diff --git a/server/services/agentCliSpawning.js b/server/services/agentCliSpawning.js
index 4f6ae6835a..7696c1f3e3 100644
--- a/server/services/agentCliSpawning.js
+++ b/server/services/agentCliSpawning.js
@@ -425,15 +425,44 @@ export async function spawnDirectly({
     env: childEnv
   });
 
-  // spawn() can hand back a handle with no pid or stdio when command lookup
-  // fails. Listen immediately so that failure cannot become an unhandled error
-  // while the async setup below is still yielding.
+  // Every child listener is registered HERE, in the same tick as spawn(), into
+  // forwarding shims that buffer until the real handler bodies exist further
+  // down (#5791). Those bodies close over state built by the async setup that
+  // follows — and that setup yields for real state-file I/O (`await
+  // updateAgent(..., { pid })`), not just a microtask. A CLI that starts and
+  // exits immediately (bad flag, instant auth refusal) emits its entire life
+  // cycle inside that window: without the shims its stdout/stderr is lost and
+  // its `close` lands on nothing, leaving the run record, the execution lane
+  // and the `activeAgents` entry non-terminal until the orphan reaper notices.
+  // Buffering rather than hoisting the handler bodies keeps the change local.
+  //
+  // spawn() can also hand back a handle with no pid or stdio when command
+  // lookup fails, hence the optional chaining on the stream registrations.
   let pendingSpawnError = null;
   let handleSpawnError = null;
+  // One ordered queue for both streams: buffering them separately would lose
+  // the relative order of interleaved stdout/stderr chunks in the transcript.
+  const pendingChildOutput = [];
+  let pendingClose = null;
+  let handleStdout = null;
+  let handleStderr = null;
+  let handleClose = null;
   claudeProcess.on('error', (err) => {
     if (handleSpawnError) void handleSpawnError(err);
     else pendingSpawnError = err;
   });
+  claudeProcess.stdout?.on('data', (data) => {
+    if (handleStdout) handleStdout(data);
+    else pendingChildOutput.push({ stream: 'stdout', data });
+  });
+  claudeProcess.stderr?.on('data', (data) => {
+    if (handleStderr) handleStderr(data);
+    else pendingChildOutput.push({ stream: 'stderr', data });
+  });
+  claudeProcess.on('close', (code) => {
+    if (handleClose) void handleClose(code);
+    else pendingClose = { code };
+  });
   // Same reasoning for the stdin pipe: a child that exits before reading it
   // emits EPIPE, and an unlistened stream 'error' out here would crash the
   // server. The 'error'/'exit' handlers below settle the run with the real cause.
@@ -579,7 +608,7 @@ export async function spawnDirectly({
     }
   }, 3000);
 
-  claudeProcess.stdout.on('data', (data) => {
+  handleStdout = (data) => {
     try {
       const text = data.toString();
       // Detect fallback signals SYNCHRONOUSLY, before enqueuing any transcript
@@ -616,9 +645,9 @@ export async function spawnDirectly({
     } catch (err) {
       console.error(`❌ agentCli stdout handler failed: ${err.message}`);
     }
-  });
+  };
 
-  claudeProcess.stderr.on('data', (data) => {
+  handleStderr = (data) => {
     try {
       const text = data.toString();
       // Synchronous fallback detection before the serialized write (see stdout).
@@ -640,7 +669,7 @@ export async function spawnDirectly({
     } catch (err) {
       console.error(`❌ agentCli stderr handler failed: ${err.message}`);
     }
-  });
+  };
 
   handleSpawnError = async (err) => {
     // Runs outside the request lifecycle — an uncaught throw from the awaited
@@ -682,9 +711,8 @@ export async function spawnDirectly({
       activeAgents.delete(agentId);
     }
   };
-  if (pendingSpawnError) void handleSpawnError(pendingSpawnError);
 
-  claudeProcess.on('close', async (code) => {
+  handleClose = async (code) => {
     // Runs outside the request lifecycle — a throw from outputBatcher.flush,
     // analyzeAgentFailure, or finalizeAgent would re-escape this async handler
     // as an unhandled rejection and crash the process. The inner try/finally
@@ -975,7 +1003,22 @@ export async function spawnDirectly({
         activeAgents.delete(agentId);
       }
     }
-  });
+  };
+
+  // Replay whatever the child emitted while the async setup above was yielding
+  // (#5791). Output first, so the transcript is complete before a buffered
+  // terminal event finalizes the run — and it is enqueued onto
+  // `transcriptWriteTail` here, before either terminal handler's
+  // `drainTranscriptWrites()` captures that tail.
+  for (const { stream, data } of pendingChildOutput) {
+    if (stream === 'stderr') handleStderr(data);
+    else handleStdout(data);
+  }
+  pendingChildOutput.length = 0;
+  // A failed spawn emits 'error' AND then 'close'; the error path already
+  // finalizes the run, so replay exactly one terminal event, never both.
+  if (pendingSpawnError) void handleSpawnError(pendingSpawnError);
+  else if (pendingClose) void handleClose(pendingClose.code);
 
