Return Invalid Params for unknown prompts and missing prompt arguments

lib/mcp/prompt.rb

@@ -111,8 +111,15 @@ def validate_arguments!(args)
         missing = required_args - args.keys
         return if missing.empty?
 
+        # The explicit `error_code` maps a missing prompt argument to Invalid Params (-32602) rather
+        # than the default Internal Error (-32603); a missing required argument is client input, not a
+        # server fault. `error_type: :missing_required_arguments` keeps the descriptive message and
+        # instrumentation label. Mirrors `MCP::Server::ResourceNotFoundError`.
         raise MCP::Server::RequestHandlerError.new(
-          "Missing required arguments: #{missing.join(", ")}", nil, error_type: :missing_required_arguments
+          "Missing required arguments: #{missing.join(", ")}",
+          nil,
+          error_type: :missing_required_arguments,
+          error_code: JsonRpcHandler::ErrorCode::INVALID_PARAMS,
         )
       end
 

lib/mcp/server.rb

@@ -1260,7 +1260,16 @@ def get_prompt(request, session: nil, related_request_id: nil, cancellation: nil
       prompt = @prompts[prompt_name]
       unless prompt
         add_instrumentation_data(error: :prompt_not_found)
-        raise RequestHandlerError.new("Prompt not found #{prompt_name}", request, error_type: :prompt_not_found)
+        # The explicit `error_code` maps an unknown prompt to Invalid Params (-32602) rather than
+        # the default Internal Error (-32603), matching the `tools/call`, `resources/read`, and
+        # `completion/complete` siblings for the same not-found condition, while `error_type:
+        # :prompt_not_found` keeps the descriptive message and instrumentation label.
+        raise RequestHandlerError.new(
+          "Prompt not found #{prompt_name}",
+          request,
+          error_type: :prompt_not_found,
+          error_code: JsonRpcHandler::ErrorCode::INVALID_PARAMS,
+        )
       end
 
       add_instrumentation_data(prompt_name: prompt_name)

test/mcp/server_test.rb

@@ -1395,6 +1395,9 @@ class Example < Tool
       }
 
       response = @server.handle(request)
+      # An unknown prompt is client input, so it maps to Invalid Params (-32602), not the
+      # default Internal Error (-32603); matches the tools/call and completion/complete siblings.
+      assert_equal JsonRpcHandler::ErrorCode::INVALID_PARAMS, response.dig(:error, :code)
       assert_equal("Prompt not found unknown_prompt", response[:error][:data])
       assert_instrumentation_data({ method: "prompts/get", error: :prompt_not_found })
     end
@@ -1411,6 +1414,9 @@ class Example < Tool
       }
 
       response = @server.handle(request)
+      # A missing required argument is client input, so it maps to Invalid Params (-32602),
+      # not the default Internal Error (-32603).
+      assert_equal JsonRpcHandler::ErrorCode::INVALID_PARAMS, response.dig(:error, :code)
       assert_equal "Missing required arguments: test_argument", response[:error][:data]
       assert_instrumentation_data({
         method: "prompts/get",