Validate tool output fails for time.Time and other types that get serialized to strings in JSON

[iwyrkore]

When the output type for a tool is a struct containing a field of type time.Time, the schema properly reports the type as string but the validation fails:

timestamp: type: 2021-01-01 00:00:00 +0000 UTC has type "object", want "string"

This is because the JSON schema validation is being done on the struct and not the marshalled JSON.

To Reproduce
Steps to reproduce the behavior:

1. Create a tool that returns a struct with a time.Time as structured content:

type MyResult struct {
	Timestamp time.Time `json:"timestamp"`
}

func MyTestToolHandler(
	ctx context.Context,
	req *mcp.CallToolRequest,
	args any) (*mcp.CallToolResult, *MyResult, error) {
	result := MyResult{Timestamp: time.Now()}
	return &mcp.CallToolResult{
		Content: []mcp.Content{
			&mcp.TextContent{Text: "Testing"},
		},
	}, &result, nil
}

var MyTestTool = mcp.Tool{
	Name:        "MyTestTool",
	Description: "Testing tool",
}

mcp.AddTool(mcpServer, &MyTestTool, MyTestToolHandler)

2. Call the tool MyTestTool and get:

data: {"jsonrpc":"2.0","id":2,"error":{"code":0,"message":"tool output: validating\n\t0x14000106770\nagainst\n\t {\"type\":\"object\",\"required\":[\"timestamp\"],\"properties\":{\"timestamp\":{\"type\":\"string\"}},\"additionalProperties\":false}:\n validating root: validating /properties/timestamp: type: 2025-09-10 16:41:03.191738 -0400 EDT m=+7.869338376 has type \"object\", want \"string\""}}

Expected behavior

Expect to validate against the actual JSON produced and pass and get the following output:

data: {"jsonrpc":"2.0","id":2,"result":{"content":[{"type":"text","text":"Testing"}],"structuredContent":{"timestamp":"2025-09-10T16:49:16.712372-04:00"}}}

Logs

"error":"tool output: validating\n\t0x14000106770\nagainst\n\t {\"type\":\"object\",\"required\":[\"timestamp\"],\"properties\":{\"timestamp\":{\"type\":\"string\"}},\"additionalProperties\":false}:\n validating root: validating /properties/timestamp: type: 2025-09-10 16:41:03.191738 -0400 EDT m=+7.869338376 has type \"object\", want \"string\""

[findleyr]

Thanks for the report!

This was an oversight. To fix this, I'm going to add slow paths to validation if any reachable types are json.Marshaler or json.Unmarshaler (the direction depends on whether we're validating input or output). In that case, we'll perform a double marshal/unmarshal to ensure we validate against the actual JSON structure of the type.

[iwyrkore]

Sounds good. It would be nice to be able to configure output validation on/off. For MCP servers which are API proxies it can be problematic.

[findleyr]

@iwyrkore understood. I have been working on a proposal to make tool handing more composable.

In the meantime, it is definitely possible to write your own tool constructor that has some or all of the features of mcp.AddTool, if you want to roll your own handling behavior. examples/server/toolschemas/main.go has an example of a tool that does manual validation and parsing. At the very least, more documentation on that incoming shortly.

[iwyrkore]

Thank you! The examples/server/toolschemas/main.go example is very useful!

It would be great if jsonrpc could expose a function IsWireError(err)