fix(thinking): handle SPA fallback responses

2 files changed, 48 insertions, 0 deletions

diff --git a/src/app/store.svelte.ts b/src/app/store.svelte.ts
index 8997ce0..8191b47 100644
--- a/src/app/store.svelte.ts
+++ b/src/app/store.svelte.ts
@@ -779,6 +779,11 @@ export function createAppStore(opts?: CreateAppStoreOptions): AppStore {
     try {
       const res = await fetchImpl(`${httpBase}/conversations/${encodeURIComponent(id)}/thinking`);
       if (!res.ok) return;
+      // Guard against the SPA fallback: when DISPATCH_WEB_DIR is set (production),
+      // unknown API routes get index.html with HTTP 200 + text/html instead of a
+      // 404. Treat any non-JSON 200 as "endpoint not shipped" → leave null (ON).
+      const contentType = res.headers.get("content-type") ?? "";
+      if (!contentType.includes("application/json")) return;
       const data = (await res.json()) as ThinkingResponse;
       // Guard a slow response losing a race with a conversation switch.
       if (workspaceConversationId() === id) thinking = data.thinking ?? null;
@@ -1700,6 +1705,14 @@ export function createAppStore(opts?: CreateAppStoreOptions): AppStore {
             error: errBody?.error ?? `Set thinking failed (HTTP ${res.status})`,
           };
         }
+        // Guard against the SPA fallback (200 + text/html when the endpoint
+        // is not shipped). Optimistically apply locally — the backend can't
+        // persist it, but the UI reflects the user's intent for this session.
+        const contentType = res.headers.get("content-type") ?? "";
+        if (!contentType.includes("application/json")) {
+          if (workspaceConversationId() === id) thinking = enabled;
+          return { ok: true, thinking: enabled };
+        }
         const data = (await res.json()) as ThinkingResponse;
         const next = data.thinking ?? enabled;
         if (workspaceConversationId() === id) thinking = next;
diff --git a/src/app/store.test.ts b/src/app/store.test.ts
index 47c3977..518a7b2 100644
--- a/src/app/store.test.ts
+++ b/src/app/store.test.ts
@@ -898,6 +898,7 @@ describe("createAppStore", () => {
       if (url.endsWith("/thinking")) {
         return new Response(JSON.stringify({ conversationId: "x", thinking: false }), {
           status: 200,
+          headers: { "content-type": "application/json" },
         });
       }
       return base(input, init);
@@ -946,6 +947,38 @@ describe("createAppStore", () => {
     store.dispose();
   });
 
+  it("treats a SPA-fallback response (200 + text/html) as 'never set' (null ⇒ ON)", async () => {
+    // In production (DISPATCH_WEB_DIR set), unknown API routes get the SPA
+    // fallback: index.html with HTTP 200 + text/html. This must NOT crash
+    // res.json() — it should leave `thinking` null (ON, the default).
+    const base = fakeFetchImpl();
+    const fetchImpl: typeof fetch = async (input, init) => {
+      const url = typeof input === "string" ? input : input instanceof URL ? input.href : input.url;
+      if (url.endsWith("/thinking")) {
+        return new Response("<!DOCTYPE html><html><body>SPA fallback</body></html>", {
+          status: 200,
+          headers: { "content-type": "text/html; charset=utf-8" },
+        });
+      }
+      return base(input, init);
+    };
+    const ws = fakeSocket();
+    const store = createAppStore({
+      socketFactory: () => ws,
+      fetchImpl,
+      localStorage: createFakeStorage(),
+    });
+    ws.resolveOpen();
+
+    // give the (HTML) fetch a tick to settle
+    await vi.waitFor(() => {
+      expect(store.reasoningEffort).not.toBe(undefined);
+    });
+    expect(store.thinking).toBeNull();
+
+    store.dispose();
+  });
+
   it("setThinking PUTs the flag and updates local state from the echo", async () => {
     const calls: { url: string; method: string; body: string | undefined }[] = [];
     const base = fakeFetchImpl();
@@ -956,11 +989,13 @@ describe("createAppStore", () => {
         const sent = JSON.parse(init.body as string) as { thinking: boolean };
         return new Response(JSON.stringify({ conversationId: "x", thinking: sent.thinking }), {
           status: 200,
+          headers: { "content-type": "application/json" },
         });
       }
       if (url.endsWith("/thinking")) {
         return new Response(JSON.stringify({ conversationId: "x", thinking: null }), {
           status: 200,
+          headers: { "content-type": "application/json" },
         });
       }
       return base(input, init);