FE-873: Web-as-driver streaming chat transport (topology A) by lunelson · Pull Request #225 · hashintel/brunch

lunelson · 2026-06-16T15:11:20Z

Summary

Add SessionEventRelay so the TUI-started sidecar can stream the live in-process AgentSession event feed over the existing /rpc WebSocket as brunch.sessionEvent, multiplexed with brunch.updated and without a second canonical store.
Add the narrow web re-entry seams: session.driveTurn for plain prompts and session.answerExchange via LiveExchangeBroker for live request_answer exchanges.
Land the topology-A streaming battery and reconcile SPEC/PLAN/RPC/session docs around observer relay, reconnect, fan-out, command intake, and live answer convergence.

Verification

npm run check

lunelson · 2026-06-16T15:11:39Z

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cursor · 2026-06-17T08:09:15Z

PR Summary

Medium Risk
Introduces live session driving and in-turn exchange answering over WebSocket sidecar RPC; mitigated by handle-gated discovery, named JSON-RPC errors, transcript-backed truth (D19-L), and a broad tier-2 oracle battery on real runBrunchTui wiring.

Overview
Topology A observer + command path for the TUI-started web sidecar: live AgentSession events forward through SessionEventRelay as brunch.sessionEvent on the existing /rpc WebSocket, multiplexed with brunch.updated, with no replay to late subscribers (reconnect is projection refetch + new live frames).

Sidecar RPC is split via createWebSidecarRpcHandlers: read methods stay read-only; session.driveTurn and session.answerExchange appear only when process-local handles are attached. request_answer can await a LiveExchangeBroker when there is no interactive UI, resolved by session.answerExchange.

runBrunchTui owns the relay, live session ref, turn driver, and broker and passes them into startWebHost. Tier-2 web-driver-streaming.* tests cover relay/differential/multiplex, reconnect, fan-out, command intake, and live request_answer convergence; memory/PLAN.md and memory/SPEC.md record D84-L / A28-L / A29-L status.

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Copilot

Pull request overview

Implements the Topology A observer relay seam to forward in-process Pi AgentSession events over the existing TUI sidecar /rpc WebSocket as brunch.sessionEvent JSON-RPC notifications, multiplexed alongside existing brunch.updated domain invalidations.

Changes:

Add SessionEventRelay (brunch.sessionEvent frames with {seq, event}) plus unit tests.
Wire the relay through the TUI boot path into the web sidecar WebSocket transport, and add an end-to-end dev test that proves relay↔transcript reduction and multiplexing.
Update RPC docs + SPEC/PLAN to record the validated/built slice and decision D84-L.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/rpc/websocket.ts	Subscribes to session-event relay and forwards frames over the `/rpc` WebSocket transport.
src/rpc/web-host.ts	Threads optional `SessionEventRelay` into the web host transport wiring.
src/rpc/session-event-relay.ts	Introduces the process-local relay that envelopes Pi events as `brunch.sessionEvent` notifications with monotonic seq.
src/rpc/session-event-relay.test.ts	Unit coverage for envelope shape, no replay, and explicit unsubscription.
src/rpc/README.md	Documents `brunch.sessionEvent` as an observer-only, non-canonical notification.
src/dev/tests/web-driver-streaming.relay.test.ts	End-to-end proof: real `runBrunchTui` host relays live events over ws, multiplexed with `brunch.updated`, and streamed text appears in JSONL.
src/app/brunch-tui.ts	Creates the relay, threads it into the sidecar, and attaches the live `AgentSession` after creation.
memory/SPEC.md	Records A28-L as validated and adds D84-L decision for the relay seam.
memory/PLAN.md	Updates FE-873 status/certainty and topology materialization notes to reflect the built observer relay slice.

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+  const unsubscribeSessionEvents = options.sessionEvents?.subscribe((frame) => {
+    publishNotification(JSON.stringify(frame));
+  });


…uash merge The FE-870 squash merge (#217, e4a63fd) reintroduced the exhausted renderer-golden-coverage--graph-render-migration.md scope card in its stale pre-build form (Cards 0-3 shown blocked/not-done), though Cards 0-3 shipped and PLAN records the scope card as retired. Re-delete it so HEAD matches bkp/ln/fe-870-renderer-golden-context-tools and PLAN's "scope card retired" state. No other canonical-doc drift from the restack.

