sandbox-agent/research/acp/rfds-vs-extensions.md

ACP RFDs vs Extensions

Status date: 2026-02-10

Feature Area	Approach	Short-Term Extension Path	Notes
Generic non-permission HITL question/input flow (agent request + client response)	RFD	_sandboxagent/session/request_question request + structured JSON-RPC response (answered/rejected/cancelled).	Needs standard ACP request/response shape beyond permission prompts.
Standard model listing support (including pre-session discovery and/or stable session model state)	RFD	_sandboxagent/session/list_models response with availableModels, currentModelId, plus variant fields (defaultVariant, variants[]) in payload or _meta.	Includes model variant semantics (defaultVariant, variants[]).
Stabilize session/list	RFD	Use ACP unstable session/list directly (no custom extension alias).	Currently unstable ACP method.
Stabilize session/fork	RFD	Use ACP unstable session/fork directly (no custom extension alias).	Currently unstable ACP method.
Stabilize session/resume	RFD	Use ACP unstable session/resume directly (no custom extension alias).	Currently unstable ACP method.
Stabilize session/set_model	RFD	Use ACP unstable session/set_model directly; fallback to session/set_config_option where available.	Currently unstable ACP method.
Standard session metadata-at-creation semantics (portable title/tags-style hints)	RFD	Use session/new.params._meta["sandboxagent.dev"] for create-time metadata; optional _sandboxagent/session/set_metadata for post-create updates.	Reduces custom _meta usage for common metadata.
Sandbox Agent robust HTTP filesystem API	Extension	Already extension (/v2/fs/* custom HTTP surface).	Separate custom HTTP API; not ACP core agent-session protocol.
skills payload and behavior	Extension	Already extension via _meta["sandboxagent.dev"].skills and optional _sandboxagent/session/set_metadata.	Product-specific behavior.
Client-requested session alias (requestedSessionId) behavior	Extension	Already extension via _meta["sandboxagent.dev"].requestedSessionId.	Product-specific metadata/ID behavior.
Agent version pin/hint behavior during bootstrap/session creation	Extension	Already extension via _meta["sandboxagent.dev"].agentVersionRequested.	Product-specific runtime selection behavior.
Agent control-plane inventory/install state (list/install/credentialsAvailable/installed/path)	Extension	Already extension as custom control-plane HTTP (/v2/agents, /v2/agents/{agent}/install).	Control-plane functionality outside ACP core.
Runtime daemon/process status and related capabilities (serverStatus, sharedProcess, commandExecution, errorEvents)	Extension	Already extension as control-plane fields (/v2/agents, /v2/health) and capability _meta hints.	Runtime/orchestrator-specific state.
Sandbox Agent control-plane APIs (/v2/health, /v2/agents, /v2/agents/{agent}/install)	Extension	Already extension (custom non-ACP API surface).	Custom non-ACP HTTP surface.
OpenCode compatibility bridging behavior (/opencode/*)	Extension	Already extension as product compatibility bridge layer.	Product compatibility layer, not ACP core protocol.