[BUG] 2.1.270: session transcript JSONL omits most assistant text written before a tool call (interactive CLI)

What happens

Since upgrading from 2.1.267 to 2.1.270, the session transcript (projects/<project>/<session>.jsonl) is missing most assistant text blocks that come before a tool_use in the same response. The text is shown in the terminal as normal. thinking and tool_use rows are still written, the latter with full input. End-of-turn text is affected much less.

The text is not elsewhere in the file: a phrase from a reply shown in the UI appears only inside later tool inputs that quote it.

Measurements

All assistant rows over six days on one machine, one model (claude-opus-5), all entrypoint: cli, split by the two CLAUDE_CONFIG_DIR profiles used here:

| version | profile | assistant rows | rows with a text block | share |
|---|---|---:|---:|---:|
| 2.1.267 | A | 6,712 | 1,771 | 26.4% |
| 2.1.267 | B | 12,427 | 3,427 | 27.6% |
| 2.1.270 | A | 1,970 | 253 | 12.8% |
| 2.1.270 | B | 2,536 | 74 | 2.9% |

• Text rows with stop_reason: tool_use, as a share of tool calls: 51.7% → 13.6% in profile A and 53.7% → 1.9% in profile B, 2.1.267 → 2.1.270.
• One session resumed across the upgrade went from 372 text blocks in 1,262 assistant rows (2.1.263) to 3 in 40 (2.1.270).
• Haiku sessions that never call tools record text on every response, on both versions.
• Both profiles share settings.json. Profile B signs in with forceLoginMethod: "claudeai" to an organisation account; I have not established whether that is why it is worse.

Why it matters

Hooks that read transcript_path to check what the model said before a tool call can no longer see it, and the PreToolUse payload carries no assistant text. A PreToolUse hook here that requires a written marker before certain commands became impossible to satisfy on 2.1.270.

Related

#65051 (daemon sessions, 2.1.161) and #76668 (desktop app) describe the same shape on other entrypoints; this is the interactive CLI.

Environment: Claude Code 2.1.270, macOS 26.5 (Darwin 25.5.0), zsh, iTerm2.