fix(prompt-caching): align _can_carry_marker with last-part-dict marking

Follow-up to the salvaged #57845 fix. _can_carry_marker used
any(isinstance(part, dict)) but _apply_cache_marker only marks the LAST
content part, so a list whose last element is a non-dict passed the carrier
gate yet received no marker — wasting one of the four breakpoints. Tighten
the predicate to require content[-1] to be a dict (mirroring the apply
logic) and add a regression test. Flagged by a 3-agent review.

agent/prompt_caching.py

@@ -65,7 +65,11 @@ def _can_carry_marker(msg: dict, native_anthropic: bool) -> bool:
     if content is None or content == "":
         return False
     if isinstance(content, list):
-        return any(isinstance(part, dict) for part in content)
+        # _apply_cache_marker only marks the LAST content part, so the carrier
+        # predicate must agree: a list whose last element isn't a dict cannot
+        # actually receive a marker and would waste a breakpoint. Mirror the
+        # `content` truthiness + last-element-dict check in _apply_cache_marker.
+        return bool(content) and isinstance(content[-1], dict)
     return isinstance(content, str)
 
 

tests/agent/test_prompt_caching.py

@@ -98,6 +98,25 @@ class TestCanCarryMarker:
         assert _can_carry_marker({"role": "tool", "content": "result"}, native_anthropic=False) is True
         assert _can_carry_marker({"role": "tool", "content": ""}, native_anthropic=False) is False
 
+    def test_openrouter_list_carrier_requires_last_part_dict(self):
+        """Carrier predicate must agree with _apply_cache_marker, which only marks
+        the LAST content part. A list whose last element isn't a dict cannot carry
+        a marker and must not consume a breakpoint."""
+        # Last part is a dict -> carrier.
+        assert _can_carry_marker(
+            {"role": "user", "content": [{"type": "text", "text": "a"}]},
+            native_anthropic=False,
+        ) is True
+        # Last part is a non-dict (stray raw string) -> NOT a carrier, even though
+        # an earlier part is a dict. Previously this passed the gate but got no
+        # marker, wasting a breakpoint.
+        assert _can_carry_marker(
+            {"role": "user", "content": [{"type": "text", "text": "a"}, "trailing raw"]},
+            native_anthropic=False,
+        ) is False
+        # Empty list -> not a carrier.
+        assert _can_carry_marker({"role": "user", "content": []}, native_anthropic=False) is False
+
 
 class TestApplyAnthropicCacheControl:
     def test_empty_messages(self):