diff --git a/.changeset/proud-sloths-cheer.md b/.changeset/proud-sloths-cheer.md
new file mode 100644
index 000000000..c72d65a92
--- /dev/null
+++ b/.changeset/proud-sloths-cheer.md
@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 4015
+---
+**A killed test chunk now names the file that was hanging.** `scripts/run-tests.cjs` logged only chunk starts, so a chunk killed at the 600s cap printed ~55 basenames and left the operator to guess which one hung — and every timing figure had to be reconstructed from CI log timestamps. It now emits per-chunk elapsed time on every path, names the files still in flight on a kill with how stale the last event is (hang vs. merely slow), and ranks the chunk by known weight, flagging files missing from the timings table. (#4012)
diff --git a/bin/install.js b/bin/install.js
index 62fa3da5a..c9311d08d 100755
--- a/bin/install.js
+++ b/bin/install.js
@@ -411,7 +411,7 @@ const GSD_CHANGESET_FILES = [
   'github-release-notes.cjs', 'lint.cjs', 'new.cjs',
   'README.md', // documentation only — not user-authored
 ];
-const GSD_SCRIPTS_LIB_FILES = ['cli-exit.cjs', 'allowlist-ratchet.cjs', 'drift-scan.cjs', 'alias-drift-families.cjs', 'exit-code-registry.cjs'];
+const GSD_SCRIPTS_LIB_FILES = ['cli-exit.cjs', 'allowlist-ratchet.cjs', 'drift-scan.cjs', 'alias-drift-families.cjs', 'exit-code-registry.cjs', 'ndjson-reporter.cjs'];
 
 /**
  * Resolve a runtime's shared-hooks directory name from its descriptor.
diff --git a/scripts/lib/ndjson-reporter.cjs b/scripts/lib/ndjson-reporter.cjs
new file mode 100644
index 000000000..6f48de57d
--- /dev/null
+++ b/scripts/lib/ndjson-reporter.cjs
@@ -0,0 +1,119 @@
+'use strict';
+
+// Machine-readable companion reporter for scripts/run-tests.cjs (#3889).
+//
+// node:test's built-in reporters (spec/tap) are human-formatted and
+// scripts/run-tests.cjs spawns the child with `stdio: 'inherit'` — by design,
+// per #3597/#1051, to avoid the maxBuffer and live-output risks of piping —
+// so the parent process has no way to see WHICH file was executing when a
+// per-chunk timeout kills the child. This reporter runs ALONGSIDE the normal
+// human reporter (a second `--test-reporter` on the same invocation, per
+// Node's documented multi-reporter pairing) and appends one JSON object per
+// line to a path supplied via the GSD_RUN_TESTS_EVENTS_FILE env var. On a
+// timeout, run-tests.cjs reads that file back to name the file(s) still
+// in flight (a `test:dequeue` with no matching `test:pass`/`test:fail`).
+//
+// Durability, not `--test-reporter-destination` (#3889 root cause): a
+// reporter that YIELDS strings has them piped by Node into a
+// `fs.WriteStream` targeting the destination path, and that stream buffers.
+// The parent's `execFileSync` timeout SIGKILLs the child on a hang, and
+// SIGKILL is uncatchable and gives the process zero chance to flush — so a
+// yield-based reporter can lose every event still sitting in the stream's
+// buffer, which is exactly the case this feature exists to diagnose (proven
+// live: a chunk killed at 2006ms produced a `killed after 2006ms` line from
+// the TIMER, which lives in the parent, but zero usable events from the
+// reporter, which lives in the child and never flushed). Writing each event
+// with `fs.appendFileSync` — synchronous and unbuffered — makes it durable
+// the instant it happens, before the process can be killed out from under
+// it. The reporter therefore yields NOTHING; it is a pure side-effecting
+// sink. Node still requires a `--test-reporter-destination` to pair with
+// this `--test-reporter` (see run-tests.cjs's reporterArgsFor), but that
+// destination is a throwaway sink that stays empty by design — the durable
+// path is GSD_RUN_TESTS_EVENTS_FILE, not the destination Node manages.
+//
+// Contract targeted: Node's "Custom reporters" contract
+// (https://nodejs.org/api/test.html#custom-reporters) — a reporter module's
+// default export is a function receiving the test runner's event stream (an
+// AsyncIterable of `{ type, data }` objects). Node feeds that function to
+// `stream.compose` as the stream's "body". When the body is an async
+// FUNCTION (not an `async function*` generator), `stream.compose`'s own
+// contract (https://nodejs.org/api/stream.html#streamcomposestreams)
+// requires it to return nully (undefined/null) — returning anything else,
+// including an array, throws `ERR_INVALID_RETURN_VALUE` ("Expected nully to
+// be returned from the 'body' function but got an instance of Array")
+// exactly once the promise resolves. Verified directly against
+// `stream.compose` in this repo's Node: calling it with a body that
+// `return`s `[]` reproduces that same TypeError. A generator form
+// (`async function*`) is the one that yields an iterable; the plain
+// `async function` form used here is the one that must return nully. This
+// repo's `engines.node` requires >=24.0.0 (package.json), where both
+// contracts have been stable since Node 20.
+//
+// Kept intentionally tiny: only the five event types run-tests.cjs needs are
+// handled — `test:enqueue`/`test:dequeue` (emitted by the RUNNER as it queues
+// and begins each spawned test-file child, independent of whether anything
+// inside that file ever completes) plus `test:start`/`test:pass`/`test:fail`
+// (emitted per-subtest, once the child reports it). `test:dequeue` is the
+// event that actually means "in flight": a subtest inside a file that hangs
+// forever never reaches `test:start`/`test:pass`/`test:fail` at all, because
+// node:test only surfaces those to the parent once the child COMPLETES that
+// test — a hang, by definition, never completes. Recording `test:dequeue`
+// closes that gap: it fires the moment the runner begins the file, so a
+// killed hang still leaves a durable "this file was running" record.
+// Everything else (diagnostics, plans, coverage) is ignored so a truncated
+// events file (the process is SIGKILLed mid-`appendFileSync` on timeout — an
+// individual write is unbuffered but not atomic, so the OS can still
+// interleave a partial write with the kill) never leaves more than one
+// dangling unparsable trailing line.
+module.exports = async function ndjsonEventReporter(source) {
+  const eventsPath = process.env.GSD_RUN_TESTS_EVENTS_FILE;
+  // #3889: an init marker, written as this reporter's FIRST action — before
+  // the `for await` loop even begins consuming the event stream — so the
+  // events file's mere existence (and its exact contents) can distinguish
+  // "the reporter module never loaded in the child at all" (file absent)
+  // from "it loaded fine but no test:start reached it before the kill"
+  // (file contains only this one line) from "it's working" (file contains
+  // more than this line). Same appendFileSync durability rationale as every
+  // other write in this file: synchronous and unbuffered, so it survives an
+  // uncatchable SIGKILL landing a moment later.
+  if (eventsPath) {
+    try {
+      require('fs').appendFileSync(eventsPath, `${JSON.stringify({ type: 'reporter:init', ts: Date.now() })}\n`);
+    } catch {
+      // Best-effort, same as every other write below — must never crash the
+      // test run this reporter is only observing.
+    }
+  }
+  for await (const event of source) {
+    if (!eventsPath) continue; // no destination configured — nothing to record
+    if (
+      event.type === 'test:enqueue' ||
+      event.type === 'test:dequeue' ||
+      event.type === 'test:start' ||
+      event.type === 'test:pass' ||
+      event.type === 'test:fail'
+    ) {
+      const { file, name, nesting, testNumber } = event.data || {};
+      const line = `${JSON.stringify({
+        type: event.type,
+        file,
+        name,
+        nesting,
+        testNumber,
+        ts: Date.now(),
+      })}\n`;
+      try {
+        require('fs').appendFileSync(eventsPath, line);
+      } catch {
+        // Best-effort: a write failure here (e.g. the events dir vanished)
+        // must never crash the test run this reporter is only observing.
+      }
+    }
+  }
+  // Falls through to an implicit `return undefined` (nully): this reporter is
+  // a pure side-effecting sink (see the durability note above), never a
+  // source of reporter OUTPUT, and `stream.compose` requires its async
+  // FUNCTION body to return nully — returning an iterable (e.g. `[]`) here
+  // raises `ERR_INVALID_RETURN_VALUE` (observed live: this exact `return []`
+  // crashed every chunk on the real remote run this regresses).
