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        freeze the prefix · melt the bill

A Claude Code plugin that keeps DeepSeek's prompt cache hitting when Claude Code would otherwise bust it.

cache-stable passthrough proxy · deterministic tool order · env freeze+delta · cold-anchor coalescing · live hit-rate statusline · zero deps

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Pointing Claude Code at DeepSeek's Anthropic-compatible endpoint is cheap, and a vanilla session already hits DeepSeek's cache ~90% on its own — CC's system prompt is shared across users, so it stays warm. The cache breaks when your tool list isn't stable: MCP servers connect and reshuffle it, and because tools render first in the cached prefix, any reorder busts everything from byte 0. Permafrost sits between CC and DeepSeek and rewrites the cache-relevant bytes so the tools + system anchor stays byte-identical turn after turn — then streams the reply straight back.

  Claude Code ──Anthropic /v1/messages──▶ Permafrost ──▶ DeepSeek /anthropic
   (unchanged)        127.0.0.1:8787     freeze prefix       (both speak Anthropic,
                                          + record hits        no translation)

Do you actually need it?

We ran bare DeepSeek vs Permafrost head-to-head on the live API and read DeepSeek's real cache tokens. The honest result is two-sided:

Your Claude Code session	bare DeepSeek	+ Permafrost
Vanilla — stable tools, no MCP (10-turn task)	89.6% hit	89.6% — no difference
Tool list churns — MCP servers reshuffle it	33% hit	71% hit

If you run plain single-agent CC, you don't need this — DeepSeek already does the job. Permafrost earns its keep on the second row: tool churn busts the prefix at byte 0 and bare collapses to 33%; the deterministic sort holds it at 71% (reproduce: e2e/tool_order_ab.py). So it's for users with MCP servers, heavily customized setups whose anchor isn't the shared-warm one, or parallel subagent fan-outs on cold anchors.

On cost: DeepSeek is ~11× cheaper than Claude on pricing alone (cache-hit input $0.0028 vs $0.30 /1M) — that's DeepSeek, not us, the moment you switch endpoints. Permafrost's additional contribution is keeping the cache hitting when it would otherwise bust (the 33% → 71% roughly halves the bill on a tool-churning workload; on a vanilla session it adds ~0%). Price-for-identical- traffic, not a model-quality claim — deepseek-v4-flash is not Sonnet 4.6.

Quick start

git clone https://github.com/jianzhichun/permafrost && cd permafrost
export ANTHROPIC_API_KEY=sk-your-deepseek-key
./cli/permafrost wrap     # starts the proxy, sets env for the child only, execs claude

wrap sets ANTHROPIC_BASE_URL + ENABLE_TOOL_SEARCH=true for the child claude process only — it never touches your shell or ~/.claude/settings.json.

As a plugin (gives you /permafrost:* commands, statusline, auto-start hook):

/plugin marketplace add jianzhichun/permafrost
/plugin install permafrost@permafrost

Persistent: copy the env block from settings.example.json into ~/.claude/settings.json, run permafrost up, start claude normally. (CC reads its env once at launch — set these before it starts.)

What it does

On every /v1/messages request, the pipeline (proxy/permafrost_align.py) keeps the tools+system anchor byte-stable, then reads DeepSeek's real prompt_cache_hit_tokens:

1. sorts tools deterministically — late-binding MCP servers can't reshuffle position 0 of the prefix. (This is the one that matters most — see above.)
2. freezes the env block + emits deltas — pins the first-seen env/context block (cwd, date, git status) into the anchor; later turns send only changed lines on the tail. PERMAFROST_FREEZE_ENV=0 relocates the whole block instead.
3. strips Claude Code's TodoWrite reminder — removes the non-semantic messages[] system nudge before forwarding. PERMAFROST_STRIP_TODOWRITE_REMINDERS=0 disables this.
4. strips cache_control and serializes canonically — DeepSeek ignores the markers; canonical bytes remove serialization drift.

