From 2c06cf5b6f8a09504874dd6a2d8bbefa2c3b5389 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jo=C3=A3o=20Salavisa?= <joao.salavisa@gmail.com>
Date: Tue, 22 Sep 2026 08:15:41 +0100
Subject: [PATCH] fix: keep file tree paths workspace-relative (#3855)

Co-authored-by: Kandev Agent <agent@kandev.dev>
---
 .../agent/handlers/workspace_file_handlers.go |   4 +
 .../handlers/workspace_file_handlers_test.go  |  42 ++++
 .../server/process/workspace_files.go         |   4 +
 .../server/process/workspace_files_test.go    |  40 ++++
 .../internal/common/workspacepath/tree.go     |  61 ++++++
 .../common/workspacepath/tree_test.go         |  35 ++++
 .../chat/messages/tool-edit-message.test.tsx  |  18 ++
 .../task/chat/messages/tool-edit-message.tsx  |   3 +-
 .../task/file-browser-apply-changes.test.ts   |  54 ++++-
 .../web/components/task/file-browser-hooks.ts |   9 +-
 .../file-browser-restore-expanded.test.tsx    |  16 ++
 .../components/task/file-browser-restore.ts   |   3 +-
 .../components/task/file-tree-reveal.test.ts  |  20 ++
 apps/web/components/task/file-tree-reveal.ts  |  16 +-
 apps/web/e2e/helpers/session.ts               |  18 ++
 .../chat-read-absolute-workspace-path.spec.ts | 101 +++++++++
 .../e2e/tests/task/mobile-file-viewer.spec.ts |  29 +--
 apps/web/hooks/use-file-editors.test.tsx      |  26 +++
 apps/web/hooks/use-file-editors.ts            |  10 +-
 apps/web/hooks/use-lsp-file-opener.ts         |   6 +-
 apps/web/lib/workspace-file-path.test.ts      |  90 ++++++++
 apps/web/lib/workspace-file-path.ts           |  73 +++++++
 docs/plans/file-tree-path-scope/plan.md       | 184 +++++++++++++++++
 .../task-01-normalize-tool-file-paths.md      | 145 +++++++++++++
 .../task-02-enforce-tree-request-scope.md     | 172 ++++++++++++++++
 .../ui/requirements/file-tree-path-scope.md   |  96 +++++++++
 .../ui/system-design/file-tree-path-scope.md  | 192 ++++++++++++++++++
 27 files changed, 1432 insertions(+), 35 deletions(-)
 create mode 100644 apps/backend/internal/common/workspacepath/tree.go
 create mode 100644 apps/backend/internal/common/workspacepath/tree_test.go
 create mode 100644 apps/web/e2e/tests/task/chat-read-absolute-workspace-path.spec.ts
 create mode 100644 apps/web/lib/workspace-file-path.test.ts
 create mode 100644 apps/web/lib/workspace-file-path.ts
 create mode 100644 docs/plans/file-tree-path-scope/plan.md
 create mode 100644 docs/plans/file-tree-path-scope/task-01-normalize-tool-file-paths.md
 create mode 100644 docs/plans/file-tree-path-scope/task-02-enforce-tree-request-scope.md
 create mode 100644 docs/specs/ui/requirements/file-tree-path-scope.md
 create mode 100644 docs/specs/ui/system-design/file-tree-path-scope.md

diff --git a/apps/backend/internal/agent/handlers/workspace_file_handlers.go b/apps/backend/internal/agent/handlers/workspace_file_handlers.go
index 3b616125b2..dfbaf6fa2d 100644
--- a/apps/backend/internal/agent/handlers/workspace_file_handlers.go
+++ b/apps/backend/internal/agent/handlers/workspace_file_handlers.go
@@ -10,6 +10,7 @@ import (
 	agentctl "github.com/kandev/kandev/internal/agent/runtime/agentctl"
 	"github.com/kandev/kandev/internal/agentctl/types/streams"
 	"github.com/kandev/kandev/internal/common/logger"
+	"github.com/kandev/kandev/internal/common/workspacepath"
 	ws "github.com/kandev/kandev/pkg/websocket"
 	"go.uber.org/zap"
 )
@@ -62,6 +63,9 @@ func (h *WorkspaceFileHandlers) wsGetFileTree(ctx context.Context, msg *ws.Messa
 	if req.SessionID == "" {
 		return ws.NewError(msg.ID, msg.Action, ws.ErrorCodeValidation, "session_id is required", nil)
 	}
+	if err := workspacepath.ValidateTreePath(req.Path); err != nil {
+		return ws.NewError(msg.ID, msg.Action, ws.ErrorCodeValidation, err.Error(), nil)
+	}
 	// Get agent execution for this session
 	execution, err := h.lifecycle.GetOrEnsureExecution(ctx, req.SessionID)
 	if err != nil {
diff --git a/apps/backend/internal/agent/handlers/workspace_file_handlers_test.go b/apps/backend/internal/agent/handlers/workspace_file_handlers_test.go
index b066ea4a7e..dcda89a5bf 100644
--- a/apps/backend/internal/agent/handlers/workspace_file_handlers_test.go
+++ b/apps/backend/internal/agent/handlers/workspace_file_handlers_test.go
@@ -19,6 +19,19 @@ import (
 	"go.uber.org/zap/zaptest/observer"
 )
 
+type countingWorkspaceExecutionLookup struct {
+	calls int
+}
+
+func (l *countingWorkspaceExecutionLookup) GetExecutionBySessionID(string) (*lifecycle.AgentExecution, bool) {
+	return nil, false
+}
+
+func (l *countingWorkspaceExecutionLookup) GetOrEnsureExecution(context.Context, string) (*lifecycle.AgentExecution, error) {
+	l.calls++
+	return nil, context.Canceled
+}
+
 func TestWorkspaceFileHandlersValidateRequestsBeforeLookup(t *testing.T) {
 	tests := []struct {
 		name    string
@@ -50,6 +63,35 @@ func TestWorkspaceFileHandlersValidateRequestsBeforeLookup(t *testing.T) {
 	}
 }
 
+func TestWorkspaceFileHandlersRejectAbsoluteTreePathBeforeLookup(t *testing.T) {
+	core, logs := observer.New(zapcore.DebugLevel)
+	log, err := commonlogger.NewFromZap(zap.New(core))
+	if err != nil {
+		t.Fatal(err)
+	}
+	lookup := &countingWorkspaceExecutionLookup{}
+	handler := NewWorkspaceFileHandlers(lookup, log)
+	msg, err := ws.NewRequest("id", ws.ActionWorkspaceFileTreeGet, map[string]any{
+		"session_id": "s",
+		"path":       "/home/jcfs/project/public/assets",
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	response, err := handler.wsGetFileTree(context.Background(), msg)
+	if err != nil {
+		t.Fatalf("wsGetFileTree returned error: %v", err)
+	}
+	assertWorkspaceContentSearchErrorCode(t, response, ws.ErrorCodeValidation)
+	if lookup.calls != 0 {
+		t.Fatalf("execution lookup calls = %d, want 0", lookup.calls)
+	}
+	if logs.FilterLevelExact(zap.ErrorLevel).Len() != 0 {
+		t.Fatalf("error logs = %v, want none", logs.FilterLevelExact(zap.ErrorLevel).All())
+	}
+}
+
 func TestWorkspaceFileHandlersSuccessContracts(t *testing.T) {
 	tests := []struct {
 		name      string
diff --git a/apps/backend/internal/agentctl/server/process/workspace_files.go b/apps/backend/internal/agentctl/server/process/workspace_files.go
index b5b8f1d77f..5514e562b1 100644
--- a/apps/backend/internal/agentctl/server/process/workspace_files.go
+++ b/apps/backend/internal/agentctl/server/process/workspace_files.go
@@ -22,6 +22,7 @@ import (
 	"github.com/kandev/kandev/internal/agentctl/types"
 	"github.com/kandev/kandev/internal/common/readselector"
 	"github.com/kandev/kandev/internal/common/subproc"
+	"github.com/kandev/kandev/internal/common/workspacepath"
 	storageworkspaces "github.com/kandev/kandev/internal/system/storage/workspaces"
 	"go.uber.org/zap"
 )
@@ -126,6 +127,9 @@ func (wt *WorkspaceTracker) getFileListClass(ctx context.Context, class subproc.
 
 // GetFileTree returns the file tree for a given path and depth
 func (wt *WorkspaceTracker) GetFileTree(reqPath string, depth int) (*types.FileTreeNode, error) {
+	if err := workspacepath.ValidateTreePath(reqPath); err != nil {
+		return nil, err
+	}
 	// Resolve the full path with path traversal protection
 	safePath := filepath.Join(wt.workDir, filepath.Clean(reqPath))
 	cleanWorkDir := filepath.Clean(wt.workDir)
diff --git a/apps/backend/internal/agentctl/server/process/workspace_files_test.go b/apps/backend/internal/agentctl/server/process/workspace_files_test.go
index 9bcce1a039..b2f07cf725 100644
--- a/apps/backend/internal/agentctl/server/process/workspace_files_test.go
+++ b/apps/backend/internal/agentctl/server/process/workspace_files_test.go
@@ -13,6 +13,7 @@ import (
 
 	"github.com/kandev/kandev/internal/agentctl/types"
 	"github.com/kandev/kandev/internal/common/logger"
+	"github.com/kandev/kandev/internal/common/workspacepath"
 	storageworkspaces "github.com/kandev/kandev/internal/system/storage/workspaces"
 	"go.uber.org/zap"
 )
@@ -509,6 +510,45 @@ func TestGetFileTree_HidesOnlyRootOwnershipMarker(t *testing.T) {
 	}
 }
 
