Read tool can overflow context window, making autocompact unrecoverable

Summary

A single Read tool call on a large file can spike token count past the entire context window, causing autocompact to fail with "Conversation too long" and the session to become unrecoverable.

Steps to reproduce

1. Start a Claude Code session (Opus 4.6, 200k context / 180k effective window)
2. Work normally until context is at ~50-90k tokens (well under the 167k autocompact threshold)
3. Have Claude read a large file (in my case, a 367KB / 7,889-line TypeScript file)
4. Session immediately shows "Context limit reached" despite having ~110k tokens free moments earlier

Debug log evidence

From ~/.claude/debug/<session-id>.txt:

17:53:56.721 autocompact: tokens=91760   threshold=167000 effectiveWindow=180000
17:54:37.914 executePreToolHooks called for tool: Read
17:54:37.929 autocompact: tokens=464024  threshold=167000 effectiveWindow=180000
17:54:37.935 Forked agent [compact] finished: 1 messages, types=[assistant], totalUsage: input=0 output=0
17:54:37.936 [ERROR] Error: Conversation too long. Press esc twice to go up a few messages and try again.

Token count jumped from 91k → 464k in one step (a +372k spike from a single Read result). The autocompact agent immediately failed because the conversation was already 2.5x the context window.

What the user sees

• /context shows only 27% usage (54k/200k) — but this is after the failed compact dropped the conversation back down
• The "Context limit reached" message persists even though there's 110k+ tokens of free space
• The session is effectively stuck — user must /clear

Root cause

Claude Code doesn't pre-validate whether a Read tool result will fit in the remaining context before appending it. A single large file (367KB ≈ 372k tokens) exceeded the entire 180k effective window, and autocompact cannot recover because it needs to send the full conversation to the API to generate a summary.

Suggested fixes

1. Pre-check file size before reading: Estimate token count from file size and auto-apply offset/limit if it would overflow remaining context
2. Truncate oversized tool results: Cap tool result size to remaining context space (with a warning to the user)
3. Allow autocompact to drop the offending result: If a single tool result caused the overflow, autocompact could remove/truncate it before attempting compression
4. Warn before reading large files: Show a confirmation prompt for files estimated to consume >50% of remaining context

Environment

• Claude Code version: latest (as of 2026-02-10)
• Model: claude-opus-4-6 (200k context window)
• Platform: macOS Darwin 25.3.0
• The file that caused it: a 367KB / 7,889-line single React component (app.tsx)