diff --git a/.changeset/quiet-v2-ajv-unknown-formats.md b/.changeset/quiet-v2-ajv-unknown-formats.md
new file mode 100644
index 0000000000..eb530a565f
--- /dev/null
+++ b/.changeset/quiet-v2-ajv-unknown-formats.md
@@ -0,0 +1,7 @@
+---
+'@modelcontextprotocol/core-internal': patch
+'@modelcontextprotocol/client': patch
+'@modelcontextprotocol/server': patch
+---
+
+Suppress unknown-format warnings in the default Ajv validators for every supported schema dialect. Vendor-specific formats remain ignored, known formats remain validated, and other diagnostics and caller-supplied Ajv instances are unchanged.
diff --git a/packages/core-internal/src/validators/ajvProvider.ts b/packages/core-internal/src/validators/ajvProvider.ts
index e33adb741f..0ad6df254a 100644
--- a/packages/core-internal/src/validators/ajvProvider.ts
+++ b/packages/core-internal/src/validators/ajvProvider.ts
@@ -32,7 +32,18 @@ function createDefaultAjvInstance(engineClass: typeof Ajv2020 | typeof Ajv2019 |
         strict: false,
         validateFormats: true,
         validateSchema: false,
-        allErrors: true
+        allErrors: true,
+        logger: {
+            log: (...args: unknown[]) => console.log(...args),
+            warn: (...args: unknown[]) => {
+                // Unknown formats are ignored under strict: false; avoid noise for each schema path.
+                if (typeof args[0] === 'string' && args[0].startsWith('unknown format')) {
+                    return;
+                }
+                console.warn(...args);
+            },
+            error: (...args: unknown[]) => console.error(...args)
+        }
     });
     addFormats(ajv);
     return ajv;
@@ -92,7 +103,9 @@ export class AjvJsonSchemaValidator implements jsonSchemaValidator {
      * `strict: false`, `validateFormats: true`, `validateSchema: false`, `allErrors: true`, and
      * `ajv-formats` registered — **lazily, on the first {@linkcode getValidator} call needing each**, so
      * constructing the provider (e.g. as the default validator of a `Client`/`Server` that never
-     * validates a JSON Schema) does not pay the ajv + ajv-formats instantiation cost. The parameter
+     * validates a JSON Schema) does not pay the ajv + ajv-formats instantiation cost.
+     * Unknown-format warnings are suppressed by the default engines; all other diagnostics are
+     * forwarded to the console. Pass a custom engine to retain unknown-format warnings. The parameter
      * is typed structurally so consumers who don't pass an instance need not have `ajv` installed.
      */
     constructor(ajv?: AjvLike) {
diff --git a/packages/core-internal/test/validators/ajvUnknownFormat.test.ts b/packages/core-internal/test/validators/ajvUnknownFormat.test.ts
new file mode 100644
index 0000000000..34e2689a99
--- /dev/null
+++ b/packages/core-internal/test/validators/ajvUnknownFormat.test.ts
@@ -0,0 +1,93 @@
+import { Ajv as Draft7Ajv } from 'ajv';
+import { Ajv2019 } from 'ajv/dist/2019.js';
+import { Ajv2020 } from 'ajv/dist/2020.js';
+import { afterEach, describe, expect, it, vi } from 'vitest';
+
+import { AjvJsonSchemaValidator } from '../../src/validators/ajvProvider';
+import type { JsonSchemaType } from '../../src/validators/types';
+
+const vendorFormatSchema: JsonSchemaType = {
+    type: 'object',
+    properties: {
+        a: { type: 'string', format: 'google-duration' },
+        b: { type: 'string', format: 'vendor-specific-format' },
+        nested: { type: 'object', properties: { c: { type: 'string', format: 'google-duration' } } }
+    },
+    required: ['a']
+};
+
+const dialects = [
+    { name: 'default', schema: undefined, Engine: Ajv2020 },
+    { name: '2020-12', schema: 'https://json-schema.org/draft/2020-12/schema', Engine: Ajv2020 },
