diff --git a/acceptance/lib/handlers.js b/acceptance/lib/handlers.js
index 3b54ce4..edf4722 100644
--- a/acceptance/lib/handlers.js
+++ b/acceptance/lib/handlers.js
@@ -2,27 +2,33 @@
   Project step handlers for the psf-memo-client acceptance pipeline.
 
   These handlers connect Gherkin step text to real project behavior
-  (src/services/memo-post.js and src/services/new-post.js), driving them
-  through small injected adapters (a fake wallet, a fake feed, and a fake
-  navigator) so the acceptance run is deterministic and offline.
+  (src/services/memo-post.js, src/services/new-post.js, src/services/memo-reply.js,
+  src/services/reply-thread-page.js, src/services/memo-set-name.js, and
+  src/services/set-name-page.js), driving them through small injected adapters
+  (a fake wallet, a fake feed, a fake thread, and a fake navigator) so the
+  acceptance run is deterministic and offline.
 
   Regex matching with placeholder-name capture is the default style: a single
   handler pattern captures the placeholder name (e.g. <message>) and fetches
   the example value from the scenario example store.
 
-  The handlers serve both specs/post-memo.feature and specs/memo-new.feature,
-  whose wording differs but which share the same underlying Memo post behavior.
+  The handlers serve specs/post-memo.feature, specs/memo-new.feature,
+  specs/reply-memo.feature, and specs/set-name.feature, whose wording differs
+  but which share the same underlying Memo action/page-controller behavior.
 */
 
 'use strict'
 
 const MemoPost = require('../../src/services/memo-post')
 const NewPostPage = require('../../src/services/new-post')
+const MemoReply = require('../../src/services/memo-reply')
+const ReplyThreadPage = require('../../src/services/reply-thread-page')
 const MemoSetName = require('../../src/services/memo-set-name')
 const SetNamePage = require('../../src/services/set-name-page')
 const AccountPage = require('../../src/services/account-page')
 
 const MEMO_POST_PREFIX = MemoPost.MEMO_POST_PREFIX
+const MEMO_REPLY_PREFIX = MemoReply.MEMO_REPLY_PREFIX
 const MEMO_SET_NAME_PREFIX = MemoSetName.MEMO_SET_NAME_PREFIX
 
 // A fake wallet exposing the minimal-slp-wallet adapter surface the app uses.
@@ -63,15 +69,29 @@ function makeProfiles () {
   }
 }
 
+// A fake thread store recording replies added to a post thread.
+function makeThread () {
+  const replies = []
+  return {
+    rootTxid: null,
+    replies,
+    addReply: (r) => replies.push(r)
+  }
+}
+
 // Fresh world/state object for a single scenario execution.
 function createWorld () {
   const wallet = makeWallet('')
   const feed = makeFeed()
   const memoPost = new MemoPost({ wallet, feed })
+  const thread = makeThread()
+  const memoReply = new MemoReply({ wallet, thread })
   const world = {
     wallet,
     feed,
+    thread,
     memoPost,
+    memoReply,
     currentPath: null,
     menuOpen: false
   }
@@ -84,6 +104,13 @@ function createWorld () {
     menuLinks: []
   })
 
+  // The Reply Thread Page controller wraps the memo reply behavior. It does
+  // not navigate on success so the user stays in the thread modal.
+  world.replyPage = new ReplyThreadPage({
+    memoReply,
+    navigate: () => {}
+  })
+
   // The Set Name Page and Account Page controllers share a profile store so
   // a name set on one page is visible on the other.
   const profiles = makeProfiles()
@@ -101,6 +128,14 @@ function createWorld () {
   return world
 }
 
+// Decode a raw reply payload into its parent txid (hex) and reply text.
+function decodeReplyPayload (raw) {
+  const buf = Buffer.from(raw)
+  const parentTxid = buf.slice(0, 32).toString('hex')
+  const text = buf.slice(32).toString('utf8')
+  return { parentTxid, text }
+}
+
 // Handler registry. Each entry: { pattern, run }.
 // run receives (match, exampleStore, world, step).
 const handlers = [
@@ -213,6 +248,63 @@ const handlers = [
       await world.setNamePage.submit()
     }
   },
+  {
+    name: 'open reply thread',
+    pattern: /^I open the thread for the post with txid (.+)$/,
+    run (m, example, world) {
+      const txid = m[1].trim()
+      world.thread.rootTxid = txid
+      world.replyPage.setParent(txid)
+    }
+  },
+  {
+    name: 'type reply text',
+    pattern: /^I type a reply with the text "<([A-Za-z0-9_]+)>"$/,
+    run (m, example, world) {
+      const param = m[1]
+      if (!(param in example)) {
+        throw new Error(`Missing example value for "${param}"`)
+      }
+      world.replyPage.setInput(example[param])
+      world.replyPage.setParent(world.thread.rootTxid)
+    }
+  },
+  {
+    name: 'type reply to nested reply',
+    pattern: /^I type a reply to the nested reply with the text "<([A-Za-z0-9_]+)>"$/,
+    run (m, example, world) {
+      const param = m[1]
+      if (!(param in example)) {
+        throw new Error(`Missing example value for "${param}"`)
+      }
+      world.replyPage.setInput(example[param])
+      if (!world.nestedTxid) {
+        throw new Error('No nested reply has been selected.')
+      }
+      world.replyPage.setParent(world.nestedTxid)
+    }
+  },
+  {
+    name: 'submit reply',
+    pattern: /^I submit the reply$/,
+    async run (m, example, world) {
+      await world.replyPage.submit()
+    }
+  },
+  {
+    name: 'thread shows nested reply',
+    pattern: /^the thread shows a nested reply with the txid (.+)$/,
+    run (m, example, world) {
+      const txid = m[1].trim()
+      world.nestedTxid = txid
+      world.thread.addReply({
+        txid,
+        address: 'someone-else',
+        text: 'nested reply',
+        parentTxid: world.thread.rootTxid
+      })
+    }
+  },
   {
     name: 'click Set Name button',
     pattern: /^I click the Set Name button$/,
@@ -254,6 +346,68 @@ const handlers = [
       }
     }
   },
+  {
+    name: 'broadcasts OP_RETURN with Memo reply prefix',
+    pattern: /^(?:the wallet|the app) broadcasts an OP_RETURN transaction with the Memo reply prefix$/,
+    run (m, example, world) {
+      const broadcasts = world.wallet.broadcasts
+      if (!broadcasts.length) {
+        throw new Error('No OP_RETURN transaction was broadcast.')
+      }
+      const last = broadcasts[broadcasts.length - 1]
+      if (last.prefix !== MEMO_REPLY_PREFIX) {
+        throw new Error(`Expected Memo reply prefix ${MEMO_REPLY_PREFIX}, got "${last.prefix}".`)
+      }
+      const { parentTxid, text } = decodeReplyPayload(last.msg)
+      if (parentTxid !== world.replyPage.parentTxid) {
+        throw new Error('Broadcast parent txid did not match the expected reply target.')
+      }
+      if (text !== world.replyPage.input) {
+        throw new Error('Broadcast reply text did not match the typed reply.')
+      }
+    }
+  },
+  {
+    name: 'thread shows new reply from my address',
+    pattern: /^the thread shows a new reply from my address with the text "<([A-Za-z0-9_]+)>"$/,
+    run (m, example, world) {
+      const param = m[1]
+      const expectedText = example[param]
+      const myAddress = world.wallet.walletInfo.cashAddress
+      const found = world.thread.replies.find(
+        (r) => r.text === expectedText && r.address === myAddress
+      )
+      if (!found) {
+        throw new Error(`Thread does not show the new reply with text "${expectedText}".`)
+      }
+    }
+  },
+  {
+    name: 'thread shows validation/length error',
+    pattern: /^the thread shows a (validation|length) error$/,
+    run (m, example, world) {
+      const kind = m[1]
+      const expectedCode = kind === 'validation' ? 'reply_validation' : 'reply_length'
+      if (world.replyPage.submitError !== expectedCode) {
+        throw new Error(`Expected ${expectedCode}, got ${world.replyPage.submitError}.`)
+      }
+    }
+  },
+  {
+    name: 'thread remaining byte count',
+    pattern: /^the thread shows a remaining byte count of <([A-Za-z0-9_]+)>$/,
+    run (m, example, world) {
+      const param = m[1]
+      const expected = parseInt(example[param], 10)
+      if (Number.isNaN(expected)) {
+        throw new Error(`Invalid expected count for "${param}".`)
+      }
+      const actual = world.replyPage.remainingCount()
+      if (actual !== expected) {
+        throw new Error(`Expected ${expected} remaining bytes, got ${actual}.`)
+      }
+    }
+  },
   {
     name: 'feed shows new post from my address',
     pattern: /^the feed shows a new post from my address with the text "<([A-Za-z0-9_]+)>"$/,
diff --git a/specifier-prompt.md b/specifier-prompt.md
index a5956a8..b07bbca 100644
--- a/specifier-prompt.md
+++ b/specifier-prompt.md
@@ -104,12 +104,20 @@ Saved (and updated) at `specs/feature-backlog.md`. memo protocol action bytes be
 
