Journey: Extend Without Forking: Plugins & WASM

You need a domain tool (an internal lookup) and a custom compressor for your data shape. Forking the engine and maintaining a patched build is a non-starter. LeanCTX gives you two ways to extend it, neither of which touches its source — and one command that installs either.

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1. A tool the agent can call — declare an MCP server

Your tool already speaks MCP, or you write it in whatever language you like and give it an MCP interface. It stays yours: your repo, your release cadence, your process. LeanCTX only needs to know how to run it.

# lean-ctx-addon.toml
[addon]
name = "crm"
version = "0.1.0"
description = "Look up an account in the CRM"

[mcp]
transport = "stdio"
command   = "crm-bin"
args      = ["mcp", "serve"]
sha256    = "…"              # optional pin; checked before every spawn
integration = "memory"        # optional: fold results into ctx_knowledge

lean-ctx addon add ./crm-0.1.0.ctxpkg

LeanCTX prints the exact command it would spawn, says plainly that the server runs as a normal process with your privileges, and asks. It does not install the binary for you — no uv tool install, no npx. Fetching crm-bin stays your step, where your own package manager’s trust model applies.

2. A custom compressor or context provider — WASM

This one has to run inside the pipeline, so it is a sandboxed WASM module compiled from any language that targets wasm32-unknown-unknown:

lean-ctx addon release ./my-compressor   # signed .ctxpkg, module embedded
lean-ctx addon add ./my-compressor-1.0.0.ctxpkg

The module travels inside the signed package. No artifact host, no checksum files, no CI of your own: the signature covers the executable bytes, so there is nothing external to serve and nothing to download at install time.

/v1/capabilities → extensions.compressors: ["identity","markdown","prose","whitespace","my_ext"]
conformance scorecard → [ok] extensions/compressor:my_ext

While authoring, skip packaging entirely:

export LEAN_CTX_WASM_DIR=~/.local/share/lean-ctx/wasm    # *.wasm → registered compressors
lean-ctx conformance                                     # checked like a built-in

That override is a developer convenience with none of the install-time verification — not a distribution path.

3. Under the hood — core/context_package + core/wasm_ext.rs

Install verifies the package signature locally rather than trusting its source, checks every module against its pinned SHA-256 and the WebAssembly magic bytes, and writes modules read-only, never executable — they are fed to an interpreter, not to the OS loader.

At runtime the WASM host implements wasm-abi-v1 (alloc + lctx_compress / lctx_provider_fetch) with a fresh store per call, no ambient environment, and a byte budget the host applies after decoding, so a faulty or hostile guest can never overrun it. Declared MCP servers go through the gateway instead: global-only, opt-in, and pinned to a binary digest when the manifest says so.

Payoff

The engine is extensible in any language, verified at install, discoverable and conformance-checked — your tools and transforms are first-class without ever touching LeanCTX’s source, and without needing a release pipeline of your own.