diff --git a/packages/client/src/client/auth.ts b/packages/client/src/client/auth.ts
index 1a021be18..a0ad7f415 100644
--- a/packages/client/src/client/auth.ts
+++ b/packages/client/src/client/auth.ts
@@ -25,6 +25,14 @@ import {
 } from '@modelcontextprotocol/core';
 import pkceChallenge from 'pkce-challenge';
 
+/**
+ * Deduplicates concurrent auth() calls for the same provider.
+ * When multiple requests hit a 401 simultaneously, only one auth refresh
+ * runs — all callers share the same Promise and receive the same result.
+ * This prevents race conditions with rotating refresh tokens (RFC 6819 §5.2.2.3).
+ */
+const pendingAuth = new Map<OAuthClientProvider, Promise<AuthResult>>();
+
 /**
  * Function type for adding client authentication to token requests.
  */
@@ -547,23 +555,40 @@ export async function auth(
         fetchFn?: FetchLike;
     }
 ): Promise<AuthResult> {
-    try {
-        return await authInternal(provider, options);
-    } catch (error) {
-        // Handle recoverable error types by invalidating credentials and retrying
-        if (error instanceof OAuthError) {
-            if (error.code === OAuthErrorCode.InvalidClient || error.code === OAuthErrorCode.UnauthorizedClient) {
-                await provider.invalidateCredentials?.('all');
-                return await authInternal(provider, options);
-            } else if (error.code === OAuthErrorCode.InvalidGrant) {
-                await provider.invalidateCredentials?.('tokens');
-                return await authInternal(provider, options);
+    // Deduplicate concurrent auth calls for the same provider.
+    // This prevents a race condition where two simultaneous 401 responses both
+    // trigger auth(), both call refreshAuthorization() with the same rotating
+    // refresh token, and the second one gets InvalidGrant because the server
+    // already consumed the token (RFC 6819 §5.2.2.3 replay detection).
+    const pending = pendingAuth.get(provider);
+    if (pending !== undefined) {
+        return pending;
+    }
+
+    const promise = (async () => {
+        try {
+            return await authInternal(provider, options);
+        } catch (error) {
+            // Handle recoverable error types by invalidating credentials and retrying
+            if (error instanceof OAuthError) {
+                if (error.code === OAuthErrorCode.InvalidClient || error.code === OAuthErrorCode.UnauthorizedClient) {
+                    await provider.invalidateCredentials?.('all');
+                    return await authInternal(provider, options);
+                } else if (error.code === OAuthErrorCode.InvalidGrant) {
+                    await provider.invalidateCredentials?.('tokens');
+                    return await authInternal(provider, options);
+                }
             }
+
+            // Throw otherwise
+            throw error;
+        } finally {
+            pendingAuth.delete(provider);
         }
+    })();
 
-        // Throw otherwise
-        throw error;
-    }
+    pendingAuth.set(provider, promise);
+    return promise;
 }
 
 async function authInternal(
@@ -1439,6 +1464,10 @@ 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,