fix(markdown): MULTI_FRONTMATTER stops at the first non-frontmatter line

Review catch on #3163: the fence-pairing scan walked arbitrarily far past
body prose to find a second ---, and one colon-prefixed prose line
(`Note: …`, `TODO: …`) was enough to reject legitimate content at
import. Issue #2743's spec is leading-only detection that stops at the
first non-frontmatter character — the walk now breaks on the first line
that isn't YAML-shaped (key:, list item, comment, indented continuation,
or blank), so an hrule followed by prose is body content, never a
stacked block. Real stacked blocks (adjacent, blank-separated,
list-valued keys) still reject.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/cli.ts

@@ -365,6 +365,12 @@ async function main() {
     if (def.required && params[key] === undefined) {
       if (queryHasAlt && key === 'query') continue;
       const cliName = op.cliHints?.name || op.name;
+      // #2822: when the missing param is the op's stdin-fed one, the usage
+      // line alone is misleading (the positionals may all be present — the
+      // pipe was just empty). Name the real problem.
+      if (op.cliHints?.stdin === key) {
+        console.error(`Error: required "${key}" is missing — stdin was empty or not piped. Pipe content on stdin or pass --${key.replace(/_/g, '-')}.`);
+      }
       const positional = op.cliHints?.positional || [];
       const usage = positional.map(p => `<${p}>`).join(' ');
       console.error(`Usage: gbrain ${cliName} ${usage}`);
@@ -761,6 +767,10 @@ export function parseOpArgs(op: Operation, args: string[]): Record<string, unkno
   const params: Record<string, unknown> = {};
   const positional = op.cliHints?.positional || [];
   let posIdx = 0;
+  // #2822: track which params came from positionals so a later flag that
+  // silently discards one (`gbrain put CONTENT --slug foo` — CONTENT was
+  // parsed as the slug) gets a stderr warning instead of vanishing.
+  const positionallySet = new Set<string>();
 
   for (let i = 0; i < args.length; i++) {
     const arg = args[i];
@@ -778,13 +788,20 @@ export function parseOpArgs(op: Operation, args: string[]): Record<string, unkno
       if (paramDef?.type === 'boolean') {
         params[key] = true;
       } else if (i + 1 < args.length) {
+        if (positionallySet.has(key) && params[key] !== args[i + 1]) {
+          console.error(`Warning: ${arg} overrides the positional <${key}> value ${JSON.stringify(params[key])}.`);
+        }
         params[key] = args[++i];
         if (paramDef?.type === 'number') params[key] = Number(params[key]);
       }
     } else if (posIdx < positional.length) {
       const key = positional[posIdx++];
       const paramDef = op.params[key];
+      if (params[key] !== undefined && params[key] !== (paramDef?.type === 'number' ? Number(arg) : arg)) {
+        console.error(`Warning: positional <${key}> overrides the earlier --${key.replace(/_/g, '-')} value ${JSON.stringify(params[key])}.`);
+      }
       params[key] = paramDef?.type === 'number' ? Number(arg) : arg;
+      positionallySet.add(key);
     }
   }
 
@@ -796,7 +813,11 @@ export function parseOpArgs(op: Operation, args: string[]): Record<string, unkno
       console.error(`Error: stdin content exceeds ${MAX_STDIN} bytes. Split into smaller inputs.`);
       process.exit(1);
     }
-    params[op.cliHints.stdin] = stdinContent;
+    // #2822: empty/whitespace-only stdin (cron with no input, broken pipe)
+    // stays UNSET so the required-param check rejects the call instead of
+    // silently writing an empty page (0 chunks, invisible to search and
+    // embed --stale).
+    if (stdinContent.trim().length > 0) params[op.cliHints.stdin] = stdinContent;
   }
 
   return params;

src/commands/lint.ts

@@ -48,6 +48,7 @@ const FRONTMATTER_RULE_NAMES: Record<ParseValidationCode, string> = {
   NESTED_QUOTES: 'frontmatter-nested-quotes',
   NON_STRING_FIELD: 'frontmatter-non-string-field',
   EMPTY_FRONTMATTER: 'frontmatter-empty',
+  MULTI_FRONTMATTER: 'frontmatter-multi',
 };
 
