diff --git a/build/tasks/verify/verify-json.mts b/build/tasks/verify/verify-json.mts
index 0c0d8ad9..f96fd03d 100644
--- a/build/tasks/verify/verify-json.mts
+++ b/build/tasks/verify/verify-json.mts
@@ -5,7 +5,13 @@
  * @module {type ES6Module} build/tasks/verify/verify-json
  */
 
-import { exec, glob, matched, quote } from '@openinf/.github/build/utils';
+import {
+  exec,
+  glob,
+  matched,
+  quote,
+  reportFormattingFixes,
+} from '@openinf/.github/build/utils';
 
 const EXCLUDED = ['!lib/', '!node_modules/'];
 
@@ -31,5 +37,9 @@ const scripts = [
 for (const element of scripts) {
   exitCode = await exec(element);
 
-  if (exitCode !== 0) process.exitCode = exitCode;
+  if (exitCode !== 0) {
+    process.exitCode = exitCode;
+
+    if (element.startsWith('prettier')) reportFormattingFixes(json5Files);
+  }
 }
diff --git a/build/tasks/verify/verify-md.mts b/build/tasks/verify/verify-md.mts
index 522c5a5c..306fd104 100644
--- a/build/tasks/verify/verify-md.mts
+++ b/build/tasks/verify/verify-md.mts
@@ -5,7 +5,13 @@
  * @module {type ES6Module} build/tasks/verify/verify-md
  */
 
-import { exec, glob, matched, quote } from '@openinf/.github/build/utils';
+import {
+  exec,
+  glob,
+  matched,
+  quote,
+  reportFormattingFixes,
+} from '@openinf/.github/build/utils';
 
 const markdownFiles = await glob([
   '**/*.md',
@@ -28,5 +34,9 @@ const scripts = matched(markdownFiles, '**/*.md')
 for (const element of scripts) {
   exitCode = await exec(element);
 
-  if (exitCode !== 0) process.exitCode = exitCode;
+  if (exitCode !== 0) {
+    process.exitCode = exitCode;
+
+    if (element.startsWith('prettier')) reportFormattingFixes(markdownFiles);
+  }
 }
diff --git a/build/tasks/verify/verify-yaml.mts b/build/tasks/verify/verify-yaml.mts
index 6d1e4199..b68296e0 100644
--- a/build/tasks/verify/verify-yaml.mts
+++ b/build/tasks/verify/verify-yaml.mts
@@ -5,7 +5,13 @@
  * @module {type ES6Module} build/tasks/verify/verify-yaml
  */
 
-import { exec, glob, matched, quote } from '@openinf/.github/build/utils';
+import {
+  exec,
+  glob,
+  matched,
+  quote,
+  reportFormattingFixes,
+} from '@openinf/.github/build/utils';
 
 const yamlFiles = await glob([
   '**/*.yml',
@@ -24,5 +30,9 @@ const scripts = matched(yamlFiles, '**/*.yml, **/*.yaml')
 for (const element of scripts) {
   exitCode = await exec(element);
 
-  if (exitCode !== 0) process.exitCode = exitCode;
+  if (exitCode !== 0) {
+    process.exitCode = exitCode;
+
+    if (element.startsWith('prettier')) reportFormattingFixes(yamlFiles);
+  }
 }
diff --git a/build/utils.mts b/build/utils.mts
index 5ea86abc..03041540 100644
--- a/build/utils.mts
+++ b/build/utils.mts
@@ -9,6 +9,7 @@
 // Requirements
 // -----------------------------------------------------------------------------
 
