diff --git a/frontend/src/plugins/builtin/core-drawing/index.ts b/frontend/src/plugins/builtin/core-drawing/index.ts
index a15009a..2c48c5b 100644
--- a/frontend/src/plugins/builtin/core-drawing/index.ts
+++ b/frontend/src/plugins/builtin/core-drawing/index.ts
@@ -891,6 +891,137 @@ export const rayTool: ToolExtensionV2 = {
   },
 };
 
+// ─────────────────────────────────────────────────────────────
+// Task CAD-5: MLINE (Mehrfachlinie) — zwei parallele Linien
+// (Wand/Rohr) mit gap-Offset ±gap/2 entlang der Normalen.
+// Optionale Endkappen verbinden die beiden Linien an Start/Ende.
+// Klick-Sequenz wie polyline; ENTER committet ab 2 Punkten;
+// ALLE Teile in EINER Transaktion = 1 Undo-Schritt.
+// ─────────────────────────────────────────────────────────────
+
+let mlinePoints: Pt[] = [];
+
+/**
+ * Normalen-Offset-Punkte für eine Punktliste ±d (CAD-5, rein).
+ * An inneren Punkten wird die Richtung aus dem Mittel der
+ * angrenzenden Segment-Normalen bestimmt (Miter wie bei Wänden).
+ */
+function offsetPolyline(pts: Pt[], d: number): Pt[] {
+  if (pts.length < 2) return [...pts];
+  const out: Pt[] = [];
+  for (let i = 0; i < pts.length; i++) {
+    let nx = 0, ny = 0;
+    if (i > 0) {
+      const dx = pts[i].x - pts[i - 1].x;
+      const dy = pts[i].y - pts[i - 1].y;
+      const len = Math.hypot(dx, dy) || 1;
+      nx += -dy / len; ny += dx / len;
+    }
+    if (i < pts.length - 1) {
+      const dx = pts[i + 1].x - pts[i].x;
+      const dy = pts[i + 1].y - pts[i].y;
+      const len = Math.hypot(dx, dy) || 1;
+      nx += -dy / len; ny += dx / len;
+    }
+    const nLen = Math.hypot(nx, ny) || 1;
+    out.push({ x: pts[i].x + (nx / nLen) * d, y: pts[i].y + (ny / nLen) * d });
+  }
+  return out;
+}
+
+export const mlineTool: ToolExtensionV2 = {
+  manifest: {
+    id: 'mline',
+    label: 'Mehrfachlinie',
+    icon: '\u2551',
+    ribbonTab: 'canvas',
+    tags: ['basic'],
+    description: 'Doppel-Linie (Wand): Punkte klicken, ENTER beendet (CAD-5)',
+  },
+  optionsSchema: [
+    { key: 'gap', label: 'Gesamtstaerke', type: 'number', min: 1 },
+    { key: 'cap', label: 'Endkappen', type: 'checkbox' },
+    { key: 'strokeColor', label: 'Farbe', type: 'color' },
+  ],
+  handlers: {
+    down(e, ctx) {
+      mlinePoints.push(e.world);
+      ctx.setStatus(`Mehrfachlinie: ${mlinePoints.length} Punkte — ENTER beendet`);
+    },
+    move(e, ctx) {
+      const c = ctx as FullToolContext;
+      if (mlinePoints.length === 0) return;
+      const gap = (c.options.gap as number | undefined) ?? 20;
+      const pts = [...mlinePoints, e.world];
+      const a = offsetPolyline(pts, gap / 2);
+      const b = offsetPolyline(pts, -gap / 2);
+      ctx.setPreview?.({
+        id: '__preview_mline__',
+        type: 'polyline',
+        layerId: 'layer-0',
+        x: e.world.x, y: e.world.y, width: 0, height: 0,
+        properties: { points: a, stroke: '#e0e0f0', strokeWidth: 1, mline: true },
+      } as unknown as CADElement);
+    },
+    up() { /* Commit nur via ENTER */ },
+    key(e, ctx) {
+      if (e.key !== 'Enter' && e.key !== 'ENTER') return false;
+      const c = ctx as FullToolContext;
+      if (mlinePoints.length < 2) {
+        ctx.setStatus(`Mehrfachlinie benötigt mindestens 2 Punkte (aktuell ${mlinePoints.length})`);
+        return true;
+      }
+      if (!c.doc) return false;
+      const gap = Math.max(1, (c.options.gap as number | undefined) ?? 20);
