diff --git a/apps/web/src/components/chat/HeaderActions.test.tsx b/apps/web/src/components/chat/HeaderActions.test.tsx
index ee0de270f..07a260422 100644
--- a/apps/web/src/components/chat/HeaderActions.test.tsx
+++ b/apps/web/src/components/chat/HeaderActions.test.tsx
@@ -120,7 +120,9 @@ vi.mock("@/stores/diffStore", async (importOriginal) => {
 });
 
 vi.mock("./OpenInAppButton", () => ({
-  OpenInAppButton: () => <div data-testid="open-in-app" />,
+  OpenInAppButton: ({ dirPath }: { dirPath: string | null }) => (
+    <div data-testid="open-in-app" data-dir-path={dirPath ?? ""} />
+  ),
 }));
 
 vi.mock("./CreatePrDialog", () => ({
@@ -545,6 +547,27 @@ describe("HeaderActions - consolidated header", () => {
     expect(icon.querySelector('path[d="M12 12H3.75M12 12L19.5 19.5M12 12L19.5 4.5"]')).toBeInTheDocument();
   });
 
+  it("does not fall back to the workspace checkout for a worktree thread without a worktree path", () => {
+    renderHeaderActions(makeThread({ mode: "worktree", worktree_path: null }));
+
+    expect(screen.getByTestId("open-in-app")).toHaveAttribute("data-dir-path", "");
+    expect(screen.getByTestId("thread-overview-local-path")).toHaveTextContent("Unavailable");
+    expect(screen.getByTestId("thread-overview-local-path")).not.toHaveTextContent("/test");
+  });
+
+  it("opens the workspace checkout for a direct thread", () => {
+    renderHeaderActions(makeThread({ mode: "direct", worktree_path: null }));
+
+    expect(screen.getByTestId("open-in-app")).toHaveAttribute("data-dir-path", "/test");
+    expect(screen.getByTestId("thread-overview-local-path")).toHaveTextContent("/test");
+  });
+
+  it("opens the thread worktree for a worktree thread", () => {
+    renderHeaderActions(makeThread({ mode: "worktree", worktree_path: "/repo/worktrees/feat-x" }));
+
+    expect(screen.getByTestId("open-in-app")).toHaveAttribute("data-dir-path", "/repo/worktrees/feat-x");
+  });
+
   it("keeps older cached recap visible and exposes coverage times through the affordance", async () => {
     const messages = [
       createMockMessage({
diff --git a/apps/web/src/components/chat/HeaderActions.tsx b/apps/web/src/components/chat/HeaderActions.tsx
index 6cb12ca39..dfd37476a 100644
--- a/apps/web/src/components/chat/HeaderActions.tsx
+++ b/apps/web/src/components/chat/HeaderActions.tsx
@@ -4,6 +4,7 @@ import { OpenInAppButton } from "./OpenInAppButton";
 import { ThreadOverview } from "./ThreadOverview";
 import { useDiffStore } from "@/stores/diffStore";
 import { useWorkspaceStore } from "@/features/projects/state/workspaceStore";
+import { resolveThreadDirPath } from "@/lib/worktree";
 import { toggleRightPanelAdaptive } from "@/lib/right-panel-layout";
 import { Button } from "@/components/ui/button";
 import { Tooltip, TooltipContent, TooltipTrigger } from "@/components/ui/tooltip";
@@ -48,7 +49,7 @@ export function HeaderActions({ thread, threadPaneWidth }: HeaderActionsProps) {
     <div className="flex items-center justify-end gap-1">
       <div className="flex items-center gap-0.5 bg-muted/20 rounded-md px-1 py-0.5">
         <OpenInAppButton
-          dirPath={thread.worktree_path ?? workspacePath}
+          dirPath={resolveThreadDirPath(thread, workspacePath)}
           threadId={thread.id}
           threadOverride={thread.default_open_in_app ?? null}
         />
diff --git a/apps/web/src/hooks/useThreadGitActions.ts b/apps/web/src/hooks/useThreadGitActions.ts
index 9de9fdfd7..5b2baadc2 100644
--- a/apps/web/src/hooks/useThreadGitActions.ts
+++ b/apps/web/src/hooks/useThreadGitActions.ts
@@ -4,6 +4,7 @@ import { useHasCommitsAhead } from "@/hooks/useHasCommitsAhead";
 import { useWorkspaceStore } from "@/features/projects/state/workspaceStore";
 import { useComposerDraftStore } from "@/stores/composerDraftStore";
 import { isPrable } from "@/lib/is-prable";
+import { resolveThreadDirPath } from "@/lib/worktree";
 import { openGitHubUrl } from "@/features/preview/navigation/open-url-in-preview";
 import type { Thread } from "@/transport";
 