 /** Codes whose lint findings are fixable by `gbrain frontmatter validate --fix`. */

src/core/import-file.ts

@@ -301,6 +301,17 @@ export async function importFromContent(
   // silently fabricated a duplicate at (default, slug) — causing later
   // bare-slug subqueries (getTags, deleteChunks, etc.) to crash with 21000.
   const sourceId = opts.sourceId;
+  // #2822: reject empty/whitespace-only content before any work happens. An
+  // empty page writes 0 chunks — invisible to search AND to `embed --stale`
+  // (nothing to embed), so the mistake never surfaces. Empty content is
+  // always a caller bug (empty piped stdin, bad shell substitution). Thrown
+  // (not returned) so every wrapper site surfaces the message, matching the
+  // ContentSanityBlockError flow.
+  if (content.trim().length === 0) {
+    throw new Error(
+      `Content for "${slug}" is empty; refusing to write an empty page (0 chunks would be invisible to search and embed --stale).`,
+    );
+  }
   // Reject oversized payloads before any parsing, chunking, or embedding happens.
   // Uses Buffer.byteLength to count UTF-8 bytes the same way disk size would,
   // so the network path behaves identically to the file path.
@@ -314,7 +325,17 @@ export async function importFromContent(
     };
   }
 
-  const parsed = parseMarkdown(content, slug + '.md', { activePack: opts.activePack });
+  const parsed = parseMarkdown(content, slug + '.md', { activePack: opts.activePack, validate: true });
+
+  // #2743: reject stacked frontmatter (the double-put corruption class —
+  // already-serialized markdown re-wrapped in fresh frontmatter). gray-matter
+  // parses only the first block; the second would land verbatim in the body
+  // and poison every subsequent round-trip. Only MULTI_FRONTMATTER rejects
+  // here — the other validation codes keep their lint-only semantics.
+  const multiFm = parsed.errors?.find(e => e.code === 'MULTI_FRONTMATTER');
+  if (multiFm) {
+    throw new Error(`MULTI_FRONTMATTER: ${multiFm.message} (slug "${slug}", line ${multiFm.line})`);
+  }
 
   // v0.42 (#1699 trust boundary): strip gate-owned markers from UNTRUSTED
   // input. parseMarkdown preserves every frontmatter key except type/title/

src/core/markdown.ts

@@ -11,7 +11,8 @@ export type ParseValidationCode =
   | 'NULL_BYTES'
   | 'NESTED_QUOTES'
   | 'NON_STRING_FIELD'
-  | 'EMPTY_FRONTMATTER';
+  | 'EMPTY_FRONTMATTER'
+  | 'MULTI_FRONTMATTER';
 
 export interface ParseValidationError {
   code: ParseValidationCode;
@@ -331,6 +332,50 @@ function collectValidationErrors(
       });
     }
   }
+
+  // 9. MULTI_FRONTMATTER (#2743) — a second ---…--- block right after the
+  //    closing fence is stacked frontmatter (the double-put corruption class:
+  //    already-serialized markdown re-wrapped in fresh frontmatter).
+  //    gray-matter parses only the first block and silently leaves the second
+  //    in the body. Heuristic: first non-empty line after the close is `---`,
+  //    a later `---` closes it, EVERY line between is frontmatter-shaped
+  //    (YAML `key:`, `- ` list item, `#` comment, indented continuation, or
+  //    blank — the issue's "stop at the first non-frontmatter character"
+  //    spec), and at least one is a `key:` line. A lone `---` stays a
+  //    markdown horizontal rule, and an hrule followed by prose — even
+  //    colon-prefixed prose like `Note: …` mixed with plain lines — is body
+  //    content, not a stacked block.
+  let afterClose = closeLine + 1;
+  while (afterClose < lines.length && lines[afterClose].trim().length === 0) afterClose++;
+  if (afterClose < lines.length && lines[afterClose].trim() === '---') {
+    let secondClose = -1;
+    for (let i = afterClose + 1; i < lines.length; i++) {
+      const trimmed = lines[i].trim();
+      if (trimmed === '---') {
+        secondClose = i;
+        break;
+      }
+      const yamlShaped =
+        trimmed.length === 0 ||
+        /^[A-Za-z_][\w-]*\s*:/.test(trimmed) ||
+        trimmed.startsWith('- ') ||
+        trimmed === '-' ||
+        trimmed.startsWith('#') ||
+        /^\s/.test(lines[i]);
+      if (!yamlShaped) break; // first non-frontmatter line → body prose, not a stacked block
+    }
+    if (
+      secondClose > afterClose + 1 &&
+      lines.slice(afterClose + 1, secondClose).some(l => /^\s*[A-Za-z_][\w-]*\s*:/.test(l))
+    ) {
+      errors.push({
+        code: 'MULTI_FRONTMATTER',
+        message:
+          'Stacked frontmatter: a second ---…--- block follows the frontmatter (double-put corruption); merge into a single frontmatter block',
+        line: afterClose + 1,
+      });
+    }
+  }
 }
 
