feat: show NIP-32 hashtag labels from follows on posts

Posts now display a '#tag added by <user>' row (tag icon + clickable
hashtag + labeler attribution) in feeds and the thread view whenever a
followed user - or the logged-in user - attached a hashtag to the post
via a kind 1985 label event. Hashtags the author already used are
skipped, and labels from non-follows are never shown.

The labels are fetched by a new filter family in the per-note
EventWatcherSubAssembler: kind 1985 requests routed through the outbox
model (account.followsPerRelay), so each follow's labels are requested
from that follow's own outbox relays with the filter's authors
restricted to the follows that actually write there.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MjCyWTm3FqEMYiNUH8pEyE

amethyst/src/main/java/com/vitorpamplona/amethyst/service/relayClient/reqCommand/event/EventObservers.kt

@@ -265,6 +265,19 @@ fun observeNoteZaps(
     return flow.collectAsStateWithLifecycle()
 }
 
+@Composable
+fun observeNoteLabels(
+    note: Note,
+    accountViewModel: AccountViewModel,
+): State<NoteState?> {
+    // Subscribe in the relay for changes in this note.
+    EventFinderFilterAssemblerSubscription(note, accountViewModel)
+
+    // Subscribe in the LocalCache for changes that arrive in the device
+    val flow = remember(note) { note.flow().labels.stateFlow }
+    return flow.collectAsStateWithLifecycle()
+}
+
 @Composable
 fun observeNoteReposts(
     note: Note,

amethyst/src/main/java/com/vitorpamplona/amethyst/service/relayClient/reqCommand/event/watchers/EventWatcherSubAssembler.kt

@@ -58,6 +58,8 @@ class EventWatcherSubAssembler(
             return null
         }
 
+        val account = keys.first().account
+
         lastNotesOnFilter = keys.map { it.note }
 
         return groupByRelayPresence(lastNotesOnFilter, latestEOSEs)
@@ -65,10 +67,12 @@ class EventWatcherSubAssembler(
                 if (group.isNotEmpty()) {
                     val addressables = group.filterIsInstance<AddressableNote>()
                     val events = group.mapNotNull { if (it !is AddressableNote) it else null }
+                    val eventEoses = findMinimumEOSEs(events, latestEOSEs)
 
                     listOfNotNull(
-                        filterRepliesAndReactionsToNotes(events, findMinimumEOSEs(events, latestEOSEs)),
+                        filterRepliesAndReactionsToNotes(events, eventEoses),
                         filterRepliesAndReactionsToAddresses(addressables, findMinimumEOSEs(addressables, latestEOSEs)),
+                        filterLabelsByFollowsToNotes(account, events, eventEoses),
                     ).flatten()
                 } else {
                     emptyList()

amethyst/src/main/java/com/vitorpamplona/amethyst/service/relayClient/reqCommand/event/watchers/FilterLabelsByFollowsToNotes.kt

@@ -0,0 +1,60 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.amethyst.service.relayClient.reqCommand.event.watchers
+
+import com.vitorpamplona.amethyst.model.Account
+import com.vitorpamplona.amethyst.model.Note
+import com.vitorpamplona.amethyst.service.relays.SincePerRelayMap
+import com.vitorpamplona.quartz.nip01Core.relay.client.pool.RelayBasedFilter
+import com.vitorpamplona.quartz.nip01Core.relay.filters.Filter
+import com.vitorpamplona.quartz.nip32Labeling.LabelEvent
+
+/**
+ * NIP-32: watches for kind 1985 label events that tag the notes on screen, restricted to
+ * the accounts the user follows and routed through the outbox model: each follow's labels
+ * are requested from THAT follow's outbox relays, with each relay's filter limited to the
+ * follows that actually write there. Labels from non-follows are never requested.
+ */
+fun filterLabelsByFollowsToNotes(
+    account: Account,
+    notes: List<Note>,
+    since: SincePerRelayMap?,
+): List<RelayBasedFilter> {
+    if (notes.isEmpty()) return emptyList()
+
+    val noteIds = notes.map { it.idHex }.sorted()
+
+    return account.followsPerRelay.value.mapNotNull { (relay, authors) ->
+        if (authors.isEmpty()) return@mapNotNull null
+        RelayBasedFilter(
+            relay = relay,
+            filter =
+                Filter(
+                    kinds = listOf(LabelEvent.KIND),
+                    authors = authors.sorted(),
+                    tags = mapOf("e" to noteIds),
+                    since = since?.get(relay)?.time,
+                    // Labels are rare; this only bounds pathological cases.
+                    limit = 500,
+                ),
+        )
+    }
+}

