diff --git a/packages/sdk/src/client.ts b/packages/sdk/src/client.ts
index cbeac0f5d..9f06ac83e 100644
--- a/packages/sdk/src/client.ts
+++ b/packages/sdk/src/client.ts
@@ -269,17 +269,45 @@ export class AgentRelayClient {
     });
     client.child = child;
 
-    // Broker may still be connecting to Relaycast. Retry getSession
-    // with backoff if we get 503 (broker warming up).
+    // The broker prints "API listening on …" the moment its TCP listener is
+    // bound, but it still needs to complete a Relaycast handshake before
+    // `getSession()` will return. Two failure modes to handle:
+    //
+    //   1. Broker is alive and warming up — the startup-only API responds
+    //      503 until the handshake completes. Poll until it succeeds.
+    //   2. Broker died during the handshake (e.g. Relaycast unreachable) —
+    //      the in-flight fetch sees the socket drop as `TypeError: fetch
+    //      failed`, which is uninformative on its own.
+    //
+    // We race each `getSession()` against `brokerExited` so case (2) reports
+    // as the actual broker exit (with its stderr tail and exit code), not as
+    // a mystery network error. No backoff for the death case — we know it
+    // immediately. 503 polling stays simple at 1s intervals.
+    const brokerExited = new Promise<never>((_, reject) => {
+      child.once('exit', (code) => {
+        reject(
+          new Error(
+            formatBrokerStartupError(
+              `Broker process exited with code ${code} during initial handshake`,
+              child,
+              { binaryPath, args, cwd, stdoutLines, stderrLines }
+            )
+          )
+        );
+      });
+    });
+    // Suppress unhandledRejection if the race is won by getSession before
+    // the broker exits later (e.g. on normal shutdown).
+    brokerExited.catch(() => {});
+
     let session: SessionInfo | undefined;
     for (let attempt = 0; attempt < 10; attempt++) {
       try {
-        session = await client.getSession();
+        session = await Promise.race([client.getSession(), brokerExited]);
         break;
       } catch (err) {
-        const is503 =
-          err instanceof Error &&
-          (err.message.includes('503') || err.message.includes('Service Unavailable'));
+        const message = err instanceof Error ? err.message : String(err);
+        const is503 = message.includes('503') || message.includes('Service Unavailable');
         if (!is503 || attempt >= 9) throw err;
         await new Promise((resolve) => setTimeout(resolve, 1000));
       }
diff --git a/src/cli/lib/broker-lifecycle.test.ts b/src/cli/lib/broker-lifecycle.test.ts
new file mode 100644
index 000000000..cb83a5c1e
--- /dev/null
+++ b/src/cli/lib/broker-lifecycle.test.ts
@@ -0,0 +1,98 @@
+import { describe, expect, it } from 'vitest';
+
+import {
+  classifyBrokerStartError,
+  classifyBrokerStartStage,
+  describeError,
+} from './broker-lifecycle.js';
+
+describe('describeError', () => {
+  it('returns plain message for a bare Error', () => {
+    expect(describeError(new Error('boom'))).toBe('boom');
+  });
+
+  it('unwraps the Node fetch failed cause and surfaces the network code', () => {
+    // Mirror the shape Node 22 produces: TypeError with a cause carrying .code.
+    const cause = Object.assign(new Error('connect ECONNREFUSED 127.0.0.1:3889'), {
+      code: 'ECONNREFUSED',
+    });
+    const err = new TypeError('fetch failed', { cause });
+
+    const result = describeError(err);
+    expect(result).toContain('fetch failed');
+    expect(result).toContain('ECONNREFUSED');
+    expect(result).toContain('127.0.0.1');
+  });
+
+  it('unwraps DNS errors (ENOTFOUND)', () => {
+    const cause = Object.assign(new Error('getaddrinfo ENOTFOUND api.agentrelay.com'), {
+      code: 'ENOTFOUND',
+    });
+    const err = new TypeError('fetch failed', { cause });
+
+    const result = describeError(err);
+    expect(result).toContain('ENOTFOUND');
+    expect(result).toContain('agentrelay.com');
+  });
+
+  it('handles non-Error values without throwing', () => {
+    expect(describeError('something went wrong')).toBe('something went wrong');
+    expect(describeError(undefined)).toBe('undefined');
+    expect(describeError(null)).toBe('null');
+  });
+
+  it('caps the cause-chain walk so a cycle cannot loop forever', () => {
+    const a = new Error('a') as Error & { cause?: unknown };
+    const b = new Error('b') as Error & { cause?: unknown };
+    a.cause = b;
+    b.cause = a;
+    // Just needs to terminate — the assertion is the absence of a hang.
