feat(community): validate posts against NIP-9A community rules in composer

When the comment composer is replying into a NIP-72 community
(`replyingTo.event is CommunityDefinitionEvent`), subscribe to the
community's latest `kind:34551` rules document, run
`CommunityRulesValidator.validate(...)` on every draft change, render an
inline banner above the bottom action row when the draft would be
rejected, and disable the post button until the violation is resolved.

- `CommunityRulesFilterSubAssembler` is added to the existing
  `CommunityFilterAssembler.group` so any screen that already mounts
  `CommunityFilterAssemblerSubscription` (community feed, this composer)
  also pulls the latest rules event from the community's relays. Filter
  is `kinds=[34551]`, `authors=<owner+moderators>`, `#a=<addressTag>`,
  matching the NIP-72 trust model.
- `CommentPostViewModel` observes `kind:34551` via
  `LocalCache.observeEvents` keyed on the reply target, picks the latest
  `created_at` matching the community address, and re-runs the validator
  on every draft change. The validator's `postsTodayByKind` and `wot`
  callbacks are intentionally null for this PR (per-day quota lookups
  and NIP-02 follow-graph traversal are deferred to follow-ups; the
  validator skips those checks cleanly).
- Draft size is conservatively estimated from `content.toByteArray(UTF-8)`
  — tags add bytes, so this can under-count on the boundary, but relays
  still enforce the real cap. Good enough for a pre-send preview.
- `CommunityRulesViolationBanner` renders the first
  `CommunityRulesValidator.Violation` with a localized message; new
  strings cover all 7 sealed-violation variants.
- `CommentPostViewModelTest` covers valid drafts, kind-not-allowed,
  oversize, denied-author, the under-size boundary, and multibyte UTF-8
  size accounting.

Compose state — not StateFlow — backs `validationResult` and
`communityRules` so `canPost()` and the banner recompose without an
explicit `collectAsState` site at the top bar (`isActive` reads
`validationResult` directly).

Refs nostr-protocol/nips#2331
Closes #2759

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/note/nip22Comments/CommentPostViewModel.kt

@@ -71,6 +71,7 @@ import com.vitorpamplona.quartz.experimental.nip95.data.FileStorageEvent
 import com.vitorpamplona.quartz.experimental.nip95.header.FileStorageHeaderEvent
 import com.vitorpamplona.quartz.nip01Core.core.AddressableEvent
 import com.vitorpamplona.quartz.nip01Core.core.Event
+import com.vitorpamplona.quartz.nip01Core.relay.filters.Filter
 import com.vitorpamplona.quartz.nip01Core.signers.EventTemplate
 import com.vitorpamplona.quartz.nip01Core.signers.SignerExceptions
 import com.vitorpamplona.quartz.nip01Core.tags.geohash.geohash
@@ -96,6 +97,8 @@ import com.vitorpamplona.quartz.nip57Zaps.splits.zapSplits
 import com.vitorpamplona.quartz.nip57Zaps.zapraiser.zapraiser
 import com.vitorpamplona.quartz.nip57Zaps.zapraiser.zapraiserAmount
 import com.vitorpamplona.quartz.nip72ModCommunities.definition.CommunityDefinitionEvent
+import com.vitorpamplona.quartz.nip72ModCommunities.rules.CommunityRulesEvent
+import com.vitorpamplona.quartz.nip72ModCommunities.rules.CommunityRulesValidator
 import com.vitorpamplona.quartz.nip73ExternalIds.ExternalId
 import com.vitorpamplona.quartz.nip73ExternalIds.scope
 import com.vitorpamplona.quartz.nip92IMeta.IMetaTagBuilder
@@ -113,6 +116,7 @@ import com.vitorpamplona.quartz.nip94FileMetadata.thumbhash
 import com.vitorpamplona.quartz.utils.TimeUtils
 import kotlinx.collections.immutable.ImmutableList
 import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.Job
 import kotlinx.coroutines.flow.StateFlow
 import kotlinx.coroutines.flow.collectLatest
 import kotlinx.coroutines.launch
@@ -151,6 +155,17 @@ open class CommentPostViewModel :
 
     var notifying by mutableStateOf<List<User>?>(null)
 