Plus three runtime features (depth in docs/HOW-IT-WORKS.md):

• Cold-anchor coalescing — parallel Task subagents sharing a cold prefix would all pay the miss price (DeepSeek's write is async); one leader warms the cache, followers wait for its first byte. Live: 0% → 73%. PERMAFROST_COALESCE=0 off.
• Idle keepalive (opt-in) — replays each conversation's last request, unchanged in body and headers, after PERMAFROST_KEEPALIVE_S idle, so a long think-gap doesn't lose the prefix to eviction. Live: 99.9% on replay.
• Diagnostics — /permafrost:status :doctor :benchmark, a savings statusline (❄ 88% cache hit · $0.12 saved), per-session + per-lineage stats, and byte-level anchor diffs (how we caught CC's per-request cch nonce).

The full catalogue of CC cache-busters it defends against: docs/cache-busters.md.

Validation

e2e/run_full_suite.sh drives headless claude -p through Permafrost against real DeepSeek across six phases (alignment, coalescing, keepalive+resume, warm, sessions, anchor-diff) — latest run 15/15 passed. That proves the mechanisms work end-to-end; the value vs bare is the head-to-head table above. What these runs taught us about DeepSeek's cache (it re-renders before caching; cache identity includes the client header fingerprint; it keys on tool order but tolerates the cch nonce) is in docs/e2e-findings.md.

Offline benchmark (no API key)

Emulator faithful to the live findings — caches the rendered request (tools → system → messages), unit-quantized, with params + header identity. 12-turn CC-shaped conversation:

Scenario	off	aggressive
Tool churn + live git/env	0% hit	88% hit

off caches nothing (reshuffled tools bust byte 0); aggressive (sort) holds 88%. python3 benchmarks/bench.py --turns 12 to reproduce; --real adds a live probe. Details + the raw-byte-vs-rendered cross-check: benchmarks/RESULTS.md.

Reference

Modes (PERMAFROST_MODE): aggressive (default: sort + freeze + strip + canonical) · safe (no content moved) · off (passthrough, still meters).

Env hardening — set before launching claude:

Variable	Why
ANTHROPIC_BASE_URL=http://127.0.0.1:8787	route CC through Permafrost
ENABLE_TOOL_SEARCH=true	critical — without it a custom base URL makes CC stop deferring tools and re-inline the whole set every turn
ANTHROPIC_MODEL / ANTHROPIC_SMALL_FAST_MODEL	pin to one DeepSeek SKU (claude-opus*→v4-pro, *sonnet/haiku*→v4-flash)

Config (env, all optional):

Env	Default	Meaning
PERMAFROST_PORT	8787	listen port
PERMAFROST_UPSTREAM	…/anthropic	upstream (any Anthropic-compatible endpoint)
PERMAFROST_FREEZE_ENV	1	freeze env + delta (0 = relocate whole block)
PERMAFROST_STRIP_TODOWRITE_REMINDERS	1	remove Claude Code's non-semantic TodoWrite reminder from messages[]
PERMAFROST_COALESCE / _SETTLE_MS	1 / 2500	coalescing + async-write settle
PERMAFROST_KEEPALIVE_S	0 (off)	idle-replay interval
PERMAFROST_PRICES	V4 Flash	"hit,miss,output" USD/1M for the cost readout

The proxy is zero-dependency stdlib Python, pools upstream connections (no per-request TLS), and refuses non-loopback clients on its /permafrost/* endpoints.

Tests

# offline (no key, in CI): alignment, proxy units, CC-fixture, emulator + 3 smokes
python3 tests/test_alignment.py && python3 tests/test_proxy_units.py
python3 tests/test_cc_fixture.py && python3 tests/test_emulator.py
bash tests/proxy_smoke.sh && bash tests/coalesce_smoke.sh && bash tests/keepalive_smoke.sh

# live (funded DEEPSEEK_API_KEY, a few cents):
bash e2e/run_claude_code.sh    # one real CC task
bash e2e/run_full_suite.sh     # every feature, asserted
python3 e2e/tool_order_ab.py   # the decisive bare-vs-Permafrost test

Credits

Generalizes techniques from DeepSeek-Reasonix (prefix-cache discipline, prompt_cache_hit_tokens accounting) and Headroom (proxy interception, canonical forwarding, the ENABLE_TOOL_SEARCH finding). Where Headroom's CacheAligner detects volatile content, Permafrost rewrites it. Compared to CCR / LiteLLM / Headroom / Reasonix: docs/landscape.md.

MIT — see LICENSE.