diff --git a/.changeset/quiet-unknown-format-warnings.md b/.changeset/quiet-unknown-format-warnings.md
new file mode 100644
index 0000000000..956565b6db
--- /dev/null
+++ b/.changeset/quiet-unknown-format-warnings.md
@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/sdk': patch
+---
+
+Silence unknown-format warnings from the default AJV validator. Servers using custom formats no longer spam one warning per subschema on listTools; other warnings and known-format validation are unchanged.
diff --git a/src/validation/ajv-provider.ts b/src/validation/ajv-provider.ts
index 3c2967c3a1..404475cbac 100644
--- a/src/validation/ajv-provider.ts
+++ b/src/validation/ajv-provider.ts
@@ -11,7 +11,18 @@ function createDefaultAjvInstance(): Ajv {
         strict: false,
         validateFormats: true,
         validateSchema: false,
-        allErrors: true
+        allErrors: true,
+        // Servers may use custom formats Ajv does not know (e.g. google-duration).
+        // Ajv skips validation for those with one warning per subschema, which
+        // spams listTools output. Drop only that noise; keep other warnings.
+        logger: {
+            log: console.log.bind(console),
+            error: console.error.bind(console),
+            warn: (message: string, ...params: unknown[]) => {
+                if (message.startsWith('unknown format')) return;
+                console.warn(message, ...params);
+            }
+        }
     });
 
     const addFormats = _addFormats as unknown as typeof _addFormats.default;
diff --git a/test/validation/ajv-unknown-format.test.ts b/test/validation/ajv-unknown-format.test.ts
new file mode 100644
index 0000000000..85b0cdf7c5
--- /dev/null
+++ b/test/validation/ajv-unknown-format.test.ts
@@ -0,0 +1,39 @@
+/**
+ * Unknown custom formats (e.g. google-duration) should not spam the console.
+ * The default AJV instance skips validation for formats it does not know;
+ * it should do so quietly while still enforcing known formats.
+ */
+
+import { vi } from 'vitest';
+
+import { AjvJsonSchemaValidator } from '../../src/validation/ajv-provider.js';
+import type { JsonSchemaType } from '../../src/validation/types.js';
+
+describe('AjvJsonSchemaValidator unknown formats', () => {
+    it('compiles custom formats without warning and skips their validation', () => {
+        const warn = vi.spyOn(console, 'warn').mockImplementation(() => {});
+        try {
+            const schema: JsonSchemaType = {
+                type: 'object',
+                properties: {
+                    alignmentPeriod: { type: 'string', format: 'google-duration' }
+                }
+            };
+            const validator = new AjvJsonSchemaValidator().getValidator(schema);
+
+            expect(warn).not.toHaveBeenCalled();
+            expect(validator({ alignmentPeriod: '3600s' }).valid).toBe(true);
+            expect(validator({ alignmentPeriod: 'bogus' }).valid).toBe(true);
+        } finally {
+            warn.mockRestore();
+        }
+    });
+
+    it('still enforces known formats', () => {
+        const schema: JsonSchemaType = { type: 'string', format: 'email' };
+        const validator = new AjvJsonSchemaValidator().getValidator(schema);
+
+        expect(validator('user@example.com').valid).toBe(true);
+        expect(validator('invalid-email').valid).toBe(false);
+    });
+});