From 3729867e9c788bfd3984a25093dfac4085cd3e36 Mon Sep 17 00:00:00 2001
From: Jason Chen <jason@slab.com>
Date: Sat, 9 Aug 2025 17:14:01 -0400
Subject: [PATCH] don't validate tool error responses

---
 lib/hermes/server/handlers/tools.ex           |  4 ++++
 .../component/tool_annotations_test.exs       | 20 +++++++++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/lib/hermes/server/handlers/tools.ex b/lib/hermes/server/handlers/tools.ex
index 74261cc..cdb4e8d 100644
--- a/lib/hermes/server/handlers/tools.ex
+++ b/lib/hermes/server/handlers/tools.ex
@@ -79,6 +79,10 @@ defmodule Hermes.Server.Handlers.Tools do
     {:reply, Response.to_protocol(resp), frame}
   end
 
+  defp maybe_validate_output_schema(_tool, %Response{isError: true} = resp, frame) do
+    {:reply, Response.to_protocol(resp), frame}
+  end
+
   defp maybe_validate_output_schema(%Tool{} = tool, %Response{structured_content: nil}, frame) do
     metadata = %{tool_name: tool.name}
     {:error, Error.execution(@output_schema_err, metadata), frame}
diff --git a/test/hermes/server/component/tool_annotations_test.exs b/test/hermes/server/component/tool_annotations_test.exs
index d36fe69..e3e86f6 100644
--- a/test/hermes/server/component/tool_annotations_test.exs
+++ b/test/hermes/server/component/tool_annotations_test.exs
@@ -103,6 +103,10 @@ defmodule Hermes.Server.Component.ToolAnnotationsTest do
     end
 
     @impl true
+    def execute(%{query: "tool error"}, frame) do
+      {:reply, Response.error(Response.tool(), "Tool error"), frame}
+    end
+
     def execute(%{query: query}, frame) do
       results = %{
         results: [
@@ -392,6 +396,22 @@ defmodule Hermes.Server.Component.ToolAnnotationsTest do
       assert result["isError"] == false
     end
 
+    test "tool error response skips output schema validation", %{
+      server: server,
+      session_id: session_id
+    } do
+      request =
+        build_request("tools/call", %{
+          "name" => "tool_with_output_schema",
+          "arguments" => %{"query" => "tool error"}
+        })
+
+      {:ok, response_string} =
+        GenServer.call(server, {:request, request, session_id, %{}})
+
+      assert {:ok, [%{"result" => %{"isError" => true}}]} = Message.decode(response_string)
+    end
+
     test "tool without output schema works normally", %{
       server: server,
       session_id: session_id