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/note/NoteCompose.kt

@@ -79,6 +79,7 @@ import com.vitorpamplona.amethyst.ui.navigation.routes.routeEditDraftTo
 import com.vitorpamplona.amethyst.ui.navigation.routes.routeFor
 import com.vitorpamplona.amethyst.ui.note.creators.zapsplits.DisplayZapSplits
 import com.vitorpamplona.amethyst.ui.note.elements.DisplayEditStatus
+import com.vitorpamplona.amethyst.ui.note.elements.DisplayFollowLabelsInPost
 import com.vitorpamplona.amethyst.ui.note.elements.DisplayFollowingCommunityInPost
 import com.vitorpamplona.amethyst.ui.note.elements.DisplayFollowingHashtagsInPost
 import com.vitorpamplona.amethyst.ui.note.elements.DisplayLocation
@@ -788,6 +789,8 @@ fun InnerNoteWithReactions(
                     Spacer(modifier = HalfDoubleVertSpacer)
                     DisplayZapSplits(noteEvent, false, accountViewModel, nav)
                 }
+
+                DisplayFollowLabelsInPost(baseNote, accountViewModel, nav)
             }
 
             RenderApprovalIfNeeded(baseNote, accountViewModel, nav)

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/note/elements/DisplayFollowLabelsInPost.kt

