Proof Key for Code Exchange (PKCE) is a security extension for OAuth 2.0’s Authorization Code flow that addresses vulnerabilities related to authorization code interception and Cross-Site Request Forgery (CSRF) attacks. While the standard Authorization Code flow is considered secure, it cannot guarantee that the app exchanging the authorization code is the same one that requested it, leaving room for security breaches. PKCE enhances this flow by introducing a "code verifier," which creates a "code challenge" to authenticate the client, ensuring the app requesting the tokens is indeed the one that initiated the request. This mechanism is crucial for public clients, such as single-page and native mobile applications, which cannot securely store client secrets, but it also benefits confidential clients by providing additional protection against potential threats. PKCE is now mandatory in the OAuth 2.1 specification, underscoring its importance in securing authorization processes across different client types. The adoption of PKCE is further emphasized in the Model Context Protocol (MCP) for AI agents and Large Language Models, ensuring secure integration with external tools and APIs.