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Support completion/complete per MCP specification

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## Motivation and Context

The MCP specification defines `completion/complete` for providing autocompletion
suggestions for prompt arguments and resource template URIs.
The Ruby SDK previously had only a no-op handler that returned empty results,
with no way for users to define custom completion logic.

This aligns the Ruby SDK with the Python and TypeScript SDKs,
both of which support user-defined completion handlers.

## How Has This Been Tested?

Server tests cover: default handler, custom handler for `ref/prompt` and `ref/resource`,
context argument passing, 100-item truncation, and error responses for nonexistent prompts,
nonexistent resource templates, and invalid ref types.

Client tests cover: request structure, context parameter inclusion,
and fallback when result is missing.

## Breaking Changes

None. The existing default no-op handler behavior is preserved.
The `completion_handler` method is purely additive. The only behavioral change
is that `completion/complete` requests now validate that the referenced prompt
or resource template exists before calling the handler,
returning an `invalid_params` error for unknown references.
This commit is contained in:
Koichi ITO 2026-04-01 01:03:23 +09:00
commit 88e1f9a840
1 changed files with 681 additions and 2 deletions
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16
lib/mcp/client.rb
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@ -147,6 +147,22 @@ module MCP
response.fetch("result", {})
end
# Requests completion suggestions from the server for a prompt argument or resource template URI.
#
# @param ref [Hash] The reference, e.g. `{ type: "ref/prompt", name: "my_prompt" }`
# or `{ type: "ref/resource", uri: "file:///{path}" }`.
# @param argument [Hash] The argument being completed, e.g. `{ name: "language", value: "py" }`.
# @param context [Hash, nil] Optional context with previously resolved arguments.
# @return [Hash] The completion result with `"values"`, `"hasMore"`, and optionally `"total"`.
def complete(ref:, argument:, context: nil)
params = { ref: ref, argument: argument }
params[:context] = context if context
response = request(method: "completion/complete", params: params)
response.dig("result", "completion") || { "values" => [], "hasMore" => false }
end
private
def request(method:, params: nil)