diff --git a/scripts/envelope-to-gbrain.mjs b/scripts/envelope-to-gbrain.mjs
index 84721ef1e..ac83a042c 100644
--- a/scripts/envelope-to-gbrain.mjs
+++ b/scripts/envelope-to-gbrain.mjs
@@ -64,7 +64,11 @@ for (const [i, c] of conversations.entries()) {
   // other. The date only leads as a human/chronological sort prefix; the id
   // carries uniqueness. Positional fallback keeps names unique and deterministic
   // when an envelope omits an id.
-  const convId = (typeof c.id === 'string' && c.id.trim()) ? c.id.trim() : `conv-${i + 1}`;
+  // One predicate for "this conversation carries its own id", shared by the
+  // filename and the frontmatter below. Keeping it in a single place is what
+  // stops the two from disagreeing about whether an id exists.
+  const hasId = typeof c.id === 'string' && c.id.trim() !== '';
+  const convId = hasId ? c.id.trim() : `conv-${i + 1}`;
   const name = `${date || '0000-00-00'}-${slug(convId, `conv-${i + 1}`)}.md`;
   // gbrain reads YAML frontmatter + markdown body; keep provenance in frontmatter.
   // Emit `type: conversation` so gbrain stores these as conversation pages rather
@@ -77,8 +81,15 @@ for (const [i, c] of conversations.entries()) {
     'type: conversation',
     `title: ${JSON.stringify(c.title || 'Untitled conversation')}`,
     `date: ${date || 'null'}`,
-    `source: ${env.meta?.source_provider || 'unknown'}`,
-    `memvelope_conversation_id: ${JSON.stringify(c.id)}`,
+    // Every interpolated value is quoted. An envelope is a third-party file, so
+    // a provider string carrying a newline would otherwise close this scalar and
+    // inject arbitrary frontmatter keys into the page gbrain ingests.
+    `source: ${JSON.stringify(env.meta?.source_provider || 'unknown')}`,
+    // Omit the key entirely when the envelope carries no id, rather than
+    // emitting the literal `undefined` or a synthesized `conv-N` — the positional
+    // fallback names the file, but it is not a memvelope conversation id and
+    // must not be recorded as one.
+    ...(hasId ? [`memvelope_conversation_id: ${JSON.stringify(convId)}`] : []),
     'origin: memvelope/envelope-v0',
     '---',
     '',
diff --git a/test/envelope-to-gbrain.test.ts b/test/envelope-to-gbrain.test.ts
index 733e683e9..29361216c 100644
--- a/test/envelope-to-gbrain.test.ts
+++ b/test/envelope-to-gbrain.test.ts
@@ -6,6 +6,9 @@ import { afterAll, describe, expect, test } from 'bun:test';
 import { mkdtempSync, rmSync, readdirSync, readFileSync, writeFileSync } from 'node:fs';
 import { tmpdir } from 'node:os';
 import { join } from 'node:path';
+// The same parser gbrain uses to ingest frontmatter (src/core/markdown.ts), so
+// the injection test asserts against the real consumer rather than a substring.
+import { safeLoad as yamlSafeLoad } from 'js-yaml';
 
 const SCRIPT_PATH = join(import.meta.dir, '..', 'scripts', 'envelope-to-gbrain.mjs');
 const FIXTURE_PATH = join(import.meta.dir, 'fixtures', 'memvelope', 'sample.mve.json');
@@ -70,7 +73,7 @@ describe('envelope-to-gbrain importer', () => {
     expect(page).toContain('type: conversation');
     expect(page).toContain('title: "Onboarding Checklist Draft"');
     expect(page).toContain('date: 2025-11-02');
-    expect(page).toContain('source: chatgpt');
+    expect(page).toContain('source: "chatgpt"');
     expect(page).toContain('memvelope_conversation_id: "c-3f9a2b"');
     expect(page).toContain('origin: memvelope/envelope-v0');
   });
@@ -156,4 +159,68 @@ describe('envelope-to-gbrain importer', () => {
     expect(result.exitCode).toBe(0);
     expect(markdownFiles(result.outDir)).toEqual(['2025-11-02-conv-1.md']);
   });
+
+  test('missing conversation id omits the provenance key rather than emitting a value', async () => {
+    const inputDir = tempDir();
+    const envelopePath = join(inputDir, 'missing-id-frontmatter.mve.json');
+    writeFileSync(envelopePath, JSON.stringify({
+      memvelope: 'envelope-v0',
+      meta: { source_provider: 'chatgpt' },
+      conversations: [
+        {
+          title: 'Missing id example',
+          created_at: '2025-11-02T14:22:51.000Z',
+          messages: [{ id: 'm1', role: 'user', ts: '2025-11-02T14:22:51.000Z', text: 'alice-example asked about frontmatter.' }],
+        },
+      ],
+    }));
+
+    const result = await runImporter(envelopePath);
+    const page = readOnlyMarkdown(result.outDir);
+
+    expect(result.exitCode).toBe(0);
+    // Absent means absent: never the literal string `undefined`, and never the
+    // positional filename fallback masquerading as a real conversation id.
+    expect(page).not.toContain('memvelope_conversation_id');
+    expect(page).not.toContain('undefined');
+    expect(page).toContain('source: "chatgpt"');
+  });
+
+  test('a provider string carrying a newline cannot inject frontmatter keys', async () => {
+    const inputDir = tempDir();
+    const envelopePath = join(inputDir, 'injecting-provider.mve.json');
+    writeFileSync(envelopePath, JSON.stringify({
+      memvelope: 'envelope-v0',
+      meta: { source_provider: 'chatgpt\ntype: injected\nowner: attacker' },
+      conversations: [
+        {
+          id: 'c-inject',
+          title: 'Injection attempt',
+          created_at: '2025-11-02T14:22:51.000Z',
+          messages: [{ id: 'm1', role: 'user', ts: '2025-11-02T14:22:51.000Z', text: 'alice-example sent a hostile provider string.' }],
+        },
+      ],
+    }));
+
+    const result = await runImporter(envelopePath);
+    const page = readOnlyMarkdown(result.outDir);
+    const frontmatter = page.split('---')[1] ?? '';
+    const parsed = yamlSafeLoad(frontmatter) as Record<string, unknown>;
+
+    expect(result.exitCode).toBe(0);
+    // The newline is escaped inside a quoted scalar, so the hostile text stays
+    // one value instead of becoming keys. Asserted structurally: a substring
+    // check cannot tell a real key from the same characters inside a quoted
+    // value, and would pass for the wrong reason.
+    expect(Object.keys(parsed).sort()).toEqual([
+      'date',
+      'memvelope_conversation_id',
+      'origin',
+      'source',
+      'title',
+      'type',
+    ]);
+    expect(parsed.type).toBe('conversation');
+    expect(parsed.source).toBe('chatgpt\ntype: injected\nowner: attacker');
+  });
 });