diff --git a/frontend/src/components/BlockLibraryTree.tsx b/frontend/src/components/BlockLibraryTree.tsx
index bec4e51..c7d7dc3 100644
--- a/frontend/src/components/BlockLibraryTree.tsx
+++ b/frontend/src/components/BlockLibraryTree.tsx
@@ -13,6 +13,9 @@ import {
   exportLibrary,
   importLibrary,
 } from '../services/api';
+import { getActiveDocument } from '../kernel/document/documentService';
+import { generateBlockSvg } from '../utils/blockThumbnail';
+import type { CADElement } from '../types/cad.types';
 
 interface BlockLibraryTreeProps {
   token: string;
@@ -76,6 +79,38 @@ const BlockLibraryTree: React.FC<BlockLibraryTreeProps> = ({ token, onBlockDragS
     loadData();
   }, [loadData]);
 
+  // ─── Task F3: Auswahl als Block speichern ─────────────────
+  const handleSaveSelectionAsBlock = useCallback(async () => {
+    const bridge = (window as unknown as { __v2GetSelection?: () => { ids: string[] } }).__v2GetSelection;
+    const ids = bridge?.().ids ?? [];
+    if (ids.length === 0) {
+      window.alert('Keine Auswahl im Canvas — erst Elemente wählen (V2-Tools).');
+      return;
+    }
+    const doc = getActiveDocument();
+    const elements: CADElement[] = [];
+    for (const id of ids) {
+      const el = doc?.getElement(id);
+      if (el) elements.push(el);
+    }
+    if (elements.length === 0) {
+      window.alert('Auswahl enthält keine Elemente.');
+      return;
+    }
+    const name = window.prompt('Blockname:', `Block-${new Date().toISOString().slice(0, 10)}`);
+    if (!name?.trim()) return;
+    try {
+      await createGlobalBlock(token, {
+        name: name.trim(),
+        block_data: JSON.stringify(elements),
+        svg_data: generateBlockSvg(elements),
+      });
+      await loadData();
+    } catch (err) {
+      window.alert(`Speichern fehlgeschlagen: ${err instanceof Error ? err.message : String(err)}`);
+    }
+  }, [token, loadData]);
+
   // ─── Task F1: Library-Paket Export/Import ────────────────
 
   const handleLibraryExport = useCallback(async () => {
@@ -459,6 +494,13 @@ const BlockLibraryTree: React.FC<BlockLibraryTreeProps> = ({ token, onBlockDragS
     <div className="global-lib-tree" onContextMenu={(e) => handleContextMenu(e, 'root', null, null)}>
       <div className="global-lib-header">
         <span className="global-lib-title">🌐 Globale Bibliothek</span>
+        <button
+          className="global-lib-add-btn"
+          title="Canvas-Auswahl als Block speichern (F3)"
+          onClick={() => void handleSaveSelectionAsBlock()}
+        >
+          💾
+        </button>
         <button
           className="global-lib-add-btn"
           title="Bibliothek als .wcadlib exportieren"
diff --git a/frontend/src/utils/blockThumbnail.ts b/frontend/src/utils/blockThumbnail.ts
new file mode 100644
index 0000000..eb4d307
--- /dev/null
+++ b/frontend/src/utils/blockThumbnail.ts
@@ -0,0 +1,109 @@
+/**
+ * blockThumbnail – SVG-Thumbnail-Generierung für Block-Bibliothek (Task F3).
+ *
+ * Statt canvas.toDataURL werden die Elemente über die bereits vorhandene,
+ * testbare Shape-Fabrik elementToPrimitives (Task C2) beschrieben und als
+ * eigenständiges SVG serialisiert. Vorteile:
+ * - Konsistent zu den svg_data-Thumbnails der globalen Bibliothek (Tree/Grid)
+ * - Unit-testbar ohne Canvas/DOM
+ * - Skalierbar & themefarben-identisch zur Zeichenfläche
+ *
+ * BBox-Konvention wie elementDrawers: rect x/y = Zentrum der BBox,
+ * circle/arc radius via center ± r.
