diff --git a/package-lock.json b/package-lock.json
index 4dde0ea..5b96e37 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -10,6 +10,7 @@
       "dependencies": {
         "@fontsource/ibm-plex-mono": "^5.2.7",
         "@fontsource/ibm-plex-sans": "^5.2.8",
+        "json-stringify-pretty-compact": "^4.0.0",
         "monaco-editor": "^0.54.0",
         "react": "^19.2.7",
         "react-dom": "^19.2.7",
diff --git a/package.json b/package.json
index ac1c658..bb335ff 100644
--- a/package.json
+++ b/package.json
@@ -26,6 +26,7 @@
   "dependencies": {
     "@fontsource/ibm-plex-mono": "^5.2.7",
     "@fontsource/ibm-plex-sans": "^5.2.8",
+    "json-stringify-pretty-compact": "^4.0.0",
     "monaco-editor": "^0.54.0",
     "react": "^19.2.7",
     "react-dom": "^19.2.7",
diff --git a/src/app/components/SpecEditor.tsx b/src/app/components/SpecEditor.tsx
index 0eb9f43..a3f7675 100644
--- a/src/app/components/SpecEditor.tsx
+++ b/src/app/components/SpecEditor.tsx
@@ -23,6 +23,7 @@ import * as monaco from 'monaco-editor/esm/vs/editor/edcore.main';
 import 'monaco-editor/esm/vs/language/json/monaco.contribution';
 import '../infrastructure/monaco-env'; // side-effect: wire workers before create
 import { configureVegaLiteJson } from '../infrastructure/monaco-schema';
+import { configureJsonFormatter, installFormatOnPaste } from '../infrastructure/monaco-format';
 import { openModal } from '../modals/ModalCoordinator';
 import { useAppStore } from '../stores/AppStore';
 import { confirm } from '../stores/ConfirmStore';
@@ -134,6 +135,8 @@ function EditorSettings() {
 // Register the bundled Vega-Lite schema once: resolves `$schema` locally (no
 // network warning) and powers validation, autocomplete, and hover docs.
 configureVegaLiteJson();
+// Register the compact JSON formatter once (Format Document + format-on-paste, §03A).
+configureJsonFormatter();
 
 function EditorToolbar() {
   const activeId = useSnippetStore((s) => s.activeSnippetId);
@@ -267,6 +270,11 @@ export function SpecEditor() {
       }
     });
 
+    // Pasting reindents the whole spec to keep it consistently formatted (§03A).
+    // The reformat fires onDidChangeModelContent above, so the draft buffer picks
+    // up the formatted text. No-op on the read-only published view / invalid JSON.
+    const pasteSub = installFormatOnPaste(editor);
+
     // Cmd/Ctrl+S is owned globally by the EventRouter (docs/architecture/04 →
     // "bind listeners in exactly one place"), which publishes before the
     // interactive-context gate so it works while the editor has focus. Monaco
@@ -274,6 +282,7 @@ export function SpecEditor() {
 
