contextpaw 0.2.1

Make LLM context overflow honest: stop Ollama truncating your prompt in silence, stop llama.cpp's 400 from killing your agent.

🐾 ContextPaw

Your LLM server lies to you when the context window overflows. ContextPaw makes it stop.

Two servers, two opposite lies, same broken agent:

	What it does on overflow	Why that's bad
Ollama	Silently drops the front of your prompt, returns 200 OK	Your agent loses its system prompt and tool definitions, then confidently makes something up. Nothing is logged. You will never know.
llama.cpp	Returns a hard 400 exceed_context_size_error	Honest — but it kills the agent turn. The run is dead and the work is lost.

ContextPaw sits in front of either one and turns overflow into something an agent can actually survive: fit the window, keep what matters, and say out loud what was lost.

Prior art, stated plainly. Ollama's silent truncation is not a new discovery — it has been reported and left open for years: #3839 ("Detect Truncation Due to Exceeding Context Size", open since April 2024), #14259 ("truncation happens silently with no user-visible indication"), #9208. What ContextPaw adds is (a) a measurement of what the silence actually costs you, and (b) a fix you can install today without waiting for upstream.

your app ──► contextpaw :11434 ──► ollama    :11435
                              └──► llama.cpp :8091

Drop-in: it speaks Ollama's API and the OpenAI API, so it takes port 11434 and nothing in your stack needs to change.

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The proof

A secret is placed at the start of a prompt that overflows the window. Then we ask for it back. Same model (gemma-4-abliterated:12b), same prompt, same num_ctx.

Ollama, direct:

HTTP 200
prompt_eval_count: 16387          <- it read 16k of a 160k-token prompt. It did not say so.
answer: "Le mot de passe secret est : **remplissage**."
                                  ^^^^^^^^^^^^ it invented this

Ollama, through ContextPaw:

HTTP 200
contextpaw: compacted=True  160689 -> 26024 tokens  (budget 32452)
            strategy: middle-out (head+tail preserved)
            evicted : 134727 tokens
answer: "Le mot de passe secret est **ANANAS-7734**."
                                     ^^^^^^^^^^^ correct

And on llama.cpp, where the same prompt is simply fatal:

direct        -> HTTP 400 exceed_context_size_error   (agent turn is dead)
via contextpaw-> HTTP 200, 53589 -> 3094 tokens, 50556 evicted, answer correct

Reproduce both: python3 demo_needle.py and python3 demo_400.py.

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Install & run

pip install contextpaw

# move the real Ollama off 11434 so ContextPaw can take it
OLLAMA_HOST=127.0.0.1:11435 ollama serve

contextpaw --port 11434 --ollama http://127.0.0.1:11435

Nothing else changes. Your clients keep pointing at :11434.

curl localhost:11434/contextpaw/health   # policy, stats, calibration

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Two rules it never breaks

1. Never rewrite the head. Both servers cache the prompt prefix. Compacting from the front invalidates that cache on every turn and your TTFT explodes — and it throws away the system prompt, the tool schemas, and the task goal, which is exactly what the agent cannot work without. ContextPaw evicts from the middle and keeps both ends. (Trimming the head is precisely what Ollama's built-in truncation does. It is why it produces confident nonsense.)

2. Never evict silently. Every eviction is reported back — in the response body (contextpaw field), in headers (X-ContextPaw-Compacted, X-ContextPaw-Evicted-Tokens), and inline to the model itself:

[contextpaw: 12431 tokens of earlier conversation and tool output elided to fit the context window. This information is GONE from your context — if you need it, fetch it again rather than guessing.]

An agent that knows it lost the output of tool call #7 can go read the file again. An agent that was never told will hallucinate over the hole. The silence is the bug.

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What gets evicted, in what order

For chat messages (/api/chat, /v1/chat/completions):

Priority		Treatment
PINNED	system messages	never touched — tool defs, instructions
RECENT	last --keep-recent (default 4)	never deleted
EVICTABLE	everything else	dropped biggest-first

Biggest-first is deliberate: in an agent loop the giant messages are almost always stale tool output — a file read, an HTTP body. Dropping one 12k-token file dump recovers more room than dropping twenty turns of dialogue, and loses far less of the reasoning thread.

