `model`, `reasoning`,
+  and `fast`, **WHEN** the session is resumed under an agent that advertises only
+  `mode, model, context, reasoning, fast`, **THEN** the backend applies
+  `reasoning` and `fast`, does not call `SetConfigOption` for `effort` or
+  `thinking`, and emits no "could not be applied at startup" warning for them.
+- **GIVEN** the same resumed session, **WHEN** the frontend evaluates the
+  model-selector gate after the agent reports its config options, **THEN**
+  `configHydrated` is `true` and the model/mode selector renders.
+- **GIVEN** a resumed session whose persisted options are all advertised by the
+  current agent, **WHEN** the session resumes, **THEN** all persisted options are
+  applied exactly as today and the selector renders (no behavior change).
+- **GIVEN** a resumed session where the agent advertises `reasoning` but rejects
+  the persisted `reasoning` value, **WHEN** replay runs, **THEN** `reasoning`
+  still appears in the startup warning (real failure preserved).
+- **GIVEN** a running session under one agent, **WHEN** the user switches model
+  and the agent re-advertises a different option set, **THEN** the live
+  `session.models_updated` path updates the selector as it does today (unchanged).
+
+## Out of scope
+
+- Migrating or deleting stale option keys already persisted in session metadata.
+- Changing how provider option catalogs are discovered or the sessionless
+  resolver in `docs/specs/agents/dynamic-provider-options.md`.
+- Per-model nesting of persisted config options.
+- Any change to the live in-session `session.models_updated` protocol.
+
+## Open questions
+
+- None.