diff --git a/src/lib/bns.js b/src/lib/bns.js
index d7a0851..0127bdb 100644
--- a/src/lib/bns.js
+++ b/src/lib/bns.js
@@ -296,6 +296,7 @@ export async function buildIndex(provider, { enforceTld = false, cosignGate = tr
       const entry = {
         name, category: mint.tokenData.category, records: payload.r ?? {},
         txid: tx_hash, height, updatedTxid: null,
+        ...(won.commit ? { commit: { height: won.commit.height, txid: won.commit.txid } } : {}),
       };
       names.set(name, entry);
       byCategory.set(entry.category, entry);
diff --git a/src/lib/registrar.js b/src/lib/registrar.js
index 4c2d7ad..844af12 100644
--- a/src/lib/registrar.js
+++ b/src/lib/registrar.js
@@ -109,11 +109,21 @@ export async function checkAvailability(client, name) {
   // (PROTOCOL-ADDENDUM-release.md, the one-block gap).
   const releasedAt = index.released?.get(key);
   const releasePending = releasedAt !== undefined && !(releasedAt > 0);
+  // A registration can still be displaced, for contestBlocks after it is
+  // mined, by one whose commit is older (PROTOCOL-ADDENDUM-commit-reveal.md).
+  // `contest` says whether that window is open and what a challenger must beat.
+  const contestBlocks = NETWORK.commitReveal?.contestBlocks ?? 0;
+  const contest = entry ? {
+    open: !(entry.height > 0) || (index.height ?? 0) + 1 <= entry.height + contestBlocks,
+    finalAt: entry.height > 0 ? entry.height + contestBlocks + 1 : null,
+    rank: entry.commit ? [entry.commit.height, entry.commit.txid] : [entry.height > 0 ? entry.height : Infinity, entry.txid],
+  } : null;
   return {
     name: key,
     rawName: raw,
     available: !entry && !releasePending,
     entry,
+    contest,
     releasedAt: releasedAt ?? null,
     releasePending,
     reason: entry ? "registered" : releasePending ? "release-pending" : null,
@@ -332,19 +342,27 @@ export async function registerWithBuiltInWallet(client, {
   commit = null,
   onProgress = () => {},
 }) {
+  let committed = null;
   if (commit) {
     if (normalizeName(commit.name) !== normalizeName(name)) throw new Error("this commit was made for another name");
     if (commit.lock !== binToHex(addressToLockingBytecode(wallet.address))) throw new Error("this commit was made for another owner");
-    const state = await commitState(client, commit);
+    const state = committed = await commitState(client, commit);
     if (!state.mature) throw new Error(state.expired
       ? "this commit is too old to reveal — commit again"
       : "this commit is not in a block yet — the registration can be sent after the next block");
   }
   onProgress("checking-availability");
   const availability = await checkAvailability(client, name);
-  if (!availability.available) throw new Error(availability.releasePending
-    ? `"${availability.name}" was just released — it can be registered once that release is in a block`
-    : `"${availability.name}" is already registered`);
+  if (!availability.available) {
+    // Somebody registered it first — perhaps by copying this very name out of
+    // the mempool. An older commit still takes it while the contest is open.
+    const c = availability.contest;
+    const older = committed && c && (committed.height < c.rank[0] || (committed.height === c.rank[0] && commit.txid < c.rank[1]));
+    if (!(c?.open && older)) throw new Error(availability.releasePending
+      ? `"${availability.name}" was just released — it can be registered once that release is in a block`
+      : `"${availability.name}" is already registered`);
+    onProgress("contesting", { incumbent: availability.entry.txid, finalAt: c.finalAt });
+  }
 
   onProgress("loading-coins");
   let utxos = await getUtxosForAddresses(client, wallet.watchedAddresses);
diff --git a/src/lib/resolver-web.js b/src/lib/resolver-web.js
index f628c5c..ccce46a 100644
--- a/src/lib/resolver-web.js
+++ b/src/lib/resolver-web.js
@@ -811,6 +811,8 @@ export async function buildIndex({ network = NETWORK, electrum = getNetwork(netw
         names.set(key, {
           name: key, category: mint.tokenData.category, records: payload.r ?? {},
           txid: tx_hash, height, updatedTxid: null,
+          // the commit this registration revealed, when it had one
+          ...(won.commit ? { commit: { height: won.commit.height, txid: won.commit.txid } } : {}),
           // The mint output's scriptPubKey.address is the initial owner —
           // needed for the signed-records-manifest write path (POST
           // /api/records/<name>) so the gateway can reject a signed
@@ -918,6 +920,7 @@ export function buildIndexFromSnapshot({ snapshot, enforceTld = false, tldMap =
       names.set(key, {
         name: key, category: mint.tokenData.category, records: payload.r ?? {},
         txid: tx_hash, height, updatedTxid: null,
+        ...(won.commit ? { commit: { height: won.commit.height, txid: won.commit.txid } } : {}),
         owner: ownerFromVout(mint),
       });
       // registered straight into the registry covenant?