+func TestGetFileTree_RejectsAbsolutePathBeforeFilesystemAccess(t *testing.T) {
+	workspace := t.TempDir()
+	external := t.TempDir()
+	log, err := logger.NewFromZap(zap.NewNop())
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_, err = (&WorkspaceTracker{workDir: workspace, logger: log}).GetFileTree(external, 1)
+	if !errors.Is(err, workspacepath.ErrTreePathNotRelative) {
+		t.Fatalf("GetFileTree error = %v, want %v", err, workspacepath.ErrTreePathNotRelative)
+	}
+}
+
+func TestGetFileTree_AllowsLiteralColonInRelativePath(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("Windows does not permit literal colons in path components")
+	}
+	workspace := t.TempDir()
+	directory := filepath.Join(workspace, "config:dev")
+	if err := os.Mkdir(directory, 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(directory, "settings.json"), []byte("{}"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	tree, err := (&WorkspaceTracker{workDir: workspace}).GetFileTree("config:dev", 1)
+	if err != nil {
+		t.Fatalf("GetFileTree failed: %v", err)
+	}
+	if tree.Path != "config:dev" {
+		t.Fatalf("GetFileTree root path = %q, want %q", tree.Path, "config:dev")
+	}
+	if findChild(tree, "settings.json") == nil {
+		t.Fatal("file beneath colon-containing directory should be visible")
+	}
+}
+
 func TestGetFileList_HidesOnlyRootOwnershipMarker(t *testing.T) {
 	taskRoot := createOwnershipMarkerFixture(t)
 	initGitRepoAt(t, taskRoot)
diff --git a/apps/backend/internal/common/workspacepath/tree.go b/apps/backend/internal/common/workspacepath/tree.go
new file mode 100644
index 0000000000..4b78659d03
--- /dev/null
+++ b/apps/backend/internal/common/workspacepath/tree.go
@@ -0,0 +1,61 @@
+// Package workspacepath validates path identities used by workspace services.
+package workspacepath
+
+import (
+	"errors"
+	pathpkg "path"
+	"path/filepath"
+	"strings"
+)
+
+// ErrTreePathNotRelative identifies a path that cannot address the workspace tree.
+var ErrTreePathNotRelative = errors.New("file tree path must be workspace-relative")
+
+// ValidateTreePath accepts the empty workspace root and canonical slash-separated relative paths.
+func ValidateTreePath(requested string) error {
+	if requested == "" {
+		return nil
+	}
+	if strings.IndexByte(requested, 0) >= 0 ||
+		strings.Contains(requested, `\`) ||
+		filepath.IsAbs(requested) ||
+		pathpkg.IsAbs(filepath.ToSlash(requested)) ||
+		filepath.VolumeName(requested) != "" ||
+		hasWindowsDrivePrefix(requested) ||
+		hasAbsoluteURIForm(requested) {
+		return ErrTreePathNotRelative
+	}
+	cleaned := pathpkg.Clean(requested)
+	if cleaned != requested || cleaned == "." || cleaned == ".." || strings.HasPrefix(cleaned, "../") {
+		return ErrTreePathNotRelative
+	}
+	return nil
+}
+
+func hasAbsoluteURIForm(path string) bool {
+	colon := strings.IndexByte(path, ':')
+	if colon <= 0 || !isASCIILetter(path[0]) {
+		return false
+	}
+	for index := 1; index < colon; index++ {
+		char := path[index]
+		if !isASCIILetter(char) && (char < '0' || char > '9') && char != '+' && char != '-' && char != '.' {
+			return false
+		}
+	}
+	remainder := path[colon+1:]
+	return strings.HasPrefix(remainder, "//") ||
+		strings.EqualFold(path[:colon], "file") && strings.HasPrefix(remainder, "/")
+}
+
+func hasWindowsDrivePrefix(path string) bool {
+	if len(path) < 2 || path[1] != ':' {
+		return false
+	}
+	first := path[0]
+	return isASCIILetter(first)
+}
+
+func isASCIILetter(char byte) bool {
+	return char >= 'A' && char <= 'Z' || char >= 'a' && char <= 'z'
+}
diff --git a/apps/backend/internal/common/workspacepath/tree_test.go b/apps/backend/internal/common/workspacepath/tree_test.go
new file mode 100644
index 0000000000..03f6cb52ce
--- /dev/null
+++ b/apps/backend/internal/common/workspacepath/tree_test.go
@@ -0,0 +1,35 @@
+package workspacepath
+
+import "testing"
+
+func TestValidateTreePath(t *testing.T) {
+	for _, path := range []string{"", "src", "src/components", ".codex/agents", "config:dev", "dir/.env:"} {
+		t.Run("valid_"+path, func(t *testing.T) {
+			if err := ValidateTreePath(path); err != nil {
+				t.Fatalf("ValidateTreePath(%q) = %v, want nil", path, err)
+			}
+		})
+	}
+
+	for _, path := range []string{
+		"/home/jcfs/project/src",
+		`C:\workspace\src`,
+		"C:/workspace/src",
+		"file:///workspace/src",
+		"file:/workspace/src",
+		".",
+		"..",
+		"../src",
+		"src/../outside",
+		"src//components",
+		"src/components/",
+		"src\\components",
+		"src\x00components",
+	} {
+		t.Run("invalid_"+path, func(t *testing.T) {
+			if err := ValidateTreePath(path); err != ErrTreePathNotRelative {
+				t.Fatalf("ValidateTreePath(%q) = %v, want %v", path, err, ErrTreePathNotRelative)
+			}
+		})
+	}
+}
diff --git a/apps/web/components/task/chat/messages/tool-edit-message.test.tsx b/apps/web/components/task/chat/messages/tool-edit-message.test.tsx
index 3799eb8909..7475d0274c 100644
--- a/apps/web/components/task/chat/messages/tool-edit-message.test.tsx
+++ b/apps/web/components/task/chat/messages/tool-edit-message.test.tsx
@@ -57,3 +57,21 @@ describe("ToolEditMessage write summary", () => {
     expect(html).not.toContain("line)");
   });
 });
+
+describe("ToolEditMessage file action", () => {
+  // @covers AC-UI-FILE-TREE-PATH-SCOPE-001.1
+  it("does not treat a workspace prefix collision as an in-workspace file", () => {
+    const comment = writeComment("Write utils.ts", "only line");
+    const metadata = comment.metadata as {
+      normalized: { modify_file: { file_path: string } };
+    };
+    metadata.normalized.modify_file.file_path = "/workspace-old/apps/web/lib/utils.ts";
+
+    const html = renderToStaticMarkup(
+      <ToolEditMessage comment={comment} worktreePath="/workspace" onOpenFile={() => undefined} />,
+    );
+
+    expect(html).toContain('title="Copy path"');
+    expect(html).not.toContain('title="Open file"');
+  });
+});
diff --git a/apps/web/components/task/chat/messages/tool-edit-message.tsx b/apps/web/components/task/chat/messages/tool-edit-message.tsx
index 2effbb671a..aca3682ff2 100644
--- a/apps/web/components/task/chat/messages/tool-edit-message.tsx
+++ b/apps/web/components/task/chat/messages/tool-edit-message.tsx
@@ -18,6 +18,7 @@ import { ExpandableRow } from "./expandable-row";
 import { transformFileMutation, type FileMutation } from "@/lib/diff";
 import { useExpandState } from "./use-expand-state";
 import { useOpenFileAtLine } from "@/hooks/use-file-editors";
+import { workspaceRelativeFilePath } from "@/lib/workspace-file-path";
 import { useTranslation } from "react-i18next";
 import type { TFunction } from "i18next";
 
@@ -79,7 +80,7 @@ function FileActionButton({
   onCopyPath,
 }: FileActionButtonProps) {
   const { t } = useTranslation();
-  const isFileInWorktree = worktreePath && filePath.startsWith(worktreePath);
+  const isFileInWorktree = workspaceRelativeFilePath(filePath, worktreePath) !== null;
   if (onOpenFile && isFileInWorktree) {
     return (
       <button
diff --git a/apps/web/components/task/file-browser-apply-changes.test.ts b/apps/web/components/task/file-browser-apply-changes.test.ts
index 7edc2c3dfd..4e78bfde53 100644
--- a/apps/web/components/task/file-browser-apply-changes.test.ts
+++ b/apps/web/components/task/file-browser-apply-changes.test.ts
@@ -20,6 +20,8 @@ const SESSION_ID = "sess";
 const REFRESH_OP = "refresh";
 const THM_OLD = "thm/old.txt";
 const THM_NEW = "thm/new.txt";
+const SRC_PATH = "src";
+const SRC_COMPONENTS_PATH = `${SRC_PATH}/components`;
 
 beforeEach(async () => {
   vi.resetModules();
@@ -84,7 +86,7 @@ describe("applyFileChanges — refresh operation expands to all expanded folders
     applyFileChanges({
       client: client(),
       sessionId: SESSION_ID,
-      expandedPaths: new Set(["src", "src/components"]),
+      expandedPaths: new Set([SRC_PATH, SRC_COMPONENTS_PATH]),
       changes: [{ path: "", operation: REFRESH_OP }],
       setTree: vi.fn(),
       setLoadState: vi.fn(),
@@ -92,8 +94,8 @@ describe("applyFileChanges — refresh operation expands to all expanded folders
     await new Promise<void>((r) => setTimeout(r, 0));
     expect(requestFileTreeMock.mock.calls.map((c) => c[2]).sort()).toEqual([
       "",
-      "src",
-      "src/components",
+      SRC_PATH,
+      SRC_COMPONENTS_PATH,
     ]);
   });
 
@@ -151,6 +153,52 @@ describe("applyFileChanges — refresh operation expands to all expanded folders
   });
 });
 
+it("skips invalid expanded paths during a generic refresh", async () => {
+  mockEmptyTree();
+  applyFileChanges({
+    client: client(),
+    sessionId: SESSION_ID,
+    expandedPaths: new Set([SRC_PATH, SRC_COMPONENTS_PATH, "/home/jcfs/project/src"]),
+    changes: [{ path: "", operation: REFRESH_OP }],
+    setTree: vi.fn(),
+    setLoadState: vi.fn(),
+  });
+  await new Promise<void>((r) => setTimeout(r, 0));
+  expect(requestFileTreeMock.mock.calls.map((c) => c[2]).sort()).toEqual([
+    "",
+    SRC_PATH,
+    SRC_COMPONENTS_PATH,
+  ]);
+});
+
+it("refreshes an expanded path containing a literal colon", async () => {
+  mockEmptyTree();
+  applyFileChanges({
+    client: client(),
+    sessionId: SESSION_ID,
+    expandedPaths: new Set(["config:dev"]),
+    changes: [{ path: "", operation: REFRESH_OP }],
+    setTree: vi.fn(),
+    setLoadState: vi.fn(),
+  });
+  await new Promise<void>((r) => setTimeout(r, 0));
+  expect(requestFileTreeMock.mock.calls.map((c) => c[2]).sort()).toEqual(["", "config:dev"]);
+});
+
+it("ignores a specific change with an invalid path", async () => {
+  mockEmptyTree();
+  applyFileChanges({
+    client: client(),
+    sessionId: SESSION_ID,
+    expandedPaths: new Set([SRC_PATH]),
+    changes: [{ path: "/home/jcfs/project/src/app.ts", operation: "create" }],
+    setTree: vi.fn(),
+    setLoadState: vi.fn(),
+  });
+  await new Promise<void>((r) => setTimeout(r, 0));
+  expect(requestFileTreeMock).not.toHaveBeenCalled();
+});
+
 // Regression (#982): refresh scoped to one repo must not wipe sibling repos' loaded subtrees.
 describe("applyFileChanges — cross-repo subtree preservation", () => {
   it("preserves kandev's loaded children when refresh is scoped to thm", async () => {
diff --git a/apps/web/components/task/file-browser-hooks.ts b/apps/web/components/task/file-browser-hooks.ts
index 2d7dd9d005..10c9fac9da 100644
--- a/apps/web/components/task/file-browser-hooks.ts
+++ b/apps/web/components/task/file-browser-hooks.ts
@@ -13,6 +13,7 @@ import { compareTreeNodes, sortRootChildren } from "./file-tree-utils";
 import { restoredExpandedPaths } from "./file-browser-restore";
 import { useTreeLoader } from "./file-browser-tree-loader";
 import { createDebugLogger, isDebug } from "@/lib/debug/log";
+import { isWorkspaceTreePath } from "@/lib/workspace-file-path";
 
 const debugLoad = createDebugLogger("file-browser:load");
 const debugChanges = createDebugLogger("file-browser:changes");
@@ -153,6 +154,11 @@ function nearestExpandedFolder(parentPath: string, expandedPaths: ReadonlySet<st
   return "";
 }
 
