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.

Functions

normalize_completion ^{<a href="https://github.com/PrefectHQ/fastmcp/blob/main/fastmcp_slim/fastmcp/server/completions.py#L54" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a>}

normalize_completion(result: CompletionValues) -> mcp_types.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.