diff --git a/src/Socket/messages-recv.ts b/src/Socket/messages-recv.ts
index 52a742dd..970f511c 100644
--- a/src/Socket/messages-recv.ts
+++ b/src/Socket/messages-recv.ts
@@ -2345,10 +2345,15 @@ export const makeMessagesRecvSocket = (config: SocketConfig) => {
 		//
 		// HISTORICAL: this restores the intent of d73cd28d39 (2026-02-03) which was
 		// partially reverted by c3fc792351 the same day due to a race-condition concern
-		// with migrateSession (kept sync here). storeLIDPNMappings was over-protected.
+		// with migrateSession (kept sync here). storeLIDPNMappings was over-protected:
+		// it persists a mapping that downstream consumers can re-derive from key.*Alt,
+		// while migrateSession actually moves the Signal session record that decrypt()
+		// will load microseconds later — those two have very different criticality.
 		//
-		// DO NOT make migrateSession async — decrypt() depends on it.
-		// See Downloads/InfiniteAPI-Inbound-Latency-Fix-Documentation.md for full context.
+		// DO NOT make migrateSession async — decrypt() depends on the session being at
+		// the correct identifier (LID vs PN) when it runs. Other code paths (USync
+		// device lookup in messages-send.ts) create LID/PN mappings without migrating
+		// the session, so we cannot skip migration even when the mapping already exists.
 		if (!!alt) {
 			const altServer = jidDecode(alt)?.server
 			const primaryJid = msg.key.participant || msg.key.remoteJid!
diff --git a/src/Utils/process-message.ts b/src/Utils/process-message.ts
index e76b64e8..846441eb 100644
--- a/src/Utils/process-message.ts
+++ b/src/Utils/process-message.ts
@@ -82,6 +82,47 @@ const REAL_MSG_REQ_ME_STUB_TYPES = new Set([WAMessageStubType.GROUP_PARTICIPANT_
  * (TC_TOKEN_INDEX_KEY) via buildMergedTcTokenIndexWrite, so the 24h prune sweep in
  * messages-recv picks them up across sessions.
  */
+/**
+ * Single-concurrency queue for `storeTcTokensFromHistorySync` calls.
+ *
+ * Why: the function does read-then-write merges (`keyStore.get('tctoken', ...)` →
+ * compute → `keyStore.set(...)`) which are NOT atomic at the store level. If two
+ * history-sync chunks invoke this concurrently (common during reconnect / QR
+ * scan), an older chunk that started first can `keyStore.set` AFTER a newer
+ * chunk, overwriting the newer entry — and worse, the merged `__index` write
+ * can drop JIDs the other chunk just added. Result: stale tcTokens / repeat 463
+ * sends until the next opportunistic refetch.
+ *
+ * Serialising via a chained Promise keeps the runs ordered while still freeing
+ * the calling `processMessage` to emit `messaging-history.set` immediately
+ * (the chain is fire-and-forget at the call site). Errors don't break the chain
+ * — each `catch` resets it to `Promise.resolve()` so a single failure can't
+ * stall future runs.
+ *
+ * The chain is module-scoped (one per Node process). Multiple Baileys instances
+ * sharing this module will serialise across instances too, but their writes
+ * target different keyStores so there's no correctness gain — only a tiny loss
+ * of inter-instance parallelism for tcToken syncs, which is acceptable given
+ * how rarely this runs vs. how rare cross-instance contention is.
+ */
+let historyTcTokenChain: Promise<void> = Promise.resolve()
+
+function scheduleHistoryTcTokenSync(
+	chats: Chat[],
+	signalRepository: SignalRepositoryWithLIDStore,
+	keyStore: SignalKeyStoreWithTransaction,
+	logger?: ILogger
+): void {
+	historyTcTokenChain = historyTcTokenChain
+		.catch(() => {
+			/* swallow prior error so chain stays alive */
+		})
+		.then(() => storeTcTokensFromHistorySync(chats, signalRepository, keyStore, logger))
+		.catch(err => {
+			logger?.warn({ err }, 'background tctoken history-sync persistence failed')
+		})
+}
+
 async function storeTcTokensFromHistorySync(
 	chats: Chat[],
 	signalRepository: SignalRepositoryWithLIDStore,
@@ -565,22 +606,28 @@ const processMessage = async (
 						}
 					}
 
-					// Persist tctokens carried by history-sync chats in BACKGROUND.
+					// Persist tctokens carried by history-sync chats in BACKGROUND, serialised.
+					//
 					// Originally awaited (PR #386) to avoid 463 on first multi-device send, but in
-					// production this drains the event buffer per-chunk and adds visible delivery
-					// latency (especially after restart / QR scan when many chunks arrive at once).
+					// production this drained the event buffer per-chunk and added visible delivery
+					// latency (especially after restart / QR scan when many chunks arrived at once).
+					//
+					// `scheduleHistoryTcTokenSync` enqueues onto a single-concurrency promise chain
+					// (see definition above) — chunks persist sequentially in the order they were
+					// emitted, preserving timestamp monotonicity AND keeping the `__index` write
+					// safe from concurrent merge clobbers. The call returns immediately so the
+					// `messaging-history.set` emit is not blocked.
 					//
 					// TRADE-OFF: a listener that fires an outbound send IMMEDIATELY after the emit
-					// may race the background persistence and get a 463 on that specific send. The
-					// existing 463 handler in messages-recv.ts triggers a getPrivacyTokens() refetch
-					// that auto-recovers within seconds. Net result is much better UX than per-chunk
-					// stalls.
+					// may race the still-pending persistence and get a 463 on that specific send.
+					// The existing 463 handler in messages-recv.ts triggers a getPrivacyTokens()
+					// refetch that auto-recovers within seconds. Net result is much better UX than
+					// per-chunk stalls.
 					//
-					// DO NOT add `await` back here without re-evaluating production latency.
-					// See Downloads/InfiniteAPI-Inbound-Latency-Fix-Documentation.md for full context.
-					storeTcTokensFromHistorySync(data.chats, signalRepository, keyStore, logger).catch(err =>
-						logger?.warn({ err }, 'background tctoken history-sync persistence failed')
-					)
+					// DO NOT add `await` back here without re-evaluating production latency, AND
+					// DO NOT call storeTcTokensFromHistorySync directly — it must go through the
+					// chain to preserve write ordering across overlapping chunks.
+					scheduleHistoryTcTokenSync(data.chats, signalRepository, keyStore, logger)
 
 					ev.emit('messaging-history.set', {
 						...data,