 **Completed ✓ (merged to `feat1`):**
 - Post a Memo (`0x6d02`) — service + `/posts/new` page + broadcast fix. FULLY DONE.
+- Set display name (`0x6d01`) — `/account` + `/memo/set-name` pages, byte counter (77 bytes). DONE.
 
 **Tier P1 — Core social verbs (write + read) — do these next, in order:**
 1. ✅ Post a Memo (`0x6d02`) — DONE
-2. Set display name (`0x6d01`) — **NEXT** (breadcrumb: the feed already renders
-   display names; the write action is missing)
-3. Reply to a Memo (`0x6d03`) — thread already renders; add reply broadcast
+2. ✅ Set display name (`0x6d01`) — DONE
+3. Reply to a Memo (`0x6d03`) — **NEXT** (thread already renders; add reply broadcast)
+   - **User-approved decisions (2026-08-26, from memo.cash UI review):**
+     - Reply max = **184 bytes** (UTF-8 byte count, memo.cash `MaxSize.Reply`).
+     - Reply form lives **inside the thread modal** (not inline in the feed).
+     - Keep the existing comment-icon behavior (opens the thread modal); put the reply
+       form in the modal.
+     - Replicate the live `[remaining]` byte counter (turns red when over limit).
+     - Update the thread **optimistically** after broadcast (no refresh).
+     - Users can **reply to a reply** (nested), not just the root post.
 4. Like / tip a Memo (`0x6d04`)
 5. Set profile text / bio (`0x6d05`)
 6. Set profile picture (`0x6d0a`)
@@ -228,11 +236,24 @@ specing reply/like/follow.
    (`Failed to broadcast: <msg>`). Keep that behavior in specs.
 6. **memo.cash pages are behind Cloudflare** — `/memo/new` etc. are hard to scrape; rely
    on user-provided behavior details and the protocol spec.
-7. **Byte vs char:** the 217 limit and the counter currently count characters
-   (`input.length`, UTF-16), not bytes. The user is aware; multi-byte unicode may
-   diverge. Ask/decide per feature.
+7. **Byte vs char:** the 217 post limit and its counter count characters (`input.length`,
+   UTF-16), not bytes. The user is aware; multi-byte unicode may diverge. **Set Name
+   (`0x6d01`) uses BYTE counting (77 bytes) for memo.cash parity** — its byte counter and
+   length check use UTF-8 byte length. Ask/decide per feature.
 8. **Live backend for e2e:** `https://memo-api.fullstackcash.net/` (prod memo-db). The
    user can provide BCH for real broadcasts.
+9. **memo.cash login is Cloudflare-blocked for automation:** the `/login` page shows a
+   hard Turnstile challenge that does not auto-resolve, even with a persistent Playwright
+   profile. The public pages (home, `/all` feed, `/post/<txid>`) DO resolve with a
+   persistent profile (`launchPersistentContext` + `--headless=new` +
+   `--disable-blink-features=AutomationControlled` + realistic UA). To explore the
+   logged-in UI, either solve Turnstile (real session / captcha service) or get
+   screenshots/HTML from the user. The reply UI was captured from public feed/post pages
+   plus reverse-engineering `https://memo.cash/js/min.js`:
+   - login flow `POST /login/submit {username,password,rid,loginToken}` → `SessionKey`;
+   - reply submit `memo/reply-submit` with `{txHash,message}`;
+   - `MaxSize.Reply = 184`; reply form = Message label + `[remaining]` byte counter +
+     textarea + "Post Reply"/"Cancel" + "Creating..."/"Processing..." states.
 
