studio: expose chat_only in unauth /api/health for SPA first-load routing

PR #5406's launcher contract withheld chat_only because the diagnostic dict gated it behind a valid bearer. The SPA's first-load router needs chat_only *before* any bearer is available (to decide whether /studio and /export redirect to /chat); the Windows + Linux UI smokes' chat_only inference fallback was incorrectly classifying non-chat-only Studios as chat-only when the bearered probe came back without the field (post-bootstrap tokens still carry must_change_password=True).

chat_only is a hardware-shape capability bit (training vs inference-only) -- the same category as supports_desktop_auth and desktop_manageability_version which already ship unauth -- so promoting it is consistent with the existing contract. The truly sensitive diagnostic stays gated: version, studio_version, device_type, native_path_leases_supported, desktop_owner. Tests + the playwright_extra_ui chat_only probe updated to match the new contract.

studio/backend/main.py

@@ -528,9 +528,20 @@ async def health_check(request: Request):
         "timestamp": datetime.now().isoformat(),
         # Launcher / preflight contract: stable identity + capability bits.
         # Safe to expose unauthenticated -- studio_root_id is a hex digest
-        # of the install path, the desktop flags are non-sensitive booleans.
+        # of the install path, the desktop flags and chat_only are
+        # non-sensitive feature-shape booleans (same category as
+        # ``supports_desktop_auth``).
+        #
+        # chat_only is part of the contract because the SPA's first-load
+        # router needs it to decide whether to redirect /studio + /export
+        # to /chat *before* any bearer is available; the Playwright UI
+        # tests rely on the same signal so they don't have to maintain a
+        # heuristic ("did the URL change after goto?"). Withholding it
+        # broke the Windows + Linux UI smokes and the change-password
+        # bootstrap flow.
         "service": "Unsloth UI Backend",
         "studio_root_id": _studio_root_id(),
+        "chat_only": _hw_module.CHAT_ONLY,
         "desktop_protocol_version": 1,
         "desktop_manageability_version": 1,
         "supports_desktop_auth": True,
@@ -561,13 +572,14 @@ async def health_check(request: Request):
         **minimal,
         # Sensitive diagnostic fields. Gated on a valid bearer because:
         # - version / studio_version reveal patch-level CVE exposure;
-        # - device_type / chat_only reveal training-vs-inference shape;
+        # - device_type reveals the training-vs-inference shape;
         # - desktop_owner reveals which UID/process owns the desktop lease;
         # - native_path_leases_supported reveals filesystem capability.
+        # chat_only is intentionally NOT gated; see the comment on the
+        # ``minimal`` dict above.
         "version": UNSLOTH_VERSION,
         "studio_version": STUDIO_VERSION,
         "device_type": device_type,
-        "chat_only": _hw_module.CHAT_ONLY,
         "native_path_leases_supported": native_path_leases_supported(),
         **({"desktop_owner": owner} if (owner := _desktop_owner()) else {}),
     }

studio/backend/tests/test_health_unauth_contract.py

@@ -41,6 +41,7 @@ LAUNCHER_KEYS = {
     "timestamp",
     "service",
     "studio_root_id",
+    "chat_only",
     "desktop_protocol_version",
     "desktop_manageability_version",
     "supports_desktop_auth",
@@ -51,7 +52,6 @@ GATED_KEYS = {
     "version",
     "studio_version",
     "device_type",
-    "chat_only",
     "native_path_leases_supported",
 }
 

studio/backend/tests/test_middleware.py

@@ -245,12 +245,13 @@ class TestHealthAuthGate:
         "timestamp",
         "service",
         "studio_root_id",
+        "chat_only",
         "desktop_protocol_version",
         "desktop_manageability_version",
         "supports_desktop_auth",
         "supports_desktop_backend_ownership",
     }
-    GATED_KEYS = ("version", "device_type", "chat_only", "native_path_leases_supported")
+    GATED_KEYS = ("version", "device_type", "native_path_leases_supported")
 
     def test_no_auth_returns_launcher_payload(self, health_app):
         c = TestClient(health_app)

tests/studio/playwright_extra_ui.py

@@ -298,16 +298,14 @@ with sync_playwright() as p:
     composer = page.locator('textarea[aria-label="Message input"]')
     composer.wait_for(state = "visible", timeout = 60_000)
 
-    # Detect chat-only mode: /api/health.chat_only is the source of truth,
-    # but the field is gated behind a valid bearer (along with version /
-    # device_type / desktop_owner) -- unauthenticated callers only see the
-    # launcher contract (status, service, studio_root_id, desktop_*).
-    # Pass the access token we already minted above so the test always
-    # observes chat_only directly rather than guessing from URL behavior.
+    # Detect chat-only mode: /api/health.chat_only is the source of truth.
+    # The field is part of the unauthenticated launcher contract so the
+    # SPA's first-load router can decide whether to redirect /studio +
+    # /export to /chat *before* any bearer exists. Bearered or not, the
+    # field is always present on a healthy backend.
     health_resp = evaluate_fetch(
         page,
         f"{BASE}/api/health",
-        headers = {"Authorization": f"Bearer {token}"},
         timeout_ms = FETCH_TIMEOUT_MS,
     )
     if health_resp.get("error"):
@@ -315,13 +313,14 @@ with sync_playwright() as p:
         sys.exit(1)
     health = health_resp.get("body") or {}
     if "chat_only" not in health:
-        # Older Studio builds (pre-auth gating) returned chat_only in the
-        # minimal payload. Tolerate that by probing /studio: a 3xx to /chat
-        # is the equivalent runtime signal.
-        page.goto(f"{BASE}/studio")
-        page.wait_for_timeout(500)
-        chat_only = "/studio" not in page.url
-        info(f"chat_only inferred from /studio URL: {chat_only}")
+        # Defensive: an older Studio build without the launcher-contract
+        # patch may omit chat_only when called unauthenticated. Default
+        # to non-chat-only and log so the probe is not silently wrong.
+        chat_only = False
+        info(
+            "WARN /api/health did not return chat_only; defaulting to "
+            "chat_only=False (older Studio build)"
+        )
     else:
         chat_only = bool(health.get("chat_only"))
         info(f"chat_only mode: {chat_only}")