+      const cap = (c.options.cap as boolean | undefined) ?? false;
+      const stroke = (c.options.strokeColor as string | undefined) ?? '#e0e0f0';
+      const layerId = (c.options.layerId as string | undefined) ?? 'layer-0';
+      const a = offsetPolyline(mlinePoints, gap / 2);
+      const b = offsetPolyline(mlinePoints, -gap / 2);
+      const mkLine = (pts: Pt[]): CADElement => ({
+        id: uid('mline'),
+        type: 'polyline',
+        layerId,
+        x: pts.reduce((s, p) => s + p.x, 0) / pts.length,
+        y: pts.reduce((s, p) => s + p.y, 0) / pts.length,
+        width: Math.max(...pts.map((p) => p.x)) - Math.min(...pts.map((p) => p.x)),
+        height: Math.max(...pts.map((p) => p.y)) - Math.min(...pts.map((p) => p.y)),
+        properties: { points: pts, stroke, strokeWidth: 1, mline: true, gap },
+      } as unknown as CADElement);
+      const els: CADElement[] = [mkLine(a), mkLine(b)];
+      if (cap) {
+        const capAt = (i: number): CADElement => ({
+          id: uid('mlinecap'),
+          type: 'line',
+          layerId,
+          x: (a[i].x + b[i].x) / 2,
+          y: (a[i].y + b[i].y) / 2,
+          width: Math.abs(b[i].x - a[i].x),
+          height: Math.abs(b[i].y - a[i].y),
+          properties: {
+            x1: a[i].x, y1: a[i].y, x2: b[i].x, y2: b[i].y,
+            stroke, strokeWidth: 1, mlineCap: true, mline: true,
+          },
+        } as unknown as CADElement);
+        els.push(capAt(0), capAt(a.length - 1));
+      }
+      c.doc.transact(() => {
+        for (const el of els) c.doc!.addElement(el);
+      });
+      const count = mlinePoints.length;
+      mlinePoints = [];
+      ctx.setPreview?.(null);
+      ctx.setStatus(`Mehrfachlinie erstellt (${count} Punkte${cap ? ', mit Endkappen' : ''})`);
+      return true;
+    },
+    cancel(ctx) {
+      mlinePoints = [];
+      ctx.setPreview?.(null);
+      ctx.setStatus('Mehrfachlinie abgebrochen');
+    },
+  },
+};
+
 export const coreDrawingPlugin: PluginV2 = {
   manifest: {
     id: 'core-drawing',
@@ -901,5 +1032,5 @@ export const coreDrawingPlugin: PluginV2 = {
     category: 'tools',
     enabledByDefault: true,
   },
-  tools: [lineTool, rectTool, circleTool, arcTool, polylineTool, polygonTool, revcloudTool, ellipseTool, splineTool, pointTool, xlineTool, rayTool],
+  tools: [lineTool, rectTool, circleTool, arcTool, polylineTool, polygonTool, revcloudTool, ellipseTool, splineTool, pointTool, xlineTool, rayTool, mlineTool],
 };
diff --git a/frontend/src/plugins/builtin/core-modify/index.ts b/frontend/src/plugins/builtin/core-modify/index.ts
index ea65cf9..5b69698 100644
--- a/frontend/src/plugins/builtin/core-modify/index.ts
+++ b/frontend/src/plugins/builtin/core-modify/index.ts
@@ -25,6 +25,9 @@ import {
   extendElement,
   filletElements,
 } from '../../../tools/modification/geometry';
+import { openBlockForEdit, saveBlockEdit, discardBlockEdit } from '../../../services/blockEditorService';
+import { BLOCK_EDIT_MARKER } from '../../../services/blockEditorService';
+export { BLOCK_EDIT_MARKER };
 import type { Pt } from '../../../tools/modification/geometry';
 
 const uid = (prefix: string): string =>
@@ -1861,6 +1864,57 @@ export const measureTool: ToolExtensionV2 = {
   },
 };
 
+// ─────────────────────────────────────────────────────────────
+// Task CAD-5: Block-Editor — Blockdefinition in-place bearbeiten.