+    expect(typeof describeError(a)).toBe('string');
+  });
+});
+
+describe('classifyBrokerStartError', () => {
+  it('prefers the underlying network code over the constructor name', () => {
+    const cause = Object.assign(new Error('connect ECONNREFUSED'), { code: 'ECONNREFUSED' });
+    const err = new TypeError('fetch failed', { cause });
+
+    expect(classifyBrokerStartError(err)).toBe('ECONNREFUSED');
+  });
+
+  it('falls back to the constructor name when no code is present', () => {
+    expect(classifyBrokerStartError(new Error('whatever'))).toBe('Error');
+    expect(classifyBrokerStartError(new TypeError('boom'))).toBe('TypeError');
+  });
+
+  it('handles non-Error values', () => {
+    expect(classifyBrokerStartError('oops')).toBe('string');
+    expect(classifyBrokerStartError(undefined)).toBe('undefined');
+  });
+});
+
+describe('classifyBrokerStartStage', () => {
+  it('marks dashboard port conflicts when the dashboard was requested', () => {
+    const err = Object.assign(new Error('listen EADDRINUSE'), { code: 'EADDRINUSE' });
+    expect(classifyBrokerStartStage(err, 'listen EADDRINUSE', true)).toBe('dashboard_port');
+  });
+
+  it('marks already-running brokers from the message text', () => {
+    const message = 'another broker instance is already running in this directory (/tmp/x)';
+    expect(classifyBrokerStartStage(new Error(message), message, false)).toBe('already_running');
+  });
+
+  it('classifies fetch failures as connect-stage errors', () => {
+    const cause = Object.assign(new Error('ECONNREFUSED'), { code: 'ECONNREFUSED' });
+    const err = new TypeError('fetch failed', { cause });
+    expect(classifyBrokerStartStage(err, 'fetch failed', false)).toBe('connect');
+  });
+
+  it('classifies broker-exited-before-ready as a spawn failure', () => {
+    const message = 'Broker process exited with code 1 before becoming ready (pid=123; …)';
+    expect(classifyBrokerStartStage(new Error(message), message, false)).toBe('spawn');
+  });
+
+  it('falls back to startup for everything else', () => {
+    expect(classifyBrokerStartStage(new Error('???'), '???', false)).toBe('startup');
+  });
+});
diff --git a/src/cli/lib/broker-lifecycle.ts b/src/cli/lib/broker-lifecycle.ts
index 8c23a145a..de341cef2 100644
--- a/src/cli/lib/broker-lifecycle.ts
+++ b/src/cli/lib/broker-lifecycle.ts
@@ -2,9 +2,11 @@ import fs from 'node:fs';
 import os from 'node:os';
 import path from 'node:path';
 import { AgentRelayClient } from '@agent-relay/sdk';
+import { track } from '@agent-relay/telemetry';
 
 import type { CoreDependencies, CoreProjectPaths, CoreRelay, SpawnedProcess } from '../commands/core.js';
 import { buildBundledRelaycastMcpCommand } from './relaycast-mcp-command.js';
+import { errorClassName } from './telemetry-helpers.js';
 
 type UpOptions = {
   dashboard?: boolean;
@@ -86,6 +88,84 @@ function toErrorMessage(err: unknown): string {
   return err instanceof Error ? err.message : String(err);
 }
 
+type ErrorWithCode = { code?: unknown };
+
+function errorCode(err: unknown): string | undefined {
+  if (!err || typeof err !== 'object') return undefined;
+  const code = (err as ErrorWithCode).code;
+  return typeof code === 'string' ? code : undefined;
+}
+
+/**
+ * Extract a human-meaningful detail string from an error, walking `err.cause`.
