fix(ai-chat): raise undici's 300s stream timeout for long agent turns (#175)

Long research turns failed mid-task with "Lost connection to the AI provider".
Node's global fetch (undici) defaults BOTH headersTimeout and bodyTimeout to
300_000ms, and the chat provider + the external-MCP dispatcher both ran on it
with no override, so:
  - the z.ai chat stream dropped when a late step's huge accumulated context
    pushed the model's time-to-first-token past 5 min (the model reasons
    server-side with NO streamed reasoning, so the connection is silent until the
    first answer token — reproduced: even a trivial glm-5.2 query has a ~4-8s
    first-chunk gap; a long run reaches 400k+-token steps), or a reasoning model
    paused >5 min between chunks (bodyTimeout);
  - the crawl4ai SSE transport, held open across the whole turn, dropped when it
    idled >5 min between tool calls.

Fix: a dedicated undici dispatcher whose stream timeouts are raised to a
generous-but-FINITE silence timeout (default 15 min, AI_STREAM_TIMEOUT_MS) on
each path. NOT disabled (0): that would let a genuinely hung provider — with the
client still connected — hang forever, since the turn's abortSignal only fires on
client disconnect. The timeout bounds SILENCE (time-to-first-byte and the gap
BETWEEN chunks), NOT total turn duration, so an arbitrarily long turn that keeps
streaming is never cut; only a stream quiet for >15 min is treated as a hang.
  - ai-streaming-fetch.ts: createStreamingFetch() + streamTimeoutMs() /
    streamingDispatcherOptions() (the shared, configurable timeout).
  - ai.service: the chat provider fetch is createStreamingFetch(), wrapped by the
    existing passive ECONNRESET telemetry (createDiagnosticFetch gained an
    optional baseFetch) so the telemetry observes the SAME transport.
  - mcp-clients: the SSRF-pinned Agent uses streamingDispatcherOptions().

Investigation: reproduced the transport mechanism against the real z.ai endpoint
(a 1ms headersTimeout throws UND_ERR_HEADERS_TIMEOUT — the exact drop) and ran
the actual research agent to a ~428k-token context. Verified the fixed path
streams cleanly live (glm-5.2 turns finish; telemetry confirms the streaming
fetch is in use).

Tests: ai-streaming-fetch.spec (default 15m + env override + invalid fallback +
both-timeouts + streams a delayed response); ai-http-diagnostics + ai/mcp specs
green. server tsc clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/server/src/core/ai-chat/external-mcp/mcp-clients.service.ts

@@ -6,6 +6,7 @@ import { createMCPClient } from '@ai-sdk/mcp';
 import { Agent, type Dispatcher } from 'undici';
 import { AiMcpServerRepo } from '@docmost/db/repos/ai-chat/ai-mcp-server.repo';
 import { AiMcpServer } from '@docmost/db/types/entity.types';
+import { streamingDispatcherOptions } from '../../../integrations/ai/ai-streaming-fetch';
 import { SecretBoxService } from '../../../integrations/crypto/secret-box';
 import { isUrlAllowed, isIpAllowed } from './ssrf-guard';
 
@@ -400,6 +401,16 @@ export function validateResolvedAddresses(
  */
 function buildPinnedDispatcher(): Agent {
   return new Agent({
+    // Raise undici's default 300s headers/body timeouts on external MCP traffic
+    // to the same generous-but-finite silence timeout the chat fetch uses (#175).
+    // A long agent turn keeps an SSE transport (e.g. crawl4ai's /mcp/sse) open
+    // across the whole turn; that connection can idle BETWEEN tool calls longer
+    // than 5 min, and undici's bodyTimeout would otherwise sever it mid-task — a
+    // tool-call failure that aborts the streamed turn and shows the user "Lost
+    // connection to the AI provider". A slow single tool call (a crawl) can
+    // likewise exceed headersTimeout. The timeout stays FINITE so a genuinely
+    // hung server is still broken eventually.
+    ...streamingDispatcherOptions(),
     connect: {
       lookup: (hostname, _options, callback) => {
         // Always resolve ALL addresses ourselves; do not trust the caller's

