🤖 fix: propagate backend startTime to fix negative TTFT on replay

When the frontend reconnected to an active stream (via replayStream()),
timestamps were mismatched:
- Frontend set startTime = Date.now() when receiving stream-start
- Backend sent delta timestamps from when they were originally created
- Result: firstTokenTime < startTime = negative TTFT

Fixed by adding startTime to StreamStartEvent schema and propagating
the backend's original stream start time to the frontend.

This also fixes inflated tool execution time during replay, which was
caused by pendingToolStarts containing historical timestamps while
getActiveStreamTimingStats() calculated duration using Date.now().

Change-Id: I67813d7127913c1aa19894a81c7b330953383be4
Signed-off-by: Thomas Kosiewski <[email protected]>

Author: ThomasK33

src/browser/stores/WorkspaceStore.test.ts

@@ -328,6 +328,7 @@ describe("WorkspaceStore", () => {
           messageId: "msg1",
           model: "claude-opus-4",
           workspaceId: "test-workspace",
+          startTime: Date.now(),
         };
         await new Promise<void>((resolve) => {
           setTimeout(resolve, 10);
@@ -433,6 +434,7 @@ describe("WorkspaceStore", () => {
           messageId: "msg1",
           model: "claude-sonnet-4",
           workspaceId: "test-workspace",
+          startTime: Date.now(),
         };
         await new Promise<void>((resolve) => {
           setTimeout(resolve, 10);
@@ -476,6 +478,7 @@ describe("WorkspaceStore", () => {
           messageId: "msg1",
           model: "claude-sonnet-4",
           workspaceId: "test-workspace",
+          startTime: Date.now(),
         };
         await new Promise<void>((resolve) => {
           setTimeout(resolve, 10);

src/browser/stories/mockFactory.ts

@@ -605,6 +605,7 @@ export function createStreamingChatHandler(opts: {
         messageId: opts.streamingMessageId,
         model: opts.model,
         historySequence: opts.historySequence,
+        startTime: Date.now(),
       });
 
       // Send text delta if provided

src/browser/utils/messages/ChatEventProcessor.test.ts

@@ -13,6 +13,7 @@ describe("ChatEventProcessor - Reasoning Delta", () => {
       messageId,
       model: "gpt-4",
       historySequence: 1,
+      startTime: Date.now(),
     });
 
     // Send reasoning deltas
@@ -69,6 +70,7 @@ describe("ChatEventProcessor - Reasoning Delta", () => {
       messageId,
       model: "gpt-4",
       historySequence: 1,
+      startTime: Date.now(),
     });
 
     // Reasoning 1

src/browser/utils/messages/StreamingMessageAggregator.status.test.ts

@@ -113,6 +113,7 @@ describe("StreamingMessageAggregator - Agent Status", () => {
       messageId,
       model: "test-model",
       historySequence: 1,
+      startTime: Date.now(),
     });
 
     // Add a status_set tool call
@@ -155,6 +156,7 @@ describe("StreamingMessageAggregator - Agent Status", () => {
       messageId,
       model: "test-model",
       historySequence: 1,
+      startTime: Date.now(),
     });
 
     // First status_set
@@ -218,6 +220,7 @@ describe("StreamingMessageAggregator - Agent Status", () => {
       messageId,
       model: "test-model",
       historySequence: 1,
+      startTime: Date.now(),
     });
 
     // Set status
@@ -269,6 +272,7 @@ describe("StreamingMessageAggregator - Agent Status", () => {
       messageId,
       model: "test-model",
       historySequence: 1,
+      startTime: Date.now(),
     });
 
     // Add a status_set tool call
@@ -308,6 +312,7 @@ describe("StreamingMessageAggregator - Agent Status", () => {
       messageId: "msg1",
       model: "test-model",
       historySequence: 1,
+      startTime: Date.now(),
     });
 
     aggregator.handleToolCallStart({
@@ -370,6 +375,7 @@ describe("StreamingMessageAggregator - Agent Status", () => {
       messageId,
       model: "test-model",
       historySequence: 1,
+      startTime: Date.now(),
     });
 
     // Add a status_set tool call with invalid emoji
@@ -429,6 +435,7 @@ describe("StreamingMessageAggregator - Agent Status", () => {
       messageId,
       model: "test-model",
       historySequence: 1,
+      startTime: Date.now(),
     });
 
