diff --git a/packages/harness/src/runner.mjs b/packages/harness/src/runner.mjs
index 974038ea3..14441c52b 100644
--- a/packages/harness/src/runner.mjs
+++ b/packages/harness/src/runner.mjs
@@ -386,30 +386,39 @@ export function toolServer({ path, tools, client, local }) {
 
 // #1873 r127: exit 127 is the shell's "command not found". The turn's stderr then holds the
 // shell's own line for it, the last one nearest the exit:
-//   bash  X: line N: CMD: No such file or directory | X: line N: [exec: ]CMD: [command ]not found
+//   bash  X: line N: [exec: ]CMD: No such file or directory | [command ]not found
 //   dash  X: N: [exec: ]CMD: not found
-//   env   /usr/bin/env: 'CMD': No such file or directory
+//   env   [/usr/bin/]env: 'CMD': No such file or directory
+// Only those shapes count, so another program's missing file (cat: F: No such file…) doesn't.
 // The answer is CMD's basename, which the person reads in the failure; never its host path, and
-// nothing that isn't a plain program name.
-const NOT_FOUND = /(?:^|: )(?:exec: )?['\u2018]?([^\s:'\u2019]+)['\u2019]?: (?:(?:command )?not found|No such file or directory)$/;
+// nothing that isn't a plain program name. A run of 16 letters and digits looks like a key, not a
+// program, so it stays in the run's private stderr.
+const NOT_FOUND = [
+  /(?:^|: )line \d+: (?:exec: )?([^\s:]+): (?:(?:command )?not found|No such file or directory)$/,
+  /: \d+: (?:exec: )?([^\s:]+): not found$/,
+  /^(?:\S*\/)?env: ['\u2018]([^\s:'\u2019]+)['\u2019]: No such file or directory$/,
+];
 export function missingCommand(text) {
   let name = null;
-  for (const line of String(text).split("\n")) {
-    const m = NOT_FOUND.exec(line);
+  for (const line of String(text).split(/\r?\n/)) {
+    const m = NOT_FOUND.map((re) => re.exec(line)).find(Boolean);
     if (m) name = basename(m[1]);
   }
-  return name && /^[A-Za-z0-9._+-]{1,64}$/.test(name) ? name : null;
+  return name && /^[A-Za-z0-9._+-]{1,64}$/.test(name) && !/[A-Za-z0-9]{16}/.test(name) ? name : null;
 }
 
-// The last 16 KiB of a turn's stderr file, or "" when it can't be read.
+// The last 16 KiB of a turn's stderr file from a line start, or "" when it can't be read.
+const TAIL = 16384;
 function stderrTail(path) {
   let fd;
   try {
     fd = openSync(path, "r");
-    const size = fstatSync(fd).size, length = Math.min(size, 16384);
+    const size = fstatSync(fd).size, length = Math.min(size, TAIL);
     const buf = Buffer.alloc(length);
     readSync(fd, buf, 0, length, size - length);
-    return buf.toString("utf8");
+    const text = buf.toString("utf8");
+    // A window that starts mid-file starts mid-line; that partial line is dropped.
+    return size > TAIL ? text.slice(text.indexOf("\n") + 1) : text;
   } catch {
     return "";
   } finally {
diff --git a/packages/harness/tests/runner.test.mjs b/packages/harness/tests/runner.test.mjs
index cd8e51b5f..e732e4cf4 100644
--- a/packages/harness/tests/runner.test.mjs
+++ b/packages/harness/tests/runner.test.mjs
@@ -354,6 +354,30 @@ test("r127 missingCommand reads the shells' not-found lines and gives a safe bas
   // A name that isn't a plain program name is left out, never echoed.
   assert.equal(missingCommand("a: 1: exec: $(rm -rf x): not found\n"), null);
   assert.equal(missingCommand(`a: 1: exec: ${"x".repeat(80)}: not found\n`), null);
+  // Review r127 P3-5: tokens without spaces reach the name check, and it refuses them.
+  assert.equal(missingCommand("a: 1: exec: $(id): not found\n"), null);
+  assert.equal(missingCommand("a: line 2: x;id: command not found\n"), null);
+  assert.equal(missingCommand("a: line 2: `id`: command not found\n"), null);
+  // P3-2: a long run of letters and digits looks like a key, not a program; it stays in stderr.
+  assert.equal(missingCommand("adapter.sh: line 3: sk-proj-abcDEF1234567890xyz: command not found\n"), null);
+  assert.equal(missingCommand("adapter.sh: line 3: 0123456789abcdef0123: command not found\n"), null);
+  // P3-1: only a shell's or env's own line counts; other programs' missing files don't.
+  assert.equal(missingCommand("cat: /nope/file: No such file or directory\n"), null);
+  assert.equal(missingCommand("w.sh: line 1: /opt/x/claude: No such file or directory\ncat: /run/state.json: No such file or directory\n"), "claude");
+  // P3-4: CRLF lines read the same.
+  assert.equal(missingCommand("sh: line 1: pi: command not found\r\n"), "pi");
+});
+
+test("r127 a not-found line cut by the 16 KiB stderr window is not read as a name", async t => {
+  const ctx = await setup(t, { session: { harness: 'pi', turnEnd: 'exit' } });
+  ctx.inbox = [];
+  // 52 + 16351 + 1 bytes: the 16 KiB window starts inside "claude", at "aude: No such file…".
+  writeFileSync(join(ctx.dir, 'adapter.sh'),
+    `printf 'x: line 1: /opt/x/claude: No such file or directory\\n' >&2\nhead -c 16351 /dev/zero | tr '\\0' 'z' >&2\nprintf '\\n' >&2\nexit 127\n`);
+  const r = startRunner(ctx.runDir, ctx.cap);
+  t.after(async () => { r.child.kill('SIGTERM'); await r.exited; });
+  const reply = await ask(ctx);
+  assert.equal(reply.body, "The turn failed: adapter exited 127");
 });
 
 // Claude's private stream reader owns model activity; final result stdout alone