diff --git a/.changeset/silence-ajv-unknown-format-warnings.md b/.changeset/silence-ajv-unknown-format-warnings.md
new file mode 100644
index 0000000000..7b9b43f940
--- /dev/null
+++ b/.changeset/silence-ajv-unknown-format-warnings.md
@@ -0,0 +1,5 @@
+---
+'@modelcontextprotocol/sdk': patch
+---
+
+Silence Ajv's "unknown format ... ignored" warnings in the default `AjvJsonSchemaValidator` logger so vendor-specific `format` annotations (e.g. `google-duration`) no longer flood the console on every `listTools` call. Validation semantics are unchanged.
diff --git a/src/validation/ajv-provider.ts b/src/validation/ajv-provider.ts
index 3c2967c3a1..61bf0a0880 100644
--- a/src/validation/ajv-provider.ts
+++ b/src/validation/ajv-provider.ts
@@ -6,12 +6,34 @@ import Ajv from 'ajv';
 import _addFormats from 'ajv-formats';
 import type { JsonSchemaType, JsonSchemaValidator, JsonSchemaValidatorResult, jsonSchemaValidator } from './types.js';
 
+/**
+ * Logger for the default Ajv instance. JSON Schema treats `format` as an annotation,
+ * so schemas are allowed to declare formats this validator does not know about (e.g.
+ * vendor-specific ones like `google-duration`), and unknown formats must be ignored
+ * silently. Ajv's default logger emits one "unknown format ... ignored" warning per
+ * schema path on every compile, so a single server-side format could flood the console
+ * on each `listTools` call. Other warnings and errors are still surfaced on the console.
+ * Callers who prefer to see (or map) unknown-format diagnostics can pass their own
+ * pre-configured Ajv instance instead.
+ */
+const defaultAjvLogger = {
+    log: (...args: unknown[]) => console.log(...args),
+    warn: (...args: unknown[]) => {
+        const message = args[0];
+        if (!(typeof message === 'string' && message.startsWith('unknown format'))) {
+            console.warn(...args);
+        }
+    },
+    error: (...args: unknown[]) => console.error(...args)
+};
+
 function createDefaultAjvInstance(): Ajv {
     const ajv = new Ajv({
         strict: false,
         validateFormats: true,
         validateSchema: false,
-        allErrors: true
+        allErrors: true,
+        logger: defaultAjvLogger
     });
 
     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..cd875fde78
--- /dev/null
+++ b/test/validation/ajv-unknown-format.test.ts
@@ -0,0 +1,81 @@
+import { afterEach, describe, expect, it, vi } from 'vitest';
+
+import { AjvJsonSchemaValidator } from '../../src/validation/ajv-provider.js';
+import type { JsonSchemaType } from '../../src/validation/types.js';
+
+/**
+ * Servers may declare vendor-specific string `format` values that this SDK does not
+ * know about (e.g. `google-duration` used by the Google Monitoring MCP server).
+ * JSON Schema treats `format` as an annotation, so those schemas must still validate,
+ * and Ajv's "unknown format ... ignored" warnings must not flood the console every
+ * time tool outputSchemas are compiled (see issue #2855).
+ */
+describe('AjvJsonSchemaValidator with unknown formats', () => {
+    afterEach(() => {
+        vi.restoreAllMocks();
+    });
+
+    const schemaWithUnknownFormat: JsonSchemaType = {
+        type: 'object',
+        properties: {
+            d: {
+                type: 'string',
+                format: 'google-duration'
+            }
+        }
+    };
+
+    it('does not warn about unknown formats', () => {
+        const warnSpy = vi.spyOn(console, 'warn').mockImplementation(() => {});
+        const validator = new AjvJsonSchemaValidator();
+
+        validator.getValidator(schemaWithUnknownFormat);
+
+        const unknownFormatWarnings = warnSpy.mock.calls.filter(
+            ([message]) => typeof message === 'string' && message.startsWith('unknown format')
+        );
+        expect(unknownFormatWarnings).toHaveLength(0);
+    });
+
+    it('stays quiet across repeated compiles of the same schema', () => {
+        const warnSpy = vi.spyOn(console, 'warn').mockImplementation(() => {});
+        const validator = new AjvJsonSchemaValidator();
+
+        for (let i = 0; i < 10; i++) {
+            validator.getValidator(schemaWithUnknownFormat);
+        }
+
+        expect(warnSpy).not.toHaveBeenCalled();
+    });
+
+    it('still validates data when the format is unknown', () => {
+        vi.spyOn(console, 'warn').mockImplementation(() => {});
+        const validator = new AjvJsonSchemaValidator();
+        const validate = validator.getValidator<{ d: string }>(schemaWithUnknownFormat);
+
+        const result = validate({ d: '1800s' });
+        expect(result.valid).toBe(true);
+        expect(result.data).toEqual({ d: '1800s' });
+
+        const invalidResult = validate({ d: 5 });
+        expect(invalidResult.valid).toBe(false);
+        expect(invalidResult.errorMessage).toBeDefined();
+    });
+
+    it('still validates known formats', () => {
+        const validator = new AjvJsonSchemaValidator();
+        const validate = validator.getValidator<{ email: string }>({
+            type: 'object',
+            properties: {
+                email: {
+                    type: 'string',
+                    format: 'email'
+                }
+            },
+            required: ['email']
+        });
+
+        expect(validate({ email: 'user@example.com' }).valid).toBe(true);
+        expect(validate({ email: 'not-an-email' }).valid).toBe(false);
+    });
+});