@memberjunction/ai-bridge-server

Server-tier coordination + execution for MemberJunction Realtime Bridges. This package completes the realtime engine’s long-deferred unified media-transport seam — the pipe that connects a bridge driver’s media plane to a realtime agent session — and adds the bot-session lifecycle, host affinity + janitor, participant tracking, and turn-taking integration around it.

It is the server half of the AIBridgeEngineBase / AIBridgeEngine pair, exactly mirroring how @memberjunction/aiengine’s AIEngine builds on @memberjunction/ai-engine-base’s AIEngineBase. The cross-platform, metadata-only base (provider/identity/channel cache, the abstract BaseRealtimeBridge driver, TurnTakingPolicy, media-track types) lives in @memberjunction/ai-bridge-base; this package adds everything that actually runs a bridged session.

See the architecture plan at /plans/realtime/realtime-bridges-architecture.md (§1 transport seam, §2 layer cake, §7 lifecycle, §10 security/janitor) for the full design.

What’s in the box

Export	Purpose
AIBridgeEngine	The server engine. Extends AIBridgeEngineBase; starts/stops bridged sessions, wires the transport seam, tracks participants, integrates turn-taking, reconciles orphans. Singleton via AIBridgeEngine.Instance.
LoopbackBridge	An in-memory, platform-free bridge driver (@RegisterClass(BaseRealtimeBridge, 'LoopbackBridge')) that echoes outbound media back inbound — proves the seam round-trips with zero external infrastructure.
StartBridgeSessionParams / ActiveBridgeSession	The start contract + the live in-memory session handle.
IHostInstanceIdentity / DefaultHostInstanceIdentity	The injectable host-instance identity used for HostInstanceID stamping + janitor affinity (host apps inject the real one; a default is provided for standalone use).
LoadLoopbackBridge	Tree-shaking-prevention loader for the loopback driver.

Installation

npm install @memberjunction/ai-bridge-server

The transport seam (the heart)

The realtime session contract (IRealtimeSession in @memberjunction/ai) is already media-agnostic — SendInput(frame) is what the agent hears (and sees), OnOutput(handler) is what it says, OnVideoFrame(handler) is the video it shows — but it had no client-facing pipe. A bridge is that pipe. AIBridgeEngine.wireTransportSeam connects the two directions:

// Inbound: endpoint media → the agent hears (and, for a video model, sees) it.
bridge.OnMedia((frame) => {
    const chunk = frameToArrayBuffer(frame);     // unwrap BridgeMediaFrame → ArrayBuffer
    const kind = mediaKindOf(frame.Track);       // 'video' for video-in and screen-in, else 'audio'
    if (chunk) realtimeSession.SendInput({ Data: chunk, Kind: kind, MimeType: frame.MimeType, TimestampMs: frame.TimestampMs });
});

// Outbound: the agent speaks → into the meeting/call.
realtimeSession.OnOutput((chunk) => {
    bridge.SendMedia('audio-out', arrayBufferToFrame(chunk, 'audio-out'));
});

// Outbound video: the agent's typed video frames → video-out, with their MIME type.
realtimeSession.OnVideoFrame?.((frame) => {
    bridge.SendMedia('video-out', { Track: 'video-out', Bytes: frame.Data, MimeType: frame.MimeType });
});

The snippet leaves out the video gate, the notes to the model and the floor gate; see Video. The engine never constructs the realtime model — the IRealtimeSession is injected via StartBridgeSessionParams.RealtimeSession, so the only coupling is the seam itself.

Video

What the driver is told

StartBridgeSession passes the driver its Configuration plus facts read from the realtime session: its audio rates (InboundSampleRate, OutboundSampleRate) and its video facts, InboundVideoStreams (how many camera or screen sources it takes at once; 0 when it takes no video) and InboundVideoRate (frames per second; absent when the model declares none). The video facts come from the session’s declared capabilities (InboundVideoStreamsOf, InboundVideoRateOf in @memberjunction/ai), so the rate comes from the model, not the driver.

Video in: the gate and the notes

• The gate. A video-in or screen-in frame goes only to a session whose capabilities declare an inbound video track. Any other session would read the image as audio, so the engine drops the frame (one verbose log line per session). No inbound frame is sent while the model session is being replaced.
• “[You can now see: …]” goes to the model right before the first frame of a source new to the current model session: the first source, the next one after a switch, and a source seen again after it ended. A replacement model session (after a drop, or the audio-only one below) starts with no sources seen, so it gets the note with its first frame. Sources are told apart by SourceID; a frame without one gets no note.
• “[You can no longer see: …]” goes when the driver reports through OnVideoSourceEnded that a source whose frames reached the current model session stopped, a switch to another source included. A source the model never saw gets no note.
• The label. Both notes name the source by its SourceLabel (“Ada’s camera”). A source without one is “a participant’s camera” or “a participant’s screen”, never its SourceID.
• The notes go through the session’s optional SendContextNote, and a driver may hold them: Gemini Live queues a note until the agent’s reply ends but sends frames at once, so while the agent talks, a new source’s first frames can reach the model before its note.

VideoSourceSeenNote and VideoSourceEndedNote build the two notes.