 ---
 
@@ -256,4 +277,8 @@ At the end of each session, update this file:
 - Mark features completed in the backlog (§5).
 - Add any new gotchas to §10.
 - Note the current `feat1` HEAD commit.
-- State the next feature to work on (currently: **Set display name, `0x6d01`**).
+- State the next feature to work on (currently: **Reply to a Memo, `0x6d03`**).
+
+Current `display-name` HEAD: `230618d` (Set Name buffer fix merged).
+Next feature: **Reply to a Memo, `0x6d03`** — spec decisions captured in §5; Gherkin
+not yet written; awaiting user approval before handoff to coder.
diff --git a/specs/feature-backlog.md b/specs/feature-backlog.md
index 7ba39d1..4da416d 100644
--- a/specs/feature-backlog.md
+++ b/specs/feature-backlog.md
@@ -63,7 +63,7 @@ action plus its read/display surface. This is the recommended first development
 | # | Feature | Memo action | Write | Read surface |
 |---|---------|-------------|-------|--------------|
 | 1 | Post a Memo | `0x6d02` | Compose + `sendOpReturn` | Appears in recent feed & own profile after indexing |
-| 2 | Set display name | `0x6d01` | Broadcast name | Name shown on posts, profiles, feed |
+| 2 | Set display name | `0x6d01` | Broadcast name | Name shown on posts, profiles, feed | ✅ DONE |
 | 3 | Reply to a Memo | `0x6d03` | Broadcast reply to parent txid | Nested thread view |
 | 4 | Like a Memo | `0x6d04` | Broadcast like for a post txid | Like count + liked state on post |
 | 5 | Set profile text (bio) | `0x6d05` | Broadcast bio | Shown on profile page |
@@ -77,9 +77,14 @@ follower/following lists; name + profile + avatar joined into feed/profile respo
 
 ## Priority order within P1
 
-1. **Post a Memo** — the primary verb; unblocks all others.
-2. **Set display name** — makes the feed readable and gives identity.
+1. **Post a Memo** — the primary verb; unblocks all others. ✅ DONE
+2. **Set display name** — makes the feed readable and gives identity. ✅ DONE
 3. **Reply to a Memo** — core conversation; extends the existing thread modal.
+   - **Decisions (2026-08-26, from memo.cash UI review):** reply max = **184 bytes**
+     (UTF-8 byte count); reply form **inside the thread modal**; keep the existing
+     comment-icon behavior (opens the thread modal); replicate the live `[remaining]`
+     byte counter (turns red when over); update the thread **optimistically** after
+     broadcast; users can **reply to a reply** (nested).
 4. **Like a Memo** — social signal; needs like-count API.
 5. **Set profile text** — bio for the profile page.
 6. **Set profile picture** — avatar for posts/profiles.
diff --git a/specs/reply-memo.feature b/specs/reply-memo.feature
new file mode 100644
index 0000000..c4ff571
--- /dev/null
+++ b/specs/reply-memo.feature
@@ -0,0 +1,61 @@
+# Scenarios: Reply to a Memo - 1, Reply to a Memo - 2, Reply to a Memo - 3, Reply to a Memo - 4, Reply to a Memo - 5
+Feature: Reply to a Memo
+
+  Background:
+    Given a wallet authenticated for the address bitcoincash:qqlrzp23w08434twmvr4fxw672whkjy0py26r63g3d
+    Given the wallet has spendable output to pay the transaction fee
+    Given I open the thread for the post with txid aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+
+  Scenario Outline: Reply to a Memo - 1 a valid reply to a post is broadcast and shown in the thread
+    When I type a reply with the text "<message>"
+    When I submit the reply
+    Then the wallet broadcasts an OP_RETURN transaction with the Memo reply prefix
+    Then the thread shows a new reply from my address with the text "<message>"
+
+    Examples:
+      | message |
+      | hello memo |
+      | a longer reply with several words. |
+
+  Scenario Outline: Reply to a Memo - 2 an empty reply is rejected
+    When I type a reply with the text "<message>"
+    When I submit the reply
+    Then the thread shows a validation error
+    Then the wallet does not broadcast any transaction
+
+    Examples:
+      | message |
+      |  |
+
+  Scenario Outline: Reply to a Memo - 3 an over-long reply is rejected
+    When I type a reply with the text "<message>"
+    When I submit the reply
+    Then the thread shows a length error
+    Then the wallet does not broadcast any transaction
+
+    Examples:
+      | message |
+      | aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa |
+      | bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb |
+
+  Scenario Outline: Reply to a Memo - 4 the byte counter counts down from the reply limit
+    When I type a reply with the text "<message>"
+    Then the thread shows a remaining byte count of <count>
+
+    Examples:
+      | message | count |
+      |         | 184   |
+      | hello   | 179   |
+      | aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa | 0     |
+
+  Scenario Outline: Reply to a Memo - 5 a reply to a nested reply is broadcast
+    And the thread shows a nested reply with the txid bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb
+    When I type a reply to the nested reply with the text "<message>"
+    When I submit the reply
+    Then the wallet broadcasts an OP_RETURN transaction with the Memo reply prefix
+    Then the thread shows a new reply from my address with the text "<message>"
+
+    Examples:
+      | message |
+      | hello nested |
+      | a longer nested reply. |
diff --git a/src/components/app-body/set-name/index.js b/src/components/app-body/set-name/index.js
index 8d6a849..787737d 100644
--- a/src/components/app-body/set-name/index.js
+++ b/src/components/app-body/set-name/index.js
@@ -12,6 +12,7 @@ import { useNavigate } from 'react-router-dom'
 // Local libraries
 import MemoSetName from '../../../services/memo-set-name'
 import SetNamePage from '../../../services/set-name-page'
+import { byteLength } from '../../../services/utf8'
 
 function SetName (props) {
   const { appData } = props
@@ -22,7 +23,7 @@ function SetName (props) {
   const [err, setErr] = useState('')
   const [settingName, setSettingName] = useState(false)
 
-  const remaining = maxBytes - Buffer.byteLength(input, 'utf8')
+  const remaining = maxBytes - byteLength(input)
 