+    { name: '2019-09', schema: 'https://json-schema.org/draft/2019-09/schema', Engine: Ajv2019 },
+    { name: 'draft-07', schema: 'http://json-schema.org/draft-07/schema#', Engine: Draft7Ajv },
+    { name: 'draft-06', schema: 'http://json-schema.org/draft-06/schema#', Engine: Draft7Ajv }
+];
+
+afterEach(() => vi.restoreAllMocks());
+
+describe.each(dialects)('unknown formats ($name)', ({ schema: dialect, Engine }) => {
+    const schema: JsonSchemaType = { ...vendorFormatSchema, ...(dialect ? { $schema: dialect } : {}) };
+
+    it('suppresses every warning across distinct schema compilations while preserving validation', () => {
+        const warn = vi.spyOn(console, 'warn').mockImplementation(() => {});
+        const provider = new AjvJsonSchemaValidator();
+        for (let i = 0; i < 2; i++) {
+            const validate = provider.getValidator({ ...schema, $id: `urn:test:unknown-format:${i}` });
+            expect(validate({ a: 'not-a-duration', b: 'anything', nested: { c: 'anything' } }).valid).toBe(true);
+            expect(validate({ a: 42 }).valid).toBe(false);
+            expect(validate({}).valid).toBe(false);
+        }
+        expect(warn).not.toHaveBeenCalled();
+    });
+
+    it('still enforces known formats', () => {
+        const validate = new AjvJsonSchemaValidator().getValidator({
+            ...(dialect ? { $schema: dialect } : {}),
+            type: 'string',
+            format: 'email'
+        });
+        expect(validate('user@example.com').valid).toBe(true);
+        expect(validate('not-an-email').valid).toBe(false);
+    });
+
+    it('preserves caller-supplied diagnostics, including two warnings per subschema path', () => {
+        const warn = vi.spyOn(console, 'warn').mockImplementation(() => {});
+        const engine = new Engine({ strict: false, validateFormats: true, validateSchema: false, allErrors: true });
+        const logger = engine.logger;
+        new AjvJsonSchemaValidator(engine).getValidator(schema);
+        expect(engine.logger).toBe(logger);
+        // Three paths each produce two warnings with the pinned Ajv version.
+        expect(warn).toHaveBeenCalledTimes(6);
+        for (const [format, path] of [
+            ['google-duration', '#/properties/a'],
+            ['vendor-specific-format', '#/properties/b'],
+            ['google-duration', '#/properties/nested/properties/c']
+        ]) {
+            const message = `unknown format "${format}" ignored in schema at path "${path}"`;
+            expect(warn.mock.calls.filter(([value]) => value === message)).toHaveLength(2);
+        }
+    });
+});
+
+it('forwards other logger messages and their arguments unchanged', () => {
+    const warn = vi.spyOn(console, 'warn').mockImplementation(() => {});
+    const log = vi.spyOn(console, 'log').mockImplementation(() => {});
+    const error = vi.spyOn(console, 'error').mockImplementation(() => {});
+    const detail = { schema: 'example' };
+    const compile = Ajv2020.prototype.compile;
+    vi.spyOn(Ajv2020.prototype, 'compile').mockImplementation(function (this: Ajv2020, schema) {
+        this.logger.warn('another warning', detail);
+        this.logger.warn(detail, 'extra');
+        this.logger.log('unknown format diagnostic', detail);
+        this.logger.error('unknown format diagnostic', detail);
+        return compile.call(this, schema);
+    });
+    new AjvJsonSchemaValidator().getValidator({ type: 'string' });
+    expect(warn.mock.calls).toEqual([
+        ['another warning', detail],
+        [detail, 'extra']
+    ]);
+    expect(log).toHaveBeenCalledExactlyOnceWith('unknown format diagnostic', detail);
+    expect(error).toHaveBeenCalledExactlyOnceWith('unknown format diagnostic', detail);
+});