+};
diff --git a/scripts/run-tests.cjs b/scripts/run-tests.cjs
index 476ad6bf0..4541f80e7 100644
--- a/scripts/run-tests.cjs
+++ b/scripts/run-tests.cjs
@@ -35,8 +35,10 @@
 // See docs/TESTING-SUITES.md for full grouping policy.
 'use strict';
 
-const { readdirSync, readFileSync } = require('fs');
+const { readdirSync, readFileSync, mkdtempSync, rmSync, unlinkSync, writeFileSync } = require('fs');
 const { join, basename } = require('path');
+const { tmpdir } = require('os');
+const { pathToFileURL } = require('url');
 const { execFileSync } = require('child_process');
 const { ExitError, runMain } = require('./lib/cli-exit.cjs');
 const {
@@ -682,6 +684,117 @@ function selectExplicitFiles(allFiles, filesValue, filesFrom) {
   return { files: [...new Set(selected)] };
 }
 
+// #3889: reads back the ndjson companion reporter's destination file for one
+// chunk and reduces it to "which file(s) were in flight" — a `test:dequeue`
+// with no matching `test:pass`/`test:fail` FOR THE SAME FILE. `test:dequeue`
+// is emitted by the RUNNER the moment it begins a spawned test-file child,
+// independent of whether anything inside that file ever completes — it is
+// the correct "in flight" signal precisely BECAUSE a hang never completes.
+// `test:start`/`test:pass`/`test:fail` are per-SUBTEST events that node:test
+// only surfaces to the parent once the child reports that subtest, which
+// happens on completion — a subtest that hangs forever inside `new
+// Promise(() => {})` never reports `test:start` either, so those three event
+// types alone can NEVER see a genuine hang (this was the root cause of the
+// feature never working: the three original event types are precisely the
+// ones a hang guarantees are never emitted). `test:start` is still tracked
+// here as a SECONDARY in-flight signal (a file can legitimately produce
+// both), never as the primary one. Tracked per FILE, not per (file, nesting,
+// testNumber) — dequeue/enqueue are file-level events with no subtest
+// identity, so file is the only key both event families share.
+//
+// Tolerant of a destination file that is missing (reporter never flushed
+// anything before the kill) or whose last line is truncated mid-write (the
+// child is SIGKILLed, not given a chance to finish a buffered write) — both
+// degrade to "no in-flight file identified" rather than throwing, since this
+// is a best-effort diagnostic layered on top of, never a precondition for,
+// the timeout it explains.
+function analyzeChunkEvents(eventsPath) {
+  let raw;
+  try {
+    raw = readFileSync(eventsPath, 'utf8');
+  } catch {
+    // The events file never existed — either GSD_RUN_TESTS_EVENTS_FILE never
+    // resolved to a writable path, or the reporter module never loaded in the
+    // child at all (the `reporter:init` marker below is the reporter's very
+    // FIRST action, before any test can run, so its absence pins the failure
+    // to reporter load/resolution, not to the tests). Distinct from "file
+    // exists but only the init marker was written" (readError=false,
+    // sawInitMarker=true, anyDequeued=false) so the diagnostic below can say
+    // explicitly WHICH of the two happened, rather than silently collapsing
+    // both into "no in-flight file identified".
+    return {
+      files: [],
+      staleMs: null,
+      sawAnyEvent: false,
+      sawInitMarker: false,
+      anyDequeued: false,
+      readError: true,
+    };
+  }
+  // Map preserves insertion order; re-`set`ting an existing key moves it to
+  // the END, so iterating this map's keys yields "most recently
+  // dequeued/started" LAST — reversed below to report most-recent-first.
+  const dequeuedAt = new Map(); // file -> last dequeue/start ts seen
+  const terminated = new Set(); // files with an observed test:pass/test:fail
+  let lastTs = null;
+  let sawInitMarker = false;
+  let sawAnyEvent = false;
+  for (const line of raw.split('\n')) {
+    if (line.trim() === '') continue;
+    let evt;
+    try {
+      evt = JSON.parse(line);
+    } catch {
+      continue; // truncated trailing line from a mid-write kill
+    }
+    if (typeof evt.ts === 'number') lastTs = evt.ts;
+    if (evt.type === 'reporter:init') {
+      sawInitMarker = true;
+      continue; // not a test event — never tracked in inFlight, never proof a test ran
+    }
+    sawAnyEvent = true;
+    if (!evt.file) continue; // defensive: every recorded type carries `file`
+    if (evt.type === 'test:dequeue' || evt.type === 'test:start') {
+      dequeuedAt.delete(evt.file); // move to most-recent position
+      dequeuedAt.set(evt.file, evt.ts);
+      terminated.delete(evt.file); // a re-dequeue (retry) puts it back in flight
+    } else if (evt.type === 'test:pass' || evt.type === 'test:fail') {
+      terminated.add(evt.file);
+    }
+    // test:enqueue is intentionally NOT a start signal here: "enqueued" means
+    // "queued to run", not "running" — test:dequeue is the runner actually
+    // picking the file up, which is the moment that matters for "in flight".
+  }
+  const files = [...dequeuedAt.keys()].filter((f) => !terminated.has(f)).reverse();
+  return {
+    files,
+    staleMs: lastTs !== null ? Date.now() - lastTs : null,
+    sawAnyEvent,
+    sawInitMarker,
+    anyDequeued: dequeuedAt.size > 0,
+    readError: false,
+  };
+}
+
+// #3889: ranks a killed chunk's files heaviest-first using the same weigher
+// the packer used to build the chunk, and flags any file the timings table
+// has no measurement for at all (as opposed to one that IS measured but
+// happens to be cheap) — an unmeasured file is an unknown quantity, not a
+// known-light one, and the table itself is advisory/stale (see the
+// loadTestTimings header), so this is presented as a hint, never a verdict.
+function rankChunkFilesByWeight(files, weightOf, timingsTable) {
+  return [...files]
+    .map((f) => ({ base: basename(f), weight: weightOf(f) }))
+    .sort((a, b) => b.weight - a.weight)
+    .map(({ base, weight }, idx) => {
+      const measured = timingsTable ? Object.hasOwn(timingsTable.timings, base) : false;
+      return `  ${idx + 1}. ${base} (weight=${weight.toFixed(2)}${
+        measured ? '' : ', UNMEASURED — absent from tests/test-timings.json (table is advisory'
+          + ' and stale; treat this file as an unknown cost, not a cheap one)'
+      })`;
+    });
+}
+
 function main() {
   const args = process.argv.slice(2);
   const parsed = parseArgs(args);
@@ -960,7 +1073,83 @@ function main() {
   const nodeMajor = Number(process.versions.node.split('.')[0]);
   const forceExit = nodeMajor >= 22 && !process.env.RUN_TESTS_NO_FORCE_EXIT;
 
-  const FIXED_OVERHEAD = process.execPath.length + '--test'.length + concurrency.length + (forceExit ? '--test-force-exit'.length + 1 : 0) + 8;
+  // #3889: a second, machine-readable reporter runs ALONGSIDE the normal
+  // human one so a chunk timeout can name the file that was in flight (see
+  // scripts/lib/ndjson-reporter.cjs for the full contract writeup, its
+  // Node-docs citation, and — load-bearing — why it writes via
+  // `fs.appendFileSync` to a path passed through GSD_RUN_TESTS_EVENTS_FILE
+  // instead of yielding strings for Node to pipe through
+  // `--test-reporter-destination`: that destination is backed by an
+  // `fs.WriteStream`, which BUFFERS, and execFileSync's timeout SIGKILLs the
+  // child — uncatchable, zero chance to flush — so a yield-based reporter can
+  // lose every event still sitting in the stream's buffer, which is exactly
+  // the case this feature exists to diagnose (confirmed live: chunk killed at
+  // 2006ms produced a timer-based "killed after 2006ms" line — which lives in
+  // this parent process — but zero usable in-flight-file events from the
+  // reporter, which lives in the child and never flushed). Passing
+  // --test-reporter at all replaces node's implicit default reporter
+  // entirely, so the human reporter must also be named explicitly,
+  // reproducing node's own default selection (spec on a TTY, tap otherwise)
+  // so visible stdout output is unchanged.