+ *
+ * Node's native `fetch()` throws `TypeError: fetch failed` for any network
+ * problem and stuffs the real reason (ECONNREFUSED, ENOTFOUND, AbortError,
+ * UND_ERR_CONNECT_TIMEOUT, …) into `err.cause`. Without unwrapping, every
+ * outbound HTTP failure looks identical to the user.
+ *
+ * Exported for testing.
+ */
+export function describeError(err: unknown): string {
+  const top = toErrorMessage(err);
+  if (!(err instanceof Error) || !err.cause) return top;
+
+  // Walk the cause chain and collect the deepest message + any error codes.
+  const codes: string[] = [];
+  let detail: string | undefined;
+  let cursor: unknown = err.cause;
+  let depth = 0;
+  while (cursor && depth < 5) {
+    const code = errorCode(cursor);
+    if (code && !codes.includes(code)) codes.push(code);
+    if (cursor instanceof Error && cursor.message) {
+      detail = cursor.message;
+    }
+    cursor = cursor instanceof Error ? cursor.cause : undefined;
+    depth += 1;
+  }
+
+  const parts = [top];
+  if (detail && detail !== top) parts.push(detail);
+  if (codes.length > 0) parts.push(`[${codes.join(', ')}]`);
+  return parts.join(' — ');
+}
+
+/**
+ * Pick the best `error_class` for telemetry. Prefer a network-style code from
+ * `err.cause` (ECONNREFUSED etc.) over the generic constructor name (TypeError)
+ * — a code is more actionable in PostHog and matches the schema's example
+ * values for `BrokerStartFailedEvent.error_class`.
+ *
+ * Exported for testing.
+ */
+export function classifyBrokerStartError(err: unknown): string {
+  let cursor: unknown = err;
+  let depth = 0;
+  while (cursor && depth < 5) {
+    const code = errorCode(cursor);
+    if (code) return code;
+    cursor = cursor instanceof Error ? cursor.cause : undefined;
+    depth += 1;
+  }
+  return errorClassName(err) ?? 'Error';
+}
+
+/** Exported for testing. */
+export function classifyBrokerStartStage(
+  err: unknown,
+  message: string,
+  wantsDashboard: boolean
+): string {
+  if (errorCode(err) === 'EADDRINUSE' && wantsDashboard) return 'dashboard_port';
+  if (isBrokerAlreadyRunningError(message)) return 'already_running';
+  if (/fetch failed/i.test(message)) return 'connect';
+  if (/Broker did not report API port/i.test(message)) return 'spawn';
+  if (/Broker process exited with code/i.test(message)) return 'spawn';
+  if (/ENOENT/i.test(message) && /broker/i.test(message)) return 'resolve_binary';
+  return 'startup';
+}
+
 async function resolveApiPortWithFallback(
   startApiPort: number,
   maxAttempts: number,
@@ -1126,14 +1206,18 @@ export async function runUpCommand(options: UpOptions, deps: CoreDependencies):
     await deps.holdOpen();
   } catch (err: unknown) {
     await shutdownOnce();
-    const withCode = err as { code?: string };
     const message = toErrorMessage(err);
-    if (withCode.code === 'EADDRINUSE' && wantsDashboard) {
+    const stage = classifyBrokerStartStage(err, message, wantsDashboard);
+    track('broker_start_failed', {
+      stage,
+      error_class: classifyBrokerStartError(err),
+    });
+    if (errorCode(err) === 'EADDRINUSE' && wantsDashboard) {
       deps.error(`Dashboard port ${dashboardPort} is already in use.`);
     } else if (isBrokerAlreadyRunningError(message)) {
       reportAlreadyRunningError(message, paths.dataDir, deps);
     } else {
-      deps.error(`Failed to start broker: ${message}`);
+      deps.error(`Failed to start broker: ${describeError(err)}`);
     }
     deps.exit(1);
   }