diff --git a/package.json b/package.json
index cb38dfc..13e9949 100644
--- a/package.json
+++ b/package.json
@@ -26,6 +26,7 @@
     "start": "react-scripts start",
     "build": "react-scripts build",
     "test": "node --test \"test/unit/*.test.js\"",
+    "test:property": "node --test \"test/property/*.test.js\"",
     "test:acceptance": "node acceptance/acceptance.js",
     "eject": "react-scripts eject",
     "lint": "standard --env mocha --fix",
diff --git a/test/property/harness.js b/test/property/harness.js
new file mode 100644
index 0000000..0ccf9af
--- /dev/null
+++ b/test/property/harness.js
@@ -0,0 +1,48 @@
+/*
+  Small property-testing harness for psf-memo-client.
+
+  Node's built-in test runner has no property-based generator, so this module
+  provides a tiny deterministic, seeded pseudo-random generator plus a helper
+  to run a property across many samples and report a counterexample. All
+  generation is seeded, so runs are reproducible.
+*/
+
+'use strict'
+
+const assert = require('node:assert/strict')
+
+// A small deterministic PRNG (mulberry32). Same seed => same stream.
+function seededRandom (seed = 12345) {
+  let a = seed >>> 0
+  return function next () {
+    a |= 0
+    a = (a + 0x6d2b79f5) | 0
+    let t = Math.imul(a ^ (a >>> 15), 1 | a)
+    t = (t + Math.imul(t ^ (t >>> 7), 61 | t)) ^ t
+    return ((t ^ (t >>> 14)) >>> 0) / 4294967296
+  }
+}
+
+// Run a property across N samples. `gen` returns a fresh input; `check`
+// returns true when the property holds. Asserts a counterexample on failure.
+async function forAll (gen, check, { samples = 500, label = 'property' } = {}) {
+  for (let i = 0; i < samples; i++) {
+    const input = gen(i)
+    const ok = await check(input)
+    assert.ok(ok, `${label} failed at sample ${i} for input: ${JSON.stringify(input)}`)
+  }
+}
+
+// Generate a random ASCII string of a given length using a seeded RNG.
+function makeStringGen (rng) {
+  return (length) => {
+    const chars = []
+    for (let i = 0; i < length; i++) {
+      // Mix of printable ASCII (32..126).
+      chars.push(String.fromCharCode(32 + Math.floor(rng() * 95)))
+    }
+    return chars.join('')
+  }
+}
+
+module.exports = { seededRandom, forAll, makeStringGen }
diff --git a/test/property/memo-post.property.test.js b/test/property/memo-post.property.test.js
new file mode 100644
index 0000000..b848f56
--- /dev/null
+++ b/test/property/memo-post.property.test.js
@@ -0,0 +1,112 @@
+/*
+  Property tests for the Memo post / New Post behavior slices.
+
+  These assert useful invariants that unit tests cover only at a few fixed
+  points:
+    - Validation classification is stable across a broad input range: any
+      non-blank string up to the length limit is accepted, any longer string is
+      rejected with a length error, and blank/non-string input is a validation
+      error.
+    - The New Post character counter conserves its relationship to input
+      length: input.length + remainingCount() === MAX_MEMO_CHARS for any string.
+    - setInput round-trips the exact draft text.
+*/
+
+'use strict'
+
+const test = require('node:test')
+
+const { seededRandom, forAll, makeStringGen } = require('./harness')
+
+const MemoPost = require('../../src/services/memo-post')
+const NewPostPage = require('../../src/services/new-post')
+
+const MAX = MemoPost.MAX_MEMO_CHARS // 217
+
+const rng = seededRandom(20260826)
+const stringOf = makeStringGen(rng)
+
+function buildPage () {
+  return new NewPostPage({
+    memoPost: new MemoPost({}),
+    navigate: () => {},
+    menuLinks: []
+  })
+}
+
+test('memo validation: any non-blank string at or below the limit is valid', async () => {
+  await forAll(
+    (i) => {
+      const len = 1 + Math.floor(rng() * MAX) // 1..MAX
+      return stringOf(len)
+    },
+    (msg) => {
+      // A random ASCII string may occasionally be all whitespace; whitespace-only
+      // input is a validation error, so only assert for non-blank strings.
+      if (msg.trim().length === 0) return true
+      const result = new MemoPost({}).validate(msg)
+      return result.ok === true
+    },
+    { label: 'valid length' }
+  )
+})
+
+test('memo validation: any string above the limit is a length error', async () => {
+  await forAll(
+    (i) => stringOf(MAX + 1 + Math.floor(rng() * 100)), // > MAX
+    (msg) => {
+      const result = new MemoPost({}).validate(msg)
+      return result.ok === false && result.type === 'length'
+    },
+    { label: 'over-long rejected as length' }
+  )
+})
+
+test('memo validation: blank and non-string input are validation errors', async () => {
+  await forAll(
+    (i) => (i % 2 === 0 ? '   ' : null),
+    (msg) => {
+      const result = new MemoPost({}).validate(msg)
+      return result.ok === false && result.type === 'validation'
+    },
+    { label: 'blank/non-string rejected as validation' }
+  )
+})
+
+test('character counter conserves length: remaining === MAX - input.length', async () => {
+  await forAll(
+    (i) => stringOf(Math.floor(rng() * (MAX + 8))),
+    (msg) => {
+      const page = buildPage()
+      page.setInput(msg)
+      return page.remainingCount() === MAX - msg.length
+    },
+    { label: 'counter conservation' }
+  )
+})
+
+test('setInput round-trips the draft text exactly', async () => {
+  await forAll(
+    (i) => stringOf(Math.floor(rng() * 50)),
+    (msg) => {
+      const page = buildPage()
+      page.setInput(msg)
+      return page.input === msg
+    },
+    { label: 'setInput round-trip' }
+  )
+})
+
+test('menu link registration is idempotent', async () => {
+  await forAll(
+    (i) => `/posts/${i}`,
+    (path) => {
+      const page = buildPage()
+      page.addMenuLink(path)
+      page.addMenuLink(path)
+      page.addMenuLink(path)
+      return page.menuLinks.filter((p) => p === path).length === 1
+    },
+    { label: 'menu link idempotence' }
+  )
+})