diff --git a/api/auth/nonce.js b/api/auth/nonce.js
new file mode 100644
index 0000000..1d3347d
--- /dev/null
+++ b/api/auth/nonce.js
@@ -0,0 +1,18 @@
+'use strict';
+
+const crypto = require('node:crypto');
+
+const NONCE_BYTES = 16;
+
+module.exports = async function handler(req, res) {
+  res.setHeader('Content-Type', 'application/json; charset=utf-8');
+  res.setHeader('Cache-Control', 'no-store');
+
+  if (req.method !== 'GET') {
+    res.setHeader('Allow', 'GET');
+    return res.status(405).json({ ok: false, status: 'AUTH_FAILED', error: 'Method not allowed. Use GET.' });
+  }
+
+  const nonce = crypto.randomBytes(NONCE_BYTES).toString('hex');
+  return res.status(200).json({ ok: true, nonce });
+};
diff --git a/api/auth/verify.js b/api/auth/verify.js
new file mode 100644
index 0000000..47c6bb1
--- /dev/null
+++ b/api/auth/verify.js
@@ -0,0 +1,65 @@
+'use strict';
+
+const { URL } = require('node:url');
+
+const ALLOWED_CHAIN_IDS = new Set((process.env.SIWE_ALLOWED_CHAIN_IDS || '1,8453,11155111').split(',').map((v) => Number(v.trim())).filter(Number.isFinite));
+const ALLOWED_DOMAIN = process.env.SIWE_ALLOWED_DOMAIN || '';
+const ALLOWED_URI = process.env.SIWE_ALLOWED_URI || '';
+const REQUIRED_STATEMENT = 'CommandLayer Claim activation';
+
+function getHost(req) {
+  return String((req.headers && (req.headers['x-forwarded-host'] || req.headers.host)) || '').split(',')[0].trim().toLowerCase();
+}
+
+module.exports = async function handler(req, res) {
+  res.setHeader('Content-Type', 'application/json; charset=utf-8');
+  res.setHeader('Cache-Control', 'no-store');
+
+  if (req.method !== 'POST') {
+    res.setHeader('Allow', 'POST');
+    return res.status(405).json({ ok: false, status: 'AUTH_FAILED', error: 'Method not allowed. Use POST.' });
+  }
+
+  const body = req.body || {};
+  const message = typeof body.message === 'string' ? body.message : '';
+  const signature = typeof body.signature === 'string' ? body.signature : '';
+  if (!message || !signature) {
+    return res.status(400).json({ ok: false, status: 'AUTH_FAILED', error: 'Missing SIWE message or signature.' });
+  }
+
+  let SiweMessage;
+  try {
+    ({ SiweMessage } = require('siwe'));
+  } catch {
+    return res.status(503).json({ ok: false, status: 'AUTH_FAILED', error: 'SIWE verification dependency unavailable on server.' });
+  }
+
+  try {
+    const parsed = new SiweMessage(message);
+    const expectedDomain = ALLOWED_DOMAIN || getHost(req);
+    if (expectedDomain && String(parsed.domain || '').toLowerCase() !== expectedDomain) {
+      return res.status(400).json({ ok: false, status: 'AUTH_FAILED', error: 'SIWE domain mismatch.' });
+    }
+
+    if (ALLOWED_URI && parsed.uri !== ALLOWED_URI) {
+      return res.status(400).json({ ok: false, status: 'AUTH_FAILED', error: 'SIWE URI mismatch.' });
+    }
+
+    if (!ALLOWED_CHAIN_IDS.has(Number(parsed.chainId))) {
+      return res.status(400).json({ ok: false, status: 'AUTH_FAILED', error: 'Unsupported SIWE chainId.' });
+    }
+
+    if (!String(parsed.statement || '').toLowerCase().includes(REQUIRED_STATEMENT.toLowerCase())) {
+      return res.status(400).json({ ok: false, status: 'AUTH_FAILED', error: 'Invalid SIWE statement for claim activation.' });
+    }
+
+    const result = await parsed.verify({ signature, domain: expectedDomain, nonce: parsed.nonce });
+    if (!result.success) {
+      return res.status(401).json({ ok: false, status: 'AUTH_FAILED', error: 'SIWE signature verification failed.' });
+    }
+
+    return res.status(200).json({ ok: true, status: 'AUTHENTICATED', address: result.data.address, chainId: Number(result.data.chainId), ens: null });
+  } catch (error) {
+    return res.status(400).json({ ok: false, status: 'AUTH_FAILED', error: error && error.message ? error.message : 'Invalid SIWE payload.' });
+  }
+};
diff --git a/public/claim.html b/public/claim.html
index 16a75b2..caf0307 100644
--- a/public/claim.html
+++ b/public/claim.html
@@ -428,9 +428,20 @@ <h3>Browser-side key generation</h3>
       Wallet connection is required to write records under your ENS name.<br>
       Payment/provisioning is required for CommandLayer-native namespace activation.
     </div>
+
+    <div style="margin-top:16px;padding:14px;border-radius:12px;border:1px solid var(--border);background:#fff">
+      <div style="font-weight:700;margin-bottom:6px">Sign in with Ethereum</div>
+      <div style="font-size:13px;color:var(--text-2);margin-bottom:10px">Authenticate the wallet that will submit this activation request.</div>
+      <div style="font-size:12px;color:var(--muted);margin-bottom:10px">Your Ethereum wallet authenticates the claim request. Your Ed25519 key signs agent receipts.</div>
+      <button class="btn btn-secondary" id="siweAuthBtn" onclick="signInWithEthereum()">Sign-In with Ethereum</button>
+      <div id="siweStatus" style="font-size:13px;color:var(--text-2);margin-top:10px">Status: Not authenticated</div>
+      <div id="siweWallet" style="font-family:var(--mono);font-size:12px;color:var(--muted);margin-top:4px"></div>
+      <div id="siweModeHint" style="font-size:12px;color:var(--muted);margin-top:8px"></div>
+    </div>
+
     <div class="btn-row">
       <button class="btn btn-ghost" onclick="goToStep(4)">← Back</button>
-      <button class="btn btn-primary" onclick="goToStep(6)">Continue →</button>
+      <button class="btn btn-primary" onclick="goStep5Auth()">Continue →</button>
     </div>
   </div>
 
