diff --git a/.changeset/node-codegen-fallback.md b/.changeset/node-codegen-fallback.md
new file mode 100644
index 0000000000..7ad3e3b7be
--- /dev/null
+++ b/.changeset/node-codegen-fallback.md
@@ -0,0 +1,6 @@
+---
+'@modelcontextprotocol/server': patch
+'@modelcontextprotocol/client': patch
+---
+
+On Node with code generation from strings disallowed (`--disallow-code-generation-from-strings`), the default JSON Schema validator is now the cf-worker validator. Ajv, the default otherwise, threw an `EvalError` on the first schema, so `fromJsonSchema` and the server and client constructors failed on such hosts.
diff --git a/packages/client/src/shimsNode.ts b/packages/client/src/shimsNode.ts
index 5c411d731b..dea37df5fb 100644
--- a/packages/client/src/shimsNode.ts
+++ b/packages/client/src/shimsNode.ts
@@ -3,7 +3,27 @@
  *
  * This file is selected via package.json export conditions when running in Node.js.
  */
-export { AjvJsonSchemaValidator as DefaultJsonSchemaValidator } from '@modelcontextprotocol/core-internal/validators/ajv';
+import { AjvJsonSchemaValidator } from '@modelcontextprotocol/core-internal/validators/ajv';
+import { CfWorkerJsonSchemaValidator } from '@modelcontextprotocol/core-internal/validators/cfWorker';
+
+/**
+ * Ajv compiles validators with `new Function`. A Node process can disallow that
+ * (`--disallow-code-generation-from-strings`, or a host that sets it), and Ajv
+ * then throws an `EvalError` on the first schema. Such a process gets the
+ * validator the workerd and browser shims use, which does not generate code.
+ */
+function canGenerateCode(): boolean {
+    try {
+        new Function('');
+        return true;
+    } catch {
+        return false;
+    }
+}
+
+export const DefaultJsonSchemaValidator: typeof AjvJsonSchemaValidator | typeof CfWorkerJsonSchemaValidator = canGenerateCode()
+    ? AjvJsonSchemaValidator
+    : CfWorkerJsonSchemaValidator;
 
 /**
  * Whether `fetch()` may throw `TypeError` due to CORS. CORS is a browser-only concept —
diff --git a/packages/client/test/client/codegenDisallowed.test.ts b/packages/client/test/client/codegenDisallowed.test.ts
new file mode 100644
index 0000000000..0fc77a092a
--- /dev/null
+++ b/packages/client/test/client/codegenDisallowed.test.ts
@@ -0,0 +1,44 @@
+/**
+ * The node shim's default JSON Schema validator depends on whether the process
+ * may generate code from strings. A process started with
+ * `--disallow-code-generation-from-strings` throws an EvalError from
+ * `new Function`; the test stands that in by replacing the global `Function`
+ * while the shim module loads.
+ */
+import { afterEach, describe, expect, test, vi } from 'vitest';
+
+const RealFunction = globalThis.Function;
+
+async function loadShim(): Promise<typeof import('../../src/shimsNode')> {
+    vi.resetModules();
+    return import('../../src/shimsNode');
+}
+
+describe('node shim default JSON Schema validator', () => {
+    afterEach(() => {
+        globalThis.Function = RealFunction;
+    });
+
+    test('is Ajv when code generation is allowed', async () => {
+        const { DefaultJsonSchemaValidator } = await loadShim();
+        // Modules reload per test, so the classes are compared by name.
