Merge pull request #289 from koic/feature_completion_complete

Support `completion/complete` per MCP specification

README.md

@@ -50,6 +50,7 @@ It implements the Model Context Protocol specification, handling model context r
 - `resources/list` - Lists all registered resources and their schemas
 - `resources/read` - Retrieves a specific resource by name
 - `resources/templates/list` - Lists all registered resource templates and their schemas
+- `completion/complete` - Returns autocompletion suggestions for prompt arguments and resource URIs
 
 ### Custom Methods
 
@@ -183,6 +184,53 @@ The `server_context.report_progress` method accepts:
 - `report_progress` is a no-op when no `progressToken` was provided by the client
 - Supports both numeric and string progress tokens
 
+### Completions
+
+MCP spec includes [Completions](https://modelcontextprotocol.io/specification/latest/server/utilities/completion),
+which enable servers to provide autocompletion suggestions for prompt arguments and resource URIs.
+
+To enable completions, declare the `completions` capability and register a handler:
+
+```ruby
+server = MCP::Server.new(
+  name: "my_server",
+  prompts: [CodeReviewPrompt],
+  resource_templates: [FileTemplate],
+  capabilities: { completions: {} },
+)
+
+server.completion_handler do |params|
+  ref = params[:ref]
+  argument = params[:argument]
+  value = argument[:value]
+
+  case ref[:type]
+  when "ref/prompt"
+    values = case argument[:name]
+    when "language"
+      ["python", "pytorch", "pyside"].select { |v| v.start_with?(value) }
+    else
+      []
+    end
+    { completion: { values: values, hasMore: false } }
+  when "ref/resource"
+    { completion: { values: [], hasMore: false } }
+  end
+end
+```
+
+The handler receives a `params` hash with:
+
+- `ref` - The reference (`{ type: "ref/prompt", name: "..." }` or `{ type: "ref/resource", uri: "..." }`)
+- `argument` - The argument being completed (`{ name: "...", value: "..." }`)
+- `context` (optional) - Previously resolved arguments (`{ arguments: { ... } }`)
+
+The handler must return a hash with a `completion` key containing `values` (array of strings), and optionally `total` and `hasMore`.
+The SDK automatically enforces the 100-item limit per the MCP specification.
+
+The server validates that the referenced prompt, resource, or resource template is registered before calling the handler.
+Requests for unknown references return an error.
+
 ### Logging
 
 The MCP Ruby SDK supports structured logging through the `notify_log_message` method, following the [MCP Logging specification](https://modelcontextprotocol.io/specification/latest/server/utilities/logging).
@@ -298,7 +346,6 @@ transport = MCP::Server::Transports::StreamableHTTPTransport.new(server, session
 ### Unsupported Features (to be implemented in future versions)
 
 - Resource subscriptions
-- Completions
 - Elicitation
 
 ### Usage
@@ -1056,6 +1103,7 @@ This class supports:
 - Resource reading via the `resources/read` method (`MCP::Client#read_resources`)
 - Prompt listing via the `prompts/list` method (`MCP::Client#prompts`)
 - Prompt retrieval via the `prompts/get` method (`MCP::Client#get_prompt`)
+- Completion requests via the `completion/complete` method (`MCP::Client#complete`)
 - Automatic JSON-RPC 2.0 message formatting
 - UUID request ID generation
 

conformance/server.rb

@@ -488,6 +488,7 @@ module Conformance
       server.server_context = server
 
       configure_resources_read_handler(server)
+      configure_completion_handler(server)
     end
 
     def configure_resources_read_handler(server)
@@ -528,6 +529,35 @@ module Conformance
       end
     end
 
+    def configure_completion_handler(server)
+      server.completion_handler do |params|
+        ref = params[:ref]
+        argument = params[:argument]
+        value = argument[:value].to_s
+
+        case ref[:type]
+        when "ref/prompt"
+          case ref[:name]
+          when "test_prompt_with_arguments"
+            candidates = case argument[:name]
+            when "arg1"
+              ["value1", "value2", "value3"]
+            when "arg2"
+              ["optionA", "optionB", "optionC"]
+            else
+              []
+            end
+            values = candidates.select { |v| v.start_with?(value) }
+            { completion: { values: values, hasMore: false } }
+          else
+            { completion: { values: [], hasMore: false } }
+          end
+        else
+          { completion: { values: [], hasMore: false } }
+        end
+      end
+    end
+
     def build_rack_app(transport)
       mcp_app = proc do |env|
         request = Rack::Request.new(env)

lib/mcp/client.rb

@@ -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)

lib/mcp/server.rb

@@ -24,6 +24,12 @@ module MCP
     UNSUPPORTED_PROPERTIES_UNTIL_2025_06_18 = [:description, :icons].freeze
     UNSUPPORTED_PROPERTIES_UNTIL_2025_03_26 = [:title, :websiteUrl].freeze
 
