diff --git a/app-setup/templates/transmission-done.sh b/app-setup/templates/transmission-done.sh
index 4216ac0..e4d4dd0 100755
--- a/app-setup/templates/transmission-done.sh
+++ b/app-setup/templates/transmission-done.sh
@@ -231,11 +231,12 @@ log() {
   fi
   local timestamp
   timestamp=$(date '+%Y-%m-%d %H:%M:%S')
-  # In test mode, only write to log file (don't output to stdout)
+  # stderr, never stdout: functions that return data by echo are captured with
+  # $(...), and a log line in that capture corrupts the returned value.
   if [[ "${TEST_MODE}" == "true" ]]; then
     printf '[%s] %s\n' "${timestamp}" "$1" >>"${LOG_FILE}"
   else
-    printf '[%s] %s\n' "${timestamp}" "$1" | tee -a "${LOG_FILE}"
+    printf '[%s] %s\n' "${timestamp}" "$1" | tee -a "${LOG_FILE}" >&2
   fi
 }
 
diff --git a/tests/transmission-filebot/unit/test_type_detection.bats b/tests/transmission-filebot/unit/test_type_detection.bats
index bf7335d..6e1fe16 100755
--- a/tests/transmission-filebot/unit/test_type_detection.bats
+++ b/tests/transmission-filebot/unit/test_type_detection.bats
@@ -199,3 +199,31 @@ load ../test_helper
   run cat "${LOG_FILE}"
   assert_output_contains "Pattern counts" "${output}"
 }
+
+# Production runs with TEST_MODE=false, where log() also prints to the
+# terminal. That output must not reach stdout: callers capture this function
+# with $(...), and on 2026-09-24 the captured log lines made media_type differ
+# from "tv", so an SNW episode previewed with strict auto-detection and failed.
+@test "detect_media_type_heuristic: returns only the type when TEST_MODE=false" {
+  local file="${TEST_TEMP_DIR}/star.trek.strange.new.worlds.s04e10.1080p.web.h264-cakes[EZTVx.to].mkv"
+  touch "${file}"
+  export TR_TORRENT_FILES="${file}"
+  export TEST_MODE=false
+
+  local result
+  result=$(detect_media_type_heuristic "${file}")
+
+  assert_equal "tv" "${result}"
+}
+
+@test "log: writes to LOG_FILE and not to stdout when TEST_MODE=false" {
+  export TEST_MODE=false
+  : >"${LOG_FILE}"
+
+  local captured
+  captured=$(log "probe message" 2>/dev/null)
+
+  assert_equal "" "${captured}"
+  run cat "${LOG_FILE}"
+  assert_output_contains "probe message" "${output}"
+}