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Coming soon. Current thinking, not a shipped contract. Today the only runtime is claude; custom runtimes are the extensibility seam — claude itself is just the first runtime built on this contract.
A runtime is a container image that implements the runtime contract. Any agent harness — your own, or a lab’s — becomes a runtime by meeting it; you then select it per agent with runtime: "<name>", and nothing else about the session changes.

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The contract

The platform invokes your image once per turn and passes it (via env) the session id, the read cursor, the events API URL, and a fenced turn token. Per turn the image:
  1. reads the new input from the event log at the cursor;
  2. drives its own agent loop;
  3. streams events back to the session over the authenticated events API — the turn token is fenced, so if the platform has superseded this instance its writes are rejected and a stale machine can’t corrupt the log;
  4. performs all side effects through the remote sandbox tools — it has no local disk or network of its own; the sandbox is the boundary;
  5. exits 0 when it has nothing left to do (yield, don’t block). A non-zero exit is treated as a crash, and the platform restarts the turn in place.

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State and recovery

The image carries no durable state. The durable record is the event log plus the runtime’s on-box journal. Keep your resumable state in the journal directory the platform provides and you inherit the same durability as claude: a crash restarts from the journal, an idle session hibernates and is checkpointed, and a lost machine is recreated and restored.

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Models

The model is configuration, not part of the image. A runtime declares which providers it can drive; switching across providers needs an image that drives the target (the planned opencode aims to drive all).

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Distribution

You register a runtime with oc runtime push <image> and select it with runtime: "<name>". Runtimes are versioned and a session pins the version it runs, so pushing a new build never affects in-flight sessions — the same mechanism behind the prebuilt claude, codex, and opencode images.