+    DEFAULT_COMPLETION_RESULT = { completion: { values: [], hasMore: false } }.freeze
+
+    # Servers return an array of completion values ranked by relevance, with maximum 100 items per response.
+    # https://modelcontextprotocol.io/specification/2025-11-25/server/utilities/completion#completion-results
+    MAX_COMPLETION_VALUES = 100
+
     class RequestHandlerError < StandardError
       attr_reader :error_type
       attr_reader :original_error
@@ -100,12 +106,12 @@ module MCP
         Methods::PING => ->(_) { {} },
         Methods::NOTIFICATIONS_INITIALIZED => ->(_) {},
         Methods::NOTIFICATIONS_PROGRESS => ->(_) {},
+        Methods::COMPLETION_COMPLETE => ->(_) { DEFAULT_COMPLETION_RESULT },
         Methods::LOGGING_SET_LEVEL => method(:configure_logging_level),
 
         # No op handlers for currently unsupported methods
         Methods::RESOURCES_SUBSCRIBE => ->(_) { {} },
         Methods::RESOURCES_UNSUBSCRIBE => ->(_) { {} },
-        Methods::COMPLETION_COMPLETE => ->(_) { { completion: { values: [], hasMore: false } } },
         Methods::ELICITATION_CREATE => ->(_) {},
       }
       @transport = transport
@@ -208,6 +214,15 @@ module MCP
       @handlers[Methods::RESOURCES_READ] = block
     end
 
+    # Sets a custom handler for `completion/complete` requests.
+    # The block receives the parsed request params and should return completion values.
+    #
+    # @yield [params] The request params containing `:ref`, `:argument`, and optionally `:context`.
+    # @yieldreturn [Hash] A hash with `:completion` key containing `:values`, optional `:total`, and `:hasMore`.
+    def completion_handler(&block)
+      @handlers[Methods::COMPLETION_COMPLETE] = block
+    end
+
     private
 
     def validate!
@@ -307,6 +322,8 @@ module MCP
             { resourceTemplates: @handlers[Methods::RESOURCES_TEMPLATES_LIST].call(params) }
           when Methods::TOOLS_CALL
             call_tool(params, session: session)
+          when Methods::COMPLETION_COMPLETE
+            complete(params)
           when Methods::LOGGING_SET_LEVEL
             configure_logging_level(params, session: session)
           else
@@ -481,6 +498,14 @@ module MCP
       @resource_templates.map(&:to_h)
     end
 
+    def complete(params)
+      validate_completion_params!(params)
+
+      result = @handlers[Methods::COMPLETION_COMPLETE].call(params)
+
+      normalize_completion_result(result)
+    end
+
     def report_exception(exception, server_context = {})
       configuration.exception_reporter.call(exception, server_context)
     end
@@ -539,5 +564,56 @@ module MCP
         server_context
       end
     end
+
+    def validate_completion_params!(params)
+      unless params.is_a?(Hash)
+        raise RequestHandlerError.new("Invalid params", params, error_type: :invalid_params)
+      end
+
+      ref = params[:ref]
+      if ref.nil? || ref[:type].nil?
+        raise RequestHandlerError.new("Missing or invalid ref", params, error_type: :invalid_params)
+      end
+
+      argument = params[:argument]
+      if argument.nil? || argument[:name].nil? || !argument.key?(:value)
+        raise RequestHandlerError.new("Missing argument name or value", params, error_type: :invalid_params)
+      end
+
+      case ref[:type]
+      when "ref/prompt"
+        unless @prompts[ref[:name]]
+          raise RequestHandlerError.new("Prompt not found: #{ref[:name]}", params, error_type: :invalid_params)
+        end
+      when "ref/resource"
+        uri = ref[:uri]
+        found = @resource_index.key?(uri) || @resource_templates.any? { |t| t.uri_template == uri }
+        unless found
+          raise RequestHandlerError.new("Resource not found: #{uri}", params, error_type: :invalid_params)
+        end
+      else
+        raise RequestHandlerError.new("Invalid ref type: #{ref[:type]}", params, error_type: :invalid_params)
+      end
+    end
+
+    def normalize_completion_result(result)
+      return DEFAULT_COMPLETION_RESULT unless result.is_a?(Hash)
+
+      completion = result[:completion] || result["completion"]
+      return DEFAULT_COMPLETION_RESULT unless completion.is_a?(Hash)
+
+      values = completion[:values] || completion["values"] || []
+      total = completion[:total] || completion["total"]
+      has_more = completion[:hasMore] || completion["hasMore"] || false
+
+      count = values.length
+      if count > MAX_COMPLETION_VALUES
+        has_more = true
+        total ||= count
+        values = values.first(MAX_COMPLETION_VALUES)
+      end
+
+      { completion: { values: values, total: total, hasMore: has_more }.compact }
+    end
   end
 end

