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fix: return protocol errors for invalid arguments and server errors

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Per the MCP spec, invalid arguments and server errors are protocol
errors (JSON-RPC error responses), not tool execution errors
(isError: true). PR #165 moved all error cases to isError: true,
and PR #231 only restored protocol errors for unknown tools.

This fixes the remaining two cases: invalid arguments (-32602) and
server errors (-32603).

Ref: https://modelcontextprotocol.io/specification/2024-11-05/server/tools#error-handling
This commit is contained in:
Sarah Pickett 2026-04-01 11:08:05 -04:00
commit 8dd0379f16
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GPG key ID: 9EFE291931498465
3 changed files with 59 additions and 59 deletions
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22
lib/mcp/server.rb
View file

@ -316,13 +316,12 @@ module MCP
add_instrumentation_data(client: client) if client
result
rescue RequestHandlerError => e
report_exception(e.original_error || e, { request: request })
add_instrumentation_data(error: e.error_type)
raise e
rescue => e
report_exception(e, { request: request })
if e.is_a?(RequestHandlerError)
add_instrumentation_data(error: e.error_type)
raise e
end
add_instrumentation_data(error: :internal_error)
raise RequestHandlerError.new("Internal error handling #{method} request", request, original_error: e)
end
@ -421,7 +420,7 @@ module MCP
add_instrumentation_data(error: :missing_required_arguments)
missing = tool.input_schema.missing_required_arguments(arguments).join(", ")
return error_tool_response("Missing required arguments: #{missing}")
raise RequestHandlerError.new("Missing required arguments: #{missing}", request, error_type: :invalid_params)
end
if configuration.validate_tool_call_arguments && tool.input_schema
@ -430,7 +429,7 @@ module MCP
rescue Tool::InputSchema::ValidationError => e
add_instrumentation_data(error: :invalid_schema)
return error_tool_response(e.message)
raise RequestHandlerError.new(e.message, request, error_type: :invalid_params)
end
end
@ -440,9 +439,12 @@ module MCP
rescue RequestHandlerError
raise
rescue => e
report_exception(e, request: request)
error_tool_response("Internal error calling tool #{tool_name}: #{e.message}")
raise RequestHandlerError.new(
"Internal error calling tool #{tool_name}: #{e.message}",
request,
error_type: :internal_error,
original_error: e,
)
end
def list_prompts(request)

10
test/mcp/server_context_test.rb
View file

@ -199,7 +199,7 @@ module MCP
response[:result][:content][0][:content]
end
test "tool with required server_context fails when server has no context" do
test "tool with required server_context returns protocol error in JSON-RPC format when server has no context" do
server_no_context = Server.new(
name: "test_server_no_context",
tools: [ToolWithRequiredContext],
@ -217,10 +217,10 @@ module MCP
response = server_no_context.handle(request)
assert_nil response[:error], "Expected no JSON-RPC error"
assert response[:result][:isError]
assert_equal "text", response[:result][:content][0][:type]
assert_match(/Internal error calling tool tool_with_required_context: /, response[:result][:content][0][:text])
assert_nil response[:result]
assert_equal(-32603, response[:error][:code])
assert_equal "Internal error", response[:error][:message]
assert_match(/Internal error calling tool tool_with_required_context: /, response[:error][:data])
end
test "call_tool_with_args correctly detects server_context parameter presence" do