+    // NIP-9A: latest community rules document for the community we're posting into.
+    // Null when the reply target is not a community, or no rules have been observed yet.
+    var communityRules: CommunityRulesEvent? by mutableStateOf(null)
+        private set
+    private var rulesObserverJob: Job? = null
+
+    // NIP-9A: latest validation outcome for the current draft.
+    // Null = no violation (or rules not yet known); non-null = first violation found.
+    var validationResult: CommunityRulesValidator.Violation? by mutableStateOf(null)
+        private set
+
     override val message = TextFieldState()
 
     val urlPreviews = PreviewState()
@@ -243,6 +258,90 @@ open class CommentPostViewModel :
     open fun reply(post: Note) {
         this.replyingTo = post
         this.externalIdentity = (post.event as? CommentEvent)?.scope()
+        observeCommunityRules(post)
+    }
+
+    /**
+     * Subscribes to the latest [CommunityRulesEvent] when [target] is a NIP-72
+     * community, and clears the rules state otherwise. Re-evaluates the current
+     * draft against any rules that arrive (NIP-9A composer-side validation).
+     */
+    private fun observeCommunityRules(target: Note) {
+        rulesObserverJob?.cancel()
+        rulesObserverJob = null
+        communityRules = null
+        validationResult = null
+
+        val targetEvent = target.event as? CommunityDefinitionEvent ?: return
+        val communityAddress = targetEvent.addressTag()
+        val authors = targetEvent.moderatorKeys()
+
+        val filter =
+            Filter(
+                kinds = listOf(CommunityRulesEvent.KIND),
+                authors = authors.sorted(),
+                tags = mapOf("a" to listOf(communityAddress)),
+                limit = 5,
+            )
+
+        rulesObserverJob =
+            viewModelScope.launch(Dispatchers.IO) {
+                LocalCache.observeEvents<CommunityRulesEvent>(filter).collectLatest { events ->
+                    val latest =
+                        events
+                            .filter { it.communityAddress() == communityAddress }
+                            .maxByOrNull { it.createdAt }
+                    communityRules = latest
+                    revalidateDraft()
+                }
+            }
+    }
+
+    /**
+     * Runs the NIP-9A validator against the current draft and updates
+     * [validationResult]. Web-of-trust gates and per-day quota checks are deferred
+     * to a follow-up (see NIP-9A track), so [postsTodayByKind] and `wot` are null
+     * here; the validator skips those checks cleanly.
+     */
+    private fun revalidateDraft() {
+        val rules = communityRules
+        if (rules == null) {
+            validationResult = null
+            return
+        }
+
+        if (!::account.isInitialized) return
+
+        validationResult =
+            validateDraft(
+                rules = rules,
+                author = account.signer.pubKey,
+                draftContent = message.text.toString(),
+            )
+    }
+
+    companion object {
+        /**
+         * Pure helper that runs the NIP-9A validator for a `kind:1111` reply with
+         * [draftContent]. Sizes are estimated from the content bytes; tags add bytes,
+         * so this can under-count on the boundary, but relays still enforce the real
+         * cap. Good enough for a pre-send preview.
+         *
+         * Web-of-trust gates and per-day quota lookups are deferred to a follow-up;
+         * see the NIP-9A track in the issue tracker.
+         */
+        internal fun validateDraft(
+            rules: CommunityRulesEvent,
+            author: String,
+            draftContent: String,
+        ): CommunityRulesValidator.Violation? =
+            CommunityRulesValidator(rules).validate(
+                author = author,
+                kind = CommentEvent.KIND,
+                sizeBytes = draftContent.toByteArray(Charsets.UTF_8).size,
+                postsTodayByKind = null,
+                wot = null,
+            )
     }
 
     open fun quote(quote: Note) {
@@ -316,6 +415,7 @@ open class CommentPostViewModel :
                 replyingTo = LocalCache.getOrCreateNote(it)
             }
         }
+        replyingTo?.let { observeCommunityRules(it) }
 
         wantsToAddGeoHash = draftEvent.hasGeohashes()
 
@@ -588,6 +688,11 @@ open class CommentPostViewModel :
 
         message.setTextAndPlaceCursorAtEnd("")
 
+        rulesObserverJob?.cancel()
+        rulesObserverJob = null
+        communityRules = null
+        validationResult = null
+
         replyingTo = null
         externalIdentity = null
 