   async function handleSubmit (event) {
     event.preventDefault()
diff --git a/src/services/memo-reply.js b/src/services/memo-reply.js
new file mode 100644
index 0000000..c5d2a14
--- /dev/null
+++ b/src/services/memo-reply.js
@@ -0,0 +1,112 @@
+/*
+  Memo reply behavior: compose, validate, and broadcast a Memo "reply" message.
+
+  A Memo reply is an OP_RETURN Bitcoin Cash transaction carrying the Memo reply
+  protocol prefix (0x6d03) followed by the parent transaction hash (32 bytes)
+  and the reply message text. Broadcasting is done through a wallet that
+  exposes the minimal-slp-wallet adapter surface (walletInfo, getUtxos(),
+  sendOpReturn()).
+
+  The wallet and thread are injected so this module stays testable and free of
+  network/UI concerns; environmentally unsuitable I/O lives behind those small
+  adapter boundaries.
+
+  Constants
+    MEMO_REPLY_PREFIX : hex prefix for the Memo "reply" action (0x6d03)
+    MAX_REPLY_BYTES   : maximum allowed reply text length (184 bytes)
+*/
+
+const MemoAction = require('./memo-action')
+const { byteLength } = require('./utf8')
+
+const MEMO_REPLY_PREFIX = '6d03'
+const MAX_REPLY_BYTES = 184
+const PARENT_TXID_BYTES = 32
+
+class MemoReply extends MemoAction {
+  static config = {
+    prefix: MEMO_REPLY_PREFIX,
+    walletRequiredMsg: 'Memo reply requires a wallet.',
+    lengthMessage: `Reply is too long. Maximum is ${MAX_REPLY_BYTES} bytes.`,
+    emptyMessage: 'Reply must not be empty.',
+    lengthCode: 'reply_length',
+    validationCode: 'reply_validation'
+  }
+
+  constructor (deps = {}) {
+    super(deps)
+    this.thread = deps.thread
+  }
+
+  // A reply is over-length when its UTF-8 byte count exceeds the limit.
+  isTooLong (message) {
+    return byteLength(message) > MAX_REPLY_BYTES
+  }
+
+  // Compose and broadcast a Memo reply for the given message and parent txid.
+  // Resolves with the transaction id, or rejects with a typed error.
+  async reply (message, parentTxid) {
+    const check = this.validate(message)
+    this._throwIfInvalid(check)
+
+    if (!this.wallet) {
+      throw new Error(this.walletRequiredMsg)
+    }
+
+    // Refresh the wallet's spendable UTXO store so the broadcast has inputs.
+    await this.wallet.getUtxos()
+
+    // Build the raw payload: parent txid bytes followed by UTF-8 message bytes.
+    const raw = buildReplyPayload(parentTxid, message)
+    const txid = await this.wallet.sendOpReturn(raw, this.prefix)
+
+    // Reflect the result on the injected thread once broadcast succeeds.
+    this.reflect(txid, message, parentTxid)
+
+    return txid
+  }
+
+  // Record the new reply on the injected thread store when one is present.
+  reflect (txid, message, parentTxid) {
+    if (this.thread && typeof this.thread.addReply === 'function') {
+      this.thread.addReply({
+        txid,
+        address: this.wallet.walletInfo.cashAddress,
+        text: message,
+        parentTxid
+      })
+    }
+  }
+}
+
+// Build the raw OP_RETURN message payload for a reply.
+// The protocol wire format is: <parent txid 32 bytes><reply text UTF-8 bytes>.
+function buildReplyPayload (parentTxid, message) {
+  const parentBytes = hexToBytes(parentTxid)
+  const textBytes = new TextEncoder().encode(message)
+  const raw = new Uint8Array(parentBytes.length + textBytes.length)
+  raw.set(parentBytes, 0)
+  raw.set(textBytes, parentBytes.length)
+  return raw
+}
+
+// Decode a 64-character hex transaction id into 32 raw bytes.
+function hexToBytes (hex) {
+  if (typeof hex !== 'string' || hex.length !== PARENT_TXID_BYTES * 2) {
+    throw new Error('Parent txid must be a 64-character hex string.')
+  }
+  const bytes = new Uint8Array(PARENT_TXID_BYTES)
+  for (let i = 0; i < hex.length; i += 2) {
+    const byte = parseInt(hex.substr(i, 2), 16)
+    if (Number.isNaN(byte)) {
+      throw new Error('Parent txid must be a valid hex string.')
+    }
+    bytes[i / 2] = byte
+  }
+  return bytes
+}
+
+MemoReply.MEMO_REPLY_PREFIX = MEMO_REPLY_PREFIX
+MemoReply.MAX_REPLY_BYTES = MAX_REPLY_BYTES
+
+module.exports = MemoReply
diff --git a/src/services/memo-set-name.js b/src/services/memo-set-name.js
index 4f87773..2763c55 100644
--- a/src/services/memo-set-name.js
+++ b/src/services/memo-set-name.js
@@ -17,6 +17,7 @@
 */
 
 const MemoAction = require('./memo-action')
+const { byteLength } = require('./utf8')
 
 const MEMO_SET_NAME_PREFIX = '6d01'
 const MAX_NAME_BYTES = 77
@@ -38,7 +39,7 @@ class MemoSetName extends MemoAction {
 
   // A name is over-length when it exceeds the byte limit.
   isTooLong (name) {
-    return Buffer.byteLength(name, 'utf8') > MAX_NAME_BYTES
+    return byteLength(name) > MAX_NAME_BYTES
   }
 
   // Compose and broadcast a Memo set-name transaction for the given name.
diff --git a/src/services/page-controller.js b/src/services/page-controller.js
index 083becd..3b31bc1 100644
--- a/src/services/page-controller.js
+++ b/src/services/page-controller.js
@@ -33,7 +33,9 @@ class PageController {
 
     try {
       const txid = await this._perform(this.input)
-      this.navigate(this.successPath)
+      if (this.successPath) {
+        this.navigate(this.successPath)
+      }
       this._setBusy(false)
       return { ok: true, txid }
     } catch (err) {
diff --git a/src/services/reply-thread-page.js b/src/services/reply-thread-page.js
new file mode 100644
index 0000000..a65f480
--- /dev/null
+++ b/src/services/reply-thread-page.js
@@ -0,0 +1,59 @@
+/*
+  Reply Thread Page behavior: compose and submit a reply inside a thread
+  modal, with a live byte counter that counts down from the reply limit.
+
+  This is the testable controller behind the Reply form in the React thread
+  modal. It wraps the Memo reply behavior (src/services/memo-reply.js) and
+  adds page-level concerns: holding the current input, tracking the parent txid
+  being replied to, computing the remaining byte count, and surfacing
+  validation/length/broadcast errors.
+
+  The memoReply and navigate concerns are injected so this module stays free of
+  UI/network concerns; environmentally unsuitable I/O lives behind those small
+  adapter boundaries.
+*/
+
+const PageController = require('./page-controller')
+const MemoReply = require('./memo-reply')
+const { byteLength } = require('./utf8')
+
+const REPLY_THREAD_PATH = '/posts/thread'
+
+class ReplyThreadPage extends PageController {
+  constructor (deps = {}) {
+    super(deps)
+    this.memoReply = deps.memoReply || null
+    this.replying = false
+    this.successPath = deps.successPath || null
+    this.validationCodes = ['reply_validation', 'reply_length']
+    this.parentTxid = deps.parentTxid || null
+  }
+
+  // Set the parent txid that the next reply will be attached to.
+  setParent (txid) {
+    this.parentTxid = txid
+    return this
+  }
+
+  // Bytes remaining before the reply byte limit is reached.
+  remainingCount () {
+    return MemoReply.MAX_REPLY_BYTES - byteLength(this.input)
+  }
+
+  // Set the in-flight replying flag.
+  _setBusy (value) {
+    this.replying = value
+  }
+
+  // Run the memo reply action for the current input against the current parent.
+  async _perform (input) {
+    if (!this.memoReply) {
+      throw new Error('Reply thread requires a memo reply handler.')
+    }
+    return this.memoReply.reply(input, this.parentTxid)
+  }
+}
+
+ReplyThreadPage.REPLY_THREAD_PATH = REPLY_THREAD_PATH
+
+module.exports = ReplyThreadPage
diff --git a/src/services/set-name-page.js b/src/services/set-name-page.js
index 571a8cc..1337a61 100644
--- a/src/services/set-name-page.js
+++ b/src/services/set-name-page.js
@@ -15,6 +15,7 @@
 
 const PageController = require('./page-controller')
 const MemoSetName = require('./memo-set-name')
+const { byteLength } = require('./utf8')
 
 const SET_NAME_PATH = '/memo/set-name'
 const ACCOUNT_PATH = '/account'
@@ -30,7 +31,7 @@ class SetNamePage extends PageController {
 
