Check Cache Bugs

Check Cache Bugs (CC#40524)

Audit your Claude Code setup for cache and cost bugs discovered in March-April 2026.

Time: ~30 seconds | Scope: version, config files, CLAUDE.md, skills, hooks, shell profiles, all claude binaries

Note on cache contamination: This skill loads content containing cch= strings into the current session's message array. For cleanest results, run this command at the very start of a fresh session, or via claude -p "$(cat .claude/commands/check-cache-bugs.md)" as a one-shot print-mode invocation.

Reference: anthropics/claude-code#40524 | Discovered by: @jmarianski + @whiletrue0x | Extended by: @ArkNill (ArkNill/claude-code-cache-analysis, April 2026)

Background

Fix Status (current as of v2.1.92)

Original three bugs (CC#40524)

• Bug 1 (FIXED v2.1.91): Bun's native HTTP layer did a same-length byte replacement of the cch=00000 attestation placeholder after JSON.stringify but before TLS. Triggered only if cch=00000 appeared literally in messages[] content. Confirmed closed; npm and standalone binary are now equivalent on v2.1.91+.

• Bug 2 (FIXED v2.1.90): The session JSONL writer stripped deferred_tools_delta attachment records before writing to disk. On --resume, those records were absent; the deferred tools layer had no prior history and re-announced all tools from scratch, shifting every message position and breaking the messages-level cache prefix entirely. Each resume rebuilt 87-118K tokens as cache_creation. Anthropic tracked internally as inc-4747.

• Bug 3 (ACTIVE, v2.1.69+): Claude Code injects a billing header as the first block of the system prompt. The header contains a 3-character SHA-256 hash derived from characters [4, 7, 20] of the first user message + CC version, unique per session, per subagent, per side-query. Since the cache is prefix-based, this causes a cold miss on ~12K system prompt tokens on every invocation. A session with 5 subagents = 6 cold misses. Empirical: 48% -> 99.98% cache hit with env var workaround.

Additional bugs (ArkNill, April 2026)

• B4: Microcompact (all versions): Three mechanisms silently replace tool results with [Old tool result content cleared] before sending to the API. Controlled by server-side GrowthBook flags, bypasses DISABLE_AUTO_COMPACT. The /export command shows original context, NOT what the model received. 327 clearing events measured in one session.

• B5: Tool result budget cap (all versions): applyToolResultBudget() truncates tool results when aggregate chars exceed 200K. Built-in tools (Read, Bash, Grep, Glob, Edit) are affected. MCP tools can override via _meta["anthropic/maxResultSizeChars"] (v2.1.91+).

Cost impact summary: 2-5x per session start/subagent for Bug 3 in isolation; 10-20x per turn for Bug 2 with 10+ skills and 3-4 resumes (both estimates are correct in their respective context).

Instructions

You are an auditor. Run all phases in order, collect every result, and produce the final report. Do not skip phases or stop early.

Phase 1: Claude Code version and install method

# Version check
claude --version

# All installed claude binaries
which -a claude 2>/dev/null

# Check if active binary is standalone or npm
file $(which claude) 2>/dev/null
ls -la $(which claude) 2>/dev/null

• Version < 2.1.90 -> flag BUG 2 RISK (resume/DTD; fixed in v2.1.90)
• Version < 2.1.91 AND standalone binary -> flag BUG 1 MECHANISM (sentinel; fixed in v2.1.91)
• Version >= 2.1.69 -> flag BUG 3 RISK (attribution header; no fix yet, check Phase 4)
• Binary is Mach-O / ELF executable -> standalone (relevant for Bug 1 on v2.1.36-v2.1.90)
• Binary is a symlink to node_modules or contains cli.js -> npm/npx

Phase 2: Sentinel scan (Bug 1)

Search for the literal string cch= in all static config files. Exclude .jsonl files (ephemeral conversation history) and this command file itself.

# Global config files
grep -r "cch=" \
  ~/.claude/CLAUDE.md \
  ~/.claude/MEMORY.md \
  ~/.claude/TONE.md \
  ~/.claude/FLAGS.md \
  ~/.claude/RULES.md \
  ~/.claude/RTK.md \
  ~/.claude/ANTI_AI.md \
  2>/dev/null

# Global skills, commands, agents, hooks (excluding this command file)
grep -rl "cch=" ~/.claude/skills/ 2>/dev/null
grep -rl "cch=" ~/.claude/commands/ --exclude="check-cache-bugs.md" 2>/dev/null
grep -rl "cch=" ~/.claude/agents/ 2>/dev/null
grep -rl "cch=" ~/.claude/hooks/ 2>/dev/null

