mcp: validate tool output

Author: jba

mcp/server.go

@@ -260,8 +260,12 @@ func toolForErr[In, Out any](t *Tool, h ToolHandlerFor[In, Out]) (*Tool, ToolHan
 			}, nil
 		}
 
-		// TODO(v0.3.0): Validate out.
-		_ = outputResolved
+		// Validate output schema, if any.
+		// Skip if out is nil: we've removed "null" from the output schema, so nil won't validate.
+		if v := reflect.ValueOf(out); v.Kind() == reflect.Pointer && v.IsNil() {
+		} else if err := validateSchema(outputResolved, &out); err != nil {
+			return nil, err
+		}
 
 		// TODO: return the serialized JSON in a TextContent block, as per spec?
 		// https://modelcontextprotocol.io/specification/2025-06-18/server/tools#structured-content

mcp/tool.go

@@ -9,6 +9,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	// "log"
 
 	"github.com/google/jsonschema-go/jsonschema"
 )
@@ -42,13 +43,17 @@ func unmarshalSchema(data json.RawMessage, resolved *jsonschema.Resolved, v any)
 	if err := dec.Decode(v); err != nil {
 		return fmt.Errorf("unmarshaling: %w", err)
 	}
-	// TODO: test with nil args.
+	return validateSchema(resolved, v)
+}
+
+// TODO: test with nil args.
+func validateSchema(resolved *jsonschema.Resolved, value any) error {
 	if resolved != nil {
-		if err := resolved.ApplyDefaults(v); err != nil {
-			return fmt.Errorf("applying defaults from \n\t%s\nto\n\t%s:\n%w", schemaJSON(resolved.Schema()), data, err)
+		if err := resolved.ApplyDefaults(value); err != nil {
+			return fmt.Errorf("applying defaults from \n\t%s\nto\n\t%v:\n%w", schemaJSON(resolved.Schema()), value, err)
 		}
-		if err := resolved.Validate(v); err != nil {
-			return fmt.Errorf("validating\n\t%s\nagainst\n\t %s:\n %w", data, schemaJSON(resolved.Schema()), err)
+		if err := resolved.Validate(value); err != nil {
+			return fmt.Errorf("validating\n\t%v\nagainst\n\t %s:\n %w", value, schemaJSON(resolved.Schema()), err)
 		}
 	}
 	return nil