+  //
+  // Node's documented multi-reporter contract requires EVERY --test-reporter
+  // to be paired with its own --test-reporter-destination, even though the
+  // ndjson reporter yields nothing and ignores its own destination — so the
+  // pairing still needs a sink argument. Pointed at the OS null device (a
+  // FIXED-length constant, unlike the old per-chunk events path) rather than
+  // a real file: it stays empty by design, since the durable write path is
+  // now the env var below, not this destination.
+  //
+  // One tmp dir for the whole run (not per-chunk) to avoid mkdtemp churn;
+  // each chunk gets its own events file inside it so a diagnostic read never
+  // races with, or is polluted by, another chunk's events. Deleted on the
+  // success path; kept only long enough to read back on a timeout.
+  const eventsDir = mkdtempSync(join(tmpdir(), 'gsd-run-tests-events-'));
+  // #3889: Node documents `--test-reporter`'s value as "a string similar to
+  // those used in import() statements" (https://nodejs.org/api/test.html#--test-reporter),
+  // NOT a bare filesystem path — a bare absolute path is not a portable
+  // import specifier (notably on Windows, where `C:\...` is not valid
+  // import()able syntax). Converting through pathToFileURL is the fix that
+  // hardens reporter resolution against a possible root cause of #3889: the
+  // events file never being created at all because the reporter module
+  // itself never loaded in the child. This makes the resulting string
+  // LONGER than the bare path, which is why reporterOverhead/FIXED_OVERHEAD
+  // below are derived from the final reporterArgs strings, not a literal
+  // constant — they must reflect whatever this line actually produces.
+  const reporterModulePath = pathToFileURL(join(__dirname, 'lib', 'ndjson-reporter.cjs')).href;
+  const humanReporter = process.stdout.isTTY ? 'spec' : 'tap';
+  // Chunk index zero-padded to a fixed width so the events path's length is
+  // stable across chunks (kept for readability/debuggability; it no longer
+  // feeds FIXED_OVERHEAD since the path now travels via env, not argv). 3
+  // digits covers up to 999 chunks — this suite chunks into the tens, never
+  // close to that ceiling.
+  const eventsPathFor = (i) => join(eventsDir, `chunk-${String(i).padStart(3, '0')}.ndjson`);
+  // Fixed argv for every chunk: the events path moved to the environment
+  // (GSD_RUN_TESTS_EVENTS_FILE, set per-chunk below in execFileSync's `env`),
+  // which does NOT count toward the Windows 32,767-char argv ceiling — only
+  // this fixed sink destination does.
+  //
+  // The ndjson reporter yields nothing and ignores its own destination — the
+  // durable write path is GSD_RUN_TESTS_EVENTS_FILE above — but Node still
+  // opens this destination as a real fs.WriteStream and fsyncs it on close,
+  // so it MUST be a regular file. os.devNull (a character device on every
+  // platform) fails that fsync with EINVAL, crashing every chunk, not just
+  // the timeout path (confirmed live: 43/43 chunk failures, "EINVAL: invalid
+  // argument, fsync" on WriteStream close). Do not re-point this at devNull —
+  // it is the ONE destination flavor this feature cannot use. Pre-created
+  // once, alongside eventsDir, so it stays empty and its path length is
+  // identical across every chunk (see FIXED_OVERHEAD below).
+  const reporterSinkPath = join(eventsDir, 'reporter-sink.txt');
+  writeFileSync(reporterSinkPath, '');
+  const reporterArgs = [
+    `--test-reporter=${humanReporter}`,
+    '--test-reporter-destination=stdout',
+    `--test-reporter=${reporterModulePath}`,
+    `--test-reporter-destination=${reporterSinkPath}`,
+  ];
+  const reporterOverhead = reporterArgs.reduce((sum, a) => sum + a.length + 1, 0);
+
+  const FIXED_OVERHEAD = process.execPath.length + '--test'.length + concurrency.length + (forceExit ? '--test-force-exit'.length + 1 : 0) + reporterOverhead + 8;
   const chunks = packChunks(selected, {
     weightOf: fileWeightOf(),
     maxWeight: MAX_FILES_PER_CHUNK,
@@ -970,9 +1159,20 @@ function main() {
 
   // A chunk that still hangs (a leak the backstop somehow misses, or a wedged
   // subprocess) must fail loudly rather than silently burn the job's wall-clock
-  // budget until the CI runner cancels the whole job. Default 10 min per chunk:
-  // well above a healthy chunk (~4-5 min on the windows lane) but below the 20m
-  // job cap. Operator/test override via RUN_TESTS_CHUNK_TIMEOUT_MS.
+  // budget. Default 10 min per chunk. Operator/test override via
+  // RUN_TESTS_CHUNK_TIMEOUT_MS.
+  //
+  // Correction: this used to be justified as "below the 20m job cap" — that is
+  // stale. The full-test lane is now SHARDED 3x with `timeout-minutes: 45` in
+  // .github/workflows/test.yml, and that budget is asserted by
+  // tests/ci-test-job-timeout-budget.test.cjs. Consequence: the 600s
+  // per-chunk cap below is now the BINDING constraint, not the job cap.
+  // Evidence (measured 2026-08-28): windows shards ran 19m+ while every macos
+  // shard had already finished — nowhere near 45m — yet chunk 1/5 was killed
+  // at 600s on two separate runs (b351c83e0, c3e667df3), presenting as
+  // "# fail 0" with exit 1. Do not reason about a modern chunk kill using the
+  // old 20m silent-cancel model; they are different failure modes with
+  // different evidence.
   const chunkTimeoutMs = positiveNumberEnv(process.env.RUN_TESTS_CHUNK_TIMEOUT_MS, 600000);
 
   // #2665: snapshot GSD's install footprint in every LIVE runtime config dir
@@ -1003,32 +1203,107 @@ function main() {
     if (chunks.length > 1) {
       console.error(`run-tests: chunk ${i + 1}/${chunks.length} — ${chunks[i].length} files`);
     }
+    const chunkEventsPath = eventsPathFor(i);
+    const chunkStartedAt = process.hrtime.bigint();
     try {
       execFileSync(
         process.execPath,
-        ['--test', ...(forceExit ? ['--test-force-exit'] : []), concurrency, ...chunks[i]],
+        [
+          '--test',
+          ...(forceExit ? ['--test-force-exit'] : []),
+          concurrency,
+          ...reporterArgs,
+          ...chunks[i],
+        ],
         {
           stdio: 'inherit',
-          env: { ...process.env },
+          env: { ...process.env, GSD_RUN_TESTS_EVENTS_FILE: chunkEventsPath },
           timeout: chunkTimeoutMs,
         },
       );
+      const elapsedMs = Number(process.hrtime.bigint() - chunkStartedAt) / 1e6;
+      console.error(
+        `run-tests: chunk ${i + 1}/${chunks.length} completed in ${elapsedMs.toFixed(0)}ms`,
+      );
+      // Success path only (#3889): the events file has served its purpose —
+      // delete it now rather than let it accumulate across a multi-chunk run.
+      try {
+        unlinkSync(chunkEventsPath);
+      } catch {
+        // Best-effort; a missing/already-gone file is not an error here.
+      }
     } catch (err) {
+      const elapsedMs = Number(process.hrtime.bigint() - chunkStartedAt) / 1e6;
       // When the per-chunk timeout fires, execFileSync kills the child and
       // surfaces it as err.code === 'ETIMEDOUT' (POSIX) and/or err.killed === true
       // (platform-dependent). Check both so detection holds on Windows and POSIX.
       const timedOut = err.killed === true || err.code === 'ETIMEDOUT';
+      console.error(
+        `run-tests: chunk ${i + 1}/${chunks.length} ${timedOut ? 'was killed' : 'failed'} ` +
+          `after ${elapsedMs.toFixed(0)}ms`,
+      );
       if (timedOut) {
+        // #3889: name the file(s) in flight when the kill fired, using the
+        // ndjson companion reporter's destination file (stdio:'inherit' means
+        // this parent never saw the child's own stdout, so it cannot know
+        // otherwise). Falls back to "no file identified" rather than
+        // throwing when the reporter file is missing/empty/truncated.
