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+# Sandbox Runtime Requirements
+
+This document states the sandbox environment as a contract. The sandbox owner
+must meet this contract. The Paperclip runtime does not build the environment at
+exec time. The environment is a requirement, not a build step.
+
+This document states requirements. It does not state build steps.
+
+## Required on PATH
+
+- `node` must be installed and on the PATH.
+- Each agent CLI that the run uses must be installed and on the PATH. The set of
+  agent CLIs includes `claude`, `codex`, `gemini`, and similar CLIs.
+- The owner installs only the CLIs that the run uses. The owner does not need to
+  install a CLI that no run uses.
+
+## Runtime dependencies
+
+The sandbox execution and synchronization paths need more than `node` and the
+agent CLIs. The owner must also supply these:
+
+- A POSIX shell as `sh`, normally `/bin/sh`. The runtime runs each command with
+  `sh -c <script>`. The runtime uses `bash` only when the adapter sets the shell
+  to `bash`.
+- `tar`. The synchronization path extracts and creates archives with `tar`. A
+  sandbox without `tar` cannot receive or return workspace files.
+- A writable workspace directory. The runtime extracts the workspace archive
+  into this directory.
+- A writable home directory. The agent CLIs write state and credentials under
+  the home directory.
+- A writable cache directory and a writable temporary directory. The runtime and
+  the agent CLIs write intermediate files to these locations.
+
+## Detection contract
+
+Paperclip probes each CLI before launch. Paperclip uses the same detection
+pattern that the runtime Dockerfiles use:
+
+```bash
+command -v <cmd> || exit 1
+```
+
+Paperclip probes each CLI with `command -v <cmd>`. Paperclip fails loudly when
+the CLI is absent and no install command is configured for the CLI.
+
+## Optional CLI installation
+
+An adapter can configure an install command for a CLI. When an install command
+is configured, the runtime obeys this flow:
+
+1. The runtime probes the CLI with `command -v <cmd>`.
+2. If the CLI is already on the PATH, the runtime skips the install.
+3. If the CLI is absent, the runtime runs the configured install command one
+   time.
+4. A failed install is not fatal. The runtime writes a log line and continues.
+   The launch-time probe still reports a missing CLI and fails loudly.
+
+An owner who relies on a configured install command must also supply the network
+access, the filesystem write access, and the package tooling that the install
+command needs. When no install command is configured, the runtime does not
+install the CLI. The owner must supply the CLI on the PATH.
+
+## Firm rule
+
+- The Paperclip runtime never modifies the login profile. The runtime never
+  writes a profile file. The runtime never writes an rc file.
+- The Paperclip runtime never sources `nvm` on the exec path.
+- The sandbox owner supplies a ready PATH. The PATH must resolve `node` and each
+  used agent CLI without any action from the runtime, except for a configured
+  install command.