+ */
+import { elementToPrimitives } from '../render/pixi/elementDrawers';
+import type { ShapePrimitive } from '../render/pixi/elementDrawers';
+import type { CADElement } from '../types/cad.types';
+
+const PAD = 10;
+
+function fmt(n: number): string {
+  return String(Math.round(n * 100) / 100);
+}
+
+function escapeXml(s: string): string {
+  return s.replace(/&/g, '&amp;').replace(/</g, '&lt;').replace(/>/g, '&gt;').replace(/"/g, '&quot;');
+}
+
+interface BBox { minX: number; minY: number; maxX: number; maxY: number }
+
+function primExtent(p: ShapePrimitive, acc: BBox): void {
+  const add = (x: number, y: number) => {
+    if (x < acc.minX) acc.minX = x;
+    if (y < acc.minY) acc.minY = y;
+    if (x > acc.maxX) acc.maxX = x;
+    if (y > acc.maxY) acc.maxY = y;
+  };
+  switch (p.kind) {
+    case 'line':
+      add(p.from.x, p.from.y); add(p.to.x, p.to.y);
+      break;
+    case 'rect':
+      add(p.x - p.w / 2, p.y - p.h / 2); add(p.x + p.w / 2, p.y + p.h / 2);
+      break;
+    case 'circle':
+      add(p.center.x - p.radius, p.center.y - p.radius); add(p.center.x + p.radius, p.center.y + p.radius);
+      break;
+    case 'arc':
+      add(p.center.x - p.radius, p.center.y - p.radius); add(p.center.x + p.radius, p.center.y + p.radius);
+      break;
+    case 'polyline':
+      for (const pt of p.points) add(pt.x, pt.y);
+      break;
+    case 'text':
+      add(p.at.x, p.at.y - p.fontSize);
+      add(p.at.x + p.fontSize * 0.6 * p.text.length, p.at.y);
+      break;
+  }
+}
+
+function primToSvgTag(p: ShapePrimitive): string {
+  switch (p.kind) {
+    case 'line':
+      return `<line x1="${fmt(p.from.x)}" y1="${fmt(p.from.y)}" x2="${fmt(p.to.x)}" y2="${fmt(p.to.y)}" stroke="${p.stroke}" stroke-width="${p.strokeWidth}"/>`;
+    case 'rect':
+      return `<rect x="${fmt(p.x - p.w / 2)}" y="${fmt(p.y - p.h / 2)}" width="${fmt(p.w)}" height="${fmt(p.h)}" fill="${p.fill ?? 'none'}" stroke="${p.stroke}" stroke-width="${p.strokeWidth}"/>`;
+    case 'circle':
+      return `<circle cx="${fmt(p.center.x)}" cy="${fmt(p.center.y)}" r="${fmt(p.radius)}" fill="${p.fill ?? 'none'}" stroke="${p.stroke}" stroke-width="${p.strokeWidth}"/>`;
+    case 'arc': {
+      // Konvention: 0° = 3 Uhr, gegen Uhrzeigersinn (Bildschirm-Y zeigt nach unten)
+      const rad = (deg: number) => (deg * Math.PI) / 180;
+      const sx = p.center.x + p.radius * Math.cos(rad(p.startDeg));
+      const sy = p.center.y - p.radius * Math.sin(rad(p.startDeg));
+      const ex = p.center.x + p.radius * Math.cos(rad(p.endDeg));
+      const ey = p.center.y - p.radius * Math.sin(rad(p.endDeg));
+      let delta = (p.endDeg - p.startDeg) % 360;
+      if (delta < 0) delta += 360;
+      const largeArc = delta > 180 ? 1 : 0;
+      // CCW auf dem Schirm = SVG sweep-flag 0
+      return `<path d="M ${fmt(sx)} ${fmt(sy)} A ${fmt(p.radius)} ${fmt(p.radius)} 0 ${largeArc} 0 ${fmt(ex)} ${fmt(ey)}" fill="none" stroke="${p.stroke}" stroke-width="${p.strokeWidth}"/>`;
+    }
+    case 'polyline': {
