diff --git a/.changeset/diff-usage-error-to-stderr.md b/.changeset/diff-usage-error-to-stderr.md
new file mode 100644
index 0000000000..f9345aa844
--- /dev/null
+++ b/.changeset/diff-usage-error-to-stderr.md
@@ -0,0 +1,17 @@
+---
+"@objectstack/cli": patch
+---
+
+`os diff` with no path arguments no longer prints its usage error on stdout — in either face.
+
+The refusal sat **above** the command's first `if (!flags.json)`, so the face was still undecided when it ran and it fired in **both**. `printError` plus three `console.log` calls — all four writing to stdout — then `process.exit(1)`. Measured on the published entry `bin/run.js` with `NO_COLOR=1` and the streams captured separately, `os diff --json` and bare `os diff` answered byte-identically: exit 1, **141 bytes of prose on stdout, an empty stderr**, and `JSON.parse(stdout)` throwing on the one stream `--json` reserves for the machine.
+
+The diagnostic now goes to stderr, where the rest of this CLI's diagnostics already go. The 141 bytes moved intact — stdout 141 → 0, stderr 0 → 141. Nothing else moves:
+
+- **the exit code is still 1**, so a consumer branching on exit status sees no change at all;
+- **the wording is unchanged**, both usage hints included, so a human reading a terminal sees the same four lines;
+- **nothing is accepted or rejected differently** — no invocation that worked before fails now.
+
+⚠️ **No error payload is invented on this path.** What a `--json` consumer should *receive* on a refusal is an open envelope question, entangled with `os lint --eval --json`'s bare `{ error }` (no `code`, no `httpStatus`), and it is deliberately left open here — this change settles only that the machine's channel no longer carries prose. `--json` on this path emits nothing on stdout; a consumer must still read the exit status, exactly as it must today.
+
+This is the sibling of the `resolveConfigPath` repair, and a genuinely different site: that one is reached through `loadConfig()`, this one is `diff.ts`'s own usage error, raised before any config work happens. The existing pin drives `os diff` with two paths precisely so the run gets *past* this check, so it could not see this path. A new pin (`diff-usage-error-stream.e2e.test.ts`) drives the bare form in both faces, and carries a structural tripwire: across 62 command modules, 27 of which offer `--json`, `diff` was the only one with a stdout write above its guard, and the tripwire goes red if another arrives.
diff --git a/packages/cli/src/commands/diff.ts b/packages/cli/src/commands/diff.ts
index 89307dc468..16518ee6cf 100644
--- a/packages/cli/src/commands/diff.ts
+++ b/packages/cli/src/commands/diff.ts
@@ -8,6 +8,7 @@ import {
   printSuccess,
   printWarning,
   printError,
+  printErrorToStderr,
   printInfo,
   printStep,
   createTimer,
@@ -177,11 +178,32 @@ export default class Diff extends Command {
     const beforePath: string | undefined = args.before || flags.before;
     const afterPath: string | undefined = args.after || flags.after;
 
+    // This refusal goes to STDERR, and it is the one write in this file that
+    // has to (#15697).
+    //
+    // It sits ABOVE the first `if (!flags.json)` below, so it is reached with
+    // the face still undecided and fires in BOTH — the text face and the
+    // machine face alike. Measured on the published entry `bin/run.js` with
+    // `NO_COLOR=1` and the streams captured separately, it used to answer
+    // `os diff --json` with **exit 1, 141 bytes of prose on stdout and an empty
+    // stderr**: `JSON.parse(stdout)` threw, on the one stream `--json` reserves
+    // for the machine (`utils/json-stdout.ts`). Both faces measured identically,
+    // because there is no branch here to tell them apart.
+    //
+    // ⚠️ Every other diagnostic in this file stays on stdout deliberately: they
+    // sit INSIDE a `!flags.json` branch, i.e. the command has already decided it
+    // is rendering its text face, which is exactly the case `printError` is for
+    // (see the note on {@link printErrorToStderr}).
