diff --git a/js/test/fixtures.mjs b/js/test/fixtures.mjs
new file mode 100644
index 0000000..6efa1c5
--- /dev/null
+++ b/js/test/fixtures.mjs
@@ -0,0 +1,26 @@
+// Test fixtures copied verbatim from mpl_codes js/site.js @ 5b50cc0 when
+// the interpreter test suite migrated into this repo. These are the exact
+// programs the site ships; the entries' third field is the site's i18n
+// label key, preserved untouched so nothing about the facts changed in the
+// migration.
+
+export const SYMBOLS = [
+ ['✎','\\trace','sym_trace'],['λ','\\lambda','sym_lambda'],['≜','\\coloneq','sym_def'],
+ ['←','\\leftarrow','sym_assign'],['⟹','\\implies','sym_implies'],['|','|','sym_bar'],
+ ['∀','\\forall','sym_forall'],['∈','\\in','sym_in'],['∧','\\and','sym_and'],
+ ['∨','\\or','sym_or'],['≤','\\leq','sym_leq'],['≥','\\geq','sym_geq'],
+ ['≠','\\neq','sym_neq'],['×','\\times','sym_times'],['÷','\\div','sym_div'],
+ ['⊥','\\bot','sym_bot']
+];
+
+export const EXERCISES = [
+ {t:'ex1_t',d:'ex1_d',lvl:'lvl1',code:'✎ "Hello, World!";\n✎ "Jambo!";\n✎ "你好!";\n✎ "مرحبا!";'},
+ {t:'ex2_t',d:'ex2_d',lvl:'lvl1',code:'length ← 5;\nwidth ← 3;\n✎("Area = " + length × width);'},
+ {t:'ex3_t',d:'ex3_d',lvl:'lvl2',code:'fact ≜ λn: (n ≤ 1 ⟹ 1) | (n × fact(n - 1));\n✎("5! = " + fact(5));'},
+ {t:'ex4_t',d:'ex4_d',lvl:'lvl2',code:'total ← 0;\n∀ n ∈ [1, 2, 3, 4, 5]: total ← total + n × n;\n✎("Σ = " + total);'},
+ {t:'ex5_t',d:'ex5_d',lvl:'lvl2',code:'even ≜ λn: (n = 0 ⟹ true) | ((n = 1 ⟹ false) | even(n - 2));\n∀ n ∈ [1, 2, 3, 4, 5, 6, 7, 8]: (even(n) ⟹ ✎(n)) | ⊥;'},
+ {t:'ex6_t',d:'ex6_d',lvl:'lvl3',code:'twice ≜ λf: λx: f(f(x));\ninc ≜ λn: n + 1;\n✎ twice(inc)(40);'}
+];
+
+export const DEFAULT_PROGRAM = '-- سلام · 你好 · Hola · Hello\ngreet ≜ λname: ✎("Salaam, " + name + "!");\ngreet("Fatima");\n\nfact ≜ λn: (n ≤ 1 ⟹ 1) | (n × fact(n - 1));\n✎("5! = " + fact(5));';
+export const HERO_PROGRAM = 'fact ≜ λn: (n ≤ 1 ⟹ 1) | (n × fact(n - 1));\n✎("5! = " + fact(5));';
diff --git a/js/test/load.mjs b/js/test/load.mjs
new file mode 100644
index 0000000..377811f
--- /dev/null
+++ b/js/test/load.mjs
@@ -0,0 +1,11 @@
+// Single shared loader for the interpreter artifact. js/mpl.js must stay
+// byte-identical to the file served at mpl.codes, so the loader adapts to
+// the artifact — never the other way around. The artifact ends with a
+// CommonJS export guard, so createRequire is sufficient today; if that
+// guard ever changes, adapt here (e.g. node:vm), do not edit js/mpl.js.
+import { createRequire } from 'node:module';
+
+const require = createRequire(import.meta.url);
+const { runMPL, ESCAPES } = require('../mpl.js');
+
+export { runMPL, ESCAPES };
diff --git a/js/test/mpl.test.mjs b/js/test/mpl.test.mjs
new file mode 100644
index 0000000..7df95b8
--- /dev/null
+++ b/js/test/mpl.test.mjs
@@ -0,0 +1,144 @@
+'use strict';
+/* Interpreter tests — run with: node --test "js/test/*.test.mjs"
+ * Asserts EXACT outputs for every program mpl.codes ships, so the site can
+ * never claim an example works that doesn't. Migrated from the mpl_codes
+ * test suite (test/mpl.test.js @ 5b50cc0) with every assertion's input and
+ * expected value preserved exactly; only the runner mechanics changed.
