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Merge pull request #165 from Jflick58/fix/make-tool-error-spec-compliant

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Fix tool error handling to follow MCP spec
This commit is contained in:
Koichi ITO 2025-10-22 10:58:29 +09:00 • committed by GitHub
commit 61d48cd6d9
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3 changed files with 71 additions and 39 deletions
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41
lib/mcp/server.rb
View file

@ -258,10 +258,17 @@ module MCP
def call_tool(request)
tool_name = request[:name]
tool = tools[tool_name]
unless tool
add_instrumentation_data(error: :tool_not_found)
raise RequestHandlerError.new("Tool not found #{tool_name}", request, error_type: :tool_not_found)
add_instrumentation_data(tool_name:, error: :tool_not_found)
return Tool::Response.new(
[{
type: "text",
text: "Tool not found: #{tool_name}",
}],
error: true,
).to_h
end
arguments = request[:arguments] || {}
@ -269,11 +276,14 @@ module MCP
if tool.input_schema&.missing_required_arguments?(arguments)
add_instrumentation_data(error: :missing_required_arguments)
raise RequestHandlerError.new(
"Missing required arguments: #{tool.input_schema.missing_required_arguments(arguments).join(", ")}",
request,
error_type: :missing_required_arguments,
)
missing = tool.input_schema.missing_required_arguments(arguments).join(", ")
return Tool::Response.new(
[{
type: "text",
text: "Missing required arguments: #{missing}",
}],
error: true,
).to_h
end
if configuration.validate_tool_call_arguments && tool.input_schema
@ -281,14 +291,27 @@ module MCP
tool.input_schema.validate_arguments(arguments)
rescue Tool::InputSchema::ValidationError => e
add_instrumentation_data(error: :invalid_schema)
raise RequestHandlerError.new(e.message, request, error_type: :invalid_schema)
return Tool::Response.new(
[{
type: "text",
text: e.message,
}],
error: true,
).to_h
end
end
begin
call_tool_with_args(tool, arguments)
rescue => e
raise RequestHandlerError.new("Internal error calling tool #{tool_name}", request, original_error: e)
report_exception(e, { request: request })
Tool::Response.new(
[{
type: "text",
text: "Internal error calling tool #{tool_name}: #{e.message}",
}],
error: true,
).to_h
end
end

7
test/mcp/server_context_test.rb
View file

@ -152,9 +152,10 @@ module MCP
response = server_no_context.handle(request)
assert response[:error]
# The error is wrapped as "Internal error calling tool..."
assert_equal "Internal error", response[:error][:message]
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])
end
test "call_tool_with_args correctly detects server_context parameter presence" do

62
test/mcp/server_test.rb
View file

@ -255,7 +255,7 @@ module MCP
assert_instrumentation_data({ method: "tools/call", tool_name: })
end
test "#handle tools/call returns error if required tool arguments are missing" do
test "#handle tools/call returns error response with isError true if required tool arguments are missing" do
tool_with_required_argument = Tool.define(
name: "test_tool",
title: "Test tool",
@ -279,8 +279,10 @@ module MCP
response = server.handle(request)
assert_equal "Missing required arguments: message", response[:error][:data]
assert_equal "Internal error", response[:error][:message]
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]
end
test "#handle_json tools/call executes tool and returns result" do
@ -340,17 +342,12 @@ module MCP
assert_equal({ content: [{ type: "text", content: "OK" }], isError: false }, response[:result])
end
test "#handle tools/call returns internal error and reports exception if the tool raises an error" do
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: {
jsonrpc: "2.0",
method: "tools/call",
params: { name: "tool_that_raises", arguments: { message: "test" } },
id: 1,
},
request: { name: "tool_that_raises", arguments: { message: "test" } },
},
server_context,
)
@ -368,12 +365,14 @@ module MCP
response = @server.handle(request)
assert_equal "Internal error", response[:error][:message]
assert_equal "Internal error calling tool tool_that_raises", response[:error][:data]
assert_instrumentation_data({ method: "tools/call", tool_name: "tool_that_raises", error: :internal_error })
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" })
end
test "#handle_json returns internal error and reports exception if the tool raises an error" do
test "#handle_json returns error response with isError true if the tool raises an error" do
request = JSON.generate({
jsonrpc: "2.0",
method: "tools/call",
@ -385,12 +384,14 @@ module MCP
})
response = JSON.parse(@server.handle_json(request), symbolize_names: true)
assert_equal "Internal error", response[:error][:message]
assert_equal "Internal error calling tool tool_that_raises", response[:error][:data]
assert_instrumentation_data({ method: "tools/call", tool_name: "tool_that_raises", error: :internal_error })
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" })
end
test "#handle tools/call returns error for unknown tool" do
test "#handle tools/call returns error response with isError true for unknown tool" do
request = {
jsonrpc: "2.0",
method: "tools/call",
@ -402,12 +403,14 @@ module MCP
}
response = @server.handle(request)
assert_equal "Internal error", response[:error][:message]
assert_equal "Tool not found unknown_tool", response[:error][:data]
assert_instrumentation_data({ method: "tools/call", error: :tool_not_found })
assert_nil response[:error], "Expected no JSON-RPC error"
assert response[:result][:isError]
assert_equal "text", response[:result][:content][0][:type]
assert_equal "Tool not found: unknown_tool", response[:result][:content][0][:text]
assert_instrumentation_data({ method: "tools/call", tool_name: "unknown_tool", error: :tool_not_found })
end
test "#handle_json returns error for unknown tool" do
test "#handle_json returns error response with isError true for unknown tool" do
request = JSON.generate({
jsonrpc: "2.0",
method: "tools/call",
@ -419,7 +422,10 @@ module MCP
})
response = JSON.parse(@server.handle_json(request), symbolize_names: true)
assert_equal "Internal error", response[:error][:message]
assert_nil response[:error], "Expected no JSON-RPC error"
assert response[:result][:isError]
assert_equal "text", response[:result][:content][0][:type]
assert_equal "Tool not found: unknown_tool", response[:result][:content][0][:text]
end
test "#tools_call_handler sets the tools/call handler" do
@ -945,8 +951,9 @@ module MCP
assert_equal "2.0", response[:jsonrpc]
assert_equal 1, response[:id]
assert_equal(-32603, response[:error][:code])
assert_includes response[:error][:data], "Missing required arguments"
assert_nil response[:error], "Expected no JSON-RPC error"
assert response[:result][:isError]
assert_includes response[:result][:content][0][:text], "Missing required arguments"
end
test "tools/call validates arguments against input schema when validate_tool_call_arguments is true" do
@ -969,8 +976,9 @@ module MCP
assert_equal "2.0", response[:jsonrpc]
assert_equal 1, response[:id]
assert_equal(-32603, response[:error][:code])
assert_includes response[:error][:data], "Invalid arguments"
assert_nil response[:error], "Expected no JSON-RPC error"
assert response[:result][:isError]
assert_includes response[:result][:content][0][:text], "Invalid arguments"
end
test "tools/call skips argument validation when validate_tool_call_arguments is false" do