And the case everyone gets wrong: when the newest message is itself a huge tool dump, it sits in the protected window and would deadlock compaction. Being protected from deletion must not mean being protected from shrinking — so ContextPaw shrinks it in place, middle-out, keeping its head and tail. (This exact bug was found by the test suite, not by luck. See test_huge_recent_tool_output_is_shrunk_not_deadlocked.)

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Summarize what you evict (--summarize)

A marker saying "12431 tokens were elided" tells the agent a hole exists. A digest tells it what was in the hole.

contextpaw --summarize --summarizer-model gemma3:1b

Three facts were buried in the middle of an over-long prompt — squarely inside the region compaction evicts. Then we asked for them back:

	facts recovered	the model said
evict only	0 / 3	"Production_Server … erreur de syntaxe … Marc"
--summarize	3 / 3	"Orion-7 … certificats TLS expirés … Marie-Claude"

Read the first row again. The model hallucinated even though the marker explicitly told it the information was gone. Telling an agent "you lost something" does not stop it inventing — you have to give the content back. That is the whole case for the summarizer, and we only know it because we measured it.

Implementation notes that matter:

• Map-reduce over the whole span, never head+tail. The first version trimmed the span to fit the summarizer's own window — and scored 0/3, because the facts were in the middle of the evicted span, so it dropped them again. It reproduced, inside the summarizer, the exact bug this project exists to fix. Now every chunk is summarized and merged. (test_summarizer_chunks_cover_the_whole_span)
• Cached by content hash. In an agent loop the same old turns are evicted every single turn; without a cache you would pay for the identical digest forever.
• Best-effort, never fatal. If the summarizer is down, times out, or returns nothing, we fall back to the plain marker. A rescue tool that fails the request it was rescuing is worse than no tool at all.

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One runtime owns the GPU (--arbitrate)

A 24 GB card cannot hold Ollama's model and a llama.cpp server. Measured on an RTX 4090: gemma-4-12b in Ollama at 4×32k = 15.1 GB; qwythos-9b in llama.cpp at 4×32k = 11.0 GB. 26.1 GB > 24 GB. They do not coexist.

The dangerous part: nothing tells you. Ollama will load a model on top of a running llama.cpp server and — with GGML_CUDA_ENABLE_UNIFIED_MEMORY=1 — spill to system RAM silently, 20× slower, while ollama ps still cheerfully reports 100% GPU. Same disease as silent truncation: the system would rather lie than say no.

ContextPaw sees every request, so it knows which runtime the caller wants. It evicts the other one first.

contextpaw --arbitrate \
  --llamacpp-cmd '/path/to/llama-server -m model.gguf --parallel 4 -c 32768 -ngl 99 --port 8091'

$ curl localhost:11434/contextpaw/runtime
{"active": "llamacpp", "llamacpp_alive": true, "ollama_models_loaded": [], "stats": {"switches": 2}}

Measured switch cost: llama.cpp → Ollama 5.1s, Ollama → llama.cpp 2.3s. Cheap enough to do on demand. --min-hold (default 20s) stops two callers flip-flopping the card and spending all their time reloading models instead of generating.

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Policies

contextpaw --policy compact   # (default) shrink it, report it
contextpaw --policy strict    # refuse loudly with 413 rather than compact
contextpaw --policy off       # pure passthrough

strict is the one to reach for while debugging: it gives you the clean, machine-readable error that Ollama should have given you in the first place.

Token counting

By default ContextPaw ships no tokenizer and needs none. It estimates, then calibrates itself against the prompt_eval_count every server already reports — and it only ever ratchets the ratio upward. Over-counting is safe (you compact a little early); under-counting is what gets your prompt silently truncated. It refuses to learn an unsafe ratio.

Want exact counts? Opt in:

pip install contextpaw[exact]
contextpaw --tokenizer google/gemma-3-1b-it

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Limitations (v0.1)

• On streaming responses the eviction report is delivered in headers only — the body is proxied through untouched.
• The summarizer adds latency on the turn where it runs (5 calls to a 1B model, ~10s for a 17k-token span) — then it is cached. Enable it for agent loops, not for chat.
• Raw-prompt compaction (/api/generate) is head+tail; only chat messages get true semantic, per-message eviction. Structure your calls as messages if you can.
• Character-proportional slicing, not token-offset slicing, when using the calibrated counter. The safety margin absorbs it; use --tokenizer if you want it tight.

Tests

python3 -m pytest tests/ -q     # 14 passed

MIT.