+ */
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import fs from 'node:fs';
+import path from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+import { runMPL, ESCAPES } from './load.mjs';
+import { SYMBOLS, EXERCISES, DEFAULT_PROGRAM, HERO_PROGRAM } from './fixtures.mjs';
+
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
+
+function run(src) {
+  const out = [];
+  runMPL(src, s => out.push(s));
+  return out;
+}
+
+test('hero program computes factorial', () => {
+  assert.deepEqual(run(HERO_PROGRAM), ['5! = 120']);
+});
+
+test('default playground program', () => {
+  assert.deepEqual(run(DEFAULT_PROGRAM), ['Salaam, Fatima!', '5! = 120']);
+});
+
+const EXPECTED_EXERCISES = [
+  ['Hello, World!', 'Jambo!', '你好!', 'مرحبا!'],
+  ['Area = 15'],
+  ['5! = 120'],
+  ['Σ = 55'],
+  ['2', '4', '6', '8'],
+  ['42'],
+];
+
+test('all six exercises produce their documented output', () => {
+  assert.equal(EXERCISES.length, EXPECTED_EXERCISES.length);
+  EXERCISES.forEach((ex, i) => {
+    assert.deepEqual(run(ex.code), EXPECTED_EXERCISES[i], `exercise ${i + 1}`);
+  });
+});
+
+test('all six type symbols lex as inert ∈-constraint atoms', () => {
+  // ℕ ℤ ℚ ℝ ℂ 𝔹 parse in constraint position and are discarded unevaluated
+  // (§5: "parsed today, not yet enforced"). 𝔹 is supplementary-plane: the
+  // lexer must treat the surrogate pair as one code point.
+  for (const T of ['ℕ', 'ℤ', 'ℚ', 'ℝ', 'ℂ', '𝔹']) {
+    assert.deepEqual(run(`f ≜ λx∈${T}: (x ⟹ 1) | 0;\n✎ f(true);`), ['1'],
+      `type symbol ${T}`);
+  }
+});
+
+test('𝔹 (U+1D539) lexes as one code point, not two surrogate halves', () => {
+  // The exact gate case:
+  assert.deepEqual(run('f ≜ λx∈𝔹: (x ⟹ 1) | 0; ✎ f(true);'), ['1']);
+  // And no half-pair matching: 𝕊 (U+1D54A) shares 𝔹's high surrogate
+  // \uD835 but is NOT a type symbol — it must be rejected whole.
+  try {
+    run('✎ 𝕊;');
+    assert.fail('𝕊 should not lex');
+  } catch (e) {
+    assert.equal(e.key, 'err_char');
+  }
+});
+
+test('vendored examples 01 and 02 run in the M0 browser core', () => {
+  // 03–10 use constructs beyond the browser core (modules, resources,
+  // channels, metaprogramming) — running them is deliberately NOT claimed.
+  const ex = n => fs.readFileSync(path.join(__dirname, '..', '..', 'examples', n), 'utf8');
+  assert.deepEqual(run(ex('01_hello_world.mpl')), ['Hello, World!']);
+  assert.deepEqual(run(ex('02_factorial.mpl')), ['120']);
+});
+
+test('undefined name reports err_undef at 1:1', () => {
+  try {
+    run('x + 1;');
+    assert.fail('should have thrown');
+  } catch (e) {
+    assert.equal(e.key, 'err_undef');
+    assert.equal(e.line, 1);
+    assert.equal(e.col, 1);
+  }
+});
+
+test('division by zero reports err_div0', () => {
+  try {
+    run('✎ 1 ÷ 0;');
+    assert.fail('should have thrown');
+  } catch (e) {
+    assert.equal(e.key, 'err_div0');
+  }
+});
+
+test('SYMBOLS table: one escape per glyph, all escapes known to the lexer', () => {
+  const glyphs = new Set();
+  const escapes = new Set();
+  for (const [glyph, esc] of SYMBOLS) {
+    assert.ok(!glyphs.has(glyph), `duplicate glyph ${glyph}`);
+    assert.ok(!escapes.has(esc), `duplicate escape ${esc}`);
+    glyphs.add(glyph);
+    escapes.add(esc);
+    if (esc.startsWith('\\')) {
+      // the palette's escape must be exactly what the lexer expands
+      assert.equal(ESCAPES[esc.slice(1)], glyph,
+        `lexer ESCAPES['${esc.slice(1)}'] must map to ${glyph}`);
+    } else {
+      // ASCII symbols (like |) are their own spelling
+      assert.equal(esc, glyph);
+    }
+  }
+});
+
+test('every palette escape lexes identically to its glyph', () => {
+  // One program that uses all fifteen backslash-escaped glyphs.
+  const glyphProgram = [
+    'check ≜ λa, b: ((a ≤ b) ∧ (b ≥ a) ∧ (a ≠ b) ⟹ ✎ "cmp") | ⊥;',
+    'check(1, 2);',
+    't ← 0;',
+    '∀ n ∈ [1, 2, 3]: t ← t + n;',
+    '✎(t × 2);',
+    '✎(t ÷ 2);',
+    '✎((true ∨ false) ⟹ "or");',
+  ].join('\n');
+  // Verify coverage: every escaped SYMBOLS glyph appears in the program.
+  for (const [glyph, esc] of SYMBOLS) {
+    if (esc.startsWith('\\')) {
+      assert.ok(glyphProgram.includes(glyph), `probe program must use ${glyph}`);
+    }
+  }
+  // Build the escape-spelled variant (a space terminates each escape word).
+  let escProgram = glyphProgram;
+  for (const [glyph, esc] of SYMBOLS) {
+    if (esc.startsWith('\\')) escProgram = escProgram.split(glyph).join(esc + ' ');
+  }
+  const expected = ['cmp', '12', '3', 'or'];
+  assert.deepEqual(run(glyphProgram), expected);
+  assert.deepEqual(run(escProgram), expected, 'escape spelling must behave identically');
+});