test/mcp/client_test.rb

@@ -456,5 +456,86 @@ module MCP
       error = assert_raises(Client::ServerError) { client.tools }
       assert_equal("extra details", error.data)
     end
+
+    def test_complete_raises_server_error_on_error_response
+      transport = mock
+      mock_response = { "error" => { "code" => -32_602, "message" => "Invalid params" } }
+
+      transport.expects(:send_request).returns(mock_response).once
+
+      client = Client.new(transport: transport)
+      error = assert_raises(Client::ServerError) { client.complete(ref: { type: "ref/prompt", name: "missing" }, argument: { name: "arg", value: "" }) }
+      assert_equal(-32_602, error.code)
+    end
+
+    def test_complete_sends_request_and_returns_completion_result
+      transport = mock
+      mock_response = {
+        "result" => {
+          "completion" => {
+            "values" => ["python", "pytorch"],
+            "hasMore" => false,
+          },
+        },
+      }
+
+      transport.expects(:send_request).with do |args|
+        args.dig(:request, :method) == "completion/complete" &&
+          args.dig(:request, :jsonrpc) == "2.0" &&
+          args.dig(:request, :params, :ref) == { type: "ref/prompt", name: "code_review" } &&
+          args.dig(:request, :params, :argument) == { name: "language", value: "py" } &&
+          !args.dig(:request, :params).key?(:context)
+      end.returns(mock_response).once
+
+      client = Client.new(transport: transport)
+      result = client.complete(
+        ref: { type: "ref/prompt", name: "code_review" },
+        argument: { name: "language", value: "py" },
+      )
+
+      assert_equal(["python", "pytorch"], result["values"])
+      refute(result["hasMore"])
+    end
+
+    def test_complete_includes_context_when_provided
+      transport = mock
+      mock_response = {
+        "result" => {
+          "completion" => {
+            "values" => ["flask"],
+            "hasMore" => false,
+          },
+        },
+      }
+
+      transport.expects(:send_request).with do |args|
+        args.dig(:request, :params, :context) == { arguments: { language: "python" } }
+      end.returns(mock_response).once
+
+      client = Client.new(transport: transport)
+      result = client.complete(
+        ref: { type: "ref/prompt", name: "code_review" },
+        argument: { name: "framework", value: "fla" },
+        context: { arguments: { language: "python" } },
+      )
+
+      assert_equal(["flask"], result["values"])
+    end
+
+    def test_complete_returns_default_when_result_is_missing
+      transport = mock
+      mock_response = { "result" => {} }
+
+      transport.expects(:send_request).returns(mock_response).once
+
+      client = Client.new(transport: transport)
+      result = client.complete(
+        ref: { type: "ref/prompt", name: "test" },
+        argument: { name: "arg", value: "" },
+      )
+
+      assert_equal([], result["values"])
+      refute(result["hasMore"])
+    end
   end
 end

test/mcp/server_test.rb

@@ -1468,8 +1468,10 @@ module MCP
     end
 
     test "#handle completion/complete returns default completion result" do
+      prompt = Prompt.define(name: "test") {}
       server = Server.new(
         name: "test_server",
+        prompts: [prompt],
         capabilities: { completions: {} },
       )
 