+      const pts = p.points.map(pt => `${fmt(pt.x)},${fmt(pt.y)}`).join(' ');
+      if (p.close) {
+        return `<polygon points="${pts}" fill="${'none'}" stroke="${p.stroke}" stroke-width="${p.strokeWidth}"/>`;
+      }
+      return `<polyline points="${pts}" fill="none" stroke="${p.stroke}" stroke-width="${p.strokeWidth}"/>`;
+    }
+    case 'text':
+      return `<text x="${fmt(p.at.x)}" y="${fmt(p.at.y)}" font-size="${p.fontSize}" fill="${p.color}">${escapeXml(p.text)}</text>`;
+  }
+}
+
+/** Serialisiert Primitive zu einem eigenständigen SVG-String (leer bei []). */
+export function primitivesToSvg(prims: ShapePrimitive[]): string {
+  if (prims.length === 0) return '';
+  const acc: BBox = { minX: Infinity, minY: Infinity, maxX: -Infinity, maxY: -Infinity };
+  for (const p of prims) primExtent(p, acc);
+  const w = acc.maxX - acc.minX;
+  const h = acc.maxY - acc.minY;
+  const vb = `${fmt(acc.minX - PAD)} ${fmt(acc.minY - PAD)} ${fmt(w + 2 * PAD)} ${fmt(h + 2 * PAD)}`;
+  const body = prims.map(primToSvgTag).join('');
+  return `<svg xmlns="http://www.w3.org/2000/svg" viewBox="${vb}">${body}</svg>`;
+}
+
+/** Erzeugt ein SVG-Thumbnail aus einer Element-Auswahl (Task F3). */
+export function generateBlockSvg(elements: CADElement[]): string {
+  const prims = elements.flatMap(el => elementToPrimitives(el));
+  return primitivesToSvg(prims);
+}
diff --git a/frontend/tests/blockThumbnail.test.ts b/frontend/tests/blockThumbnail.test.ts
new file mode 100644
index 0000000..9b91bce
--- /dev/null
+++ b/frontend/tests/blockThumbnail.test.ts
@@ -0,0 +1,45 @@
+/**
+ * BlockThumbnail Tests (Task F3) – SVG-Thumbnail aus elementToPrimitives
+ */
+import { describe, it, expect } from 'vitest';
+import { generateBlockSvg, primitivesToSvg } from '../src/utils/blockThumbnail';
+import { elementToPrimitives } from '../src/render/pixi/elementDrawers';
+import type { CADElement } from '../src/types/cad.types';
+
+describe('blockThumbnail (F3)', () => {
+  const chair: CADElement = {
+    id: 'c1',
+    type: 'chair',
+    layerId: 'l0',
+    x: 100,
+    y: 200,
+    width: 40,
+    height: 40,
+    properties: {},
+  } as CADElement;
+
+  it('erzeugt SVG mit viewBox aus Element-BBox', () => {
+    const svg = generateBlockSvg([chair]);
+    expect(svg).toContain('<svg');
+    expect(svg).toContain('viewBox');
+  });
+
+  it('konvertiert rect-Primitive in SVG-rect', () => {
+    const prims = elementToPrimitives(chair);
+    expect(prims.length).toBeGreaterThan(0);
+    const svg = primitivesToSvg(prims);
+    expect(svg).toContain('<rect');
+  });
+
+  it('liefert leeren String für leere Auswahl', () => {
+    expect(generateBlockSvg([])).toBe('');
+  });
+
+  it('BBox umfasst mehrere Elemente korrekt', () => {
+    const a: CADElement = { ...chair, id: 'a', x: 0, y: 0 };
+    const b: CADElement = { ...chair, id: 'b', x: 500, y: 300 };
+    const svg = generateBlockSvg([a, b]);
+    // chair-Drawer erzeugt rect-Zentrum bei (el.x - w/2) → BBox -40..500/-40..300 + PAD 10
+    expect(svg).toContain('viewBox="-50 -50 560 360"');
+  });
+});