 /**

test/cli-args.test.ts

@@ -1,4 +1,8 @@
-import { describe, expect, test } from 'bun:test';
+import { afterEach, describe, expect, test } from 'bun:test';
+import { spawnSync } from 'child_process';
+import { join, resolve } from 'path';
+import { mkdtempSync, rmSync } from 'fs';
+import { tmpdir } from 'os';
 import { parseOpArgs } from '../src/cli.ts';
 import { operationsByName } from '../src/core/operations.ts';
 
@@ -20,5 +24,70 @@ describe('parseOpArgs', () => {
       source_id: 'gstack-code-repo-0e4763c9',
     });
   });
+
+  describe('positional/flag overwrite warning (#2822)', () => {
+    const errors: string[] = [];
+    const origError = console.error;
+    const captureErrors = () => {
+      console.error = (...args: unknown[]) => errors.push(args.join(' '));
+    };
+    afterEach(() => {
+      console.error = origError;
+      errors.length = 0;
+    });
+
+    test('a flag that overwrites a positional value warns to stderr', () => {
+      captureErrors();
+      const params = parseOpArgs(operationsByName.query, ['positional text', '--query', 'flag text']);
+      expect(params.query).toBe('flag text');
+      expect(errors.some(e => e.includes('Warning') && e.includes('--query'))).toBe(true);
+    });
+
+    test('a positional that overwrites an earlier flag value warns to stderr', () => {
+      captureErrors();
+      const params = parseOpArgs(operationsByName.query, ['--query', 'flag text', 'positional text']);
+      expect(params.query).toBe('positional text');
+      expect(errors.some(e => e.includes('Warning') && e.includes('<query>'))).toBe(true);
+    });
+
+    test('no warning when flag and positional agree', () => {
+      captureErrors();
+      parseOpArgs(operationsByName.query, ['same', '--query', 'same']);
+      expect(errors).toEqual([]);
+    });
+  });
 });
 
+describe('gbrain put — empty non-TTY stdin rejects (#2822)', () => {
+  const REPO = resolve(import.meta.dir, '..');
+  const CLI = join(REPO, 'src', 'cli.ts');
+
+  const runPut = (input: string) => {
+    // Isolated HOME so a regression can never write into a real brain.
+    const home = mkdtempSync(join(tmpdir(), 'gbrain-put-empty-'));
+    try {
+      return spawnSync('bun', [CLI, 'put', 'inbox/empty-stdin-test'], {
+        stdio: ['pipe', 'pipe', 'pipe'],
+        input,
+        encoding: 'utf-8',
+        timeout: 60_000,
+        env: { ...process.env, HOME: home, GBRAIN_SKIP_STARTUP_HOOKS: '1' },
+      });
+    } finally {
+      rmSync(home, { recursive: true, force: true });
+    }
+  };
+
+  test('empty stdin exits 1 and names the missing content param', () => {
+    const res = runPut('');
+    expect(res.status).toBe(1);
+    expect(res.stderr).toContain('content');
+    expect(res.stderr).toContain('stdin');
+  }, 90_000);
+
+  test('whitespace-only stdin also exits 1', () => {
+    const res = runPut('   \n\t\n');
+    expect(res.status).toBe(1);
+    expect(res.stderr).toContain('stdin');
+  }, 90_000);
+});

test/import-file.test.ts

@@ -708,3 +708,32 @@ body unchanged
     expect(shortCircuited).toBe(true);
   });
 });
+
+describe('importFromContent — empty content guard (#2822)', () => {
+  test('empty string throws instead of writing an invisible 0-chunk page', async () => {
+    const engine = mockEngine();
+    await expect(importFromContent(engine, 'inbox/empty', '', { noEmbed: true })).rejects.toThrow(/empty/i);
+    expect((engine as any)._calls.find((c: any) => c.method === 'putPage')).toBeUndefined();
+  });
+
+  test('whitespace-only content throws', async () => {
+    const engine = mockEngine();
+    await expect(importFromContent(engine, 'inbox/ws', '  \n\t \n', { noEmbed: true })).rejects.toThrow(/empty/i);
+  });
+});
+
+describe('importFromContent — stacked frontmatter rejection (#2743)', () => {
+  test('double-put shaped content (two ---…--- blocks) throws MULTI_FRONTMATTER', async () => {
+    const engine = mockEngine();
+    const md = '---\ntitle: outer\n---\n\n---\ntitle: inner\ntype: concept\n---\n\nreal body';
+    await expect(importFromContent(engine, 'inbox/double', md, { noEmbed: true })).rejects.toThrow(/MULTI_FRONTMATTER/);
+    expect((engine as any)._calls.find((c: any) => c.method === 'putPage')).toBeUndefined();
+  });
+
+  test('normal content with horizontal rules in the body still imports', async () => {
+    const engine = mockEngine();
+    const md = '---\ntitle: ok\ntype: concept\n---\n\nprose before\n\n---\n\nprose after the rule';
+    const result = await importFromContent(engine, 'inbox/hrule', md, { noEmbed: true });
+    expect(result.status).toBe('imported');
+  });
+});

