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MCP Core Concept

Short definitions of tool, resource, and prompt in MCP (and in this doc), and where they appear.

What is a tool?​

In MCP, a tool is an action the client can ask a server to run. The server advertises each tool with a name, description, and input parameters; the client invokes it with arguments and gets a result back.

  • Example: getTrainStatus(train_no), getPNRStatus(pnr) — the client calls them with inputs; the server runs them and returns output.
  • Where it’s mentioned in this doc:
    Intro (official definition: “tools e.g. search engines, calculators”),
    How MCP is masked PRIMER (table: “Functions” → MCP tools; invoke_action → client calls a tool; implementation sketch).

What is a resource?​

In MCP, a resource is read-only data the client can fetch by a URI. It is not an action to “invoke” — the client reads it (e.g. contents of a file, or a lookup like train name → number).

  • Example: A train name → number mapping, or a file on disk. The client requests the resource by URI and gets back the data.
  • Where it’s mentioned in this doc:
    How MCP is masked PRIMER (table: getTrainNumberMapping → MCP resource; “read-only data the client fetches by URI”; implementation sketch).

What is a prompt?​

In this documentation we use prompt in two ways:

  1. PRIMER system prompt — The text we give the model that lists the six steps (prompt_input, reason_plan, invoke_action, monitor_result, explain_output, recover_error) and the available functions. The model is instructed to respond with JSON for each step.
    Where it’s mentioned:
    What is PRIMER (“The prompt” section, full prompt in a code block),
    How MCP is masked PRIMER (“Prompt with part-by-part mapping”, diagram).

  2. MCP / official docs — The official MCP definition says AI apps can connect to “workflows (e.g. specialized prompts)”. So in the broader MCP world, prompts can be part of workflows (e.g. reusable prompt templates). In our PRIMER→MCP mapping, the prompt_input step is the user’s message that the client receives, not an MCP “prompt” type by itself.

Summary: In our pages, “the prompt” usually means the PRIMER system prompt. “Tool” = action to invoke; “resource” = read-only data by URI; “prompt” = that system prompt or (in MCP) workflow-related prompts.