triggerdotdev · ericallam · May 16, 2026
diff --git a/apps/webapp/app/components/runs/v3/agent/AgentView.tsx b/apps/webapp/app/components/runs/v3/agent/AgentView.tsx
@@ -1,5 +1,5 @@
 import type { UIMessage } from "@ai-sdk/react";
-import { SSEStreamSubscription } from "@trigger.dev/core/v3";
+import { ChatSnapshotV1Schema, SSEStreamSubscription } from "@trigger.dev/core/v3";
 import { useEffect, useMemo, useRef, useState } from "react";
 import { Paragraph } from "~/components/primitives/Paragraph";
 import { Spinner } from "~/components/primitives/Spinner";
@@ -27,6 +27,15 @@ export type AgentViewAuth = {
    * channel and is merged in by the AgentView subscription.
    */
   initialMessages: UIMessage[];
+  /**
+   * Presigned GET URL for the session's chat-snapshot S3 blob (written
+   * by the agent after each turn-complete; see `ChatSnapshotV1`).
+   * Optional — sessions that registered a `hydrateMessages` hook skip
+   * snapshot writes and the URL fetch will 404. In that case the
+   * dashboard falls back to seq=0 SSE (which, post-trim, shows only the
+   * most recent turn). Generated server-side by `SessionPresenter`.
+   */
+  snapshotPresignedUrl?: string;
 };
 
 /**
@@ -81,6 +90,7 @@ export function AgentView({ agentView }: { agentView: AgentViewAuth }) {
     projectSlug: project.slug,
     envSlug: environment.slug,
     initialMessages: agentView.initialMessages,
+    snapshotPresignedUrl: agentView.snapshotPresignedUrl,
   });
 
   // Sticky-bottom auto-scroll: walks up to find the inspector's scroll
@@ -120,13 +130,19 @@ export function AgentView({ agentView }: { agentView: AgentViewAuth }) {
  * - `kind: "stop"` is a stop signal — no messages, nothing to render
  *   here, so it's filtered.
  *
+ * Wire payloads are slim-wire (one new UIMessage per record, on
+ * `payload.message`). The legacy `payload.messages` array shape is kept
+ * here as a fallback so any historical records on a long-lived session
+ * still render.
+ *
  * The server wraps records in `{data, id}` and writes `data` as a JSON
  * string; SSE v2 delivers the parsed string back. {@link parseChunkPayload}
  * re-parses to recover the object.
  */
 type InputStreamChunk = {
   kind?: "message" | "stop";
   payload?: {
+    message?: { id?: string; role?: string; parts?: unknown[] };
     messages?: Array<{ id?: string; role?: string; parts?: unknown[] }>;
     trigger?: string;
   };
@@ -217,13 +233,15 @@ function useAgentSessionMessages({
   projectSlug,
   envSlug,
   initialMessages,
+  snapshotPresignedUrl,
 }: {
   sessionId: string;
   apiOrigin: string;
   orgSlug: string;
   projectSlug: string;
   envSlug: string;
   initialMessages: UIMessage[];
+  snapshotPresignedUrl?: string;
 }): UIMessage[] {
   // Seed with the user messages from the run's task payload.
   const seedMessages = useMemo(
@@ -285,27 +303,92 @@ function useAgentSessionMessages({
     const outputUrl = `${sessionBase}/out`;
     const inputUrl = `${sessionBase}/in`;
 
+    /**
+     * Try to seed `pendingRef` from the agent's S3 snapshot blob and return
+     * the snapshot's `lastOutEventId` so the `.out` SSE subscription resumes
+     * just past the snapshot. Returns undefined for sessions that don't
+     * have a snapshot (e.g. `hydrateMessages` customers, or sessions that
+     * have never completed a turn).
+     */
+    const loadSnapshot = async (): Promise<string | undefined> => {
+      if (!snapshotPresignedUrl) return undefined;
+      try {
+        const resp = await fetch(snapshotPresignedUrl, { signal: abort.signal });
+        if (!resp.ok) return undefined;
+        const json = (await resp.json()) as unknown;
+        const parsed = ChatSnapshotV1Schema.safeParse(json);
+        if (!parsed.success) return undefined;
+        const snapshot = parsed.data;
+        // Preserve the snapshot's array order in the final render by
+        // giving each message a unique, monotonically increasing
+        // timestamp from `(savedAt - count + index)`. Real chunk
+        // timestamps from the SSE path use S2 arrival ms (positive
+        // numbers in the present), so anything below `savedAt` sorts
+        // before live chunks while preserving snapshot order among
+        // themselves.
+        const count = snapshot.messages.length;
+        snapshot.messages.forEach((raw, i) => {
+          const message = raw as UIMessage;
+          if (!message?.id) return;
+          // The snapshot's seed wins over the task-payload seed for any
+          // overlapping ids (the snapshot represents the agent's
+          // canonical accumulator, post-turn).
+          pendingRef.current.set(message.id, message);
+          if (!timestampsRef.current.has(message.id)) {
+            timestampsRef.current.set(message.id, snapshot.savedAt - count + i);
+          }
+        });
+        scheduleFlush.current();
+        return snapshot.lastOutEventId;
+      } catch {
+        // 404 / network / parse / abort — fall back to seq=0 SSE
+        return undefined;
+      }
+    };
+
+    const outputSubOptions = (lastEventId: string | undefined) =>
+      ({
+        signal: abort.signal,
+        timeoutInSeconds: 120,
+        ...(lastEventId !== undefined ? { lastEventId } : {}),
+      }) as const;
+
     const commonSubOptions = {
       signal: abort.signal,
       timeoutInSeconds: 120,
     } as const;
 