+        const {
+          files: inFlightFiles,
+          staleMs,
+          sawInitMarker,
+          anyDequeued,
+          readError,
+        } = analyzeChunkEvents(chunkEventsPath);
+        const inFlightMsg = inFlightFiles.length > 0
+          ? `In flight when killed (test:dequeue with no matching pass/fail): ` +
+            `${inFlightFiles.map((f) => basename(f)).join(', ')} — last reporter event was ` +
+            `${staleMs !== null ? `${staleMs}ms` : 'an unknown time'} before this diagnostic ` +
+            `(small = output kept flowing until the kill = slow; large = it stopped early = hang).`
+          : anyDequeued
+            ? `No file was in flight when killed — every file the runner dequeued in this ` +
+              `chunk already terminated (test:pass/test:fail seen for each), so the CHILD ` +
+              `PROCESS itself hung after its last test finished (last reporter event was ` +
+              `${staleMs !== null ? `${staleMs}ms` : 'an unknown time'} before this ` +
+              `diagnostic); suspect a leaked handle outside any single test, or an ` +
+              `after-tests hook.`
+            : readError
+              ? `THE EVENTS FILE DOES NOT EXIST for this chunk — not even the reporter's own ` +
+                `\`reporter:init\` marker, which is the reporter module's first action before ` +
+                `it reads a single test event. Two possible causes, NOT distinguished by this ` +
+                `diagnostic: the ndjson reporter module never loaded in the child at all ` +
+                `(--test-reporter resolution failure), or the child was killed before the ` +
+                `reporter function was ever invoked (process/spawn startup stall). This ` +
+                `diagnostic could not identify an in-flight file.`
+              : sawInitMarker
+                ? `THE REPORTER LOADED BUT THE RUNNER NEVER DEQUEUED A SINGLE FILE — the events ` +
+                  `file contains only the reporter's own \`reporter:init\` marker (and possibly ` +
+                  `\`test:enqueue\` events with no matching \`test:dequeue\`), so the reporter ` +
+                  `module was invoked and ran, but node's test runner never began executing any ` +
+                  `file in this chunk before the kill. This is a genuinely surprising state — ` +
+                  `\`test:dequeue\` fires the instant the runner starts a file, independent of ` +
+                  `whether anything inside it ever completes. Two possible causes, NOT ` +
+                  `distinguished by this diagnostic: node --test itself stalled before ` +
+                  `dispatching any test file, or process/spawn startup stalled. This diagnostic ` +
+                  `could not identify an in-flight file.`
+                : `No reporter events were recorded before the kill — the companion reporter's ` +
+                  `events file exists but is empty/unparseable (no \`reporter:init\` marker and ` +
+                  `no test events), so even the reporter's first appendFileSync may not have ` +
+                  `completed. This diagnostic could not identify an in-flight file.`;
+
+        const table = loadTestTimings(process.env.RUN_TESTS_TIMINGS_FILE || DEFAULT_TIMINGS_PATH);
+        const ranked = rankChunkFilesByWeight(chunks[i], fileWeightOf(), table).join('\n');
+
         console.error(
           `run-tests: chunk ${i + 1}/${chunks.length} exceeded the per-chunk timeout ` +
             `of ${chunkTimeoutMs}ms and was killed. Two possible causes: (1) a test leaks ` +
             `an open handle (un-terminated Worker, un-killed child process, or ref'd timer) ` +
             `so node --test never exits — but --test-force-exit already guards that, so if it ` +
             `is enabled suspect (2) the chunk is legitimately too slow for the budget (too ` +
-            `many/too-heavy files packed together). Check whether output kept flowing until ` +
-            `the kill (slow) vs stopped early (hang) before assuming a leak. Files: ${chunks[i]
-              .map(f => f.split(/[\\/]/).pop())
-              .join(' ')}`,
+            `many/too-heavy files packed together).\n${inFlightMsg}\n` +
+            `Files in this chunk, heaviest-first by measured weight ` +
+            `(table last regenerated 2026-08-07; real Windows cost runs ~2.2x the recorded ` +
+            `figure, so treat every number as a floor):\n${ranked}`,
         );
       }
       const code = err.status || 1;
@@ -1063,6 +1338,17 @@ function main() {
       // and the first non-zero exit is reported at the end.
     }
   }
+  // #3889: sweep any events file the per-chunk success path didn't already
+  // delete (a timeout diagnostic read one but left it on disk; an aborted
+  // run may leave more that were never opened). All diagnostics that needed
+  // these files have already been printed above, so this is unconditional —
+  // best-effort, since a failure to remove a tmp dir must never mask the
+  // real chunk-loop exit code.
+  try {
+    rmSync(eventsDir, { recursive: true, force: true });
+  } catch {
+    // Non-fatal: an orphaned OS tmp dir is a cosmetic leak, not a test result.
+  }
   // #2665: post-suite hermeticity check. Runs even when tests failed — a leaked
   // global install is worth reporting alongside the failure that hid it, and
   // suppressing it on red would hide it exactly when the suite is least trusted.
@@ -1098,6 +1384,7 @@ module.exports = {
   loadTestTimings,
   makeFileWeigher,
   packChunks,
+  analyzeChunkEvents,
   DEFAULT_TIMINGS_PATH,
   // Exported so callers (tests/ci-test-scope.test.cjs) can assert the
   // suite-token resolution contract in-process rather than through a timed
diff --git a/tests/fixtures/install-tree/antigravity.json b/tests/fixtures/install-tree/antigravity.json
index ff646f413..0b6ec95e5 100644
--- a/tests/fixtures/install-tree/antigravity.json
+++ b/tests/fixtures/install-tree/antigravity.json
@@ -433,6 +433,7 @@
   "scripts/lib/cli-exit.cjs",
   "scripts/lib/drift-scan.cjs",
   "scripts/lib/exit-code-registry.cjs",
+  "scripts/lib/ndjson-reporter.cjs",
   "skills/gsd-add-tests/SKILL.md",
   "skills/gsd-ai-integration-phase/SKILL.md",
   "skills/gsd-audit-fix/SKILL.md",
diff --git a/tests/fixtures/install-tree/augment.json b/tests/fixtures/install-tree/augment.json
index 92e060da9..33ef834fe 100644
--- a/tests/fixtures/install-tree/augment.json
+++ b/tests/fixtures/install-tree/augment.json
@@ -503,6 +503,7 @@
   "scripts/lib/cli-exit.cjs",
   "scripts/lib/drift-scan.cjs",
   "scripts/lib/exit-code-registry.cjs",
+  "scripts/lib/ndjson-reporter.cjs",
   "skills/gsd-ns-context/SKILL.md",
   "skills/gsd-ns-context/skills/docs-update/SKILL.md",
   "skills/gsd-ns-context/skills/extract-learnings/SKILL.md",
diff --git a/tests/fixtures/install-tree/claude-local.json b/tests/fixtures/install-tree/claude-local.json
index 23e4ffa1b..d180006de 100644
--- a/tests/fixtures/install-tree/claude-local.json
+++ b/tests/fixtures/install-tree/claude-local.json
@@ -502,5 +502,6 @@
   "scripts/lib/allowlist-ratchet.cjs",
   "scripts/lib/cli-exit.cjs",
   "scripts/lib/drift-scan.cjs",
-  "scripts/lib/exit-code-registry.cjs"
+  "scripts/lib/exit-code-registry.cjs",
+  "scripts/lib/ndjson-reporter.cjs"
 ]
