diff --git a/src/radius-mcp-sdk.ts b/src/radius-mcp-sdk.ts
index c141581..24372ef 100644
--- a/src/radius-mcp-sdk.ts
+++ b/src/radius-mcp-sdk.ts
@@ -10,7 +10,7 @@ import {
   recoverTypedDataAddress,
   type TypedData,
 } from 'viem';
-import type { CacheConfig, RadiusConfig, MCPHandler, MCPRequest, MCPResponse, EVMAuthErrorResponse, EVMAuthProof, ProofErrorCode } from './types/index.js';
+import type { CacheConfig, RadiusConfig, MCPRequest, MCPResponse, EVMAuthErrorResponse, EVMAuthProof, ProofErrorCode } from './types/index.js';
 import { RadiusError } from './types/errors.js';
 
 const ERC1155_ABI = [
@@ -148,10 +148,10 @@ export class RadiusMcpSdk {
     }
   }
 
-  protect(tokenId: number | number[], handler: MCPHandler): MCPHandler {
+  protect<T extends (...args: any[]) => any>(tokenId: number | number[], handler: T): T {
     const tokenIds = Array.isArray(tokenId) ? tokenId : [tokenId];
 
-    return async (request: MCPRequest, extra?: unknown): Promise<MCPResponse> => {
+    return (async (request: MCPRequest, extra?: unknown): Promise<MCPResponse> => {
       const authFlowId = `${Date.now()}-${Math.random().toString(36).substring(2, 11)}`;
 
       if (this.config.debug) {
@@ -274,7 +274,7 @@ export class RadiusMcpSdk {
         }
         return this.handleError(error as Error, tokenIds, toolName);
       }
-    };
+    }) as T;
   }
 
   private extractProof(request: MCPRequest): EVMAuthProof | null {
diff --git a/src/types/errors.ts b/src/types/errors.ts
index df45c9f..b0e3951 100644
--- a/src/types/errors.ts
+++ b/src/types/errors.ts
@@ -1,3 +1,23 @@
+export const RadiusErrorCode = {
+  INVALID_CONFIG: 'INVALID_CONFIG',
+  PROOF_INVALID: 'PROOF_INVALID',
+  PROOF_EXPIRED: 'PROOF_EXPIRED',
+  PROOF_MALFORMED: 'PROOF_MALFORMED',
+  CHAIN_MISMATCH: 'CHAIN_MISMATCH',
+  CONTRACT_ERROR: 'CONTRACT_ERROR',
+  NETWORK_ERROR: 'NETWORK_ERROR',
+  SIGNATURE_INVALID: 'SIGNATURE_INVALID',
+  CONTRACT_MISMATCH: 'CONTRACT_MISMATCH',
+  SIGNER_MISMATCH: 'SIGNER_MISMATCH',
+  NONCE_INVALID: 'NONCE_INVALID',
+  PROOF_MISSING: 'PROOF_MISSING',
+  AUTH_PROOF_MISSING: 'AUTH_PROOF_MISSING',
+  AUTH_INVALID: 'AUTH_INVALID',
+  TOKEN_MISSING: 'TOKEN_MISSING'
+} as const;
+
+export type RadiusErrorCode = typeof RadiusErrorCode[keyof typeof RadiusErrorCode];
+
 export class RadiusError extends Error {
   constructor(
     public code: RadiusErrorCode,
@@ -8,17 +28,3 @@ export class RadiusError extends Error {
     this.name = 'RadiusError';
   }
 }
-
-export type RadiusErrorCode =
-  | 'INVALID_CONFIG'
-  | 'PROOF_INVALID'
-  | 'PROOF_EXPIRED'
-  | 'PROOF_MALFORMED'
-  | 'CHAIN_MISMATCH'
-  | 'CONTRACT_ERROR'
-  | 'NETWORK_ERROR'
-  | 'SIGNATURE_INVALID'
-  | 'CONTRACT_MISMATCH'
-  | 'SIGNER_MISMATCH'
-  | 'NONCE_INVALID'
-  | 'PROOF_MISSING';