Check Cache Bugs (CC#40524)

Audit your Claude Code setup for three cache bugs discovered in March 2026 that can silently inflate API costs by 10-20x.

Time: ~20 seconds | Scope: version, config files, CLAUDE.md, skills, hooks, shell profiles, all claude binaries

Reference: anthropics/claude-code#40524 | Discovered by: u/skibidi-toaleta-2137 (r/ClaudeAI) + @whiletrue0x (GitHub)

Background

Three independent bugs cause Claude Code's prompt cache to break silently:

Bug 1 (standalone binary, v2.1.36+, edge case): A native string replacement in Anthropic's custom Bun/Zig fork is baked into the Zig HTTP header builder at the native layer — invisible from JavaScript, fires after JSON.stringify but before TLS. On every /v1/messages request, it searches the JSON body for the first occurrence of cch=00000 and replaces it with a 5-char body hash. In normal usage only system[0] is affected (which has cache_control: null), so no cache impact. It only breaks cache if cch=00000 leaks into messages[] — from a CLAUDE.md discussing the mechanism, a tool reading the CC bundle source, or the user typing it literally. 62 real-project transcripts verified: zero accidental leaks in normal usage.
Bug 2 (v2.1.69+): The deferred_tools_delta attachment introduced in v2.1.69 causes messages[0] to differ between fresh and resumed sessions, independently breaking cache prefix matching on every --resume or --continue. Confirmed still present in v2.1.88.
Bug 3 (v2.1.69+, widest impact): cli.js injects x-anthropic-billing-header as the first system prompt block, containing a per-conversation hash (3-char SHA-256 of first user message + version) that changes on every new session and every subagent call. This causes a full cache_creation on the system prompt (~12K tokens) instead of cache_read on every session start and Agent invocation. A/B tested: cache hit ratio goes from 48% to 99.98% with the env var fix.

Status: partial fix shipped in v2.1.88 (tool schema bytes). Bugs 2 and 3 still active as of v2.1.88.

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

bash

# 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 >= 2.1.36 AND standalone binary → flag BUG 1 MECHANISM PRESENT (edge case, only triggers if sentinel in messages)
Version >= 2.1.69 → flag BUG 2 RISK and BUG 3 RISK
Binary is Mach-O / ELF executable → standalone → Bug 1 mechanism present
Binary is a symlink to node_modules or contains cli.js → npm/npx → Bug 1 does not apply

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.

bash

# 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.

Phase 3 — Resume/continue usage (Bug 2)

bash

# 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

Any hit in hooks/settings → flag BUG 2 AUTOMATED (constant exposure)
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.

bash

# 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

bash

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 >= 2.1.69 as at risk for all three bugs. Note stale npm 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+, edge case)
**Status**: [SAFE / AT RISK / NOT APPLICABLE]

**Mechanism active (standalone binary >= v2.1.36)**: [YES / NO]
**Trigger sentinel found in static config**: [YES — locations | NO]

[If AT RISK] `cch=00000` found in static config files:
- [file path]: [matching line]
→ Fix: remove the `cch=00000` string from those files.
→ Temporary workaround: `npx @anthropic-ai/claude-code` (npm package uses standard Bun, no replacement).
→ Note: sentinel in `.jsonl` conversation history is normal and harmless — only static config matters.

[If SAFE] Mechanism present in binary but no trigger in static config. Normal usage unaffected.
[If NOT APPLICABLE] npm/npx install — Bug 1 mechanism absent at the binary level.

---

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

**Version in affected range (>= 2.1.69)**: [YES / NO]
**Automated usage (hooks/settings)**: [YES — locations | NO]
**Manual usage (commands/skills/scripts)**: [YES — locations | NO]

**Root cause**: `deferred_tools_delta` attachment introduced in v2.1.69 causes `messages[0]` to differ between fresh and resumed sessions, breaking cache prefix matching independently of Bug 1.

[If AT RISK]
→ Avoid `--resume` and `--continue` until fix ships.
→ Downgrade workaround: `npm install -g @anthropic-ai/claude-code@2.1.68` (last version before regression)

[If NOT APPLICABLE] Version < 2.1.69 — not affected.

---

### Bug 3 — Attribution header per-session hash (widest impact)
**Status**: [ACTIVE / MITIGATED / NOT APPLICABLE]

**Version in affected range (>= 2.1.69)**: [YES / NO]
**CLAUDE_CODE_ATTRIBUTION_HEADER=false already set**: [YES / NO]

[If ACTIVE] Every session start and every subagent call misses the system prompt cache (~12K tokens rebuilt at cache_creation rate).
→ Fix: add to ~/.claude/settings.json:
  {
    "env": {
      "CLAUDE_CODE_ATTRIBUTION_HEADER": "false",
      "ENABLE_TOOL_SEARCH": "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.

---

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

---

### Summary

| Bug | Impact | Status | Action |
|-----|--------|--------|--------|
| Bug 1 — sentinel in config | Edge case | [SAFE / AT RISK / N/A] | [action or "none"] |
| Bug 2 — --resume/--continue | Per-resume cache miss | [SAFE / AT RISK / N/A] | [action or "none"] |
| Bug 3 — attribution header | Every session + subagent | [ACTIVE / MITIGATED / N/A] | [action or "none"] |
| 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/N/A]
✅ No active exposure. Re-run after updating Claude Code.

⚠️ Track fixes: github.com/anthropics/claude-code/issues/40524

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