fix(agent): hoist checkpoint carrier guard above the reasoning branches

The cherry-picked guard sat inside the codex-items block, which (a) is
skipped entirely in codex_responses mode (conversation_loop passes
drop_codex_reasoning_items=False there) and (b) is unreachable for
carriers whose adapter-joined commentary populates msg['reasoning'] —
the string-reasoning branch returns True first. Hoist the checkpoint
check above every reasoning branch so no carrier shape can be dropped,
in any api_mode. Adds the two carrier-shape tests that pin exactly this
(both fail with the guard in its original position).

run_agent.py

@@ -4655,6 +4655,18 @@ class AIAgent:
                 return False
         elif content is not None and content != "":
             return False
+        # A native compaction checkpoint makes a carrier never thinking-only,
+        # regardless of api_mode or which reasoning field is populated. The
+        # checkpoint is the server-side stand-in for already-pruned history
+        # and exists in exactly one place; the codex_responses adapter also
+        # surfaces commentary text via msg["reasoning"], so the string branch
+        # below would otherwise drop a carrier before the sidecar is ever
+        # inspected. Checked here — above every reasoning branch — so no
+        # carrier shape can fall into a drop path (#82108 review finding).
+        from agent.native_compaction import has_compaction_checkpoint
+
+        if has_compaction_checkpoint(msg.get("codex_reasoning_items")):
+            return False
         reasoning = msg.get("reasoning_content") or msg.get("reasoning")
         if isinstance(reasoning, str) and reasoning.strip():
             return True
@@ -4668,16 +4680,6 @@ class AIAgent:
         # empty-turn handling instead of being dropped here.
         codex_items = msg.get("codex_reasoning_items")
         if drop_codex_reasoning_items and isinstance(codex_items, list):
-            # A native compaction checkpoint rides this same sidecar and is
-            # the server-side stand-in for already-pruned history. Dropping
-            # the turn takes the checkpoint with it — the request then carries
-            # neither the compacted history nor the checkpoint that replaces
-            # it. Compaction pruning filters items for this reason rather than
-            # popping the key; a carrier is never thinking-only.
-            from agent.native_compaction import has_compaction_checkpoint
-
-            if has_compaction_checkpoint(codex_items):
-                return False
             return any(
                 isinstance(item, dict) and item.get("type") == "reasoning"
                 for item in codex_items

tests/run_agent/test_thinking_only_sanitizer.py

@@ -225,3 +225,20 @@ class TestCompactionCheckpointCarrier:
             if item.get("type") == "compaction"
         ]
         assert surviving == [self.CHECKPOINT]
+
+    def test_reasoning_text_carrier_is_not_thinking_only(self):
+        """codex_responses adapter surfaces commentary via msg['reasoning'];
+        the string branch must not drop a checkpoint carrier (#82108 review:
+        the original guard sat below this branch and never fired)."""
+        msg = self._carrier([self.CHECKPOINT])
+        msg["reasoning"] = "some commentary the adapter joined in"
+        assert not AIAgent._is_thinking_only_assistant(msg)
+
+    def test_reasoning_text_carrier_survives_even_when_codex_items_kept(self):
+        """codex_responses mode passes drop_codex_reasoning_items=False; the
+        guard must still protect the carrier through the reasoning branch."""
+        msg = self._carrier([self.REASONING, self.CHECKPOINT])
+        msg["reasoning"] = "commentary"
+        assert not AIAgent._is_thinking_only_assistant(
+            msg, drop_codex_reasoning_items=False
+        )