diff --git a/Changelog.md b/Changelog.md
index 652474b..0657d12 100644
--- a/Changelog.md
+++ b/Changelog.md
@@ -14,6 +14,16 @@
 - `robots.txt` is served at every path, for installations that own their host
   name; behind a proxy that mounts derekovecs below a path, the site wide
   robots.txt still has to say it
+- the per request diagnostics of the vector code are off unless
+  `DEREKOVECS_DEBUG` is set. Every neighbourhood request wrote a few hundred
+  lines - one per window position, one per vocabulary lookup, the whole JSON of
+  a similar profile - which came to a million lines a day on
+  corpora.ids-mannheim.de and filled the log partition of the machine. Startup
+  and error messages are unaffected
+- the per request narration of the perl side - which word is being looked up,
+  which result comes from a cache, which static file was fetched - is logged at
+  debug level now. It repeated what the access log line of the same request
+  already says
 
 ## [0.96] - 2026-07-31
 
diff --git a/README.md b/README.md
index 346c4ea..54371a5 100644
--- a/README.md
+++ b/README.md
@@ -119,6 +119,27 @@
 That is a request, not a barrier - it is obeyed by the crawlers that care, and
 the `403` above is what stops the rest.
 
+### Logging
+
+The server logs one line per request. The per request diagnostics of the
+vector code - one line per window position, one per vocabulary lookup, the
+whole JSON of a similar profile - are off by default; a single neighbourhood
+request writes a few hundred of them, which is a million lines a day on a busy
+instance. Set `DEREKOVECS_DEBUG=1` in the environment to get them back:
+
+```bash
+DEREKOVECS_DEBUG=1 MOJO_CONFIG=$(pwd)/example.conf morbo script/derekovecs-server
+```
+
+Startup messages, the memory mapping of the model and error messages are not
+affected by it.
+
+The perl side logs one line per request. What it used to say in addition -
+which word is being looked up, which result came from a cache - is logged at
+debug level and can be turned on with `MOJO_LOG_LEVEL=debug`. Requests that are
+turned away as crawlers are logged with their user agent, so that the list can
+be checked against what actually arrives.
+
 ## Web Service API
 
 In addition to the web user interface, derekovecs also provides a web api which is however still very unsystematic and **not stable**. To figure out the meaning of still undocumented result components, have a look at the table head mouse-overs in the GUI or at the source code around [here](https://korap.ids-mannheim.de/gerrit/plugins/gitiles/ids-kl/derekovecs/+/refs/heads/master/templates/index.html.ep#684).
diff --git a/lib/IDS/DeReKoVecs/Read.pm b/lib/IDS/DeReKoVecs/Read.pm
index f75ca90..0b396ec 100644
--- a/lib/IDS/DeReKoVecs/Read.pm
+++ b/lib/IDS/DeReKoVecs/Read.pm
@@ -89,19 +89,19 @@
 
   my $pipe;
   if($compare_to ne "") {
-    $c->app->log->info("comparing syn neighbours to: $compare_to/getClassicCollocators?w=$word");
+    $c->app->log->debug("comparing syn neighbours to: $compare_to/getClassicCollocators?w=$word");
     open $pipe, "lwp-request $compare_to/getClassicCollocators?w=$word |";
   }
 
   my $collocators = $opt_C ? undef : $cccache->get($word);
   if(!defined $collocators) {
-    $c->app->log->info("Getting classic collocates of $word.");
+    $c->app->log->debug("Getting classic collocates of $word.");
     $collocators = getClassicCollocators($word);
     $collocators =~ s/:(-?)(nan|inf)/:"${1}${2}"/g;
     $collocators =~ s/"""/"\\""/g;
     $cccache->set($word => $collocators) unless $opt_C;
   } else {
-    $c->app->log->info("Getting classic collocates for $word from cache.");
+    $c->app->log->debug("Getting classic collocates for $word from cache.");
   }
 
   if(defined($pipe)) {
@@ -145,7 +145,7 @@
     $profiles = getSimilarProfiles($word);
     $spcache->set($word => $profiles) unless $opt_C;
   } else {
-    $c->app->log->info("Getting similar profiles for $word from cache:");
+    $c->app->log->debug("Getting similar profiles for $word from cache:");
   }
   return $profiles;
 }
diff --git a/lib/IDS/DeReKoVecs/derekovecs-server.c b/lib/IDS/DeReKoVecs/derekovecs-server.c
index 56e037c..b1aa341 100644
--- a/lib/IDS/DeReKoVecs/derekovecs-server.c
+++ b/lib/IDS/DeReKoVecs/derekovecs-server.c
@@ -19,6 +19,26 @@
 #define MAX_EXP 6
 #define MIN_RESP 0.50
 
