fix(sdk): chat.agent transcript follow-ups

- Drop the compaction prefix fingerprint. A compacted lane is reused while
  its throughId still exists in the transcript, so a rollback that crosses
  the compaction point rebuilds from the transcript and an in-place edit of
  a summarized message is tolerated (unsupported by design). A custom
  TranscriptStorage no longer has to preserve exact message JSON for a
  compaction to survive.
- Skip persisting a content-less assistant response, so a turn that errors
  before the model writes anything no longer leaves an empty assistant
  bubble in the transcript or the next turn's context.
- Fall back to a generic error message when a thrown error has no message,
  instead of surfacing a blank error to the client.
- The test harness seedSessionInTail stamps the harness clientData onto
  seeded records, matching how production preserves each record's metadata,
  so recovery re-dispatch is testable for clientDataSchema agents.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VG39FXXkFFU24U5EtJMwPi

Author: ericallam

.changeset/transcript-storage-followups.md

@@ -0,0 +1,5 @@
+---
+"@trigger.dev/sdk": patch
+---
+
+chat.agent transcript fixes: a turn that errors before the model produces any content no longer stores an empty assistant message, an error thrown without a message now shows a generic error instead of a blank one, and a custom transcript storage no longer needs to preserve exact message JSON for a compaction to survive a continuation.

packages/trigger-sdk/src/v3/ai.ts

@@ -81,7 +81,6 @@ import {
   defaultStorage,
   diffTranscript,
   parseTranscriptRuntimeState,
-  prefixFingerprint,
   restoreModelLane,
   type TranscriptChange,
   type TranscriptChangeReason,
@@ -1350,7 +1349,8 @@ async function reportChatCustomAgentClientDataError(
   error: unknown,
   options: { writeToStream: boolean; callHandler?: boolean }
 ): Promise<void> {
-  const errorText = error instanceof Error ? error.message : "An unexpected error occurred";
+  const errorText =
+    error instanceof Error && error.message ? error.message : "An unexpected error occurred";
   logger.warn("chat.customAgent: clientData validation failed", {
     chatId: payload.chatId,
     trigger: payload.trigger,
@@ -7171,7 +7171,6 @@ function chatAgent<
                       compaction: {
                         modelMessages: accumulatedMessages,
                         throughId,
-                        fingerprint: prefixFingerprint(shadow, throughId),
                       },
                     }
                   : {}),
@@ -9175,56 +9174,63 @@ function chatAgent<
                       } as TUIMessage;
                       locals.set(chatResponsePartsKey, []);
                     }
-                    // Tool-approval continuations: the AI SDK reuses the trailing
-                    // assistant's ID (via originalMessages) so the captured response
-                    // carries the same ID as an existing message. Replace in place
-                    // instead of pushing a duplicate. For action turns this never
-                    // matches because originalMessages is omitted (fresh ID).
-                    const existingIdx = capturedResponseMessage.id
-                      ? accumulatedUIMessages.findIndex((m) => m.id === capturedResponseMessage!.id)
-                      : -1;
-                    const previousAtIdx =
-                      existingIdx !== -1 ? accumulatedUIMessages[existingIdx] : undefined;
-                    if (existingIdx !== -1) {
-                      accumulatedUIMessages[existingIdx] = capturedResponseMessage;
-                    } else {
-                      accumulatedUIMessages.push(capturedResponseMessage);
-                    }
-                    turnNewUIMessages.push(capturedResponseMessage);
-                    locals.set(chatCurrentUIMessagesKey, accumulatedUIMessages);
-                    // Record toolCallId → head messageId so a HITL
-                    // continuation next turn can recover the head id
-                    // even if the AI SDK regenerates it. See
-                    // `chatToolCallToMessageIdKey` for the full
-                    // rationale (TRI-9137).
-                    recordToolCallIdsFromMessage(capturedResponseMessage);
-                    try {
-                      const responseModelMessages = await toModelMessages([
-                        stripProviderMetadata(capturedResponseMessage),
-                      ]);
+                    const responseHasContent = capturedResponseMessage.parts.some(
+                      (part) => part.type !== "step-start"
+                    );
+                    if (responseHasContent) {
+                      // Tool-approval continuations: the AI SDK reuses the trailing
+                      // assistant's ID (via originalMessages) so the captured response
+                      // carries the same ID as an existing message. Replace in place
+                      // instead of pushing a duplicate. For action turns this never
+                      // matches because originalMessages is omitted (fresh ID).
+                      const existingIdx = capturedResponseMessage.id
+                        ? accumulatedUIMessages.findIndex(
+                            (m) => m.id === capturedResponseMessage!.id
+                          )
+                        : -1;
+                      const previousAtIdx =
+                        existingIdx !== -1 ? accumulatedUIMessages[existingIdx] : undefined;
                       if (existingIdx !== -1) {
-                        const ok =
-                          previousAtIdx !== undefined &&
-                          (await replaceModelRun(
-                            accumulatedMessages,
-                            previousAtIdx,
-                            capturedResponseMessage,
-                            steerTailThisTurn
-                          ));
-                        if (!ok) {
-                          logger.warn(
-                            "chat.agent: replaced response not found at the model lane tail; reconverting the lane"
-                          );
-                          accumulatedMessages = await toModelMessages(accumulatedUIMessages);
-                          laneCompacted = false;
-                          laneInjections = [];
-                        }
+                        accumulatedUIMessages[existingIdx] = capturedResponseMessage;
                       } else {
-                        accumulatedMessages.push(...responseModelMessages);
+                        accumulatedUIMessages.push(capturedResponseMessage);
+                      }
+                      turnNewUIMessages.push(capturedResponseMessage);
+                      locals.set(chatCurrentUIMessagesKey, accumulatedUIMessages);
+                      // Record toolCallId → head messageId so a HITL
+                      // continuation next turn can recover the head id
+                      // even if the AI SDK regenerates it. See
+                      // `chatToolCallToMessageIdKey` for the full
+                      // rationale (TRI-9137).
+                      recordToolCallIdsFromMessage(capturedResponseMessage);
+                      try {
+                        const responseModelMessages = await toModelMessages([
+                          stripProviderMetadata(capturedResponseMessage),
+                        ]);
+                        if (existingIdx !== -1) {
+                          const ok =
+                            previousAtIdx !== undefined &&
+                            (await replaceModelRun(
+                              accumulatedMessages,
+                              previousAtIdx,
+                              capturedResponseMessage,
+                              steerTailThisTurn
+                            ));
+                          if (!ok) {
+                            logger.warn(
+                              "chat.agent: replaced response not found at the model lane tail; reconverting the lane"
+                            );
+                            accumulatedMessages = await toModelMessages(accumulatedUIMessages);
+                            laneCompacted = false;
+                            laneInjections = [];
+                          }
+                        } else {
+                          accumulatedMessages.push(...responseModelMessages);
+                        }
+                        turnNewModelMessages.push(...responseModelMessages);
+                      } catch {
+                        // Conversion failed — skip accumulation for this turn
                       }
-                      turnNewModelMessages.push(...responseModelMessages);
-                    } catch {
-                      // Conversion failed — skip accumulation for this turn
                     }
                   }
                   // If there's no captured response (manual pipe mode) but there are
@@ -9757,7 +9763,9 @@ function chatAgent<
             try {
               await withChatWriter(async (writer) => {
                 const errorText =
-                  turnError instanceof Error ? turnError.message : "An unexpected error occurred";
+                  turnError instanceof Error && turnError.message
+                    ? turnError.message
+                    : "An unexpected error occurred";
                 writer.write({ type: "error", errorText } as any);
               });
               // Signal turn complete so the client knows this turn is done