@@ -630,6 +735,7 @@ open class CommentPostViewModel :
 
     override fun onMessageChanged() {
         urlPreviews.update(message.text.toString())
+        revalidateDraft()
 
         if (message.selection.collapsed) {
             val lastWord = message.currentWord()
@@ -707,7 +813,8 @@ open class CommentPostViewModel :
             !mediaUploadTracker.isUploading &&
             !wantsInvoice &&
             (!wantsZapraiser || zapRaiserAmount.value != null) &&
-            multiOrchestrator == null
+            multiOrchestrator == null &&
+            validationResult == null
 
     fun insertAtCursor(newElement: String) {
         message.insertUrlAtCursor(newElement)

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/note/nip22Comments/CommunityRulesViolationBanner.kt

@@ -0,0 +1,119 @@
+/*
+ * 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.nip22Comments
+
+import androidx.compose.foundation.background
+import androidx.compose.foundation.layout.Row
+import androidx.compose.foundation.layout.Spacer
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.layout.size
+import androidx.compose.foundation.layout.width
+import androidx.compose.foundation.shape.RoundedCornerShape
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.draw.clip
+import androidx.compose.ui.unit.dp
+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.ui.stringRes
+import com.vitorpamplona.quartz.nip72ModCommunities.rules.CommunityRulesValidator
+
+/**
+ * Inline banner shown above the composer's send button when the current draft
+ * violates the latest NIP-9A `kind:34551` rules document for the community
+ * being posted into. The send button is disabled in lockstep — see
+ * `CommentPostViewModel.canPost`.
+ */
+@Composable
+fun CommunityRulesViolationBanner(violation: CommunityRulesValidator.Violation) {
+    Row(
+        modifier =
+            Modifier
+                .fillMaxWidth()
+                .padding(horizontal = 10.dp, vertical = 6.dp)
+                .clip(RoundedCornerShape(8.dp))
+                .background(MaterialTheme.colorScheme.errorContainer)
+                .padding(horizontal = 12.dp, vertical = 10.dp),
+        verticalAlignment = Alignment.CenterVertically,
+    ) {
+        Icon(
+            symbol = MaterialSymbols.Warning,
+            contentDescription = stringRes(R.string.community_rules_violation_icon),
+            modifier = Modifier.size(20.dp),
+            tint = MaterialTheme.colorScheme.onErrorContainer,
+        )
+        Spacer(Modifier.width(10.dp))
+        Text(
+            text = describeViolation(violation),
+            style = MaterialTheme.typography.bodyMedium,
+            color = MaterialTheme.colorScheme.onErrorContainer,
+        )
+    }
+}
+
+@Composable
+private fun describeViolation(violation: CommunityRulesValidator.Violation): String =
+    when (violation) {
+        is CommunityRulesValidator.Violation.AuthorDenied -> {
+            stringRes(R.string.community_rules_violation_author_denied)
+        }
+
+        is CommunityRulesValidator.Violation.KindNotAllowed -> {
+            stringRes(R.string.community_rules_violation_kind_not_allowed, violation.kind)
+        }
+
+        is CommunityRulesValidator.Violation.KindSizeExceeded -> {
+            stringRes(
+                R.string.community_rules_violation_kind_size_exceeded,
+                violation.sizeBytes,
+                violation.maxBytes,
+            )
+        }
+
+        is CommunityRulesValidator.Violation.MaxSizeExceeded -> {
+            stringRes(
+                R.string.community_rules_violation_max_size_exceeded,
+                violation.sizeBytes,
+                violation.maxBytes,
+            )
+        }
+
+        is CommunityRulesValidator.Violation.QuotaExceeded -> {
+            stringRes(
+                R.string.community_rules_violation_quota_exceeded,
+                violation.postsToday,
+                violation.maxPerDay,
+            )
+        }
+
+        is CommunityRulesValidator.Violation.WotGateFailed -> {
+            stringRes(R.string.community_rules_violation_wot_gate_failed)
+        }
+
+        is CommunityRulesValidator.Violation.StaleRules -> {
+            stringRes(R.string.community_rules_violation_stale_rules)
+        }
+    }

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/note/nip22Comments/GenericCommentPostScreen.kt

