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) | Bug | Versions affected | Status | |-----|------------------|--------| | Bug 1: cch sentinel (standalone binary) | v2.1.36-v2.1.90 | **FIXED in v2.1.91** | | Bug 2: deferred_tools_delta on --resume | v2.1.69-v2.1.89 | **FIXED in v2.1.90** | | Bug 3: attribution header per-session hash | v2.1.69+ | **Active** (env var workaround) | | B4: Microcompact / silent context stripping | all versions through v2.1.92 | **Active** (GrowthBook controlled) | | B5: Tool result budget cap 200K | all versions through v2.1.92 | **Active** (MCP tools exempted) | ### 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 ```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 < 2.1.90 -> 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. ```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 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) ```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 ``` - 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. ```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 < 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 |