@@ -573,7 +584,8 @@ <h3>Payment and provisioning are coming next.</h3>
   capMode:'packs', selectedPack:null, cherryVerbs:[],
   pubKeyB64:'', privKeyB64:'', kid:'',
   keyGenerated:false, keyDownloaded:false, keyAcked:false,
-  _cardJson:''
+  _cardJson:'',
+  authenticatedAddress:'', authStatus:'NOT_AUTHENTICATED', authChainId:null
 };
 
 // ── HELPERS ───────────────────────────────────────────────────────────────────
@@ -665,6 +677,7 @@ <h3>Payment and provisioning are coming next.</h3>
     document.getElementById('mode-'+id).classList.toggle('selected', id===m);
   });
   updateNamePreview();
+  updateSiweModeHint();
 }
 
 function updateNamePreview() {
@@ -685,11 +698,13 @@ <h3>Payment and provisioning are coming next.</h3>
   if (!state.activationMode) { alert('Please choose an activation mode.'); return; }
   // default to CL mode if none selected
   if (!document.querySelector('.mode-card.selected')) selectMode('cl');
+updateSiweModeHint();
   goToStep(3);
 }
 
 // Auto-select CL mode on load
 selectMode('cl');
+updateSiweModeHint();
 
 // ── STEP 3: CAPABILITIES ──────────────────────────────────────────────────────
 function setCapMode(m) {
@@ -754,6 +769,63 @@ <h3>Payment and provisioning are coming next.</h3>
   goToStep(4);
 }
 
