diff --git a/packages/web/src/components/ide/EditorArea.vue b/packages/web/src/components/ide/EditorArea.vue
new file mode 100644
index 0000000..e62db1d
--- /dev/null
+++ b/packages/web/src/components/ide/EditorArea.vue
@@ -0,0 +1,253 @@
+<template>
+  <div class="flex h-full min-h-0 flex-col">
+    <EditorTabs />
+
+    <template v-if="activeTab">
+      <!-- barre : bascule éditeur/diff + chemin + save -->
+      <div class="flex items-center gap-2 border-b border-border px-2 py-1 text-xs">
+        <SegmentedControl v-model="viewModel" :options="viewOptions" />
+        <span class="min-w-0 flex-1 truncate font-mono text-fg-subtle" :title="activeTab.file">{{ activeTab.file }}</span>
+        <BaseButton
+          v-if="activeTab.view === 'editor'"
+          size="sm"
+          variant="ghost"
+          :icon="Save"
+          :loading="saving"
+          :disabled="!ide.isDirty(activeTab.id) || !ready"
+          @click="save"
+        >
+          {{ t('editor.save') }}
+        </BaseButton>
+      </div>
+
+      <!-- bannière de conflit (le fichier a changé sur le disque depuis le chargement) -->
+      <div
+        v-if="conflict && activeTab.view === 'editor'"
+        class="flex flex-wrap items-center gap-2 border-b border-amber-700/40 bg-amber-950/30 px-3 py-1.5 text-xs text-amber-200"
+      >
+        <TriangleAlert :size="14" /> {{ t('editor.conflict') }}
+        <div class="ml-auto flex items-center gap-2">
+          <BaseButton size="sm" variant="ghost" @click="reload">{{ t('editor.reload') }}</BaseButton>
+          <BaseButton size="sm" variant="danger" :loading="saving" @click="overwrite">{{ t('editor.overwrite') }}</BaseButton>
+        </div>
+      </div>
+      <p v-if="loadError && activeTab.view === 'editor'" class="px-3 py-1 text-xs text-rose-400">{{ loadError }}</p>
+
+      <div class="relative min-h-0 flex-1">
+        <div v-show="activeTab.view === 'editor'" ref="host" class="h-full min-h-0" />
+        <DiffViewer
+          v-if="activeTab.view === 'diff'"
+          :key="`diff:${activeTab.id}`"
+          :repo-id="activeTab.repoId"
+          :wt="activeTab.wtPath"
+          :file="activeTab.file"
+          :staged="!!activeTab.diffStaged"
+          :version="0"
+          class="h-full overflow-auto"
+        />
+      </div>
+    </template>
+
+    <EmptyState v-else :icon="FileCode" :title="t('ide.noFileOpen')" :hint="t('ide.noFileHint')" class="m-6" />
+  </div>
+</template>
+
+<script setup lang="ts">
+// Zone centrale de l'IDE : UNE seule instance Monaco partagée entre tous les onglets, avec un cache
+// de modèles (préserve contenu, historique undo et position du curseur au changement d'onglet).
+// Les onglets « diff » réutilisent DiffViewer. Le dirty est remonté au store IDE.