   return agentId;
 }
diff --git a/server/services/agentCliSpawning.test.js b/server/services/agentCliSpawning.test.js
index e39e285c8c..6b5655af3b 100644
--- a/server/services/agentCliSpawning.test.js
+++ b/server/services/agentCliSpawning.test.js
@@ -680,6 +680,44 @@ describe('stream error containment', () => {
     });
   });
 
+  // ─── Fast-exiting child: events emitted during the post-spawn await (#5791) ─
+
+  it('does not drop stdout or close emitted while the post-spawn await is still yielding', async () => {
+    // The child's whole life cycle can land between spawn() and the point where
+    // the real stdout/stderr/close handler bodies are installed: the
+    // `updateAgent(agentId, { pid })` in between is real state-file I/O, so it
+    // yields for a macrotask, not just a microtask. A CLI that dies instantly
+    // (bad flag, instant auth refusal) exits inside that window — before the fix
+    // its output was lost and its `close` landed on nothing, leaving the run
+    // record, the execution lane and the activeAgents entry non-terminal until
+    // the orphan reaper eventually noticed.
+    const { activeAgents } = await import('./agentState.js');
+    const { finalizeAgent } = await import('./agentFinalization.js');
+    activeAgents.clear();
+    finalizeAgent.mockClear();
+    agentStateMocks.updateAgent.mockImplementationOnce(async () => {
+      // Emit from inside the same macrotask yield the real updateAgent creates.
+      await new Promise((r) => setImmediate(r));
+      fakeProcess.stdout.emit('data', Buffer.from(
+        '{"type":"stream_event","event":{"type":"content_block_delta","delta":{"type":"text_delta","text":"unknown flag --nope\\n"}}}\n'
+      ));
+      fakeProcess.emit('close', 2);
+    });
+
+    await expect(spawnDirectly(minimalArgs)).resolves.toBe('agent-test');
+    await new Promise((r) => setTimeout(r, 30));
+
+    expect(finalizeAgent).toHaveBeenCalledWith(expect.objectContaining({
+      agentId: 'agent-test',
+      exitCode: 2,
+      success: false,
+      // The output the child managed to write before exiting is exactly what
+      // explains the failure, so it has to survive the buffering too.
+      outputBuffer: expect.stringContaining('unknown flag --nope'),
+    }));
+    expect(activeAgents.has('agent-test')).toBe(false);
+  });
+
   // ─── Lifecycle ledger — the first-output boundary (#4540) ─────────────────
 
   it('records ONE run.output on the first real byte, and never again', async () => {
diff --git a/server/services/agentManagement.test.js b/server/services/agentManagement.test.js
index d1bf487217..9b5f0aa034 100644
--- a/server/services/agentManagement.test.js
+++ b/server/services/agentManagement.test.js
@@ -1350,15 +1350,18 @@ describe('close-handler skip-finalization — source contract', () => {
   }
 
   it('CLI close handler guards with pausedAgents.has and returns before finalizeAgent', () => {
-    // The guard appears in the claudeProcess.on('close', ...) callback.
-    const closeIdx = AGENT_CLI_SRC.indexOf("claudeProcess.on('close'");
-    expect(closeIdx, "claudeProcess 'close' handler must exist").toBeGreaterThan(-1);
+    // The real body lives in `handleClose`, not in the `claudeProcess.on('close')`
+    // registration — that registration is a forwarding shim attached in the same
+    // tick as spawn() so a fast-exiting child's close event isn't dropped while
+    // the async setup is still yielding (#5791).
+    const closeIdx = AGENT_CLI_SRC.indexOf('handleClose = async (code)');
+    expect(closeIdx, 'CLI handleClose handler must exist').toBeGreaterThan(-1);
 
     // Extract the full callback body via brace-balancing rather than a fixed
     // slice — a try/catch crash-guard wrapper can push finalizeAgent past any
     // fixed window (see #1825).
-    const closeBody = extractFunctionBody(AGENT_CLI_SRC, "claudeProcess.on('close'");
-    expect(closeBody, "claudeProcess 'close' handler body must be extractable").toBeTruthy();
+    const closeBody = extractFunctionBody(AGENT_CLI_SRC, 'handleClose = async (code)');
+    expect(closeBody, 'CLI handleClose handler body must be extractable').toBeTruthy();
 
     // Guard present
     expect(closeBody).toMatch(/pausedAgents\.has\(agentId\)/);