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mcp: statically type the server-side tool params (#378)

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Introduce CallToolParamsRaw on the server side, so that tool handlers
see in the type system that a tool's arguments are a json.RawMessage.

Fixes #377.
This commit is contained in:
Jonathan Amsterdam 2025-08-30 08:16:06 -04:00 • committed by GitHub
commit 7bfde44981
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GPG key ID: B5690EEEBB952194
4 changed files with 17 additions and 22 deletions
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5
mcp/mcp_test.go
View file

@ -494,7 +494,6 @@ func TestEndToEnd(t *testing.T) {
if result.Action != "accept" {
t.Errorf("got action %q, want %q", result.Action, "accept")
}
})
// Disconnect.
@ -1638,7 +1637,7 @@ func TestPointerArgEquivalence(t *testing.T) {
//
// We handle a few different types of results, to assert they behave the
// same in all cases.
AddTool(s, &Tool{Name: "pointer"}, func(_ context.Context, req *ServerRequest[*CallToolParams], in *input) (*CallToolResult, *output, error) {
AddTool(s, &Tool{Name: "pointer"}, func(_ context.Context, req *CallToolRequest, in *input) (*CallToolResult, *output, error) {
switch in.In {
case "":
return nil, nil, fmt.Errorf("must provide input")
@ -1652,7 +1651,7 @@ func TestPointerArgEquivalence(t *testing.T) {
panic("unreachable")
}
})
AddTool(s, &Tool{Name: "nonpointer"}, func(_ context.Context, req *ServerRequest[*CallToolParams], in input) (*CallToolResult, output, error) {
AddTool(s, &Tool{Name: "nonpointer"}, func(_ context.Context, req *CallToolRequest, in input) (*CallToolResult, output, error) {
switch in.In {
case "":
return nil, output{}, fmt.Errorf("must provide input")

26
mcp/protocol.go
View file

@ -40,6 +40,7 @@ type Annotations struct {
Priority float64 `json:"priority,omitempty"`
}
// CallToolParams is used by clients to call a tool.
type CallToolParams struct {
// This property is reserved by the protocol to allow clients and servers to
// attach additional metadata to their responses.
@ -48,20 +49,13 @@ type CallToolParams struct {
Arguments any `json:"arguments,omitempty"`
}
// When unmarshalling CallToolParams on the server side, we need to delay unmarshaling of the arguments.
func (c *CallToolParams) UnmarshalJSON(data []byte) error {
var raw struct {
Meta `json:"_meta,omitempty"`
Name string `json:"name"`
RawArguments json.RawMessage `json:"arguments,omitempty"`
}
if err := json.Unmarshal(data, &raw); err != nil {
return err
}
c.Meta = raw.Meta
c.Name = raw.Name
c.Arguments = raw.RawArguments
return nil
// CallToolParamsRaw is passed to tool handlers on the server.
type CallToolParamsRaw struct {
// This property is reserved by the protocol to allow clients and servers to
// attach additional metadata to their responses.
Meta `json:"_meta,omitempty"`
Name string `json:"name"`
Arguments json.RawMessage `json:"arguments,omitempty"`
}
// The server's response to a tool call.
@ -115,6 +109,10 @@ func (x *CallToolParams) isParams() {}
func (x *CallToolParams) GetProgressToken() any { return getProgressToken(x) }
func (x *CallToolParams) SetProgressToken(t any) { setProgressToken(x, t) }
func (x *CallToolParamsRaw) isParams() {}
func (x *CallToolParamsRaw) GetProgressToken() any { return getProgressToken(x) }
func (x *CallToolParamsRaw) SetProgressToken(t any) { setProgressToken(x, t) }
type CancelledParams struct {
// This property is reserved by the protocol to allow clients and servers to
// attach additional metadata to their responses.

2
mcp/requests.go
View file

@ -7,7 +7,7 @@
package mcp
type (
CallToolRequest = ServerRequest[*CallToolParams]
CallToolRequest = ServerRequest[*CallToolParamsRaw]
CompleteRequest = ServerRequest[*CompleteParams]
GetPromptRequest = ServerRequest[*GetPromptParams]
InitializedRequest = ServerRequest[*InitializedParams]

6
mcp/server.go
View file

@ -9,7 +9,6 @@ import (
"context"
"encoding/base64"
"encoding/gob"
"encoding/json"
"fmt"
"iter"
"maps"
@ -234,10 +233,9 @@ func toolForErr[In, Out any](t *Tool, h ToolHandlerFor[In, Out]) (*Tool, ToolHan
th := func(ctx context.Context, req *CallToolRequest) (*CallToolResult, error) {
// Unmarshal and validate args.
rawArgs := req.Params.Arguments.(json.RawMessage)
var in In
if rawArgs != nil {
if err := unmarshalSchema(rawArgs, inputResolved, &in); err != nil {
if req.Params.Arguments != nil {
if err := unmarshalSchema(req.Params.Arguments, inputResolved, &in); err != nil {
return nil, err
}
}