Backfill the subagent display name from the second start event

The dispatch-time subagent_start fires before the trigger args (and therefore
the name parameter) have streamed; the phase-3 start re-announces the lane with
the name. The block builder was dropping that duplicate wholesale, losing the
name on streaming providers — now it backfills subagentName onto the existing
block instead. (The home turn-model path already reconciled this case.)

apps/sim/lib/copilot/request/go/stream.test.ts

@@ -936,4 +936,70 @@ describe('copilot go stream helpers', () => {
     expect(subagentBlock?.parentSpanId).toBe('S1')
     expect(subagentBlock?.parentToolCallId).toBe('tc-deploy-inner')
   })
+
+  it('backfills the display name when only the second subagent start carries it', async () => {
+    const scope = {
+      lane: 'subagent' as const,
+      agentId: 'research',
+      parentToolCallId: 'tc-research',
+      spanId: 'S3',
+      parentSpanId: 'S1',
+    }
+    vi.mocked(fetch).mockResolvedValueOnce(
+      createSseResponse([
+        // Dispatch-time start: fires before the trigger args stream, so no name.
+        createEvent({
+          streamId: 'stream-1',
+          cursor: '1',
+          seq: 1,
+          requestId: 'req-1',
+          type: MothershipStreamV1EventType.span,
+          scope,
+          payload: {
+            kind: 'subagent',
+            event: 'start',
+            agent: 'research',
+            data: { tool_call_id: 'tc-research' },
+          },
+        }),
+        // Phase-3 start re-announces the lane WITH the orchestrator-chosen name.
+        createEvent({
+          streamId: 'stream-1',
+          cursor: '2',
+          seq: 2,
+          requestId: 'req-1',
+          type: MothershipStreamV1EventType.span,
+          scope,
+          payload: {
+            kind: 'subagent',
+            event: 'start',
+            agent: 'research',
+            data: { tool_call_id: 'tc-research', name: 'Pricing research' },
+          },
+        }),
+        createEvent({
+          streamId: 'stream-1',
+          cursor: '3',
+          seq: 3,
+          requestId: 'req-1',
+          type: MothershipStreamV1EventType.complete,
+          payload: { status: MothershipStreamV1CompletionStatus.complete },
+        }),
+      ])
+    )
+
+    const context = createStreamingContext()
+    const execContext: ExecutionContext = {
+      userId: 'user-1',
+      workflowId: 'workflow-1',
+    }
+
+    await runStreamLoop('https://example.com/mothership/stream', {}, context, execContext, {
+      timeout: 1000,
+    })
+
+    const subagentBlocks = context.contentBlocks.filter((block) => block.type === 'subagent')
+    expect(subagentBlocks).toHaveLength(1)
+    expect(subagentBlocks[0]?.subagentName).toBe('Pricing research')
+  })
 })

apps/sim/lib/copilot/request/go/stream.ts

@@ -433,25 +433,36 @@ export async function runStreamLoop(
               context.subAgentToolCalls[toolCallId] ??= []
             }
             if (toolCallId && subagentName) {
+              const payloadData = streamEvent.payload.data
+              const rawName =
+                payloadData && typeof payloadData === 'object' && !Array.isArray(payloadData)
+                  ? (payloadData as Record<string, unknown>).name
+                  : undefined
+              const displayName = typeof rawName === 'string' && rawName ? rawName : undefined
               const openParents = (context.openSubagentParents ??= new Set<string>())
               if (!openParents.has(toolCallId)) {
                 openParents.add(toolCallId)
-                const payloadData = streamEvent.payload.data
-                const displayName =
-                  payloadData && typeof payloadData === 'object' && !Array.isArray(payloadData)
-                    ? (payloadData as Record<string, unknown>).name
-                    : undefined
                 context.contentBlocks.push({
                   type: 'subagent',
                   content: subagentName,
-                  ...(typeof displayName === 'string' && displayName
-                    ? { subagentName: displayName }
-                    : {}),
+                  ...(displayName ? { subagentName: displayName } : {}),
                   parentToolCallId: toolCallId,
                   ...(spanId ? { spanId } : {}),
                   ...(parentSpanId ? { parentSpanId } : {}),
                   timestamp: Date.now(),
                 })
+              } else if (displayName) {
+                // The lane was opened by the dispatch-time start, which fires
+                // before the trigger args (and therefore the name) exist. The
+                // phase-3 start re-announces the lane WITH the name; backfill
+                // it instead of dropping the duplicate wholesale.
+                for (let i = context.contentBlocks.length - 1; i >= 0; i--) {
+                  const b = context.contentBlocks[i]
+                  if (b.type === 'subagent' && b.parentToolCallId === toolCallId) {
+                    if (!b.subagentName) b.subagentName = displayName
+                    break
+                  }
+                }
               }
             } else {
               logger.warn('subagent start missing toolCallId or agent name', {