     // ---- Output stream: assistant messages ---------------------------------
     //
-    // The output stream delivers UIMessageChunks interleaved with
-    // Trigger-specific control chunks (`trigger:turn-complete`, etc.). We
-    // filter the control chunks and fold everything else into an assistant
-    // `UIMessage` via our own `applyOutputChunk` accumulator — the AI SDK's
-    // `readUIMessageStream` helper is only available in `ai@6`, and the
-    // webapp is pinned to `ai@4`, so we re-implement just the chunk types
-    // that `renderPart` actually displays.
+    // The output stream delivers data records (UIMessageChunks) interleaved
+    // with Trigger control records (`turn-complete`, `upgrade-required`) and
+    // S2 command records (`trim`). Control + command records ride on
+    // `record.headers` with empty bodies; the SSE parser strips S2 command
+    // records entirely, and control records arrive with `value.chunk ===
+    // undefined`, which `parseChunkPayload` drops below.
+    //
+    // We fold everything else into an assistant `UIMessage` via our own
+    // `applyOutputChunk` accumulator — the AI SDK's `readUIMessageStream`
+    // helper is only available in `ai@6`, and the webapp is pinned to
+    // `ai@4`, so we re-implement just the chunk types that `renderPart`
+    // actually displays.
     //
     // We capture the **server timestamp of each assistant message's first
     // `start` chunk** so later sort-by-timestamp merges with the input
     // stream correctly.
     const runOutput = async () => {
       try {
-        const sub = new SSEStreamSubscription(outputUrl, commonSubOptions);
+        // Seed messages from the snapshot first (if available), then
+        // resume the SSE from the snapshot's last event id so we don't
+        // re-stream chunks already represented in the snapshot. If no
+        // snapshot exists (no URL, 404, parse failure), the SSE opens
+        // at seq=0 — which, post-trim, contains roughly one turn of
+        // records (acceptable fallback for `hydrateMessages` sessions
+        // and fresh sessions).
+        const snapshotLastEventId = await loadSnapshot();
+        if (abort.signal.aborted) return;
+
+        const sub = new SSEStreamSubscription(outputUrl, outputSubOptions(snapshotLastEventId));
         const raw = await sub.subscribe();
         const reader = raw.getReader();
 
@@ -318,6 +401,12 @@ function useAgentSessionMessages({
 
             const chunk = parseChunkPayload(value.chunk) as OutputChunk | null;
             if (!chunk || typeof chunk.type !== "string") continue;
+            // Legacy belt-and-suspenders: prior versions of the SDK
+            // emitted `trigger:turn-complete` / `trigger:upgrade-required`
+            // as data records (`type` field). Current versions use
+            // header-form control records, which `parseChunkPayload`
+            // drops above. Keep this filter to handle any in-flight
+            // sessions whose `.out` was populated by the older SDK.
             if (chunk.type.startsWith("trigger:")) continue;
 
             if (chunk.type === "start") {
@@ -413,9 +502,18 @@ function useAgentSessionMessages({
             const chunk = parseChunkPayload(value.chunk) as InputStreamChunk | null;
             if (!chunk || chunk.kind !== "message") continue;
             const payload = chunk.payload;
-            if (!payload || !Array.isArray(payload.messages)) continue;
-
-            const incomingUsers = payload.messages.filter(
+            if (!payload) continue;
+
+            // Slim-wire is one UIMessage on `payload.message`; legacy
+            // payloads carried an array on `payload.messages`. Accept
+            // either so historical records on a long-lived session still
+            // render.
+            const candidates = Array.isArray(payload.messages)
+              ? payload.messages
+              : payload.message
+                ? [payload.message]
+                : [];
+            const incomingUsers = candidates.filter(
               (m): m is UIMessage =>
                 m != null && (m as { role?: string }).role === "user" && typeof m.id === "string"
             );
@@ -454,7 +552,7 @@ function useAgentSessionMessages({
         pendingTimerRef.current = null;
       }
     };
-  }, [sessionId, apiOrigin, orgSlug, projectSlug, envSlug]);
+  }, [sessionId, apiOrigin, orgSlug, projectSlug, envSlug, snapshotPresignedUrl]);
 
