refactor(app): fold error prefix into validateStandardSchema; clarify registerAppTool runtime caveat

Author: ochafik

src/app.ts

@@ -403,34 +403,23 @@ export class App extends ProtocolWithEvents<
         if (!registeredTool.enabled) {
           throw new Error(`Tool ${name} is disabled`);
         }
-        let parsedArgs = rawArgs;
-        if (config.inputSchema) {
-          try {
-            parsedArgs = await validateStandardSchema(
+        const parsedArgs = config.inputSchema
+          ? await validateStandardSchema(
               config.inputSchema,
               rawArgs,
-            );
-          } catch (e) {
-            throw new Error(
-              `Invalid input for tool ${name}: ${(e as Error).message}`,
-            );
-          }
-        }
+              `Invalid input for tool ${name}: `,
+            )
+          : rawArgs;
         const result = await (cb as AppToolCallback<StandardSchemaWithJSON>)(
           parsedArgs,
           extra,
         );
         if (config.outputSchema) {
-          try {
-            result.structuredContent = (await validateStandardSchema(
-              config.outputSchema,
-              result.structuredContent,
-            )) as CallToolResult["structuredContent"];
-          } catch (e) {
-            throw new Error(
-              `Invalid output for tool ${name}: ${(e as Error).message}`,
-            );
-          }
+          result.structuredContent = (await validateStandardSchema(
+            config.outputSchema,
+            result.structuredContent,
+            `Invalid output for tool ${name}: `,
+          )) as CallToolResult["structuredContent"];
         }
         return result;
       },

src/server/index.ts

@@ -225,11 +225,10 @@ export function registerAppTool<
     inputSchema?: InputArgs;
     outputSchema?: OutputArgs;
   },
-  // ToolCallback in sdk@1.x is constrained to zod; widen here so callers using
-  // other Standard Schema libs type-check. Runtime is fully delegated to
-  // McpServer.registerTool, which in 1.29.0 already handles anything zod-shaped
-  // (and zod ≥3.25 is a Standard Schema). Non-zod callers get correct arg
-  // inference here; the cast below bridges the 1.x SDK type until v2.
+  // The widened constraint signals the v2 API shape, but NOTE: McpServer in
+  // sdk@1.x still calls zod internals at runtime, so non-zod schemas will fail
+  // here until we depend on sdk v2. Zod (which all current callers use) is
+  // unaffected. The cast below bridges the 1.x type signature.
   cb: ToolCallback<
     InputArgs extends undefined | ZodRawShapeCompat | AnySchema
       ? InputArgs

src/standard-schema.ts

@@ -47,11 +47,12 @@ export function standardSchemaToJsonSchema(
 
 /**
  * Validate a value against a Standard Schema. Returns the parsed value on
- * success or throws with a formatted issue list on failure.
+ * success or throws with a formatted issue list (optionally prefixed).
  */
 export async function validateStandardSchema<S extends StandardSchemaV1>(
   schema: S,
   value: unknown,
+  errorPrefix = "",
 ): Promise<StandardSchemaV1.InferOutput<S>> {
   const result = await schema["~standard"].validate(value);
   if (result.issues) {
@@ -63,7 +64,7 @@ export async function validateStandardSchema<S extends StandardSchemaV1>(
         return path ? `${path}: ${i.message}` : i.message;
       })
       .join("; ");
-    throw new Error(msg);
+    throw new Error(errorPrefix + msg);
   }
   return result.value;
 }