diff --git a/tests/models/test_response_terminal.py b/tests/models/test_response_terminal.py
new file mode 100644
index 0000000000..0801de2a5e
--- /dev/null
+++ b/tests/models/test_response_terminal.py
@@ -0,0 +1,98 @@
+from __future__ import annotations
+
+from types import SimpleNamespace
+from typing import cast
+
+from openai.types.responses import Response
+
+from agents.exceptions import ModelBehaviorError
+from agents.models._response_terminal import (
+    format_response_error_event,
+    format_response_terminal_failure,
+    response_error_event_failure_error,
+    response_terminal_failure_error,
+)
+
+
+def test_format_response_terminal_failure_without_response() -> None:
+    message = format_response_terminal_failure("response.failed", None)
+
+    assert message == "Responses stream ended with terminal event `response.failed`."
+
+
+def test_format_response_terminal_failure_includes_all_details() -> None:
+    response = SimpleNamespace(
+        status="incomplete",
+        error="boom",
+        incomplete_details="max_output_tokens",
+    )
+
+    message = format_response_terminal_failure("response.incomplete", cast(Response, response))
+
+    assert message == (
+        "Responses stream ended with terminal event `response.incomplete`. "
+        "status=incomplete; error=boom; incomplete_details=max_output_tokens."
+    )
+
+
+def test_format_response_terminal_failure_omits_falsy_details() -> None:
+    response = SimpleNamespace(status=None, error="boom", incomplete_details=None)
+
+    message = format_response_terminal_failure("response.failed", cast(Response, response))
+
+    assert message == "Responses stream ended with terminal event `response.failed`. error=boom."
+
+
+def test_format_response_terminal_failure_with_no_details_keeps_base_message() -> None:
+    response = SimpleNamespace(status=None, error=None, incomplete_details=None)
+
+    message = format_response_terminal_failure("response.failed", cast(Response, response))
+
+    assert message == "Responses stream ended with terminal event `response.failed`."
+
+
+def test_format_response_error_event_includes_all_details() -> None:
+    event = SimpleNamespace(code="rate_limit", message="slow down", param="model")
+
+    message = format_response_error_event("error", event)
+
+    assert message == (
+        "Responses stream ended with terminal event `error`. "
+        "code=rate_limit; message=slow down; param=model."
+    )
+
+
+def test_format_response_error_event_omits_falsy_details() -> None:
+    event = SimpleNamespace(code=None, message="slow down", param=None)
+
+    message = format_response_error_event("error", event)
+
+    assert message == "Responses stream ended with terminal event `error`. message=slow down."
+
+
+def test_format_response_error_event_with_no_details_keeps_base_message() -> None:
+    event = SimpleNamespace(code=None, message=None, param=None)
+
+    message = format_response_error_event("error", event)
+
+    assert message == "Responses stream ended with terminal event `error`."
+
+
+def test_response_terminal_failure_error_wraps_formatted_message() -> None:
+    response = SimpleNamespace(status="failed", error=None, incomplete_details=None)
+
+    error = response_terminal_failure_error("response.failed", cast(Response, response))
+
+    assert isinstance(error, ModelBehaviorError)
+    assert str(error) == (
+        "Responses stream ended with terminal event `response.failed`. status=failed."
+    )
+
+
+def test_response_error_event_failure_error_wraps_formatted_message() -> None:
+    event = SimpleNamespace(code="rate_limit", message=None, param=None)
+
+    error = response_error_event_failure_error("error", event)
+
+    assert isinstance(error, ModelBehaviorError)
+    assert str(error) == ("Responses stream ended with terminal event `error`. code=rate_limit.")