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>

lib/agent/ask/__tests__/ask-service.test.ts

@@ -16,6 +16,7 @@ vi.mock('../snapshot', () => ({
 }))
 
 import { answerAssistantQuestion } from '../ask-service'
+import { EmptyModelAnswerError } from '../errors'
 
 function supabaseWith(company: { name?: string; entity_type?: string } | null): SupabaseClient {
   const chain = {
@@ -116,6 +117,22 @@ describe('answerAssistantQuestion', () => {
     expect(call.prompt).toContain('Status: momsregistrerad (momsperiod: quarterly).')
   })
 
+  it('defaults maxTokens to 5400 (the streaming chat reply ceiling; 1500 caused empty max_tokens answers)', async () => {
+    await answerAssistantQuestion({ supabase: supabaseWith(null), companyId: 'c1', question: 'Hej?' })
+    expect(generateText.mock.calls[0][0].maxTokens).toBe(5400)
+  })
+
+  it('rejects with the typed empty-answer error when the model returns no visible text', async () => {
+    generateText.mockResolvedValue({ text: '  \n', model: 'claude-sonnet-5', usage: {} })
+    const promise = answerAssistantQuestion({
+      supabase: supabaseWith(null),
+      companyId: 'c1',
+      question: 'Hej?',
+    })
+    await expect(promise).rejects.toBeInstanceOf(EmptyModelAnswerError)
+    await expect(promise).rejects.toMatchObject({ code: 'empty_model_answer' })
+  })
+
   it('honours a custom maxSteps', async () => {
     buildLedgerTools.mockReturnValue([
       { name: 'gnubok_get_vat_report', description: 'd', jsonSchema: {}, execute: vi.fn() },