Shubhra/set ttft and ttfb to -1 when queue is empty (#475)

Author: Shubhrakanti

.changeset/cold-tomatoes-eat.md

@@ -0,0 +1,5 @@
+---
+'@livekit/agents': patch
+---
+
+set ttft/ttfb to -1 when output is empty

agents/src/llm/llm.ts

@@ -78,14 +78,14 @@ export abstract class LLMStream implements AsyncIterableIterator<ChatChunk> {
 
   protected async monitorMetrics() {
     const startTime = process.hrtime.bigint();
-    let ttft: bigint | undefined;
+    let ttft: bigint = BigInt(-1);
     let requestId = '';
     let usage: CompletionUsage | undefined;
 
     for await (const ev of this.queue) {
       this.output.put(ev);
       requestId = ev.requestId;
-      if (!ttft) {
+      if (ttft === BigInt(-1)) {
         ttft = process.hrtime.bigint() - startTime;
       }
       if (ev.usage) {
@@ -98,7 +98,7 @@ export abstract class LLMStream implements AsyncIterableIterator<ChatChunk> {
     const metrics: LLMMetrics = {
       timestamp: Date.now(),
       requestId,
-      ttft: Math.trunc(Number(ttft! / BigInt(1000000))),
+      ttft: ttft === BigInt(-1) ? -1 : Math.trunc(Number(ttft / BigInt(1000000))),
       duration: Math.trunc(Number(duration / BigInt(1000000))),
       cancelled: false, // XXX(nbsp)
       label: this.label,

agents/src/tts/tts.ts

@@ -136,7 +136,7 @@ export abstract class SynthesizeStream
         const metrics: TTSMetrics = {
           timestamp: Date.now(),
           requestId,
-          ttfb: Math.trunc(Number(ttfb! / BigInt(1000000))),
+          ttfb: Math.trunc(Number((ttfb || BigInt(0)) / BigInt(1000000))),
           duration: Math.trunc(Number(duration / BigInt(1000000))),
           charactersCount: text.length,
           audioDuration,
@@ -257,13 +257,13 @@ export abstract class ChunkedStream implements AsyncIterableIterator<Synthesized
   protected async monitorMetrics() {
     const startTime = process.hrtime.bigint();
     let audioDuration = 0;
-    let ttfb: bigint | undefined;
+    let ttfb: bigint = BigInt(-1);
     let requestId = '';
 
     for await (const audio of this.queue) {
       this.output.put(audio);
       requestId = audio.requestId;
-      if (!ttfb) {
+      if (ttfb === BigInt(-1)) {
         ttfb = process.hrtime.bigint() - startTime;
       }
       audioDuration += audio.frame.samplesPerChannel / audio.frame.sampleRate;
@@ -274,7 +274,7 @@ export abstract class ChunkedStream implements AsyncIterableIterator<Synthesized
     const metrics: TTSMetrics = {
       timestamp: Date.now(),
       requestId,
-      ttfb: Math.trunc(Number(ttfb! / BigInt(1000000))),
+      ttfb: ttfb === BigInt(-1) ? -1 : Math.trunc(Number(ttfb / BigInt(1000000))),
       duration: Math.trunc(Number(duration / BigInt(1000000))),
       charactersCount: this.#text.length,
       audioDuration,