+// Klick 1 auf Block-Instanz oeffnet (markierte Kopien erscheinen),
+// Kopien normal bearbeiten, Klick 2 speichert zurueck in die
+// Definition, cancel verwirft. (Service: blockEditorService)
+// ─────────────────────────────────────────────────────────────
+
+export const blockEditTool: ToolExtensionV2 = {
+  manifest: {
+    id: 'block-edit',
+    label: 'Block bearbeiten',
+    icon: '\u2699',
+    ribbonTab: 'canvas',
+    tags: ['pro'],
+    description: 'Block-Instanz anklicken um die Definition zu bearbeiten (CAD-5)',
+  },
+  optionsSchema: [],
+  handlers: {
+    down(e, ctx) {
+      const c = ctx as FullToolContext;
+      if (!c.doc || !c.selection) return;
+      // Offene Sitzung? → speichern
+      if ((c.doc.getAllElements() as CADElement[]).some(
+        (el) => (el.properties as Record<string, unknown>)[BLOCK_EDIT_MARKER],
+      )) {
+        const saved = saveBlockEdit(c.doc as never);
+        ctx.setStatus(saved ? `Block "${saved}" gespeichert` : 'Speichern fehlgeschlagen');
+        return;
+      }
+      const hit = c.selection.hitTest(e.world.x, e.world.y, 8);
+      if (!hit || hit.type !== 'block_instance') {
+        ctx.setStatus('Block bearbeiten: Block-Instanz anklicken');
+        return;
+      }
+      const blockId = (hit.properties as Record<string, unknown>).blockId as string | undefined;
+      if (!blockId) {
+        ctx.setStatus('Block bearbeiten: Instanz ohne Block-Referenz');
+        return;
+      }
+      const opened = openBlockForEdit(c.doc as never, blockId);
+      ctx.setStatus(opened ? `Block "${blockId}" geöffnet — Elemente bearbeiten, erneut klicken speichert` : 'Block konnte nicht geöffnet werden');
+    },
+    cancel(ctx) {
+      const c = ctx as FullToolContext;
+      if (c.doc) discardBlockEdit(c.doc as never);
+      ctx.setPreview?.(null);
+      ctx.setStatus('Block-Bearbeitung verworfen');
+    },
+  },
+};
+
 export const coreModifyPlugin: PluginV2 = {
   manifest: {
     id: 'core-modify',
@@ -1871,5 +1925,5 @@ export const coreModifyPlugin: PluginV2 = {
     category: 'tools',
     enabledByDefault: true,
   },
-  tools: [selectTool, hatchTool, moveTool, copyTool, rotateTool, scaleTool, mirrorTool, deleteTool, offsetTool, trimTool, extendTool, filletTool, arrayRectTool, arrayPolarTool, chamferTool, lengthenTool, joinTool, explodeTool, alignTool, polylineEditTool, stretchTool, breakTool, arrayPathTool, divideTool, measureTool],
+  tools: [selectTool, hatchTool, moveTool, copyTool, rotateTool, scaleTool, mirrorTool, deleteTool, offsetTool, trimTool, extendTool, filletTool, arrayRectTool, arrayPolarTool, chamferTool, lengthenTool, joinTool, explodeTool, alignTool, polylineEditTool, stretchTool, breakTool, arrayPathTool, divideTool, measureTool, blockEditTool],
 };
diff --git a/frontend/src/services/blockEditorService.ts b/frontend/src/services/blockEditorService.ts
new file mode 100644
index 0000000..e09cb30
--- /dev/null
+++ b/frontend/src/services/blockEditorService.ts
@@ -0,0 +1,94 @@
+/**
+ * Task CAD-5 – Block-Editor-Service.
+ *
+ * openBlockForEdit legt markierte KOPIEN der Block-Elemente ins
+ * Dokument (Original unangetastet); saveBlockEdit schreibt die
+ * bearbeiteten Kopien als neue Blockdefinition zurueck und
+ * raeumt die Edit-Kopien auf; discardBlockEdit verwirft alles.