   // Bytes remaining before the name limit is reached.
   remainingCount () {
-    return MemoSetName.MAX_NAME_BYTES - Buffer.byteLength(this.input, 'utf8')
+    return MemoSetName.MAX_NAME_BYTES - byteLength(this.input)
   }
 
   // Set the in-flight setting-name flag.
diff --git a/src/services/utf8.js b/src/services/utf8.js
new file mode 100644
index 0000000..52d4531
--- /dev/null
+++ b/src/services/utf8.js
@@ -0,0 +1,15 @@
+/*
+  UTF-8 byte-length helper for browser and Node.
+
+  The Node global `Buffer` is not available in the browser, so byte counting
+  (used by the Memo set-name byte counter and length check) must not depend on
+  it. TextEncoder is available in both environments and reports the UTF-8 byte
+  length of a string.
+*/
+
+// Return the number of UTF-8 bytes in a string.
+function byteLength (str) {
+  return new TextEncoder().encode(String(str)).length
+}
+
+module.exports = { byteLength }
diff --git a/test/unit/memo-action-helpers.js b/test/unit/memo-action-helpers.js
index 0db0983..b4d3f77 100644
--- a/test/unit/memo-action-helpers.js
+++ b/test/unit/memo-action-helpers.js
@@ -18,17 +18,21 @@ const { fakeWallet } = require('../helpers/fake-wallet')
 //   storeKey   - the action's store dependency key ('feed' or 'profiles')
 //   storeFactory - () => a fresh store
 //   assertStoreEmpty - (store, wallet) => asserts the store was not updated
+//   assertBroadcastMsg - (broadcast, value) => asserts the broadcast message
+//   byteBased  - true when the slice counts bytes (registers multi-byte tests)
+//   extraArgs  - extra arguments passed to the broadcast method after the value
 function registerMemoActionTests (cfg) {
-  const { Action, method, MAX, lengthCode, validationCode, label, storeKey, storeFactory, assertStoreEmpty } = cfg
+  const { Action, method, MAX, lengthCode, validationCode, label, storeKey, storeFactory, assertStoreEmpty, assertBroadcastMsg, byteBased = false, extraArgs = [] } = cfg
+  const checkBroadcastMsg = assertBroadcastMsg || ((broadcast, value) => assert.equal(broadcast.msg, value))
 
   test(`${label} at the maximum length (${MAX}) is accepted`, async () => {
     const wallet = fakeWallet()
     const action = new Action({ wallet })
 
     const value = 'x'.repeat(MAX)
-    const txid = await action[method](value)
+    const txid = await action[method](value, ...extraArgs)
     assert.equal(txid, 'fake-txid')
-    assert.equal(wallet.broadcasts[0].msg, value)
+    checkBroadcastMsg(wallet.broadcasts[0], value)
   })
 
   test(`${label} over the limit (${MAX + 1}) throws a length error and broadcasts nothing`, async () => {
@@ -37,7 +41,7 @@ function registerMemoActionTests (cfg) {
     const action = new Action({ wallet, [storeKey]: store })
 
     await assert.rejects(
-      action[method]('y'.repeat(MAX + 1)),
+      action[method]('y'.repeat(MAX + 1), ...extraArgs),
       (err) => err.code === lengthCode
     )
     assert.equal(wallet.broadcasts.length, 0)
@@ -50,12 +54,54 @@ function registerMemoActionTests (cfg) {
       const action = new Action({ wallet })
 
       await assert.rejects(
-        action[method](invalid),
+        action[method](invalid, ...extraArgs),
         (err) => err.code === validationCode
       )
       assert.equal(wallet.broadcasts.length, 0)
     }
   })
+
+  test(`${label} that is empty throws a validation error and broadcasts nothing`, async () => {
+    const wallet = fakeWallet()
+    const store = storeFactory()
+    const action = new Action({ wallet, [storeKey]: store })
+
+    await assert.rejects(
+      action[method]('', ...extraArgs),
+      (err) => err.code === validationCode
+    )
+    assert.equal(wallet.broadcasts.length, 0)
+    assertStoreEmpty(store, wallet)
+  })
+
+  if (byteBased) {
+    test(`${label} with multi-byte characters at the byte limit is accepted`, async () => {
+      const wallet = fakeWallet()
+      const action = new Action({ wallet })
+
+      // 'é' encodes to 2 UTF-8 bytes, so floor(MAX/2) characters reach the limit.
+      const count = Math.floor(MAX / 2)
+      const value = 'é'.repeat(count)
+      assert.equal(Buffer.byteLength(value, 'utf8'), count * 2)
+      const txid = await action[method](value, ...extraArgs)
+      assert.equal(txid, 'fake-txid')
+    })
+
+    test(`${label} with multi-byte characters that exceed the byte limit throws a length error`, async () => {
+      const wallet = fakeWallet()
+      const action = new Action({ wallet })
+
+      const count = Math.floor(MAX / 2) + 1
+      const value = 'é'.repeat(count)
+      assert.ok(Buffer.byteLength(value, 'utf8') > MAX)
+
+      await assert.rejects(
+        action[method](value, ...extraArgs),
+        (err) => err.code === lengthCode
+      )
+      assert.equal(wallet.broadcasts.length, 0)
+    })
+  }
 }
 
 module.exports = { registerMemoActionTests }
diff --git a/test/unit/memo-post.test.js b/test/unit/memo-post.test.js
index b04f92d..148b382 100644
--- a/test/unit/memo-post.test.js
+++ b/test/unit/memo-post.test.js
@@ -59,19 +59,6 @@ test('posting a valid memo broadcasts an OP_RETURN with the Memo post prefix and
   assert.equal(feed.posts[0].address, wallet.walletInfo.cashAddress)
 })
 