+import { computed, onBeforeUnmount, onMounted, ref, shallowRef, useTemplateRef, watch } from 'vue';
+import { useI18n } from 'vue-i18n';
+import { FileCode, Save, TriangleAlert } from '@lucide/vue';
+import type * as Monaco from 'monaco-editor';
+import { useIdeStore, type EditorTab } from '../../stores/ide';
+import { gitApi } from '../../lib/git-api';
+import { ApiError } from '../../lib/api';
+import { loadMonaco } from '../../composables/useMonaco';
+import EditorTabs from './EditorTabs.vue';
+import DiffViewer from '../workspace/DiffViewer.vue';
+import SegmentedControl from '../ui/SegmentedControl.vue';
+import BaseButton from '../ui/BaseButton.vue';
+import EmptyState from '../ui/EmptyState.vue';
+
+interface ModelEntry {
+  model: Monaco.editor.ITextModel;
+  baseMtime: number | undefined;
+  savedContent: string;
+}
+
+const { t } = useI18n();
+const ide = useIdeStore();
+const host = useTemplateRef<HTMLDivElement>('host');
+
+const ready = ref(false);
+const saving = ref(false);
+const conflict = ref(false);
+const loadError = ref<string | null>(null);
+
+let monaco: typeof Monaco | null = null;
+const editor = shallowRef<Monaco.editor.IStandaloneCodeEditor | null>(null);
+const entries = new Map<string, ModelEntry>();
+const viewStates = new Map<string, Monaco.editor.ICodeEditorViewState | null>();
+let shownTabId: string | null = null;
+let disposed = false;
+
+const activeTab = computed(() => ide.activeTab);
+const viewOptions = computed(() => [
+  { value: 'editor', label: t('workspace.editor') },
+  { value: 'diff', label: t('workspace.diff') },
+]);
+const viewModel = computed<string>({
+  get: () => activeTab.value?.view ?? 'editor',
+  set: (v) => {
+    if (activeTab.value) ide.setTabView(activeTab.value.id, v === 'diff' ? 'diff' : 'editor', activeTab.value.diffStaged);
+  },
+});
+
+onMounted(async () => {
+  monaco = await loadMonaco();
+  if (disposed || !host.value) return;
+  editor.value = monaco.editor.create(host.value, {
+    value: '',
+    theme: 'arboretum-dark',
+    automaticLayout: true,
+    minimap: { enabled: false },
+    wordWrap: 'on',
+    fontSize: 13,
+    scrollBeyondLastLine: false,
+    tabSize: 2,
+  });
+  editor.value.onDidChangeModelContent(() => {
+    if (!shownTabId) return;
+    const entry = entries.get(shownTabId);
+    if (entry) ide.setTabDirty(shownTabId, entry.model.getValue() !== entry.savedContent);
+  });
+  editor.value.addCommand(monaco.KeyMod.CtrlCmd | monaco.KeyCode.KeyS, () => void save());
+  ready.value = true;
+  if (activeTab.value && activeTab.value.view === 'editor') await showTab(activeTab.value);
+});
+
+async function ensureEntry(tab: EditorTab): Promise<ModelEntry | null> {
+  const cached = entries.get(tab.id);
+  if (cached) return cached;
+  if (!monaco) return null;
+  const res = await gitApi.readFile(tab.repoId, tab.wtPath, tab.file);
+  if (disposed) return null;
+  const uri = monaco.Uri.parse(`inmemory://arb/${encodeURIComponent(tab.id)}`);
+  const model = monaco.editor.getModel(uri) ?? monaco.editor.createModel(res.content, res.language, uri);
+  const entry: ModelEntry = { model, baseMtime: res.mtime, savedContent: res.content };
+  entries.set(tab.id, entry);
+  return entry;
+}
+
+async function showTab(tab: EditorTab): Promise<void> {
+  if (!editor.value) return;
+  loadError.value = null;
+  conflict.value = false;
+  // sauvegarde de la position/scroll de l'onglet quitté
+  if (shownTabId && shownTabId !== tab.id) viewStates.set(shownTabId, editor.value.saveViewState());
+  try {
+    const entry = await ensureEntry(tab);
+    if (!entry || disposed || !editor.value) return;
+    editor.value.setModel(entry.model);
+    const vs = viewStates.get(tab.id);
+    if (vs) editor.value.restoreViewState(vs);
+    shownTabId = tab.id;
+    ide.setTabDirty(tab.id, entry.model.getValue() !== entry.savedContent);
+    requestAnimationFrame(() => editor.value?.layout());
+    editor.value.focus();
+  } catch (err) {
+    loadError.value = err instanceof Error ? err.message : String(err);
+  }
+}
+
+async function save(): Promise<void> {
+  const tab = activeTab.value;
+  if (!tab || tab.view !== 'editor' || saving.value || !ide.isDirty(tab.id)) return;
+  const entry = entries.get(tab.id);
+  if (!entry) return;
+  saving.value = true;
+  conflict.value = false;