+function isExpandedPathInRefreshScope(path: string, repositoryName?: string): boolean {
+  if (!isWorkspaceTreePath(path)) return false;
+  return !repositoryName || path === repositoryName || path.startsWith(`${repositoryName}/`);
+}
+
 /** Apply incoming file changes to the tree by refreshing affected folders. */
 export function applyFileChanges(ctx: {
   client: ReturnType<typeof getWebSocketClient>;
@@ -169,13 +175,14 @@ export function applyFileChanges(ctx: {
       foldersToRefresh.add("");
       const repo = change.repository_name;
       for (const exp of expandedPaths) {
-        if (!repo || exp === repo || exp.startsWith(repo + "/")) {
+        if (isExpandedPathInRefreshScope(exp, repo)) {
           foldersToRefresh.add(exp);
         }
       }
       continue;
     }
     const p = change.path;
+    if (!isWorkspaceTreePath(p)) continue;
     const lastSlash = p.lastIndexOf("/");
     const parent = lastSlash === -1 ? "" : p.substring(0, lastSlash);
     foldersToRefresh.add(nearestExpandedFolder(parent, expandedPaths));
diff --git a/apps/web/components/task/file-browser-restore-expanded.test.tsx b/apps/web/components/task/file-browser-restore-expanded.test.tsx
index d8c169e248..be93d39890 100644
--- a/apps/web/components/task/file-browser-restore-expanded.test.tsx
+++ b/apps/web/components/task/file-browser-restore-expanded.test.tsx
@@ -81,6 +81,22 @@ describe("useFileBrowserTree persisted expansion", () => {
     expect(result.current.expandedPaths).toEqual(new Set(EXPANDED_PATHS));
   });
 
+  it("discards invalid persisted paths while restoring valid siblings", async () => {
+    sessionStorage.setItem(
+      STORAGE_KEY,
+      JSON.stringify([CODEX_PATH, AGENTS_PATH, "/home/jcfs/project/public/assets"]),
+    );
+    const { result } = renderHook(() => useFileBrowserTree(SESSION, ENVIRONMENT));
+
+    await waitFor(() => expect(result.current.loadState).toBe("loaded"));
+
+    expect(requestFileTreeMock.mock.calls.map((call) => call[2])).toEqual([
+      ROOT_PATH,
+      ...EXPANDED_PATHS,
+    ]);
+    expect(result.current.expandedPaths).toEqual(new Set(EXPANDED_PATHS));
+  });
+
   it("retries a transient restored-folder failure without pruning persisted expansion", async () => {
     vi.useFakeTimers();
     let codexAttempts = 0;
diff --git a/apps/web/components/task/file-browser-restore.ts b/apps/web/components/task/file-browser-restore.ts
index 5437ab1dc9..928fe733d4 100644
--- a/apps/web/components/task/file-browser-restore.ts
+++ b/apps/web/components/task/file-browser-restore.ts
@@ -1,6 +1,7 @@
 import { getWebSocketClient } from "@/lib/ws/connection";
 import { requestFileTree } from "@/lib/ws/workspace-files";
 import type { FileTreeNode } from "@/lib/types/backend";
+import { isWorkspaceTreePath } from "@/lib/workspace-file-path";
 import { findNodeByPath, mergeTreeNodes } from "./file-tree-utils";
 import type { TreeLoadOwner } from "./file-browser-tree-loader";
 export type RestoredTree = {
@@ -12,7 +13,7 @@ export type RestoredTree = {
 export function restoredExpandedPaths(paths: string[]): string[] {
   const restored = new Set<string>();
   for (const path of paths) {
-    if (!path) continue;
+    if (!path || !isWorkspaceTreePath(path)) continue;
     const parts = path.split("/");
     for (let i = 1; i <= parts.length; i++) restored.add(parts.slice(0, i).join("/"));
   }
diff --git a/apps/web/components/task/file-tree-reveal.test.ts b/apps/web/components/task/file-tree-reveal.test.ts
index 6a67e586b9..d130b317b2 100644
--- a/apps/web/components/task/file-tree-reveal.test.ts
+++ b/apps/web/components/task/file-tree-reveal.test.ts
@@ -273,3 +273,23 @@ it("does not reopen an active branch after the user collapses it", () => {
   expect(expanded.current.size).toBe(0);
   expect(setExpandedPaths).not.toHaveBeenCalled();
 });
+
+it("ignores an absolute active file path", async () => {
+  const loadChildren = vi.fn<(node: FileTreeNode) => Promise<boolean>>();
+  const { expanded, setExpandedPaths } = expansionState();
+
+  renderHook(() =>
+    useFileTreeReveal({
+      activeFilePath: "/home/jcfs/project/src/main/Greeting.kt",
+      sessionId: "session-1",
+      tree: srcOnlyTree(),
+      setExpandedPaths,
+      isLoading: () => false,
+      loadChildren,
+    }),
+  );
+  await act(async () => Promise.resolve());
+
+  expect(expanded.current).toEqual(new Set());
+  expect(loadChildren).not.toHaveBeenCalled();
+});
diff --git a/apps/web/components/task/file-tree-reveal.ts b/apps/web/components/task/file-tree-reveal.ts
index 6379314bbc..6a7ccad04b 100644
--- a/apps/web/components/task/file-tree-reveal.ts
+++ b/apps/web/components/task/file-tree-reveal.ts
@@ -3,6 +3,7 @@
 import { useEffect, useRef, useState } from "react";
 import type React from "react";
 import type { FileTreeNode } from "@/lib/types/backend";
+import { isWorkspaceTreePath } from "@/lib/workspace-file-path";
 import { findUnloadedAncestor, getAncestorPaths } from "./file-tree-utils";
 
 const DEFAULT_RETRY_DELAYS_MS = [250, 1_000] as const;
@@ -62,16 +63,17 @@ export function useFileTreeReveal(params: FileTreeRevealParams) {
   latestRef.current = { isLoading, loadChildren, retryDelaysMs };
   const runRef = useRef<RevealRun | null>(null);
   const [revision, setRevision] = useState(0);
-  const revealKey = activeFilePath ? `${sessionId}\0${activeFilePath}` : null;
+  const targetPath = activeFilePath && isWorkspaceTreePath(activeFilePath) ? activeFilePath : null;
+  const revealKey = targetPath ? `${sessionId}\0${targetPath}` : null;
   const treeAvailable = tree !== null;
 
   useEffect(() => {
-    if (!activeFilePath || !treeAvailable) return;
-    expandAncestors(activeFilePath, setExpandedPaths);
-  }, [activeFilePath, revealKey, setExpandedPaths, treeAvailable]);
+    if (!targetPath || !treeAvailable) return;
+    expandAncestors(targetPath, setExpandedPaths);
+  }, [revealKey, setExpandedPaths, targetPath, treeAvailable]);
 
   useEffect(() => {
-    if (!activeFilePath || !revealKey || !tree) {
+    if (!targetPath || !revealKey || !tree) {
       clearRetry(runRef.current);
       runRef.current = null;
       return;
@@ -82,7 +84,7 @@ export function useFileTreeReveal(params: FileTreeRevealParams) {
       runRef.current = createRun(revealKey);
     }
     const run = runRef.current;
-    const ancestor = findUnloadedAncestor(tree, activeFilePath, getAncestorPaths(activeFilePath));
+    const ancestor = findUnloadedAncestor(tree, targetPath, getAncestorPaths(targetPath));
     if (!ancestor) {
       clearRetry(run);
       return;
@@ -125,7 +127,7 @@ export function useFileTreeReveal(params: FileTreeRevealParams) {
         run.inFlightPath = null;
         console.error("Failed to reveal file tree path", error);
       });
-  }, [activeFilePath, revealKey, revision, tree]);
+  }, [revealKey, revision, targetPath, tree]);
 
   useEffect(
     () => () => {
diff --git a/apps/web/e2e/helpers/session.ts b/apps/web/e2e/helpers/session.ts
index a35f0b58ec..cb9c1942cc 100644
--- a/apps/web/e2e/helpers/session.ts
+++ b/apps/web/e2e/helpers/session.ts
@@ -57,6 +57,24 @@ export async function waitForSessionDone(
   );
 }
 
+export async function waitForWorkspacePath(
+  apiClient: ApiClient,
+  taskId: string,
+  sessionId: string,
+  timeout = 30_000,
+): Promise<string> {
+  return pollUntil(
+    async () => {
+      const { sessions } = await apiClient.listTaskSessions(taskId);
+      const session = sessions.find((candidate) => candidate.id === sessionId);
+      return session?.workspace_path ?? session?.worktree_path ?? "";
+    },
+    (workspacePath) => workspacePath.length > 0,
+    timeout,
+    "Waiting for task workspace path",
+  );
+}
+
 export async function waitForAgentMessage(
   apiClient: ApiClient,
   sessionId: string,
diff --git a/apps/web/e2e/tests/task/chat-read-absolute-workspace-path.spec.ts b/apps/web/e2e/tests/task/chat-read-absolute-workspace-path.spec.ts
new file mode 100644
index 0000000000..e127dabbce
--- /dev/null
+++ b/apps/web/e2e/tests/task/chat-read-absolute-workspace-path.spec.ts
@@ -0,0 +1,101 @@
+import path from "node:path";
+import type { Page } from "@playwright/test";
+import { test, expect } from "../../fixtures/test-base";
+import { GitHelper, makeGitEnv, createStandardProfile } from "../../helpers/git-helper";
+import { waitForWorkspacePath } from "../../helpers/session";
+import { SessionPage } from "../../pages/session-page";
+
+function observeFileTreeRequestPaths(page: Page): string[] {
+  const paths: string[] = [];
+  page.on("websocket", (socket) => {
+    if (!socket.url().endsWith("/ws")) return;
+    socket.on("framesent", ({ payload }) => {
+      const text = typeof payload === "string" ? payload : payload.toString();
+      try {
+        const frame = JSON.parse(text) as {
+          action?: string;
+          payload?: { path?: unknown };
+        };
+        if (frame.action === "workspace.tree.get" && typeof frame.payload?.path === "string") {
+          paths.push(frame.payload.path);
+        }
+      } catch {
+        // Gateway-adjacent sockets can carry non-JSON frames.
+      }
+    });
+  });
+  return paths;
+}
+
+test.describe("Absolute workspace read links", () => {
+  // @covers AC-UI-FILE-TREE-PATH-SCOPE-001.1
+  // @covers AC-UI-FILE-TREE-PATH-SCOPE-001.3
+  test("opens with a relative editor and file-tree identity", async ({
+    testPage,
+    apiClient,
+    seedData,
+    backend,
+  }) => {
+    test.setTimeout(90_000);
+    const suffix = `${Date.now()}-${Math.random().toString(36).slice(2, 8)}`;
+    const directory = `absolute-read-${suffix}`;
+    const fileName = "case.ts";
+    const filePath = `${directory}/${fileName}`;
+    const git = new GitHelper(
+      path.join(backend.tmpDir, "repos", "e2e-repo"),
+      makeGitEnv(backend.tmpDir),
+    );
+    git.createFile(filePath, 'export const suspect = "Colonel Mustard";');
+    git.stageAll();
+    git.commit(`add ${filePath}`);
+
+    const profile = await createStandardProfile(apiClient, `absolute-read-${Date.now()}`);
+    const task = await apiClient.createTaskWithAgent(
+      seedData.workspaceId,
+      "Absolute Workspace Read",
+      profile.id,
+      {
+        description: 'e2e:message("ready")',
+        workflow_id: seedData.workflowId,
+        workflow_step_id: seedData.startStepId,
+        repository_ids: [seedData.repositoryId],
+      },
+    );
+    if (!task.session_id) throw new Error("createTaskWithAgent did not return a session_id");
+    const workspacePath = await waitForWorkspacePath(apiClient, task.id, task.session_id);
+    const absoluteFilePath = path.join(workspacePath, filePath);
+    await apiClient.seedSessionMessage(task.session_id, {
+      type: "tool_read",
+      content: "Read file",
+      metadata: {
+        status: "complete",
+        tool_call_id: "tc-absolute-workspace-read",
+        normalized: {
+          read_file: {
+            file_path: absoluteFilePath,
+            offset: 1,
+            limit: 1,
+            output: { content: 'export const suspect = "Colonel Mustard";', line_count: 1 },
+          },
+        },
+      },
+    });
+
+    const treeRequestPaths = observeFileTreeRequestPaths(testPage);
+    await testPage.goto(`/t/${task.id}`);
+    const session = new SessionPage(testPage);
+    await session.waitForLoad();
+    await session.waitForChatIdle({ timeout: 45_000 });
+
+    const link = session.activeChat().locator(`button[title="${absoluteFilePath}"]`);
+    await expect(link).toBeVisible({ timeout: 15_000 });
+    await link.click();
+    await expect(testPage.getByTestId("preview-tab-file-editor")).toBeVisible({ timeout: 15_000 });
+    await expect(testPage.locator(".dv-tab.dv-active-tab", { hasText: fileName })).toBeVisible();
+
+    await session.clickTab("Files");
+    await expect(session.fileTreeNode(filePath)).toBeVisible({ timeout: 15_000 });
+    expect(treeRequestPaths.length).toBeGreaterThan(0);
+    expect(treeRequestPaths.every((requestPath) => !path.isAbsolute(requestPath))).toBe(true);
+  });
+});
diff --git a/apps/web/e2e/tests/task/mobile-file-viewer.spec.ts b/apps/web/e2e/tests/task/mobile-file-viewer.spec.ts
index 35dbafc788..637f1cd1de 100644
--- a/apps/web/e2e/tests/task/mobile-file-viewer.spec.ts
+++ b/apps/web/e2e/tests/task/mobile-file-viewer.spec.ts
@@ -12,6 +12,7 @@ import type { ApiClient } from "../../helpers/api-client";
 import type { BackendContext } from "../../fixtures/backend";
 import { GitHelper, makeGitEnv, createStandardProfile } from "../../helpers/git-helper";
 import { selectMarkdownPreviewText } from "../../helpers/markdown-preview";
+import { waitForWorkspacePath } from "../../helpers/session";
 import { SessionPage } from "../../pages/session-page";
 