@@ -1496,6 +1498,432 @@ module MCP
       )
     end
 
+    test "#handle completion/complete with custom handler for ref/prompt" do
+      prompt = Prompt.define(
+        name: "code_review",
+        arguments: [Prompt::Argument.new(name: "language", required: true)],
+      ) {}
+      server = Server.new(
+        name: "test_server",
+        prompts: [prompt],
+        capabilities: { completions: {} },
+      )
+
+      server.completion_handler do |_params|
+        { completion: { values: ["python", "pytorch", "pyside"], total: 10, hasMore: true } }
+      end
+
+      server.handle({ jsonrpc: "2.0", method: "initialize", id: 1 })
+      server.handle({ jsonrpc: "2.0", method: "notifications/initialized" })
+
+      response = server.handle({
+        jsonrpc: "2.0",
+        id: 2,
+        method: "completion/complete",
+        params: {
+          ref: { type: "ref/prompt", name: "code_review" },
+          argument: { name: "language", value: "py" },
+        },
+      })
+
+      assert_equal(
+        {
+          jsonrpc: "2.0",
+          id: 2,
+          result: { completion: { values: ["python", "pytorch", "pyside"], total: 10, hasMore: true } },
+        },
+        response,
+      )
+    end
+
+    test "#handle completion/complete with custom handler for ref/resource" do
+      template = ResourceTemplate.new(
+        uri_template: "file:///{path}",
+        name: "file",
+      )
+      server = Server.new(
+        name: "test_server",
+        resource_templates: [template],
+        capabilities: { completions: {} },
+      )
+
+      server.completion_handler do |_params|
+        { completion: { values: ["file:///src", "file:///spec"], hasMore: false } }
+      end
+
+      server.handle({ jsonrpc: "2.0", method: "initialize", id: 1 })
+      server.handle({ jsonrpc: "2.0", method: "notifications/initialized" })
+
+      response = server.handle({
+        jsonrpc: "2.0",
+        id: 2,
+        method: "completion/complete",
+        params: {
+          ref: { type: "ref/resource", uri: "file:///{path}" },
+          argument: { name: "path", value: "s" },
+        },
+      })
+
+      assert_equal(
+        {
+          jsonrpc: "2.0",
+          id: 2,
+          result: { completion: { values: ["file:///src", "file:///spec"], hasMore: false } },
+        },
+        response,
+      )
+    end
+
+    test "#handle completion/complete passes context arguments to handler" do
+      prompt = Prompt.define(
+        name: "code_review",
+        arguments: [
+          Prompt::Argument.new(name: "language", required: true),
+          Prompt::Argument.new(name: "framework", required: false),
+        ],
+      ) {}
+      server = Server.new(
+        name: "test_server",
+        prompts: [prompt],
+        capabilities: { completions: {} },
+      )
+
+      received_params = nil
+      server.completion_handler do |params|
+        received_params = params
+        { completion: { values: ["flask"], hasMore: false } }
+      end
+
+      server.handle({ jsonrpc: "2.0", method: "initialize", id: 1 })
+      server.handle({ jsonrpc: "2.0", method: "notifications/initialized" })
+
+      server.handle({
+        jsonrpc: "2.0",
+        id: 2,
+        method: "completion/complete",
+        params: {
+          ref: { type: "ref/prompt", name: "code_review" },
+          argument: { name: "framework", value: "fla" },
+          context: { arguments: { language: "python" } },
+        },
+      })
+
+      assert_equal({ language: "python" }, received_params.dig(:context, :arguments))
+    end
+
+    test "#handle completion/complete truncates values exceeding 100 items" do
+      prompt = Prompt.define(name: "test") {}
+      server = Server.new(
+        name: "test_server",
+        prompts: [prompt],
+        capabilities: { completions: {} },
+      )
+
+      server.completion_handler do |_params|
+        { completion: { values: (1..150).map(&:to_s), hasMore: false } }
+      end
+
+      server.handle({ jsonrpc: "2.0", method: "initialize", id: 1 })
+      server.handle({ jsonrpc: "2.0", method: "notifications/initialized" })
+
+      response = server.handle({
+        jsonrpc: "2.0",
+        id: 2,
+        method: "completion/complete",
+        params: {
+          ref: { type: "ref/prompt", name: "test" },
+          argument: { name: "arg", value: "" },
+        },
+      })
+