+  try {
+    const content = entry.model.getValue();
+    const res = await gitApi.writeFile(tab.repoId, tab.wtPath, tab.file, content, entry.baseMtime);
+    entry.baseMtime = res.mtime;
+    entry.savedContent = content;
+    ide.setTabDirty(tab.id, false);
+  } catch (err) {
+    if (err instanceof ApiError && err.code === 'STALE_FILE') conflict.value = true;
+    else loadError.value = err instanceof Error ? err.message : String(err);
+  } finally {
+    saving.value = false;
+  }
+}
+
+async function overwrite(): Promise<void> {
+  const tab = activeTab.value;
+  if (!tab || saving.value) return;
+  const entry = entries.get(tab.id);
+  if (!entry) return;
+  saving.value = true;
+  try {
+    const content = entry.model.getValue();
+    const res = await gitApi.writeFile(tab.repoId, tab.wtPath, tab.file, content); // sans baseMtime : écrase
+    entry.baseMtime = res.mtime;
+    entry.savedContent = content;
+    ide.setTabDirty(tab.id, false);
+    conflict.value = false;
+  } catch (err) {
+    loadError.value = err instanceof Error ? err.message : String(err);
+  } finally {
+    saving.value = false;
+  }
+}
+
+async function reload(): Promise<void> {
+  const tab = activeTab.value;
+  if (!tab) return;
+  const entry = entries.get(tab.id);
+  if (!entry) return;
+  try {
+    const res = await gitApi.readFile(tab.repoId, tab.wtPath, tab.file);
+    entry.model.setValue(res.content);
+    entry.baseMtime = res.mtime;
+    entry.savedContent = res.content;
+    conflict.value = false;
+    ide.setTabDirty(tab.id, false);
+  } catch (err) {
+    loadError.value = err instanceof Error ? err.message : String(err);
+  }
+}
+
+// changement d'onglet actif -> affiche le bon modèle (si vue éditeur).
+watch(
+  () => [activeTab.value?.id, activeTab.value?.view] as const,
+  ([, view]) => {
+    if (ready.value && activeTab.value && view === 'editor') void showTab(activeTab.value);
+  },
+);
+
+// onglets fermés -> dispose leurs modèles et view states (évite les fuites mémoire).
+watch(
+  () => ide.editorTabs.map((tb) => tb.id),
+  (ids) => {
+    const alive = new Set(ids);
+    for (const [id, entry] of entries) {
+      if (!alive.has(id)) {
+        entry.model.dispose();
+        entries.delete(id);
+        viewStates.delete(id);
+        if (shownTabId === id) shownTabId = null;
+      }
+    }
+  },
+);
+
+onBeforeUnmount(() => {
+  disposed = true;
+  for (const entry of entries.values()) entry.model.dispose();
+  entries.clear();
+  viewStates.clear();
+  editor.value?.dispose();
+});
+</script>
diff --git a/packages/web/src/components/ide/EditorTabs.vue b/packages/web/src/components/ide/EditorTabs.vue
new file mode 100644
index 0000000..9273550
--- /dev/null
+++ b/packages/web/src/components/ide/EditorTabs.vue
@@ -0,0 +1,56 @@
+<template>
+  <div class="flex h-[var(--ide-tab-h)] shrink-0 items-stretch overflow-x-auto border-b border-border bg-surface-1">
+    <div
+      v-for="tab in ide.editorTabs"
+      :key="tab.id"
+      class="group flex cursor-pointer items-center gap-1.5 border-r border-border px-3 text-xs select-none"
+      :class="tab.id === ide.activeTabId ? 'bg-surface-0 text-fg' : 'text-fg-muted hover:bg-surface-2/50'"
+      :title="tab.file"
+      @click="ide.setActiveTab(tab.id)"
+      @mousedown.middle.prevent="requestClose(tab.id)"
+    >
+      <FileCode :size="12" class="shrink-0 text-fg-subtle" />
+      <span class="truncate font-mono">{{ basename(tab.file) }}</span>
+      <span v-if="tab.view === 'diff'" class="text-[9px] tracking-wide text-fg-subtle uppercase">diff</span>
+      <button
+        type="button"
+        class="ml-0.5 flex h-4 w-4 shrink-0 items-center justify-center rounded hover:bg-surface-3"
+        :class="closeButtonClass(tab.id)"
+        :title="pendingCloseId === tab.id ? t('editor.unsaved') : t('common.close')"
+        @click.stop="requestClose(tab.id)"
+      >
+        <component :is="pendingCloseId === tab.id ? AlertTriangle : ide.isDirty(tab.id) ? Dot : X" :size="pendingCloseId === tab.id ? 12 : 14" />
+      </button>
+    </div>
+  </div>
+</template>
+
+<script setup lang="ts">
+// Bandeau d'onglets de l'éditeur. Fermeture immédiate si propre ; garde à deux clics si modifié
+// (cohérent avec les autres confirmations de l'app), pour ne pas perdre des changements non sauvés.