 function createLongFileContent(lines = 500): string {
@@ -521,13 +522,11 @@ test.describe("Mobile file viewer panel", () => {
     await expect(viewer.getByText(filePath)).toBeVisible();
   });
 
-  // A chat read/edit file link can target a line deep in a file. Tapping it sets
-  // a pending cursor position (use-file-editors), then on mobile the file opens
-  // through MobileFileViewerPanel → FileViewerContent (CodeMirror), which must
-  // consume that pending entry and scroll the target line into view. Regression
-  // guard for the mobile half of "open-and-scroll-to-line": before the fix
-  // CodeMirror ignored the pending map and the file opened pinned at the top.
-  test("tapping a chat read link scrolls the CodeMirror viewer to the target line", async ({
+  // @covers AC-UI-FILE-TREE-PATH-SCOPE-001.1
+  // @covers AC-UI-FILE-TREE-PATH-SCOPE-001.6
+  // An absolute workspace path from a read tool must use the same relative
+  // identity for the native viewer and its pending cursor position.
+  test("tapping an absolute workspace read link scrolls to the target line", async ({
     testPage,
     apiClient,
     seedData,
@@ -562,11 +561,11 @@ test.describe("Mobile file viewer panel", () => {
       },
     );
     if (!task.session_id) throw new Error("createTaskWithAgent did not return a session_id");
+    const workspacePath = await waitForWorkspacePath(apiClient, task.id, task.session_id);
+    const absoluteFilePath = path.join(workspacePath, filePath);
 
-    // Seed a real read card pointing deep into the file. The chat read-message
-    // component sets the pending cursor position from `offset` before opening,
-    // keyed by this exact `file_path` — the same string the mobile viewer keys
-    // FileViewerContent on, so the consume matches.
+    // Seed the absolute path emitted by an agent tool. The opener must normalize
+    // it before keying the pending cursor and native viewer with the same path.
     const targetLine = 300;
     await apiClient.seedSessionMessage(task.session_id, {
       type: "tool_read",
@@ -576,7 +575,7 @@ test.describe("Mobile file viewer panel", () => {
         tool_call_id: "tc-chat-read-scroll",
         normalized: {
           read_file: {
-            file_path: filePath,
+            file_path: absoluteFilePath,
             offset: targetLine,
             limit: 20,
             output: { content: 'export const line_299 = "line 299";', line_count: 20 },
@@ -590,14 +589,16 @@ test.describe("Mobile file viewer panel", () => {
     await session.waitForLoad();
     await session.waitForChatIdle({ timeout: 45_000 });
 
-    // FilePathButton renders the openable link with the seeded path as `title`.
+    // FilePathButton preserves the recorded path in its title while displaying
+    // the relative suffix.
     const chat = session.activeChat();
-    const fileLink = chat.locator(`button[title="${filePath}"]`);
+    const fileLink = chat.locator(`button[title="${absoluteFilePath}"]`);
     await expect(fileLink).toBeVisible({ timeout: 15_000 });
     await fileLink.tap();
 
     const viewer = testPage.getByTestId("mobile-file-viewer-panel");
     await expect(viewer).toBeVisible({ timeout: 10_000 });
+    await expect(viewer.getByText(filePath)).toBeVisible();
 
     const cmScroller = viewer.locator(".cm-scroller").first();
     await expect(cmScroller).toBeVisible();
diff --git a/apps/web/hooks/use-file-editors.test.tsx b/apps/web/hooks/use-file-editors.test.tsx
index cadb0b93fe..c139d9bc1c 100644
--- a/apps/web/hooks/use-file-editors.test.tsx
+++ b/apps/web/hooks/use-file-editors.test.tsx
@@ -271,6 +271,32 @@ describe("useOpenFileAtLine", () => {
     expect(consumePendingCursorPosition(APP_PATH)).toBeUndefined();
   });
 
+  // @covers AC-UI-FILE-TREE-PATH-SCOPE-001.1
+  it("uses one workspace-relative identity for an absolute tool path", () => {
+    const openFile = vi.fn();
+    const editor = createEditor(WORKTREE_APP_PATH);
+    getMonacoInstance.mockReturnValue({ editor: { getEditors: () => [editor] } });
+    const { result } = renderHook(() => useOpenFileAtLine(openFile, 88, WORKTREE_PATH));
+
+    act(() => result.current(WORKTREE_APP_PATH));
+
+    expect(openFile).toHaveBeenCalledWith(APP_PATH);
+    expect(editor.setPosition).toHaveBeenCalledWith({ lineNumber: 88, column: 1 });
+    expect(consumePendingCursorPosition(APP_PATH)).toBeUndefined();
+    expect(consumePendingCursorPosition(WORKTREE_APP_PATH)).toBeUndefined();
+  });
+
+  it("normalizes Windows workspace aliases case-insensitively", () => {
+    const openFile = vi.fn();
+    const { result } = renderHook(() =>
+      useOpenFileAtLine(openFile, undefined, String.raw`C:\Workspace`),
+    );
+
+    act(() => result.current(String.raw`c:\workspace\src\app.ts`));
+
+    expect(openFile).toHaveBeenCalledWith(APP_PATH);
+  });
+
   it("scrolls only the requested session-qualified model", () => {
     const firstModelUri = modelUriForDocument(DOCUMENT_URI, FIRST_SESSION_ID);
     const secondModelUri = modelUriForDocument(DOCUMENT_URI, SECOND_SESSION_ID);
diff --git a/apps/web/hooks/use-file-editors.ts b/apps/web/hooks/use-file-editors.ts
index b272f61ee0..0e92039f30 100644
--- a/apps/web/hooks/use-file-editors.ts
+++ b/apps/web/hooks/use-file-editors.ts
@@ -13,6 +13,7 @@ import {
 } from "@/lib/local-storage";
 import { calculateHash } from "@/lib/utils/file-diff";
 import { getFilePreviewKind } from "@/lib/utils/file-types";
+import { normalizeWorkspaceFilePath } from "@/lib/workspace-file-path";
 import { useToast } from "@/components/toast-provider";
 import { useSessionGitStatus } from "@/hooks/domains/session/use-session-git-status";
 import { useSaveDeleteActions } from "./use-file-save-delete";
@@ -45,13 +46,14 @@ export function useOpenFileAtLine(
 ) {
   return useCallback(
     (path: string) => {
+      const openPath = normalizeWorkspaceFilePath(path, worktreePath);
       if (startLine && startLine > 0) {
-        setPendingCursorPosition(path, startLine, 1, undefined, sessionId);
-        onOpenFile?.(path);
-        scrollEditorIfMounted(path, worktreePath ?? null, startLine, 1, { sessionId });
+        setPendingCursorPosition(openPath, startLine, 1, undefined, sessionId);
+        onOpenFile?.(openPath);
+        scrollEditorIfMounted(openPath, worktreePath ?? null, startLine, 1, { sessionId });
         return;
       }
-      onOpenFile?.(path);
+      onOpenFile?.(openPath);
     },
     [onOpenFile, startLine, worktreePath, sessionId],
   );
diff --git a/apps/web/hooks/use-lsp-file-opener.ts b/apps/web/hooks/use-lsp-file-opener.ts
index d3c1e5281b..143fca6159 100644
--- a/apps/web/hooks/use-lsp-file-opener.ts
+++ b/apps/web/hooks/use-lsp-file-opener.ts
@@ -18,12 +18,10 @@ import {
   type WorkspaceFileLocation,
 } from "@/lib/lsp/file-uri";
 import { useDockviewStore } from "@/lib/state/dockview-store";
+import { normalizeWorkspaceFilePath } from "@/lib/workspace-file-path";
 
 export function toWorkspaceRelativePath(filePath: string, workspacePath: string | null): string {
-  const normalizedWorkspacePath = workspacePath?.replace(/\/+$/, "") ?? "";
-  if (!normalizedWorkspacePath) return filePath;
-  const workspacePrefix = `${normalizedWorkspacePath}/`;
-  return filePath.startsWith(workspacePrefix) ? filePath.slice(workspacePrefix.length) : filePath;
+  return normalizeWorkspaceFilePath(filePath, workspacePath);
 }
 