test/markdown-validation.test.ts

@@ -256,6 +256,56 @@ body`;
     });
   });
 
+  describe('MULTI_FRONTMATTER (#2743)', () => {
+    test('stacked frontmatter immediately after the close fence', () => {
+      const md = `${fence}\ntitle: outer\n${fence}\n${fence}\ntitle: inner\ntype: concept\n${fence}\n\nbody`;
+      const parsed = parseMarkdown(md, undefined, { validate: true });
+      expect(parsed.errors!.map(e => e.code)).toContain('MULTI_FRONTMATTER');
+    });
+
+    test('stacked frontmatter with a blank line between blocks (serializeMarkdown shape)', () => {
+      const md = `${fence}\ntitle: outer\n${fence}\n\n${fence}\ntitle: inner\n${fence}\n\nbody`;
+      const parsed = parseMarkdown(md, undefined, { validate: true });
+      expect(parsed.errors!.map(e => e.code)).toContain('MULTI_FRONTMATTER');
+    });
+
+    test('horizontal rules in the body are NOT flagged', () => {
+      const md = `${fence}\ntitle: ok\n${fence}\n\nsome prose\n\n${fence}\n\nmore prose\n\n${fence}\n\nend`;
+      const parsed = parseMarkdown(md, undefined, { validate: true });
+      expect(parsed.errors!.map(e => e.code)).not.toContain('MULTI_FRONTMATTER');
+    });
+
+    test('hrule pair at body start without YAML-shaped lines is NOT flagged', () => {
+      const md = `${fence}\ntitle: ok\n${fence}\n\n${fence}\n\nplain prose between rules\n\n${fence}\n\nend`;
+      const parsed = parseMarkdown(md, undefined, { validate: true });
+      expect(parsed.errors!.map(e => e.code)).not.toContain('MULTI_FRONTMATTER');
+    });
+
+    test('timeline sentinel form is NOT flagged', () => {
+      const md = `${fence}\ntitle: ok\n${fence}\n\nbody text\n\n${fence}\n\n## Timeline\n- 2024-01-01: thing`;
+      const parsed = parseMarkdown(md, undefined, { validate: true });
+      expect(parsed.errors!.map(e => e.code)).not.toContain('MULTI_FRONTMATTER');
+    });
+
+    test('body hrule + colon-prefixed prose (`Note: …`) mixed with plain lines is NOT flagged', () => {
+      const md = `${fence}\ntitle: ok\ntype: concept\n${fence}\n\n${fence}\n\nNote: remember to follow up\n\nlots of plain prose here\n\n${fence}\n\nmore prose`;
+      const parsed = parseMarkdown(md, undefined, { validate: true });
+      expect(parsed.errors!.map(e => e.code)).not.toContain('MULTI_FRONTMATTER');
+    });
+
+    test('fence pairing stops at the first non-frontmatter line (no far-fence pairing across prose)', () => {
+      const md = `${fence}\ntitle: ok\n${fence}\n\n${fence}\n\n${'plain prose line\n'.repeat(40)}TODO: fix the widget\n${'more prose\n'.repeat(40)}${fence}\nend`;
+      const parsed = parseMarkdown(md, undefined, { validate: true });
+      expect(parsed.errors!.map(e => e.code)).not.toContain('MULTI_FRONTMATTER');
+    });
+
+    test('stacked block with list-valued keys is still flagged', () => {
+      const md = `${fence}\ntitle: outer\n${fence}\n\n${fence}\ntitle: inner\ntags:\n  - a\n  - b\n${fence}\n\nbody`;
+      const parsed = parseMarkdown(md, undefined, { validate: true });
+      expect(parsed.errors!.map(e => e.code)).toContain('MULTI_FRONTMATTER');
+    });
+  });
+
   test('error.line is set for line-bearing errors', () => {
     const md = `${fence}\ntype: concept\n${fence}\n# Heading inline\n\nbody\x00drop`;
     const parsed = parseMarkdown(md, undefined, { validate: true });