+    //
+    // ⛔ Moving the bytes is the whole change. The exit code stays 1, the
+    // wording stays identical, and no payload is invented: what `--json` should
+    // emit on a refusal is an open envelope question (#15549) touching this
+    // command family at once, and settling it is above this fix's authority.
     if (!beforePath || !afterPath) {
-      printError('Two config file paths are required.');
-      console.log('');
-      console.log(chalk.dim('  Usage: objectstack diff <before> <after>'));
-      console.log(chalk.dim('     or: objectstack diff --before path1 --after path2'));
+      printErrorToStderr('Two config file paths are required.');
+      console.error('');
+      console.error(chalk.dim('  Usage: objectstack diff <before> <after>'));
+      console.error(chalk.dim('     or: objectstack diff --before path1 --after path2'));
       process.exit(1);
     }
 
diff --git a/packages/cli/test/diff-usage-error-stream.e2e.test.ts b/packages/cli/test/diff-usage-error-stream.e2e.test.ts
new file mode 100644
index 0000000000..4ce74314d6
--- /dev/null
+++ b/packages/cli/test/diff-usage-error-stream.e2e.test.ts
@@ -0,0 +1,261 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * `os diff` with NO path arguments ⇒ nothing unparseable on stdout, in EITHER
+ * face (#15697).
+ *
+ * ## Why this is a second file rather than a case in the sibling pin
+ *
+ * `config-miss-stdout-purity.e2e.test.ts` (#15547) drives the same command, but
+ * a different branch of it: its `FAMILY` entry is
+ * `diff: { explicit: [MISSING, MISSING], auto: null }` — two paths, supplied so
+ * the run gets *past* the usage check and down into `resolveConfigPath()`. It
+ * says so in place: "`os diff` requires two config paths, so there is no bare
+ * form that reaches the helper without one."
+ *
+ * That is precisely the branch this file drives — the bare form. The refusal
+ * here is `diff.ts`'s own usage error, reached BEFORE any config work happens,
+ * so the sibling pin structurally cannot see it and stayed green through it.
+ *
+ * ## What was measured
+ *
+ * The four writes sat ABOVE the command's first `if (!flags.json)`, so the face
+ * was still undecided when they ran and they fired in BOTH. On the published
+ * entry `bin/run.js`, `NO_COLOR=1`, streams captured separately:
+ *
+ *     os diff --json   → exit 1 · stdout 141 bytes of prose · stderr 0 bytes
+ *     os diff          → exit 1 · stdout 141 bytes of prose · stderr 0 bytes
+ *
+ * Byte-identical, because there was no branch to tell the faces apart. And
+ * `JSON.parse(stdout)` threw on the one stream `--json` reserves for the
+ * machine.
+ *
+ * ## What is asserted — and what is deliberately NOT
+ *
+ * ⛔ No error-payload shape is pinned here. Whether `--json` should emit an
+ * envelope on a refusal is an open question entangled with #15549
+ * (`os lint --eval --json`'s bare `{ error }`, no `code`, no `httpStatus`), and
+ * settling it is above this pin's authority.
+ *
+ * So the assertion is the half that needs no ruling, and it is a PROPERTY, not
+ * a string: **stdout carries nothing a machine cannot read.** Empty passes, one
+ * JSON document passes, prose fails. A pin on "141 bytes" would rot on the next
+ * wording change; this one survives it, and survives whoever settles the
+ * envelope question without their having to touch this file.
+ *
+ * The other assertions are the ones a "just silence it" regression would break:
+ * the diagnostic must still reach the operator on **stderr**, and the exit
+ * status must still be 1.
+ *
+ * ## Anti-vacuity
+ *
+ * Every assertion below is green if the command dies early for an unrelated
+ * reason — an empty stdout is "machine-readable" and a missing string is "not
+ * on stdout". So the suite refuses to report until a CONTROL has shown that
+ * this argv actually reaches this command: `os diff --help` must exit 0 and
+ * render the command's own description. If the binary were broken, the control
+ * goes red and the rest is known to be worthless rather than quietly green.