-test('posting an empty memo throws a validation error and broadcasts nothing', async () => {
-  const wallet = fakeWallet()
-  const feed = fakeFeed()
-  const memoPost = new MemoPost({ wallet, feed })
-
-  await assert.rejects(
-    memoPost.post(''),
-    (err) => err.code === 'memo_validation'
-  )
-  assert.equal(wallet.broadcasts.length, 0)
-  assert.equal(feed.posts.length, 0)
-})
-
 test('posting without a wallet reports a missing-wallet error', async () => {
   const memoPost = new MemoPost({})
   await assert.rejects(
diff --git a/test/unit/memo-reply.test.js b/test/unit/memo-reply.test.js
new file mode 100644
index 0000000..319e7fc
--- /dev/null
+++ b/test/unit/memo-reply.test.js
@@ -0,0 +1,103 @@
+/*
+  Unit tests for the Memo reply behavior slice (src/services/memo-reply.js).
+
+  These tests express the observable behavior described by
+  specs/reply-memo.feature:
+    - a valid reply broadcasts an OP_RETURN transaction carrying the Memo reply
+      prefix (0x6d03), the parent txid bytes, and the message text; the thread
+      reflects the new reply.
+    - an empty reply is rejected with a validation error and nothing is broadcast.
+    - an over-long reply is rejected with a length error and nothing is broadcast.
+*/
+
+'use strict'
+
+const test = require('node:test')
+const assert = require('node:assert/strict')
+
+const MemoReply = require('../../src/services/memo-reply')
+const { fakeWallet } = require('../helpers/fake-wallet')
+const { registerMemoActionTests } = require('./memo-action-helpers')
+
+const PARENT_TXID = 'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa'
+
+// A fake thread that records replies added to the thread view.
+function fakeThread (rootTxid = PARENT_TXID) {
+  const replies = []
+  return {
+    rootTxid,
+    replies,
+    addReply: (r) => replies.push(r)
+  }
+}
+
+// Decode the reply text from a raw Uint8Array payload (skipping the 32-byte parent txid).
+function decodeReplyText (raw) {
+  const decoder = new TextDecoder()
+  return decoder.decode(raw.slice(32))
+}
+
+// Decode the parent txid from a raw Uint8Array payload.
+function decodeParentTxid (raw) {
+  return Buffer.from(raw.slice(0, 32)).toString('hex')
+}
+
+registerMemoActionTests({
+  Action: MemoReply,
+  method: 'reply',
+  MAX: 184,
+  lengthCode: 'reply_length',
+  validationCode: 'reply_validation',
+  label: 'a reply',
+  storeKey: 'thread',
+  storeFactory: fakeThread,
+  assertStoreEmpty: (thread) => assert.equal(thread.replies.length, 0),
+  assertBroadcastMsg: (broadcast, value) => assert.equal(decodeReplyText(broadcast.msg), value),
+  byteBased: true,
+  extraArgs: [PARENT_TXID]
+})
+
+test('MEMO_REPLY_PREFIX is the Memo reply action 0x6d03', () => {
+  assert.equal(MemoReply.MEMO_REPLY_PREFIX, '6d03')
+})
+
+test('replying with a valid message broadcasts an OP_RETURN with the Memo reply prefix and payload', async () => {
+  const wallet = fakeWallet()
+  const thread = fakeThread()
+  const memoReply = new MemoReply({ wallet, thread })
+
+  const txid = await memoReply.reply('hello memo', PARENT_TXID)
+
+  assert.equal(txid, 'fake-txid')
+  assert.equal(wallet.broadcasts.length, 1)
+  const b = wallet.broadcasts[0]
+  assert.equal(b.prefix, '6d03')
+  assert.ok(b.msg instanceof Uint8Array)
+  assert.equal(decodeParentTxid(b.msg), PARENT_TXID)
+  assert.equal(decodeReplyText(b.msg), 'hello memo')
+
+  // The thread reflects the new reply from this address with this text.
+  assert.equal(thread.replies.length, 1)
+  assert.equal(thread.replies[0].text, 'hello memo')
+  assert.equal(thread.replies[0].address, wallet.walletInfo.cashAddress)
+  assert.equal(thread.replies[0].parentTxid, PARENT_TXID)
+})
+
+test('replying without a wallet reports a missing-wallet error', async () => {
+  const memoReply = new MemoReply({})
+  await assert.rejects(
+    memoReply.reply('hello memo', PARENT_TXID),
+    (err) => /wallet/i.test(err.message)
+  )
+})
+
+test('replying with an invalid parent txid reports a clear error', async () => {
+  const wallet = fakeWallet()
+  const memoReply = new MemoReply({ wallet })
+
+  await assert.rejects(
+    memoReply.reply('hello memo', 'not-a-txid'),
+    (err) => /txid/i.test(err.message)
+  )
+  assert.equal(wallet.broadcasts.length, 0)
+})
diff --git a/test/unit/memo-set-name.test.js b/test/unit/memo-set-name.test.js
index 623ff17..c902e70 100644
--- a/test/unit/memo-set-name.test.js
+++ b/test/unit/memo-set-name.test.js
@@ -28,7 +28,8 @@ registerMemoActionTests({
   label: 'setting a name',
   storeKey: 'profiles',
   storeFactory: fakeProfiles,
-  assertStoreEmpty: (profiles, wallet) => assert.equal(profiles.getName(wallet.walletInfo.cashAddress), null)
+  assertStoreEmpty: (profiles, wallet) => assert.equal(profiles.getName(wallet.walletInfo.cashAddress), null),
+  byteBased: true
 })
 test('MEMO_SET_NAME_PREFIX is the Memo set-name action 0x6d01', () => {
   assert.equal(MemoSetName.MEMO_SET_NAME_PREFIX, '6d01')
@@ -51,31 +52,6 @@ test('setting a valid name broadcasts an OP_RETURN with the Memo set-name prefix
   assert.equal(profiles.getName(wallet.walletInfo.cashAddress), 'trout')
 })
 
-test('setting a name at the maximum byte length with multi-byte characters is accepted', async () => {
-  const wallet = fakeWallet()
-  const memoSetName = new MemoSetName({ wallet })
-
-  // 38 'é' characters are 76 bytes in UTF-8.
-  const name = 'é'.repeat(38)
-  assert.equal(Buffer.byteLength(name, 'utf8'), 76)
-  const txid = await memoSetName.setName(name)
-  assert.equal(txid, 'fake-txid')
-})
-
-test('setting an over-long name in bytes (78) throws a length error even when char count is lower', async () => {
-  const wallet = fakeWallet()
-  const memoSetName = new MemoSetName({ wallet })
-
-  // 40 'é' characters are 80 bytes, exceeding the 77-byte limit.
-  const name = 'é'.repeat(40)
-  assert.ok(Buffer.byteLength(name, 'utf8') > 77)
-  await assert.rejects(
-    memoSetName.setName(name),
-    (err) => err.code === 'name_length'
-  )
-  assert.equal(wallet.broadcasts.length, 0)
-})
-
 test('setting an empty name throws a validation error and broadcasts nothing', async () => {
   const wallet = fakeWallet()
   const profiles = fakeProfiles()
diff --git a/test/unit/new-post.test.js b/test/unit/new-post.test.js
index f1944ab..dd4a0dd 100644
--- a/test/unit/new-post.test.js
+++ b/test/unit/new-post.test.js
@@ -18,7 +18,7 @@ const assert = require('node:assert/strict')
 const MemoPost = require('../../src/services/memo-post')
 const NewPostPage = require('../../src/services/new-post')
 const { fakeWallet } = require('../helpers/fake-wallet')
-const { registerPageControllerTests } = require('./page-controller-helpers')
+const { registerPageControllerTests, registerPageSubmitTests } = require('./page-controller-helpers')
 const { buildPage } = require('./page-build-helpers')
 
 const MAX = MemoPost.MAX_MEMO_CHARS // 217
@@ -70,54 +70,19 @@ test('the character counter reaches zero at the memo limit', () => {
   assert.equal(page.remainingCount(), 0)
 })
 