Video out

OnVideoFrame → SendMedia('video-out'). Each typed frame (RealtimeVideoFrame: a fragmented MP4 piece, an encoded chunk or an image) goes to the bridge with its bytes, its MimeType, so the driver can tell an avatar’s MP4 from other video, and what it says about its video (Width, Height, KeyFrame):

• an image: its own size when it has one, and KeyFrame: true;
• a chunk: its own size when it has one, and its own key-frame flag;
• an fMP4 init segment: its own size, read from the piece itself, and no KeyFrame;
• an fMP4 fragment with video: whether its first video frame is a key frame, and the size the stream’s init segment gives the video track. An audio-only fragment, a fragment before any init segment, or one the driver couldn’t read carries none of these.

When the session’s speech is floor-gated (FullDuplexTurnGate, in a room with more than one agent), its video is gated too. An fMP4 fragment counts by the duration of its audio track, which carries the agent’s voice (FullDuplexTurnGate.OnOutputDuration). An init segment passes and is kept for reading the fragments. Chunks and images carry no voice and pass. A cut flushes the bridge’s queued media.

The deprecated OnVideoOutput (untyped bytes) is still forwarded to video-out, without a MIME type.

When the avatar can’t be shown

A driver that publishes the agent’s avatar reports through OnAvatarUnavailable when it took the avatar down ('decoder-failed' or 'publish-failed'). The engine then replaces the model session once, through the host’s StartBridgeSessionParams.RecoverRealtimeSessionWithoutAvatar, with one that renders no avatar, seeded with the conversation so far, so the model stops generating video nobody sees. Inbound audio is held while the replacement opens, and the replacement must use the same audio sample rates. Nothing is replaced when the host gave no factory, or the session is ending, recovering or already replaced; if the replacement fails, the session goes on as it was. This is separate from RecoverRealtimeSession’s attempts after a dropped model session.

Model usage

The engine records no model usage. For an agent’s bridged session, RealtimeClientSessionService.WireBridgeRealtimeSession in @memberjunction/ai-agents records the model session’s OnUsage on that session’s own co-agent prompt run, so a replacement session opened through it has a run of its own.

Usage

import { AIBridgeEngine } from '@memberjunction/ai-bridge-server';
import { RegexAddressedMatcher } from '@memberjunction/ai-bridge-base';

const engine = AIBridgeEngine.Instance;
await engine.Config(false, contextUser, provider);   // load the bridge registry (inherited from base)

// `realtimeSession` is an already-open IRealtimeSession from the agent/session layer.
const active = await engine.StartBridgeSession({
    AgentSessionID: existingAgentSessionId,
    Provider: zoomProviderRow,                        // MJAIBridgeProviderEntity (DriverClass resolves the driver)
    RealtimeSession: realtimeSession,                 // injected — the engine wires its media plane
    Address: 'https://zoom.us/j/123456789',
    TurnMode: 'Passive',
    TurnMatcher: new RegexAddressedMatcher(['Sage']),
    ContextUser: contextUser,
    MetadataProvider: provider,
});

// ...later, on hang-up / host-ended:
await engine.StopBridgeSession(active.SessionBridgeID, 'Explicit');

Lifecycle

StartBridgeSession drives the AIAgentSessionBridge status state machine Pending → Connecting → Connected (and → Failed on any error), stamps HostInstanceID for node affinity, resolves the driver via ClassFactory.CreateInstance(BaseRealtimeBridge, provider.DriverClass), then wires the transport seam + participant tracking + turn-taking. StopBridgeSession disconnects the driver and transitions the row to Disconnected with a CloseReason. Both are idempotent.

Host affinity & janitor

Every bridge row is stamped with HostInstanceID. ReconcileOrphans(contextUser, provider) force-closes Connected/Connecting bridges left by a prior boot of this host (matching hostname prefix, differing instance id) with CloseReason = 'Janitor' — mirroring SessionJanitor. The scheduling (run-once-at-boot + periodic sweep) is a host-provided hook so this package carries no timer/IO of its own; call ReconcileOrphans from the host’s janitor.

Turn-taking

A TurnTakingPolicy is built per session from TurnMode (Passive default / Active / Hybrid) and fed the session’s diarized user transcript segments. Speak requests a spoken update on the session (when the provider supports RequestSpokenUpdate), PostToChat is a documented hook for the Phase-2 bridge-chat channel, and Silent does nothing.

Testing the seam without a platform

LoopbackBridge echoes every outbound frame back inbound, so with a mock IRealtimeSession you can assert media flows bridge → session and session → bridge with no external service. Its EmitVideoSourceEnded and EmitAvatarUnavailable drive the ended-source note and the audio-only replacement. See src/__tests__/transport-seam.test.ts for the round-trip, lifecycle, turn-taking, participant, janitor and video-note tests, and src/__tests__/avatar-output.test.ts for video out, the floor gate and the audio-only replacement.

Composition with the realtime engine

Layer	Package	Role
Metadata cache + abstract driver + turn policy	@memberjunction/ai-bridge-base	provider/identity/channel cache, BaseRealtimeBridge, TurnTakingPolicy, media-track types
Coordination + execution + transport seam	@memberjunction/ai-bridge-server (this)	bot lifecycle, seam wiring, participant tracking, janitor
Realtime session contract	@memberjunction/ai	IRealtimeSession / BaseRealtimeModel (injected, never constructed here)

License

Business Source License 1.1 — see LICENSE for details.