+/* Per request diagnostics. Every neighbourhood request used to write a few
+   hundred lines - one per window position, one per vocabulary lookup, the
+   whole JSON of a similar profile - and a busy instance turned that into a
+   million lines a day, which is what filled the log partition of the
+   production machine. They are off unless DEREKOVECS_DEBUG is set to
+   something other than 0 or the empty string. Startup messages and error
+   messages are not affected. */
+static int derekovecs_debug(void) {
+  static int enabled = -1;
+  if (enabled < 0) {
+    const char *e = getenv("DEREKOVECS_DEBUG");
+    enabled = (e && *e && strcmp(e, "0") != 0) ? 1 : 0;
+  }
+  return enabled;
+}
+
+#define DEBUG_PRINTF(...)  do { if (derekovecs_debug()) printf(__VA_ARGS__); } while (0)
+#define DEBUG_EPRINTF(...) do { if (derekovecs_debug()) fprintf(stderr, __VA_ARGS__); } while (0)
+#define DEBUG_FFLUSH()     do { if (derekovecs_debug()) fflush(stdout); } while (0)
+
 //the thread function
 void *connection_handler(void *);
 
@@ -422,7 +442,7 @@
     if (a >= window)
       a++;
     wpos_sum = 0;
-    printf("window pos: %ld\n", a);
+    DEBUG_PRINTF("window pos: %ld\n", a);
     if (a != window) {
       max_f = -1;
       window_offset = a * size;
@@ -458,8 +478,8 @@
             worstbest = best[N - 1].activation;
         }
       }
-      printf("%ld %.2f\n", max_target, max_f);
-      printf("%s (%.2f) ", &vocab[max_target * max_w], max_f);
+      DEBUG_PRINTF("%ld %.2f\n", max_target, max_f);
+      DEBUG_PRINTF("%s (%.2f) ", &vocab[max_target * max_w], max_f);
       if (max_f > maxmax_f) {
         maxmax_f = max_f;
         maxmax_target = max_target;
@@ -468,7 +488,7 @@
         if (best[b].position == window - a)
           best[b].cprobability = best[b].activation / wpos_sum;
     } else {
-      printf("\x1b[1m%s\x1b[0m ", &vocab[d * max_w]);
+      DEBUG_PRINTF("\x1b[1m%s\x1b[0m ", &vocab[d * max_w]);
     }
     pars->window_sums[a] = wpos_sum;
   }
@@ -555,11 +575,11 @@
   buffer[0] = '[';
   buffer[1] = 0;
   if (node < 0 || node >= sprofiles_qty) {
-    printf("Not available in precomputed profile\n");
+    DEBUG_PRINTF("Not available in precomputed profile\n");
     return newSVpv("[{\"w\":\"not available\", \"v\":0}]\n", 0);
   }
 
-  printf("******* %s ******\n", &vocab[max_w * node]);
+  DEBUG_PRINTF("******* %s ******\n", &vocab[max_w * node]);
 
   for (i = 0; i < 100 && i < sprofiles[node].len; i++) {
     sprintf(pair_buffer, "{\"w\":\"%s\", \"v\":%f},", &vocab[max_w * (sprofiles[node].nbr[i].index)], sprofiles[node].nbr[i].value);
@@ -570,7 +590,7 @@
   else
     strcat(buffer, "]");
   strcat(buffer, "\n");
-  printf("%s", buffer);
+  DEBUG_PRINTF("%s", buffer);
   return newSVpv(buffer, 0);
 }
 
@@ -623,10 +643,10 @@
     if (b == words) b = -1;
     wl->wordi[a] = b;
     if (b == -1) {
-      fprintf(stderr, "Out of dictionary word!\n");
+      DEBUG_EPRINTF("Out of dictionary word!\n");
       cn--;
     } else {
-      fprintf(stderr, "Word: \"%s\"  Position in vocabulary: %lld\n", &vocab[wl->wordi[a] * max_w], wl->wordi[a]);
+      DEBUG_EPRINTF("Word: \"%s\"  Position in vocabulary: %lld\n", &vocab[wl->wordi[a] * max_w], wl->wordi[a]);
     }
   }
   wl->length = cn;
@@ -665,7 +685,7 @@
   if (result != NULL)
     return newSVpv(result, 0);
 