packages/trigger-sdk/src/v3/test/mock-chat-agent.ts

@@ -496,7 +496,7 @@ export function mockChatAgent(
   __setReplaySessionInTailImplForTests(async () => {
     return seededSessionInMessages.map((message, i) => ({
       message,
-      metadata: undefined,
+      metadata: clientData,
       seqNum: i + 1,
     })) as never;
   });

packages/trigger-sdk/src/v3/transcriptStorage.ts

@@ -405,15 +405,17 @@ type ModelLaneInjection = { afterId: string; messages: ModelMessage[] };
  * model's context that cannot be rebuilt from the transcript. Opaque to a
  * storage; only the runtime reads it.
  *
- * `compaction` is the whole model lane after a compaction, valid for the
- * transcript prefix ending at `throughId` whose fingerprint matches, so a
- * rollback or edit of that prefix makes it unusable and the next save
- * clears it. `injections` are conversational messages `chat.inject` added,
- * anchored after the transcript message they followed.
+ * `compaction` is the whole model lane after a compaction, covering the
+ * transcript prefix ending at `throughId`. It is used as long as `throughId`
+ * still exists in the transcript, so a rollback or truncation that removes it
+ * rebuilds the lane from the transcript instead. Editing a message before
+ * `throughId` in place is unsupported and does not invalidate the lane.
+ * `injections` are conversational messages `chat.inject` added, anchored
+ * after the transcript message they followed.
  */
 export type TranscriptRuntimeState = {
   v: 1;
-  compaction?: { modelMessages: ModelMessage[]; throughId: string; fingerprint: string };
+  compaction?: { modelMessages: ModelMessage[]; throughId: string };
   injections?: ModelLaneInjection[];
   /** `chat.inject` messages queued but not yet drained into a turn when the save happened. */
   queued?: ModelMessage[];
@@ -429,13 +431,11 @@ export function parseTranscriptRuntimeState(value: unknown): TranscriptRuntimeSt
     compaction &&
     typeof compaction === "object" &&
     Array.isArray(compaction.modelMessages) &&
-    typeof compaction.throughId === "string" &&
-    typeof compaction.fingerprint === "string"
+    typeof compaction.throughId === "string"
   ) {
     out.compaction = {
       modelMessages: compaction.modelMessages as ModelMessage[],
       throughId: compaction.throughId,
-      fingerprint: compaction.fingerprint,
     };
   }
   if (Array.isArray(record.injections)) {
@@ -452,35 +452,11 @@ export function parseTranscriptRuntimeState(value: unknown): TranscriptRuntimeSt
   return out;
 }
 