+        expect(DefaultJsonSchemaValidator.name).toBe('AjvJsonSchemaValidator');
+    });
+
+    test('is the cf-worker validator when code generation is disallowed, and it validates', async () => {
+        globalThis.Function = function () {
+            throw new EvalError('Code generation from strings disallowed for this context');
+        } as unknown as FunctionConstructor;
+        const { DefaultJsonSchemaValidator } = await loadShim();
+        globalThis.Function = RealFunction;
+
+        expect(DefaultJsonSchemaValidator.name).toBe('CfWorkerJsonSchemaValidator');
+        const check = new DefaultJsonSchemaValidator().getValidator<{ a: number }>({
+            type: 'object',
+            properties: { a: { type: 'number' } },
+            required: ['a']
+        });
+        expect(check({ a: 1 }).valid).toBe(true);
+        expect(check({ a: 'x' }).valid).toBe(false);
+    });
+});
diff --git a/packages/server/src/shimsNode.ts b/packages/server/src/shimsNode.ts
index 6ec703659a..6e127f5316 100644
--- a/packages/server/src/shimsNode.ts
+++ b/packages/server/src/shimsNode.ts
@@ -3,5 +3,25 @@
  *
  * This file is selected via package.json export conditions when running in Node.js.
  */
-export { AjvJsonSchemaValidator as DefaultJsonSchemaValidator } from '@modelcontextprotocol/core-internal/validators/ajv';
+import { AjvJsonSchemaValidator } from '@modelcontextprotocol/core-internal/validators/ajv';
+import { CfWorkerJsonSchemaValidator } from '@modelcontextprotocol/core-internal/validators/cfWorker';
+
+/**
+ * Ajv compiles validators with `new Function`. A Node process can disallow that
+ * (`--disallow-code-generation-from-strings`, or a host that sets it), and Ajv
+ * then throws an `EvalError` on the first schema. Such a process gets the
+ * validator the workerd and browser shims use, which does not generate code.
+ */
+function canGenerateCode(): boolean {
+    try {
+        new Function('');
+        return true;
+    } catch {
+        return false;
+    }
+}
+
+export const DefaultJsonSchemaValidator: typeof AjvJsonSchemaValidator | typeof CfWorkerJsonSchemaValidator = canGenerateCode()
+    ? AjvJsonSchemaValidator
+    : CfWorkerJsonSchemaValidator;
 export { default as process } from 'node:process';
diff --git a/packages/server/test/server/codegenDisallowed.test.ts b/packages/server/test/server/codegenDisallowed.test.ts
new file mode 100644
index 0000000000..0fc77a092a
--- /dev/null
+++ b/packages/server/test/server/codegenDisallowed.test.ts
@@ -0,0 +1,44 @@
+/**
+ * The node shim's default JSON Schema validator depends on whether the process
+ * may generate code from strings. A process started with
+ * `--disallow-code-generation-from-strings` throws an EvalError from
+ * `new Function`; the test stands that in by replacing the global `Function`
+ * while the shim module loads.
+ */
+import { afterEach, describe, expect, test, vi } from 'vitest';
+
+const RealFunction = globalThis.Function;
+
+async function loadShim(): Promise<typeof import('../../src/shimsNode')> {
+    vi.resetModules();
+    return import('../../src/shimsNode');
+}
+
+describe('node shim default JSON Schema validator', () => {
+    afterEach(() => {
+        globalThis.Function = RealFunction;
+    });
+
+    test('is Ajv when code generation is allowed', async () => {
+        const { DefaultJsonSchemaValidator } = await loadShim();
+        // Modules reload per test, so the classes are compared by name.
+        expect(DefaultJsonSchemaValidator.name).toBe('AjvJsonSchemaValidator');
+    });
+
+    test('is the cf-worker validator when code generation is disallowed, and it validates', async () => {
+        globalThis.Function = function () {
+            throw new EvalError('Code generation from strings disallowed for this context');
+        } as unknown as FunctionConstructor;
+        const { DefaultJsonSchemaValidator } = await loadShim();
+        globalThis.Function = RealFunction;
+
+        expect(DefaultJsonSchemaValidator.name).toBe('CfWorkerJsonSchemaValidator');
+        const check = new DefaultJsonSchemaValidator().getValidator<{ a: number }>({
+            type: 'object',
+            properties: { a: { type: 'number' } },
+            required: ['a']
+        });
+        expect(check({ a: 1 }).valid).toBe(true);
+        expect(check({ a: 'x' }).valid).toBe(false);
+    });
+});