Langchain is one of many frameworks that implement the "tool calling" pattern, where LLM applications can request that a tool is run (eg "search the web for X" or "execute this Python code") and see the result of that.
MCP is a standard for describing and exposing tools to LLM applications.
So they're not the same category of thing. Langchain could implement aspects of the MCP specification, in fact it looks like they're doing that already: https://github.com/langchain-ai/langchain-mcp-adapters
So, is MCP a spec for tool calling API?
It's a spec for how external info can get into a language model's context window. Part of that is tool calling, but there's also templated input, cancellation, progress tracking, recursive generation sampling, statefulness of tool servers, etc.
There's a MCP "server" spec for writing code that exposes tool definitions and the ability to execute them, and an MCP "client" spec for building systems that can make use of those servers.
it's more than just tools. there're prompts & resources too