[Bug] Opus 4.8 reasoning regression: failed document search and tool-use persistence

Bug Description
Title: Opus 4.8 in Claude Code — sudden reasoning regression on a stable document workflow (17 Aug 2026)

Environment:

Claude Code CLI v2.1.233 (current)
Model: Opus 4.8 (explicitly pinned, after reverting from Opus 5)
macOS, MacBook Air M4
Effort level: [fill in — e.g. high / default]

Summary:
I have been editing the same .docx in Claude Code daily for a week — an advisory grant application — with consistent, good results. Today (17 Aug) the same workflow degraded sharply: shallow output, poor persistence, and a tool-use failure that never occurred previously.

Specific failure:
Asked to rewrite the answer to question 26 in a document Code already had open. Instead of reading the questions in document order and counting to the 26th, it ran a literal string-search for "26", found none (Word list numbering is style-generated, not stored as literal text), and concluded the question did not exist. It then ran repeated failing searches and asked me to paste the wording of a question that was in a file it already had open. It only located the question after I supplied a screenshot. This is a document Code has edited successfully all week.

Why this looks server-side, not user-side:

No change to the file, the prompts, the CLI version, or the model pin between the good week and today's failure.
An incident affecting Claude Code was logged on 16 Aug (~21:58 UTC, marked resolved ~22:40 UTC); this degradation is the following day.
The pattern — same model ID, same config, stable for weeks, then a sudden sustained quality drop — matches earlier reported Code harness regressions.

Impact: This is advisory/professional work, not coding. Hallucinated or shortcut output here carries direct client and reputational risk.

Expected: Reasoning and tool-use quality consistent with the prior week on the identical workflow.

Environment Info

• Platform: darwin
• Terminal: ghostty
• Version: 2.1.233
• Feedback ID: c3cdfc1d-1bb0-4ba8-aa0f-317d456aee2c

Errors

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