+ * Marker: BLOCK_EDIT_MARKER (properties-Flag + __blockEdit-IDs).
+ */
+import type { CADElement, BlockDefinition } from '../types/cad.types';
+import type { CADDocument } from '../kernel/document/CADDocument';
+
+/** Marker-Flag in properties der Edit-Kopien. */
+export const BLOCK_EDIT_MARKER = '__blockEdit';
+
+/** Aktive Edit-Sitzung (Service-State). */
+let editSession: { blockId: string; copyIds: string[] } | null = null;
+
+/**
+ * Oeffnet einen Block zur Bearbeitung: Kopien aller Elemente mit
+ * Edit-Marker werden ins Dokument gelegt (Original bleibt).
+ * @returns true wenn geoeffnet, false bei unbekannter ID oder
+ * bereits offener Sitzung.
+ */
+export function openBlockForEdit(doc: CADDocument, blockId: string): boolean {
+  if (editSession) return false;
+  const block = doc.getBlock(blockId);
+  if (!block) return false;
+  const copyIds: string[] = [];
+  doc.transact(() => {
+    for (const el of block.elements) {
+      const copy: CADElement = {
+        ...el,
+        id: `__blockEdit_${blockId}_${copyIds.length}`,
+        properties: {
+          ...(el.properties as Record<string, unknown>),
+          [BLOCK_EDIT_MARKER]: true,
+        },
+      } as unknown as CADElement;
+      doc.addElement(copy);
+      copyIds.push(copy.id);
+    }
+  });
+  editSession = { blockId, copyIds };
+  return true;
+}
+
+/**
+ * Speichert die Bearbeitung: die markierten Kopien werden als
+ * neue Elementliste in die Blockdefinition geschrieben (Marker
+ * entfernt); Edit-Kopien werden aus dem Dokument geloescht.
+ * @returns blockId der gespeicherten Definition oder null.
+ */
+export function saveBlockEdit(doc: CADDocument): string | null {
+  if (!editSession) return null;
+  const { blockId, copyIds } = editSession;
+  const elements: CADElement[] = [];
+  for (const id of copyIds) {
+    const el = doc.getElement(id);
+    if (!el) continue;
+    const { [BLOCK_EDIT_MARKER]: _marker, ...cleanProps } =
+      el.properties as Record<string, unknown>;
+    elements.push({ ...el, id: uid('blk'), properties: cleanProps } as unknown as CADElement);
+  }
+  const block = doc.getBlock(blockId);
+  if (block) {
+    doc.addBlock({ ...block, elements });
+  }
+  doc.deleteElements(copyIds);
+  editSession = null;
+  return blockId;
+}
+
+/**
+ * Verwirft die Bearbeitung: Edit-Kopien werden geloescht, die
+ * Blockdefinition bleibt unveraendert.
+ * @returns true wenn verworfen, false ohne offene Sitzung.
+ */
+export function discardBlockEdit(doc: CADDocument): boolean {
+  if (!editSession) return false;
+  doc.deleteElements(editSession.copyIds);
+  editSession = null;
+  return true;
+}
+
+/** Ist gerade eine Block-Bearbeitung offen? */
+export function isBlockEditOpen(): boolean {
+  return editSession !== null;
+}
+
+let uidCounter = 0;
+function uid(prefix: string): string {
+  return `${prefix}_${Date.now().toString(36)}_${uidCounter++}_${Math.random().toString(36).slice(2, 8)}`;
+}
diff --git a/frontend/tests/mlineBlockEditor.test.ts b/frontend/tests/mlineBlockEditor.test.ts
new file mode 100644
index 0000000..ea2a77d
--- /dev/null
+++ b/frontend/tests/mlineBlockEditor.test.ts
@@ -0,0 +1,314 @@
+/**
+ * Task CAD-5 - MLINE (Mehrfachlinie) + Block-Editor.
+ *
+ * MLINE: Zwei parallele Linien (Wand/Rohr) mit gap-Offset ±gap/2
+ * entlang der Normalen, optionale Endkappen, ENTER-Commit ab
+ * 2 Punkten, ALLES in EINER Transaktion.
