fix(tapper): stop retrying telemetry batches the hub rejects

pkg/tapper/invocation_telemetry.go

@@ -242,10 +242,24 @@ func (r *httpInvocationReporter) send(ctx context.Context, batch []InvocationEve
 	}
 }
 
+// disablesInvocationTelemetry reports whether a status means "stop trying" as
+// opposed to "try again later". Every code here says the hub will never accept
+// this client's events, so retrying only wastes requests.
+//
+// 400 is in the list because a hub older than the client rejects any field it
+// does not know — its decoder disallows unknown fields — and the client cannot
+// negotiate the payload down. Without this, a tap carrying a newly added field
+// would re-send a guaranteed-rejected batch on every flush for the life of the
+// process. Degrading to no telemetry is the correct outcome, and it is what
+// lets the client and the hub release in either order.
+//
+// 413 is deliberately absent: batch contents vary, so a too-large batch says
+// nothing about the next one.
 func disablesInvocationTelemetry(status int) bool {
 	switch status {
-	case http.StatusUnauthorized, http.StatusForbidden, http.StatusNotFound,
-		http.StatusMethodNotAllowed, http.StatusGone, http.StatusNotImplemented:
+	case http.StatusBadRequest, http.StatusUnauthorized, http.StatusForbidden,
+		http.StatusNotFound, http.StatusMethodNotAllowed, http.StatusGone,
+		http.StatusNotImplemented:
 		return true
 	default:
 		return false

pkg/tapper/invocation_telemetry_test.go

@@ -171,6 +171,28 @@ func TestHTTPInvocationReporterTimeoutAndOlderHubAreBestEffort(t *testing.T) {
 		r.Close(ctx)
 		require.Equal(t, int64(1), calls.Load())
 	})
+
+	// A hub older than this client rejects any field it does not know, and the
+	// client cannot negotiate the payload down. Retrying a guaranteed-rejected
+	// batch on every flush is pure waste, so 400 stops the process reporter the
+	// same way an unsupported endpoint does.
+	t.Run("rejected payload disables process reporter", func(t *testing.T) {
+		var calls atomic.Int64
+		client := &http.Client{Transport: telemetryRoundTripFunc(func(_ *http.Request) (*http.Response, error) {
+			calls.Add(1)
+			return &http.Response{StatusCode: http.StatusBadRequest, Body: io.NopCloser(strings.NewReader("")), Header: make(http.Header)}, nil
+		})}
+		r := newHTTPInvocationReporter("https://old.example.com/api/v1/telemetry/invocations", "token", "test", invocationReporterOptions{
+			client: client, batchSize: 1, flushInterval: time.Hour,
+		})
+		r.Report(InvocationEvent{Surface: "mcp", Tool: "first"})
+		require.Eventually(t, r.disabled.Load, time.Second, time.Millisecond)
+		r.Report(InvocationEvent{Surface: "mcp", Tool: "second"})
+		ctx, cancel := context.WithTimeout(context.Background(), time.Second)
+		defer cancel()
+		r.Close(ctx)
+		require.Equal(t, int64(1), calls.Load(), "the rejected batch must not be retried")
+	})
 }
 
 func TestHTTPInvocationReporterConcurrentReportAndClose(t *testing.T) {