diff --git a/docker/agent-runtime/README.md b/docker/agent-runtime/README.md
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+++ b/docker/agent-runtime/README.md
@@ -2,6 +2,8 @@
 
 Container images for running coding-agent harnesses in sandboxed environments (for example the kubernetes sandbox provider, stage 1 of the k8s contribution). Images are named `agent-runtime-{harness}:{version}` and published to `ghcr.io/paperclipai/` by the `agent-runtime-images` workflow. The registry is overridable: every reference flows through the `REGISTRY` bake variable.
 
+For the sandbox environment contract that the owner must meet, see [SANDBOX-REQUIREMENTS.md](SANDBOX-REQUIREMENTS.md).
+
 ## Image Lineup
 
 - **`agent-runtime-base`**: Foundation. Ubuntu 22.04 + Node 22 + git + tini + non-root user (uid 1000) + the agent shim.
diff --git a/docker/agent-runtime/SANDBOX-REQUIREMENTS.md b/docker/agent-runtime/SANDBOX-REQUIREMENTS.md
new file mode 100644
index 00000000000..34102b33147
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@@ -0,0 +1,36 @@
+# 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.
+
+## 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. Paperclip does not install the CLI. Paperclip does not repair
+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.