apps/server/src/integrations/ai/ai-http-diagnostics.ts

@@ -25,7 +25,13 @@ import { Logger } from '@nestjs/common';
  * The seq/last-call timestamps are module-level, so under concurrent turns the
  * idle-gap figure is approximate (fine for single-user reproduction).
  */
-export function createDiagnosticFetch(context: string): typeof fetch {
+export function createDiagnosticFetch(
+  context: string,
+  // The underlying fetch to instrument. Defaults to the global fetch; the chat
+  // provider passes a streaming fetch (disabled undici stream timeouts, #175) so
+  // the telemetry observes the SAME transport the long agent turn actually uses.
+  baseFetch: typeof fetch = fetch,
+): typeof fetch {
   const logger = new Logger(context);
   let callSeq = 0;
   let lastCallStartedAt: number | undefined;
@@ -46,9 +52,9 @@ export function createDiagnosticFetch(context: string): typeof fetch {
           ? body.byteLength
           : undefined;
     try {
-      // Delegate to global fetch; return the Response UNTOUCHED (never read/clone
-      // the body) so the streamed SSE response is unaffected.
-      const res = await fetch(input, init);
+      // Delegate to the base fetch; return the Response UNTOUCHED (never read/
+      // clone the body) so the streamed SSE response is unaffected.
+      const res = await baseFetch(input, init);
       logger.log(
         `provider HTTP DIAGNOSTIC: call#${callId} OK ` +
           `headersAfter=${Date.now() - startedAt}ms status=${res.status} ` +

apps/server/src/integrations/ai/ai-streaming-fetch.spec.ts

@@ -0,0 +1,78 @@
+import * as http from 'node:http';
+import {
+  createStreamingFetch,
+  streamTimeoutMs,
+  streamingDispatcherOptions,
+} from './ai-streaming-fetch';
+
+/**
+ * #175: undici's default 300s headers/body timeouts severed long agent turns.
+ * The streaming fetch raises them to a generous-but-FINITE silence timeout (not
+ * 0 — a true hang must still break). We pin: the configured value + env override,
+ * that both dispatcher timeouts use it, and that a delayed response streams.
+ */
+describe('streamTimeoutMs', () => {
+  const ORIG = process.env.AI_STREAM_TIMEOUT_MS;
+  afterEach(() => {
+    if (ORIG === undefined) delete process.env.AI_STREAM_TIMEOUT_MS;
+    else process.env.AI_STREAM_TIMEOUT_MS = ORIG;
+  });
+
+  it('defaults to a generous-but-finite 15 minutes', () => {
+    delete process.env.AI_STREAM_TIMEOUT_MS;
+    expect(streamTimeoutMs()).toBe(900_000);
+    // Finite — NOT disabled (0 would let a hung provider leak forever).
+    expect(streamTimeoutMs()).toBeGreaterThan(0);
+    expect(Number.isFinite(streamTimeoutMs())).toBe(true);
+  });
+
+  it('honours a positive AI_STREAM_TIMEOUT_MS override', () => {
+    process.env.AI_STREAM_TIMEOUT_MS = '120000';
+    expect(streamTimeoutMs()).toBe(120000);
+  });
+
+  it('ignores an invalid / non-positive override (falls back to default)', () => {
+    for (const bad of ['0', '-5', 'abc', '']) {
+      process.env.AI_STREAM_TIMEOUT_MS = bad;
+      expect(streamTimeoutMs()).toBe(900_000);
+    }
+  });
+
+  it('applies the timeout to BOTH undici stream timeouts', () => {
+    delete process.env.AI_STREAM_TIMEOUT_MS;
+    expect(streamingDispatcherOptions()).toEqual({
+      headersTimeout: 900_000,
+      bodyTimeout: 900_000,
+    });
+  });
+});
+
+describe('createStreamingFetch — against a delayed server', () => {
+  let server: http.Server;
+  let url: string;
+  // The server waits before sending ANY byte (a long time-to-first-token).
+  const DELAY = 400;
+
+  beforeAll(async () => {
+    server = http.createServer((_req, res) => {
+      setTimeout(() => {
+        res.writeHead(200, { 'Content-Type': 'text/plain' });
+        res.end('ok');
+      }, DELAY);
+    });
+    await new Promise<void>((resolve) => server.listen(0, '127.0.0.1', resolve));
+    const addr = server.address() as import('node:net').AddressInfo;
+    url = `http://127.0.0.1:${addr.port}/`;
+  });
+
+  afterAll(async () => {
+    await new Promise<void>((resolve) => server.close(() => resolve()));
+  });
+
+  it('streams the delayed response instead of timing out', async () => {
+    const streamingFetch = createStreamingFetch();
+    const res = await streamingFetch(url);
+    expect(res.status).toBe(200);
+    expect(await res.text()).toBe('ok');
+  });
+});

