diff --git a/DESIGN_SYSTEM.md b/DESIGN_SYSTEM.md
index 8278c30..cdcaf58 100644
--- a/DESIGN_SYSTEM.md
+++ b/DESIGN_SYSTEM.md
@@ -173,6 +173,7 @@ Regles UI :
 - les boutons de controle utilisent les icones `play-circle`, `pause-circle`, `record-circle`, `stop-circle`, `refresh`, `sort-time`, `scrollable`.
 - pour `Heure precise` et `Intervalle`, le refresh inline est un toggle compact de repetition automatique ; il est bloque si la frequence est inferieure a 5 minutes.
 - le formulaire `Intervalle` affiche deux champs `hh:mm:ss` compacts : duree de repetition et depart optionnel ; un depart vide signifie `Maintenant`.
+- les invalidations du formulaire countdown s'affichent en notice inline rouge compacte, portée par `useDraftForm`.
 
 Alertes timer :
 
diff --git a/tests/helpers/data-validation.mjs b/tests/helpers/data-validation.mjs
index 4eea9fe..03053ad 100644
--- a/tests/helpers/data-validation.mjs
+++ b/tests/helpers/data-validation.mjs
@@ -265,6 +265,8 @@ export function validateSiteContent(site) {
     "toolboxes.modules.timer.intervalStartTimeOption",
     "toolboxes.modules.timer.intervalStartTimeLabel",
     "toolboxes.modules.timer.addCountdownButton",
+    "toolboxes.modules.timer.requiredError",
+    "toolboxes.modules.timer.invalidError",
     "toolboxes.modules.timer.defaultCountdownLabel",
     "toolboxes.modules.timer.finishedLabel",
     "toolboxes.modules.timer.targetLabel",
diff --git a/website/public/data/site.json b/website/public/data/site.json
index 363bca8..4752927 100644
--- a/website/public/data/site.json
+++ b/website/public/data/site.json
@@ -430,6 +430,8 @@
         "intervalStartTimeOption": "Heure précise",
         "intervalStartTimeLabel": "Départ",
         "addCountdownButton": "Ajouter",
+        "requiredError": "Valeur requise.",
+        "invalidError": "Valeur invalide.",
         "defaultCountdownLabel": "Timer",
         "finishedLabel": "Terminé",
         "targetLabel": "Suivant",
diff --git a/website/src/features/toolboxes/modules/TimerModule.jsx b/website/src/features/toolboxes/modules/TimerModule.jsx
index 6166b43..10bcb1c 100644
--- a/website/src/features/toolboxes/modules/TimerModule.jsx
+++ b/website/src/features/toolboxes/modules/TimerModule.jsx
@@ -94,6 +94,27 @@ function hasTimePartValue(parts) {
   return Boolean(String(parts?.hours || "").trim() || String(parts?.minutes || "").trim() || String(parts?.seconds || "").trim());
 }
 
+function getCountdownValidationError(type, draft, textContent) {
+  const requiredError = textContent.requiredError || "Valeur requise.";
+  const invalidError = textContent.invalidError || "Valeur invalide.";
+  if (type === "duration" && timePartsToDurationMs(draft.duration) <= 0) {
+    return requiredError;
+  }
+  if (type === "daily_time" && !timePartsToString(draft.time)) {
+    return invalidError;
+  }
+  if (type === "time_pattern") {
+    if (!hasTimePartValue(draft.pattern)) return requiredError;
+    if (!timePartsToString(draft.pattern, { allowWildcard: true })) return invalidError;
+  }
+  if (type === "interval") {
+    const startTimeHasValue = hasTimePartValue(draft.intervalStartTime);
+    if (timePartsToDurationMs(draft.interval) <= 0) return requiredError;
+    if (startTimeHasValue && !timePartsToString(draft.intervalStartTime)) return invalidError;
+  }
+  return "";
+}
+
 export function TimerModule({ toolboxId, moduleId, context }) {
   const data = context.normalizeTimerData(context.getModuleData(toolboxId, moduleId, { activeTab: "stopwatch" }));
   const textContent = context.moduleText?.timer || {};
@@ -194,6 +215,8 @@ export function TimerModule({ toolboxId, moduleId, context }) {
   function createCountdown(event) {
     countdownDraft.handleSubmit(event, (draft, { updateValues }) => {
       const type = COUNTDOWN_TYPES.includes(draft.type) ? draft.type : "duration";
+      const validationError = getCountdownValidationError(type, draft, textContent);
+      if (validationError) return validationError;
       const label = draft.label.trim() || textContent.defaultCountdownLabel || "Timer";
       const now = getNowMs();
       let countdown = null;
@@ -335,6 +358,7 @@ export function TimerModule({ toolboxId, moduleId, context }) {
           <CountdownControls
             textContent={textContent}
             form={countdownDraft.values}
+            error={countdownDraft.error}
             onChange={countdownDraft.updateValues}
             onSubmit={createCountdown}
           />
@@ -441,9 +465,9 @@ function StopwatchControls({ textContent, elapsedMs, running, onStart, onPause,
   );
 }
 
