diff --git a/gitnexus/src/cli/ai-context.ts b/gitnexus/src/cli/ai-context.ts
index f2f1c52fb..c944f68b2 100644
--- a/gitnexus/src/cli/ai-context.ts
+++ b/gitnexus/src/cli/ai-context.ts
@@ -775,7 +775,7 @@ export async function refreshBaseRefLine(
     // CLI fallback when migrating guidance generated for an external store.
     const legacyRunner =
       block.match(/or `([^`\n]+?) detect-changes --scope compare --base-ref\b/)?.[1] ??
-      '.gitnexus/run.cjs';
+      'node .gitnexus/run.cjs';
     const mergeBaseGuidance = regressionReviewGuidance(defaultBranch, legacyRunner);
     let newBlock = block.replace(
       /For regression review of target branch "(?:[^"\\]|\\.)*"/,
diff --git a/gitnexus/test/unit/ai-context.test.ts b/gitnexus/test/unit/ai-context.test.ts
index 36d22a93d..4751900df 100644
--- a/gitnexus/test/unit/ai-context.test.ts
+++ b/gitnexus/test/unit/ai-context.test.ts
@@ -1636,6 +1636,24 @@ For regression review: \`detect_changes({scope: "compare", base_ref: "main"})\`
     }
   });
 
+  it('uses Node for a legacy line without a CLI fallback (#3509)', async () => {
+    const dir = await fs.mkdtemp(path.join(os.tmpdir(), 'gn-review-no-runner-'));
+    try {
+      const seed = `<!-- gitnexus:start -->
+For regression review: \`detect_changes({scope: "compare", base_ref: "main"})\`.
+<!-- gitnexus:end -->
+`;
+      await fs.writeFile(path.join(dir, 'CLAUDE.md'), seed, 'utf-8');
+
+      expect((await refreshBaseRefLine(dir, 'develop')).files).toEqual(['CLAUDE.md']);
+      const after = await fs.readFile(path.join(dir, 'CLAUDE.md'), 'utf-8');
+      expect(after).toContain('`node .gitnexus/run.cjs detect-changes');
+      expect(after).toContain('base_ref: "<merge-base SHA>"');
+    } finally {
+      await fs.rm(dir, { recursive: true, force: true });
+    }
+  });
+
   it('threads defaultBranch through generateAIContextFiles into AGENTS.md and CLAUDE.md (#243)', async () => {
     const subDir = await fs.mkdtemp(path.join(os.tmpdir(), 'gn-default-branch-'));
     const subStorage = path.join(subDir, '.gitnexus');