diff --git a/src/server/mcp.test.ts b/src/server/mcp.test.ts
index 10e550df4..4a7b2bb9d 100644
--- a/src/server/mcp.test.ts
+++ b/src/server/mcp.test.ts
@@ -937,24 +937,6 @@ describe("tool()", () => {
       }),
     );
 
-    mcpServer.registerTool(
-      "test (new api)",
-      {
-        inputSchema: {
-          name: z.string(),
-          value: z.number(),
-        },
-      },
-      async ({ name, value }) => ({
-        content: [
-          {
-            type: "text",
-            text: `${name}: ${value}`,
-          },
-        ],
-      })
-    );
-
     const [clientTransport, serverTransport] =
       InMemoryTransport.createLinkedPair();
 
@@ -977,23 +959,15 @@ describe("tool()", () => {
         },
         CallToolResultSchema,
       ),
-    ).rejects.toThrow(/Invalid arguments/);
-
-    await expect(
-      client.request(
+    ).resolves.toStrictEqual({
+      content: [
         {
-          method: "tools/call",
-          params: {
-            name: "test (new api)",
-            arguments: {
-              name: "test",
-              value: "not a number",
-            },
-          },
+          type: "text",
+          text: expect.stringMatching(/Invalid arguments for tool test/),
         },
-        CallToolResultSchema,
-      ),
-    ).rejects.toThrow(/Invalid arguments/);
+      ],
+      isError: true,
+    });
   });
 
   /***
diff --git a/src/server/mcp.ts b/src/server/mcp.ts
index 791facef1..818c45cb3 100644
--- a/src/server/mcp.ts
+++ b/src/server/mcp.ts
@@ -162,10 +162,15 @@ export class McpServer {
             request.params.arguments,
           );
           if (!parseResult.success) {
-            throw new McpError(
-              ErrorCode.InvalidParams,
-              `Invalid arguments for tool ${request.params.name}: ${parseResult.error.message}`,
-            );
+            return {
+              content: [
+                {
+                  type: "text",
+                  text: `Invalid arguments for tool ${request.params.name}: ${parseResult.error.message}`,
+                },
+              ],
+              isError: true,
+            };
           }
 
           const args = parseResult.data;