fix: trim event buffer to newest bytes instead of discarding on overflow

SantiagoDePolonia · claude · SantiagoDePolonia · commit e62ce428e976 · 2026-02-25T12:16:02.000+01:00
When the SSE event buffer exceeds maxEventBufferRemainder, keep the
newest bytes rather than resetting entirely. This preserves the tail of
in-flight partial events (where usage data in JSON is most likely found)
instead of silently dropping them.

Co-Authored-By: Claude Opus 4.6 &lt;noreply@anthropic.com&gt;
diff --git a/internal/usage/stream_wrapper.go b/internal/usage/stream_wrapper.go
@@ -64,9 +64,13 @@ func (w *StreamUsageWrapper) processCompleteEvents() {
 	// Find the last complete event boundary
 	lastBoundary := bytes.LastIndex(data, []byte("\n\n"))
 	if lastBoundary < 0 {
-		// No complete event yet — apply safety valve on remainder size
+		// No complete event yet — apply safety valve on remainder size.
+		// Trim to keep only the newest bytes so partial events are preserved.
 		if w.eventBuffer.Len() > maxEventBufferRemainder {
+			tail := w.eventBuffer.Bytes()
+			start := len(tail) - maxEventBufferRemainder
 			w.eventBuffer.Reset()
+			w.eventBuffer.Write(tail[start:])
 		}
 		return
 	}
@@ -103,10 +107,11 @@ func (w *StreamUsageWrapper) processCompleteEvents() {
 	// Keep only the remainder
 	w.eventBuffer.Reset()
 	if len(remainder) > 0 {
-		// Safety valve on remainder size
-		if len(remainder) <= maxEventBufferRemainder {
-			w.eventBuffer.Write(remainder)
+		// Safety valve on remainder size — trim to keep newest bytes
+		if len(remainder) > maxEventBufferRemainder {
+			remainder = remainder[len(remainder)-maxEventBufferRemainder:]
 		}
+		w.eventBuffer.Write(remainder)
 	}
 }
 

Original file line number	Diff line number	Diff line change
`@@ -64,9 +64,13 @@ func (w *StreamUsageWrapper) processCompleteEvents() {`
`64`	`64`	`// Find the last complete event boundary`
`65`	`65`	`lastBoundary := bytes.LastIndex(data, []byte("\n\n"))`
`66`	`66`	`if lastBoundary < 0 {`
`67`		`- // No complete event yet — apply safety valve on remainder size`
	`67`	`+ // No complete event yet — apply safety valve on remainder size.`
	`68`	`+ // Trim to keep only the newest bytes so partial events are preserved.`
`68`	`69`	`if w.eventBuffer.Len() > maxEventBufferRemainder {`
	`70`	`+ tail := w.eventBuffer.Bytes()`
	`71`	`+ start := len(tail) - maxEventBufferRemainder`
`69`	`72`	`w.eventBuffer.Reset()`
	`73`	`+ w.eventBuffer.Write(tail[start:])`
`70`	`74`	`}`
`71`	`75`	`return`
`72`	`76`	`}`
`@@ -103,10 +107,11 @@ func (w *StreamUsageWrapper) processCompleteEvents() {`
`103`	`107`	`// Keep only the remainder`
`104`	`108`	`w.eventBuffer.Reset()`
`105`	`109`	`if len(remainder) > 0 {`
`106`		`- // Safety valve on remainder size`
`107`		`- if len(remainder) <= maxEventBufferRemainder {`
`108`		`- w.eventBuffer.Write(remainder)`
	`110`	`+ // Safety valve on remainder size — trim to keep newest bytes`
	`111`	`+ if len(remainder) > maxEventBufferRemainder {`
	`112`	`+ remainder = remainder[len(remainder)-maxEventBufferRemainder:]`
`109`	`113`	`}`
	`114`	`+ w.eventBuffer.Write(remainder)`
`110`	`115`	`}`
`111`	`116`	`}`
`112`	`117`