+ * Block-Editor: openBlockForEdit (markierte Kopien), saveBlockEdit
+ * (zurueckschreiben + aufraeumen), discard, blockEditTool-Flow.
+ */
+import { describe, it, expect, beforeEach, afterEach } from 'vitest';
+import { mlineTool } from '../src/plugins/builtin/core-drawing';
+import { blockEditTool, BLOCK_EDIT_MARKER } from '../src/plugins/builtin/core-modify';
+import {
+  openBlockForEdit,
+  saveBlockEdit,
+  discardBlockEdit,
+} from '../src/services/blockEditorService';
+import { CADDocument } from '../src/kernel/document/CADDocument';
+import { createInMemoryDoc } from './helpers/inMemoryDoc';
+import type { CADElement, Pt, BlockDefinition } from '../src/types/cad.types';
+
+function mkCtx(options: Record<string, unknown> = {}) {
+  const { ydoc } = createInMemoryDoc();
+  const doc = new CADDocument(ydoc);
+  const statuses: string[] = [];
+  const previews: (CADElement | null)[] = [];
+  const ctx: any = {
+    doc,
+    options,
+    setStatus: (m: string) => statuses.push(m),
+    setPreview: (el: CADElement | null) => previews.push(el),
+  };
+  return { ctx, doc, statuses, previews };
+}
+
+function pe(x: number, y: number): { world: Pt } {
+  return { world: { x, y } } as never;
+}
+
+function key(k: string): { key: string } {
+  return { key: k } as never;
+
+}
+
+const BLOCK: BlockDefinition = {
+  id: 'blk-1', name: 'Muster', description: '', category: 'test',
+  elements: [
+    { id: 'e1', type: 'rect', layerId: 'l', x: 50, y: 25, width: 100, height: 50, properties: {} },
+    { id: 'e2', type: 'line', layerId: 'l', x: 50, y: 50, width: 100, height: 0, properties: { x1: 0, y1: 50, x2: 100, y2: 50 } },
+  ],
+};
+
+afterEach(() => {
+  // Service-State-Leak verhindern: offene Edit-Sitzungen immer verwerfen
+  // (Dummy-Doc: deleteElements ignoriert fehlende IDs still)
+  const { ydoc: y2 } = createInMemoryDoc();
+  discardBlockEdit(new CADDocument(y2));
+});
+
+beforeEach(() => {
+  mlineTool.handlers.cancel?.({ setStatus: () => {}, setPreview: () => {} } as never);
+  blockEditTool.handlers.cancel?.({ setStatus: () => {}, setPreview: () => {} } as never);
+});
+
+// ─── MLINE ────────────────────────────────────────────────────
+
+describe('CAD-5: mlineTool', () => {
+  it('horizontale Mittellinie: 2 parallele Linien mit gap/2 Abstand', () => {
+    const { ctx, doc } = mkCtx({ gap: 20 });
+    mlineTool.handlers.down!(pe(0, 0), ctx);
+    mlineTool.handlers.down!(pe(100, 0), ctx);
+    mlineTool.handlers.key!(key('Enter'), ctx);
+    const lines = doc.getAllElements().filter((e) => e.type === 'polyline');
+    expect(lines).toHaveLength(2);
+    const ys = lines.map((l) => (l.properties.points as Pt[])[0].y).sort();
+    expect(ys[0]).toBeCloseTo(-10, 5);
+    expect(ys[1]).toBeCloseTo(10, 5);
+  });
+
+  it('gap-Option steuert den Abstand (gap=10 → ±5)', () => {
+    const { ctx, doc } = mkCtx({ gap: 10 });
+    mlineTool.handlers.down!(pe(0, 0), ctx);
+    mlineTool.handlers.down!(pe(50, 0), ctx);
+    mlineTool.handlers.key!(key('Enter'), ctx);
+    const lines = doc.getAllElements().filter((e) => e.type === 'polyline');
+    const ys = lines.map((l) => (l.properties.points as Pt[])[0].y).sort();
+    expect(ys[0]).toBeCloseTo(-5, 5);