+import { ref } from 'vue';
+import { useI18n } from 'vue-i18n';
+import { AlertTriangle, Dot, FileCode, X } from '@lucide/vue';
+import { useIdeStore } from '../../stores/ide';
+
+const { t } = useI18n();
+const ide = useIdeStore();
+const pendingCloseId = ref<string | null>(null);
+
+const basename = (path: string): string => path.split('/').pop() ?? path;
+
+function closeButtonClass(id: string): string {
+  if (pendingCloseId.value === id) return 'text-amber-400 opacity-100';
+  // dirty : point visible en permanence ; propre : croix visible au survol de l'onglet.
+  return ide.isDirty(id) ? 'text-amber-400 opacity-100 group-hover:text-fg' : 'opacity-0 group-hover:opacity-100';
+}
+
+function requestClose(id: string): void {
+  if (ide.isDirty(id) && pendingCloseId.value !== id) {
+    pendingCloseId.value = id; // premier clic sur un onglet modifié : demande confirmation
+    return;
+  }
+  pendingCloseId.value = null;
+  ide.closeTab(id);
+}
+</script>
diff --git a/packages/web/src/components/ide/IdeShell.vue b/packages/web/src/components/ide/IdeShell.vue
index 744fe28..02eed44 100644
--- a/packages/web/src/components/ide/IdeShell.vue
+++ b/packages/web/src/components/ide/IdeShell.vue
@@ -8,9 +8,9 @@
       </template>
 
       <div class="flex min-w-0 flex-1 flex-col">
-        <!-- centre : éditeur à onglets (livré en B4). Placeholder pour l'instant. -->
+        <!-- centre : éditeur Monaco à onglets + diff (B4) -->
         <main class="min-h-0 flex-1 overflow-hidden">
-          <EmptyState :icon="FileCode" :title="t('ide.noFileOpen')" :hint="t('ide.noFileHint')" class="m-6" />
+          <EditorArea />
         </main>
 
         <!-- dock terminaux (livré en B5). Placeholder pour l'instant. -->
@@ -33,7 +33,7 @@
 // des ressources ouvertes contre les données live (worktrees / sessions).
 import { watch } from 'vue';
 import { useI18n } from 'vue-i18n';
-import { FileCode, SquareTerminal } from '@lucide/vue';
+import { SquareTerminal } from '@lucide/vue';
 import { useIdeStore, wtKey } from '../../stores/ide';
 import { useWorktreesStore } from '../../stores/worktrees';
 import { useSessionsStore } from '../../stores/sessions';
@@ -41,6 +41,7 @@ import ActivityBar from './ActivityBar.vue';
 import PrimarySidebar from './PrimarySidebar.vue';
 import PanelSplitter from './PanelSplitter.vue';
 import StatusBar from './StatusBar.vue';
+import EditorArea from './EditorArea.vue';
 import EmptyState from '../ui/EmptyState.vue';
 
 const { t } = useI18n();