diff --git a/src/together/error.py b/src/together/error.py
index e2883a2..97e33c8 100644
--- a/src/together/error.py
+++ b/src/together/error.py
@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import json
+from collections.abc import Mapping
 from typing import Any, Dict
 
 from requests import RequestException
@@ -8,6 +9,16 @@
 from together.types.error import TogetherErrorResponse
 
 
+def _json_safe_headers(headers: str | Dict[Any, Any] | None) -> str | Dict[Any, Any]:
+    if headers is None:
+        return {}
+    if isinstance(headers, str):
+        return headers
+    if isinstance(headers, Mapping):
+        return {str(key): value for key, value in headers.items()}
+    return str(headers)
+
+
 class TogetherException(Exception):
     def __init__(
         self,
@@ -43,8 +54,9 @@ def __repr__(self) -> str:
                 "response": self._message,
                 "status": self.http_status,
                 "request_id": self.request_id,
-                "headers": self.headers,
-            }
+                "headers": _json_safe_headers(self.headers),
+            },
+            default=str,
         )
         return "%s(%r)" % (self.__class__.__name__, repr_message)
 
diff --git a/tests/unit/test_error_repr.py b/tests/unit/test_error_repr.py
new file mode 100644
index 0000000..e94dd63
--- /dev/null
+++ b/tests/unit/test_error_repr.py
@@ -0,0 +1,108 @@
+"""Tests for TogetherException.__repr__ with non-JSON-serializable headers."""
+
+from __future__ import annotations
+
+import json
+from unittest.mock import MagicMock
+
+from multidict import CIMultiDict, CIMultiDictProxy
+
+from together.error import (
+    APIError,
+    AuthenticationError,
+    ResponseError,
+    TogetherException,
+)
+
+
+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:
+        """Real CIMultiDictProxy headers must not crash repr (issue #108)."""
+        headers = CIMultiDictProxy(
+            CIMultiDict(
+                {"Content-Type": "application/json", "X-Request-Id": "abc"}
+            )
+        )
+        exc = TogetherException(
+            message="server error",
+            headers=headers,  # type: ignore[arg-type]
+            http_status=500,
+            request_id="req-456",
+        )
+        # Before fix: TypeError: Object of type CIMultiDictProxy
+        #             is not JSON serializable
+        result = repr(exc)
+        assert "TogetherException" in result
+        assert "server error" in result
+        parsed = json.loads(result[len("TogetherException(") + 1 : -2])
+        assert parsed["headers"] == {
+            "Content-Type": "application/json",
+            "X-Request-Id": "abc",
+        }
+
+    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."""
+        headers = CIMultiDictProxy(CIMultiDict({"X-Rate-Limit": "100"}))
+
+        for ExcClass in (AuthenticationError, ResponseError, APIError):
+            exc = ExcClass(
+                message="subclass test",
+                headers=headers,  # type: ignore[arg-type]
+                http_status=429,
+            )
+            result = repr(exc)
+            assert ExcClass.__name__ in result