-  printf("Looking for biggest distances between main and merged vectors ...\n");
+  DEBUG_PRINTF("Looking for biggest distances between main and merged vectors ...\n");
   collocator *best;
   best = malloc(N * sizeof(collocator));
   memset(best, 0, N * sizeof(collocator));
@@ -720,7 +740,7 @@
     res = -1;
   else {
     res = cos_similarity(a->wordi[0], b->wordi[0]);
-    fprintf(stderr, "a: %lld b: %lld res:%f\n", a->wordi[0], b->wordi[0], res);
+    DEBUG_EPRINTF("a: %lld b: %lld res:%f\n", a->wordi[0], b->wordi[0], res);
   }
   free(a);
   free(b);
@@ -831,8 +851,8 @@
   a = posix_memalign((void **)&target_sums, 128, cutoff * sizeof(float));
   memset(target_sums, 0, cutoff * sizeof(float));
 
-  printf("Starting %d threads\n", syn_threads);
-  fflush(stdout);
+  DEBUG_PRINTF("Starting %d threads\n", syn_threads);
+  DEBUG_FFLUSH();
   for (a = 0; a < syn_threads; a++) {
     pars[a].cutoff = cutoff;
     pars[a].target_sums = target_sums;
@@ -845,11 +865,11 @@
     pars[a].upto = a + 1;
     pthread_create(&pt[a], NULL, getCollocators, (void *)&pars[a]);
   }
-  printf("Waiting for syn threads to join\n");
-  fflush(stdout);
+  DEBUG_PRINTF("Waiting for syn threads to join\n");
+  DEBUG_FFLUSH();
   for (a = 0; a < syn_threads; a++) pthread_join(pt[a], (void *)&syn_nbs[a]);
-  printf("Syn threads joint\n");
-  fflush(stdout);
+  DEBUG_PRINTF("Syn threads joint\n");
+  DEBUG_FFLUSH();
   result = malloc((maxPerPos > 0 ? maxPerPos : 1) * (max_w + 96) * syn_threads + 16);
   char *p = (char *) result;
   *p = 0;
@@ -926,8 +946,8 @@
   a = posix_memalign((void **)&target_sums, 128, cutoff * sizeof(float));
   memset(target_sums, 0, cutoff * sizeof(float));
 
-  printf("Starting %d threads for paradigmatic search\n", para_threads);
-  fflush(stdout);
+  DEBUG_PRINTF("Starting %d threads for paradigmatic search\n", para_threads);
+  DEBUG_FFLUSH();
   for (a = 0; a < para_threads; a++) {
     pars[a].cutoff = cutoff;
     pars[a].token = st1;
@@ -941,7 +961,7 @@
       pars[a].from = merge_words + a * slice;
       pars[a].upto = merge_words + ((a + 1) * slice > cutoff ? cutoff : (a + 1) * slice);
     }
-    printf("From: %ld, Upto: %ld\n", pars[a].from, pars[a].upto);
+    DEBUG_PRINTF("From: %ld, Upto: %ld\n", pars[a].from, pars[a].upto);
     pthread_create(&pt[a], NULL, _get_neighbours, (void *)&pars[a]);
   }
   if (M2) {
@@ -958,11 +978,11 @@
       pthread_create(&pt[a + para_threads], NULL, getCollocators, (void *)&pars[a + para_threads]);
     }
   }
-  printf("Waiting for para threads to join\n");
-  fflush(stdout);
+  DEBUG_PRINTF("Waiting for para threads to join\n");
+  DEBUG_FFLUSH();
   for (a = 0; a < para_threads; a++) pthread_join(pt[a], (void *)&para_nbs[a]);
-  printf("Para threads joint\n");
-  fflush(stdout);
+  DEBUG_PRINTF("Para threads joint\n");
+  DEBUG_FFLUSH();
 
   /* if(!syn_nbs[0]) */
   /* 	goto end; */
@@ -970,7 +990,7 @@
   qsort(best, N * para_threads, sizeof(collocator), cmp_activation);
 
   long long chosen[MAX_NEIGHBOURS];
-  printf("N: %d\n", N);
+  DEBUG_PRINTF("N: %d\n", N);
 