@@ -0,0 +1,134 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.amethyst.ui.note.elements
+
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.Spacer
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.runtime.getValue
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.text.font.FontWeight
+import com.vitorpamplona.amethyst.R
+import com.vitorpamplona.amethyst.commons.icons.symbols.Icon
+import com.vitorpamplona.amethyst.commons.icons.symbols.MaterialSymbols
+import com.vitorpamplona.amethyst.commons.ui.components.ClickableTextColor
+import com.vitorpamplona.amethyst.model.Note
+import com.vitorpamplona.amethyst.model.User
+import com.vitorpamplona.amethyst.service.relayClient.reqCommand.event.observeNoteLabels
+import com.vitorpamplona.amethyst.ui.navigation.navs.INav
+import com.vitorpamplona.amethyst.ui.navigation.routes.Route
+import com.vitorpamplona.amethyst.ui.note.UsernameDisplay
+import com.vitorpamplona.amethyst.ui.screen.loggedIn.AccountViewModel
+import com.vitorpamplona.amethyst.ui.stringRes
+import com.vitorpamplona.amethyst.ui.theme.HalfVertSpacer
+import com.vitorpamplona.amethyst.ui.theme.Size16Modifier
+import com.vitorpamplona.amethyst.ui.theme.StdHorzSpacer
+import com.vitorpamplona.amethyst.ui.theme.lessImportantLink
+import com.vitorpamplona.amethyst.ui.theme.placeholderText
+import com.vitorpamplona.quartz.nip01Core.tags.hashtags.isTaggedHash
+
+/**
+ * NIP-32: shows the hashtags that people the user follows (or the user themselves) attached
+ * to this post via kind 1985 label events, as "🏷 #tag added by <user>" rows. Labels from
+ * non-follows are ignored, and hashtags the author already used in the post are skipped
+ * (they render with the post itself). Renders nothing when there is nothing to show.
+ */
+@Composable
+fun DisplayFollowLabelsInPost(
+    baseNote: Note,
+    accountViewModel: AccountViewModel,
+    nav: INav,
+    modifier: Modifier = Modifier,
+) {
+    val labelState by observeNoteLabels(baseNote, accountViewModel)
+
+    val labels = labelState?.note?.labels ?: return
+    if (labels.isEmpty()) return
+
+    val follows = accountViewModel.account.followingKeySet()
+    val loggedUser = accountViewModel.account.userProfile().pubkeyHex
+
+    val visibleLabels =
+        labels.mapNotNull { (tag, labelNotes) ->
+            // The author already used the hashtag — it renders with the post itself.
+            if (baseNote.event?.isTaggedHash(tag) == true) return@mapNotNull null
+
+            val labeler =
+                labelNotes
+                    .firstOrNull {
+                        val pubkey = it.author?.pubkeyHex
+                        pubkey != null && (pubkey == loggedUser || pubkey in follows)
+                    }?.author ?: return@mapNotNull null
+
+            tag to labeler
+        }
+
+    if (visibleLabels.isEmpty()) return
+
+    Column(modifier) {
+        Spacer(HalfVertSpacer)
+        visibleLabels.forEach { (tag, labeler) ->
+            FollowLabelRow(tag, labeler, accountViewModel, nav)
+        }
+    }
+}
+
+@Composable
+private fun FollowLabelRow(
+    tag: String,
+    labeler: User,
+    accountViewModel: AccountViewModel,
+    nav: INav,
+) {
+    Row(verticalAlignment = Alignment.CenterVertically) {
+        Icon(
+            symbol = MaterialSymbols.Tag,
+            contentDescription = null,
+            modifier = Size16Modifier,
+            tint = MaterialTheme.colorScheme.placeholderText,
+        )
+        Spacer(StdHorzSpacer)
+        ClickableTextColor(
+            "#$tag",
+            linkColor = MaterialTheme.colorScheme.lessImportantLink,
+            maxLines = 1,
+        ) {
+            nav.nav(Route.Hashtag(tag))
+        }
+        Spacer(StdHorzSpacer)
+        Text(
+            stringRes(R.string.hashtag_label_added_by),
+            color = MaterialTheme.colorScheme.placeholderText,
+            maxLines = 1,
+        )
+        Spacer(StdHorzSpacer)
+        UsernameDisplay(
+            labeler,
+            fontWeight = FontWeight.Bold,
+            textColor = MaterialTheme.colorScheme.placeholderText,
+            accountViewModel = accountViewModel,
+        )
+    }
+}

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/threadview/ThreadFeedView.kt

@@ -118,6 +118,7 @@ import com.vitorpamplona.amethyst.ui.note.calculateBackgroundColor
 import com.vitorpamplona.amethyst.ui.note.creators.zapsplits.DisplayZapSplits
 import com.vitorpamplona.amethyst.ui.note.elements.DefaultImageHeader
 import com.vitorpamplona.amethyst.ui.note.elements.DefaultImageHeaderBackground
+import com.vitorpamplona.amethyst.ui.note.elements.DisplayFollowLabelsInPost
 import com.vitorpamplona.amethyst.ui.note.elements.DisplayFollowingCommunityInPost
 import com.vitorpamplona.amethyst.ui.note.elements.DisplayFollowingHashtagsInPost
 import com.vitorpamplona.amethyst.ui.note.elements.DisplayLocation
@@ -1135,6 +1136,8 @@ private fun FullBleedNoteCompose(
             }
         }
 
+        DisplayFollowLabelsInPost(baseNote, accountViewModel, nav, PaddingHorizontal12Modifier)
+
         RenderApprovalIfNeeded(baseNote, accountViewModel, nav)
 
         ReactionsRow(baseNote, showReactionDetail = true, addPadding = true, editState = editState, accountViewModel = accountViewModel, nav = nav)