diff --git a/src/minisweagent/agents/default.py b/src/minisweagent/agents/default.py
index 9f80e8fda..a2e73e1be 100644
--- a/src/minisweagent/agents/default.py
+++ b/src/minisweagent/agents/default.py
@@ -71,6 +71,35 @@ def add_messages(self, *messages: dict) -> list[dict]:
         self.messages.extend(messages)
         return list(messages)
 
+    def _completed_tool_call_ids(self) -> set[str]:
+        completed_ids = set()
+        for message in self.messages:
+            if message.get("role") == "tool" and message.get("tool_call_id"):
+                completed_ids.add(message["tool_call_id"])
+            if message.get("type") == "function_call_output" and message.get("call_id"):
+                completed_ids.add(message["call_id"])
+        return completed_ids
+
+    def _validate_new_tool_call_ids(self, message: dict) -> None:
+        completed_ids = self._completed_tool_call_ids()
+        if not completed_ids:
+            return
+        repeated_ids = sorted(
+            {
+                action["tool_call_id"]
+                for action in message.get("extra", {}).get("actions", [])
+                if action.get("tool_call_id") in completed_ids
+            }
+        )
+        if repeated_ids:
+            raise FormatError(
+                self.model.format_message(
+                    role="user",
+                    content=(f"Tool call ID(s) already completed and must not be reused: {', '.join(repeated_ids)}"),
+                    extra={"interrupt_type": "FormatError"},
+                )
+            )
+
     def handle_uncaught_exception(self, e: Exception) -> list[dict]:
         return self.add_messages(
             self.model.format_message(
@@ -146,6 +175,7 @@ def query(self) -> dict:
         self.n_calls += 1
         message = self.model.query(self.messages)
         self.cost += message.get("extra", {}).get("cost", 0.0)
+        self._validate_new_tool_call_ids(message)
         self.add_messages(message)
         return message
 
diff --git a/tests/agents/test_default.py b/tests/agents/test_default.py
index 48aa5b0be..21940e149 100644
--- a/tests/agents/test_default.py
+++ b/tests/agents/test_default.py
@@ -53,6 +53,21 @@ def is_observation_message(msg: dict) -> bool:
     return False
 
 
+class _RecordingEnvironment:
+    def __init__(self):
+        self.actions: list[dict] = []
+
+    def execute(self, action: dict) -> dict:
+        self.actions.append(action)
+        return {"output": action["command"], "returncode": 0, "exception_info": ""}
+
+    def get_template_vars(self) -> dict:
+        return {}
+
+    def serialize(self) -> dict:
+        return {}
+
+
 # --- Fixtures ---
 
 
@@ -384,6 +399,50 @@ def test_step_adds_messages(model_factory):
     assert "returncode" in get_observation_text(agent.messages[-1])
 
 
+def test_duplicate_completed_tool_call_id_is_not_executed_again(toolcall_config):
+    """Repeated provider tool-call IDs should not produce duplicate tool results."""
+
+    first_call = {
+        "id": "repeat_loop_initial",
+        "type": "function",
+        "function": {"name": "bash", "arguments": '{"command": "echo first"}'},
+    }
+    repeated_call = {
+        "id": "repeat_loop_initial",
+        "type": "function",
+        "function": {"name": "bash", "arguments": '{"command": "echo duplicate"}'},
+    }
+    env = _RecordingEnvironment()
+    agent = DefaultAgent(
+        model=DeterministicToolcallModel(
+            outputs=[
+                make_toolcall_output(
+                    None,
+                    [first_call],
+                    [{"command": "echo first", "tool_call_id": "repeat_loop_initial"}],
+                ),
+                make_toolcall_output(
+                    None,
+                    [repeated_call],
+                    [{"command": "echo duplicate", "tool_call_id": "repeat_loop_initial"}],
+                ),
+            ],
+        ),
+        env=env,
+        **toolcall_config,
+    )
+
+    agent.add_messages({"role": "system", "content": "system"}, {"role": "user", "content": "task"})
+    agent.step()
+
+    with pytest.raises(FormatError) as exc:
+        agent.step()
+
+    assert "already completed" in exc.value.messages[0]["content"]
+    assert env.actions == [{"command": "echo first", "tool_call_id": "repeat_loop_initial"}]
+    assert [msg.get("tool_call_id") for msg in agent.messages if msg.get("role") == "tool"] == ["repeat_loop_initial"]
+
+
 def test_observations_captured(model_factory):
     """Test intermediate outputs are captured correctly."""
     factory, config = model_factory
@@ -495,10 +554,14 @@ def test_repeated_format_errors_terminate_cleanly(toolcall_config):
 def test_format_error_counter_resets_on_success(toolcall_config):
     """A successful tool call between format errors resets the consecutive counter, so isolated
     errors don't accumulate to the termination threshold."""
-    good = make_tc_model([("listing", [{"command": "echo hello"}])]).config.outputs[0]
-    submit = make_tc_model(
-        [("done", [{"command": "echo 'COMPLETE_TASK_AND_SUBMIT_FINAL_OUTPUT'\necho ok"}])]
-    ).config.outputs[0]
+    tool_outputs = make_tc_model(
+        [
+            ("listing", [{"command": "echo hello"}]),
+            ("done", [{"command": "echo 'COMPLETE_TASK_AND_SUBMIT_FINAL_OUTPUT'\necho ok"}]),
+        ]
+    ).config.outputs
+    good = tool_outputs[0]
+    submit = tool_outputs[1]
     # error, success (reset), error, submit -> never 2 in a row, so it must NOT terminate early.
     outputs = [{"_format_error": True}, good, {"_format_error": True}, submit]
     agent = DefaultAgent(