apps/server/src/integrations/ai/ai-streaming-fetch.ts

@@ -0,0 +1,58 @@
+import { Agent } from 'undici';
+
+/**
+ * Default SILENCE timeout for streaming AI calls (15 min). Generous, but FINITE.
+ *
+ * Node's global fetch (undici) defaults headersTimeout and bodyTimeout to
+ * 300_000ms, which severed legitimate long agent turns mid-stream — surfacing as
+ * "Lost connection to the AI provider" (#175): a late step with a huge context
+ * pushes the model's time-to-first-token past 5 min, or a reasoning model pauses
+ * >5 min between chunks. We do NOT disable the timeout (0) — that would let a
+ * genuinely hung provider, with the client still connected, hang forever
+ * (abortSignal only fires on client disconnect). Instead we raise it well above
+ * any realistic gap while keeping it finite so a true hang is eventually broken.
+ *
+ * This bounds SILENCE (time-to-first-byte and the gap BETWEEN chunks), NOT total
+ * turn duration — so an arbitrarily long turn that keeps streaming bytes is never
+ * cut; only a stream that goes quiet for longer than this is treated as a hang.
+ */
+const DEFAULT_STREAM_TIMEOUT_MS = 900_000;
+
+/**
+ * The configured silence timeout (ms). Override with `AI_STREAM_TIMEOUT_MS`; a
+ * missing/invalid/non-positive value falls back to {@link DEFAULT_STREAM_TIMEOUT_MS}.
+ */
+export function streamTimeoutMs(): number {
+  const raw = Number(process.env.AI_STREAM_TIMEOUT_MS);
+  return Number.isFinite(raw) && raw > 0 ? raw : DEFAULT_STREAM_TIMEOUT_MS;
+}
+
+/**
+ * undici `Agent` timeout options for streaming AI traffic — both stream timeouts
+ * set to the (generous, finite) silence timeout. Shared by the chat provider
+ * fetch and the external-MCP dispatcher so they behave identically (#175).
+ */
+export function streamingDispatcherOptions(): {
+  headersTimeout: number;
+  bodyTimeout: number;
+} {
+  const t = streamTimeoutMs();
+  return { headersTimeout: t, bodyTimeout: t };
+}
+
+/**
+ * Build a `fetch` for long-lived streaming AI calls (the agent chat turn) backed
+ * by a dedicated undici dispatcher whose stream timeouts are the generous-but-
+ * finite silence timeout above (#175). A single shared dispatcher is returned
+ * (callers hold it for the service lifetime) so its connection pool is reused.
+ */
+export function createStreamingFetch(): typeof fetch {
+  const dispatcher = new Agent(streamingDispatcherOptions());
+  return ((input: Parameters<typeof fetch>[0], init?: RequestInit) =>
+    fetch(input, {
+      ...(init ?? {}),
+      // `dispatcher` is an undici-specific init field (not in the DOM RequestInit
+      // type); Node's global fetch reads it. Cast to satisfy the type.
+      dispatcher,
+    } as RequestInit & { dispatcher: Agent })) as typeof fetch;
+}

apps/server/src/integrations/ai/ai.service.ts

@@ -16,6 +16,7 @@ import { AiSttNotConfiguredException } from './ai-stt-not-configured.exception';
 import { describeProviderError } from './ai-error.util';
 // DIAGNOSTIC (provider ECONNRESET investigation) — temporary.
 import { createDiagnosticFetch } from './ai-http-diagnostics';
+import { createStreamingFetch } from './ai-streaming-fetch';
 import { AiProviderCredentialsRepo } from '@docmost/db/repos/ai-chat/ai-provider-credentials.repo';
 import { SecretBoxService } from '../crypto/secret-box';
 import { AiDriver } from './ai.types';
@@ -45,11 +46,14 @@ export interface ChatModelOverride {
 export class AiService {
   private readonly logger = new Logger(AiService.name);
 
-  // DIAGNOSTIC (provider ECONNRESET investigation) — temporary: passive
-  // instrumentation of the OpenAI-compatible provider HTTP calls (z.ai).
-  // Logs call timing/outcome only — no behavior change.
+  // Provider HTTP fetch for the chat path: a streaming fetch that DISABLES
+  // undici's 300s headers/body timeouts (#175 — long agent turns were severed
+  // mid-stream), wrapped with passive ECONNRESET-investigation telemetry so the
+  // logs observe the exact transport the turn uses. Held for the service
+  // lifetime to reuse the streaming dispatcher's connection pool.
   private readonly aiDiagnosticFetch = createDiagnosticFetch(
     'AiService:provider-http',
+    createStreamingFetch(),
   );
 
   constructor(