-function CountdownControls({ textContent, form, onChange, onSubmit }) {
+function CountdownControls({ textContent, form, error, onChange, onSubmit }) {
   return (
-    <form className="timer-control-panel timer-countdown-form" role="tabpanel" onSubmit={onSubmit}>
+    <form className="timer-control-panel timer-countdown-form" role="tabpanel" onSubmit={onSubmit} aria-describedby={error ? "timer-countdown-error" : undefined}>
       <label>
         <span>{textContent.countdownLabel || "Libellé"}</span>
         <input value={form.label} onChange={(event) => onChange({ label: event.target.value })} placeholder={textContent.countdownLabelPlaceholder || "Boss, event, craft..."} />
@@ -490,6 +514,12 @@ function CountdownControls({ textContent, form, onChange, onSubmit }) {
           </label>
         </div>
       )}
+      {error && (
+        <p id="timer-countdown-error" className="timer-form-error" role="alert">
+          <Icon name="close" />
+          <span>{error}</span>
+        </p>
+      )}
       <button className="primary" type="submit">{textContent.addCountdownButton || "Ajouter"}</button>
     </form>
   );
diff --git a/website/src/hooks/useDraftForm.js b/website/src/hooks/useDraftForm.js
index 93f3aa3..ac1361a 100644
--- a/website/src/hooks/useDraftForm.js
+++ b/website/src/hooks/useDraftForm.js
@@ -1,23 +1,31 @@
 // Rôle : centralise les petits formulaires contrôlés avec brouillon local.
-// Fournit helpers de champs, submit preventDefault et reset, sans porter de validation métier.
+// Fournit helpers de champs, submit preventDefault, reset et erreur inline retournée par le submit métier.
 // À utiliser quand plusieurs champs sont simplement saisis puis nettoyés au submit.
 import { useState } from "react";
 
 export function useDraftForm(initialValues) {
   const [values, setValues] = useState(initialValues);
+  const [error, setError] = useState("");
 
   function setField(name, value) {
+    setError("");
     setValues((current) => ({ ...current, [name]: value }));
   }
 
   function updateValues(patch) {
+    setError("");
     setValues((current) => ({ ...current, ...patch }));
   }
 
   function reset(nextValues = initialValues) {
+    setError("");
     setValues(nextValues);
   }
 
+  function clearError() {
+    setError("");
+  }
+
   function getFieldProps(name, props = {}) {
     return {
       ...props,
@@ -32,13 +40,18 @@ export function useDraftForm(initialValues) {
 
   function handleSubmit(event, onSubmit) {
     event.preventDefault();
-    onSubmit(values, { reset, setField, setValues, updateValues });
+    const result = onSubmit(values, { reset, setField, setValues, updateValues, setError, clearError });
+    if (typeof result === "string") setError(result);
+    else setError("");
   }
 
   return {
     values,
+    error,
     setField,
     setValues,
+    setError,
+    clearError,
     updateValues,
     reset,
     getFieldProps,
diff --git a/website/src/styles/_toolboxes.scss b/website/src/styles/_toolboxes.scss
index fecdb8e..97ef861 100644
--- a/website/src/styles/_toolboxes.scss
+++ b/website/src/styles/_toolboxes.scss
@@ -3609,6 +3609,34 @@ button.combo-input-token.combo-value-grab .ui-icon {
   font-size: var(--font-size-xs);
 }
 
+.timer-form-error {
+  display: inline-flex;
+  width: 100%;
+  min-width: 0;
+  align-items: center;
+  gap: 7px;
+  padding: 7px 9px;
+  border: 1px solid rgba(251, 113, 133, 0.26);
+  border-radius: var(--radius-sm);
+  background: rgba(80, 25, 42, 0.28);
+  color: var(--color-danger);
+  font-size: var(--font-size-xs);
+  font-weight: 800;
+}
+
+.timer-form-error .ui-icon {
+  width: 13px;
+  height: 13px;
+  flex: 0 0 auto;
+}
+
+.timer-form-error span {
+  min-width: 0;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
 .timer-control-panel > span,
 .timer-control-panel label span {
   color: var(--color-text-secondary);