investigate(cache): compression-rotation cache-scope gap -- needs maintainer design decision

tests/agent/transports/test_codex_cache_scope_compression_rotation_gap.py

@@ -0,0 +1,56 @@
+"""Demonstrates the compression-rotation cache-scope gap tracked by #79017.
+
+This is NOT a fix -- it's a reproduction for maintainers to look at while
+deciding whether the "logical cache-scope" redesign proposed in #79017 is
+worth doing. See the issue for the full design discussion.
+
+_cache_scope_from_session_id() (introduced by #78959 for issue #78941)
+scopes prompt_cache_key by the *physical* session_id. That's correct for
+unrelated sessions and for cron re-fires, but context-compression rotation
+mints a brand new physical session_id mid-conversation to segment the
+transcript -- so the same logical conversation goes cache-cold at every
+rotation boundary.
+"""
+
+import pytest
+
+from agent.transports import get_transport
+
+
+@pytest.fixture
+def transport():
+    import agent.transports.codex  # noqa: F401
+    return get_transport("codex_responses")
+
+
+@pytest.mark.xfail(
+    reason="#79017: cache scope has no concept of a logical conversation "
+    "identity distinct from the physical session_id, so compression "
+    "rotation always goes cache-cold. Needs a design decision, not a "
+    "one-line fix -- see the issue.",
+    strict=True,
+)
+def test_compression_rotation_preserves_cache_scope(transport):
+    root_session = "session-root-abc123"
+    # A compression rotation mints a new physical session_id for the same
+    # logical conversation (see agent/conversation_compression.py).
+    rotated_session = "session-rotated-def456"
+
+    root_kw = transport.build_kwargs(
+        model="gpt-5.4",
+        messages=[{"role": "system", "content": "You are a helpful assistant."}],
+        tools=[],
+        session_id=root_session,
+        is_codex_backend=True,
+    )
+    rotated_kw = transport.build_kwargs(
+        model="gpt-5.4",
+        messages=[{"role": "system", "content": "You are a helpful assistant."}],
+        tools=[],
+        session_id=rotated_session,
+        is_codex_backend=True,
+    )
+
+    # Desired behavior once #79017 lands: the rotated segment of the SAME
+    # conversation should keep the same cache scope as its root.
+    assert root_kw["prompt_cache_key"] == rotated_kw["prompt_cache_key"]