@@ -88,7 +89,7 @@ function useThreadPrState(thread: Thread) {
 export function useThreadGitActions(thread: Thread) {
   const [createPrOpen, setCreatePrOpen] = useState(false);
   const { workspace, prable, pr, checks, openPrDetail } = useThreadPrState(thread);
-  const dirPath = thread.worktree_path ?? workspace?.path ?? null;
+  const dirPath = resolveThreadDirPath(thread, workspace?.path ?? null);
 
   // Whether the branch has commits ahead of base (disable Create PR when it doesn't).
   const hasCommitsAhead = useHasCommitsAhead(
diff --git a/apps/web/src/lib/__tests__/worktree.test.ts b/apps/web/src/lib/__tests__/worktree.test.ts
new file mode 100644
index 000000000..e18ac2b89
--- /dev/null
+++ b/apps/web/src/lib/__tests__/worktree.test.ts
@@ -0,0 +1,40 @@
+import { describe, it, expect } from "vitest";
+import { resolveThreadDirPath } from "../worktree";
+
+describe("resolveThreadDirPath", () => {
+  it("returns the worktree path for a worktree thread", () => {
+    expect(
+      resolveThreadDirPath(
+        { mode: "worktree", worktree_path: "/repo/worktrees/feat-x" },
+        "/repo",
+      ),
+    ).toBe("/repo/worktrees/feat-x");
+  });
+
+  it("returns null for a worktree thread whose worktree path is not yet provisioned", () => {
+    expect(
+      resolveThreadDirPath({ mode: "worktree", worktree_path: null }, "/repo"),
+    ).toBeNull();
+  });
+
+  it("falls back to the workspace checkout for a direct thread", () => {
+    expect(
+      resolveThreadDirPath({ mode: "direct", worktree_path: null }, "/repo"),
+    ).toBe("/repo");
+  });
+
+  it("prefers a direct thread's worktree path when present", () => {
+    expect(
+      resolveThreadDirPath(
+        { mode: "direct", worktree_path: "/repo/worktrees/feat-x" },
+        "/repo",
+      ),
+    ).toBe("/repo/worktrees/feat-x");
+  });
+
+  it("returns null when no workspace is available", () => {
+    expect(
+      resolveThreadDirPath({ mode: "direct", worktree_path: null }, null),
+    ).toBeNull();
+  });
+});
diff --git a/apps/web/src/lib/worktree.ts b/apps/web/src/lib/worktree.ts
index 85bc6b893..62a2e3dc2 100644
--- a/apps/web/src/lib/worktree.ts
+++ b/apps/web/src/lib/worktree.ts
@@ -1,7 +1,21 @@
+import type { Thread } from "@/transport";
 import type { WorktreeInfo } from "@/transport/types";
 
 const DETACHED_WORKTREE_BRANCH = "(detached)";
 
+/**
+ * Directory the thread operates in, for open-in and "Worktree path" surfaces.
+ * A worktree thread whose path has not been persisted yet must not fall back
+ * to the workspace checkout: the base directory is a different working tree
+ * than the one the user is in.
+ */
+export function resolveThreadDirPath(
+  thread: Pick<Thread, "mode" | "worktree_path">,
+  workspacePath: string | null,
+): string | null {
+  return thread.worktree_path ?? (thread.mode === "worktree" ? null : workspacePath);
+}
+
 /** Normalizes a worktree path for UI-side matching. */
 export function normalizeWorktreePath(path: string): string {
   return path.replace(/\\/g, "/").replace(/\/$/, "").toLowerCase();