+    expect(ys[1]).toBeCloseTo(5, 5);
+  });
+
+  it('Endkappen-Option: 2 zusaetzliche Linien an Start und Ende', () => {
+    const { ctx, doc } = mkCtx({ gap: 20, cap: true });
+    mlineTool.handlers.down!(pe(0, 0), ctx);
+    mlineTool.handlers.down!(pe(100, 0), ctx);
+    mlineTool.handlers.key!(key('Enter'), ctx);
+    const caps = doc.getAllElements().filter((e) => e.properties.mlineCap);
+    expect(caps).toHaveLength(2);
+    // Startkappe verbindet beide Linien (y=±10) bei x=0 — Zuordnung a/b
+    // ist Vorzeichenkonvention, daher reihenfolge-agnostisch pruefen:
+    const startCap = caps.find((c) => (c.properties.x1 as number) === 0);
+    expect(startCap).toBeDefined();
+    const ys = [startCap!.properties.y1, startCap!.properties.y2].sort((p, q) => (p as number) - (q as number)) as number[];
+    expect(ys[0]).toBeCloseTo(-10, 5);
+    expect(ys[1]).toBeCloseTo(10, 5);
+  });
+
+  it('ohne cap: nur 2 Linien, keine Kappen', () => {
+    const { ctx, doc } = mkCtx({ gap: 20 });
+    mlineTool.handlers.down!(pe(0, 0), ctx);
+    mlineTool.handlers.down!(pe(100, 0), ctx);
+    mlineTool.handlers.key!(key('Enter'), ctx);
+    expect(doc.getAllElements().filter((e) => e.properties.mlineCap)).toHaveLength(0);
+  });
+
+  it('EINE Transaktion: 1 Undo entfernt alle MLINE-Teile', () => {
+    const { ctx, doc } = mkCtx({ gap: 20, cap: true });
+    mlineTool.handlers.down!(pe(0, 0), ctx);
+    mlineTool.handlers.down!(pe(100, 0), ctx);
+    mlineTool.handlers.key!(key('Enter'), ctx);
+    expect(doc.getAllElements().length).toBe(4); // 2 Linien + 2 Kappen
+    doc.undo();
+    expect(doc.getAllElements()).toHaveLength(0);
+  });
+
+  it('ENTER mit weniger als 2 Punkten committet NICHT', () => {
+    const { ctx, doc, statuses } = mkCtx({ gap: 20 });
+    mlineTool.handlers.down!(pe(0, 0), ctx);
+    mlineTool.handlers.key!(key('Enter'), ctx);
+    expect(doc.getAllElements()).toHaveLength(0);
+    expect(statuses[statuses.length - 1]).toContain('2');
+  });
+
+  it('L-foermiger Pfad: beide Linien folgen der Geometrie', () => {
+    const { ctx, doc } = mkCtx({ gap: 20 });
+    mlineTool.handlers.down!(pe(0, 0), ctx);
+    mlineTool.handlers.down!(pe(100, 0), ctx);
+    mlineTool.handlers.down!(pe(100, 100), ctx);
+    mlineTool.handlers.key!(key('Enter'), ctx);
+    const lines = doc.getAllElements().filter((e) => e.type === 'polyline');
+    expect(lines).toHaveLength(2);
+    for (const l of lines) {
+      const pts = l.properties.points as Pt[];
+      expect(pts).toHaveLength(3); // gleiche Punktanzahl wie Mittellinie
+    }
+  });
+
+  it('Metadaten: mline:true und gap in properties', () => {
+    const { ctx, doc } = mkCtx({ gap: 20 });
+    mlineTool.handlers.down!(pe(0, 0), ctx);
+    mlineTool.handlers.down!(pe(100, 0), ctx);
+    mlineTool.handlers.key!(key('Enter'), ctx);
+    const line = doc.getAllElements().find((e) => e.properties.mline)!;
+    expect(line.properties.mline).toBe(true);
+    expect(line.properties.gap).toBe(20);
+  });
+
+  it('cancel resettet die Sitzung', () => {
+    const { ctx, doc } = mkCtx({ gap: 20 });
+    mlineTool.handlers.down!(pe(0, 0), ctx);
+    mlineTool.handlers.cancel?.(ctx);
+    mlineTool.handlers.down!(pe(0, 0), ctx);