# Project-level config
grep -r "cch=" CLAUDE.md .claude/CLAUDE.md .claude/MEMORY.md 2>/dev/null
grep -rl "cch=" .claude/skills/ 2>/dev/null
grep -rl "cch=" .claude/commands/ --exclude="check-cache-bugs.md" 2>/dev/null
grep -rl "cch=" .claude/agents/ 2>/dev/null
grep -rl "cch=" .claude/hooks/ 2>/dev/null

# Broader scan: all CLAUDE.md files across projects
find ~ -name "CLAUDE.md" \
  -not -path "*/node_modules/*" \
  -not -path "*/.git/*" \
  2>/dev/null | xargs grep -l "cch=" 2>/dev/null

Flag as BUG 1 RISK if any match found outside .jsonl files AND version < v2.1.91. On v2.1.91+: sentinel mechanism is fixed, no risk regardless of config content.

Phase 3: Resume/continue usage (Bug 2)

# settings.json
grep -i -- "--resume\|--continue" ~/.claude/settings.json 2>/dev/null
grep -i -- "--resume\|--continue" .claude/settings.json 2>/dev/null

# Hooks
grep -rn -- "--resume\|--continue" ~/.claude/hooks/ 2>/dev/null
grep -rn -- "--resume\|--continue" .claude/hooks/ 2>/dev/null

# Commands and skills
grep -rn -- "--resume\|--continue" ~/.claude/commands/ 2>/dev/null
grep -rn -- "--resume\|--continue" .claude/commands/ 2>/dev/null

# Shell profiles (aliases, functions)
grep -n -- "--resume\|--continue" ~/.zshrc ~/.bashrc ~/.bash_profile ~/.zprofile 2>/dev/null

# Project scripts
find . -name "*.sh" -o -name "Makefile" 2>/dev/null | \
  xargs grep -l -- "--resume\|--continue" 2>/dev/null

• Version >= v2.1.90: Bug 2 is FIXED, flag hits as informational only (no active cost risk)
• Version < v2.1.90 AND any hit in hooks/settings -> flag BUG 2 AUTOMATED (constant exposure)
• Version < v2.1.90 AND any hit in commands/skills/scripts -> flag BUG 2 MANUAL (exposure when invoked)

Phase 4: Attribution header check (Bug 3)

Check whether the billing header env var is already disabled.

# Global settings
grep -i "CLAUDE_CODE_ATTRIBUTION_HEADER\|ENABLE_TOOL_SEARCH" \
  ~/.claude/settings.json 2>/dev/null

# Project settings
grep -i "CLAUDE_CODE_ATTRIBUTION_HEADER\|ENABLE_TOOL_SEARCH" \
  .claude/settings.json 2>/dev/null

# Shell profiles
grep -i "CLAUDE_CODE_ATTRIBUTION_HEADER" \
  ~/.zshrc ~/.bashrc ~/.bash_profile ~/.zprofile 2>/dev/null

• CLAUDE_CODE_ATTRIBUTION_HEADER not set to false AND version >= 2.1.69 -> flag BUG 3 ACTIVE
• Already set to false -> BUG 3 MITIGATED

Phase 5: Multiple binaries check

for b in $(which -a claude 2>/dev/null | sort -u); do
  echo "=== $b ==="
  $b --version 2>/dev/null || echo "unavailable"
  file $b 2>/dev/null
  ls -la $b 2>/dev/null
done

Flag any standalone binary < v2.1.91 as at risk for Bug 1. Flag any binary < v2.1.90 as at risk for Bug 2. Flag any binary >= v2.1.69 as at risk for Bug 3 (unless attribution header env var already set). Note stale binaries that could be mistakenly invoked.

Output Format

## Claude Code Cache Bug Audit: CC#40524

**Date**: [today]
**Active claude version**: [version]
**Install method**: [standalone binary | npm/npx | mixed]

---

### Bug 1: Sentinel replacement (standalone binary v2.1.36-v2.1.90)
**Status**: [FIXED (v2.1.91+) / SAFE / AT RISK / NOT APPLICABLE]

**Version >= v2.1.91**: [YES -> FIXED | NO -> check below]
**Mechanism active (standalone binary, v2.1.36-v2.1.90)**: [YES / NO]
**Trigger sentinel found in static config**: [YES (locations) | NO]

[If FIXED] Running v2.1.91+. Sentinel mechanism patched; npm and standalone are equivalent. No action needed.
[If AT RISK (v < 2.1.91 AND standalone AND sentinel found)]
-> Update to v2.1.91+: `npm install -g @anthropic-ai/claude-code@latest`
-> Temporary: remove `cch=00000` string from the flagged files, or use npm instead.
[If SAFE] Mechanism present in binary but no trigger in static config. Update to v2.1.91 still recommended.
[If NOT APPLICABLE] npm/npx install: Bug 1 mechanism absent.