+import { spawnSync } from 'node:child_process';
 import { glob as nodeGlob } from 'node:fs/promises';
 import { join as pathJoin, relative as pathRelative } from 'node:path';
 import { catchWrap } from '@isaacs/catcher';
@@ -172,3 +173,66 @@ export async function glob(patterns: string | string[]) {
       pathRelative(process.cwd(), pathJoin(entry.parentPath, entry.name))
     );
 }
+
+/**
+ * Says what prettier would change, as a diff, for the files it would change.
+ *
+ * `prettier --check` names a file and nothing else. Whoever reads that in a
+ * CI log is left to reproduce the run to learn whether it objected to a line
+ * that ran long, a list marker or a table, and a contributor who cannot run
+ * the tools locally has no way to learn it at all. The diff answers that on
+ * the spot, the same way a reviewer would: here is the line, here is what it
+ * should be.
+ *
+ * Tools are run without a shell, so a path reaches them as one argument
+ * whatever it contains; only the leading `./` that `quote` adds, for a name
+ * that would read as an option, is still needed. Their output is not capped:
+ * `spawnSync` kills a child that prints more than 1 MiB by default, and
+ * a large file would then lose its diff for no reason the reader could see.
+ * @param {string[]} files The files the check was handed.
+ * @returns {string} A unified diff per file prettier would change, or nothing.
+ */
+export function formattingFixes(files: string[]) {
+  const paths = files.map((path) =>
+    path.startsWith('-') ? `./${path}` : path
+  );
+  const options = {
+    encoding: 'utf8',
+    maxBuffer: Number.POSITIVE_INFINITY,
+  } as const;
+  const listed = spawnSync('prettier', ['--list-different', ...paths], options);
+  const changed = (listed.stdout ?? '').split('\n').filter(Boolean);
+  let fixes = '';
+
+  for (const path of changed) {
+    const formatted = spawnSync('prettier', [path], options);
+
+    // A file prettier cannot parse has no formatted version to compare, and
+    // the check already printed why.
+    if (formatted.status !== 0) continue;
+
+    const diff = spawnSync(
+      'diff',
+      ['-u', '--label', path, '--label', `${path} (formatted)`, path, '-'],
+      { ...options, input: formatted.stdout }
+    );
+
+    fixes += diff.stdout ?? '';
+  }
+
+  return fixes;
+}
+
+/**
+ * Prints what prettier would change, after a check of those files failed.
+ * @param {string[]} files The files the check was handed.
+ */
+export function reportFormattingFixes(files: string[]) {
+  const fixes = formattingFixes(files);
+
+  if (fixes === '') return;
+
+  console.error(
+    `\nWhat prettier would change (\`nps format.all\` applies it):\n\n${fixes}`
+  );
+}
diff --git a/build/utils.test.mts b/build/utils.test.mts
index 7752cee2..02e68296 100644
--- a/build/utils.test.mts
+++ b/build/utils.test.mts
@@ -10,7 +10,12 @@ import { mkdir, mkdtemp, readFile, writeFile } from 'node:fs/promises';
 import { tmpdir } from 'node:os';
 import { dirname, join as pathJoin } from 'node:path';
 import { after, before, describe, test } from 'node:test';
-import { exec, glob, quote } from '@openinf/.github/build/utils';
+import {
+  exec,
+  formattingFixes,
+  glob,
+  quote,
+} from '@openinf/.github/build/utils';
 
 // Every pattern a build task writes is relative to the directory the task
 // runs in, so the fixture has to become that directory.
@@ -178,3 +183,29 @@ describe('quote', () => {
     }
   });
 });
+
+describe('formattingFixes', () => {
+  before(async () => {
+    const root = await mkdtemp(pathJoin(tmpdir(), 'openinf-format-'));
+
+    await writeFile(pathJoin(root, 'untidy.md'), '#  Title\n\n* item\n');
+    await writeFile(pathJoin(root, 'tidy.md'), '# Title\n\n- item\n');
+    process.chdir(root);
+  });
+
+  after(() => {
+    process.chdir(cwd);
+  });
+
+  test('shows the lines prettier would change, and what to', () => {
+    const fixes = formattingFixes(['untidy.md', 'tidy.md']);
+
+    ok(fixes.includes('--- untidy.md\n+++ untidy.md (formatted)\n'));
+    ok(fixes.includes('\n-#  Title\n+# Title\n'));
+    ok(fixes.includes('\n-* item\n+- item\n'));
+  });
+
+  test('says nothing about a file already formatted', () => {
+    deepStrictEqual(formattingFixes(['tidy.md']), '');
+  });
+});