 export function preserveExistingEditorLocation(
diff --git a/apps/web/lib/workspace-file-path.test.ts b/apps/web/lib/workspace-file-path.test.ts
new file mode 100644
index 0000000000..c0b14becdc
--- /dev/null
+++ b/apps/web/lib/workspace-file-path.test.ts
@@ -0,0 +1,90 @@
+import { describe, expect, it } from "vitest";
+import {
+  isWorkspaceTreePath,
+  normalizeWorkspaceFilePath,
+  workspaceRelativeFilePath,
+} from "./workspace-file-path";
+
+const POSIX_WORKSPACE_ROOT = "/workspace";
+const POSIX_WORKSPACE_FILE = "/workspace/src/app.ts";
+const RELATIVE_WORKSPACE_FILE = "src/app.ts";
+
+describe("workspaceRelativeFilePath", () => {
+  it.each([
+    ["POSIX path", POSIX_WORKSPACE_FILE, POSIX_WORKSPACE_ROOT, RELATIVE_WORKSPACE_FILE],
+    ["trailing root separator", POSIX_WORKSPACE_FILE, "/workspace/", RELATIVE_WORKSPACE_FILE],
+    [
+      "Windows path",
+      String.raw`c:\workspace\src\app.ts`,
+      String.raw`C:\Workspace`,
+      RELATIVE_WORKSPACE_FILE,
+    ],
+    ["Windows drive root", String.raw`C:\src\app.ts`, "c:\\", RELATIVE_WORKSPACE_FILE],
+    ["file URI", "file:///workspace/src/app.ts", "file:///workspace", RELATIVE_WORKSPACE_FILE],
+    ["file URI root", "file:///src/app.ts", "file:///", RELATIVE_WORKSPACE_FILE],
+    ["Windows file URI", "file:///c:/workspace/SRC/App.ts", "file:///C:/Workspace", "SRC/App.ts"],
+    [
+      "UNC path",
+      String.raw`\\BUILD-SERVER\WORK\src\app.ts`,
+      String.raw`\\build-server\work`,
+      RELATIVE_WORKSPACE_FILE,
+    ],
+    [
+      "UNC file URI",
+      "file://BUILD-SERVER/WORK/src/app.ts",
+      "file://build-server/work",
+      RELATIVE_WORKSPACE_FILE,
+    ],
+  ])("resolves a contained %s", (_label, filePath, workspaceRoot, expected) => {
+    expect(workspaceRelativeFilePath(filePath, workspaceRoot)).toBe(expected);
+  });
+
+  it.each([
+    ["prefix collision", "/workspace-old/src/app.ts", POSIX_WORKSPACE_ROOT],
+    ["external absolute", "/opt/reference.md", POSIX_WORKSPACE_ROOT],
+    ["absolute traversal", "/workspace/../outside/app.ts", POSIX_WORKSPACE_ROOT],
+    ["case-distinct POSIX file URI", "file:///Workspace/src/app.ts", "file:///workspace"],
+    ["missing root", POSIX_WORKSPACE_FILE, null],
+  ])("rejects a %s", (_label, filePath, workspaceRoot) => {
+    expect(workspaceRelativeFilePath(filePath, workspaceRoot)).toBeNull();
+  });
+});
+
+describe("normalizeWorkspaceFilePath", () => {
+  it("preserves an external absolute path", () => {
+    expect(normalizeWorkspaceFilePath("/opt/reference.md", POSIX_WORKSPACE_ROOT)).toBe(
+      "/opt/reference.md",
+    );
+  });
+
+  it("canonicalizes separators in an already-relative path", () => {
+    expect(normalizeWorkspaceFilePath(String.raw`src\app.ts`, String.raw`C:\Workspace`)).toBe(
+      "src/app.ts",
+    );
+  });
+});
+
+describe("isWorkspaceTreePath", () => {
+  it.each(["", "src", "src/components", ".codex/agents", "config:dev", "dir/.env:"])(
+    "accepts the workspace-relative tree path %j",
+    (path) => {
+      expect(isWorkspaceTreePath(path)).toBe(true);
+    },
+  );
+
+  it.each([
+    "/home/jcfs/project/src",
+    String.raw`C:\workspace\src`,
+    "file:///workspace/src",
+    "file:/workspace/src",
+    ".",
+    "..",
+    "../src",
+    "src/../outside",
+    "src//components",
+    String.raw`src\components`,
+    "src\0components",
+  ])("rejects the non-canonical tree path %j", (path) => {
+    expect(isWorkspaceTreePath(path)).toBe(false);
+  });
+});
diff --git a/apps/web/lib/workspace-file-path.ts b/apps/web/lib/workspace-file-path.ts
new file mode 100644
index 0000000000..b9fb46c8de
--- /dev/null
+++ b/apps/web/lib/workspace-file-path.ts
@@ -0,0 +1,73 @@
+function normalizeSeparators(path: string): string {
+  return path.replaceAll("\\", "/");
+}
+
+function trimTrailingSeparators(path: string): string {
+  if (path === "/" || /^[A-Za-z]:\/$/.test(path) || /^file:\/\/\/(?:[A-Za-z]:\/)?$/i.test(path)) {
+    return path;
+  }
+  return path.replace(/\/+$/, "");
+}
+
+function usesWindowsPathSemantics(path: string): boolean {
+  return (
+    /^[A-Za-z]:\//.test(path) ||
+    path.startsWith("//") ||
+    /^file:\/\/\/[A-Za-z]:(?:\/|$)/i.test(path) ||
+    /^file:\/\/[^/]/i.test(path)
+  );
+}
+
+function hasAbsoluteURIForm(path: string): boolean {
+  return /^[A-Za-z][A-Za-z\d+.-]*:\/\//.test(path) || /^file:\//i.test(path);
+}
+
+function isAbsoluteFilePath(path: string): boolean {
+  return path.startsWith("/") || usesWindowsPathSemantics(path) || hasAbsoluteURIForm(path);
+}
+
+/** Return whether a path is a canonical workspace-relative tree identity. */
+export function isWorkspaceTreePath(path: string): boolean {
+  if (path === "") return true;
+  if (
+    path.startsWith("/") ||
+    path.includes("\\") ||
+    path.includes("\0") ||
+    /^[A-Za-z]:/.test(path) ||
+    hasAbsoluteURIForm(path)
+  ) {
+    return false;
+  }
+  return path.split("/").every((segment) => segment !== "" && segment !== "." && segment !== "..");
+}
+
+/** Return a workspace-relative alias for a contained path, or null when it is outside. */
+export function workspaceRelativeFilePath(
+  filePath: string,
+  workspaceRoot: string | null | undefined,
+): string | null {
+  if (!workspaceRoot) return null;
+  const path = trimTrailingSeparators(normalizeSeparators(filePath));
+  const root = trimTrailingSeparators(normalizeSeparators(workspaceRoot));
+  if (!root) return null;
+
+  const windowsSemantics = usesWindowsPathSemantics(path) || usesWindowsPathSemantics(root);
+  const comparablePath = windowsSemantics ? path.toLocaleLowerCase("en-US") : path;
+  const comparableRoot = windowsSemantics ? root.toLocaleLowerCase("en-US") : root;
+  if (comparablePath === comparableRoot) return "";
+  const rootPrefix = comparableRoot.endsWith("/") ? comparableRoot : `${comparableRoot}/`;
+  if (!comparablePath.startsWith(rootPrefix)) return null;
+  const relativePath = path.slice(rootPrefix.length);
+  return isWorkspaceTreePath(relativePath) ? relativePath : null;
+}
+
+/** Normalize a workspace-contained path while preserving external absolute paths. */
+export function normalizeWorkspaceFilePath(
+  filePath: string,
+  workspaceRoot: string | null | undefined,
+): string {
+  const relativePath = workspaceRelativeFilePath(filePath, workspaceRoot);
+  if (relativePath !== null) return relativePath;
+  const normalizedPath = normalizeSeparators(filePath);
+  return isAbsoluteFilePath(normalizedPath) ? filePath : normalizedPath;
+}
diff --git a/docs/plans/file-tree-path-scope/plan.md b/docs/plans/file-tree-path-scope/plan.md
new file mode 100644
index 0000000000..b38633d655
--- /dev/null
+++ b/docs/plans/file-tree-path-scope/plan.md
@@ -0,0 +1,184 @@
+---
+created: 2026-09-21
+status: implemented
+requirements:
+  - REQ-UI-FILE-TREE-PATH-SCOPE-001
+system_design:
+  - ../../specs/ui/system-design/file-tree-path-scope.md
+legacy_specs: []
+---
+
+# Implementation Plan: File Tree Path Scope
+
+## Overview
+
+Keep agent-referenced workspace files and the task Files tree on one relative path identity, while
+preserving the separate read-only absolute-file content contract. The implementation first
+normalizes tool-originated paths before editor and cursor state diverge. It then validates every
+source of file-tree state and rejects invalid direct requests at both backend boundaries.
+
+## Confirmed root cause
+
+- Agent read and edit metadata can contain `/home/jcfs/playground/mystery/...`, even though the
+  current task workspace is already `/home/jcfs/playground/mystery`.
+- `FilePathButton` displays the relative suffix but forwards the original absolute path.
+- `useOpenFileAtLine` keys pending cursor and editor state with that unnormalized value.
+- `FilesPanel` forwards the active editor path to `useFileTreeReveal`, which derives and persists
+  `/home`, `/home/jcfs`, and every deeper absolute ancestor.
+- A generic file refresh fans out one `workspace.tree.get` request for every expanded entry.
+- `WorkspaceTracker.GetFileTree` joins each request to `workDir`; the absolute-looking request is
+  therefore evaluated as `<workDir>/home/jcfs/...` and fails `stat`.
+- `WorkspaceFileHandlers.wsGetFileTree` logs every agentctl failure at ERROR, so one contaminated
+  expansion set produces the repeated stack traces in the incident.
+
+## Scope
+
+### In scope
+
+- Normalize agent tool file paths contained by the active task workspace before editor, tab, and
+  cursor state is written.
+- Preserve absolute external file content reads without treating them as tree nodes.
+- Validate reveal, restored expansion, and refresh-derived tree paths.
+- Clean older invalid expansion state while retaining valid entries.
+- Reject absolute and escaping tree paths before backend filesystem lookup or ERROR logging.
+- Add focused frontend, backend, desktop, and phone regression coverage.
+
+### Out of scope
+
+- Changing ADR 0016 or restricting external read-only file content.
+- Changing Files-panel or editor layout, controls, styling, copy, or navigation.
+- Mapping arbitrary external directories into a task workspace.
+- Changing Markdown source-checkout aliases or repository materialization.
+
+## Technical approach
+
+Add a pure frontend path module that owns containment and tree-path admission for POSIX and Windows
+forms. Use it in the tool file opener before cursor state is keyed, then at reveal, restore, and
+refresh boundaries. Keep external absolute opens unchanged and filter them only from tree state.
+
+Validate `workspace.tree.get` in the WebSocket handler before execution lookup so invalid client
+input returns a validation response without an ERROR log. Repeat the validation in agentctl before
+joining or statting the path. Keep current behavior for valid relative directories and genuine
+dependency failures.
+
+## ASCII UI preview
+
+### UI-01: Open a workspace file from an agent tool card
+
+Entry point: an agent read or edit card whose recorded path is absolute. State: the file is inside
+the active task workspace.
+
+Current desktop behavior:
+
+```text
+Chat                         Files                         Editor
++----------------------+    +------------------------+   +---------------------------+
+| Read public/...json  | -> | hidden state: /home/...|   | death-on-the-nile/case.json|
++----------------------+    +------------------------+   +---------------------------+
+                                  |
+                                  +-- refresh sends /home/... as tree paths
+```
+
+Proposed desktop behavior:
+
+```text
+Chat                         Files                         Editor
++----------------------+    +------------------------+   +---------------------------+
+| Read public/...json  | -> | public                 |   | public/.../case.json      |
++----------------------+    |   assets               |   +---------------------------+
+                            |     cases              |
+                            |       death-on-the-nile|
+                            +------------------------+
+```
+
+Proposed phone behavior:
+
+```text
++--------------------------------+
+| Chat                           |
+| Read public/.../case.json      |
++--------------------------------+
+              tap
+               v
++--------------------------------+
+| < Files   public/.../case.json |
+|                                |
+| existing native file viewer    |
++--------------------------------+
+```
+
+An external absolute file still opens in the existing editor or phone viewer, but the Files tree
+does not move or store its ancestors. The path identity and unchanged navigation are requirements;
+the spacing and example filenames are illustrative. No new copy or control is introduced.
+
+## Work orders