---

### Bug 2: Cache prefix mismatch on --resume / --continue (v2.1.69-v2.1.89)
**Status**: [FIXED (v2.1.90+) / AT RISK (automated) / AT RISK (manual) / NOT APPLICABLE]

**Version >= v2.1.90**: [YES -> FIXED | NO -> check below]
**Automated usage found (hooks/settings)**: [YES (locations) | NO]
**Manual usage found (commands/skills/scripts)**: [YES (locations) | NO]

**Root cause**: JSONL writer stripped `deferred_tools_delta` records before writing to disk. On `--resume`,
the deferred tools layer had no announcement history and re-announced all tools from scratch, shifting every
message position and breaking messages-level cache prefix. Each resume rebuilt 87-118K tokens as cache_creation.
Anthropic tracked as inc-4747. Fixed in v2.1.90.

[If FIXED] Running v2.1.90+. Bug resolved. `--resume` usage found in config is now safe at the cache level.
[If AT RISK]
-> Update to v2.1.90+: `npm install -g @anthropic-ai/claude-code@latest`
-> Immediate: avoid `--resume` and `--continue` until updated.
[If NOT APPLICABLE] Version < 2.1.69: not affected.

---

### Bug 3: Attribution header per-session hash (v2.1.69+, no fix yet)
**Status**: [ACTIVE / MITIGATED / NOT APPLICABLE]

**Version in affected range (>= v2.1.69)**: [YES / NO]
**CLAUDE_CODE_ATTRIBUTION_HEADER set to false/0/no/off**: [YES -> MITIGATED | NO -> ACTIVE]

[If ACTIVE] Every session start and every subagent call misses the system prompt cache (~12K tokens
rebuilt at cache_creation rate). A session with 5 subagents = 6 cold misses.
-> Fix (immediate, no restart needed): add to ~/.claude/settings.json:
  {
    "env": {
      "CLAUDE_CODE_ATTRIBUTION_HEADER": "false"
    }
  }
-> Expected impact: cache hit ratio 48% -> ~99.98% (measured, source: @whiletrue0x CC#40524)

[If MITIGATED] Header already disabled. No action needed.
[If NOT APPLICABLE] Version < 2.1.69: not affected.

---

### B4/B5: Context mutation (informational, all versions)

These bugs are controlled by server-side GrowthBook flags and cannot be mitigated client-side.
No config check is possible; report as informational.

**B4: Microcompact**: Three mechanisms silently replace tool results with `[Old tool result content cleared]`
before sending to the API. The `/export` command shows original context, NOT what the model received.
Workaround: start fresh sessions periodically to reset the tool result pool.

**B5: Tool result budget cap (200K chars aggregate)**: Built-in tools (Read, Bash, Grep, Glob, Edit)
hit a 200K character cap on aggregate tool results per session. Results are truncated beyond this threshold.
MCP tools can partially override via `_meta["anthropic/maxResultSizeChars"]` (v2.1.91+, up to 500K).

---

### Multiple binaries
[List each binary found, version, type (standalone/npm), and per-bug status]

---

### Summary

| Bug | Versions | Status | Action |
|-----|----------|--------|--------|
| Bug 1: sentinel (standalone) | v2.1.36-v2.1.90 | [FIXED / SAFE / AT RISK / N/A] | [update to v2.1.91+ or "none"] |
| Bug 2: --resume/--continue | v2.1.69-v2.1.89 | [FIXED / AT RISK / N/A] | [update to v2.1.90+ or "none"] |
| Bug 3: attribution header | v2.1.69+ | [ACTIVE / MITIGATED / N/A] | [add env var or "none"] |
| B4: microcompact | all versions | INFORMATIONAL | start fresh sessions |
| B5: tool result budget 200K | all versions | INFORMATIONAL | use MCP tools for large reads |
| Stale binaries | -- | [CLEAN / PRESENT] | [remove or none] |

[If Bug 3 ACTIVE (always show this)]
Quick win: add CLAUDE_CODE_ATTRIBUTION_HEADER=false to settings.json, immediate effect, no restart needed.

[If all SAFE/MITIGATED/FIXED/N/A on Bugs 1-3]
No active cache exposure on the original CC#40524 bugs. B4/B5 are informational, monitor session length.

Track: github.com/anthropics/claude-code/issues/40524 | github.com/ArkNill/claude-code-cache-analysis