+ */
+
+import { describe, it, expect, beforeAll, afterAll } from 'vitest';
+import { execFile } from 'node:child_process';
+import { mkdtempSync, rmSync, existsSync, readFileSync, readdirSync, statSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { join, resolve, relative, sep } from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { childEnv } from './helpers/serve-process.js';
+
+const HERE = resolve(fileURLToPath(import.meta.url), '..');
+const CLI = resolve(HERE, '../bin/run-dev.js');
+const TSX = resolve(HERE, '../../../node_modules/.bin/tsx');
+const COMMANDS_DIR = resolve(HERE, '../src/commands');
+
+/** The refusal and its two hint lines — what must be on stderr, never stdout. */
+const REFUSAL = 'Two config file paths are required.';
+const HINTS = ['Usage: objectstack diff', 'or: objectstack diff --before'] as const;
+
+interface Run {
+  code: number;
+  stdout: string;
+  stderr: string;
+}
+
+function runCli(argv: string[], cwd: string): Promise<Run> {
+  return new Promise((resolvePromise) => {
+    execFile(
+      TSX,
+      [CLI, ...argv],
+      { cwd, maxBuffer: 32 * 1024 * 1024, env: childEnv({ NO_COLOR: '1' }) },
+      (err, stdout, stderr) => {
+        resolvePromise({
+          code: err
+            ? (typeof (err as { code?: unknown }).code === 'number'
+                ? (err as unknown as { code: number }).code
+                : 1)
+            : 0,
+          stdout: String(stdout),
+          stderr: String(stderr),
+        });
+      },
+    );
+  });
+}
+
+/**
+ * Whether stdout is something a program can read: nothing at all, or exactly
+ * one JSON document. Prose is the failure — see the header for why the choice
+ * between the two passing shapes is deliberately left open.
+ */
+function stdoutIsMachineReadable(stdout: string): boolean {
+  if (stdout.trim() === '') return true;
+  try {
+    JSON.parse(stdout);
+    return true;
+  } catch {
+    return false;
+  }
+}
+
+/**
+ * The two faces, both driven with NO path arguments — the argv that reaches the
+ * usage error. `--json` is the machine face; bare is the text face. The defect
+ * fired in both, so both are pinned.
+ */
+const FACES: Record<string, string[]> = {
+  'machine face (--json)': ['diff', '--json'],
+  'text face (bare)': ['diff'],
+};
+
+let dir: string;
+let control: Run;
+let runs: Record<string, Run>;
+
+beforeAll(async () => {
+  // Deliberately EMPTY. The usage error is raised before any config work, so
+  // the cwd cannot influence it — an empty dir keeps that true rather than
+  // assumed, by making sure no stray `objectstack.config.*` is in reach.
+  dir = mkdtempSync(join(tmpdir(), 'os-diff-usage-e2e-'));
+
+  // Sequential: concurrent `tsx` starts are the kind of load that makes a
+  // shared box report timeouts instead of verdicts.
+  control = await runCli(['diff', '--help'], dir);
+  runs = {};
+  for (const [name, argv] of Object.entries(FACES)) {
+    runs[name] = await runCli(argv, dir);
+  }
+}, 900_000);
+
+afterAll(() => {
+  try { rmSync(dir, { recursive: true, force: true }); } catch { /* ignore */ }
+});
+
+describe('control — the reading below is worthless without this', () => {
+  it('resolves the dev entry it drives', () => {
+    // A run against a path that does not exist fails exactly like a true
+    // negative: empty streams, non-zero exit. Rule it out before reading
+    // anything off the runs.
+    expect(existsSync(CLI)).toBe(true);
+    expect(existsSync(TSX)).toBe(true);
+  });
+
+  it('actually reaches `os diff` with this argv', () => {
+    // If this is red, every assertion below is green for the wrong reason.
+    expect(control.code).toBe(0);
+    expect(control.stdout).toContain('Compare two ObjectStack configurations');
+  });
+});
+
+describe.each(Object.keys(FACES))('os diff, no path arguments — %s', (face) => {
+  const runOf = (): Run => runs[face];
+
+  it('leaves nothing on stdout that a machine cannot read', () => {
+    // Under the defect this was 141 bytes of `  ✗ Two config file paths are
+    // required.` plus a blank line and two usage hints — on the one stream
+    // `--json` reserves for the machine, with stderr completely empty.