+      completion = response[:result][:completion]
+      assert_equal 100, completion[:values].length
+      assert_equal "1", completion[:values].first
+      assert_equal "100", completion[:values].last
+      assert(completion[:hasMore])
+      assert_equal 150, completion[:total]
+    end
+
+    test "#handle completion/complete returns error for nonexistent prompt" do
+      server = Server.new(
+        name: "test_server",
+        capabilities: { completions: {} },
+      )
+
+      server.handle({ jsonrpc: "2.0", method: "initialize", id: 1 })
+      server.handle({ jsonrpc: "2.0", method: "notifications/initialized" })
+
+      response = server.handle({
+        jsonrpc: "2.0",
+        id: 2,
+        method: "completion/complete",
+        params: {
+          ref: { type: "ref/prompt", name: "nonexistent" },
+          argument: { name: "arg", value: "val" },
+        },
+      })
+
+      assert_equal(-32_602, response[:error][:code])
+    end
+
+    test "#handle completion/complete returns error for nonexistent resource template" do
+      server = Server.new(
+        name: "test_server",
+        capabilities: { completions: {} },
+      )
+
+      server.handle({ jsonrpc: "2.0", method: "initialize", id: 1 })
+      server.handle({ jsonrpc: "2.0", method: "notifications/initialized" })
+
+      response = server.handle({
+        jsonrpc: "2.0",
+        id: 2,
+        method: "completion/complete",
+        params: {
+          ref: { type: "ref/resource", uri: "unknown://template" },
+          argument: { name: "arg", value: "val" },
+        },
+      })
+
+      assert_equal(-32_602, response[:error][:code])
+    end
+
+    test "#handle completion/complete returns error for invalid ref type" do
+      server = Server.new(
+        name: "test_server",
+        capabilities: { completions: {} },
+      )
+
+      server.handle({ jsonrpc: "2.0", method: "initialize", id: 1 })
+      server.handle({ jsonrpc: "2.0", method: "notifications/initialized" })
+
+      response = server.handle({
+        jsonrpc: "2.0",
+        id: 2,
+        method: "completion/complete",
+        params: {
+          ref: { type: "ref/invalid" },
+          argument: { name: "arg", value: "val" },
+        },
+      })
+
+      assert_equal(-32_602, response[:error][:code])
+    end
+
+    test "#handle completion/complete returns error for missing ref" do
+      server = Server.new(
+        name: "test_server",
+        capabilities: { completions: {} },
+      )
+
+      server.handle({ jsonrpc: "2.0", method: "initialize", id: 1 })
+      server.handle({ jsonrpc: "2.0", method: "notifications/initialized" })
+
+      response = server.handle({
+        jsonrpc: "2.0",
+        id: 2,
+        method: "completion/complete",
+        params: {
+          ref: {},
+          argument: { name: "arg", value: "val" },
+        },
+      })
+
+      assert_equal(-32_602, response[:error][:code])
+    end
+
+    test "#handle completion/complete with custom handler for ref/resource with resource URI" do
+      resource = Resource.new(
+        uri: "file:///README.md",
+        name: "readme",
+      )
+      server = Server.new(
+        name: "test_server",
+        resources: [resource],
+        capabilities: { completions: {} },
+      )
+
+      server.completion_handler do |_params|
+        { completion: { values: ["file:///README.md"], hasMore: false } }
+      end
+
+      server.handle({ jsonrpc: "2.0", method: "initialize", id: 1 })
+      server.handle({ jsonrpc: "2.0", method: "notifications/initialized" })
+
+      response = server.handle({
+        jsonrpc: "2.0",
+        id: 2,
+        method: "completion/complete",
+        params: {
+          ref: { type: "ref/resource", uri: "file:///README.md" },
+          argument: { name: "path", value: "R" },
+        },
+      })
+
+      assert_equal(
+        {
+          jsonrpc: "2.0",
+          id: 2,
+          result: { completion: { values: ["file:///README.md"], hasMore: false } },
+        },
+        response,
+      )
+    end
+
+    test "#handle completion/complete returns error for missing argument" do
+      prompt = Prompt.define(name: "test") {}
+      server = Server.new(
+        name: "test_server",
+        prompts: [prompt],
+        capabilities: { completions: {} },
+      )
+
+      server.handle({ jsonrpc: "2.0", method: "initialize", id: 1 })
