fix(agent): surface empty assistant answers instead of silent stops (#1830)

The support chat showed 'Tänker' and then went quiet with no answer and
no error. Root cause: the 2026-08-21 RIP-3 cutover moved general.help to
the single-call POST /api/agent/ask, whose 1500-token default cap made
stop_reason max_tokens routine on tool-loop turns. The empty answer then
passed unlogged through the service, the route answered 200 with an
empty string, and the console appended an invisible empty bubble.

Fixes, single-call path:
- ask-service: default maxTokens 1500 -> 5400 (the streaming chat's
  reply ceiling); an empty final answer now logs model + usage and
  throws the typed EmptyModelAnswerError instead of passing through.
- /api/agent/ask: maxDuration 300, logger, empty answer maps to 502
  with 'Assistenten gav inget svar. Försök igen.'; an empty assistant
  turn is never persisted (the question stays, so retry works).
- AskConsole: a 200 with an empty answer shows the error box instead of
  appending an invisible bubble.
- anthropic-family: serialized tool results are bounded at 40000 chars
  (mirrors run-turn) so one big read cannot eat the output budget; the
  step-exhausted fallback keeps tools declared with tool_choice none,
  because replaying tool_use/tool_result without tools is an API 400.

Fixes, streaming path (same silent class):
- /api/agent/invoke: maxDuration 300 so deep thinking turns are not
  killed mid-stream at the platform default cap.
- run-turn: stop_reason max_tokens with no visible text emits an error
  event, not a bare turn_complete.
- AgentChat: an NDJSON stream that ends without turn_complete or error
  (and was not aborted) shows 'Anslutningen bröts innan svaret blev
  klart. Försök igen.'

The lib/ai request-shape tests were updated deliberately for the
fallback change; general.help stays on the single-call runtime
(founder decision, not reverted).


Claude-Session: https://claude.ai/code/session_01SyDuePXxUFowaPBKpAv8SF

Co-authored-by: Jakob Wennberg <311770904+jakobwennberg-oss@users.noreply.github.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

components/agent/AgentChat.tsx

@@ -458,6 +458,11 @@ export default function AgentChat({
     const reader = response.body.getReader()
     const decoder = new TextDecoder()
     let buffer = ''
+    // Whether the stream delivered a terminal event (turn_complete or error).
+    // A serverless function killed at its duration cap closes the stream
+    // cleanly (done: true, nothing thrown), which used to look identical to
+    // success: "Tänker" collapsed with no answer and no error.
+    let sawTerminalEvent = false
     try {
       while (true) {
         const { done, value } = await reader.read()
@@ -480,6 +485,9 @@ export default function AgentChat({
           // First user-visible event lazily mounts the bubble. `conversation`
           // is a metadata event with no visible payload so it does not.
           const ev = parsed as { kind?: string } | null
+          if (ev && (ev.kind === 'turn_complete' || ev.kind === 'error')) {
+            sawTerminalEvent = true
+          }
           if (
             ev &&
             typeof ev.kind === 'string' &&
@@ -491,6 +499,12 @@ export default function AgentChat({
           handleEvent(parsed)
         }
       }
+      // The stream ended without throwing. If no terminal event arrived and
+      // the user did not abort, the function was cut off mid-turn (duration
+      // cap, proxy drop): say so instead of presenting silence as success.
+      if (!sawTerminalEvent && !signal.aborted) {
+        setErrorMessage('Anslutningen bröts innan svaret blev klart. Försök igen.')
+      }
     } catch (err) {
       if (!signal.aborted) {
         setErrorMessage(err instanceof Error ? getUserErrorMessage(err) : 'Streamen avbröts.')

components/agent/AskConsole.tsx

@@ -185,6 +185,14 @@ export default function AskConsole({
           conversationIdRef.current = convId
           onConversationCreated?.(convId)
         }
+        if (answer.trim().length === 0) {
+          // Belt and braces with the server's 502 guard: a 200 whose answer is
+          // empty must not append an invisible bubble ("Tänker" collapses and
+          // nothing appears). Same message the server sends on 502; the thread
+          // id (if one was created) is kept above so a retry lands in it.
+          setError('Assistenten gav inget svar. Försök igen.')
+          return
+        }
         setMessages((prev) => [...prev, { role: 'assistant', text: answer }])
       } catch {
         setError('Kunde inte nå assistenten. Kontrollera anslutningen och försök igen.')