+    expect(stdoutIsMachineReadable(runOf().stdout)).toBe(true);
+  });
+
+  it('keeps the human refusal off stdout entirely', () => {
+    // Asserted separately from the parse so a regression names its cause
+    // rather than only `Unexpected token`.
+    const { stdout } = runOf();
+    expect(stdout).not.toContain(REFUSAL);
+    for (const hint of HINTS) expect(stdout).not.toContain(hint);
+  });
+
+  it('still shows the operator the refusal and both hints — on stderr', () => {
+    // Diagnostics are MOVED, never destroyed: a regression toward silencing
+    // this path goes red here. This is also the pair that makes the assertion
+    // above non-vacuous — an early death would leave stderr without these.
+    const { stderr } = runOf();
+    expect(stderr).toContain(REFUSAL);
+    for (const hint of HINTS) expect(stderr).toContain(hint);
+  });
+
+  it('still exits 1', () => {
+    expect(runOf().code).toBe(1);
+  });
+});
+
+/**
+ * The structural tripwire: `diff` was the ONLY command shaped this way, and a
+ * new one must not arrive unnoticed.
+ *
+ * ⚠️ Its bound, stated rather than implied: this is a source-text scan, so it
+ * sees only the print helpers it names, called syntactically inside `run()`
+ * above the first `flags.json` read. It does NOT follow calls into helpers —
+ * that is how #15547 was reached, through `loadConfig()` — and a command that
+ * reads `flags.json` into a local on its first line hides everything below from
+ * this check. It is a tripwire for the shape that was measured, not a proof
+ * that no other shape exists.
+ */
+describe('no new command has grown a stdout write above its --json guard', () => {
+  function commandFiles(d: string): string[] {
+    const out: string[] = [];
+    for (const entry of readdirSync(d)) {
+      const abs = join(d, entry);
+      if (statSync(abs).isDirectory()) {
+        out.push(...commandFiles(abs));
+        continue;
+      }
+      if (!entry.endsWith('.ts') || entry.endsWith('.test.ts')) continue;
+      out.push(abs);
+    }
+    return out;
+  }
+
+  const WRITES_TO_STDOUT =
+    /\b(?:console\.log|printHeader|printStep|printInfo|printSuccess|printWarning|printError)\(/;
+
+  function offenders(): string[] {
+    const found: string[] = [];
+    for (const abs of commandFiles(COMMANDS_DIR)) {
+      const lines = readFileSync(abs, 'utf-8').split('\n');
+      if (!lines.some((l) => /\bjson:\s*Flags\.boolean\(/.test(l))) continue;
+      const runIdx = lines.findIndex((l) => /async run\s*\(/.test(l));
+      if (runIdx < 0) continue;
+      const guardIdx = lines.findIndex(
+        (l, i) => i >= runIdx && /flags\.json|flags\[['"]json['"]\]/.test(l),
+      );
+      if (guardIdx < 0) continue;
+      if (lines.slice(runIdx, guardIdx).some((l) => WRITES_TO_STDOUT.test(l))) {
+        found.push(relative(COMMANDS_DIR, abs).split(sep).join('/'));
+      }
+    }
+    return found.sort();
+  }
+
+  it('scans a non-empty family — refuse to reason from zero', () => {
+    // The scan is only evidence if it looked at something. A refactor that
+    // moved or renamed the commands directory would otherwise report "no
+    // offenders" from an empty sweep.
+    const withJson = commandFiles(COMMANDS_DIR).filter((abs) =>
+      /\bjson:\s*Flags\.boolean\(/.test(readFileSync(abs, 'utf-8')),
+    );
+    expect(withJson.length).toBeGreaterThan(10);
+  });
+
+  it('finds none', () => {
+    expect(offenders()).toEqual([]);
+  });
+});