From 281affed673a91f6a4e783ff2c7fe523e9c3fafe Mon Sep 17 00:00:00 2001
From: Andrew Chen <48723787+chuenchen309@users.noreply.github.com>
Date: Thu, 16 Jul 2026 09:25:01 +0800
Subject: [PATCH] fix(agents): guard IndexError when Chat.last hits an empty
 messages list

`_run_tools`'s no-op guard checked `not chat.last`, but `Chat.last` indexes
`messages[-1]`, which raises IndexError on an empty list instead of behaving
falsy. A ChatWorkflow with no messages added (e.g. `ChatWorkflow(...).run()`)
hit this IndexError inside the guard's own condition instead of the intended
clean no-op. Check `not chat.messages` directly instead.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
---
 .../src/giskard/agents/workflow.py            |  2 +-
 .../tests/test_workflow_steps.py              | 26 +++++++++++++++++++
 2 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/libs/giskard-agents/src/giskard/agents/workflow.py b/libs/giskard-agents/src/giskard/agents/workflow.py
index 348a542a7d..51b5d3399f 100644
--- a/libs/giskard-agents/src/giskard/agents/workflow.py
+++ b/libs/giskard-agents/src/giskard/agents/workflow.py
@@ -166,7 +166,7 @@ async def execute(
 
     async def _run_tools(self, chat: Chat[Any]) -> AsyncGenerator[ToolMessage, None]:
         if (
-            not chat.last
+            not chat.messages
             or not isinstance(chat.last, AssistantMessage)
             or not chat.last.tool_calls
         ):
diff --git a/libs/giskard-agents/tests/test_workflow_steps.py b/libs/giskard-agents/tests/test_workflow_steps.py
index 4f4360b3f5..dcc7f7e05c 100644
--- a/libs/giskard-agents/tests/test_workflow_steps.py
+++ b/libs/giskard-agents/tests/test_workflow_steps.py
@@ -120,3 +120,29 @@ async def test_run_returns_successful_chat():
 
     assert not chat.failed
     assert chat.last.content == "World!"
+
+
+async def test_run_with_no_messages_does_not_raise_indexerror():
+    """A ChatWorkflow with no messages added must not crash with IndexError.
+
+    ``_run_tools``'s guard checks ``not chat.last`` before checking whether
+    there is anything to process, but ``Chat.last`` indexes ``messages[-1]``,
+    which raises ``IndexError`` on an empty list instead of behaving falsy.
+    """
+    gen = MagicMock(spec=BaseGenerator)
+    gen.complete = AsyncMock(
+        return_value=CompletionResponse(
+            choices=[
+                Choice(
+                    message=AssistantMessage(content="Hello!"),
+                    finish_reason="stop",
+                    index=0,
+                )
+            ]
+        )
+    )
+
+    chat = await ChatWorkflow(generator=gen).run()
+
+    assert not chat.failed
+    assert chat.last.content == "Hello!"