+
+
+function shortAddr(addr){return addr?`${addr.slice(0,6)}...${addr.slice(-4)}`:'';}
+
+function updateSiweModeHint(){
+  const el=document.getElementById('siweModeHint');
+  if(!el) return;
+  if(state.activationMode==='own' || state.activationMode==='single'){
+    el.textContent='ENS ownership verification comes after wallet authentication.';
+  }else{
+    el.textContent='CommandLayer namespace activation requires wallet authentication before submission.';
+  }
+}
+
+async function signInWithEthereum(){
+  if(!window.ethereum){ alert('No Ethereum wallet detected. Install a wallet extension first.'); return; }
+  updateSiweModeHint();
+  const statusEl=document.getElementById('siweStatus');
+  const walletEl=document.getElementById('siweWallet');
+  statusEl.textContent='Status: Requesting signature...';
+  const [address]=await window.ethereum.request({ method:'eth_requestAccounts' });
+  const nonceRes=await fetch('/api/auth/nonce',{method:'GET',headers:{'Accept':'application/json'}});
+  const nonceData=await nonceRes.json();
+  if(!nonceData.ok){ throw new Error('Nonce request failed'); }
+  const domain=window.location.host;
+  const uri=window.location.origin;
+  const chainHex=await window.ethereum.request({ method:'eth_chainId' });
+  const chainId=parseInt(chainHex,16);
+  const issuedAt=new Date().toISOString();
+  const msg=`${domain} wants you to sign in with your Ethereum account:
+${address}
+
+CommandLayer Claim activation
+
+URI: ${uri}
+Version: 1
+Chain ID: ${chainId}
+Nonce: ${nonceData.nonce}
+Issued At: ${issuedAt}`;
+  const signature=await window.ethereum.request({method:'personal_sign',params:[msg,address]});
+  const verifyRes=await fetch('/api/auth/verify',{method:'POST',headers:{'Content-Type':'application/json'},body:JSON.stringify({message:msg,signature})});
+  const verify=await verifyRes.json();
+  if(!verify.ok){ throw new Error(verify.error||'Authentication failed'); }
+  state.authenticatedAddress=verify.address; state.authStatus=verify.status; state.authChainId=verify.chainId;
+  statusEl.textContent='Status: Authenticated';
+  walletEl.textContent=`Connected wallet: ${shortAddr(verify.address)}`;
+}
+
+function goStep5Auth(){
+  updateSiweModeHint();
+  if(state.activationMode==='cl' && !state.authenticatedAddress){
+    alert('Sign-In with Ethereum is required before submitting CommandLayer activation request.');
+    return;
+  }
+  goToStep(6);
+}
+
 // ── STEP 4: KEY GEN ───────────────────────────────────────────────────────────
 async function generateKey() {
   const errorBox = document.getElementById('keyError');
@@ -1073,6 +1145,7 @@ <h3>Payment and provisioning are coming next.</h3>
   const err=document.getElementById('keyError'); if(err){err.classList.remove('show');err.textContent='';}
   document.getElementById('namePreview').classList.remove('show');
   selectMode('cl');
+updateSiweModeHint();
   goToStep(1);
 }
 
diff --git a/tests/api-auth.test.js b/tests/api-auth.test.js
new file mode 100644
index 0000000..04ebc07
--- /dev/null
+++ b/tests/api-auth.test.js
@@ -0,0 +1,42 @@
+'use strict';
+
+const test = require('node:test');
+const assert = require('node:assert/strict');
+
+const nonceHandler = require('../api/auth/nonce');
+const verifyHandler = require('../api/auth/verify');
+
+function makeRes() {
+  return {
+    statusCode: 200,
+    headers: {},
+    body: null,
+    setHeader(name, value) { this.headers[name.toLowerCase()] = value; },
+    status(code) { this.statusCode = code; return this; },
+    json(payload) { this.body = payload; return this; },
+  };
+}
+
+test('GET /api/auth/nonce returns nonce and randomness', async () => {
+  const r1 = makeRes(); const r2 = makeRes();
+  await nonceHandler({ method: 'GET', headers: {} }, r1);
+  await nonceHandler({ method: 'GET', headers: {} }, r2);
+  assert.equal(r1.statusCode, 200);
+  assert.equal(r1.body.ok, true);
+  assert.match(r1.body.nonce, /^[a-f0-9]{32,}$/);
+  assert.notEqual(r1.body.nonce, r2.body.nonce);
+});
+
+test('POST /api/auth/verify rejects missing signature', async () => {
+  const res = makeRes();
+  await verifyHandler({ method: 'POST', body: { message: 'x' }, headers: { host: 'localhost:3000' } }, res);
+  assert.equal(res.statusCode, 400);
+  assert.equal(res.body.ok, false);
+});
+
+test('POST /api/auth/verify rejects malformed message/signature', async () => {
+  const res = makeRes();
+  await verifyHandler({ method: 'POST', body: { message: 'invalid', signature: '0xdeadbeef' }, headers: { host: 'localhost:3000' } }, res);
+  assert.equal(res.body.ok, false);
+  assert.equal(res.body.status, 'AUTH_FAILED');
+});