diff --git a/packages/client/src/client/auth.ts b/packages/client/src/client/auth.ts
index 1a021be18..0391e35e9 100644
--- a/packages/client/src/client/auth.ts
+++ b/packages/client/src/client/auth.ts
@@ -1439,6 +1439,10 @@ export async function executeTokenRequest(
         applyClientAuthentication(authMethod, clientInformation as OAuthClientInformation, headers, tokenRequestParams);
     }
 
+    // Ensure Content-Type is always form-urlencoded for the token endpoint (OAuth 2.1 §3.2).
+    // Some addClientAuthentication implementations may have inadvertently set a different value.
+    headers.set('Content-Type', 'application/x-www-form-urlencoded');
+
     const response = await (fetchFn ?? fetch)(tokenUrl, {
         method: 'POST',
         headers,
diff --git a/packages/core/src/types/schemas.ts b/packages/core/src/types/schemas.ts
index c8f0c9978..a8cf1d77a 100644
--- a/packages/core/src/types/schemas.ts
+++ b/packages/core/src/types/schemas.ts
@@ -119,8 +119,18 @@ 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()]);
+ * Rejects integers outside Number.MAX_SAFE_INTEGER to prevent JavaScript
+ * Map/Object key precision loss that causes server hangs (Issue #1765).
+ */
+export const RequestIdSchema = z.union([
+    z.string(),
+    z
+        .number()
+        .int()
+        .refine((n) => Math.abs(n) <= Number.MAX_SAFE_INTEGER, {
+            message: `Request ID must be a safe integer (|id| ≤ ${Number.MAX_SAFE_INTEGER})`
+        })
+]);
 
 /**
  * A request that expects a response.