Follow-up per independent review of #68633 (GPT-5.6-sol-xhigh in Codex,
reviewer egilewski) on this PR.
Two real bugs in the blank-text-block filtering added by that fix:
1. `effective = blocks or content` fell back to the RAW, unfiltered
`content` variable whenever every block was filtered out as blank --
which happens precisely when the entire message content WAS the
blank/whitespace payload the filter exists to remove (a sole blank
text block, a sole cache-marked blank block, or standalone
whitespace scalar content with no tool_calls). The fallback silently
restored the exact invalid content the filtering just stripped,
leaving the message provider-invalid.
Fixed: `effective = blocks if blocks else [{"type": "text", "text":
"(empty)"}]` -- never falls back to raw `content`. Also moved the
cache_control application (both the relocated-from-a-dropped-block
marker and the message-level marker) to run against `effective`
instead of the pre-fallback `blocks`, so a cache marker on a block
that was the ONLY content still lands on the (empty) placeholder
rather than being silently lost when `blocks` was empty at the
point it would otherwise have been applied.
2. The normal-path blank-text check used `(blk.get("text") or "").strip()`,
which is not type-safe for a truthy NON-string, non-None text value
(e.g. an int or dict from an invalid upstream payload) -- `or`
doesn't substitute for a truthy value, so `(7 or "").strip()` still
raises AttributeError. Now checks `isinstance(text, str)` first,
matching the replay path's `_sanitize_replay_block()`, which the
reviewer confirmed was already correctly type-safe.
Added regression tests for: sole blank list block, sole whitespace
scalar content, sole cache-marked blank block (marker relocation to
the placeholder), a truthy non-string (int) text value both mixed with
a surviving tool_use and as the sole content, and a dict-valued text
field. 7/7 new tests pass; 193/193 in the full
tests/agent/test_anthropic_adapter.py file; 23/23 in
tests/agent/test_prompt_caching.py (unaffected, confirmed).
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