fix(acp): only deliver final_response after streaming when transformed

PR #29119 dropped the 'not streamed_message' guard unconditionally so
that plugin-transformed responses (transform_llm_output hook) would
reach ACP clients. That regressed test_prompt_does_not_duplicate_streamed_final_message:
when no transform happened, the streamed text was re-sent as a duplicate
final delivery.

Tighten the condition to mirror the gateway side: deliver after streaming
only when response_transformed=True. Otherwise keep the old guard.

Adds test_prompt_delivers_transformed_response_after_streaming so the
transformed path stays covered.

acp_adapter/server.py

@@ -1534,9 +1534,11 @@ class HermesACPAgent(acp.Agent):
                 )
             except Exception:
                 logger.debug("Failed to auto-title ACP session %s", session_id, exc_info=True)
-        if final_response and conn:
-            # Always deliver the final response — plugins may have transformed
-            # it after streaming finished (e.g. transform_llm_output hook).
+        if final_response and conn and (not streamed_message or result.get("response_transformed")):
+            # Deliver the final response when streaming did not already send it,
+            # or when a plugin hook transformed the response after streaming
+            # finished (e.g. transform_llm_output) — otherwise the appended /
+            # rewritten text never reaches the client.
             update = acp.update_agent_message_text(final_response)
             await conn.session_update(session_id, update)
 

tests/acp/test_server.py

@@ -1203,6 +1203,48 @@ class TestPrompt:
         assert len(agent_chunks) == 1
         assert agent_chunks[0].content.text == "streamed answer"
 
+    @pytest.mark.asyncio
+    async def test_prompt_delivers_transformed_response_after_streaming(self, agent):
+        """If a transform_llm_output plugin hook modifies the response after
+        streaming, ACP must deliver the transformed final_response so the
+        appended/rewritten text reaches the client.
+        """
+        new_resp = await agent.new_session(cwd=".")
+        state = agent.session_manager.get_session(new_resp.session_id)
+
+        def mock_run(*args, **kwargs):
+            state.agent.stream_delta_callback("original answer")
+            return {
+                "final_response": "original answer\n\n[plugin appended this]",
+                "response_transformed": True,
+                "messages": [],
+            }
+
+        state.agent.run_conversation = mock_run
+
+        mock_conn = MagicMock(spec=acp.Client)
+        mock_conn.session_update = AsyncMock()
+        agent._conn = mock_conn
+
+        prompt = [TextContentBlock(type="text", text="hello")]
+        await agent.prompt(prompt=prompt, session_id=new_resp.session_id)
+
+        updates = [
+            call.kwargs.get("update") or call.args[1]
+            for call in mock_conn.session_update.call_args_list
+        ]
+        # The streamed chunk and the post-stream transformed message should
+        # both be present (final delivery is a separate update_agent_message_text
+        # call carrying the full transformed text).
+        all_texts = [
+            getattr(getattr(u, "content", None), "text", None)
+            for u in updates
+        ]
+        assert any(
+            text and "[plugin appended this]" in text for text in all_texts
+        ), f"expected transformed final to be delivered, got: {all_texts!r}"
+
+
     @pytest.mark.asyncio
     async def test_prompt_auto_titles_session(self, agent):
         new_resp = await agent.new_session(cwd=".")