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fastmcp.server.completions

Server-side argument completion for FastMCP. A completion request names a reference — a specific prompt or resource template — and the argument being completed, plus a context of the argument values already supplied. The server answers with candidate string values. FastMCP surfaces this as a single server-level handler registered with @mcp.completion, mirroring the MCP SDK’s own completion/complete shape and FastMCP’s client-side Client.complete(). The handler receives the reference, the argument, and the optional context, and returns candidates for whichever reference/argument pair it recognizes.

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Functions

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normalize_completion

Coerce a handler’s return value into a wire Completion. A returned str is rejected: it is almost always a mistake (the value would iterate into one-character candidates), so it raises rather than silently producing surprising output. The MCP contract caps a completion at 100 values, so a longer result is truncated to the first 100 with has_more set — a handler that returns thousands of matches emits a conforming response rather than an oversized one that strict clients reject.