+
+- [done] [Task 01: Normalize Tool File Paths](task-01-normalize-tool-file-paths.md)
+- [done] [Task 02: Enforce Tree Request Scope](task-02-enforce-tree-request-scope.md)
+
+## Dependency order
+
+```text
+Task 01 -> Task 02
+```
+
+Task 01 establishes the shared path classifier and editor identity. Task 02 uses that classifier at
+the Files-tree boundaries and adds the backend defense in depth.
+
+## Verification strategy
+
+- Pure frontend tests prove containment and canonical identity on POSIX and Windows paths.
+- Hook tests prove editor open, pending cursor, and mounted-editor scroll use the same relative key.
+- File-tree tests prove absolute active and stored paths never reach child loads or refresh requests.
+- Backend process and handler tests prove validation happens before filesystem and lifecycle work and
+  does not produce an ERROR log.
+- A desktop Playwright scenario observes the absolute workspace path from a real seeded tool card,
+  the relative editor result, and all file-tree request payloads.
+- A phone Playwright scenario proves the same tool path and line target reach the existing native
+  viewer without a composition change.
+- Frontend typecheck/lint, backend test/lint, specification validators, and `git diff --check` cover
+  the combined package.
+
+## Risks
+
+- String-prefix containment can misclassify `/workspace-old` as inside `/workspace`. The shared
+  helper must enforce a complete path segment and test the collision.
+- Normalizing only the editor open call while keeping the cursor key absolute would break
+  scroll-to-line. Normalize before both operations.
+- Rejecting all absolute paths in the file editor would regress ADR 0016. Tree admission and file
+  content admission must remain separate.
+- Filtering reveal alone leaves stale expansion entries available to generic refresh. Restore and
+  refresh need their own guards.
+- Backend validation only in the WebSocket handler leaves direct agentctl HTTP callers exposed.
+  Validate again at `WorkspaceTracker.GetFileTree`.
+- Windows drive, UNC, and Windows file URI paths need case-insensitive root containment, while POSIX
+  paths remain case-sensitive.
+- URI detection must distinguish absolute URI forms from repository-relative names containing
+  literal colons, including `config:dev`.
+
+## Package handoff
+
+Implementation follows the two sequential TDD work orders. Begin each work order with its named
+failing regression, keep the requirement and design draft until delivery is verified, and record
+results in the work orders and this plan before creating the PR.
+
+## Results
+
+Implemented both work orders. Tool-originated workspace aliases now become canonical relative
+editor identities before cursor state is written. Reveal, restore, refresh, the WebSocket handler,
+and agentctl all reject invalid tree identities before issuing or executing a tree request.
+
+PR review aligned UNC and Windows file URI containment with existing LSP case semantics, preserved
+literal colons in canonical relative tree paths, shared the E2E workspace polling helper, and made
+the agentctl regression assert the exported validation sentinel.
+
+Validation completed with 90 focused frontend tests, frontend typecheck and changed-file lint,
+focused backend tree and handler tests, backend lint, and one desktop plus one phone Playwright
+scenario. The broad process package also exposed two unchanged process-runner fixture timing tests
+that fail on this host; the affected `TestGetFileTree` tests pass independently.
+
+Post-review verification passed the six-file focused frontend suite with 78 tests, typecheck,
+changed-file ESLint, the workspace-path and handler Go tests, backend lint, all specification
+validators, and the focused desktop and phone Playwright scenarios. The real colon-directory tree
+fixture runs on POSIX because Windows does not permit colons in path components; pure validation
+coverage remains cross-platform.
diff --git a/docs/plans/file-tree-path-scope/task-01-normalize-tool-file-paths.md b/docs/plans/file-tree-path-scope/task-01-normalize-tool-file-paths.md
new file mode 100644
index 0000000000..dd3ad4fbc4
--- /dev/null
+++ b/docs/plans/file-tree-path-scope/task-01-normalize-tool-file-paths.md
@@ -0,0 +1,145 @@
+---
+id: "01-normalize-tool-file-paths"
+title: "Normalize tool file paths"
+status: done
+wave: 1
+depends_on: []
+plan: "plan.md"
+requirements:
+  - REQ-UI-FILE-TREE-PATH-SCOPE-001
+acceptance_criteria:
+  - AC-UI-FILE-TREE-PATH-SCOPE-001.1
+  - AC-UI-FILE-TREE-PATH-SCOPE-001.2
+  - AC-UI-FILE-TREE-PATH-SCOPE-001.6
+system_design:
+  - ../../specs/ui/system-design/file-tree-path-scope.md
+---
+
+# Task 01: Normalize Tool File Paths
+
+## Summary
+
+Give tool-originated workspace files one relative identity before editor, pending-cursor, and scroll
+state is written, while leaving external absolute content reads unchanged.
+
+## In scope
+
+- Add the shared frontend workspace-path classifier and normalizer.
+- Handle POSIX and Windows roots, separator normalization, drive, UNC, and Windows file URI casing,
+  and complete-segment containment.
+- Normalize read and edit tool opens inside `useOpenFileAtLine` before every editor and cursor call.
+- Use the same containment decision for the edit-card open affordance instead of raw string prefix
+  matching.
+- Reuse the helper in the LSP opener where it replaces equivalent local path logic without changing
+  LSP behavior.
+- Add desktop and phone end-to-end coverage for an agent read card containing the active workspace's
+  absolute file path and a line target.
+
+## Out of scope
+
+- File-tree expansion, restore, refresh, or backend validation from Task 02.
+- Blocking an external absolute path from the read-only content endpoint.
+- New editor or phone-viewer UI.
+
+## Acceptance
+
+- A contained absolute tool path opens, keys its pending cursor, and scrolls as the corresponding
+  workspace-relative path.
+- A path outside the workspace or sharing only its string prefix remains absolute and does not gain
+  workspace identity.
+- Desktop and phone open the same relative workspace file at the requested line through their
+  existing editor/viewer surfaces.
+
+## TDD start
+
+First add a `useOpenFileAtLine` regression that passes `/workspace/src/app.ts` with workspace root
+`/workspace` and expects `src/app.ts` in `setPendingCursorPosition`, `onOpenFile`, and
+`scrollEditorIfMounted`. It fails today because all three receive the absolute path. Add the prefix
+collision and external absolute cases before changing production code.
+
+## ASCII UI preview
+
+Relevant excerpt from [UI-01](plan.md#ui-01-open-a-workspace-file-from-an-agent-tool-card):
+
+```text
+Desktop: Read /workspace/public/.../case.json -> Editor public/.../case.json
+Phone:   Read /workspace/public/.../case.json -> native viewer public/.../case.json
+External /opt/reference.md                    -> existing read-only viewer; Files unchanged
+```
+
+The rendered components and navigation remain unchanged. Only the path identity crossing the
+existing open action changes.
+
+## Verification
+
+Run from the repository root:
+
+```bash
+(cd apps && pnpm install --frozen-lockfile)
+(cd apps/web && pnpm exec vitest run lib/workspace-file-path.test.ts \
+  hooks/use-file-editors.test.tsx components/task/chat/messages/tool-edit-message.test.tsx \
+  hooks/use-lsp-file-opener.test.ts)
+(cd apps/web && pnpm run typecheck)
+(cd apps/web && pnpm exec eslint lib/workspace-file-path.ts hooks/use-file-editors.ts \
+  components/task/chat/messages/tool-edit-message.tsx hooks/use-lsp-file-opener.ts \
+  --max-warnings 0)
+(cd apps/web && pnpm e2e:run --project chromium \
+  tests/task/chat-read-absolute-workspace-path.spec.ts)
+(cd apps/web && pnpm e2e:run --project mobile-chrome \
+  tests/task/mobile-file-viewer.spec.ts --grep "absolute workspace read link")
+```
+
+## Files likely touched
+
+- `apps/web/lib/workspace-file-path.ts`
+- `apps/web/lib/workspace-file-path.test.ts`
+- `apps/web/hooks/use-file-editors.ts`
+- `apps/web/hooks/use-file-editors.test.tsx`
+- `apps/web/hooks/use-lsp-file-opener.ts`
+- `apps/web/hooks/use-lsp-file-opener.test.ts`
+- `apps/web/components/task/chat/messages/tool-edit-message.tsx`
+- `apps/web/components/task/chat/messages/tool-edit-message.test.tsx`
+- `apps/web/e2e/tests/task/chat-read-absolute-workspace-path.spec.ts`
+- `apps/web/e2e/tests/task/mobile-file-viewer.spec.ts`
+
+## Dependencies
+
+None.
+
+## Risks
+
+- The pending cursor map must use the normalized identity before the editor mounts.
+- Tool edit cards currently use a permissive prefix test; changing it must preserve their existing
+  copy-path fallback for external files.
+- `file:///` roots used by Monaco tests need canonical handling without treating an arbitrary URI as
+  a tree path.
+
+## Parallelism
+
+`sequential`
+
+## Inputs
+
+- `AC-UI-FILE-TREE-PATH-SCOPE-001.1`, `.2`, and `.6`.
+- Path identity and tool-file open flow in the system design.
+- ADR 0016's external absolute content-read exception.
+
+## Results
+
+Implemented the shared path classifier and normalized workspace-contained read and edit paths before
+editor, pending-cursor, and scroll state is written. The edit-card containment check and LSP alias
+conversion now use the same segment-aware helper. External absolute paths remain unchanged.
+
+PR review extended the Windows comparison rule to UNC paths, drive file URIs, and hosted file URIs,
+matching the existing LSP identity semantics. The duplicated desktop and phone workspace polling
+helper now lives in the shared E2E session helper.
+
+Validation passed:
+
+- Four focused unit files passed with 56 tests.
+- Frontend typecheck and changed-file ESLint passed.
+- The Chromium desktop regression passed with one test and observed only relative tree requests.
+- The Pixel 5 phone regression passed with one test and reached the requested line in the existing
+  native viewer.
+- Post-review verification passed the six-file focused frontend suite with 78 tests, frontend
+  typecheck and changed-file ESLint, and both focused Playwright scenarios.
diff --git a/docs/plans/file-tree-path-scope/task-02-enforce-tree-request-scope.md b/docs/plans/file-tree-path-scope/task-02-enforce-tree-request-scope.md
new file mode 100644
index 0000000000..427285e281
--- /dev/null
+++ b/docs/plans/file-tree-path-scope/task-02-enforce-tree-request-scope.md
@@ -0,0 +1,172 @@
+---
+id: "02-enforce-tree-request-scope"
+title: "Enforce tree request scope"
+status: done
+wave: 2
+depends_on:
+  - "01-normalize-tool-file-paths"
+plan: "plan.md"
+requirements:
+  - REQ-UI-FILE-TREE-PATH-SCOPE-001
+acceptance_criteria:
+  - AC-UI-FILE-TREE-PATH-SCOPE-001.2
+  - AC-UI-FILE-TREE-PATH-SCOPE-001.3
+  - AC-UI-FILE-TREE-PATH-SCOPE-001.4
+  - AC-UI-FILE-TREE-PATH-SCOPE-001.5
+system_design:
+  - ../../specs/ui/system-design/file-tree-path-scope.md
+---
+
+# Task 02: Enforce Tree Request Scope
+
+## Summary
+
+Admit only workspace-relative identities to reveal, stored expansion, and refresh requests, then
+enforce the same tree contract before backend execution lookup and filesystem access.
+
+## In scope
+
+- Return early from active-file reveal for absolute, absolute-URI, or traversing paths while
+  preserving literal colons in canonical relative names.
+- Sanitize restored expansion entries before deriving ancestors and retain valid siblings.
+- Filter refresh fan-out and specific change paths before calling `requestFileTree`.
+- Reject invalid tree paths in the WebSocket handler as validation failures without acquiring an
+  execution or writing an ERROR log.
+- Reject invalid tree paths in `WorkspaceTracker.GetFileTree` before join, stat, or filesystem
+  failure telemetry.
