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internal/mcp: generic handling of CallToolParams

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Make Tool APIs future-proof by pushing generics into CallToolParams, and
using CallToolParams throughout. This has several advantages:

- We no longer need the awkward CallToolOptions type, which is almost
  but not quite like CallToolParams.
- NewTool no longer needs to define its own handler type: it accepts a
  ToolHandler[TArgs]. This also means its signature is future-proof.

Additionally, provide a `CallTool` helper (the analog of NewTool) to
serve as a more strongly typed variant of `ClientSession.CallTool`.

+ design, protocol generation

Change-Id: I80172a25bc50734dae84a541aa4776ed9161694e
Reviewed-on: https://go-review.googlesource.com/c/tools/+/676495
Reviewed-by: Jonathan Amsterdam <jba@google.com>
Auto-Submit: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
This commit is contained in:
Rob Findley 2025-05-27 14:15:09 +00:00 • committed by Gopher Robot
commit 04ca8ac03b
1 changed files with 170 additions and 108 deletions
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21
mcp/tool.go
View file

@ -13,12 +13,12 @@ import (
)
// A ToolHandler handles a call to tools/call.
type ToolHandler func(context.Context, *ServerSession, *CallToolParams) (*CallToolResult, error)
type ToolHandler[TArgs any] func(context.Context, *ServerSession, *CallToolParams[TArgs]) (*CallToolResult, error)
// A Tool is a tool definition that is bound to a tool handler.
type ServerTool struct {
Tool *Tool
Handler ToolHandler
Handler ToolHandler[json.RawMessage]
}
// NewTool is a helper to make a tool using reflection on the given handler.
@ -34,17 +34,19 @@ type ServerTool struct {
//
// TODO: just have the handler return a CallToolResult: returning []Content is
// going to be inconsistent with other server features.
func NewTool[TReq any](name, description string, handler func(context.Context, *ServerSession, TReq) ([]*Content, error), opts ...ToolOption) *ServerTool {
func NewTool[TReq any](name, description string, handler ToolHandler[TReq], opts ...ToolOption) *ServerTool {
schema, err := jsonschema.For[TReq]()
if err != nil {
panic(err)
}
wrapped := func(ctx context.Context, cc *ServerSession, params *CallToolParams) (*CallToolResult, error) {
var v TReq
if err := unmarshalSchema(params.Arguments, schema, &v); err != nil {
return nil, err
wrapped := func(ctx context.Context, cc *ServerSession, params *CallToolParams[json.RawMessage]) (*CallToolResult, error) {
var params2 CallToolParams[TReq]
if params.Arguments != nil {
if err := unmarshalSchema(params.Arguments, schema, &params2.Arguments); err != nil {
return nil, err
}
}
content, err := handler(ctx, cc, v)
res, err := handler(ctx, cc, &params2)
// TODO: investigate why server errors are embedded in this strange way,
// rather than returned as jsonrpc2 server errors.
if err != nil {
@ -53,9 +55,6 @@ func NewTool[TReq any](name, description string, handler func(context.Context, *
IsError: true,
}, nil
}
res := &CallToolResult{
Content: content,
}
return res, nil
}
t := &ServerTool{