[Bug] Sonnet 5 omits text content and shows only thinking summary before AskUserQuestion

Title: Sonnet 5 skips writing the requested text and goes straight to AskUserQuestion, leaving only a thinking summary ("· summarized") in its place; frequency jumped on 2.1.278

Environment

• Claude Code 2.1.278, macOS (Darwin 25.5.0), Ghostty terminal
• Model: claude-sonnet-5, effort high
• Default output style, no viewMode set

What happens
In a workflow where the assistant is instructed to paste a document section into its reply in full and then call AskUserQuestion ("accept, reject, or instructions?"), it sometimes
makes the tool call without writing the section. The terminal then shows a single one-sentence line ending in a dim "· summarized" where the section should be. That line is a thinking
block. Its text reads like a claim that the work was done ("I've laid out the full list of files and paths this proposal will touch…"), but no text block containing the section exists
in the message or the turn.

If the user asks the session afterward whether it pasted the section, it says it did. I suspect the session is reading its own thinking, which describes the paste as complete, but I
haven't verified that.

Expected: the section appears as a text block before the question, as it does most of the time.

Evidence from local session transcripts
I read the JSONL transcripts for one project, grouping records by API message id. Restricting to the section-approval prompts (an AskUserQuestion whose text includes "accept, reject,
or instructions"), where a paste is always required:

| Version | Model | Prompts | No text block in the same message | Rate |
|---|---|---|---|---|
| 2.1.278 | claude-sonnet-5 | 97 | 45 (38 with a thinking block) | 46% |
| 2.1.246–2.1.277 | claude-sonnet-5 | 405 | 20 | 4.9% |
| 2.1.237, 2.1.241 | claude-opus-5 | 19 | 0 | 0% |

• By date, Sonnet 5 stayed at 0–7% per day through 2026-09-18 (3 of 30), then hit 30 of 69 on 09-19 and 15 of 33 on 09-20.
• One specific instance: record 529 of one transcript is a 259-character thinking block with a signature. The previous text block in that turn is a different section. The section that

should have been shown appears in no text block, and the next record is the AskUserQuestion call.

Caveats

• The 2.1.278 jump and the 09-19/20 dates are the same data, so I can't tell a client change from a server-side one.
• The Opus 5 rows come from older versions only, so they aren't a like-for-like comparison.
• My grouping counts a paste in an earlier API message of the same turn as "no text". Some of the 20 baseline cases may be that.
• I checked only one instance by hand.

Requests

1. Confirm whether this is a known regression in Sonnet 5 or in 2.1.278.
2. Document what "· summarized" means and when it appears. I couldn't find it in the docs or the changelog through 2.1.278.
3. Consider making a thinking-only block visually distinct from an assistant text reply. As it is, a user can approve a section they never saw.

I can supply transcript excerpts on request.