diff --git a/scripts/pm/git-history.mjs b/scripts/pm/git-history.mjs
index 838505ba352..1a54ee0b67d 100644
--- a/scripts/pm/git-history.mjs
+++ b/scripts/pm/git-history.mjs
@@ -149,6 +149,33 @@
  * what git is handed and what the receipt prints, and a complete instant is
  * parsed exactly and never consults the clock.
  *
+ * ## The fifth trap, no git at all: a piped answer is cut at one buffer-full
+ *
+ * The answer leaves in ONE `process.stdout.write`. Node completes that write
+ * at once when stdout is a FILE, but when stdout is a PIPE (on POSIX) it puts
+ * in only what the pipe takes in one go and queues the rest for the event
+ * loop. The entry used to end with `process.exit(main(...))`, which tears the
+ * process down before that queue drains — so `log … | wc -l`, the natural
+ * spelling of a count taken through `log`, read one buffer-full at exit 0
+ * (#21659). Measured at 7d0781482d on this repository's shallow clone, the
+ * same `log --since=2026-08-14T00:00:00Z --ref=3711e0b763` two ways:
+ *
+ *   | stdout                         | lines | exit |
+ *   |--------------------------------|-------|------|
+ *   | redirected to a file           | 5455  | 0    |
+ *   | piped to `wc -l`, three runs   | 346   | 0    |
+ *
+ * 346 lines is about one 64 KiB pipe buffer. `count` and `touch` print one
+ * line, which always fits, so they never showed it. The entry therefore sets
+ * `process.exitCode` and returns: the process ends when its last queued write
+ * has drained, on every stdout kind. `main()` and `touchMain()` RETURN their
+ * exit codes; the one `process.exit` left is `usage()`'s, and every call to it
+ * comes before anything is written to stdout. A reader that closes early
+ * (`| head -1`) turns the queued rest into EPIPE; that is the reader's choice,
+ * not a lost answer, so it is absorbed and the answer's own exit code stands.
+ * The self-test pins a piped `log`, many buffers long, byte for byte against
+ * the same `log` sent to a file.
+ *
  * ## Cost, measured — because a tool nobody runs fixes nothing
  *
  *   | case                                            | wall  |
@@ -162,7 +189,8 @@
  */
 
 import { execFileSync, spawnSync } from 'node:child_process';
-import { mkdirSync, mkdtempSync, readFileSync, rmSync, writeFileSync } from 'node:fs';
+import { createHash } from 'node:crypto';
+import { closeSync, mkdirSync, mkdtempSync, openSync, readFileSync, rmSync, writeFileSync } from 'node:fs';
 import { tmpdir } from 'node:os';
 import { join } from 'node:path';
 import { isEntrypoint } from '../invoked-as.mjs';
@@ -185,15 +213,16 @@ import { isEntrypoint } from '../invoked-as.mjs';
 // remedy is to find what stopped registering.
 const SELF_TEST_BATTERIES = Object.freeze({
   'pure decisions': 10,
-  'real repos': 15,
+  'real repos': 17,
   'historyHorizon: the read-only reading the #9902 adopters call': 12,
   'bare dates: the instant the proof reads is the instant git counts': 16,
   'touch: the provenance reading a shallow clone fabricates': 26,
+  'piped stdout: the answer a pipe reads is the answer a file holds': 4,
 });
 
 // DELETING an entry silences that battery's floor exactly as effectively as
 // zeroing it, so the roster's own size is pinned too.
-const SELF_TEST_BATTERY_FLOOR = 5;
+const SELF_TEST_BATTERY_FLOOR = 6;
 
 // The key an assertion is filed under when no battery is open. It is not a
 // declared battery, so it reds by the same set difference rather than silently
@@ -303,9 +332,14 @@ export function ensureWindowCovered({ cwd, ref, sinceMs, allowFetch = true, allo
   const remotes = (git(['remote'], { cwd, allowFail: true }) || '').split('\n').filter(Boolean);
   const target = splitRemoteRef(ref, remotes);
   if (!target) {
+    // The boundaries were READ above; a ref that names no remote (a bare sha, a
+    // local branch) cannot be deepened, but its floor is still known, so the
+    // refusal prints it rather than `shallow floor: unknown`.
     return {
       covered: false,
       steps,
+      shallow: shallow0,
+      boundaries: boundaries0,
       reason:
         `history is truncated inside the window and '${ref}' names no remote to deepen from ` +
         `(remotes here: ${remotes.length ? remotes.join(', ') : 'none'})`,
@@ -751,7 +785,7 @@ function main(argv) {
           + 'so no success line was printed. Exiting 0 here would report a self-test\n'
           + 'that never finished as a self-test that passed.\n',
       );
-      process.exit(1);
+      return 1;
     }
     return selfTestCode;
   }
@@ -785,7 +819,7 @@ function main(argv) {
         `   commits are invisible to git log, which reports no error (#9878).\n` +
         `   Remedy: git -C ${cwd} fetch --unshallow ${splitRemoteRef(opts.ref, ['origin']) ? 'origin' : '<remote>'}\n`,
     );
-    process.exit(2);
+    return 2;
   }
 