-test('posting a valid memo broadcasts the Memo post prefix and navigates to the feed', async () => {
-  const { wallet, store, page, navigations } = build()
-  page.setInput('hello memo')
-
-  const result = await page.submit()
-
-  assert.equal(result.ok, true)
-  // The page returns to an idle (not posting) state after success.
-  assert.equal(page.posting, false)
-  // Broadcast happened with the Memo post prefix and the exact message.
-  assert.equal(wallet.broadcasts.length, 1)
-  assert.equal(wallet.broadcasts[0].prefix, '6d02')
-  assert.equal(wallet.broadcasts[0].msg, 'hello memo')
-  // Navigated to the recent feed after posting.
-  assert.deepEqual(navigations, ['/posts/recent'])
-  // The feed reflects the new post from this address.
-  assert.equal(store.posts.length, 1)
-  assert.equal(store.posts[0].text, 'hello memo')
-})
-
-test('posting an empty memo is rejected with a validation error and nothing is broadcast', async () => {
-  const { wallet, store, page, navigations } = build()
-  page.setInput('')
-
-  const result = await page.submit()
-
-  assert.equal(result.ok, false)
-  assert.equal(result.error, 'memo_validation')
-  assert.equal(page.submitError, 'memo_validation')
-  assert.equal(page.posting, false)
-  assert.equal(wallet.broadcasts.length, 0)
-  assert.equal(store.posts.length, 0)
-  assert.deepEqual(navigations, [])
-})
-
-test('posting an over-long memo is rejected with a length error and nothing is broadcast', async () => {
-  const { wallet, store, page, navigations } = build()
-  page.setInput('y'.repeat(MAX + 1))
-
-  const result = await page.submit()
-
-  assert.equal(result.ok, false)
-  assert.equal(result.error, 'memo_length')
-  assert.equal(page.submitError, 'memo_length')
-  assert.equal(page.posting, false)
-  assert.equal(wallet.broadcasts.length, 0)
-  assert.equal(store.posts.length, 0)
-  assert.deepEqual(navigations, [])
+registerPageSubmitTests({
+  buildPage: build,
+  verb: 'posting',
+  label: 'memo',
+  busyFlag: 'posting',
+  prefix: '6d02',
+  validationCode: 'memo_validation',
+  lengthCode: 'memo_length',
+  MAX,
+  successPath: '/posts/recent',
+  assertBroadcastMsg: (broadcast) => assert.equal(broadcast.msg, 'hello memo'),
+  assertStore: (store) => assert.equal(store.posts[0].text, 'hello memo'),
+  assertStoreEmpty: (store) => assert.equal(store.posts.length, 0)
 })
 
 test('the new post page starts idle (not posting)', () => {
diff --git a/test/unit/page-build-helpers.js b/test/unit/page-build-helpers.js
index 402b571..ea48d03 100644
--- a/test/unit/page-build-helpers.js
+++ b/test/unit/page-build-helpers.js
@@ -11,14 +11,16 @@ const { fakeWallet } = require('../helpers/fake-wallet')
 //   actionKey  - the page's action dependency key ('memoPost' or 'memoSetName')
 //   storeKey   - the action's store dependency key ('feed' or 'profiles')
 //   storeFactory - () => a fresh store
-function buildPage ({ Page, Action, actionKey, storeKey, storeFactory }) {
+//   pageDeps   - extra dependencies passed to the page constructor
+function buildPage ({ Page, Action, actionKey, storeKey, storeFactory, pageDeps = {} }) {
   const wallet = fakeWallet()
   const store = storeFactory()
   const action = new Action({ wallet, [storeKey]: store })
   const navigations = []
   const page = new Page({
     [actionKey]: action,
-    navigate: (path) => navigations.push(path)
+    navigate: (path) => navigations.push(path),
+    ...pageDeps
   })
   return { wallet, store, action, page, navigations }
 }
diff --git a/test/unit/page-controller-helpers.js b/test/unit/page-controller-helpers.js
index 795b16b..3ec30c2 100644
--- a/test/unit/page-controller-helpers.js
+++ b/test/unit/page-controller-helpers.js
@@ -63,4 +63,69 @@ function registerPageControllerTests (cfg) {
   })
 }
 