     // Add a successful status_set tool call
@@ -672,6 +679,7 @@ describe("StreamingMessageAggregator - Agent Status", () => {
       messageId,
       model: "test-model",
       historySequence: 1,
+      startTime: Date.now(),
     });
 
     // Status_set with long message (would be truncated by backend)
@@ -717,6 +725,7 @@ describe("StreamingMessageAggregator - Agent Status", () => {
       messageId,
       model: "test-model",
       historySequence: 1,
+      startTime: Date.now(),
     });
 
     // Add a status_set tool call with URL
@@ -761,6 +770,7 @@ describe("StreamingMessageAggregator - Agent Status", () => {
       messageId,
       model: "test-model",
       historySequence: 1,
+      startTime: Date.now(),
     });
 
     // First status with URL
@@ -855,6 +865,7 @@ describe("StreamingMessageAggregator - Agent Status", () => {
       messageId: messageId1,
       model: "test-model",
       historySequence: 1,
+      startTime: Date.now(),
     });
 
     // Set status with URL in first stream
@@ -902,6 +913,7 @@ describe("StreamingMessageAggregator - Agent Status", () => {
       messageId: messageId2,
       model: "test-model",
       historySequence: 2,
+      startTime: Date.now(),
     });
 
     // Set new status WITHOUT URL - should use the last URL ever seen

src/browser/utils/messages/StreamingMessageAggregator.test.ts

@@ -149,6 +149,7 @@ describe("StreamingMessageAggregator", () => {
         messageId: "msg1",
         historySequence: 1,
         model: "claude-3-5-sonnet-20241022",
+        startTime: Date.now(),
       });
 
       // Simulate todo_write tool call
@@ -208,6 +209,7 @@ describe("StreamingMessageAggregator", () => {
         messageId: "msg1",
         historySequence: 1,
         model: "claude-3-5-sonnet-20241022",
+        startTime: Date.now(),
       });
 
       // Simulate todo_write
@@ -343,6 +345,7 @@ describe("StreamingMessageAggregator", () => {
         messageId: "msg1",
         historySequence: 1,
         model: "claude-3-5-sonnet-20241022",
+        startTime: Date.now(),
       });
 
       aggregator.handleToolCallStart({

src/browser/utils/messages/StreamingMessageAggregator.ts

@@ -860,7 +860,7 @@ export class StreamingMessageAggregator {
     const isCompacting = lastUserMsg?.metadata?.muxMetadata?.type === "compaction-request";
 
     const context: StreamingContext = {
-      startTime: Date.now(),
+      startTime: data.startTime,
       isComplete: false,
       isCompacting,
       model: data.model,

src/common/orpc/schemas/stream.ts

@@ -40,6 +40,9 @@ export const StreamStartEventSchema = z.object({
   historySequence: z.number().meta({
     description: "Backend assigns global message ordering",
   }),
+  startTime: z.number().meta({
+    description: "Backend timestamp when stream started (Date.now())",
+  }),
 });
 
 export const StreamDeltaEventSchema = z.object({

src/node/services/agentSession.disposeRace.test.ts

@@ -105,7 +105,8 @@ describe("AgentSession disposal race conditions", () => {
         workspaceId: "ws",
         messageId: "m1",
         model: "anthropic:claude-sonnet-4-5",
-        timestamp: Date.now(),
+        historySequence: 1,
+        startTime: Date.now(),
       })
     ).not.toThrow();
   });

src/node/services/aiService.ts

@@ -1282,6 +1282,7 @@ export class AIService extends EventEmitter {
           messageId: assistantMessageId,
           model: modelString,
           historySequence,
+          startTime: Date.now(),
         };
         this.emit("stream-start", streamStartEvent);
 
@@ -1317,6 +1318,7 @@ export class AIService extends EventEmitter {
           messageId: assistantMessageId,
           model: modelString,
           historySequence,
+          startTime: Date.now(),
         };
         this.emit("stream-start", streamStartEvent);
 

src/node/services/mock/mockScenarioPlayer.ts

@@ -272,6 +272,7 @@ export class MockScenarioPlayer {
           messageId,
           model: event.model,
           historySequence,
+          startTime: Date.now(),
         };
         this.deps.aiService.emit("stream-start", payload);
         break;