mcp: small changes to CallTool structs (#60)

- Use `any` for CallToolResult.StructuredContent.

- Change the non-generic structs into aliases for the generic ones.

Author: jba

mcp/content_test.go

@@ -7,7 +7,6 @@ package mcp_test
 import (
 	"encoding/json"
 	"fmt"
-	"log"
 	"testing"
 
 	"github.com/google/go-cmp/cmp"
@@ -94,7 +93,6 @@ func TestContent(t *testing.T) {
 			t.Errorf("json.Marshal(%v) mismatch (-want +got):\n%s", test.in, diff)
 		}
 		result := fmt.Sprintf(`{"content":[%s]}`, string(got))
-		log.Println(result)
 		var out mcp.CallToolResult
 		if err := json.Unmarshal([]byte(result), &out); err != nil {
 			t.Fatal(err)

mcp/protocol.go

@@ -39,70 +39,18 @@ type Annotations struct {
 	Priority float64 `json:"priority,omitempty"`
 }
 
-type CallToolParams struct {
-	// This property is reserved by the protocol to allow clients and servers to
-	// attach additional metadata to their responses.
-	Meta      `json:"_meta,omitempty"`
-	Arguments any    `json:"arguments,omitempty"`
-	Name      string `json:"name"`
-}
-
-func (x *CallToolParams) GetProgressToken() any  { return getProgressToken(x) }
-func (x *CallToolParams) SetProgressToken(t any) { setProgressToken(x, t) }
+type CallToolParams = CallToolParamsFor[any]
 
 type CallToolParamsFor[In any] struct {
 	// This property is reserved by the protocol to allow clients and servers to
 	// attach additional metadata to their responses.
 	Meta      `json:"_meta,omitempty"`
-	Arguments In     `json:"arguments,omitempty"`
 	Name      string `json:"name"`
+	Arguments In     `json:"arguments,omitempty"`
 }
 
 // The server's response to a tool call.
-type CallToolResult struct {
-	// This property is reserved by the protocol to allow clients and servers to
-	// attach additional metadata to their responses.
-	Meta `json:"_meta,omitempty"`
-	// A list of content objects that represent the unstructured result of the tool
-	// call.
-	Content []Content `json:"content"`
-	// Whether the tool call ended in an error.
-	//
-	// If not set, this is assumed to be false (the call was successful).
-	//
-	// Any errors that originate from the tool should be reported inside the result
-	// object, with isError set to true, not as an MCP protocol-level error
-	// response. Otherwise, the LLM would not be able to see that an error occurred
-	// and self-correct.
-	//
-	// However, any errors in finding the tool, an error indicating that the
-	// server does not support tool calls, or any other exceptional conditions,
-	// should be reported as an MCP error response.
-	IsError bool `json:"isError,omitempty"`
-	// An optional JSON object that represents the structured result of the tool
-	// call.
-	// TODO(jba,rfindley): should this be any?
-	StructuredContent map[string]any `json:"structuredContent,omitempty"`
-}
-
-// UnmarshalJSON handles the unmarshalling of content into the Content
-// interface.
-func (x *CallToolResult) UnmarshalJSON(data []byte) error {
-	type res CallToolResult // avoid recursion
-	var wire struct {
-		res
-		Content []*wireContent `json:"content"`
-	}
-	if err := json.Unmarshal(data, &wire); err != nil {
-		return err
-	}
-	var err error
-	if wire.res.Content, err = contentsFromWire(wire.Content, nil); err != nil {
-		return err
-	}
-	*x = CallToolResult(wire.res)
-	return nil
-}
+type CallToolResult = CallToolResultFor[any]
 
 type CallToolResultFor[Out any] struct {
 	// This property is reserved by the protocol to allow clients and servers to
@@ -111,6 +59,9 @@ type CallToolResultFor[Out any] struct {
 	// A list of content objects that represent the unstructured result of the tool
 	// call.
 	Content []Content `json:"content"`
+	// An optional JSON object that represents the structured result of the tool
+	// call.
+	StructuredContent Out `json:"structuredContent,omitempty"`
 	// Whether the tool call ended in an error.
 	//
 	// If not set, this is assumed to be false (the call was successful).
@@ -124,9 +75,6 @@ type CallToolResultFor[Out any] struct {
 	// server does not support tool calls, or any other exceptional conditions,
 	// should be reported as an MCP error response.
 	IsError bool `json:"isError,omitempty"`
-	// An optional JSON object that represents the structured result of the tool
-	// call.
-	StructuredContent Out `json:"structuredContent,omitempty"`
 }
 
 // UnmarshalJSON handles the unmarshalling of content into the Content
@@ -148,6 +96,9 @@ func (x *CallToolResultFor[Out]) UnmarshalJSON(data []byte) error {
 	return nil
 }
 
+func (x *CallToolParamsFor[Out]) GetProgressToken() any  { return getProgressToken(x) }
+func (x *CallToolParamsFor[Out]) 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.

mcp/tool.go

@@ -17,7 +17,7 @@ import (
 // A ToolHandler handles a call to tools/call.
 // [CallToolParams.Arguments] will contain a map[string]any that has been validated
 // against the input schema.
-// Perhaps this should be an alias for ToolHandlerFor[map[string]any, map[string]any].
+// TODO: Perhaps this should be an alias for ToolHandlerFor[map[string]any, map[string]any]?
 type ToolHandler func(context.Context, *ServerSession, *CallToolParamsFor[map[string]any]) (*CallToolResult, error)
 
 // A ToolHandlerFor handles a call to tools/call with typed arguments and results.