Rate limit when communicating with one particular person

Guidelines

• I have searched open and closed issues for duplicates
• I am submitting a bug report for existing functionality that does not work as intended
• This isn't a feature request or a discussion topic

Bug description

When communicating with one particular friend, I would some time hit rate limit and cannot send message (spinning dot do not turn into one check mark)

• This doesn't happen with any other person
• This does happen within group, but message to other people would deliver fine except said person.
• While I am waiting to send my message to said person, said person can send me message and I would recieve them just fine
• While I am waiting to send my message to said person, I can send and recieve messages from other people and group chat just fine.
• Deleting unsend message to said person does not resolve the rate limit
• This issue usually go away over night, and would randomly reoccur every couple of weeks.

Screenshots

No response

Device

Pixel 7a

Android version

16 (Graphene OS without Google Play)

Signal version

8.9.1

Link to debug log

https://debuglogs.org/android/8.9.1/d0dd8e74e5f027868a90e3c118fc577fa790a069bf3c9d0996b3df0feb0303fe

[8.9.1] [49   ] 2026-05-13 22:26:08.647 EDT D SignalServiceMessageSender: [1778725568646] Sending a typing message to 1 recipient(s) using 1:1 messages.
[8.9.1] [49   ] 2026-05-13 22:26:08.647 EDT D SignalServiceMessageSender: [1778725568646] Sending to 1 recipients.
[8.9.1] [18556] 2026-05-13 22:26:08.706 EDT D LibSignalChatConnection: [libsignal-unauth:38049743] [sendRequest] Success: 409
[8.9.1] [694  ] 2026-05-13 22:26:08.709 EDT W SignalServiceMessageSender: [sendMessage][1778725568646] Handling mismatched devices. (StatusCode: 409)
[8.9.1] [694  ] 2026-05-13 22:26:08.709 EDT W SignalServiceMessageSender: [handleMismatchedDevices] Address: ********-****-****-****-*********8db, ExtraDevices: [], MissingDevices: [2]
[8.9.1] [694  ] 2026-05-13 22:26:08.711 EDT D KeysApi: Fetching prekeys for ********-****-****-****-*********8db.2, i.e. GET /v2/keys/********-****-****-****-*********8db/2
[8.9.1] [18557] 2026-05-13 22:26:08.737 EDT D LibSignalChatConnection: [libsignal-auth:604455] [sendRequest] Success: 429
[8.9.1] [694  ] 2026-05-13 22:26:08.738 EDT W SignalServiceMessageSender: [1778725568646] Hit rate limit: ********-****-****-****-*********8db
org.whispersystems.signalservice.api.push.exceptions.RateLimitException: [429] Rate limit exceeded: 429
	at org.whispersystems.signalservice.api.NetworkResultUtil.toPreKeysLegacy(NetworkResultUtil.kt:125)
	at org.whispersystems.signalservice.api.SignalServiceMessageSender.handleMismatchedDevices(SignalServiceMessageSender.java:3024)
	at org.whispersystems.signalservice.api.SignalServiceMessageSender.lambda$sendMessageRx$17(SignalServiceMessageSender.java:2378)
	at org.whispersystems.signalservice.api.SignalServiceMessageSender.$r8$lambda$SYPQNQkQYueQagvZ5NsSwnmMvkI(SignalServiceMessageSender.java:0)
	at org.whispersystems.signalservice.api.SignalServiceMessageSender$$ExternalSyntheticLambda29.call(R8$$SyntheticClass:0)
	at io.reactivex.rxjava3.internal.operators.single.SingleFromCallable.subscribeActual(SingleFromCallable.java:43)
	at io.reactivex.rxjava3.core.Single.subscribe(Single.java:4813)
	at io.reactivex.rxjava3.internal.operators.single.SingleFlatMap.subscribeActual(SingleFlatMap.java:37)
	at io.reactivex.rxjava3.core.Single.subscribe(Single.java:4813)
	at io.reactivex.rxjava3.internal.operators.single.SingleResumeNext$ResumeMainSingleObserver.onError(SingleResumeNext.java:80)
	at io.reactivex.rxjava3.internal.operators.single.SingleFlatMap$SingleFlatMapCallback$FlatMapSingleObserver.onError(SingleFlatMap.java:117)
	at io.reactivex.rxjava3.internal.disposables.EmptyDisposable.error(EmptyDisposable.java:78)
	at io.reactivex.rxjava3.internal.operators.single.SingleError.subscribeActual(SingleError.java:41)
	at io.reactivex.rxjava3.core.Single.subscribe(Single.java:4813)
	at io.reactivex.rxjava3.internal.operators.single.SingleFlatMap$SingleFlatMapCallback.onSuccess(SingleFlatMap.java:85)
	at io.reactivex.rxjava3.internal.operators.single.SingleObserveOn$ObserveOnSingleObserver.run(SingleObserveOn.java:81)
	at io.reactivex.rxjava3.internal.schedulers.ScheduledDirectTask.call(ScheduledDirectTask.java:38)
	at io.reactivex.rxjava3.internal.schedulers.ScheduledDirectTask.call(ScheduledDirectTask.java:25)
	at java.util.concurrent.FutureTask.run(FutureTask.java:317)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1154)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:652)
	at java.lang.Thread.run(Thread.java:1563)
	at org.signal.core.util.concurrent.SignalExecutors$NumberedThreadFactory$1.run(SignalExecutors.java:100)

Start with the debug log and trace the reported path through SignalServiceMessageSender.java, especially handleMismatchedDevices, then inspect NetworkResultUtil.kt and KeysApi around the prekey request. Compare the 409 mismatched-device response with the subsequent 429 rate limit. Done means the recipient-specific send failure is explained and verified against the reported behavior.