-module.exports = { registerPageControllerTests }
+// Register the page-submit tests shared by the New Post and Reply Thread pages.
+// Both pages extend PageController and submit a single action, so the valid,
+// empty, and over-long submit behaviors are identical; only the page-specific
+// pieces differ. `cfg` supplies:
+//   buildPage  - () => ({ wallet, store, page, navigations })
+//   verb       - the action verb for test names ('posting' or 'submitting')
+//   label      - the noun for test names ('memo' or 'reply')
+//   busyFlag   - the page's in-flight flag name ('posting' or 'replying')
+//   prefix     - the broadcast prefix ('6d02' or '6d03')
+//   validationCode - the validation error code
+//   lengthCode     - the length error code
+//   MAX        - the length limit
+//   successPath    - the navigation path on success, or null for no navigation
+//   assertBroadcastMsg - (broadcast) => asserts the broadcast message (optional)
+//   assertStore  - (store, wallet) => asserts the store reflects the value
+//   assertStoreEmpty - (store, wallet) => asserts the store was not updated
+function registerPageSubmitTests (cfg) {
+  const { buildPage, verb, label, busyFlag, prefix, validationCode, lengthCode, MAX, successPath, assertBroadcastMsg, assertStore, assertStoreEmpty } = cfg
+
+  test(`${verb} a valid ${label} broadcasts the ${prefix} prefix and reflects it`, async () => {
+    const { wallet, store, page, navigations } = buildPage()
+    page.setInput('hello memo')
+
+    const result = await page.submit()
+
+    assert.equal(result.ok, true)
+    assert.equal(page[busyFlag], false)
+    assert.equal(wallet.broadcasts.length, 1)
+    assert.equal(wallet.broadcasts[0].prefix, prefix)
+    if (assertBroadcastMsg) assertBroadcastMsg(wallet.broadcasts[0])
+    assert.deepEqual(navigations, successPath ? [successPath] : [])
+    assertStore(store, wallet)
+  })
+
+  test(`${verb} an empty ${label} is rejected with a validation error and nothing is broadcast`, async () => {
+    const { wallet, store, page, navigations } = buildPage()
+    page.setInput('')
+
+    const result = await page.submit()
+
+    assert.equal(result.ok, false)
+    assert.equal(result.error, validationCode)
+    assert.equal(page.submitError, validationCode)
+    assert.equal(page[busyFlag], false)
+    assert.equal(wallet.broadcasts.length, 0)
+    assertStoreEmpty(store, wallet)
+    assert.deepEqual(navigations, [])
+  })
+
+  test(`${verb} an over-long ${label} is rejected with a length error and nothing is broadcast`, async () => {
+    const { wallet, store, page, navigations } = buildPage()
+    page.setInput('y'.repeat(MAX + 1))
+
+    const result = await page.submit()
+
+    assert.equal(result.ok, false)
+    assert.equal(result.error, lengthCode)
+    assert.equal(page.submitError, lengthCode)
+    assert.equal(page[busyFlag], false)
+    assert.equal(wallet.broadcasts.length, 0)
+    assertStoreEmpty(store, wallet)
+    assert.deepEqual(navigations, [])
+  })
+}
+
+module.exports = { registerPageControllerTests, registerPageSubmitTests }
diff --git a/test/unit/reply-thread-page.test.js b/test/unit/reply-thread-page.test.js
new file mode 100644
index 0000000..2e1f828
--- /dev/null
+++ b/test/unit/reply-thread-page.test.js
@@ -0,0 +1,113 @@
+/*
+  Unit tests for the Reply Thread Page behavior slice (src/services/reply-thread-page.js).
+
+  Expresses the observable behavior described by specs/reply-memo.feature:
+    - replying with a valid message broadcasts an OP_RETURN with the Memo reply
+      prefix and reflects the reply in the thread.
+    - an empty reply is rejected with a validation error; nothing is broadcast.
+    - an over-long reply is rejected with a length error; nothing is broadcast.
+    - the byte counter counts down from the reply limit.
+*/
+
+'use strict'
+
+const test = require('node:test')
+const assert = require('node:assert/strict')
+
+const MemoReply = require('../../src/services/memo-reply')
+const ReplyThreadPage = require('../../src/services/reply-thread-page')
+const { registerPageControllerTests, registerPageSubmitTests } = require('./page-controller-helpers')
+const { buildPage } = require('./page-build-helpers')
+
+const MAX = MemoReply.MAX_REPLY_BYTES // 184
+const PARENT_TXID = 'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa'
+
+function fakeThread (rootTxid = PARENT_TXID) {
+  const replies = []
+  return { rootTxid, replies, addReply: (r) => replies.push(r) }
+}
+
+function build () {
+  return buildPage({
+    Page: ReplyThreadPage,
+    Action: MemoReply,
+    actionKey: 'memoReply',
+    storeKey: 'thread',
+    storeFactory: fakeThread,
+    pageDeps: { parentTxid: PARENT_TXID }
+  })
+}
+
+function buildBarePage (navigations) {
+  return new ReplyThreadPage({ navigate: (p) => navigations.push(p) })
+}
+
+test('REPLY_THREAD_PATH constant', () => {
+  assert.equal(ReplyThreadPage.REPLY_THREAD_PATH, '/posts/thread')
+})
+
+test('the byte counter counts down from the reply limit for an empty reply', () => {
+  const { page } = build()
+  page.setInput('')
+  assert.equal(page.remainingCount(), MAX)
+})
+
+test('the byte counter counts down from the reply limit for a short reply', () => {
+  const { page } = build()
+  page.setInput('hello')
+  assert.equal(page.remainingCount(), MAX - 5)
+})
+
+test('the byte counter counts multi-byte characters by bytes, not characters', () => {
+  const { page } = build()
+  page.setInput('é')
+  assert.equal(page.remainingCount(), MAX - 2)
+})
+
+test('the byte counter reaches zero at the reply byte limit', () => {
+  const { page } = build()
+  page.setInput('x'.repeat(MAX))
+  assert.equal(page.remainingCount(), 0)
+})
+
+registerPageSubmitTests({
+  buildPage: build,
+  verb: 'submitting',
+  label: 'reply',
+  busyFlag: 'replying',
+  prefix: '6d03',
+  validationCode: 'reply_validation',
+  lengthCode: 'reply_length',
+  MAX,
+  successPath: null,
+  assertStore: (store) => {
+    assert.equal(store.replies[0].text, 'hello memo')
+    assert.equal(store.replies[0].parentTxid, PARENT_TXID)
+  },
+  assertStoreEmpty: (store) => assert.equal(store.replies.length, 0)
+})
+
+test('the reply page starts idle (not replying)', () => {
+  const { page } = build()
+  assert.equal(page.replying, false)
+})
+
+registerPageControllerTests({
+  buildPage: build,
+  buildBarePage,
+  busyFlag: 'replying',
+  prefix: '6d03'
+})
+
+test('replying to a nested reply uses the selected parent txid', async () => {
+  const nestedTxid = 'bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb'
+  const { store, page } = build()
+  page.setParent(nestedTxid)
+  page.setInput('hello nested')
+
+  const result = await page.submit()
+
+  assert.equal(result.ok, true)
+  assert.equal(store.replies[0].parentTxid, nestedTxid)
+  assert.equal(store.replies[0].text, 'hello nested')
+})
diff --git a/test/unit/utf8.test.js b/test/unit/utf8.test.js
new file mode 100644
index 0000000..e4e3e3a
--- /dev/null
+++ b/test/unit/utf8.test.js
@@ -0,0 +1,38 @@
+/*
+  Unit tests for the UTF-8 byte-length helper (src/services/utf8.js).
+
+  The helper must report the same UTF-8 byte length as Node's Buffer without
+  depending on the Node-only `Buffer` global, so the browser build (which has
+  no Buffer) can count bytes for the Memo set-name counter and length check.
+*/
+
+'use strict'
+
+const test = require('node:test')
+const assert = require('node:assert/strict')
+
+const { byteLength } = require('../../src/services/utf8')
+
+test('byteLength matches Buffer.byteLength for ASCII text', () => {
+  for (const s of ['', 'trout', 'a longer name with spaces', 'x'.repeat(77)]) {
+    assert.equal(byteLength(s), Buffer.byteLength(s, 'utf8'))
+  }
+})
+
+test('byteLength matches Buffer.byteLength for multi-byte characters', () => {
+  for (const s of ['é', 'é'.repeat(38), '😀', '😀'.repeat(20), '日本語']) {
+    assert.equal(byteLength(s), Buffer.byteLength(s, 'utf8'))
+  }
+})
+
+test('byteLength counts UTF-8 bytes, not characters', () => {
+  // 'é' is 1 character but 2 UTF-8 bytes; an emoji is 1 character but 4 bytes.
+  assert.equal(byteLength('é'), 2)
+  assert.equal(byteLength('😀'), 4)
+  assert.equal(byteLength('a'), 1)
+})
+
+test('byteLength coerces non-string input to a string', () => {
+  assert.equal(byteLength(42), 2)
+  assert.equal(byteLength(null), 4) // String(null) === 'null'
+})