diff --git a/src/mcp/index.ts b/src/mcp/index.ts
index e6b0f2f..acdca7d 100644
--- a/src/mcp/index.ts
+++ b/src/mcp/index.ts
@@ -239,7 +239,7 @@ server.tool(
     title: z.string().describe('Node title (becomes the filename)'),
     directory: z.string().optional().describe('Directory within vault (e.g., "Concepts", "People", "Ideas"). Omit for vault root.'),
     content: z.string().describe('Markdown content for the node body'),
-    frontmatter: z.record(z.unknown()).optional().describe('YAML frontmatter fields (type, tags, status, related, etc.)'),
+    frontmatter: z.record(z.string(), z.any()).optional().describe('YAML frontmatter fields (type, tags, status, related, etc.)'),
   },
   async ({ title, directory, content, frontmatter }) => {
     const relPath = writer.createNode({
diff --git a/test/mcp-schemas.test.ts b/test/mcp-schemas.test.ts
new file mode 100644
index 0000000..db3430d
--- /dev/null
+++ b/test/mcp-schemas.test.ts
@@ -0,0 +1,41 @@
+import { describe, it, expect } from 'vitest';
+import { z } from 'zod';
+import { toJSONSchema } from 'zod/v4-mini';
+
+/**
+ * Regression test for https://github.com/obra/knowledge-graph/issues/2
+ *
+ * Zod v4's toJSONSchema() crashes on z.record(z.unknown()) because
+ * z.unknown() lacks the _zod metadata the serializer expects.
+ * The MCP SDK calls toJSONSchema() on every tool schema during
+ * tools/list, so this crash silently hides all tools from clients.
+ */
+describe('MCP tool schema serialization', () => {
+  it('kg_create_node frontmatter schema serializes to JSON Schema', () => {
+    // This is the exact schema used by kg_create_node in src/mcp/index.ts.
+    // The original z.record(z.unknown()) crashed toJSONSchema in Zod v4.
+    const schema = z.object({
+      title: z.string(),
+      directory: z.string().optional(),
+      content: z.string(),
+      frontmatter: z.record(z.string(), z.any()).optional(),
+    });
+
+    const jsonSchema = toJSONSchema(schema);
+
+    expect(jsonSchema).toBeDefined();
+    expect(jsonSchema.type).toBe('object');
+    expect(jsonSchema.properties).toHaveProperty('frontmatter');
+  });
+
+  it('z.record(z.unknown()) would crash toJSONSchema (documents the bug)', () => {
+    const brokenSchema = z.object({
+      field: z.record(z.unknown()).optional(),
+    });
+
+    // This is the actual Zod v4 bug — z.unknown() lacks _zod metadata.
+    // If this test starts passing, the upstream bug is fixed and the
+    // workaround in index.ts could be reverted (but z.any() is fine too).
+    expect(() => toJSONSchema(brokenSchema)).toThrow();
+  });
+});