+    mlineTool.handlers.key!(key('Enter'), ctx); // nur 1 Punkt der NEUEN Sitzung
+    expect(doc.getAllElements()).toHaveLength(0);
+  });
+
+  it('Manifest: id=mline, Optionen gap und cap', () => {
+    expect(mlineTool.manifest.id).toBe('mline');
+    const keys = (mlineTool.optionsSchema ?? []).map((f) => f.key);
+    expect(keys).toContain('gap');
+    expect(keys).toContain('cap');
+  });
+});
+
+// ─── Block-Editor Service ──────────────────────────────────────
+
+describe('CAD-5: blockEditorService', () => {
+  function mkDoc(): CADDocument {
+    const { ydoc } = createInMemoryDoc();
+    const doc = new CADDocument(ydoc);
+    doc.addBlock(BLOCK);
+    return doc;
+  }
+
+  it('openBlockForEdit erzeugt markierte Kopien im Dokument', () => {
+    const doc = mkDoc();
+    expect(openBlockForEdit(doc, 'blk-1')).toBe(true);
+    const copies = doc.getAllElements().filter((e) => (e.properties as Record<string, unknown>)[BLOCK_EDIT_MARKER]);
+    expect(copies).toHaveLength(2);
+    expect(doc.getBlock('blk-1')!.elements).toHaveLength(2); // Original unangetastet
+  });
+
+  it('openBlockForEdit mit unbekannter ID: false', () => {
+    const doc = mkDoc();
+    expect(openBlockForEdit(doc, 'gibts-nicht')).toBe(false);
+  });
+
+  it('saveBlockEdit schreibt bearbeitete Elemente zurueck und raeumt auf', () => {
+    const doc = mkDoc();
+    openBlockForEdit(doc, 'blk-1');
+    // Eine Kopie bearbeiten: rect verbreitern
+    const copies = doc.getAllElements().filter((e) => (e.properties as Record<string, unknown>)[BLOCK_EDIT_MARKER]);
+    const rectCopy = copies.find((c) => c.type === 'rect')!;
+    doc.updateElement(rectCopy.id, { width: 200 });
+    const saved = saveBlockEdit(doc);
+    expect(saved).toBe('blk-1');
+    // Blockdefinition hat die neue Breite
+    const savedRect = doc.getBlock('blk-1')!.elements.find((e) => e.type === 'rect')!;
+    expect(savedRect.width).toBe(200);
+    // Edit-Kopien sind aus dem Dokument entfernt
+    const remaining = doc.getAllElements().filter((e) => (e.properties as Record<string, unknown>)[BLOCK_EDIT_MARKER]);
+    expect(remaining).toHaveLength(0);
+  });
+
+  it('saveBlockEdit ohne offene Sitzung: null', () => {
+    const doc = mkDoc();
+    expect(saveBlockEdit(doc)).toBeNull();
+  });
+
+  it('discardBlockEdit verwirft Aenderungen (Block unveraendert, Kopien weg)', () => {
+    const doc = mkDoc();
+    openBlockForEdit(doc, 'blk-1');
+    const copies = doc.getAllElements().filter((e) => (e.properties as Record<string, unknown>)[BLOCK_EDIT_MARKER]);
+    doc.updateElement(copies[0].id, { width: 999 });
+    expect(discardBlockEdit(doc)).toBe(true);
+    expect(doc.getBlock('blk-1')!.elements.find((e) => e.type === 'rect')!.width).toBe(100);
+    expect(doc.getAllElements().filter((e) => (e.properties as Record<string, unknown>)[BLOCK_EDIT_MARKER])).toHaveLength(0);
+  });
+
+  it('saveBlockEdit entfernt Marker aus den gespeicherten Elementen', () => {
+    const doc = mkDoc();
+    openBlockForEdit(doc, 'blk-1');
+    saveBlockEdit(doc);
+    for (const el of doc.getBlock('blk-1')!.elements) {
+      expect((el.properties as Record<string, unknown>)[BLOCK_EDIT_MARKER]).toBeUndefined();
+    }
+  });
+});
+
