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feat: add opt-in tool output validation

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This commit is contained in:
Ates Goral 2026-05-11 22:04:15 -04:00
commit 407b064140
5 changed files with 245 additions and 2 deletions
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13
README.md
View file

@ -645,6 +645,19 @@ MCP spec for the [Output Schema](https://modelcontextprotocol.io/specification/l
The output schema follows standard JSON Schema format and helps ensure consistent data exchange between MCP servers and clients.
By default, server-side validation of tool results against `output_schema` is disabled for backwards compatibility. To validate successful tool responses, enable `validate_tool_call_results`:
```ruby
configuration = MCP::Configuration.new(validate_tool_call_results: true)
server = MCP::Server.new(
name: "example_server",
tools: [WeatherTool],
configuration: configuration
)
```
When enabled, successful tool responses for tools with an `output_schema` must include `structured_content` that conforms to the schema. Error responses are not validated against the output schema.
### Tool Responses with Structured Content
Tools can return structured data alongside text content using the `structured_content` parameter.

23
lib/mcp/configuration.rb
View file

@ -16,7 +16,7 @@ module MCP
attr_writer :instrumentation_callback
def initialize(exception_reporter: nil, around_request: nil, instrumentation_callback: nil, protocol_version: nil,
validate_tool_call_arguments: true)
validate_tool_call_arguments: true, validate_tool_call_results: false)
@exception_reporter = exception_reporter
@around_request = around_request
@instrumentation_callback = instrumentation_callback
@ -25,8 +25,10 @@ module MCP
validate_protocol_version!(protocol_version)
end
validate_value_of_validate_tool_call_arguments!(validate_tool_call_arguments)
validate_value_of_validate_tool_call_results!(validate_tool_call_results)
@validate_tool_call_arguments = validate_tool_call_arguments
@validate_tool_call_results = validate_tool_call_results
end
def protocol_version=(protocol_version)
@ -41,6 +43,12 @@ module MCP
@validate_tool_call_arguments = validate_tool_call_arguments
end
def validate_tool_call_results=(validate_tool_call_results)
validate_value_of_validate_tool_call_results!(validate_tool_call_results)
@validate_tool_call_results = validate_tool_call_results
end
def protocol_version
@protocol_version || LATEST_STABLE_PROTOCOL_VERSION
end
@ -80,11 +88,16 @@ module MCP
end
attr_reader :validate_tool_call_arguments
attr_reader :validate_tool_call_results
def validate_tool_call_arguments?
!!@validate_tool_call_arguments
end
def validate_tool_call_results?
!!@validate_tool_call_results
end
def merge(other)
return self if other.nil?
@ -113,6 +126,7 @@ module MCP
end
validate_tool_call_arguments = other.validate_tool_call_arguments
validate_tool_call_results = other.validate_tool_call_results
Configuration.new(
exception_reporter: exception_reporter,
@ -120,6 +134,7 @@ module MCP
instrumentation_callback: instrumentation_callback,
protocol_version: protocol_version,
validate_tool_call_arguments: validate_tool_call_arguments,
validate_tool_call_results: validate_tool_call_results,
)
end
@ -138,6 +153,12 @@ module MCP
end
end
def validate_value_of_validate_tool_call_results!(validate_tool_call_results)
unless validate_tool_call_results.is_a?(TrueClass) || validate_tool_call_results.is_a?(FalseClass)
raise ArgumentError, "validate_tool_call_results must be a boolean"
end
end
def default_exception_reporter
@default_exception_reporter ||= ->(exception, server_context) {}
end

12
lib/mcp/server.rb
View file

@ -586,9 +586,11 @@ module MCP
progress_token = request.dig(:_meta, :progressToken)
call_tool_with_args(
result = call_tool_with_args(
tool, arguments, server_context_with_meta(request), progress_token: progress_token, session: session, related_request_id: related_request_id, cancellation: cancellation
)
validate_tool_call_result!(tool, result)
result
rescue RequestHandlerError, CancelledError
# CancelledError is intentionally not wrapped so `handle_request` can turn it into
# `JsonRpcHandler::NO_RESPONSE` per the MCP cancellation spec.
@ -745,6 +747,14 @@ module MCP
).to_h
end
def validate_tool_call_result!(tool, result)
return unless configuration.validate_tool_call_results
return unless tool.output_schema
return if result[:isError]
tool.output_schema.validate_result(result[:structuredContent])
end
# Whether a tool/prompt handler opts in to receiving an `MCP::ServerContext`.
# Recognizes `:keyrest` (`**kwargs`) because tools are invoked without a positional argument
# (`tool.call(**args, server_context:)`), soa `**kwargs`-only signature safely captures `server_context:`.

