Run code in the sandbox: languages and runtimes

Your agent often reads a whole file or a whole log to answer one small question. With ctx_execute, the agent writes a short script, the script runs on your machine, and only what it prints reaches the model. It runs shell, PowerShell, JavaScript, TypeScript, Python or any runtime on your PATH. The script runs as a normal process on your machine: it runs as you, with your files, your network and your own agent permissions in front. Here "sandbox" means the output stays in the sandbox, away from the model.

Free · Local · Private · Yours to keep

Context Mode Engine is source-available under the Elastic License 2.0. Use it, copy it, change it and share it at zero cost, for as long as you follow the licence terms. It runs on your machine, sends zero usage data, and keeps your data in ~/.context-mode.

One question, one whole file

You ask how many TODO lines are in src. The agent opens every file and reads it into the context. A lint report or a test log goes the same way. The answer is one line, and the context window fills with the rest.

Ask for a short script

Ask the agent to use ctx_execute. The agent writes a few lines of code that find the answer. Context Mode Engine runs the code in your project folder and sends back only what the code prints.

{ "language": "shell", "code": "grep -c TODO -r src" }

The call takes these fields: language, code, and the optional file, intent, timeoutMs and background. With file, the code reads the path from the CTX_FILE variable. With intent, the reply holds the parts of the output that match it, and the full output goes to the index.

Languages and runtimes

Pick one of five named languages. Each call looks up the runtime on your PATH, so a runtime you install shows up on the next call.

• shell: sh. On Windows, Git Bash from your Git for Windows install.
• powershell: pwsh, pwsh.exe or powershell.exe.
• javascript and typescript: node. Node 24.15.0 or later runs the TypeScript file.
• python: python3, then python. On Windows, python.exe, py.exe, then python.
• Any other runtime: pass its name as the language, for example ruby or bash. Shells such as bash, zsh and fish get the code with -c.

When PATH lacks the runtime, the call returns NO_RUNTIME with the language name. Install the runtime, and the next call finds it.

{ "language": "shell", "code": "grep -c TODO -r src" }

{ "language": "powershell", "code": "(Select-String -Path src\* -Pattern TODO).Count" }

{ "language": "javascript", "code": "console.log(require('fs').readdirSync('src').length)" }

{ "language": "typescript", "code": "const n: number = 42; console.log(n)" }

{ "language": "python", "code": "import json; print(len(json.load(open('data.json'))))" }

Any other runtime on your PATH works too: pass its name as the language.

What comes back to the model

The reply holds what the code prints, at most 16,384 B, cut on a whole character. When the output is bigger, the reply shows a preview of at most 4,096 B. The Engine keeps the full output in the project index and ends the reply with a ctx_search call, so the agent finds the rest on demand. Colour codes stay off, so the reply holds plain text.

Timeout and long jobs

The default and maximum timeout is 600,000 ms, ten minutes. Set timeoutMs for a shorter run. At the timeout, the Engine stops the whole process tree: the process and every child it started. For a server or a watcher, set background to true. The call then starts the process and returns its process id.

Who runs the code

Runs as you, in your project, with your own agent permissions in front. Only what the code prints reaches the model. Your agent asks you before each tool call, as it does today. Read more on the security page.

What you get

In the plugin benchmark dated 2026-10-31, the agent ran a short script over a JSON payload of 30 Next.js issues. The raw payload was 214.0 KB, and 0.8 KB reached the model. The number is the median of 5 runs on an Apple M1 Pro with Node 26.8.1, and it counts bytes of tool output (row on GitHub). Run it yourself with cd bench && node run.mjs --runs 5 in the plugin repo.