+// ─── blockEditTool ─────────────────────────────────────────────
+
+describe('CAD-5: blockEditTool', () => {
+  function mkToolCtx() {
+    const { ydoc } = createInMemoryDoc();
+    const doc = new CADDocument(ydoc);
+    doc.addBlock(BLOCK);
+    doc.addElement({
+      id: 'inst-1', type: 'block_instance', layerId: 'l',
+      x: 0, y: 0, width: 0, height: 0,
+      properties: { blockId: 'blk-1' },
+    } as unknown as CADElement);
+    const statuses: string[] = [];
+    const ctx: any = {
+      doc,
+      options: {},
+      setStatus: (m: string) => statuses.push(m),
+      setPreview: (_el: CADElement | null) => {},
+      selection: {
+        getIds: () => [] as string[],
+        setIds: (_ids: string[]) => {},
+        hitTest: (_x: number, _y: number, _tol: number) => doc.getElement('inst-1'),
+      },
+    };
+    return { ctx, doc, statuses };
+  }
+
+  it('erster Klick auf Instanz oeffnet den Block-Editor (Kopien erscheinen)', () => {
+    const { ctx, doc, statuses } = mkToolCtx();
+    blockEditTool.handlers.down!(pe(0, 0), ctx);
+    expect(doc.getAllElements().filter((e) => (e.properties as Record<string, unknown>)[BLOCK_EDIT_MARKER])).toHaveLength(2);
+    expect(statuses[statuses.length - 1]).toContain('Block');
+  });
+
+  it('zweiter Klick speichert (Block aktualisiert, Kopien weg)', () => {
+    const { ctx, doc } = mkToolCtx();
+    blockEditTool.handlers.down!(pe(0, 0), ctx);
+    // Kopie bearbeiten
+    const copy = doc.getAllElements().find((e) => (e.properties as Record<string, unknown>)[BLOCK_EDIT_MARKER] && e.type === 'rect')!;
+    doc.updateElement(copy.id, { width: 150 });
+    blockEditTool.handlers.down!(pe(0, 0), ctx); // speichern
+    expect(doc.getBlock('blk-1')!.elements.find((e) => e.type === 'rect')!.width).toBe(150);
+    expect(doc.getAllElements().filter((e) => (e.properties as Record<string, unknown>)[BLOCK_EDIT_MARKER])).toHaveLength(0);
+  });
+
+  it('cancel verwirft die Bearbeitung', () => {
+    const { ctx, doc } = mkToolCtx();
+    blockEditTool.handlers.down!(pe(0, 0), ctx);
+    const copy = doc.getAllElements().find((e) => (e.properties as Record<string, unknown>)[BLOCK_EDIT_MARKER] && e.type === 'rect')!;
+    doc.updateElement(copy.id, { width: 999 });
+    blockEditTool.handlers.cancel?.(ctx);
+    expect(doc.getBlock('blk-1')!.elements.find((e) => e.type === 'rect')!.width).toBe(100);
+    expect(doc.getAllElements().filter((e) => (e.properties as Record<string, unknown>)[BLOCK_EDIT_MARKER])).toHaveLength(0);
+  });
+
+  it('Klick ohne Instanz-Treffer: Status erklaert, nichts passiert', () => {
+    const { ydoc } = createInMemoryDoc();
+    const doc = new CADDocument(ydoc);
+    doc.addBlock(BLOCK);
+    const statuses: string[] = [];
+    const ctx: any = {
+      doc,
+      options: {},
+      setStatus: (m: string) => statuses.push(m),
+      setPreview: () => {},
+      selection: { getIds: () => [], setIds: () => {}, hitTest: () => null },
+    };
+    blockEditTool.handlers.down!(pe(0, 0), ctx);
+    expect(doc.getAllElements()).toHaveLength(0);
+    expect(statuses[statuses.length - 1]).toContain('Block-Instanz');
+  });
+
+  it('Manifest: id=block-edit', () => {
+    expect(blockEditTool.manifest.id).toBe('block-edit');
+  });
+});