diff --git a/tests/fixtures/install-tree/claude.json b/tests/fixtures/install-tree/claude.json
index 72ac7de4d..e4d4d20c8 100644
--- a/tests/fixtures/install-tree/claude.json
+++ b/tests/fixtures/install-tree/claude.json
@@ -432,6 +432,7 @@
   "scripts/lib/cli-exit.cjs",
   "scripts/lib/drift-scan.cjs",
   "scripts/lib/exit-code-registry.cjs",
+  "scripts/lib/ndjson-reporter.cjs",
   "skills/gsd-add-tests/SKILL.md",
   "skills/gsd-ai-integration-phase/SKILL.md",
   "skills/gsd-audit-fix/SKILL.md",
diff --git a/tests/fixtures/install-tree/cline.json b/tests/fixtures/install-tree/cline.json
index e644569c7..9061cad27 100644
--- a/tests/fixtures/install-tree/cline.json
+++ b/tests/fixtures/install-tree/cline.json
@@ -395,6 +395,7 @@
   "scripts/lib/cli-exit.cjs",
   "scripts/lib/drift-scan.cjs",
   "scripts/lib/exit-code-registry.cjs",
+  "scripts/lib/ndjson-reporter.cjs",
   "skills/gsd-ns-context/SKILL.md",
   "skills/gsd-ns-context/skills/docs-update/SKILL.md",
   "skills/gsd-ns-context/skills/extract-learnings/SKILL.md",
diff --git a/tests/fixtures/install-tree/codebuddy.json b/tests/fixtures/install-tree/codebuddy.json
index 451de0a20..80033205e 100644
--- a/tests/fixtures/install-tree/codebuddy.json
+++ b/tests/fixtures/install-tree/codebuddy.json
@@ -503,6 +503,7 @@
   "scripts/lib/cli-exit.cjs",
   "scripts/lib/drift-scan.cjs",
   "scripts/lib/exit-code-registry.cjs",
+  "scripts/lib/ndjson-reporter.cjs",
   "skills/gsd-add-tests/SKILL.md",
   "skills/gsd-ai-integration-phase/SKILL.md",
   "skills/gsd-audit-fix/SKILL.md",
diff --git a/tests/fixtures/install-tree/codex.json b/tests/fixtures/install-tree/codex.json
index 1a9deb7fc..33d634a39 100644
--- a/tests/fixtures/install-tree/codex.json
+++ b/tests/fixtures/install-tree/codex.json
@@ -433,5 +433,6 @@
   "scripts/lib/allowlist-ratchet.cjs",
   "scripts/lib/cli-exit.cjs",
   "scripts/lib/drift-scan.cjs",
-  "scripts/lib/exit-code-registry.cjs"
+  "scripts/lib/exit-code-registry.cjs",
+  "scripts/lib/ndjson-reporter.cjs"
 ]
diff --git a/tests/fixtures/install-tree/copilot.json b/tests/fixtures/install-tree/copilot.json
index 9a5a81d2d..a2dfc0a36 100644
--- a/tests/fixtures/install-tree/copilot.json
+++ b/tests/fixtures/install-tree/copilot.json
@@ -395,6 +395,7 @@
   "scripts/lib/cli-exit.cjs",
   "scripts/lib/drift-scan.cjs",
   "scripts/lib/exit-code-registry.cjs",
+  "scripts/lib/ndjson-reporter.cjs",
   "skills/gsd-add-tests/SKILL.md",
   "skills/gsd-ai-integration-phase/SKILL.md",
   "skills/gsd-audit-fix/SKILL.md",
diff --git a/tests/fixtures/install-tree/cursor.json b/tests/fixtures/install-tree/cursor.json
index 04021fb09..5f4870a96 100644
--- a/tests/fixtures/install-tree/cursor.json
+++ b/tests/fixtures/install-tree/cursor.json
@@ -406,6 +406,7 @@
   "scripts/lib/cli-exit.cjs",
   "scripts/lib/drift-scan.cjs",
   "scripts/lib/exit-code-registry.cjs",
+  "scripts/lib/ndjson-reporter.cjs",
   "skills/gsd-add-tests/SKILL.md",
   "skills/gsd-ai-integration-phase/SKILL.md",
   "skills/gsd-audit-fix/SKILL.md",
diff --git a/tests/fixtures/install-tree/hermes.json b/tests/fixtures/install-tree/hermes.json
index 9dbaaf556..9243a9537 100644
--- a/tests/fixtures/install-tree/hermes.json
+++ b/tests/fixtures/install-tree/hermes.json
@@ -432,6 +432,7 @@
   "scripts/lib/cli-exit.cjs",
   "scripts/lib/drift-scan.cjs",
   "scripts/lib/exit-code-registry.cjs",
+  "scripts/lib/ndjson-reporter.cjs",
   "skills/gsd/DESCRIPTION.md",
   "skills/gsd/gsd-ns-context/SKILL.md",
   "skills/gsd/gsd-ns-context/skills/docs-update/SKILL.md",
diff --git a/tests/fixtures/install-tree/kilo.json b/tests/fixtures/install-tree/kilo.json
index 5dba68c68..d447453d7 100644
--- a/tests/fixtures/install-tree/kilo.json
+++ b/tests/fixtures/install-tree/kilo.json
@@ -506,6 +506,7 @@
   "scripts/lib/cli-exit.cjs",
   "scripts/lib/drift-scan.cjs",
   "scripts/lib/exit-code-registry.cjs",
+  "scripts/lib/ndjson-reporter.cjs",
   "skills/gsd-add-tests/SKILL.md",
   "skills/gsd-ai-integration-phase/SKILL.md",
   "skills/gsd-audit-fix/SKILL.md",
diff --git a/tests/fixtures/install-tree/kimi-code.json b/tests/fixtures/install-tree/kimi-code.json
index 9cc5918ec..9cd3a0c33 100644
--- a/tests/fixtures/install-tree/kimi-code.json
+++ b/tests/fixtures/install-tree/kimi-code.json
@@ -433,6 +433,7 @@
   "scripts/lib/cli-exit.cjs",
   "scripts/lib/drift-scan.cjs",
   "scripts/lib/exit-code-registry.cjs",
+  "scripts/lib/ndjson-reporter.cjs",
   "skills/gsd-add-tests/SKILL.md",
   "skills/gsd-ai-integration-phase/SKILL.md",
   "skills/gsd-audit-fix/SKILL.md",
diff --git a/tests/fixtures/install-tree/kimi.json b/tests/fixtures/install-tree/kimi.json
index 12301f844..8de8044da 100644
--- a/tests/fixtures/install-tree/kimi.json
+++ b/tests/fixtures/install-tree/kimi.json
@@ -430,6 +430,7 @@
   "scripts/lib/cli-exit.cjs",
   "scripts/lib/drift-scan.cjs",
   "scripts/lib/exit-code-registry.cjs",
+  "scripts/lib/ndjson-reporter.cjs",
   "skills/gsd-add-tests/SKILL.md",
   "skills/gsd-ai-integration-phase/SKILL.md",
   "skills/gsd-audit-fix/SKILL.md",
diff --git a/tests/fixtures/install-tree/opencode.json b/tests/fixtures/install-tree/opencode.json
index 4a06d99f2..b49094e86 100644
--- a/tests/fixtures/install-tree/opencode.json
+++ b/tests/fixtures/install-tree/opencode.json
@@ -506,6 +506,7 @@
   "scripts/lib/cli-exit.cjs",
   "scripts/lib/drift-scan.cjs",
   "scripts/lib/exit-code-registry.cjs",
+  "scripts/lib/ndjson-reporter.cjs",
   "skills/gsd-add-tests/SKILL.md",
   "skills/gsd-ai-integration-phase/SKILL.md",
   "skills/gsd-audit-fix/SKILL.md",
diff --git a/tests/fixtures/install-tree/pi.json b/tests/fixtures/install-tree/pi.json
index b8334ecc4..a178f40e4 100644
--- a/tests/fixtures/install-tree/pi.json
+++ b/tests/fixtures/install-tree/pi.json
@@ -398,5 +398,6 @@
   "scripts/lib/allowlist-ratchet.cjs",
   "scripts/lib/cli-exit.cjs",
   "scripts/lib/drift-scan.cjs",
-  "scripts/lib/exit-code-registry.cjs"
+  "scripts/lib/exit-code-registry.cjs",
+  "scripts/lib/ndjson-reporter.cjs"
 ]
diff --git a/tests/fixtures/install-tree/qwen.json b/tests/fixtures/install-tree/qwen.json
index 10c88e76d..323ed882c 100644
--- a/tests/fixtures/install-tree/qwen.json
+++ b/tests/fixtures/install-tree/qwen.json
@@ -432,6 +432,7 @@
   "scripts/lib/cli-exit.cjs",
   "scripts/lib/drift-scan.cjs",
   "scripts/lib/exit-code-registry.cjs",
+  "scripts/lib/ndjson-reporter.cjs",
   "skills/gsd-ns-context/SKILL.md",
   "skills/gsd-ns-context/skills/docs-update/SKILL.md",
   "skills/gsd-ns-context/skills/extract-learnings/SKILL.md",
diff --git a/tests/fixtures/install-tree/trae.json b/tests/fixtures/install-tree/trae.json
index 40e159155..027c10f81 100644
--- a/tests/fixtures/install-tree/trae.json
+++ b/tests/fixtures/install-tree/trae.json
@@ -393,6 +393,7 @@
   "scripts/lib/cli-exit.cjs",
   "scripts/lib/drift-scan.cjs",
   "scripts/lib/exit-code-registry.cjs",
+  "scripts/lib/ndjson-reporter.cjs",
   "skills/gsd-ns-context/SKILL.md",
   "skills/gsd-ns-context/skills/docs-update/SKILL.md",
   "skills/gsd-ns-context/skills/extract-learnings/SKILL.md",
diff --git a/tests/fixtures/install-tree/windsurf.json b/tests/fixtures/install-tree/windsurf.json
index 5cdaa1497..a6716c4e1 100644
--- a/tests/fixtures/install-tree/windsurf.json
+++ b/tests/fixtures/install-tree/windsurf.json
@@ -395,5 +395,6 @@
   "scripts/lib/allowlist-ratchet.cjs",
   "scripts/lib/cli-exit.cjs",
