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internal/mcp: reorganize the tool API, keeping generics

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CallToolParams.Arguments is any.

Clients can send whatever they want.

Tool authors who create their own ToolHandler (bypassing NewTool)
will receive the arguments as a map[string]any which has been
validated against the input schema.

Tool authors who call NewTool can choose the type that the arguments
unmarshal into.

There is no double-unmarshaling and no downcasting.

There is no way for tool authors to avoid the unmarshal, and no way to
avoid the validation unless the omit an input schema, which is probably
bad for LLMs. If tool authors want this optimization, we can provide
it later.

Change-Id: Ieb201e56c7fb8a23bb1a93b27e946630ec9e79ad
Reviewed-on: https://go-review.googlesource.com/c/tools/+/679595
TryBot-Bypass: Jonathan Amsterdam <jba@google.com>
Auto-Submit: Jonathan Amsterdam <jba@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
This commit is contained in:
Jonathan Amsterdam 2025-06-05 16:17:03 -04:00
commit f211cc8e4a
1 changed files with 328 additions and 228 deletions
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77
mcp/tool_test.go
View file

@ -6,8 +6,6 @@ package mcp_test
import (
"context"
"encoding/json"
"strings"
"testing"
"github.com/google/go-cmp/cmp"
@ -17,7 +15,7 @@ import (
)
// testToolHandler is used for type inference in TestNewTool.
func testToolHandler[T any](context.Context, *mcp.ServerSession, *mcp.CallToolParams[T]) (*mcp.CallToolResult, error) {
func testToolHandler[T any](context.Context, *mcp.ServerSession, *mcp.CallToolParamsFor[T]) (*mcp.CallToolResultFor[any], error) {
panic("not implemented")
}
@ -90,76 +88,3 @@ func TestNewTool(t *testing.T) {
}
}
}
func TestNewToolValidate(t *testing.T) {
// Check that the tool returned from NewTool properly validates its input schema.
type req struct {
I int
B bool
S string `json:",omitempty"`
P *int `json:",omitempty"`
}
dummyHandler := func(context.Context, *mcp.ServerSession, *mcp.CallToolParams[req]) (*mcp.CallToolResult, error) {
return nil, nil
}
tool := mcp.NewTool("test", "test", dummyHandler)
for _, tt := range []struct {
desc string
args map[string]any
want string // error should contain this string; empty for success
}{
{
"both required",
map[string]any{"I": 1, "B": true},
"",
},
{
"optional",
map[string]any{"I": 1, "B": true, "S": "foo"},
"",
},
{
"wrong type",
map[string]any{"I": 1.5, "B": true},
"cannot unmarshal",
},
{
"extra property",
map[string]any{"I": 1, "B": true, "C": 2},
"unknown field",
},
{
"value for pointer",
map[string]any{"I": 1, "B": true, "P": 3},
"",
},
{
"null for pointer",
map[string]any{"I": 1, "B": true, "P": nil},
"",
},
} {
t.Run(tt.desc, func(t *testing.T) {
raw, err := json.Marshal(tt.args)
if err != nil {
t.Fatal(err)
}
_, err = tool.Handler(context.Background(), nil,
&mcp.CallToolParams[json.RawMessage]{Arguments: json.RawMessage(raw)})
if err == nil && tt.want != "" {
t.Error("got success, wanted failure")
}
if err != nil {
if tt.want == "" {
t.Fatalf("failed with:\n%s\nwanted success", err)
}
if !strings.Contains(err.Error(), tt.want) {
t.Fatalf("got:\n%s\nwanted to contain %q", err, tt.want)
}
}
})
}
}