   const receipt =
@@ -1055,6 +1089,21 @@ function selfTest() {
     t('the clone is still shallow after that answer — proving the predicate is the floor, '
       + 'not the shallow flag', isShallow(shallowDeep) === true);
 
+    // A ref that names no remote — a bare sha, the way a tree is usually quoted —
+    // cannot be deepened, so a window crossing its floor is refused. The floor
+    // printed beside that refusal is the one already READ, never `unknown`.
+    // Nothing is fetched on this path, so `shallowDeep` keeps its depth.
+    const shallowDeepTip = g(['rev-parse', 'origin/main'], shallowDeep).trim();
+    const bareShaRefusal = runCliAllowFail(['count', `--since=${WINDOW_SINCE}`, `--ref=${shallowDeepTip}`], shallowDeep);
+    t('a bare-sha --ref whose window crosses the shallow floor is REFUSED, exit 2 with EMPTY stdout, '
+      + 'because it names no remote to deepen from',
+      bareShaRefusal.code === 2 && bareShaRefusal.stdout.trim() === ''
+        && /names no remote to deepen from/.test(bareShaRefusal.stderr), JSON.stringify(bareShaRefusal));
+    t('and the refusal prints the floor it READ, 2026-07-06, not `shallow floor: unknown` — the no-remote '
+      + 'branch used to return without the boundaries it had already read',
+      /shallow floor: 2026-07-06 /.test(bareShaRefusal.stderr) && !/shallow floor: unknown/.test(bareShaRefusal.stderr),
+      bareShaRefusal.stderr);
+
     // ── historyHorizon: the read-only reading the #9902 adopters call ───────
   battery('historyHorizon: the read-only reading the #9902 adopters call');
     const hFull = historyHorizon({ cwd: full, ref: 'origin/main', sinceMs: Date.parse(WINDOW_SINCE) });
@@ -1373,6 +1422,90 @@ function selfTest() {
       never.code === 1 && never.stdout.trim() === '' && /no commit on/.test(never.stderr), JSON.stringify(never));
     const noPath = runCliAllowFail(['touch'], full);
     t('touch without --path is usage', noPath.code === 1 && noPath.stdout.trim() === '', JSON.stringify(noPath));
+
+    // ── piped stdout: the answer a pipe reads is the answer a file holds ────
+    // The fifth trap in the header. A file takes the whole answer in one write;
+    // a pipe takes what fits and the rest waits on the event loop, which the
+    // entry's old `process.exit()` ended. Every answer above fits in one
+    // buffer-full, so none of them could see it. This one is padded to many:
+    // `%<(16384)` pads a field to 16384 columns (git ignores wider), four to a
+    // line, 21 lines. A child's pipe here is a socketpair, so a buffer-full is
+    // the socket's send buffer (212992 bytes by default on Linux) rather than a
+    // FIFO's 64 KiB; the answer is sized well past both. `full` is read-only
+    // here, so this battery's place after the deepening ones changes nothing.
+    battery('piped stdout: the answer a pipe reads is the answer a file holds');
+    const wideArgs = [
+      'log', `--since=${WINDOW_SINCE}`, `--until=${WINDOW_UNTIL}`, `--format=${'%<(16384)%H'.repeat(4)}`, `--cwd=${full}`,
+    ];
+    const newlines = (buf) => buf.reduce((n, b) => n + (b === 10 ? 1 : 0), 0);
+
+    const wideFile = join(root, 'wide-answer.txt');
+    const wideFd = openSync(wideFile, 'w');
+    let toFile;
+    try {
+      toFile = spawnSync(process.execPath, [self, ...wideArgs], { cwd: full, encoding: 'utf8', stdio: ['ignore', wideFd, 'pipe'] });
+    } finally {
+      closeSync(wideFd);
+    }
+    const fileBytes = readFileSync(wideFile);
+    const fileLines = newlines(fileBytes);
+    t('FIXTURE — the wide `log` sent to a FILE holds the 21 answer lines, exit 0, and is over 1 MiB: many times '
+      + 'what a pipe takes in one go. Read first, because an answer that fits one buffer-full passes the piped '
+      + 'pins below with or without the fix',
+      toFile.status === 0 && fileLines === 21 && fileBytes.length > 1024 * 1024,
+      `exit ${toFile.status} lines ${fileLines} bytes ${fileBytes.length} stderr ${JSON.stringify(toFile.stderr)}`);
