fix(sdk): preserve compaction and avoid double-persist on the error path

When committing the errored turn's state to the accumulator, append only the
new tail's model messages (errored user message and/or recovered partial)
instead of reconverting the whole UI history, so a prior turn's model-only
compaction survives an errored turn. Skip re-adding the partial once the
response was already committed (a later compaction may have folded it into a
summary), and roll the event/snapshot views back if the model conversion
fails so they never advertise a partial the accumulator didn't take.

Author: matt-aitken

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

@@ -7956,19 +7956,18 @@ function chatAgent<
             if (partialResponse && !partialResponse.id) {
               partialResponse = { ...partialResponse, id: generateMessageId() } as TUIMessage;
             }
-            const erroredUIMessagesWithPartial: TUIMessage[] = !partialResponse
+            const includePartial = partialResponse != null && !responseCommitted;
+            let erroredUIMessagesWithPartial: TUIMessage[] = !includePartial
               ? erroredUIMessages
               : partialIdx === -1
                 ? [...erroredUIMessages, partialResponse]
                 : (erroredUIMessages.map((m, i) =>
                     i === partialIdx ? partialResponse : m
                   ) as TUIMessage[]);
 
-            const erroredNewUIMessages: TUIMessage[] = erroredWireMessage
-              ? [erroredWireMessage]
-              : [];
-            if (partialResponse && !responseCommitted) {
-              erroredNewUIMessages.push(partialResponse);
+            let erroredNewUIMessages: TUIMessage[] = erroredWireMessage ? [erroredWireMessage] : [];
+            if (includePartial) {
+              erroredNewUIMessages.push(partialResponse!);
             }
 
             let erroredNewModelMessages: ModelMessage[] = [];
@@ -7984,18 +7983,17 @@ function chatAgent<
             // the success path's append branch); otherwise reconvert from UI.
             // Guard the conversion so a secondary failure can't crash the run.
             if (!responseCommitted && erroredUIMessagesWithPartial !== accumulatedUIMessages) {
-              const onlyAppendedPartial =
-                partialResponse != null &&
-                partialIdx === -1 &&
-                erroredUIMessages === accumulatedUIMessages;
               try {
-                if (partialResponse) {
+                if (includePartial) {
                   erroredNewModelMessages = await toModelMessages([
-                    stripProviderMetadata(partialResponse),
+                    stripProviderMetadata(partialResponse!),
                   ]);
                 }
-                if (onlyAppendedPartial) {
-                  accumulatedMessages.push(...erroredNewModelMessages);
+                if (partialIdx === -1) {
+                  const appended = erroredUIMessagesWithPartial.slice(accumulatedUIMessages.length);
+                  accumulatedMessages.push(
+                    ...(await toModelMessages(appended.map((m) => stripProviderMetadata(m))))
+                  );
                 } else {
                   accumulatedMessages = await toModelMessages(erroredUIMessagesWithPartial);
                 }
@@ -8004,6 +8002,8 @@ function chatAgent<
               } catch {
                 // Keep the prior model accumulator if conversion fails.
                 erroredNewModelMessages = [];
+                erroredUIMessagesWithPartial = erroredUIMessages;
+                erroredNewUIMessages = erroredWireMessage ? [erroredWireMessage] : [];
               }
             }