+      server.handle({ jsonrpc: "2.0", method: "notifications/initialized" })
+
+      response = server.handle({
+        jsonrpc: "2.0",
+        id: 2,
+        method: "completion/complete",
+        params: {
+          ref: { type: "ref/prompt", name: "test" },
+        },
+      })
+
+      assert_equal(-32_602, response[:error][:code])
+    end
+
+    test "#handle completion/complete returns error for missing argument value" do
+      prompt = Prompt.define(name: "test") {}
+      server = Server.new(
+        name: "test_server",
+        prompts: [prompt],
+        capabilities: { completions: {} },
+      )
+
+      server.handle({ jsonrpc: "2.0", method: "initialize", id: 1 })
+      server.handle({ jsonrpc: "2.0", method: "notifications/initialized" })
+
+      response = server.handle({
+        jsonrpc: "2.0",
+        id: 2,
+        method: "completion/complete",
+        params: {
+          ref: { type: "ref/prompt", name: "test" },
+          argument: { name: "arg" },
+        },
+      })
+
+      assert_equal(-32_602, response[:error][:code])
+    end
+
+    test "#handle completion/complete returns default when handler returns nil" do
+      prompt = Prompt.define(name: "test") {}
+      server = Server.new(
+        name: "test_server",
+        prompts: [prompt],
+        capabilities: { completions: {} },
+      )
+
+      server.completion_handler do |_params|
+        nil
+      end
+
+      server.handle({ jsonrpc: "2.0", method: "initialize", id: 1 })
+      server.handle({ jsonrpc: "2.0", method: "notifications/initialized" })
+
+      response = server.handle({
+        jsonrpc: "2.0",
+        id: 2,
+        method: "completion/complete",
+        params: {
+          ref: { type: "ref/prompt", name: "test" },
+          argument: { name: "arg", value: "" },
+        },
+      })
+
+      assert_equal(
+        {
+          jsonrpc: "2.0",
+          id: 2,
+          result: { completion: { values: [], hasMore: false } },
+        },
+        response,
+      )
+    end
+
+    test "#handle completion/complete with string-keyed handler result" do
+      prompt = Prompt.define(name: "test") {}
+      server = Server.new(
+        name: "test_server",
+        prompts: [prompt],
+        capabilities: { completions: {} },
+      )
+
+      server.completion_handler do |_params|
+        { "completion" => { "values" => ["alpha", "beta"], "hasMore" => true } }
+      end
+
+      server.handle({ jsonrpc: "2.0", method: "initialize", id: 1 })
+      server.handle({ jsonrpc: "2.0", method: "notifications/initialized" })
+
+      response = server.handle({
+        jsonrpc: "2.0",
+        id: 2,
+        method: "completion/complete",
+        params: {
+          ref: { type: "ref/prompt", name: "test" },
+          argument: { name: "arg", value: "" },
+        },
+      })
+
+      assert_equal ["alpha", "beta"], response[:result][:completion][:values]
+      assert response[:result][:completion][:hasMore]
+    end
+
+    test "#handle completion/complete returns invalid params for non-Hash params" do
+      server = Server.new(
+        name: "test_server",
+        prompts: [],
+        capabilities: { completions: {} },
+      )
+
+      server.handle({ jsonrpc: "2.0", method: "initialize", id: 1 })
+      server.handle({ jsonrpc: "2.0", method: "notifications/initialized" })
+
+      response = server.handle({
+        jsonrpc: "2.0",
+        id: 2,
+        method: "completion/complete",
+        params: "invalid",
+      })
+
+      assert_equal(-32602, response[:error][:code])
+    end
+
+    test "#handle completion/complete returns error when completions capability is not declared" do
+      server = Server.new(
+        name: "test_server",
+        prompts: [],
+      )
+
+      server.handle({ jsonrpc: "2.0", method: "initialize", id: 1 })
+      server.handle({ jsonrpc: "2.0", method: "notifications/initialized" })
+
+      response = server.handle({
+        jsonrpc: "2.0",
+        id: 2,
+        method: "completion/complete",
+        params: {
+          ref: { type: "ref/prompt", name: "test" },
+          argument: { name: "arg", value: "" },
+        },
+      })
+
+      assert response[:error]
+      assert_includes response[:error][:data], "completions"
+    end
+
     test "#handle resources/subscribe returns empty result" do
       server = Server.new(
         name: "test_server",