LeanCTX SDK

Embed LeanCTX context control into your application.

LeanCTX is the Context Gateway for AI Systems. Control what your AI can see. The SDK lets an application select task-relevant context, apply the controls supported by its integration, and inspect the resulting output and evidence. Your application keeps its model, workflow, and user interface.

SELECT → CONTROL → PROVE

• Select: gather relevant context with supported search, read, tree, compose, and lifecycle APIs.
• Control: apply the permissions and command policy available in the chosen integration and Engine release.
• Prove: inspect returned results and measurements; the stable lifecycle also exposes receipts. A context-only integration cannot observe what the host later sends to its model.

Choose your path

• Coding agent: use Stable AgentContext or AsyncAgentContext for local context tools with explicit permissions; your host keeps the model and agent loop.
• Enterprise copilot: use only the five Stable lifecycle primitives: ContextSession, ContextSource, ContextView, ContextPlan, and ContextReceipt.
• Product builder: embed the SDK and Engine in your application. The source license permits non-production use; production use, OEM embedding, and commercial redistribution require a written agreement signed by Thinkery AG.

Build with the SDK when your application needs its own LeanCTX integration. Use the Engine's CLI or MCP integration directly when an existing supported agent already provides the context tools you need.

The SDK does not contain a second implementation of the Engine. It starts one verified local Engine process and exposes its negotiated capabilities as stable language-native methods.

your model / agent loop
          ↓
AgentContext or AsyncAgentContext
          ↓  versioned local Agent Tools Interface
LeanCTX | OSS (release-specific; see [Compatibility](COMPATIBILITY.md))
          ↓
project-jailed files, cache, search, patches, approved commands

Install

The public SDK 1.1.1 release uses Engine 3.10.1 and its companion wheels. The current main source targets Engine 3.12.2. Install the public Python SDK and Engine together with:

These commands install the published SDK 1.1.1, which requires Engine 3.10.1. Current main sources target Engine 3.12.2; package version metadata alone does not identify a published artifact. See COMPATIBILITY.md.

Standard Engine:

python -m pip install "thinkery-leanctx-sdk[agent]==1.1.1"

With the certified OpenAI Agents integration:

python -m pip install "thinkery-leanctx-sdk[agent,openai-agents]==1.1.1"

CUDA and Windows-GNU builds use the documented agent-cuda and agent-windows-gnu extras. The core SDK remains pure Python.

Language SDKs

All six language SDKs implement the five Stable lifecycle primitives and the separate Stable Agent Tools surface. The table describes current main source compatibility. The published SDK 1.1.1 release requires Engine 3.10.1; current main targets Engine 3.12.2. Registry publication is per artifact and must be verified at that registry.

Runtime	Package source	Package identity
Python 3.9–3.14	repository root	thinkery-leanctx-sdk
Node.js 22+ / TypeScript	packages/typescript	@thinkery/leanctx-sdk
Go 1.24+	packages/go	github.com/Thinkery-AG/leanctx-sdk/packages/go
Rust 1.77+	packages/rust	thinkery-leanctx-sdk
Java 21 / Kotlin 2.1	packages/jvm	com.leanctx:leanctx-sdk
.NET 8+	packages/dotnet	Thinkery.LeanCtx

Each package follows the shared protocol and release gate. The Engine remains a separate local binary. Python's Preview workspace APIs live only under leanctx_sdk.preview; they are outside the Stable v1 guarantee.

Registry releases use language-scoped tags from the same commit: packages/typescript/vX.Y.Z, packages/go/vX.Y.Z, packages/rust/vX.Y.Z, packages/jvm/vX.Y.Z, and packages/dotnet/vX.Y.Z. See docs/LANGUAGE-PACKAGES.md.

Go follows the standard monorepo module convention and is tagged packages/go/vX.Y.Z. Registry-backed packages use their native registries: PyPI, npm, crates.io, Maven Central, and NuGet.

Five-minute custom agent

from leanctx_sdk import AgentContext


def my_model(task: str, context: str) -> str:
    # Replace with any model or framework call.
    return f"{task}\n\nRelevant project context:\n{context}"


with AgentContext(".", task="Explain the public API") as ctx:
    files = ctx.tree(depth=2)
    matches = ctx.search("class AgentContext", path="src")
    source = ctx.read("src/leanctx_sdk/agent.py", mode="signatures")
    answer = my_model(ctx.task, "\n".join((files.text, matches.text, source.text)))
    print(answer)
    print(f"saved tokens: {ctx.metrics.saved_tokens}")

Default permissions are read-only. A coding agent must opt in explicitly:

from leanctx_sdk import AgentContext, AgentPermissions, ExecutionPolicy

with AgentContext(
    ".",
    permissions=AgentPermissions(write=True, execute=True),
    execution_policy=ExecutionPolicy(allowed_executables=("git", "pytest")),
) as ctx:
    ctx.replace_unique("app.py", "old_name", "new_name")
    tests = ctx.run(("pytest", "-q"), timeout=30)

The permission policy is immutable for the session and is enforced again by the Engine. call() cannot bypass it, and process tools must use run(argv).