49
test/mcp/configuration_test.rb
View file

@ -74,6 +74,14 @@ module MCP
assert_equal("validate_tool_call_arguments must be a boolean", exception.message)
end
test "raises ArgumentError when validate_tool_call_results is not a boolean value" do
config = Configuration.new
exception = assert_raises(ArgumentError) do
config.validate_tool_call_results = "true"
end
assert_equal("validate_tool_call_results must be a boolean", exception.message)
end
test "merges protocol version from other configuration" do
config1 = Configuration.new(protocol_version: "2025-03-26")
config2 = Configuration.new(protocol_version: "2025-06-18")
@ -123,6 +131,40 @@ module MCP
refute merged.validate_tool_call_arguments
end
test "defaults validate_tool_call_results to false" do
config = Configuration.new
refute config.validate_tool_call_results
end
test "can set validate_tool_call_results to true" do
config = Configuration.new(validate_tool_call_results: true)
assert config.validate_tool_call_results
end
test "validate_tool_call_results? returns false when not set" do
config = Configuration.new
refute config.validate_tool_call_results?
end
test "validate_tool_call_results? returns true when set" do
config = Configuration.new(validate_tool_call_results: true)
assert config.validate_tool_call_results?
end
test "merge preserves validate_tool_call_results from other config" do
config1 = Configuration.new(validate_tool_call_results: true)
config2 = Configuration.new
merged = config1.merge(config2)
refute merged.validate_tool_call_results?
end
test "merge preserves validate_tool_call_results from self when other set" do
config1 = Configuration.new(validate_tool_call_results: true)
config2 = Configuration.new
merged = config2.merge(config1)
assert merged.validate_tool_call_results
end
test "initializes with a default pass-through around_request" do
config = Configuration.new
called = false
@ -183,5 +225,12 @@ module MCP
end
assert_equal("validate_tool_call_arguments must be a boolean", exception.message)
end
test "raises ArgumentError when validate_tool_call_results is not a boolean" do
exception = assert_raises(ArgumentError) do
Configuration.new(validate_tool_call_results: "true")
end
assert_equal("validate_tool_call_results must be a boolean", exception.message)
end
end
end

150
test/mcp/server_test.rb
View file

@ -1767,6 +1767,156 @@ module MCP
assert_includes response[:result][:content][0][:text], "Invalid arguments"
end
test "tools/call skips output schema validation by default" do
tool = Tool.define(
name: "invalid_structured_content_tool",
output_schema: {
type: "object",
properties: { result: { type: "string" } },
required: ["result"],
},
) do
Tool::Response.new(
[{ type: "text", text: "ok" }],
structured_content: { result: 123 },
)
end
server = Server.new(tools: [tool])
response = server.handle({
jsonrpc: "2.0",
id: 1,
method: "tools/call",
params: { name: "invalid_structured_content_tool" },
})
assert_nil response[:error]
assert_equal({ result: 123 }, response[:result][:structuredContent])
end
test "tools/call validates structuredContent against output schema when enabled" do
tool = Tool.define(
name: "valid_structured_content_tool",
output_schema: {
type: "object",
properties: { result: { type: "string" } },
required: ["result"],
},
) do
Tool::Response.new(
[{ type: "text", text: "ok" }],
structured_content: { result: "success" },
)
end
server = Server.new(
tools: [tool],
configuration: Configuration.new(validate_tool_call_results: true),
)
response = server.handle({
jsonrpc: "2.0",
id: 1,
method: "tools/call",
params: { name: "valid_structured_content_tool" },
})
assert_nil response[:error]
assert_equal({ result: "success" }, response[:result][:structuredContent])
end
test "tools/call returns JSON-RPC error for invalid structuredContent when output schema validation is enabled" do
tool = Tool.define(
name: "invalid_structured_content_tool",
output_schema: {
type: "object",
properties: { result: { type: "string" } },
required: ["result"],
},
) do
Tool::Response.new(
[{ type: "text", text: "ok" }],
structured_content: { result: 123 },
)
end
server = Server.new(
tools: [tool],
configuration: Configuration.new(validate_tool_call_results: true),
)
response = server.handle({
jsonrpc: "2.0",
id: 1,
method: "tools/call",
params: { name: "invalid_structured_content_tool" },
})
assert_nil response[:result]
assert_equal(-32603, response[:error][:code])
assert_equal "Internal error", response[:error][:message]
assert_match(/Internal error calling tool invalid_structured_content_tool: Invalid result:/, response[:error][:data])
end
test "tools/call returns JSON-RPC error when output schema validation is enabled and structuredContent is missing" do
tool = Tool.define(
name: "missing_structured_content_tool",
output_schema: {
type: "object",
properties: { result: { type: "string" } },
required: ["result"],
},
) do
Tool::Response.new([{ type: "text", text: "ok" }])
end
server = Server.new(
tools: [tool],
configuration: Configuration.new(validate_tool_call_results: true),
)
response = server.handle({
jsonrpc: "2.0",
id: 1,
method: "tools/call",
params: { name: "missing_structured_content_tool" },
})
assert_nil response[:result]
assert_equal(-32603, response[:error][:code])
assert_equal "Internal error", response[:error][:message]
assert_match(/Internal error calling tool missing_structured_content_tool: Invalid result:/, response[:error][:data])
end
test "tools/call skips output schema validation for error responses" do
tool = Tool.define(
name: "error_response_tool",
output_schema: {
type: "object",
properties: { result: { type: "string" } },
required: ["result"],
},
) do
Tool::Response.new(
[{ type: "text", text: "failed" }],
error: true,
structured_content: { result: 123 },
)
end
server = Server.new(
tools: [tool],
configuration: Configuration.new(validate_tool_call_results: true),
)
response = server.handle({
jsonrpc: "2.0",
id: 1,
method: "tools/call",
params: { name: "error_response_tool" },
})
assert_nil response[:error]
assert response[:result][:isError]
assert_equal({ result: 123 }, response[:result][:structuredContent])
end
test "tools/call returns JSON-RPC -32602 protocol error when tool is not found" do
server = Server.new(
tools: [TestTool],