diff --git a/src/lib/chat-project-access.test.ts b/src/lib/chat-project-access.test.ts
index 82e8d1111..bd5106c6e 100644
--- a/src/lib/chat-project-access.test.ts
+++ b/src/lib/chat-project-access.test.ts
@@ -1,5 +1,8 @@
 // @ts-nocheck
 import assert from "node:assert/strict";
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
 
 import { chatProjectAccessId, taskWorktreeProjectAccessId } from "./chat-project-access.ts";
 
@@ -211,3 +214,64 @@ assert.equal(
 );
 
 console.log("chat-project-access tests passed");
+
+// SECURITY: the requested root is client-supplied, so a `.worktrees/<name>`
+// symlink pointing outside the project would otherwise borrow the parent
+// project's grant while the harness ran elsewhere. The realpathed cwd must
+// land under the same prefix.
+assert.equal(
+  chatProjectAccessId({
+    projects,
+    requestedProjectRoot: "/Users/me/dev/cave/.worktrees/evil",
+    resolvedCwd: "/Users/me/victim-ungranted-project",
+  }),
+  "unregistered:/Users/me/dev/cave/.worktrees/evil",
+  "a symlinked worktree request whose real cwd escapes the parent project fails closed",
+);
+
+// REGRESSION: `resolvedCwd` arrives realpath-resolved from
+// resolveLocalRuntimeCwd, so a lexically-resolved prefix built from a
+// symlink-registered project root could never match it, and every legitimate
+// worktree chat under that project fail-closed as unregistered.
+{
+  const realBase = fs.realpathSync(fs.mkdtempSync(path.join(os.tmpdir(), "cpa-")));
+  const realProject = path.join(realBase, "real", "cave");
+  fs.mkdirSync(path.join(realProject, ".worktrees", "feat-x"), { recursive: true });
+  const linkedProject = path.join(realBase, "linked-cave");
+  fs.symlinkSync(realProject, linkedProject);
+
+  const linkedProjects = [
+    {
+      id: "proj-linked",
+      name: "Cave",
+      root: linkedProject,
+      createdAt: "2026-01-01T00:00:00.000Z",
+      updatedAt: "2026-01-01T00:00:00.000Z",
+    },
+  ];
+
+  assert.equal(
+    chatProjectAccessId({
+      projects: linkedProjects,
+      requestedProjectRoot: path.join(linkedProject, ".worktrees", "feat-x"),
+      resolvedCwd: path.join(realProject, ".worktrees", "feat-x"),
+    }),
+    "proj-linked",
+    "a worktree under a symlink-registered project still authorizes against that project",
+  );
+
+  fs.mkdirSync(path.join(realBase, "elsewhere"), { recursive: true });
+  assert.equal(
+    chatProjectAccessId({
+      projects: linkedProjects,
+      requestedProjectRoot: path.join(linkedProject, ".worktrees", "feat-x"),
+      resolvedCwd: path.join(realBase, "elsewhere"),
+    }),
+    `unregistered:${path.join(linkedProject, ".worktrees", "feat-x")}`,
+    "canonicalizing the project root does not widen containment",
+  );
+
+  fs.rmSync(realBase, { recursive: true, force: true });
+}
+
+console.log("chat-project-access worktree cwd containment tests passed");
diff --git a/src/lib/chat-project-access.ts b/src/lib/chat-project-access.ts
index 09e7737b7..86207d064 100644
--- a/src/lib/chat-project-access.ts
+++ b/src/lib/chat-project-access.ts
@@ -2,6 +2,7 @@ import path from "node:path";
 
 import type { CaveProject } from "./cave-projects-types.ts";
 import { projectById, projectForRoot } from "./cave-projects.ts";
+import { realpathOrResolve } from "./server/canonical-path.ts";
 
 export type ChatProjectAccessArgs = {
   projects: CaveProject[];
@@ -16,17 +17,48 @@ export type ChatProjectAccessArgs = {
 };
 
 /**
- * The registered project whose `.worktrees/` directory contains `root`, if
- * any. Separator-exact and traversal-safe: the candidate is `path.resolve`d
- * (collapsing `..` escapes) and must sit strictly BELOW
- * `<project>/.worktrees/`, so `/proj-evil/...`, `/proj/.worktrees` itself,
- * and `/proj/.worktrees/../..` all miss.
+ * The registered project whose `.worktrees/` directory contains BOTH the
+ * requested root and the cwd the runtime actually resolved for this turn.
+ *
+ * Separator-exact and traversal-safe: every path is canonicalized with
+ * `realpathOrResolve` (collapsing `..` escapes AND symlinks) and must sit
+ * strictly BELOW `<project>/.worktrees/`, so `/proj-evil/...`,
+ * `/proj/.worktrees` itself, and `/proj/.worktrees/../..` all miss.
+ *
+ * Checking the resolved cwd is the security half: the requested root is
+ * client-supplied, so on its own it only proves the CLIENT spelled a path
+ * under a registered project. A symlink at `<project>/.worktrees/<name>`
+ * pointing anywhere else would otherwise hand the caller the parent project's
+ * grant while the harness ran outside it. `resolvedCwd` arrives realpathed
+ * from `resolveLocalRuntimeCwd`, so requiring it under the same prefix pins
+ * authorization to where the work will actually happen.
+ *
+ * Canonicalizing the project root is the correctness half, and it is required
+ * for the check above to be usable: `project.root` is stored as registered,
+ * so a project registered through a symlinked path — or any root with a
+ * symlinked ancestor, `/var -> /private/var` on macOS being the everyday case
+ * — builds a prefix the realpathed cwd can never start with, and every
+ * legitimate `.worktrees/<branch>` chat fails closed as unregistered. Putting
+ * both sides in one namespace never widens containment: a symlink that
+ * escapes the project resolves outside the prefix and still misses.
  */
-function worktreeParentProject(root: string, projects: CaveProject[]): CaveProject | null {
-  const resolved = path.resolve(root);
+function worktreeParentProject(
+  root: string,
+  resolvedCwd: string,
+  projects: CaveProject[],
+): CaveProject | null {
+  const requested = realpathOrResolve(root);
+  const realCwd = realpathOrResolve(resolvedCwd);
   for (const project of projects) {
-    const prefix = path.resolve(project.root) + path.sep + ".worktrees" + path.sep;
-    if (resolved.startsWith(prefix) && resolved.length > prefix.length) return project;
+    const prefix = realpathOrResolve(project.root) + path.sep + ".worktrees" + path.sep;
+    if (
+      requested.startsWith(prefix) &&
+      requested.length > prefix.length &&
+      realCwd.startsWith(prefix) &&
+      realCwd.length > prefix.length
+    ) {
+      return project;
+    }
   }
   return null;
 }
@@ -111,7 +143,7 @@ export function chatProjectAccessId(args: ChatProjectAccessArgs): string | null
     projectForRoot(args.resolvedCwd, args.projects);
   if (project) return project.id;
 
-  const worktreeParent = worktreeParentProject(projectRoot, args.projects);
+  const worktreeParent = worktreeParentProject(projectRoot, args.resolvedCwd, args.projects);
   if (worktreeParent) return worktreeParent.id;
 
   return `unregistered:${projectRoot}`;