   "scripts/lib/drift-scan.cjs",
-  "scripts/lib/exit-code-registry.cjs"
+  "scripts/lib/exit-code-registry.cjs",
+  "scripts/lib/ndjson-reporter.cjs"
 ]
diff --git a/tests/fixtures/install-tree/zcode.json b/tests/fixtures/install-tree/zcode.json
index bc0bb4d56..943441131 100644
--- a/tests/fixtures/install-tree/zcode.json
+++ b/tests/fixtures/install-tree/zcode.json
@@ -464,6 +464,7 @@
   "scripts/lib/cli-exit.cjs",
   "scripts/lib/drift-scan.cjs",
   "scripts/lib/exit-code-registry.cjs",
+  "scripts/lib/ndjson-reporter.cjs",
   "skills/gsd-ns-context/SKILL.md",
   "skills/gsd-ns-context/skills/docs-update/SKILL.md",
   "skills/gsd-ns-context/skills/extract-learnings/SKILL.md",
diff --git a/tests/run-tests-harness.test.cjs b/tests/run-tests-harness.test.cjs
index 100886070..aeadca169 100644
--- a/tests/run-tests-harness.test.cjs
+++ b/tests/run-tests-harness.test.cjs
@@ -16,7 +16,7 @@
 
 'use strict';
 
-const { describe, test, beforeEach, afterEach } = require('node:test');
+const { describe, test, before, after, beforeEach, afterEach } = require('node:test');
 const assert = require('node:assert/strict');
 const fs = require('fs');
 const path = require('path');
@@ -24,6 +24,7 @@ const path = require('path');
 const { runNode } = require('./helpers/process-seam.cjs');
 const { toLegacyResult } = require('./helpers/git-fixture.cjs');
 const { createTempDir, cleanup, CONFIG_LOCATION_ENV_KEYS } = require('./helpers.cjs');
+const { splitLines } = require('../gsd-core/bin/lib/text-lines.cjs');
 
 const HARNESS = path.join(__dirname, '..', 'scripts', 'run-tests.cjs');
 
@@ -814,6 +815,274 @@ test('noop', () => {});
       );
     });
   });
+
+  describe('chunk-timeout instrumentation (#3889)', () => {
+    // Consolidation (CI cost, #4015): this describe block used to spawn the
+    // harness once PER assertion group (3 success-path runs + 2 timeout-path
+    // runs = 5 subprocess boots). Each boot is expensive — run-tests.cjs
+    // starts, globs the suite, then spawns `node --test` children — so on a
+    // CI shard already within ~39s of its 15-minute cap, paying for 5 boots
+    // to check facts that all hold against the SAME run is wasted spend.
+    // Below, exactly ONE successful run and ONE timed-out run are captured
+    // once (via `before`) and every assertion group below reads from those
+    // captured results instead of spawning its own. This is the whole
+    // savings — no assertion is weakened or removed.
+    const HANGS_FOREVER_BODY = `'use strict';
+const { test } = require('node:test');
+test('hangs forever', () => new Promise(() => {}));
+`;
+
+    let successDir;
+    let successRun;
+    let eventsDirBefore;
+    let eventsDirAfter;
+    let timeoutDir;
+    let timeoutRun;
+
+    before(() => {
+      // Single SUCCESS-path run, reused by T2/T3/T5 below.
+      successDir = createTempDir('gsd-3889-success-');
+      seed(successDir, ['a.test.cjs']);
+      eventsDirBefore = fs.readdirSync(require('os').tmpdir())
+        .filter((n) => n.startsWith('gsd-run-tests-events-'));
+      successRun = runHarness(successDir, []);
+      eventsDirAfter = fs.readdirSync(require('os').tmpdir())
+        .filter((n) => n.startsWith('gsd-run-tests-events-'));
+
+      // Single TIMEOUT-path run, reused by T1/T4 below. 2000ms is kept —
+      // it is already the smallest value this suite used anywhere for the
+      // per-chunk timeout, and going lower risks flaking on a loaded CI
+      // box that has to boot node --test, register the hang, and observe
+      // the kill inside the window.
+      timeoutDir = createTempDir('gsd-3889-timeout-');
+      fs.writeFileSync(path.join(timeoutDir, 'hangs.test.cjs'), HANGS_FOREVER_BODY, 'utf8');
+      timeoutRun = runHarness(timeoutDir, [], {
+        RUN_TESTS_NO_FORCE_EXIT: '1',
+        RUN_TESTS_CHUNK_TIMEOUT_MS: '2000',
+      });
+    });
+
+    after(() => {
+      cleanup(successDir);
+      cleanup(timeoutDir);
+    });
+
+    // T2: per-chunk elapsed timing appears on the normal SUCCESS path, not
+    // only when something goes wrong — this is what makes "which chunk is
+    // drifting toward the cap" readable across ordinary green runs.
+    test('a successful chunk prints its elapsed time', () => {
+      assert.strictEqual(successRun.status, 0, `expected a clean pass; STDERR:\n${successRun.stderr}`);
+      assert.match(
+        successRun.stderr,
+        /run-tests: chunk 1\/1 completed in \d+ms/,
+        `expected a per-chunk completion timing line; STDERR:\n${successRun.stderr}`,
+      );
+    });
+
+    // T3: the ndjson companion reporter's destination file is a temp
+    // artifact of the instrumentation, not a product output — it must not
+    // survive a successful run. Assert against the OS temp root's own
+    // "gsd-run-tests-events-*" prefix (scripts/run-tests.cjs's mkdtemp
+    // prefix) rather than any run-tests-owned directory, since that IS the
+    // leak surface being guarded.
+    test('the ndjson reporter temp dir is cleaned up after a successful run', () => {
+      assert.strictEqual(successRun.status, 0, `expected a clean pass; STDERR:\n${successRun.stderr}`);
+      assert.deepStrictEqual(
+        eventsDirAfter,
+        eventsDirBefore,
+        `expected no leaked gsd-run-tests-events-* temp dir after a successful run; ` +
+          `before=${JSON.stringify(eventsDirBefore)} after=${JSON.stringify(eventsDirAfter)}`,
+      );
+    });
+
+    // T5 (regression): the human reporter's --test-reporter-destination
+    // pairing must be a regular file, not os.devNull. devNull is a character
+    // device; Node opens the reporter destination as an fs.WriteStream and
+    // fsyncs it on close, and fsync on a character device fails with EINVAL
+    // — surfaced as "Emitted 'error' event on WriteStream instance" /
+    // "EINVAL: invalid argument, fsync", which crashed EVERY chunk on the
+    // real remote run this regresses (43/43 failures), not only the timeout
+    // path. The argv construction lives entirely inside main() with no
+    // exported seam to unit-test directly (see NOTES), so this asserts the
+    // closest real, externally-observable consequence: a normal successful
+    // run must not surface that error text, and must still complete and
+    // exit 0 — both of which a reintroduced devNull destination would break
+    // on any platform where fsync(devNull) actually returns EINVAL (this
+    // suite's own bench platform, historically).
+    test('a successful run never surfaces the devNull fsync/EINVAL reporter crash', () => {
+      assert.strictEqual(successRun.status, 0, `expected a clean pass; STDERR:\n${successRun.stderr}`);
+      assert.doesNotMatch(
+        successRun.stderr,
+        /EINVAL|invalid argument, fsync|WriteStream instance/i,
+        `expected no reporter-destination fsync crash; STDERR:\n${successRun.stderr}`,
+      );
+    });
+
+    // T1: on a chunk timeout, the diagnostic must NAME the file that was
+    // still executing — not merely list every file the chunk contained (the
+    // pre-instrumentation behavior). A test that hangs INSIDE its own body
+    // (never resolving) keeps its test:start event unmatched by any
+    // test:pass/test:fail in the ndjson companion reporter's output, which
+    // is exactly the signal the diagnostic reads back on timeout.
