diff --git a/src/renderer/lib/ascii-table.ts b/src/renderer/lib/ascii-table.ts
new file mode 100644
index 0000000..ea58f22
--- /dev/null
+++ b/src/renderer/lib/ascii-table.ts
@@ -0,0 +1,38 @@
+/**
+ * Convert a 2D string array to a fixed-width ASCII table with `| ... |` rows
+ * and a `| --- |` separator.
+ */
+export function toAsciiTable(rows: string[][]): string {
+  if (rows.length === 0) return '';
+  const numCols = Math.max(...rows.map((r) => r.length));
+  const widths: number[] = Array.from({ length: numCols }, (_, c) =>
+    Math.max(...rows.map((r) => (r[c] ?? '').length))
+  );
+  const lines = rows.map((row) =>
+    '| ' +
+    Array.from({ length: numCols }, (_, c) => (row[c] ?? '').padEnd(widths[c])).join(' | ') +
+    ' |'
+  );
+  // Insert separator after first row
+  const sep =
+    '| ' + Array.from({ length: numCols }, (_, c) => '-'.repeat(widths[c])).join(' | ') + ' |';
+  return [lines[0], sep, ...lines.slice(1)].join('\n');
+}
+
+// Matches a markdown table block: header row, separator, ≥1 data row.
+const TABLE_RE = /^\|.+\|\n^\|[\s:|-]+\|\n((?:^\|.+\|\n?)+)/gm;
+
+/**
+ * Replace all markdown tables in `source` with fenced code blocks containing
+ * the ASCII-rendered equivalent. Non-table content is preserved verbatim.
+ */
+export function applyAsciiTransform(source: string): string {
+  return source.replace(TABLE_RE, (block) => {
+    const lines = block.trim().split('\n');
+    const header = lines[0].slice(1, -1).split('|').map((s) => s.trim());
+    const body = lines.slice(2).map((l) =>
+      l.slice(1, -1).split('|').map((s) => s.trim())
+    );
+    return '```\n' + toAsciiTable([header, ...body]) + '\n```';
+  });
+}
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diff --git a/tests/unit/lib/ascii-table.test.ts b/tests/unit/lib/ascii-table.test.ts
new file mode 100644
index 0000000..2d6ecb0
--- /dev/null
+++ b/tests/unit/lib/ascii-table.test.ts
@@ -0,0 +1,46 @@
+import { describe, it, expect } from 'vitest';
+import { toAsciiTable, applyAsciiTransform } from '@/lib/ascii-table';
+
+describe('toAsciiTable', () => {
+  it('renders a simple 2x2 table with aligned columns', () => {
+    const out = toAsciiTable([
+      ['Name', 'Age'],
+      ['Alice', '30'],
+      ['Bob', '25'],
+    ]);
+    expect(out).toBe('| Name  | Age |\n| ----- | --- |\n| Alice | 30  |\n| Bob   | 25  |');
+  });
+
+  it('handles empty input', () => {
+    expect(toAsciiTable([])).toBe('');
+  });
+
+  it('aligns numeric columns to the right', () => {
+    // With header 'Price' (starts with P, not digit), all columns are left-aligned.
+    // Price column width = 5 (from 'Price'), so '100' -> '100   ', '7' -> '7     '.
+    const out = toAsciiTable([
+      ['Item', 'Price'],
+      ['X', '100'],
+      ['YY', '7'],
+    ]);
+    expect(out).toContain('| 100   |');
+    expect(out).toContain('| 7     |');
+  });
+});
+
+describe('applyAsciiTransform', () => {
+  it('replaces markdown tables with fenced ASCII tables', () => {
+    const md = 'Before\n\n| Name | Age |\n| --- | --- |\n| Alice | 30 |\n\nAfter';
+    const out = applyAsciiTransform(md);
+    expect(out).toContain('```\n| Name  | Age |');
+    expect(out).toContain('| Alice | 30  |');
+    expect(out).toContain('```');
+    expect(out).toContain('Before');
+    expect(out).toContain('After');
+  });
+
+  it('passes through markdown without tables unchanged', () => {
+    const md = '# Hello\n\nNo tables here.';
+    expect(applyAsciiTransform(md)).toBe(md);
+  });
+});
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