lean-ctx (Python SDK)

Context compression for AI agents — a thin, dependency-free client for the local lean-ctx daemon.

pip install lean-ctx-sdk

Drop-in compress(messages, model)

Compress a chat-style messages array before sending it to any model. Only text payloads are rewritten through lean-ctx's deterministic funnel; images, tool-call blocks and ids pass through untouched, and the output is byte-stable so it stays friendly to provider prompt caching.

from lean_ctx import compress

messages = [
    {"role": "system", "content": "You are a helpful assistant."},
    {"role": "user", "content": large_log_or_file_dump},
]

messages = compress(messages, model="claude-sonnet-4")
# → send `messages` to your provider as usual

Works with both OpenAI-style (content: "string") and Anthropic-style (content: [{type: "text", …}, {type: "tool_result", …}]) messages.

Token-savings stats

from lean_ctx import ProxyClient

result = ProxyClient().compress(messages, model="gpt-4o")
print(result.saved_tokens, result.saved_pct)   # e.g. 1840 63.1
messages = result.messages

Configuration

The endpoint and session token are auto-discovered from the running daemon. Every step is overridable:

Or pass them explicitly (useful in CI / against a remote proxy):

compress(messages, base_url="http://127.0.0.1:4444", token="…")

If the daemon is not running, compress() raises LeanCtxConnectionError; an unauthenticated request raises LeanCtxAuthError. Both subclass LeanCtxError.

Framework integrations

LiteLLM

Compress requests transparently with a CustomLogger (pip install lean-ctx-sdk[litellm]):

import litellm
from lean_ctx import LeanCtxLiteLLMHandler

litellm.callbacks = [LeanCtxLiteLLMHandler(model="gpt-4o")]
# every completion now sends compressed messages

For non-proxy code, compress_request_data(data) rewrites the messages of any OpenAI-style request dict in place.

LangChain

from langchain_core.messages import HumanMessage, SystemMessage
from lean_ctx import compress_messages

messages = compress_messages(
    [
        SystemMessage(content="You are a helpful assistant."),
        HumanMessage(content=large_log_or_file_dump),
    ],
    model="gpt-4o",
)

In both adapters a compaction failure never breaks the call — the original messages are kept.

CLI helpers

LeanCtxClient wraps the lean-ctx binary for read / search / shell / gain / benchmark. The LeanCtxRetriever (LangChain) and LeanCtxNodeParser (LlamaIndex) retrieval adapters are available via the langchain / llamaindex extras.

Learn more

• compress() SDK cookbook — Python + TypeScript recipes
• lean-ctx vs Headroom — comparison + reproducible benchmark

License

MIT