fix(gateway): avoid duplicate delta flush when buffer unchanged

Track the text length at the time of the last broadcast. The flush in
emitChatFinal now only sends a delta if the buffer has grown since the
last broadcast, preventing duplicate sends when the final delta passed
the 150ms throttle and was already broadcast.

src/agents/pi-embedded-runner/run.ts

@@ -1295,7 +1295,11 @@ export async function runEmbeddedPiAgent(
               aborted,
               systemPromptReport: attempt.systemPromptReport,
               // Handle client tool calls (OpenResponses hosted tools)
-              stopReason: attempt.clientToolCall ? "tool_calls" : undefined,
+              // Propagate the LLM stop reason so callers (lifecycle events,
+              // ACP bridge) can distinguish end_turn from max_tokens.
+              stopReason: attempt.clientToolCall
+                ? "tool_calls"
+                : (lastAssistant?.stopReason as string | undefined),
               pendingToolCalls: attempt.clientToolCall
                 ? [
                     {

src/commands/agent.ts

@@ -873,6 +873,10 @@ async function agentCommandInternal(
       fallbackProvider = fallbackResult.provider;
       fallbackModel = fallbackResult.model;
       if (!lifecycleEnded) {
+        const stopReason = result.meta.stopReason;
+        if (stopReason && stopReason !== "end_turn") {
+          console.error(`[agent] run ${runId} ended with stopReason=${stopReason}`);
+        }
         emitAgentEvent({
           runId,
           stream: "lifecycle",
@@ -881,6 +885,7 @@ async function agentCommandInternal(
             startedAt,
             endedAt: Date.now(),
             aborted: result.meta.aborted ?? false,
+            stopReason,
           },
         });
       }

src/gateway/server-chat.ts

@@ -158,6 +158,8 @@ export type ChatRunState = {
   registry: ChatRunRegistry;
   buffers: Map<string, string>;
   deltaSentAt: Map<string, number>;
+  /** Length of text at the time of the last broadcast, used to avoid duplicate flushes. */
+  deltaLastBroadcastLen: Map<string, number>;
   abortedRuns: Map<string, number>;
   clear: () => void;
 };
@@ -166,12 +168,14 @@ export function createChatRunState(): ChatRunState {
   const registry = createChatRunRegistry();
   const buffers = new Map<string, string>();
   const deltaSentAt = new Map<string, number>();
+  const deltaLastBroadcastLen = new Map<string, number>();
   const abortedRuns = new Map<string, number>();
 
   const clear = () => {
     registry.clear();
     buffers.clear();
     deltaSentAt.clear();
+    deltaLastBroadcastLen.clear();
     abortedRuns.clear();
   };
 
@@ -179,6 +183,7 @@ export function createChatRunState(): ChatRunState {
     registry,
     buffers,
     deltaSentAt,
+    deltaLastBroadcastLen,
     abortedRuns,
     clear,
   };
@@ -318,6 +323,7 @@ export function createAgentEventHandler({
       return;
     }
     chatRunState.deltaSentAt.set(clientRunId, now);
+    chatRunState.deltaLastBroadcastLen.set(clientRunId, cleaned.length);
     const payload = {
       runId: clientRunId,
       sessionKey,
@@ -355,9 +361,10 @@ export function createAgentEventHandler({
     // Flush any throttled delta so streaming clients receive the complete text
     // before the final event.  The 150 ms throttle in emitChatDelta may have
     // suppressed the most recent chunk, leaving the client with stale text.
+    // Only flush if the buffer has grown since the last broadcast to avoid duplicates.
     if (text && !shouldSuppressSilent) {
-      const lastSent = chatRunState.deltaSentAt.get(clientRunId) ?? 0;
+      const lastBroadcastLen = chatRunState.deltaLastBroadcastLen.get(clientRunId) ?? 0;
-      if (lastSent > 0) {
+      if (text.length > lastBroadcastLen) {
         const flushPayload = {
           runId: clientRunId,
           sessionKey,
@@ -373,6 +380,7 @@ export function createAgentEventHandler({
         nodeSendToSession(sessionKey, "chat", flushPayload);
       }
     }
+    chatRunState.deltaLastBroadcastLen.delete(clientRunId);
     chatRunState.buffers.delete(clientRunId);
     chatRunState.deltaSentAt.delete(clientRunId);
     if (jobState === "done") {