     return () => {
       sub.dispose();
+      pasteSub.dispose();
       editor.dispose();
       editorRef.current = null;
     };
diff --git a/src/app/infrastructure/monaco-format.ts b/src/app/infrastructure/monaco-format.ts
new file mode 100644
index 0000000..2922874
--- /dev/null
+++ b/src/app/infrastructure/monaco-format.ts
@@ -0,0 +1,51 @@
+/**
+ * Monaco JSON formatting (spec §03A; docs/architecture/08 → compact formatter).
+ *
+ * Two pieces, both backed by the portable core pretty-printer (`core/json-format`):
+ *   - `configureJsonFormatter()` registers a document-formatting provider so
+ *     "Format Document" (Shift+Alt+F) produces Vega's compact style.
+ *   - `installFormatOnPaste(editor)` reindents the whole document after a paste,
+ *     satisfying the spec's "pasting … triggers automatic reformatting".
+ *
+ * Both no-op when the document is not valid JSON, so a partial/in-progress paste
+ * is never mangled (mirrors auto-save's "skip when unparseable" rule). Paste
+ * handling is wired directly via `onDidPaste` rather than Monaco's `formatOnPaste`
+ * option so it reformats the entire spec (not just the pasted range) and does not
+ * depend on which registered formatter Monaco happens to pick.
+ */
+
+import * as monaco from 'monaco-editor/esm/vs/editor/edcore.main';
+import { formatJson } from '@core/json-format';
+
+let registered = false;
+
+/** Register the compact JSON document formatter once. Idempotent. */
+export function configureJsonFormatter(): void {
+  if (registered) return;
+  registered = true;
+  monaco.languages.registerDocumentFormattingEditProvider('json', {
+    provideDocumentFormattingEdits: (model, options) => {
+      const current = model.getValue();
+      const formatted = formatJson(current, { indent: options.tabSize });
+      if (formatted === null || formatted === current) return [];
+      return [{ range: model.getFullModelRange(), text: formatted }];
+    },
+  });
+}
+
+/**
+ * Reformat the whole document after each paste. Returns a disposable; callers
+ * dispose it when the editor unmounts.
+ */
+export function installFormatOnPaste(
+  editor: monaco.editor.IStandaloneCodeEditor,
+): monaco.IDisposable {
+  return editor.onDidPaste(() => {
+    const model = editor.getModel();
+    if (!model) return;
+    const current = model.getValue();
+    const formatted = formatJson(current, { indent: model.getOptions().tabSize });
+    if (formatted === null || formatted === current) return;
+    editor.executeEdits('format-on-paste', [{ range: model.getFullModelRange(), text: formatted }]);
+  });
+}
diff --git a/src/core/json-format.test.ts b/src/core/json-format.test.ts
new file mode 100644
index 0000000..3fd6128
--- /dev/null
+++ b/src/core/json-format.test.ts
@@ -0,0 +1,45 @@
+import { describe, expect, it } from 'vitest';
+import { formatJson } from './json-format';
+
+describe('formatJson', () => {
+  it('pretty-prints a valid object with the default indent', () => {
+    expect(formatJson('{"a":1,"b":2}')).toBe('{"a": 1, "b": 2}');
+  });
+
+  it('keeps short arrays compact (Vega house style)', () => {
+    expect(formatJson('{"values":[1,2,3]}')).toBe('{"values": [1, 2, 3]}');
+  });
+
+  it('wraps content that exceeds the line-length budget', () => {
+    expect(formatJson('{"a":1,"b":2,"c":3}', { maxLength: 5 })).toBe(
+      '{\n  "a": 1,\n  "b": 2,\n  "c": 3\n}',
+    );
+  });
+
+  it('respects a custom indent width', () => {
+    const out = formatJson('{"a":1,"b":2}', { indent: 4, maxLength: 5 });
+    expect(out).toBe('{\n    "a": 1,\n    "b": 2\n}');
+  });
+
+  it('returns null for invalid JSON so callers can skip', () => {
+    expect(formatJson('{"a":')).toBeNull();
+    expect(formatJson('not json')).toBeNull();
+  });
+
+  it('returns null for empty or whitespace-only input', () => {
+    expect(formatJson('')).toBeNull();
+    expect(formatJson('   \n  ')).toBeNull();
+  });
+
+  it('is idempotent — formatting formatted text changes nothing', () => {
+    const once = formatJson('{"a":1,"b":[1,2,3],"c":{"d":4}}');
+    expect(once).not.toBeNull();
+    expect(formatJson(once!)).toBe(once);
+  });
+
+  it('handles top-level arrays and primitives', () => {
+    expect(formatJson('[1,2,3]')).toBe('[1, 2, 3]');
+    expect(formatJson('42')).toBe('42');
+    expect(formatJson('true')).toBe('true');
+  });
+});
diff --git a/src/core/json-format.ts b/src/core/json-format.ts
new file mode 100644
index 0000000..9553331
--- /dev/null
+++ b/src/core/json-format.ts
@@ -0,0 +1,41 @@
+/**
+ * JSON pretty-printer for Vega-Lite specs (spec §03A; docs/architecture/08 →
+ * "compact formatter").
+ *
+ * Uses `json-stringify-pretty-compact` for Vega's house style — arrays/objects
+ * stay on one line until they exceed a width budget, then wrap — which reads far
+ * better for VL specs than `JSON.stringify(…, null, 2)`. Pure and portable: no
+ * Monaco, no browser. The Monaco formatting adapter
+ * (`infrastructure/monaco-format`) is a thin wrapper over this.
+ */
+
+import stringify from 'json-stringify-pretty-compact';
+
+/** Width budget before an array/object wraps onto multiple lines. */
+export const DEFAULT_MAX_LINE = 80;
+
+/** Default indent (spaces) — matches the editor's default tab size (spec §07). */
+const DEFAULT_INDENT = 2;
+
+/**
+ * Reformat `text` as consistently-indented JSON, or return `null` when it is not
+ * valid JSON. Returning `null` (rather than throwing) lets callers skip a no-op
+ * rather than mangle an in-progress edit, mirroring auto-save's "skip when
+ * unparseable" rule (spec §03B). The result is idempotent: formatting
+ * already-formatted text returns it unchanged.
+ */
+export function formatJson(
+  text: string,
+  opts: { indent?: number; maxLength?: number } = {},
+): string | null {
+  let parsed: unknown;
+  try {
+    parsed = JSON.parse(text);
+  } catch {
+    return null;
+  }
+  return stringify(parsed, {
+    indent: opts.indent ?? DEFAULT_INDENT,
+    maxLength: opts.maxLength ?? DEFAULT_MAX_LINE,
+  });
+}