SBP Conformance Levels

An SBP implementation declares a conformance level from L1 to L6. Levels are cumulative — an L3 implementation also satisfies L1 and L2.

Overview

L1 — Stateful Proxy

The gateway maintains persistent agent sessions.

Required

• POST /v1/sessions — create a session with a session_token
• POST /v1/completions (or equivalent) — run a turn; persist messages
• GET /v1/sessions/{id} — retrieve session status and step count
• Messages MUST be persisted across gateway restarts
• Agent identity MUST be stable across calls

Optional

• Streaming completions
• Memory (episodic, semantic, procedural)
• Tool execution

What is NOT required

• WebSocket
• Tether buffering
• Roaming

L2 — Tether + Resume

The gateway buffers agent output when the surface is offline and delivers it on reconnect.

Required (in addition to L1)

• wss://<host>/v1/sbp/ws/{session_id} — WebSocket endpoint
• ATTACH_SESSION / SESSION_ATTACHED handshake
• TETHER_TURN delivery on re-attach
• TURN_CHUNK + TURN_COMPLETE for live streaming
• DETACH and unclean-disconnect handling
• The Tether backend MUST survive gateway process restart (durable storage — SQLite, PostgreSQL, Redis Streams, Temporal, or equivalent)
• tether_turns_pending count in SESSION_ATTACHED

Tether backend options

Any of the following satisfy L2 durability:

L3 — Roaming

Sessions are portable as content-addressed bundles that any compliant gateway can import.

Required (in addition to L2)

• POST /v1/sbp/sessions/{id}/export — returns { roaming_token, bundle_cid, ... }
• POST /v1/sbp/sessions/import — accepts roaming_token, creates new session
• Bundle MUST include: sbp_version, bundle_cid, session, messages, memory, metadata
• bundle_cid MUST equal sha256(json.dumps(bundle, sort_keys=True).encode('utf-8')) before the field is inserted into the bundle
• Importing gateway MUST verify bundle_cid on receipt
• Default token consumption: single-use (one import per export)
• allow_reuse: true MUST produce a fork (both sessions coexist independently)

Recommended

• POST /v1/sbp/sessions/{id}/handoff — transfer to another agent
• POST /v1/sbp/sessions/{id}/fork — branch at current checkpoint
• GET /v1/sbp/sessions/{id}/lineage — full family tree (exports, handoffs, forks)

L4 — Surface Negotiation

The gateway adapts its output to the surface’s device type and capabilities.

Required (in addition to L3)

• ATTACH_SESSION MUST accept a surface field (SurfaceContext)
• Gateway MUST store SurfaceContext for the duration of the attachment
• Gateway MUST pass device_type and max_output_tokens to output generation
• Gateway MUST handle surface: null gracefully (treat as device_type: "unknown")

SurfaceContext fields

Recommended

• Contextual Translation Pipeline (CTP): second LLM pass to reformat output for mobile/IoT
• Streaming CTP output as TURN_CHUNK frames (not blocking until full)

L5 — MCP Bridge

Agents can invoke tools that run locally on the surface.

Required (in addition to L4)

• Gateway MUST register surface-declared mcp_tools as callable tools in the agent’s tool schema
• Gateway MUST emit TOOL_CALL frames and await TOOL_RESULT responses
• call_id MUST be unique per invocation (UUID)
• Timeout MUST be enforced (RECOMMENDED: 30 seconds)
• On timeout or disconnection, gateway MUST return an error tool result to the agent (not deadlock)
• Surface MUST respond with TOOL_RESULT matching the call_id

Multi-replica deployments

• In single-replica: in-process Future rendezvous is acceptable
• In multi-replica: cross-process pub/sub (Redis pub/sub, Temporal signal, etc.) MUST be used to route TOOL_RESULT to the correct pod

L6 — Gateway Federation (Optional)

Independent gateways discover each other and exchange bundles by CID.

Two roles

An L6 node operates in one of two roles (or both):

• Gateway: hosts live sessions, serves bundles by CID. The default.
• Tracker: indexes which Gateway holds which bundle_cid. Optional, no session content.

A deployment MAY combine both roles in a single process. Public SBP infrastructure SHOULD separate them so Trackers can be operated cheaply at scale without privileged access to session content.

Required (in addition to L5)

• GET /.well-known/sbp — discovery endpoint including a role field
• GET /v1/sbp/bundles/{bundle_cid} — bundle resolution by CID (Gateways only)
• Bundle resolution MUST verify bundle_cid before returning
• Federation token (signed JWT or mutual TLS) MUST be required for bundle resolution
• Trackers MUST NOT proxy bundle content; they return only references
• Trackers MUST sign their registry responses with the same key advertised in /.well-known/sbp

Discovery endpoint response

{
  "sbp_version": "1.2",
  "gateway_id": "<string>",
  "federation": true,
  "public_key": "<base64-DER>",
  "endpoints": {
    "ws": "wss://<host>/v1/sbp/ws/{session_id}",
    "bundle": "https://<host>/v1/sbp/bundles/{bundle_cid}",
    "import": "https://<host>/v1/sbp/sessions/import"
  }
}

Recommended

• Static peer list in gateway config
• DNS SRV peer discovery (_sbp._tcp.<domain>)
• Automatic session migration: surface connects to nearest L6 gateway; gateway fetches bundle from origin peer transparently

L7 — Direct Data Plane (Reserved, v0.2)

Optional WebRTC surface↔surface streaming with Gateway-mediated signalling.

L7 is reserved in SBP v1.2 and not normative. It will be specified separately once a reference implementation demonstrates its operational tradeoffs.

Motivating use case

Two surfaces on the same local network (a phone and a watch, a desktop and a tablet) exchanging live agent output without round-tripping through the cloud Gateway. The Gateway remains the source of truth for the session bundle and Tether, but acts only as a signalling server during the WebRTC handshake.

Why it’s reserved

• WebRTC requires STUN/TURN infrastructure that inflates minimum viable implementations
• NAT traversal failure modes are subtle and add a long error surface
• The use case is real but narrow — most deployments do not need it

Implementations that ship a WebRTC data plane MAY self-describe as “L7-experimental” but MUST NOT claim L7 conformance until the specification is finalised.

Self-certification

To claim a conformance level, run the SBP conformance test suite against your server URL:

npx sbp-conformance --url https://your-gateway.example.com --level L5

The test suite is available at: statebridge-protocol/sbp-conformance

Reference Implementations

SBP Conformance Levels — v1.2 — © 2026 Silkbridge, Inc. — Apache-2.0