diff --git a/agent/auxiliary_client.py b/agent/auxiliary_client.py
index d66d5d81acc15..a4b531209fe77 100644
--- a/agent/auxiliary_client.py
+++ b/agent/auxiliary_client.py
@@ -1316,6 +1316,7 @@ def create(self, **kwargs) -> Any:
         # out of cache-key routing entirely — for those hosts, skip it here.
         try:
             from agent.transports.codex import (
+                _cache_scope_from_session_id,
                 _content_cache_key,
                 _default_prompt_cache_retention_for_request,
             )
@@ -1328,7 +1329,15 @@ def create(self, **kwargs) -> Any:
                 or base_url_host_matches(_host_src, "models.github.ai")
             )
             if not _is_xai and not _is_github and "prompt_cache_key" not in resp_kwargs:
-                _cache_key = _content_cache_key(instructions, resp_kwargs.get("tools"))
+                # Scope by the owning turn's session so two unrelated sessions
+                # with the same instructions/tools (e.g. compression, MoA,
+                # flush_memories firing back-to-back on different sessions)
+                # don't bucket-share a prompt cache slot (#78941). The main
+                # transport (agent/transports/codex.py::build_kwargs) does the
+                # same; this adapter had no session handle before
+                # set_runtime_main() started threading one through.
+                _scope = _cache_scope_from_session_id(_runtime_main_value("session_id"))
+                _cache_key = _content_cache_key(instructions, resp_kwargs.get("tools"), _scope)
                 if _cache_key:
                     resp_kwargs["prompt_cache_key"] = _cache_key
             if "prompt_cache_retention" not in resp_kwargs:
@@ -3049,6 +3058,7 @@ def set_runtime_main(
     api_key: Any = "",
     api_mode: str = "",
     auth_mode: str = "",
+    session_id: str = "",
 ) -> contextvars.Token:
     """Record the current context's live main runtime for auxiliary routing.
 
@@ -3070,6 +3080,7 @@ def set_runtime_main(
         ),
         "api_mode": (api_mode or "").strip(),
         "auth_mode": (auth_mode or "").strip().lower(),
+        "session_id": (session_id or "").strip(),
     }
     # Publish authoritative context before updating locked compatibility
     # mirrors; concurrent sessions never read those mirrors at runtime.
diff --git a/agent/turn_context.py b/agent/turn_context.py
index def497b158983..45c18cec10032 100644
--- a/agent/turn_context.py
+++ b/agent/turn_context.py
@@ -393,6 +393,7 @@ def build_turn_context(
             api_key=getattr(agent, "api_key", "") or "",
             api_mode=getattr(agent, "api_mode", "") or "",
             auth_mode=getattr(agent, "auth_mode", "") or "",
+            session_id=getattr(agent, "session_id", "") or "",
         )
     except Exception:
         pass
diff --git a/tests/agent/test_auxiliary_client.py b/tests/agent/test_auxiliary_client.py
index efbd2e2b88dcd..65ed2f54c7d50 100644
--- a/tests/agent/test_auxiliary_client.py
+++ b/tests/agent/test_auxiliary_client.py
@@ -3187,6 +3187,69 @@ def create(self, **kwargs):
         assert time.monotonic() - started < 0.14
 
 
+class TestCodexAuxiliaryAdapterCacheScope:
+    """Regression for issue #78941: auxiliary Codex calls (compression,
+    flush_memories, MoA, session_search) must not bucket-share a prompt
+    cache slot across unrelated sessions just because their instructions
+    and tools happen to match.
+    """
+
+    def _create_and_capture(self, *, session_id):
+        import agent.auxiliary_client as aux
+
+        class _FakeCreateStream:
+            def __iter__(self):
+                return iter([
+                    SimpleNamespace(
+                        type="response.output_item.done",
+                        item=SimpleNamespace(
+                            type="message",
+                            content=[SimpleNamespace(type="output_text", text="ok")],
+                        ),
+                    ),
+                    SimpleNamespace(type="response.completed", response=SimpleNamespace(
+                        status="completed", id="r1", usage=None,
+                    )),
+                ])
+
+            def close(self):
+                pass
+
+        class FakeResponses:
+            def __init__(self):
+                self.kwargs = None
+
+            def create(self, **kwargs):
+                self.kwargs = kwargs
+                return _FakeCreateStream()
+
+        fake_client = SimpleNamespace(responses=FakeResponses(), base_url="")
+        adapter = aux._CodexCompletionsAdapter(fake_client, "gpt-5.5")
+        token = aux.set_runtime_main("openai", "gpt-5.5", session_id=session_id)
+        try:
+            adapter.create(
+                messages=[
+                    {"role": "system", "content": "You are a memory summarizer."},
+                    {"role": "user", "content": "Summarize the last turn."},
+                ],
+            )
+        finally:
+            aux.reset_runtime_main(token)
+        return fake_client.responses.kwargs["prompt_cache_key"]
+
+    def test_different_sessions_get_different_cache_keys(self):
+        key_a = self._create_and_capture(session_id="session-A")
+        key_b = self._create_and_capture(session_id="session-B")
+        assert key_a != key_b
+
+    def test_cron_refires_of_the_same_job_share_a_cache_key(self):
+        first = self._create_and_capture(session_id="cron_job42_20260801_090000")
+        second = self._create_and_capture(session_id="cron_job42_20260802_090000")
+        other_job = self._create_and_capture(session_id="cron_job99_20260801_090000")
+        assert first == second
+        assert first != other_job
+
+
 class TestCodexAuxiliaryToolMessageConversion:
     """Regression for issue #5709.