+
+    // The reader is its own process, so the tool's stdout is a real pipe whose
+    // far end nothing else drains. It attaches its listener at once (Node
+    // discards a child's unread output when the child exits, which would look
+    // like a truncation of the reader's own making), holds the stream paused
+    // for 200 ms so the tool fills the pipe and has to wait, then reads to EOF —
+    // or, in `close-early` mode, takes one chunk and closes, the shape of `| head -1`.
+    const PIPE_READER = [
+      "const { spawn } = require('node:child_process');",
+      "const { createHash } = require('node:crypto');",
+      'const [mode, ...cmd] = process.argv.slice(1);',
+      "const child = spawn(process.execPath, cmd, { stdio: ['ignore', 'pipe', 'pipe'] });",
+      "const hash = createHash('sha256');",
+      "let bytes = 0; let lines = 0; let stderr = '';",
+      "child.stderr.on('data', (d) => { stderr += d; });",
+      'const take = (d) => { bytes += d.length; lines += d.reduce((n, b) => n + (b === 10 ? 1 : 0), 0); hash.update(d); };',
+      "if (mode === 'close-early') child.stdout.once('data', (d) => { take(d); child.stdout.destroy(); });",
+      "else child.stdout.on('data', take);",
+      'child.stdout.pause();',
+      'setTimeout(() => child.stdout.resume(), 200);',
+      "child.on('close', (code, signal) => process.stdout.write(JSON.stringify({ code, signal, bytes, lines, sha256: hash.digest('hex'), stderr })));",
+    ].join('\n');
+    const readThroughPipe = (mode) => {
+      const r = spawnSync(process.execPath, ['-e', PIPE_READER, mode, self, ...wideArgs], {
+        cwd: full,
+        encoding: 'utf8',
+        stdio: ['ignore', 'pipe', 'pipe'],
+        timeout: 60_000,
+      });
+      try {
+        return JSON.parse(r.stdout);
+      } catch {
+        return { code: null, readerExit: r.status, readerSignal: r.signal, readerError: String(r.error ?? ''),
+          readerStdout: String(r.stdout).slice(0, 300), stderr: String(r.stderr).slice(0, 300) };
+      }
+    };
+
+    const piped = readThroughPipe('slow');
+    t('the same `log` through a pipe whose reader holds off reads the SAME 21 lines, exit 0 — where the entry\'s '
+      + 'old process.exit() delivered one buffer-full and still exited 0',
+      piped.code === 0 && piped.lines === fileLines, JSON.stringify(piped));
+    t('and the piped bytes ARE the file\'s bytes: the same length and the same sha256, not just the same line count',
+      piped.bytes === fileBytes.length && piped.sha256 === createHash('sha256').update(fileBytes).digest('hex'),
+      `piped ${piped.bytes} bytes ${piped.sha256} · file ${fileBytes.length} bytes`);
+
+    const early = readThroughPipe('close-early');
+    t('a reader that closes after its first chunk (the `| head -1` shape, proved by reading less than the file '
+      + 'holds) costs the tool no `write EPIPE` trace: it exits with the answer\'s code, 0, and its stderr is the '
+      + 'one-line method receipt',
+      early.code === 0 && early.bytes > 0 && early.bytes < fileBytes.length
+        && /^method: git log [^\n]*\n$/.test(early.stderr ?? '') && !/EPIPE/.test(early.stderr ?? ''),
+      JSON.stringify(early));
   } finally {
     rmSync(root, { recursive: true, force: true });
   }
@@ -1429,5 +1562,13 @@ function selfTest() {
 
 const invokedDirectly = isEntrypoint(import.meta.url);
 if (invokedDirectly) {
-  process.exit(main(process.argv.slice(2)) || 0);
+  // The fifth trap in the header: `process.exit()` here ended the process with
+  // most of a piped `log` answer still queued. Setting the exit code and
+  // returning lets the queued writes drain first, on every stdout kind.
+  process.stdout.on('error', (err) => {
+    // A reader that stopped reading (`| head -1`) took what it wanted; the rest
+    // of the answer has nowhere to go. Anything else is a real failure.
+    if (err?.code !== 'EPIPE') throw err;
+  });
+  process.exitCode = main(process.argv.slice(2)) || 0;
 }