diff --git a/src/rai_core/rai/agents/langchain/core/tool_runner.py b/src/rai_core/rai/agents/langchain/core/tool_runner.py
index 216748e2d..4f7dc71c3 100644
--- a/src/rai_core/rai/agents/langchain/core/tool_runner.py
+++ b/src/rai_core/rai/agents/langchain/core/tool_runner.py
@@ -73,9 +73,20 @@ def run_one(call: ToolCall):
             self.logger.info(f"Running tool: {call['name']}, args: {call['args']}")
             artifact = None
 
+            tool = self.tools_by_name.get(call["name"])
+            if tool is None:
+                error_message = f'Unknown tool: "{call["name"]}"'
+                self.logger.info(error_message)
+                return ToolMessage(
+                    content=error_message,
+                    name=call["name"],
+                    tool_call_id=call["id"],
+                    status="error",
+                )
+
             try:
                 ts = time.perf_counter()
-                output = self.tools_by_name[call["name"]].invoke(call, config)  # type: ignore
+                output = tool.invoke(call, config)  # type: ignore
                 te = time.perf_counter() - ts
                 self.logger.info(
                     f"Tool {call['name']} completed in {te:.2f} seconds. Tool output: {str(output.content)[:100]}{'...' if len(str(output.content)) > 100 else ''}"
diff --git a/tests/agents/langchain/test_tool_runner_unknown.py b/tests/agents/langchain/test_tool_runner_unknown.py
new file mode 100644
index 000000000..e8f5f01f9
--- /dev/null
+++ b/tests/agents/langchain/test_tool_runner_unknown.py
@@ -0,0 +1,20 @@
+# Copyright (C) 2026 Robotec.AI
+from langchain_core.messages import AIMessage, ToolMessage
+
+from rai.agents.langchain.core.tool_runner import ToolRunner
+
+
+def test_tool_runner_unknown_tool_returns_error_message():
+    runner = ToolRunner(tools=[])
+    ai = AIMessage(
+        content="",
+        tool_calls=[{"name": "missing_tool", "args": {}, "id": "call_1", "type": "tool_call"}],
+    )
+    out = runner.invoke({"messages": [ai]})
+    msgs = out["messages"]
+    assert len(msgs) == 2
+    err = msgs[-1]
+    assert isinstance(err, ToolMessage)
+    assert err.status == "error"
+    assert "Unknown tool" in err.content
+    assert err.tool_call_id == "call_1"