+    test('a chunk timeout names the file that was in flight when killed', () => {
+      assert.notStrictEqual(
+        timeoutRun.status,
+        0,
+        `expected non-zero exit from a timed-out chunk; got status=${timeoutRun.status}\nSTDERR:\n${timeoutRun.stderr}`,
+      );
+      assert.match(
+        timeoutRun.stderr,
+        /In flight when killed.*hangs\.test\.cjs/s,
+        `expected the diagnostic to NAME the in-flight file, not just list the chunk; STDERR:\n${timeoutRun.stderr}`,
+      );
+    });
+
+    // Regression (#3889 recurrence): the reporter module itself, called
+    // directly with no subprocess, must return nully — this is the exact
+    // contract violation (`return []`) that crashed every chunk on the real
+    // remote run this file regresses ("Expected nully to be returned from
+    // the 'body' function but got an instance of Array", thrown by
+    // node:stream's `compose` when its async-function body returns an
+    // iterable instead of undefined/null). Also pins the NDJSON side effect:
+    // only the two handled event types are appended, verbatim, one per line.
+    test('the reporter returns nully and appends only the handled event types as NDJSON', async () => {
+      const reporter = require('../scripts/lib/ndjson-reporter.cjs');
+      const eventsFile = path.join(tmpDir, 'ndjson-reporter-events.ndjson');
+      const savedEventsFile = process.env.GSD_RUN_TESTS_EVENTS_FILE;
+      process.env.GSD_RUN_TESTS_EVENTS_FILE = eventsFile;
+      try {
+        async function* fakeEvents() {
+          yield { type: 'test:start', data: { file: 'a.test.cjs', name: 't', nesting: 0, testNumber: 1 } };
+          yield { type: 'test:diagnostic', data: { message: 'ignored' } };
+          yield { type: 'test:pass', data: { file: 'a.test.cjs', name: 't', nesting: 0, testNumber: 1 } };
+        }
+        const result = await reporter(fakeEvents());
+        assert.strictEqual(
+          result ?? null,
+          null,
+          `expected the reporter to return nully (undefined/null) per stream.compose's ` +
+            `async-function body contract; got ${JSON.stringify(result)}`,
+        );
+        const rawContent = fs.readFileSync(eventsFile, 'utf8');
+        const lines = splitLines(rawContent.trim()).filter((l) => l.length > 0);
+        assert.strictEqual(lines.length, 3, `expected exactly 3 NDJSON lines (init marker + 2 handled events); got:\n${lines.join('\n')}`);
+        const [init, start, pass] = lines.map((l) => JSON.parse(l));
+        assert.strictEqual(init.type, 'reporter:init');
+        assert.strictEqual(start.type, 'test:start');
+        assert.strictEqual(start.file, 'a.test.cjs');
+        assert.strictEqual(pass.type, 'test:pass');
+        assert.strictEqual(pass.file, 'a.test.cjs');
+      } finally {
+        if (savedEventsFile === undefined) {
+          delete process.env.GSD_RUN_TESTS_EVENTS_FILE;
+        } else {
+          process.env.GSD_RUN_TESTS_EVENTS_FILE = savedEventsFile;
+        }
+        cleanup(eventsFile);
+      }
+    });
+
+    // Regression (#3889 root cause): a hang inside a test body NEVER produces
+    // a `test:start`/`test:pass`/`test:fail` for that subtest (node:test only
+    // surfaces those to the parent once the child reports completion), so
+    // those three event types alone can never see a hang. `test:enqueue` and
+    // `test:dequeue` are emitted by the RUNNER as it queues/begins a file,
+    // independent of completion — this pins that the reporter now records
+    // both, verbatim, for exactly the "enqueue then dequeue, then nothing"
+    // shape a real hang produces.
+    test('the reporter records test:enqueue and test:dequeue for the hang shape (enqueue, dequeue, nothing else)', async () => {
+      const reporter = require('../scripts/lib/ndjson-reporter.cjs');
+      const eventsFile = path.join(tmpDir, 'ndjson-reporter-hang-shape.ndjson');
+      const savedEventsFile = process.env.GSD_RUN_TESTS_EVENTS_FILE;
+      process.env.GSD_RUN_TESTS_EVENTS_FILE = eventsFile;
+      try {
+        async function* hangShapeEvents() {
+          yield { type: 'test:enqueue', data: { file: 'hangs.test.cjs', name: 'hangs.test.cjs', nesting: 0 } };
+          yield { type: 'test:dequeue', data: { file: 'hangs.test.cjs', name: 'hangs.test.cjs', nesting: 0 } };
+          // Never yields test:start/test:pass/test:fail — this IS the hang.
+        }
+        const result = await reporter(hangShapeEvents());
+        assert.strictEqual(result ?? null, null);
+        const rawContent = fs.readFileSync(eventsFile, 'utf8');
+        const lines = splitLines(rawContent.trim()).filter((l) => l.length > 0);
+        assert.strictEqual(
+          lines.length,
+          3,
+          `expected exactly 3 NDJSON lines (init marker + enqueue + dequeue); got:\n${lines.join('\n')}`,
+        );
+        const [init, enqueue, dequeue] = lines.map((l) => JSON.parse(l));
+        assert.strictEqual(init.type, 'reporter:init');
+        assert.strictEqual(enqueue.type, 'test:enqueue');
+        assert.strictEqual(enqueue.file, 'hangs.test.cjs');
+        assert.strictEqual(dequeue.type, 'test:dequeue');
+        assert.strictEqual(dequeue.file, 'hangs.test.cjs');
+      } finally {
+        if (savedEventsFile === undefined) {
+          delete process.env.GSD_RUN_TESTS_EVENTS_FILE;
+        } else {
+          process.env.GSD_RUN_TESTS_EVENTS_FILE = savedEventsFile;
+        }
+        cleanup(eventsFile);
+      }
+    });
+
+    // #3889: the init marker is the reporter's FIRST action, written before
+    // the `for await` loop even begins — so it must land even when the
+    // source event stream yields ZERO events (e.g. the child is killed
+    // before node:test emits anything). This pins the marker's whole
+    // purpose: its presence alone proves the reporter module loaded and was
+    // invoked, independent of whether any test ever started.
+    test('the reporter writes only the init marker when the source yields zero events', async () => {
+      const reporter = require('../scripts/lib/ndjson-reporter.cjs');
+      const eventsFile = path.join(tmpDir, 'ndjson-reporter-init-only.ndjson');
+      const savedEventsFile = process.env.GSD_RUN_TESTS_EVENTS_FILE;
+      process.env.GSD_RUN_TESTS_EVENTS_FILE = eventsFile;
+      try {
+        async function* emptyEvents() {}
+        const result = await reporter(emptyEvents());
+        assert.strictEqual(
+          result ?? null,
+          null,
+          `expected the reporter to return nully even with zero source events; got ${JSON.stringify(result)}`,
+        );
+        const rawContent = fs.readFileSync(eventsFile, 'utf8');
+        const lines = splitLines(rawContent.trim()).filter((l) => l.length > 0);
+        assert.strictEqual(
+          lines.length,
+          1,
+          `expected exactly 1 NDJSON line (the init marker only); got:\n${lines.join('\n')}`,
+        );
+        const [init] = lines.map((l) => JSON.parse(l));
+        assert.strictEqual(init.type, 'reporter:init');
+        assert.strictEqual(typeof init.ts, 'number');
+      } finally {
+        if (savedEventsFile === undefined) {
+          delete process.env.GSD_RUN_TESTS_EVENTS_FILE;
+        } else {
+          process.env.GSD_RUN_TESTS_EVENTS_FILE = savedEventsFile;
+        }
+        cleanup(eventsFile);
+      }
+    });
+
+    // T4: the pre-existing timeout / abort / force-exit behavior (#1051)
+    // still holds with the reporter instrumentation wired in — the new
+    // --test-reporter flags must not change detection, the abort-on-timeout
+    // control flow, or the exit code.