   return useMemo(() => {
     const timestamps = timestampsRef.current;

diff --git a/apps/webapp/app/presenters/v3/SessionPresenter.server.ts b/apps/webapp/app/presenters/v3/SessionPresenter.server.ts
@@ -1,8 +1,11 @@
 import { type Span } from "@opentelemetry/api";
+import { chatSnapshotKeySuffix } from "@trigger.dev/core/v3";
 import { type PrismaClientOrTransaction } from "@trigger.dev/database";
 import { env } from "~/env.server";
 import { findDisplayableEnvironment } from "~/models/runtimeEnvironment.server";
 import { resolveSessionByIdOrExternalId } from "~/services/realtime/sessions.server";
+import { logger } from "~/services/logger.server";
+import { generatePresignedUrl } from "~/v3/objectStore.server";
 import { ServiceValidationError } from "~/v3/services/baseService.server";
 import { startActiveSpan } from "~/v3/tracer.server";
 
@@ -15,6 +18,8 @@ export class SessionPresenter {
     userId: string;
     environmentId: string;
     sessionParam: string;
+    projectExternalRef: string;
+    environmentSlug: string;
   }) {
     return startActiveSpan(
       "SessionPresenter.call",
@@ -33,10 +38,14 @@ export class SessionPresenter {
       userId,
       environmentId,
       sessionParam,
+      projectExternalRef,
+      environmentSlug,
     }: {
       userId: string;
       environmentId: string;
       sessionParam: string;
+      projectExternalRef: string;
+      environmentSlug: string;
     },
     rootSpan: Span
   ) {
@@ -112,6 +121,48 @@ export class SessionPresenter {
     // unused — kept here to match the existing `AgentViewAuth` shape.
     const addressingKey = session.externalId ?? session.friendlyId;
 
+    // Presign a GET URL for the agent's S3 snapshot blob. The browser
+    // fetches it directly, parses + validates, and seeds the
+    // TriggerChatTransport with the full history + lastEventId before
+    // opening the SSE. Presign succeeds regardless of whether the blob
+    // exists; the frontend handles 404 gracefully.
+    //
+    // Snapshots are only written when no `hydrateMessages` hook is
+    // registered — sessions that use `hydrateMessages` will 404 here
+    // and the dashboard falls back to seq=0 SSE (which, post-trim,
+    // shows only the most recent turn — accepted, those customers
+    // have their own DB-backed dashboards).
+    // The agent writes snapshots keyed on the session's friendlyId (the
+    // `session_*` form), which matches what the SDK's `chat.agent` payload
+    // carries as `sessionId`. Use the same key shape here so the dashboard
+    // hits the same S3 object.
+    let snapshotPresignedUrl: string | undefined;
+    try {
+      const signed = await startActiveSpan(
+        "SessionPresenter.presignSnapshot",
+        async () =>
+          generatePresignedUrl(
+            projectExternalRef,
+            environmentSlug,
+            chatSnapshotKeySuffix(session.friendlyId),
+            "GET"
+          )
+      );
+      if (signed.success) {
+        snapshotPresignedUrl = signed.url;
+      } else {
+        logger.warn("SessionPresenter: snapshot presign failed", {
+          sessionId: session.id,
+          error: signed.error,
+        });
+      }
+    } catch (error) {
+      logger.warn("SessionPresenter: snapshot presign threw", {
+        sessionId: session.id,
+        error: error instanceof Error ? error.message : String(error),
+      });
+    }
+
     return {
       id: session.id,
       friendlyId: session.friendlyId,
@@ -147,6 +198,7 @@ export class SessionPresenter {
         apiOrigin: env.API_ORIGIN || env.LOGIN_ORIGIN,
         sessionId: addressingKey,
         initialMessages: [],
+        snapshotPresignedUrl,
       },
     };
   }

diff --git a/...s.$organizationSlug.projects.$projectParam.env.$envParam.sessions.$sessionParam/route.tsx b/...s.$organizationSlug.projects.$projectParam.env.$envParam.sessions.$sessionParam/route.tsx
@@ -79,6 +79,8 @@ export const loader = async ({ request, params }: LoaderFunctionArgs) => {
     userId,
     environmentId: environment.id,
     sessionParam,
+    projectExternalRef: project.externalRef,
+    environmentSlug: environment.slug,
   });
 
   if (!session) {