   AV *array = newAV();
   int i, j;
@@ -983,7 +1003,7 @@
       for (j = 0; j < i && !filtered; j++)
         if (strcasestr(&vocab[c * max_w], &vocab[chosen[j] * max_w]) ||
             strcasestr(&vocab[chosen[j] * max_w], &vocab[c * max_w])) {
-          printf("filtering %s %s\n", &vocab[chosen[j] * max_w], &vocab[c * max_w]);
+          DEBUG_PRINTF("filtering %s %s\n", &vocab[chosen[j] * max_w], &vocab[c * max_w]);
           filtered = 1;
         }
       if (filtered)
@@ -1036,16 +1056,16 @@
   float total_activation = 0;
 
   if (M2) {
-    printf("Waiting for syn threads to join\n");
-    fflush(stdout);
+    DEBUG_PRINTF("Waiting for syn threads to join\n");
+    DEBUG_FFLUSH();
     for (a = 0; a < syn_threads; a++) pthread_join(pt[a + para_threads], (void *)&syn_nbs[a]);
     for (a = 0; a <= syn_threads; a++) {
       if (a == window) continue;
       total_activation += window_sums[a];
-      printf("window pos: %ld, sum: %f\n", a, window_sums[a]);
+      DEBUG_PRINTF("window pos: %ld, sum: %f\n", a, window_sums[a]);
     }
-    printf("syn threads joint\n");
-    fflush(stdout);
+    DEBUG_PRINTF("syn threads joint\n");
+    DEBUG_FFLUSH();
 
     for (b = 0; b < syn_nbs[0]->length; b++) {
       memcpy(best + b, &syn_nbs[0]->best[b], sizeof(collocator));
@@ -1079,7 +1099,7 @@
     }
     sort_by = 0;                         // ALWAYS AUTO-FOCUS
     if (sort_by != 1 && sort_by != 2) {  // sort by auto focus mean
-      printf("window: %d  -  syn_threads: %d, %d\n", window, syn_threads, (1 << syn_threads) - 1);
+      DEBUG_PRINTF("window: %d  -  syn_threads: %d, %d\n", window, syn_threads, (1 << syn_threads) - 1);
       int wpos;
       int bits_set = 0;
       for (i = 0; i < found_index; i++) {
diff --git a/script/derekovecs-server b/script/derekovecs-server
index bc66b01..9f5dbec 100755
--- a/script/derekovecs-server
+++ b/script/derekovecs-server
@@ -222,7 +222,7 @@
 	my $c = shift;
 	my $url = $c->req->url;
 	$url =~ s@/derekovecs/@/@g;
-  $c->app->log->info("GET: " . $url);
+  $c->app->log->debug("GET: " . $url);
 	$c->reply->static($url);
 } => 'js';
 
@@ -230,7 +230,7 @@
 	my $c = shift;
 	my $url = $c->req->url;
 	$url =~ s@/derekovecs/@/@g;
-  $c->app->log->info("GET: " . $url);
+  $c->app->log->debug("GET: " . $url);
 	$c->reply->static($url);
 } => 'css';
 
@@ -412,13 +412,13 @@
 	my $c = shift;
 	my $url = $c->req->url;
 	$url =~ s@/derekovecs@@g;
-  $c->app->log->info("GET: " . $url);
+  $c->app->log->debug("GET: " . $url);
 	$c->reply->static($url);
 };
 
 get '/' => sub {
   my $c    = shift;
-  $c->app->log->info("get: ".$c->req->url->to_abs);
+  $c->app->log->debug("get: ".$c->req->url->to_abs);
 	my $word=$c->param('word');
   my $no_nbs=$c->param('n') || ($opt_m? 50 : 100);
   my $no_iterations=$c->param('N') || 2000;
@@ -452,9 +452,9 @@
       my $key = join("\x1c", $w, $cutoff, $no_nbs, $sort, $dedupe, $searchBaseVocabFirst, $nosp);
       $res = $opt_C ? undef : $cache->get($key);
       if (defined $res) {
-        $c->app->log->info("Getting $w results from cache");
+        $c->app->log->debug("Getting $w results from cache");
       } else {
-        $c->app->log->info('Looking for neighbours of '.$w);
+        $c->app->log->debug('Looking for neighbours of '.$w);
         if($opt_i) {
           $res = get_neighbours(encode("iso-8859-1", $w), $no_nbs, $sort, $searchBaseVocabFirst, $cutoff, $dedupe, $nosp);
         } else {