diff --git a/c/glm.c b/c/glm.c
index cea69b7..1e10044 100644
--- a/c/glm.c
+++ b/c/glm.c
@@ -4070,6 +4070,7 @@ static int mux_submit(Model *m, Tok *T, ServeCtx *ctx, ServeReq *req, int nctx,
         kv_disk_truncate(m,sc->len); }
     int add=nt-sc->len;
     if(add>0) memcpy(sc->hist+sc->len,tmp+sc->len,(size_t)add*sizeof(int));
+    fprintf(stderr,"[API] KV slot %d prefix %d/%d token, prefill %d\n",sub.slot,sc->len,nt,add);
     free(tmp);
     float *logit = add>0 ? step(m,sc->hist+sc->len,add,sc->len)
                          : step(m,sc->hist+sc->len-1,1,sc->len-1);