fix(core): fall back to z.toJSONSchema for zod schemas without ~standard.jsonSchema

Schemas from zod <4.2.0 implement StandardSchemaV1 but not StandardJSONSchemaV1,
so standardSchemaToJsonSchema crashed at `undefined[io]` on tools/list. Detect
`vendor: 'zod'` without `jsonSchema` and fall back to the bundled z.toJSONSchema()
with a one-time warning. Non-zod libraries without jsonSchema get a clear error
pointing at fromJsonSchema(). Bumps catalog zod to ^4.2.0.

Author: felixweinberger

.changeset/zod-jsonschema-fallback.md

@@ -0,0 +1,6 @@
+---
+'@modelcontextprotocol/server': patch
+'@modelcontextprotocol/client': patch
+---
+
+Fix runtime crash on `tools/list` when a tool's `inputSchema` comes from zod 4.0–4.1. The SDK requires `~standard.jsonSchema` (StandardJSONSchemaV1, added in zod 4.2.0); previously a missing `jsonSchema` crashed at `undefined[io]`. `standardSchemaToJsonSchema` now detects zod 4 schemas lacking `jsonSchema` and falls back to the SDK-bundled `z.toJSONSchema()`, emitting a one-time console warning. zod 3 schemas (which the bundled zod 4 converter cannot introspect) and non-zod schema libraries without `jsonSchema` get a clear error pointing to `fromJsonSchema()`. The workspace zod catalog is also bumped to `^4.2.0`.

packages/core/src/util/standardSchema.ts

@@ -6,6 +6,8 @@
 
 /* eslint-disable @typescript-eslint/no-namespace */
 
+import * as z from 'zod/v4';
+
 // Standard Schema interfaces — vendored from https://standardschema.dev (spec v1, Jan 2025)
 
 export interface StandardTypedV1<Input = unknown, Output = Input> {
@@ -138,6 +140,8 @@ export function isStandardSchemaWithJSON(schema: unknown): schema is StandardSch
 
 // JSON Schema conversion
 
+let warnedZodFallback = false;
+
 /**
  * Converts a StandardSchema to JSON Schema for use as an MCP tool/prompt schema.
  *
@@ -149,7 +153,35 @@ export function isStandardSchemaWithJSON(schema: unknown): schema is StandardSch
  * since that cannot satisfy the MCP spec.
  */
 export function standardSchemaToJsonSchema(schema: StandardJSONSchemaV1, io: 'input' | 'output' = 'input'): Record<string, unknown> {
-    const result = schema['~standard'].jsonSchema[io]({ target: 'draft-2020-12' });
+    const std = schema['~standard'];
+    let result: Record<string, unknown>;
+    if (std.jsonSchema) {
+        result = std.jsonSchema[io]({ target: 'draft-2020-12' });
+    } else if (std.vendor === 'zod') {
+        // zod 4.0–4.1 implements StandardSchemaV1 but not StandardJSONSchemaV1 (`~standard.jsonSchema`).
+        // The SDK already bundles zod 4, so fall back to its converter rather than crashing on tools/list.
+        // zod 3 schemas (which also report vendor 'zod') have `_def` but not `_zod`; the SDK-bundled
+        // zod 4 `z.toJSONSchema()` cannot introspect them, so throw a clear error instead of crashing.
+        if (!('_zod' in (schema as object))) {
+            throw new Error(
+                'Schema appears to be from zod 3, which the SDK cannot convert to JSON Schema. ' +
+                    'Upgrade to zod >=4.2.0, or wrap your JSON Schema with fromJsonSchema().'
+            );
+        }
+        if (!warnedZodFallback) {
+            warnedZodFallback = true;
+            console.warn(
+                '[@modelcontextprotocol/sdk] Your zod version does not implement `~standard.jsonSchema` (added in zod 4.2.0). ' +
+                    'Falling back to z.toJSONSchema(). Upgrade to zod >=4.2.0 to silence this warning.'
+            );
+        }
+        result = z.toJSONSchema(schema as unknown as z.ZodType, { target: 'draft-2020-12', io }) as Record<string, unknown>;
+    } else {
+        throw new Error(
+            `Schema library "${std.vendor}" does not implement StandardJSONSchemaV1 (\`~standard.jsonSchema\`). ` +
+                `Upgrade to a version that does, or wrap your JSON Schema with fromJsonSchema().`
+        );
+    }
     if (result.type !== undefined && result.type !== 'object') {
         throw new Error(
             `MCP tool and prompt schemas must describe objects (got type: ${JSON.stringify(result.type)}). ` +

packages/core/test/util/standardSchema.zodFallback.test.ts

@@ -0,0 +1,37 @@
+import { describe, expect, it, vi } from 'vitest';
+import * as z from 'zod/v4';
+import { standardSchemaToJsonSchema } from '../../src/util/standardSchema.js';
+
+type SchemaArg = Parameters<typeof standardSchemaToJsonSchema>[0];
+
+describe('standardSchemaToJsonSchema — zod fallback paths', () => {
+    it('falls back to z.toJSONSchema for zod 4.0–4.1 (vendor=zod, no ~standard.jsonSchema, has _zod)', () => {
+        const warn = vi.spyOn(console, 'warn').mockImplementation(() => {});
+        const real = z.object({ a: z.string() });
+        // Simulate zod 4.0–4.1: shadow `~standard` on the real instance with `jsonSchema` removed.
+        // Keeps the rest of the zod 4 object (including `_zod`) intact so z.toJSONSchema can introspect it.
+        const { jsonSchema: _drop, ...stdNoJson } = real['~standard'] as unknown as Record<string, unknown>;
+        void _drop;
+        Object.defineProperty(real, '~standard', { value: { ...stdNoJson, vendor: 'zod' }, configurable: true });
+
+        const result = standardSchemaToJsonSchema(real as unknown as SchemaArg);
+        expect(result.type).toBe('object');
+        expect((result.properties as unknown as Record<string, unknown>)?.a).toBeDefined();
+        expect(warn).toHaveBeenCalledOnce();
+        expect(warn.mock.calls[0]?.[0]).toContain('zod 4.2.0');
+        warn.mockRestore();
+    });
+
+    it('throws a clear error for zod 3 (vendor=zod, no ~standard.jsonSchema, no _zod)', () => {
+        // zod 3.24+ reports `~standard.vendor === 'zod'` but has no `_zod` internal marker.
+        const zod3ish = { _def: {}, '~standard': { version: 1, vendor: 'zod', validate: () => ({ value: {} }) } };
+        expect(() => standardSchemaToJsonSchema(zod3ish as unknown as SchemaArg)).toThrow(/zod 3/);
+        expect(() => standardSchemaToJsonSchema(zod3ish as unknown as SchemaArg)).toThrow(/4\.2\.0/);
+    });
+
+    it('throws a clear error for non-zod libraries without ~standard.jsonSchema', () => {
+        const fake = { '~standard': { version: 1, vendor: 'mylib', validate: () => ({ value: {} }) } };
+        expect(() => standardSchemaToJsonSchema(fake as unknown as SchemaArg)).toThrow(/mylib/);
+        expect(() => standardSchemaToJsonSchema(fake as unknown as SchemaArg)).toThrow(/fromJsonSchema/);
+    });
+});

pnpm-lock.yaml

@@ -50,7 +50,7 @@ catalogs:
         ajv-formats: ^3.0.1
         json-schema-typed: ^8.0.2
         pkce-challenge: ^5.0.0
-        zod: ^4.0
+        zod: ^4.2.0
 
 enableGlobalVirtualStore: false