+- Preserve root, valid nested directory, symlink, missing relative path, and subtree merge behavior.
+
+## Out of scope
+
+- Editor path normalization delivered by Task 01.
+- Changing file-content path rules.
+- Suppressing logs for genuine backend dependency or relative filesystem failures.
+
+## Acceptance
+
+- The reported `/home/jcfs/...` active path never adds absolute ancestors, loads a tree node, or
+  reaches `requestFileTree`, including after a generic refresh.
+- Stored invalid entries are removed while valid relative expansion restores and persists normally.
+- Direct absolute or parent-escaping tree requests fail validation before lifecycle or filesystem
+  work and produce no ERROR log.
+
+## TDD start
+
+Add these failing regressions before production changes:
+
+1. `file-tree-reveal.test.ts` supplies the reported absolute active file and expects no expansion or
+   child load.
+2. `file-browser-restore-expanded.test.tsx` stores valid relative paths beside the absolute chain and
+   expects only the valid paths to be requested and retained.
+3. `file-browser-apply-changes.test.ts` sends a generic refresh with absolute expanded paths and
+   expects only `""` plus valid relative folders.
+4. `workspace_files_test.go` calls `GetFileTree` with an existing absolute directory and expects a
+   validation error rather than a joined lookup.
+5. `workspace_file_handlers_test.go` supplies an absolute tree path with no lifecycle and expects
+   `ErrorCodeValidation` and zero ERROR entries.
+
+## ASCII UI preview
+
+Relevant state transition from [UI-01](plan.md#ui-01-open-a-workspace-file-from-an-agent-tool-card):
+
+```text
+Workspace file active: public/.../case.json -> Files reveals public > ... > case.json
+External file active:  /opt/reference.md     -> Files keeps its current expansion
+Stored mixed state:    src, src/ui, /home/... -> restore src and src/ui only
+```
+
+No rendered layout or control changes. Desktop and phone consume the same sanitized Files-tree
+state.
+
+## Verification
+
+Run from the repository root:
+
+```bash
+(cd apps/web && pnpm exec vitest run components/task/file-tree-reveal.test.ts \
+  components/task/file-browser-restore-expanded.test.tsx \
+  components/task/file-browser-restore-loader.test.tsx \
+  components/task/file-browser-apply-changes.test.ts)
+(cd apps/web && pnpm run typecheck)
+(cd apps/web && pnpm exec eslint components/task/file-tree-reveal.ts \
+  components/task/file-browser-restore.ts components/task/file-browser-hooks.ts \
+  --max-warnings 0)
+(cd apps/backend && go test ./internal/agentctl/server/process \
+  -run 'TestGetFileTree' -count=1)
+(cd apps/backend && go test ./internal/agent/handlers \
+  -run 'TestWorkspaceFileHandlers.*Tree' -count=1)
+(cd apps/backend && go test ./internal/agentctl/server/process \
+  ./internal/agent/handlers -count=1)
+make -C apps/backend lint
+```
+
+After both work orders:
+
+```bash
+python3 scripts/list-docs.py validate
+python3 scripts/lint-spec-files.test.py
+python3 scripts/lint-spec-files.py --all
+git diff --check
+git status --short
+```
+
+## Files likely touched
+
+- `apps/web/components/task/file-tree-reveal.ts`
+- `apps/web/components/task/file-tree-reveal.test.ts`
+- `apps/web/components/task/file-browser-restore.ts`
+- `apps/web/components/task/file-browser-restore-expanded.test.tsx`
+- `apps/web/components/task/file-browser-restore-loader.test.tsx`
+- `apps/web/components/task/file-browser-hooks.ts`
+- `apps/web/components/task/file-browser-apply-changes.test.ts`
+- `apps/backend/internal/agent/handlers/workspace_file_handlers.go`
+- `apps/backend/internal/agent/handlers/workspace_file_handlers_test.go`
+- `apps/backend/internal/agentctl/server/process/workspace_files.go`
+- `apps/backend/internal/agentctl/server/process/workspace_files_test.go`
+
+## Dependencies
+
+- Task 01 supplies the shared frontend path classifier and prevents new tool opens from contaminating
+  expansion state.
+
+## Risks
+
+- The empty root path is valid and must not be filtered with invalid empty segments.
+- Hidden directories beginning with `.` are valid; only standalone `.` and `..` segments are
+  rejected.
+- A repository-scoped refresh must keep its current sibling-subtree preservation behavior.
+- The handler guard is for invalid client input. Dependency failures after a valid request must keep
+  their operational error signal.
+
+## Parallelism
+
+`sequential`
+
+## Inputs
+
+- `AC-UI-FILE-TREE-PATH-SCOPE-001.2` through `.5`.
+- Tree-state admission, refresh flow, and backend validation in the system design.
+- Existing persisted-expansion and cross-repository refresh regressions.
+
+## Results
+
+Implemented canonical tree-path admission for active-file reveal, stored expansion restoration,
+generic refresh fan-out, and specific file-change refreshes. A shared Go validator now rejects the
+same invalid identities before WebSocket execution lookup and before agentctl joins or stats a
+filesystem path. Validation failures produce no operational ERROR log.
+
+PR review narrowed URI detection to absolute URI forms so repository-relative names such as
+`config:dev` remain valid in both frontend and backend validation. The agentctl regression now
+asserts the exported validation sentinel with `errors.Is`.
+
+Validation passed:
+
+- The combined focused frontend suite passed 90 tests across eight files.
+- `go test ./internal/common/workspacepath -count=1` passed.
+- Focused `TestGetFileTree` and `TestWorkspaceFileHandlers.*Tree` runs passed.
+- The complete handler package passed, and `make -C apps/backend lint` reported zero issues.
+- Post-review coverage passed a real frontend refresh and POSIX agentctl tree read for `config:dev`,
+  plus cross-platform validator cases, the focused Go packages, and backend lint.
+- The full process package reached two unchanged runner-fixture timing failures on this host:
+  `TestProcessRunnerCapturesOutput` and `TestProcessRunnerStopLogsSignalAttempts`. The branch has no
+  changes to those tests or their production code; both fail identically when run alone.
diff --git a/docs/specs/ui/requirements/file-tree-path-scope.md b/docs/specs/ui/requirements/file-tree-path-scope.md
new file mode 100644
index 0000000000..a721b483e6
--- /dev/null
+++ b/docs/specs/ui/requirements/file-tree-path-scope.md
@@ -0,0 +1,96 @@
+---
+status: active
+system: ui
+created: 2026-09-21
+owners:
+  - kandev
+---
+
+# File Tree Path Scope Requirements
+
+## Overview
+
+Agent tool messages can identify a file by its absolute host path, while the task Files tree and
+`workspace.tree.get` address nodes relative to the task workspace root. Users must be able to open
+an agent-referenced workspace file without that absolute identity entering file-tree expansion,
+persistence, or refresh state and producing repeated failed tree requests.
+
+The UI system owns this contract because it owns file-open identity, Files-panel expansion state,
+and the requests derived from that state. The workspace file service remains the final validation
+boundary for tree reads. Its separate read-only absolute-file content contract remains unchanged.
+
+## Terminology
+
+- **Workspace file path:** A normalized path relative to the active task workspace root, using `/`
+  separators and containing no parent traversal. Literal colons that the task host can represent
+  remain valid path characters unless the value has an absolute URI or Windows drive form.
+- **Workspace absolute alias:** An absolute path contained beneath the active task workspace root
+  on a complete path-segment boundary.
+- **External absolute path:** An absolute path outside the active task workspace. It can be opened
+  read-only under ADR 0016, but it has no node identity in the task Files tree.
+- **Tree request path:** The `path` supplied to `workspace.tree.get`. The empty string addresses the
+  root; every other value is a workspace file path for a directory.
+
+## Requirements
+
+### REQ-UI-FILE-TREE-PATH-SCOPE-001: Keep file-tree paths workspace-relative
+
+**Intent:** Opening files referenced by an agent must not corrupt file-tree state or cause repeated
+backend errors.
+
+**User story:** As a user inspecting an agent-referenced file, I want the file and Files tree to use
+the active task workspace consistently, so that opening or refreshing the file does not issue
+invalid directory requests.
+
+#### Acceptance criteria
+
+- **AC-UI-FILE-TREE-PATH-SCOPE-001.1:** When an agent read or edit message identifies a file by a
+  workspace absolute alias, activating that file shall open the corresponding workspace file path
+  and preserve any requested line target.
+- **AC-UI-FILE-TREE-PATH-SCOPE-001.2:** When the active editor displays an external absolute path,
+  the Files tree shall leave its expansion state unchanged and shall not attempt to reveal, restore,
+  or refresh that path. The existing read-only external file content behavior shall remain
+  available.
+- **AC-UI-FILE-TREE-PATH-SCOPE-001.3:** Every tree request derived from active-file reveal,
+  persisted expansion, or a workspace refresh shall use either the empty root path or a normalized
+  workspace file path. Absolute filesystem paths, absolute URI forms, Windows drive prefixes, and
+  paths containing parent traversal shall not cross the frontend tree-request boundary. A literal
+  colon in an otherwise canonical relative path shall remain valid.
+- **AC-UI-FILE-TREE-PATH-SCOPE-001.4:** When stored expansion state contains an invalid tree path,
+  the Files tree shall discard that path and its derived ancestors before hydration or refresh,
+  while retaining valid expanded paths for the same task environment.
+- **AC-UI-FILE-TREE-PATH-SCOPE-001.5:** The backend tree endpoint shall reject an absolute or
+  escaping request path as a validation failure before filesystem lookup. A rejected client path
+  shall not be reported as an operational ERROR log.
+- **AC-UI-FILE-TREE-PATH-SCOPE-001.6:** Desktop and phone shall keep their existing editor and file
+  viewer compositions. A workspace absolute alias shall display and reopen with its workspace file
+  path on both, with no new control or navigation step.
+
+## Scenarios
+
+- **GIVEN** the task root `/workspace` and an agent read of
+  `/workspace/public/assets/case.json`, **WHEN** the user activates the read card, **THEN** the editor
+  opens `public/assets/case.json` and the Files tree may reveal only that relative path.
+- **GIVEN** a read-only editor for `/opt/reference/guide.md`, **WHEN** the Files panel is mounted or a
+  refresh event arrives, **THEN** the editor remains usable and the Files tree does not expand or
+  request `/opt` or any descendant.
+- **GIVEN** stored expansion entries `src`, `src/components`, and `/home/user/project`, **WHEN** the
+  Files tree restores, **THEN** it restores the two valid entries and removes the absolute entry
+  without requesting it.
+- **GIVEN** the relative directory `config:dev` in a POSIX task workspace, **WHEN** the user expands
+  or refreshes it, **THEN** the Files tree requests `config:dev` without treating the literal colon
+  as a URI scheme.
+- **GIVEN** a direct `workspace.tree.get` request with `/home/user/project`, **WHEN** the backend
+  validates it, **THEN** it returns a validation response without constructing
+  `<workspace-root>/home/user/project`, calling `stat`, or emitting an ERROR log.
+
+## Out of scope
+
+- Removing the read-only absolute-file content behavior accepted by ADR 0016.
+- Mapping an external host directory into the task Files tree.
+- Changing file-tree layout, row styling, editor chrome, phone navigation, or localized copy.
+- Changing workspace source aliases used by Markdown links.
+
+## Implementation plan
+
