fix: prevent TogetherException repr crash on non-JSON-serializable headers

When headers contain non-JSON-serializable objects (e.g. aiohttp's
CIMultiDictProxy), `json.dumps()` in `__repr__` raises TypeError,
making it impossible to print or log the exception.

Add `default=str` to json.dumps so non-serializable objects fall back
to their string representation instead of crashing.

Fixes #108

Signed-off-by: Giulio Leone <6887247+giulio-leone@users.noreply.github.com>

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: giulio-leone

src/together/error.py

@@ -44,7 +44,8 @@ def __repr__(self) -> str:
                 "status": self.http_status,
                 "request_id": self.request_id,
                 "headers": self.headers,
-            }
+            },
+            default=str,
         )
         return "%s(%r)" % (self.__class__.__name__, repr_message)
 

tests/unit/test_error_repr.py

@@ -0,0 +1,122 @@
+"""Tests for TogetherException.__repr__ with non-JSON-serializable headers."""
+
+from __future__ import annotations
+
+import json
+from collections import OrderedDict
+from typing import Any, Iterator
+from unittest.mock import MagicMock
+
+import pytest
+
+from together.error import (
+    TogetherException,
+    AuthenticationError,
+    ResponseError,
+    APIError,
+)
+
+
+class FakeMultiDictProxy:
+    """Simulates aiohttp's CIMultiDictProxy — not JSON serializable."""
+
+    def __init__(self, data: dict[str, str]) -> None:
+        self._data = data
+
+    def __iter__(self) -> Iterator[str]:
+        return iter(self._data)
+
+    def __len__(self) -> int:
+        return len(self._data)
+
+    def __getitem__(self, key: str) -> str:
+        return self._data[key]
+
+    def __repr__(self) -> str:
+        return f"<FakeMultiDictProxy({self._data!r})>"
+
+
+class TestExceptionReprNonSerializable:
+    """Verify __repr__ doesn't crash on non-JSON-serializable headers (issue #108)."""
+
+    def test_repr_with_dict_headers(self) -> None:
+        """Normal dict headers should still work fine."""
+        exc = TogetherException(
+            message="test error",
+            headers={"Content-Type": "application/json"},
+            http_status=400,
+            request_id="req-123",
+        )
+        result = repr(exc)
+        assert "TogetherException" in result
+        assert "test error" in result
+
+    def test_repr_with_multidict_proxy_headers(self) -> None:
+        """CIMultiDictProxy-like headers must not crash repr (issue #108)."""
+        fake_headers = FakeMultiDictProxy(
+            {"Content-Type": "application/json", "X-Request-Id": "abc"}
+        )
+        exc = TogetherException(
+            message="server error",
+            headers=fake_headers,  # type: ignore[arg-type]
+            http_status=500,
+            request_id="req-456",
+        )
+        # Before fix: TypeError: Object of type FakeMultiDictProxy
+        #             is not JSON serializable
+        result = repr(exc)
+        assert "TogetherException" in result
+        assert "server error" in result
+
+    def test_repr_with_none_headers(self) -> None:
+        """None headers (default) should work."""
+        exc = TogetherException(message="no headers")
+        result = repr(exc)
+        assert "TogetherException" in result
+
+    def test_repr_with_string_headers(self) -> None:
+        """String headers should work."""
+        exc = TogetherException(
+            message="string headers", headers="raw-header-string"
+        )
+        result = repr(exc)
+        assert "TogetherException" in result
+
+    def test_repr_with_nested_non_serializable(self) -> None:
+        """Dict headers containing non-serializable values should not crash."""
+        exc = TogetherException(
+            message="nested issue",
+            headers={"key": MagicMock()},  # type: ignore[dict-item]
+            http_status=502,
+        )
+        result = repr(exc)
+        assert "TogetherException" in result
+
+    def test_repr_output_is_valid_after_fix(self) -> None:
+        """repr should produce parseable output (class name + JSON string)."""
+        exc = TogetherException(
+            message="validation error",
+            headers={"Authorization": "Bearer ***"},
+            http_status=422,
+            request_id="req-789",
+        )
+        result = repr(exc)
+        assert result.startswith("TogetherException(")
+        # The inner string should be valid JSON
+        inner = result[len("TogetherException(") + 1 : -2]  # strip quotes
+        parsed = json.loads(inner)
+        assert parsed["status"] == 422
+        assert parsed["request_id"] == "req-789"
+
+    def test_subclass_repr_with_non_serializable_headers(self) -> None:
+        """Subclasses should also benefit from the fix."""
+        fake_headers = FakeMultiDictProxy({"X-Rate-Limit": "100"})
+
+        for ExcClass in (AuthenticationError, ResponseError, APIError):
+            exc = ExcClass(
+                message="subclass test",
+                headers=fake_headers,  # type: ignore[arg-type]
+                http_status=429,
+            )
+            result = repr(exc)
+            assert ExcClass.__name__ in result