diff --git a/packages/sdk/src/named-gate-lowering.ts b/packages/sdk/src/named-gate-lowering.ts
index 8a4ac503..56200f6b 100644
--- a/packages/sdk/src/named-gate-lowering.ts
+++ b/packages/sdk/src/named-gate-lowering.ts
@@ -125,10 +125,17 @@ process.exit(result.status===0?0:1);`;
       // The other gate that spawns a child. Its `wc` stderr was piped and then
       // discarded, so a broken or absent `wc` was reported as a word count out
       // of bounds. A bounded suffix of what it said travels with the cause.
+      //
+      // EPIPE is not "could not run": it means wc ran and exited before
+      // reading all of `input` (it crashed, was killed, or never reads
+      // stdin). spawnSync still reports its status, signal and output, so
+      // those decide the verdict. Whether the write raced wc's exit is
+      // timing, so it must not change the verdict; reporting it hid what wc
+      // did (#611).
       body = `const result=cp.spawnSync('wc',['-w'],{input:text,encoding:'utf8',env:{...process.env,LC_ALL:'C'}});
 const noise=(result.stderr||'').trim().slice(-200);
 const said=noise===''?'':': '+noise;
-if(result.error)fail('could not run wc -w: '+(result.error.code||result.error.message)+said);
+if(result.error&&result.error.code!=='EPIPE')fail('could not run wc -w: '+(result.error.code||result.error.message)+said);
 if(result.signal)fail('wc -w was terminated by '+result.signal+said);
 if(result.status!==0)fail('wc -w exited '+result.status+said);
 const count=result.stdout.trim();
diff --git a/packages/sdk/tests/authored-parallel-agents.test.ts b/packages/sdk/tests/authored-parallel-agents.test.ts
index 12ef531e..c3503368 100644
--- a/packages/sdk/tests/authored-parallel-agents.test.ts
+++ b/packages/sdk/tests/authored-parallel-agents.test.ts
@@ -1,4 +1,4 @@
-import { mkdirSync, readFileSync, symlinkSync, writeFileSync } from 'node:fs';
+import { existsSync, mkdirSync, readFileSync, symlinkSync, writeFileSync } from 'node:fs';
 import { join, resolve } from 'node:path';
 import { afterEach, describe, expect, it } from 'vitest';
 import { flow } from '@relayflows/surface';
@@ -19,12 +19,14 @@ async function slowAgents(capacity: number, failFirstSession = false) {
   const fixture = chainFixture();
   closes.push(() => fixture.close());
   const spans = join(fixture.root, 'spans.jsonl');
+  const started = join(fixture.root, 'started');
   writeFileSync(fixture.wrapper, `#!/usr/bin/env node
 import { receiveWrapperRequest } from ${JSON.stringify(resolve('../../testdata/preflight/wrapper-session.mjs'))};
 import { appendFileSync, existsSync, writeFileSync } from 'node:fs';
 if (process.argv[2] === 'auth') process.exit(0);
 const request = await receiveWrapperRequest();
 if (request) {
+  writeFileSync(${JSON.stringify(started)}, '');
   const start = Date.now();
   await new Promise(done => setTimeout(done, 400));
   appendFileSync(${JSON.stringify(spans)}, JSON.stringify({ start, end: Date.now() }) + '\\n');
@@ -50,7 +52,14 @@ if (request) {
   closes.push(() => llm.close());
   const readSpans = () => readFileSync(spans, 'utf8').trim().split('\n')
     .map(line => JSON.parse(line) as { start: number; end: number });
-  return { fixture, client, agent, readSpans };
+  /** Resolves once a session has received its request: an agent holds a slot and is running. */
+  const firstStarted = async () => {
+    for (const deadline = Date.now() + 5_000; !existsSync(started);) {
+      if (Date.now() > deadline) throw new Error('no agent session started within 5s');
+      await new Promise(done => setTimeout(done, 10));
+    }
+  };
+  return { fixture, client, agent, readSpans, firstStarted };
 }
 
 function peakOverlap(spans: Array<{ start: number; end: number }>): number {
@@ -135,11 +144,16 @@ describe('authored steps under local workers with capacity', () => {
   });
 
   it('never starts queued agents once the body has failed', async () => {
-    const { fixture, client, agent, readSpans } = await slowAgents(1);
+    const { fixture, client, agent, readSpans, firstStarted } = await slowAgents(1);
     const failing = flow('fails-while-queued', async f => {
       await Promise.all([
         ...['a', 'b', 'c'].map(lens => f.agent(`review-${lens}`, { task: `Review for ${lens}` })),
-        new Promise((_, reject) => setTimeout(() => reject(new Error('body failed')), 100)),
+        // Fail once one agent is really running, not after a fixed delay. Each
+        // call runs its preflight before it asks for a slot; under load that
+        // outlasted 100ms, so the body failed while no agent held the slot,
+        // all three were (correctly) refused, and spans.jsonl was never
+        // written (#611).
+        firstStarted().then(() => { throw new Error('body failed'); }),
       ]);
       f.done('success');
     });
diff --git a/packages/sdk/tests/named-gate-diagnostics.test.ts b/packages/sdk/tests/named-gate-diagnostics.test.ts
index b35c60eb..d5f661d6 100644
--- a/packages/sdk/tests/named-gate-diagnostics.test.ts
+++ b/packages/sdk/tests/named-gate-diagnostics.test.ts
@@ -279,4 +279,37 @@ describe('word_count_bounds reports why its own child failed', () => {
     expect(capture.status).toBe(1);
     expect(capture.stderr).toContain('SIGKILL');
   });
+
+  // #611: a wc that exits without reading stdin made spawnSync's input write
+  // fail with EPIPE, reported as "could not run wc -w: EPIPE" in place of what
+  // wc did. With 'one two' that depended on who won the race. spawnSync's
+  // stdin is a socketpair: on macOS its buffer is small, so this input makes
+  // wc exit first every time; on Linux the buffer holds more than one env
+  // string can carry, so the race is not forced there. These hold either way.
+  describe('when wc exits before reading all of its input', () => {
+    // Below Linux's 128 KiB limit on one env string.
+    const longText = 'word '.repeat(20_000);
+    const run = (script: string) =>
+      runGate(command(), { output: deterministicEnvelope(longText) }, { path: stubWordCount(script) });
+
+    it('still reports output that is not a count', () => {
+      const capture = run("printf 'not a number'");
+      expect(capture.status).toBe(1);
+      expect(capture.stderr).toContain('not a number');
+      expect(capture.stderr).not.toContain('EPIPE');
+    });
+
+    it('still reports its exit status and what it said', () => {
+      const capture = run("printf 'wc: read error' >&2; exit 2");
+      expect(capture.status).toBe(1);
+      expect(capture.stderr).toContain('exited 2');
+      expect(capture.stderr).toContain('wc: read error');
+    });
+
+    it('still reports the signal that ended it', () => {
+      const capture = run('kill -9 $$');
+      expect(capture.status).toBe(1);
+      expect(capture.stderr).toContain('SIGKILL');
+    });
+  });
 });