+- [File tree path scope](../../../plans/file-tree-path-scope/plan.md)
diff --git a/docs/specs/ui/system-design/file-tree-path-scope.md b/docs/specs/ui/system-design/file-tree-path-scope.md
new file mode 100644
index 0000000000..d92daf982c
--- /dev/null
+++ b/docs/specs/ui/system-design/file-tree-path-scope.md
@@ -0,0 +1,192 @@
+---
+status: current
+system: ui
+requirements:
+  - REQ-UI-FILE-TREE-PATH-SCOPE-001
+---
+
+# File Tree Path Scope System Design
+
+## Purpose and boundaries
+
+The UI owns the path identity passed from agent tool cards into file editors and the relative
+directory identities retained by the task Files tree. The agentctl workspace file service owns the
+filesystem boundary and provides defense in depth for direct tree requests.
+
+This design distinguishes two existing contracts. File content reads may use an external absolute
+path under [ADR 0016](../../../decisions/0016-observed-external-file-reads.md). File-tree reads are
+directory enumeration inside the task workspace and accept only workspace-relative paths. An
+absolute editor identity therefore never implies an absolute tree identity.
+
+## Requirement mapping
+
+| Acceptance criterion               | Design sections                                                                              |
+| ---------------------------------- | -------------------------------------------------------------------------------------------- |
+| `AC-UI-FILE-TREE-PATH-SCOPE-001.1` | [Path identities](#path-identities), [Tool-file open flow](#tool-file-open-flow)             |
+| `AC-UI-FILE-TREE-PATH-SCOPE-001.2` | [Tree-state admission](#tree-state-admission), [Failure and recovery](#failure-and-recovery) |
+| `AC-UI-FILE-TREE-PATH-SCOPE-001.3` | [Tree-state admission](#tree-state-admission), [Refresh flow](#refresh-flow)                 |
+| `AC-UI-FILE-TREE-PATH-SCOPE-001.4` | [Persistence](#persistence)                                                                  |
+| `AC-UI-FILE-TREE-PATH-SCOPE-001.5` | [Backend validation](#backend-validation), [Observability](#observability)                   |
+| `AC-UI-FILE-TREE-PATH-SCOPE-001.6` | [Responsive behavior](#responsive-behavior), [Verification](#verification)                   |
+
+## Components and responsibilities
+
+- A shared pure frontend path helper classifies POSIX and Windows absolute paths, performs
+  segment-boundary containment against the active task workspace root, converts contained aliases
+  to slash-separated relative paths, and validates paths admitted to tree state.
+- `useOpenFileAtLine` in `apps/web/hooks/use-file-editors.ts` normalizes tool-originated paths before
+  it keys pending cursor state, calls the editor opener, or scrolls an existing editor. Tool read and
+  edit cards therefore use one path identity for file content, tabs, cursor state, and the Files
+  panel.
+- `useFileTreeReveal` admits only valid workspace-relative active paths before calculating ancestors
+  or loading children.
+- `restoredExpandedPaths` validates stored entries before synthesizing their ancestor chains.
+- `applyFileChanges` filters expanded paths through the same tree-path predicate before a generic
+  refresh fans out `workspace.tree.get` requests.
+- `WorkspaceFileHandlers.wsGetFileTree` rejects invalid client paths before execution lookup and
+  returns a WebSocket validation error without logging an operational failure.
+- `WorkspaceTracker.GetFileTree` repeats the absolute and traversal check before joining the request
+  to `workDir`, so direct agentctl HTTP callers cannot reach a filesystem lookup with an invalid
+  tree path.
+
+## Path identities
+
+The frontend path helper returns one of these outcomes:
+
+| Input                       | Workspace root | Open identity           | Tree eligible |
+| --------------------------- | -------------- | ----------------------- | ------------- |
+| `src/app.ts`                | `/workspace`   | `src/app.ts`            | yes           |
+| `/workspace/src/app.ts`     | `/workspace`   | `src/app.ts`            | yes           |
+| `/workspace-old/src/app.ts` | `/workspace`   | unchanged absolute path | no            |
+| `/opt/reference.md`         | `/workspace`   | unchanged absolute path | no            |
+| `../sibling/app.ts`         | `/workspace`   | unchanged invalid path  | no            |
+
+Containment uses a complete path segment, normalizes `\` to `/`, and compares Windows drive, UNC,
+drive-file-URI, and hosted-file-URI paths case-insensitively. POSIX paths and hostless non-drive file
+URIs remain case-sensitive. A sibling whose prefix happens to share the workspace root string is not
+contained. The empty string is valid only as the tree root. A relative tree path rejects empty
+segments introduced by a leading slash, `.` and `..` segments, absolute URI forms, and Windows
+drive roots. Literal colons in otherwise canonical relative segments remain valid.
+The host filesystem can still reject names it cannot represent; the real `config:dev` tree fixture
+therefore runs on POSIX, while the pure frontend and Go validators cover the path contract on every
+platform.
+
+The helper does not rewrite a legitimate external absolute file into a task-relative lookalike.
+That path continues to the read-only content endpoint unchanged and remains ineligible for the tree.
+
+## Tool-file open flow
+
+1. The normalized ACP read or edit message supplies its recorded file path and optional source line.
+2. The tool renderer passes the path through `useOpenFileAtLine` with the active session workspace
+   root.
+3. If the path is contained by that root, the hook derives the workspace-relative identity before
+   writing pending cursor state and calling `onOpenFile`.
+4. The existing file editor requests content, creates or focuses the tab, and scrolls using the same
+   normalized identity.
+5. `FilesPanel` observes the active relative editor identity and may reveal its valid ancestor chain.
+
+An external absolute path follows the same file-content route without step 3's rewrite. The editor
+can display it, while the reveal boundary ignores it.
+
+## Tree-state admission
+
+Tree state uses one predicate at each source boundary rather than assuming every caller has already
+normalized its input:
+
+- Active-file reveal returns before changing expansion state when the path is invalid.
+- Restored session storage drops invalid entries before deriving parents.
+- Generic refresh ignores invalid entries already present in memory and always retains the valid
+  root refresh.
+- Specific file-change events continue to refresh the nearest valid expanded ancestor. Invalid
+  change paths are ignored rather than converted into a host path.
+
+The predicate applies to state and request identities, not ordinary filenames shown in labels.
+Hidden paths such as `.codex/agents` remain valid.
+
+## Refresh flow
+
+A generic file refresh starts with the root request `""`. For each valid expanded directory in the
+affected repository scope, `applyFileChanges` schedules one relative child request. Invalid entries
+do not enter the request set. Responses continue to merge with loaded descendants through the
+existing `mergeTreeNodes` behavior.
+
+This filter is required even after tool-path normalization because an existing browser tab may hold
+state written by an older build or by another editor entry point.
+
+## Backend validation
+
+The WebSocket handler validates the tree path before it acquires an agentctl client. Invalid paths
+return `ErrorCodeValidation`; they are client contract failures and do not emit the
+`failed to get file tree` ERROR log.
+
+The agentctl process validates again because its HTTP endpoint is independently callable. It rejects
+an absolute path or a cleaned parent escape before `filepath.Join`, `os.Stat`, or filesystem-failure
+telemetry. Valid relative paths retain the existing containment check, missing-path behavior, depth
+handling, symlink behavior, and tree construction.
+
+## Failure and recovery
+
+- An invalid active editor path has no tree side effect. The editor's own content result remains the
+  only user-visible success or failure.
+- Invalid stored entries are pruned locally; valid siblings restore normally, and the cleaned set is
+  written back through the existing session-storage effect after load.
+- A malformed direct WebSocket request receives a validation response. A direct agentctl HTTP
+  request receives the existing 400 response with a bounded validation message.
+- A valid relative tree path that disappears can still return the existing missing-path error. That
+  operational behavior is outside this repair.
+
+## Persistence
+
+No storage schema changes. Files-panel expansion remains scoped by task environment in
+`sessionStorage`. Restore sanitization is a forward cleanup for entries produced by older clients;
+the next normal persistence pass replaces the contaminated array with the accepted relative set.
+
+Open external absolute file tabs retain their existing persistence and read-only reopen behavior.
+They are not copied into expansion storage.
+
+## Security
+
+Frontend validation limits accidental and stale requests but is not an authorization boundary. The
+agentctl process remains authoritative and rejects absolute or escaping tree enumeration. Symlink
+containment continues through existing workspace tracker behavior.
+
+ADR 0016 remains limited to current-file content reads. This design does not expand absolute access
+to directory enumeration, mutation, search, Git-ref reads, or any other workspace operation.
+
+## Responsive behavior
+
+No component anatomy, styling, touch target, copy, focus path, or scroll owner changes. Desktop tool
+links continue to open or focus the Dockview editor. Phone tool links continue to switch to the
+native `MobileFileViewerPanel`. Both receive the same normalized workspace-relative identity and
+line target.
+
+The phone regression uses the existing chat-read-to-CodeMirror flow as its nearest exemplar. A
+separate mobile layout implementation is unnecessary because normalization happens before the
+desktop and phone openers diverge.
+
+## Verification
+
+- Pure helper tests cover POSIX paths, Windows drive and UNC paths, segment-boundary prefix
+  collisions, separator normalization, external absolutes, Windows and POSIX file URI casing,
+  literal colons, and traversal.
+- `useOpenFileAtLine` tests first reproduce the defect by expecting `/workspace/src/app.ts` to open
+  and scroll as `src/app.ts`; this assertion fails before the production change. They also prove an
+  external absolute path remains unchanged.
+- Reveal, restore, and refresh tests seed the reported absolute expansion chain and assert no load or
+  request receives `/home`, `/home/user`, or descendants while valid relative siblings remain.
+- Backend process tests assert invalid paths fail before lookup. Handler tests assert an absolute
+  path returns `ErrorCodeValidation`, does not consult lifecycle, and produces no ERROR log.
+- Desktop and phone Playwright scenarios seed an agent read card with the active workspace's
+  absolute file path, activate it, and assert the existing editor/viewer opens the relative path at
+  the requested line. The desktop scenario also observes `workspace.tree.get` traffic and fails if
+  any request carries an absolute path.
+
+## Observability
+
+No new metric or routine log is needed. Invalid direct tree requests are represented by their
+validation response. Genuine dependency or filesystem failures keep their current logs. The
+absence of absolute tree requests is enforced by frontend and backend regression tests.
+
+## Related decisions
+
+- [ADR 0016: Read-Only Absolute File Paths](../../../decisions/0016-observed-external-file-reads.md)