86
test/mcp/server_test.rb
View file

@ -312,7 +312,7 @@ module MCP
assert_instrumentation_data({ method: "tools/call", tool_name: tool_name, tool_arguments: tool_args })
end
test "#handle tools/call returns error response with isError true if required tool arguments are missing" do
test "#handle tools/call returns protocol error in JSON-RPC format if required tool arguments are missing" do
tool_with_required_argument = Tool.define(
name: "test_tool",
title: "Test tool",
@ -336,10 +336,10 @@ module MCP
response = server.handle(request)
assert_nil response[:error], "Expected no JSON-RPC error"
assert response[:result][:isError]
assert_equal "text", response[:result][:content][0][:type]
assert_equal "Missing required arguments: message", response[:result][:content][0][:text]
assert_nil response[:result]
assert_equal(-32602, response[:error][:code])
assert_equal "Invalid params", response[:error][:message]
assert_includes response[:error][:data], "Missing required arguments: message"
end
test "#handle_json tools/call executes tool and returns result" do
@ -407,17 +407,7 @@ module MCP
assert_equal({ content: [{ type: "text", content: "OK" }], isError: false }, response[:result])
end
test "#handle tools/call returns error response with isError true if the tool raises an error" do
@server.configuration.exception_reporter.expects(:call).with do |exception, server_context|
assert_not_nil exception
assert_equal(
{
request: { name: "tool_that_raises", arguments: { message: "test" } },
},
server_context,
)
end
test "#handle tools/call returns protocol error in JSON-RPC format if the tool raises an uncaught exception" do
request = {
jsonrpc: "2.0",
method: "tools/call",
@ -428,13 +418,18 @@ module MCP
id: 1,
}
@server.configuration.exception_reporter.expects(:call).with do |exception, server_context|
refute_kind_of MCP::Server::RequestHandlerError, exception
assert_equal({ request: request }, server_context)
end
response = @server.handle(request)
assert_nil response[:error], "Expected no JSON-RPC error"
assert response[:result][:isError]
assert_equal "text", response[:result][:content][0][:type]
assert_match(/Internal error calling tool tool_that_raises: /, response[:result][:content][0][:text])
assert_instrumentation_data({ method: "tools/call", tool_name: "tool_that_raises", tool_arguments: { message: "test" } })
assert_nil response[:result]
assert_equal(-32603, response[:error][:code])
assert_equal "Internal error", response[:error][:message]
assert_match(/Internal error calling tool tool_that_raises: /, response[:error][:data])
assert_instrumentation_data({ method: "tools/call", tool_name: "tool_that_raises", tool_arguments: { message: "test" }, error: :internal_error })
end
test "registers tools with the same class name in different namespaces" do
@ -475,7 +470,7 @@ module MCP
MESSAGE
end
test "#handle_json returns error response with isError true if the tool raises an error" do
test "#handle_json returns protocol error in JSON-RPC format if the tool raises an uncaught exception" do
request = JSON.generate({
jsonrpc: "2.0",
method: "tools/call",
@ -487,14 +482,14 @@ module MCP
})
response = JSON.parse(@server.handle_json(request), symbolize_names: true)
assert_nil response[:error], "Expected no JSON-RPC error"
assert response[:result][:isError]
assert_equal "text", response[:result][:content][0][:type]
assert_match(/Internal error calling tool tool_that_raises: /, response[:result][:content][0][:text])
assert_instrumentation_data({ method: "tools/call", tool_name: "tool_that_raises", tool_arguments: { message: "test" } })
assert_nil response[:result]
assert_equal(-32603, response[:error][:code])
assert_equal "Internal error", response[:error][:message]
assert_match(/Internal error calling tool tool_that_raises: /, response[:error][:data])
assert_instrumentation_data({ method: "tools/call", tool_name: "tool_that_raises", tool_arguments: { message: "test" }, error: :internal_error })
end
test "#handle tools/call returns error response with isError true if input_schema raises an error during validation" do
test "#handle tools/call returns protocol error in JSON-RPC format if input_schema raises an error during validation" do
tool = Tool.define(
name: "tool_with_faulty_schema",
title: "Tool with faulty schema",
@ -518,10 +513,10 @@ module MCP
response = server.handle(request)
assert_nil response[:error], "Expected no JSON-RPC error"
assert response[:result][:isError]
assert_equal "text", response[:result][:content][0][:type]
assert_match(/Internal error calling tool tool_with_faulty_schema: Unexpected schema error/, response[:result][:content][0][:text])
assert_nil response[:result]
assert_equal(-32603, response[:error][:code])
assert_equal "Internal error", response[:error][:message]
assert_match(/Internal error calling tool tool_with_faulty_schema: Unexpected schema error/, response[:error][:data])
end
test "#handle tools/call returns JSON-RPC error for unknown tool" do
@ -1310,7 +1305,7 @@ module MCP
refute response[:result].key?(:instructions)
end
test "tools/call handles missing arguments field" do
test "tools/call returns protocol error in JSON-RPC format for missing arguments" do
server = Server.new(
tools: [TestTool],
configuration: Configuration.new(validate_tool_call_arguments: true),
@ -1329,12 +1324,13 @@ module MCP
assert_equal "2.0", response[:jsonrpc]
assert_equal 1, response[:id]
assert_nil response[:error], "Expected no JSON-RPC error"
assert response[:result][:isError]
assert_includes response[:result][:content][0][:text], "Missing required arguments"
assert_nil response[:result]
assert_equal(-32602, response[:error][:code])
assert_equal "Invalid params", response[:error][:message]
assert_includes response[:error][:data], "Missing required arguments"
end
test "tools/call validates arguments against input schema when validate_tool_call_arguments is true" do
test "tools/call returns protocol error in JSON-RPC format for invalid arguments when validate_tool_call_arguments is true" do
server = Server.new(
tools: [TestTool],
configuration: Configuration.new(validate_tool_call_arguments: true),
@ -1354,9 +1350,10 @@ module MCP
assert_equal "2.0", response[:jsonrpc]
assert_equal 1, response[:id]
assert_nil response[:error], "Expected no JSON-RPC error"
assert response[:result][:isError]
assert_includes response[:result][:content][0][:text], "Invalid arguments"
assert_nil response[:result]
assert_equal(-32602, response[:error][:code])
assert_equal "Invalid params", response[:error][:message]
assert_includes response[:error][:data], "Invalid arguments"
end
test "tools/call skips argument validation when validate_tool_call_arguments is false" do
@ -1441,7 +1438,7 @@ module MCP
assert_equal "OK", response[:result][:content][0][:content]
end
test "tools/call disallows additional properties when additionalProperties set to false" do
test "tools/call returns protocol error in JSON-RPC format when additionalProperties set to false" do
server = Server.new(
tools: [TestToolWithAdditionalPropertiesSetToFalse],
configuration: Configuration.new(validate_tool_call_arguments: true),
@ -1464,9 +1461,10 @@ module MCP
assert_equal "2.0", response[:jsonrpc]
assert_equal 1, response[:id]
assert_nil response[:error], "Expected no JSON-RPC error"
assert response[:result][:isError]
assert_includes response[:result][:content][0][:text], "Invalid arguments"
assert_nil response[:result]
assert_equal(-32602, response[:error][:code])
assert_equal "Invalid params", response[:error][:message]
assert_includes response[:error][:data], "Invalid arguments"
end
test "#handle completion/complete returns default completion result" do