diff --git a/apps/server/src/core/ai-chat/external-mcp/mcp-clients.service.ts b/apps/server/src/core/ai-chat/external-mcp/mcp-clients.service.ts
index 30e94dc0..cdc949e2 100644
--- a/apps/server/src/core/ai-chat/external-mcp/mcp-clients.service.ts
+++ b/apps/server/src/core/ai-chat/external-mcp/mcp-clients.service.ts
@@ -400,6 +400,17 @@ export function validateResolvedAddresses(
  */
 function buildPinnedDispatcher(): Agent {
   return new Agent({
+    // Disable undici's default 300s headers/body timeouts on external MCP
+    // traffic. 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" (#175). A slow single
+    // tool call (a crawl) can likewise exceed headersTimeout. Connection
+    // lifetime is bounded by the turn (clients are closed in onFinish/onError/
+    // onAbort), so disabling the wall-clock cap is safe.
+    headersTimeout: 0,
+    bodyTimeout: 0,
     connect: {
       lookup: (hostname, _options, callback) => {
         // Always resolve ALL addresses ourselves; do not trust the caller's
diff --git a/apps/server/src/integrations/ai/ai-http-diagnostics.ts b/apps/server/src/integrations/ai/ai-http-diagnostics.ts
index eb9beeb2..0761d050 100644
--- a/apps/server/src/integrations/ai/ai-http-diagnostics.ts
+++ b/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=${res.status} ` +
diff --git a/apps/server/src/integrations/ai/ai-streaming-fetch.spec.ts b/apps/server/src/integrations/ai/ai-streaming-fetch.spec.ts
new file mode 100644
index 00000000..2db93cbc
--- /dev/null
+++ b/apps/server/src/integrations/ai/ai-streaming-fetch.spec.ts
@@ -0,0 +1,50 @@
+import * as http from 'node:http';
+import {
+  createStreamingFetch,
+  STREAMING_DISPATCHER_OPTIONS,
+} from './ai-streaming-fetch';
+
+/**
+ * #175: Node's global fetch (undici) defaults headers/body timeouts to 300s and
+ * severs a long agent turn mid-stream. createStreamingFetch must DISABLE those
+ * timeouts (0). We pin the option contract deterministically AND confirm the
+ * built fetch actually streams a deliberately-delayed response.
+ */
+describe('createStreamingFetch', () => {
+  it('disables BOTH undici stream timeouts (the #175 contract)', () => {
+    expect(STREAMING_DISPATCHER_OPTIONS.headersTimeout).toBe(0);
+    expect(STREAMING_DISPATCHER_OPTIONS.bodyTimeout).toBe(0);
+  });
+
+  describe('against a delayed server', () => {
+    let server: http.Server;
+    let url: string;
+    // The server waits before sending ANY byte (a long time-to-first-token).
+    const DELAY = 400;
+
+    beforeAll(async () => {
+      server = http.createServer((_req, res) => {
+        setTimeout(() => {
+          res.writeHead(200, { 'Content-Type': 'text/plain' });
+          res.end('ok');
+        }, DELAY);
+      });
+      await new Promise<void>((resolve) =>
+        server.listen(0, '127.0.0.1', resolve),
+      );
+      const addr = server.address() as import('node:net').AddressInfo;
+      url = `http://127.0.0.1:${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');
+    });
+  });
+});
diff --git a/apps/server/src/integrations/ai/ai-streaming-fetch.ts b/apps/server/src/integrations/ai/ai-streaming-fetch.ts
new file mode 100644
index 00000000..a3a42d34
--- /dev/null
+++ b/apps/server/src/integrations/ai/ai-streaming-fetch.ts
@@ -0,0 +1,45 @@
+import { Agent } from 'undici';
+
+/**
+ * Build a `fetch` for LONG-LIVED streaming AI calls (the agent chat turn).
+ *
+ * Node's global fetch (undici) defaults BOTH `headersTimeout` and `bodyTimeout`
+ * to 300_000ms. A legitimate long agent turn trips that and is severed
+ * mid-stream — surfacing to the user as "Lost connection to the AI provider"
+ * (issue #175):
+ *   - headersTimeout (time to the first response byte) is exceeded when a late
+ *     step sends a huge accumulated context and the model's time-to-first-token
+ *     grows past 5 min;
+ *   - bodyTimeout (max gap BETWEEN body chunks) is exceeded when a reasoning
+ *     model pauses to "think" for more than 5 min between emitted chunks.
+ *
+ * This dispatcher disables both timeouts (0). Cancellation is NOT lost: the
+ * agent turn is bound to the request's abortSignal, which fires when the client
+ * disconnects (see ai-chat.controller), and the agent loop is bounded by
+ * MAX_AGENT_STEPS — so a turn still terminates; it just no longer dies at an
+ * arbitrary 5-minute wall-clock. keepAlive (undici default) is kept so the
+ * sequential per-step calls of one turn reuse the connection.
+ *
+ * A single shared dispatcher is returned (callers hold it for the service
+ * lifetime) so its connection pool is reused across requests.
+ */
+/**
+ * undici Agent options for streaming AI calls: both stream timeouts DISABLED (0)
+ * so a long turn is never severed at undici's 300s default (#175). Exported so a
+ * test can pin the contract without a timing-dependent assertion.
+ */
+export const STREAMING_DISPATCHER_OPTIONS = {
+  headersTimeout: 0,
+  bodyTimeout: 0,
+} as const;
+
+export function createStreamingFetch(): typeof fetch {
+  const dispatcher = new Agent({ ...STREAMING_DISPATCHER_OPTIONS });
+  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;
+}
diff --git a/apps/server/src/integrations/ai/ai.service.ts b/apps/server/src/integrations/ai/ai.service.ts
index 4f72d23b..65bdda5c 100644
--- a/apps/server/src/integrations/ai/ai.service.ts
+++ b/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(