+    test('existing timeout diagnostic and abort behavior are unchanged', () => {
+      assert.notStrictEqual(timeoutRun.status, 0, `expected non-zero exit; STDERR:\n${timeoutRun.stderr}`);
+      assert.match(
+        timeoutRun.stderr,
+        /exceeded the per-chunk timeout/,
+        `expected the original timeout diagnostic wording to survive; STDERR:\n${timeoutRun.stderr}`,
+      );
+      assert.match(
+        timeoutRun.stderr,
+        /run-tests: chunk 1\/1 was killed after \d+ms/,
+        `expected the new killed/elapsed line; STDERR:\n${timeoutRun.stderr}`,
+      );
+    });
+  });
 });
 
 // Pure partition contract for the shard selector (#1212). Imported directly
@@ -2139,3 +2408,131 @@ describe('chunk packing weights measured cost (#2456)', () => {
     });
   });
 });
+
+// ─── analyzeChunkEvents (#3889 durability fix) ──────────────────────────────
+//
+// scripts/lib/ndjson-reporter.cjs now writes durably (fs.appendFileSync to a
+// GSD_RUN_TESTS_EVENTS_FILE path) instead of yielding strings for Node to
+// pipe through a buffered --test-reporter-destination WriteStream that a
+// SIGKILL can wipe out before it flushes. These tests exercise the READER
+// side (analyzeChunkEvents) directly against a hand-written events file, so
+// they pin the reader's contract independently of whether the writer side
+// managed to flush anything in a given subprocess run.
+const { analyzeChunkEvents } = require('../scripts/run-tests.cjs');
+
+describe('analyzeChunkEvents (#3889)', () => {
+  let tmpDir;
+
+  beforeEach(() => {
+    tmpDir = createTempDir('gsd-3889-events-');
+  });
+
+  afterEach(() => {
+    cleanup(tmpDir);
+  });
+
+  test('a missing events file is reported as an explicit read error, not silently as "no events"', () => {
+    const missingPath = path.join(tmpDir, 'does-not-exist.ndjson');
+    const result = analyzeChunkEvents(missingPath);
+    assert.strictEqual(result.readError, true, 'a missing file must set readError=true');
+    assert.strictEqual(result.sawAnyEvent, false);
+    assert.deepStrictEqual(result.files, []);
+  });
+
+  test('an existing-but-empty events file is distinguished from a missing one (readError=false)', () => {
+    const emptyPath = path.join(tmpDir, 'empty.ndjson');
+    fs.writeFileSync(emptyPath, '', 'utf8');
+    const result = analyzeChunkEvents(emptyPath);
+    assert.strictEqual(result.readError, false, 'an existing empty file must NOT be reported as a read error');
+    assert.strictEqual(result.sawAnyEvent, false);
+    assert.deepStrictEqual(result.files, []);
+  });
+
+  test('a truncated final line does not crash the reader and earlier complete lines are still reported', () => {
+    const truncatedPath = path.join(tmpDir, 'truncated.ndjson');
+    const complete = [
+      JSON.stringify({ type: 'test:start', file: 'a.test.cjs', name: 'first', nesting: 0, testNumber: 1, ts: 1000 }),
+      JSON.stringify({ type: 'test:pass', file: 'a.test.cjs', name: 'first', nesting: 0, testNumber: 1, ts: 1010 }),
+      JSON.stringify({ type: 'test:start', file: 'b.test.cjs', name: 'hangs', nesting: 0, testNumber: 1, ts: 1020 }),
+    ].join('\n');
+    // Simulate a SIGKILL mid-appendFileSync: the trailing line is cut off
+    // partway through a JSON object, exactly as an unbuffered but non-atomic
+    // write can be interrupted.
+    const truncatedTrailer = '\n{"type":"test:start","file":"c.test.cjs","name":"cut off mid-writ';
+    fs.writeFileSync(truncatedPath, complete + truncatedTrailer, 'utf8');
+
+    const result = analyzeChunkEvents(truncatedPath);
+    assert.strictEqual(result.readError, false, 'a readable-but-truncated file must not be a read error');
+    // The complete test:start (b.test.cjs) with no matching pass/fail is
+    // still identified as in-flight despite the unparsable trailing line.
+    assert.deepStrictEqual(result.files, ['b.test.cjs']);
+    // a.test.cjs completed (start+pass), so it must NOT show as in-flight.
+    assert.ok(!result.files.includes('a.test.cjs'));
+    // c.test.cjs never parsed (truncated line), so it cannot appear either.
+    assert.ok(!result.files.includes('c.test.cjs'));
+    assert.strictEqual(result.sawAnyEvent, true, 'the complete lines before the truncation must still count as events');
+  });
+
+  // Regression (#3889 root cause): test:start/test:pass/test:fail are the
+  // exact three event types a genuine hang guarantees are never emitted —
+  // node:test only surfaces a subtest event to the parent once the child
+  // reports it, which happens on completion. test:dequeue is the RUNNER's
+  // own "began this file" signal and fires independent of completion; these
+  // four cases pin analyzeChunkEvents' dequeue-based in-flight rule directly
+  // against the synthetic event shapes a real hang, and a real finish, produce.
+  test('a dequeued file with no terminal event is reported as in flight', () => {
+    const eventsPath = path.join(tmpDir, 'dequeue-only.ndjson');
+    const lines = [
+      JSON.stringify({ type: 'reporter:init', ts: 900 }),
+      JSON.stringify({ type: 'test:enqueue', file: 'a.test.cjs', ts: 1000 }),
+      JSON.stringify({ type: 'test:dequeue', file: 'a.test.cjs', ts: 1010 }),
+    ].join('\n');
+    fs.writeFileSync(eventsPath, lines, 'utf8');
+
+    const result = analyzeChunkEvents(eventsPath);
+    assert.deepStrictEqual(result.files, ['a.test.cjs']);
+    assert.strictEqual(result.anyDequeued, true);
+    assert.strictEqual(result.sawInitMarker, true);
+  });
+
+  test('a dequeued file that also terminates reports nothing in flight (all files finished)', () => {
+    const eventsPath = path.join(tmpDir, 'dequeue-then-pass.ndjson');
+    const lines = [
+      JSON.stringify({ type: 'reporter:init', ts: 900 }),
+      JSON.stringify({ type: 'test:enqueue', file: 'a.test.cjs', ts: 1000 }),
+      JSON.stringify({ type: 'test:dequeue', file: 'a.test.cjs', ts: 1010 }),
+      JSON.stringify({ type: 'test:pass', file: 'a.test.cjs', ts: 1020 }),
+    ].join('\n');
+    fs.writeFileSync(eventsPath, lines, 'utf8');
+
+    const result = analyzeChunkEvents(eventsPath);
+    assert.deepStrictEqual(result.files, [], 'a terminated file must not show as in flight');
+    assert.strictEqual(result.anyDequeued, true, 'the file WAS dequeued — "all files finished" is a distinct state from "nothing ran"');
+  });
+
+  test('one terminated file followed by a second dequeued-but-unterminated file reports only the second', () => {
+    const eventsPath = path.join(tmpDir, 'two-files.ndjson');
+    const lines = [
+      JSON.stringify({ type: 'reporter:init', ts: 900 }),
+      JSON.stringify({ type: 'test:dequeue', file: 'a.test.cjs', ts: 1000 }),
+      JSON.stringify({ type: 'test:pass', file: 'a.test.cjs', ts: 1010 }),
+      JSON.stringify({ type: 'test:dequeue', file: 'b.test.cjs', ts: 1020 }),
+    ].join('\n');
+    fs.writeFileSync(eventsPath, lines, 'utf8');
+
+    const result = analyzeChunkEvents(eventsPath);
+    assert.deepStrictEqual(result.files, ['b.test.cjs']);
+    assert.ok(!result.files.includes('a.test.cjs'));
+  });
+
+  test('an init marker with no dequeue at all is distinguished (anyDequeued=false) from "all finished"', () => {
+    const eventsPath = path.join(tmpDir, 'init-only.ndjson');
+    fs.writeFileSync(eventsPath, JSON.stringify({ type: 'reporter:init', ts: 900 }), 'utf8');
+
+    const result = analyzeChunkEvents(eventsPath);
+    assert.deepStrictEqual(result.files, []);
+    assert.strictEqual(result.anyDequeued, false);
+    assert.strictEqual(result.sawInitMarker, true);
+    assert.strictEqual(result.sawAnyEvent, false);
+  });
+});