SDK 1.1 Stable Agent Tools

• read, search, glob, tree, compose, and symbol
• safe create_file, patch, and replace_unique
• allowlisted argv execution with compressed output
• persistent per-agent cache and aggregate token measurements
• synchronous and asynchronous APIs
• capability negotiation and typed fail-closed errors
• optional OpenAI Agents 0.8.4 function tools

The SDK supplies the tool substrate, not an autonomous planner, model, hosted service, or universal quality guarantee. Savings are measured per call against the Engine's raw-output baseline; they are not a promise for every workload.

Compatibility

The five SDK 1.0 lifecycle primitives remain available unchanged: ContextSession, ContextSource, ContextView, ContextPlan, and ContextReceipt. Stable Agent Tools is a separate API surface. The published SDK 1.1.1 requires Agent Tools Interface v1 from Engine 3.10.1; current main targets Engine 3.12.2. The older context-view/recover Engine Interface v1 remains separate.

See:

• Custom agents
• Unreleased v4 Engine planning
• Quickstart
• Compatibility
• Security
• Errors
• Migration
• Stable public surface

Deprecated names (until 2.0)

SDK 1.3 calls a running LeanCTX | OSS an instance. The Python names below are primary; the old Engine names still import and resolve to the same objects (so except EngineUnavailable still catches), but emit a DeprecationWarning. Wire/JSON fields such as engine_interface_version and the stable error codes (engine_unavailable, ...) are unchanged.

Deprecated (Python)	Use
EngineError, EngineUnavailable, EngineTimeout, EngineCrashed	InstanceError, InstanceUnavailable, InstanceTimeout, InstanceCrashed
EngineProtocolError, EngineExecutionError, EngineRejected	InstanceProtocolError, InstanceExecutionError, InstanceRejected
UnsupportedEngineError	UnsupportedInstanceError
EngineStatus	InstanceStatus
EngineClient, SubprocessEngineClient, EnginePlanningClient	InstanceClient, SubprocessInstanceClient, InstancePlanningClient
EnterpriseEngineClient, EngineContextClient, EngineContextReadResult	EnterpriseInstanceClient, InstanceContextClient, InstanceContextReadResult
EnginePlanningRequest, EngineSource	InstancePlanningRequest, InstanceSource
preview.LocalCheckpointPackageEngine	preview.LocalCheckpointPackageInstance
modules leanctx_sdk.engine, leanctx_sdk.enterprise_engine	leanctx_sdk.instance, leanctx_sdk.enterprise_instance
engine_binary= (AgentContext, AsyncAgentContext, SubprocessInstanceClient)	instance_binary=
engine= (ContextSession, checkpoint package functions)	instance=
LEANCTX_ENGINE_BIN, LEANCTX_ENGINE_BINARY, LEANCTX_ENGINE_PROOF_FILE, LEANCTX_REQUIRE_ENGINE, LEANCTX_TEST_ENGINE_BINARY	LEANCTX_INSTANCE_BIN, LEANCTX_INSTANCE_BINARY, LEANCTX_INSTANCE_PROOF_FILE, LEANCTX_REQUIRE_INSTANCE, LEANCTX_TEST_INSTANCE_BINARY (old one is a fallback, warned once)

License

The SDK is source-available under a license that permits non-production use. Production use, OEM embedding, and commercial redistribution require a separate written agreement signed by Thinkery AG. LeanCTX | OSS (lean-ctx) is licensed separately under Apache-2.0; that license does not grant SDK production or OEM rights.

Process containment (session starter)

The SDK starts the Engine binary with LEAN_CTX_SESSION_CONTAINMENT=1. An Engine binary that has a session starter answers with one line on standard output before the Engine itself starts ({"leanctx_session_containment":1}). The SDK removes that line and reports one of two states:

• engine-starter: the process the SDK started is the Engine's session starter. It runs the Engine in a process group of its own, ends that group when the Engine exits, when the SDK ends the starter or when the SDK's process is gone, and exits only afterwards.
• unavailable: no acknowledgement. The SDK's own process-group handling is all there is; what an Engine that exited first left behind may survive.

Read it from AgentContext.session_containment (also on AsyncAgentContext) and, for the one-shot client, from SubprocessInstanceClient.last_session_containment (the most recent completed run).

On Windows there is no starter process: an Engine that has the mode joins a kill-on-close job of its own before it starts anything and acknowledges with the same line (reported as the same state); everything it started ends when the Engine process ends. A host job that forbids nesting leaves it unacknowledged.

Not covered: descendants that leave their process group (setsid, setpgid), and a graceful stop — the SDK ends the starter, and the Engine's group is ended right after, which can be a few milliseconds after cancel()/close() returned. Whether a given Engine build has the starter is told only by the reported state.

Context Store preview source

The preview source adds read-only task lineage and policy evidence within one tenant/project scope. See the guide, the contract and the decision record. The source proposal requires an Engine serving the documented operations; binding tests, installed qualification and pairing acceptance remain separate.