-/**
- * A 32-bit FNV-1a hash over the fingerprints of the messages up to and
- * including `throughId`, in order. Cheap enough to compute on every save
- * because the per-message fingerprints already exist in the shadow.
- */
-export function prefixFingerprint(shadow: TranscriptShadow, throughId: string): string {
-  let hash = 0x811c9dc5;
-  if (throughId === "") return hash.toString(16).padStart(8, "0");
-  const mix = (s: string) => {
-    for (let i = 0; i < s.length; i++) {
-      hash ^= s.charCodeAt(i);
-      hash = Math.imul(hash, 0x01000193) >>> 0;
-    }
-  };
-  for (const id of shadow.ids) {
-    mix(id);
-    mix("");
-    mix(shadow.fingerprints.get(id) ?? "");
-    mix("
");
-    if (id === throughId) return hash.toString(16).padStart(8, "0");
-  }
-  return "";
-}
-
 /**
  * Rebuild the model lane for a transcript at boot. Uses the persisted
- * compacted lane when the transcript prefix it covers is unchanged, then
- * converts the rest of the transcript, re-inserting persisted injections
- * after the messages they followed.
+ * compacted lane while the transcript still contains its `throughId`
+ * boundary, then converts the rest of the transcript, re-inserting persisted
+ * injections after the messages they followed.
  */
 export async function restoreModelLane<TUIMessage extends UIMessage>(
   messages: TUIMessage[],
@@ -494,11 +470,7 @@ export async function restoreModelLane<TUIMessage extends UIMessage>(
   if (state?.compaction) {
     const throughId = state.compaction.throughId;
     const idx = throughId === "" ? -1 : messages.findIndex((m) => m.id === throughId);
-    if (
-      (idx !== -1 || throughId === "") &&
-      prefixFingerprint(createTranscriptShadow(messages), throughId) ===
-        state.compaction.fingerprint
-    ) {
+    if (idx !== -1 || throughId === "") {
       lane.push(...state.compaction.modelMessages);
       start = idx + 1;
       compacted = true;

packages/trigger-sdk/test/recovery-boot.test.ts

@@ -7,6 +7,7 @@ import { simulateReadableStream, streamText } from "ai";
 import { MockLanguageModelV3 } from "ai/test";
 import { TestSessionStreamManager } from "@trigger.dev/core/v3/test";
 import { describe, expect, it, vi } from "vitest";
+import { z } from "zod/v4";
 import type { RecoveryBootEvent, RecoveryBootResult } from "../src/v3/ai.js";
 import { __setReplaySessionOutTailImplForTests, chat } from "../src/v3/ai.js";
 
@@ -609,4 +610,33 @@ describe("continuation boot — the message that resumed the run", () => {
       await harness.close();
     }
   });
+
+  it("re-dispatches a recovered in-flight user for a clientData-scoped agent", async () => {
+    let modelCalls = 0;
+    const model = new MockLanguageModelV3({
+      doStream: async () => {
+        modelCalls++;
+        return { stream: textStream("answered") };
+      },
+    });
+    const u1 = userMessage("the interrupted question", "u-1");
+    const agent = chat.agent({
+      id: "recovery-boot.clientdata-scoped",
+      clientDataSchema: z.object({ userId: z.string() }),
+      run: async ({ messages, signal }) => streamText({ model, messages, abortSignal: signal }),
+    });
+    const harness = mockChatAgent(agent, {
+      chatId: "clientdata-scoped",
+      continuation: true,
+      previousRunId: "run_prior",
+      clientData: { userId: "u_123" },
+    });
+    harness.seedSessionInTail([u1 as never]);
+    try {
+      await new Promise((r) => setTimeout(r, 100));
+      expect(modelCalls).toBe(1);
+    } finally {
+      await harness.close();
+    }
+  });
 });