…pology A) SessionEventRelay forwards the in-process AgentSession event stream over the existing TUI sidecar /rpc WebSocket as brunch.sessionEvent frames, multiplexed with brunch.updated. Ephemeral/process-local; no second canonical store (D19-L, D84-L). Battery claims 1-4 production-wired; claims 5-7 are follow-on slices. Signed-off-by: Lu Nelson <ln@hash.ai>

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cursor · 2026-06-17T17:45:49Z

+      if (ctx.hasUI && typeof ctx.ui.editor === 'function') {
+        answer = await ctx.ui.editor(params.prompt);
+      } else if (answerBroker) {
+        answer = await answerBroker.awaitAnswer({ exchangeId: params.exchangeId });


Broker bypassed when TUI UI

High Severity

In the default runBrunchTui host, request_answer checks ctx.hasUI before the live-exchange broker, so mid-turn exchanges block on the terminal editor even though LiveExchangeBroker and session.answerExchange are wired. Web-side session.answerExchange then resolves an unused broker while the tool never awaits it, so live web answering fails in topology A.

^{Reviewed by Cursor Bugbot for commit 1c702f1. Configure here.}

cursor · 2026-06-17T17:45:49Z

+        return createJsonRpcFailure(requestId, -32010, NO_LIVE_AGENT_SESSION_DRIVER_MESSAGE);
+      }
+      return createJsonRpcSuccess(requestId, { status: 'completed' });
+    },


Sidecar allows observer driveTurn

Medium Severity

Every WebSocket client on the TUI sidecar shares the same handler registry, so session.driveTurn is callable from any connection. The fan-out acceptance model treats extra clients as read-only observers, but this method is not restricted to a single driver role and there is no rejection path for observer sockets.

^{Reviewed by Cursor Bugbot for commit 1c702f1. Configure here.}

This was referenced Jun 16, 2026

FE-870: Renderer golden coverage and agent-context tools #217

Merged

FE-868: Pi runtime rehabilitation and dependency alignment #215

Merged

lunelson mentioned this pull request Jun 16, 2026

FE-869: Runtime topology and prompt-resource ownership cleanup #216

Merged

lunelson changed the base branch from ln/fe-870-renderer-golden-context-tools to graphite-base/225 June 16, 2026 16:48

lunelson marked this pull request as ready for review June 17, 2026 08:09

Copilot AI review requested due to automatic review settings June 17, 2026 08:09

Copilot started reviewing on behalf of lunelson June 17, 2026 08:09 View session

Copilot AI reviewed Jun 17, 2026

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Comment thread src/rpc/websocket.ts

Comment on lines +39 to +41

const unsubscribeSessionEvents = options.sessionEvents?.subscribe((frame) => {

publishNotification(JSON.stringify(frame));

});

lunelson added 4 commits June 17, 2026 18:24

update agent system prompt paths

414ec13

FE-873: reconcile web-driver plan after restack

1c702f1

lunelson force-pushed the graphite-base/225 branch from ec02a3d to e50a407 Compare June 17, 2026 17:44

lunelson force-pushed the ln/fe-873-web-as-driver branch from 291c5d9 to 1c702f1 Compare June 17, 2026 17:44

lunelson changed the base branch from graphite-base/225 to ln/fe-870-renderer-ii June 17, 2026 17:44

lunelson mentioned this pull request Jun 17, 2026

FE-870: Renderer golden coverage and agent-context tools #228

Merged

cursor Bot reviewed Jun 17, 2026

View reviewed changes

lunelson changed the title ~~FE-873: observer relay seam for live AgentSession event streaming (topology A)~~ FE-873: Web-as-driver streaming chat transport (topology A) Jun 17, 2026

Base automatically changed from ln/fe-870-renderer-ii to next June 17, 2026 20:20

lunelson merged commit 8c3748f into next Jun 17, 2026
18 checks passed

lunelson deleted the ln/fe-873-web-as-driver branch June 17, 2026 20:20

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FE-873: Web-as-driver streaming chat transport (topology A)#225

FE-873: Web-as-driver streaming chat transport (topology A)#225
lunelson merged 4 commits into
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Broker bypassed when TUI UI

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Sidecar allows observer driveTurn

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lunelson commented Jun 16, 2026 •

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