@@ -52,6 +52,7 @@ import androidx.lifecycle.viewmodel.compose.viewModel
 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.model.AddressableNote
 import com.vitorpamplona.amethyst.ui.actions.StrippingFailureDialog
 import com.vitorpamplona.amethyst.ui.actions.uploads.SelectFromFiles
 import com.vitorpamplona.amethyst.ui.actions.uploads.SelectFromGallery
@@ -85,6 +86,7 @@ import com.vitorpamplona.amethyst.ui.note.creators.zapsplits.ForwardZapTo
 import com.vitorpamplona.amethyst.ui.note.creators.zapsplits.ForwardZapToButton
 import com.vitorpamplona.amethyst.ui.note.types.ReplyRenderType
 import com.vitorpamplona.amethyst.ui.screen.loggedIn.AccountViewModel
+import com.vitorpamplona.amethyst.ui.screen.loggedIn.communities.datasource.CommunityFilterAssemblerSubscription
 import com.vitorpamplona.amethyst.ui.stringRes
 import com.vitorpamplona.amethyst.ui.theme.Size10dp
 import com.vitorpamplona.amethyst.ui.theme.Size30Modifier
@@ -148,6 +150,16 @@ fun GenericCommentPostScreen(
 ) {
     WatchAndLoadMyEmojiList(accountViewModel)
 
+    // NIP-9A: when replying into a NIP-72 community, mount the community feed
+    // subscription so the latest kind:34551 rules document is fetched and
+    // observed by the ViewModel for composer-side validation. No-op for
+    // hashtag/geohash composers (replyingTo is null or not addressable there).
+    (postViewModel.replyingTo as? AddressableNote)?.let { addressable ->
+        if (addressable.event is com.vitorpamplona.quartz.nip72ModCommunities.definition.CommunityDefinitionEvent) {
+            CommunityFilterAssemblerSubscription(addressable, accountViewModel.dataSources().community)
+        }
+    }
+
     StrippingFailureDialog(postViewModel.strippingFailureConfirmation)
 
     BackHandler {
@@ -411,6 +423,9 @@ private fun GenericCommentPostBody(
             )
         }
 
+        // NIP-9A: surface the first community-rules violation found in the current draft.
+        postViewModel.validationResult?.let { CommunityRulesViolationBanner(it) }
+
         BottomRowActions(postViewModel)
     }
 }

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/communities/datasource/CommunityFilterAssembler.kt

@@ -35,6 +35,7 @@ class CommunityFilterAssembler(
     val group =
         listOf(
             CommunityFeedFilterSubAssembler(client, ::allKeys),
+            CommunityRulesFilterSubAssembler(client, ::allKeys),
         )
 
     override fun invalidateKeys() = invalidateFilters()

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/communities/datasource/CommunityRulesFilterSubAssembler.kt

@@ -0,0 +1,80 @@
+/*
+ * 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.screen.loggedIn.communities.datasource
+
+import com.vitorpamplona.amethyst.commons.relayClient.eoseManagers.SingleSubEoseManager
+import com.vitorpamplona.amethyst.model.LocalCache
+import com.vitorpamplona.amethyst.service.relays.SincePerRelayMap
+import com.vitorpamplona.quartz.nip01Core.relay.client.INostrClient
+import com.vitorpamplona.quartz.nip01Core.relay.client.pool.RelayBasedFilter
+import com.vitorpamplona.quartz.nip01Core.relay.filters.Filter
+import com.vitorpamplona.quartz.nip72ModCommunities.definition.CommunityDefinitionEvent
+import com.vitorpamplona.quartz.nip72ModCommunities.rules.CommunityRulesEvent
+
+/**
+ * Subscribes to the latest NIP-9A [CommunityRulesEvent] (kind:34551) for each
+ * tracked community.
+ *
+ * Reuses the existing [CommunityQueryState] keyspace so any screen that already
+ * subscribes to the community feed (via [CommunityFilterAssemblerSubscription])
+ * also pulls the rules document. Only events signed by the community owner or a
+ * declared moderator are accepted, matching the trust model of NIP-72 approvals.
+ */
+class CommunityRulesFilterSubAssembler(
+    client: INostrClient,
+    allKeys: () -> Set<CommunityQueryState>,
+) : SingleSubEoseManager<CommunityQueryState>(client, allKeys) {
+    override fun updateFilter(
+        keys: List<CommunityQueryState>,
+        since: SincePerRelayMap?,
+    ): List<RelayBasedFilter> {
+        if (keys.isEmpty()) return emptyList()
+
+        return keys.flatMap { key ->
+            val commEvent = key.community.event
+            if (commEvent !is CommunityDefinitionEvent) return@flatMap emptyList()
+
+            val signers = commEvent.moderatorKeys()
+            val communityRelays =
+                (
+                    commEvent.relayUrls().ifEmpty {
+                        LocalCache.relayHints.hintsForAddress(commEvent.addressTag())
+                    } + key.community.relayUrls()
+                ).toSet()
+
+            communityRelays.map { relay ->
+                RelayBasedFilter(
+                    relay = relay,
+                    filter =
+                        Filter(
+                            kinds = listOf(CommunityRulesEvent.KIND),
+                            authors = signers.sorted(),
+                            tags = mapOf("a" to listOf(commEvent.addressTag())),
+                            limit = 5,
+                            since = since?.get(relay)?.time,
+                        ),
+                )
+            }
+        }
+    }
+
+    override fun distinct(key: CommunityQueryState) = key.community.idHex
+}

