What's the difference between tool calling and MCP?

Tool calling is the model choosing a function and filling in its arguments; MCP is a protocol that decides where that function’s definition and execution actually live, hosted on a separate server rather than baked into your own application code. An MCP server advertises what it offers over a standard request and executes the call when asked, so the same server can serve tools to any MCP-aware client without custom integration code per client.

From the model’s side, nothing changes: it still emits the same structured call naming a tool and its arguments, whether that tool is a local function or one an MCP server exposes. What MCP adds is a layer the model never sees and that still has to work correctly: a cached tool list that goes stale when a server changes its schema, a timeout for a server that never answers, and a protocol-level error path that’s separate from the tool simply failing. Tool calling is what the model does; MCP is one way of supplying and running the tools it calls.

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More on this.

What's the difference between tool calling and RAG?Tool calling is the model deciding, mid-response, to fetch something or act; RAG retrieves documents automatically before the model answers, no decision involved.answer →What's the difference between tool calling and structured output?Tool calling lets the model invoke a function mid-response and get a result back; structured output only constrains its final answer's JSON shape.answer →What's the difference between tool calling and the ReAct pattern?Tool calling is the model emitting a structured call naming a function and its arguments; ReAct is an older prompting pattern that interleaves a reasoning step with each action.answer →Everything on tool callingThe definition, and every question answered under tool calling.concept →

Two ways to run Tessary.

Tessary is an open-source agent reliability platform. Cloud and self-hosted run the same workflow on the OpenTelemetry traces your agent already emits.

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Self-host Tessary for me by following https://github.com/tessaryai/tessary/blob/main/setup.md

docker compose -f oci://docker.io/tessaryai/tessary:compose up -d -y