diff --git a/.changeset/fix-unsafe-request-id.md b/.changeset/fix-unsafe-request-id.md
new file mode 100644
index 000000000..adb9b163a
--- /dev/null
+++ b/.changeset/fix-unsafe-request-id.md
@@ -0,0 +1,5 @@
+---
+"@modelcontextprotocol/core": patch
+---
+
+fix: reject request IDs exceeding Number.MAX_SAFE_INTEGER
diff --git a/packages/core/src/types/schemas.ts b/packages/core/src/types/schemas.ts
index 309b6ade2..cca62be1e 100644
--- a/packages/core/src/types/schemas.ts
+++ b/packages/core/src/types/schemas.ts
@@ -119,7 +119,15 @@ export const ResultSchema = z.looseObject({
 /**
  * A uniquely identifying ID for a request in JSON-RPC.
  */
-export const RequestIdSchema = z.union([z.string(), z.number().int()]);
+export const RequestIdSchema = z.union([
+    z.string(),
+    z
+        .number()
+        .int()
+        .refine(n => n >= Number.MIN_SAFE_INTEGER && n <= Number.MAX_SAFE_INTEGER, {
+            message: 'Request ID must be within Number.MAX_SAFE_INTEGER range'
+        })
+]);
 
 /**
  * A request that expects a response.
diff --git a/packages/core/test/types.test.ts b/packages/core/test/types.test.ts
index 429b3ecdd..9222838a3 100644
--- a/packages/core/test/types.test.ts
+++ b/packages/core/test/types.test.ts
@@ -1,4 +1,5 @@
 import {
+    RequestIdSchema,
     CallToolResultSchema,
     ClientCapabilitiesSchema,
     CompleteRequestSchema,
@@ -984,3 +985,23 @@ describe('Types', () => {
         });
     });
 });
+
+describe('RequestIdSchema', () => {
+    test('should accept string IDs', () => {
+        expect(RequestIdSchema.parse('abc-123')).toBe('abc-123');
+    });
+
+    test('should accept safe integer IDs', () => {
+        expect(RequestIdSchema.parse(1)).toBe(1);
+        expect(RequestIdSchema.parse(Number.MAX_SAFE_INTEGER)).toBe(Number.MAX_SAFE_INTEGER);
+    });
+
+    test('should reject IDs exceeding MAX_SAFE_INTEGER', () => {
+        expect(() => RequestIdSchema.parse(Number.MAX_SAFE_INTEGER + 1)).toThrow();
+        expect(() => RequestIdSchema.parse(9007199254740992)).toThrow();
+    });
+
+    test('should reject non-integer numeric IDs', () => {
+        expect(() => RequestIdSchema.parse(1.5)).toThrow();
+    });
+});