diff --git a/src/minisweagent/agents/default.py b/src/minisweagent/agents/default.py
index da54249a8..d70dbdfb8 100644
--- a/src/minisweagent/agents/default.py
+++ b/src/minisweagent/agents/default.py
@@ -12,7 +12,7 @@
 
 from minisweagent import Environment, Model, __version__
 from minisweagent.exceptions import InterruptAgentFlow, LimitsExceeded
-from minisweagent.utils.serialize import recursive_merge
+from minisweagent.utils.serialize import recursive_merge, to_jsonable
 
 
 class AgentConfig(BaseModel):
@@ -151,5 +151,5 @@ def save(self, path: Path | None, *extra_dicts) -> dict:
         data = self.serialize(*extra_dicts)
         if path:
             path.parent.mkdir(parents=True, exist_ok=True)
-            path.write_text(json.dumps(data, indent=2))
+            path.write_text(json.dumps(to_jsonable(data), indent=2))
         return data
diff --git a/src/minisweagent/models/litellm_model.py b/src/minisweagent/models/litellm_model.py
index b40bea46c..2a7d603a8 100644
--- a/src/minisweagent/models/litellm_model.py
+++ b/src/minisweagent/models/litellm_model.py
@@ -9,6 +9,7 @@
 import litellm
 from pydantic import BaseModel
 
+from minisweagent.exceptions import FormatError
 from minisweagent.models import GLOBAL_MODEL_STATS
 from minisweagent.models.utils.actions_toolcall import (
     BASH_TOOL,
@@ -84,8 +85,24 @@ def query(self, messages: list[dict[str, str]], **kwargs) -> dict:
         cost_output = self._calculate_cost(response)
         GLOBAL_MODEL_STATS.add(cost_output["cost"])
         message = response.choices[0].message.model_dump()
+        try:
+            actions = self._parse_actions(response)
+        except FormatError as e:
+            # Preserve raw assistant response for debugging (appears in live + final trajectories)
+            debug_message = {
+                "role": "assistant",
+                "content": message.get("content"),
+                "tool_calls": message.get("tool_calls"),
+                "extra": {
+                    "parse_error": True,
+                    "response": response.model_dump(),
+                    **cost_output,
+                    "timestamp": time.time(),
+                },
+            }
+            raise FormatError(debug_message, *e.messages) from e
         message["extra"] = {
-            "actions": self._parse_actions(response),
+            "actions": actions,
             "response": response.model_dump(),
             **cost_output,
             "timestamp": time.time(),
diff --git a/src/minisweagent/utils/serialize.py b/src/minisweagent/utils/serialize.py
index e5280d999..987b53aaa 100644
--- a/src/minisweagent/utils/serialize.py
+++ b/src/minisweagent/utils/serialize.py
@@ -24,3 +24,26 @@ def recursive_merge(*dictionaries: dict | None) -> dict:
             else:
                 result[key] = value
     return result
+
+
+def to_jsonable(value: Any) -> Any:
+    """Convert values to something JSON-serializable for logging."""
+    if value is None or isinstance(value, (str, int, float, bool)):
+        return value
+    if isinstance(value, (bytes, bytearray)):
+        return value.decode("utf-8", errors="replace")
+    if isinstance(value, dict):
+        return {str(k): to_jsonable(v) for k, v in value.items()}
+    if isinstance(value, (list, tuple, set)):
+        return [to_jsonable(v) for v in value]
+    if hasattr(value, "model_dump"):
+        try:
+            return to_jsonable(value.model_dump())
+        except Exception:
+            pass
+    if hasattr(value, "__dict__"):
+        try:
+            return to_jsonable(vars(value))
+        except Exception:
+            pass
+    return str(value)
diff --git a/tests/utils/test_serialize.py b/tests/utils/test_serialize.py
index d058d01a4..943ff661d 100644
--- a/tests/utils/test_serialize.py
+++ b/tests/utils/test_serialize.py
@@ -1,4 +1,4 @@
-from minisweagent.utils.serialize import UNSET, recursive_merge
+from minisweagent.utils.serialize import UNSET, recursive_merge, to_jsonable
 
 
 def test_empty_input():
@@ -136,3 +136,57 @@ def test_unset_values_skipped():
         {"a": {"x": 1, "y": 2}},
         {"a": {"y": UNSET, "z": 3}, "b": UNSET, "c": 4},
     ) == {"a": {"x": 1, "y": 2, "z": 3}, "c": 4}
+
+
+def test_to_jsonable_basic_types():
+    """Test that JSON-native types pass through unchanged."""
+    assert to_jsonable(None) is None
+    assert to_jsonable("value") == "value"
+    assert to_jsonable(7) == 7
+    assert to_jsonable(3.5) == 3.5
+    assert to_jsonable(True) is True
+
+
+def test_to_jsonable_bytes_and_collections():
+    """Test conversion of bytes and collection types."""
+    assert to_jsonable(b"hello") == "hello"
+    assert to_jsonable(bytearray(b"world")) == "world"
+    assert to_jsonable({1: "a", "b": 2}) == {"1": "a", "b": 2}
+    assert to_jsonable([1, 2, 3]) == [1, 2, 3]
+    assert to_jsonable((1, 2)) == [1, 2]
+    assert to_jsonable({3, 4}) in ([3, 4], [4, 3])
+
+
+def test_to_jsonable_model_dump():
+    """Test that objects with model_dump are serialized via model_dump."""
+
+    class DummyModel:
+        def model_dump(self):
+            return {"key": "value", "nested": {"num": 1}}
+
+    assert to_jsonable(DummyModel()) == {"key": "value", "nested": {"num": 1}}
+
+
+def test_to_jsonable_dict_fallback():
+    """Test fallback to __dict__ when model_dump fails."""
+
+    class Dummy:
+        def __init__(self):
+            self.value = 42
+
+        def model_dump(self):
+            raise RuntimeError("boom")
+
+    assert to_jsonable(Dummy()) == {"value": 42}
+
+
+def test_to_jsonable_string_fallback():
+    """Test fallback to string for objects without __dict__."""
+
+    class SlotOnly:
+        __slots__ = ()
+
+        def __str__(self) -> str:
+            return "slot-only"
+
+    assert to_jsonable(SlotOnly()) == "slot-only"