amethyst/src/main/res/values/strings.xml

@@ -2772,4 +2772,13 @@
     <string name="emoji_public_explainer">Public emojis are visible to everyone and appear in your reaction menu and \":\" autocomplete picker when this pack is in your emoji list.</string>
     <string name="emoji_private_explainer">Private emojis are stored encrypted on relays and visible only to you. They appear in your reaction menu and \":\" autocomplete just like public ones.</string>
     <string name="gif">Gif</string>
+    <!-- NIP-9A community rules composer-side validation. -->
+    <string name="community_rules_violation_icon">Community rule violation</string>
+    <string name="community_rules_violation_author_denied">You are on this community\'s deny-list. Posts will not be accepted.</string>
+    <string name="community_rules_violation_kind_not_allowed">This community does not accept events of kind %1$d.</string>
+    <string name="community_rules_violation_kind_size_exceeded">Draft is %1$d bytes; this community caps this kind at %2$d bytes.</string>
+    <string name="community_rules_violation_max_size_exceeded">Draft is %1$d bytes; this community caps events at %2$d bytes.</string>
+    <string name="community_rules_violation_quota_exceeded">You have already posted %1$d events of this kind today (limit %2$d).</string>
+    <string name="community_rules_violation_wot_gate_failed">You don\'t pass this community\'s web-of-trust requirements.</string>
+    <string name="community_rules_violation_stale_rules">The latest community rules document was rejected as stale.</string>
 </resources>

amethyst/src/test/java/com/vitorpamplona/amethyst/ui/note/nip22Comments/CommentPostViewModelTest.kt

@@ -0,0 +1,143 @@
+/*
+ * 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.nip22Comments
+
+import com.vitorpamplona.quartz.nip22Comments.CommentEvent
+import com.vitorpamplona.quartz.nip72ModCommunities.rules.CommunityRulesEvent
+import com.vitorpamplona.quartz.nip72ModCommunities.rules.CommunityRulesValidator
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNull
+import org.junit.Assert.assertTrue
+import org.junit.Test
+
+/**
+ * Unit tests for the pure NIP-9A draft-validation helper exposed on
+ * [CommentPostViewModel]'s companion. The ViewModel itself is too coupled to
+ * Compose state, the relay subscription stack, and signers to construct in a
+ * unit test, but the helper is the only place where draft size, kind, and
+ * author flow into the validator — so testing it covers what matters.
+ */
+class CommentPostViewModelTest {
+    private val community = "a".repeat(64)
+    private val author = "b".repeat(64)
+    private val denied = "c".repeat(64)
+
+    private fun rules(
+        allowedKind: Int = CommentEvent.KIND,
+        maxBytes: Int? = null,
+        deny: List<String> = emptyList(),
+    ): CommunityRulesEvent {
+        val tags =
+            buildList<Array<String>> {
+                add(arrayOf("d", "rules-1"))
+                add(arrayOf("a", "34550:$community:my-community"))
+                if (maxBytes != null) {
+                    add(arrayOf("k", allowedKind.toString(), maxBytes.toString()))
+                } else {
+                    add(arrayOf("k", allowedKind.toString()))
+                }
+                deny.forEach { add(arrayOf("p", it, "deny")) }
+            }.toTypedArray()
+        return CommunityRulesEvent(
+            id = "0".repeat(64),
+            pubKey = community,
+            createdAt = 100L,
+            tags = tags,
+            content = "",
+            sig = "0".repeat(128),
+        )
+    }
+
+    @Test
+    fun `valid draft passes without violation`() {
+        val violation =
+            CommentPostViewModel.validateDraft(
+                rules = rules(),
+                author = author,
+                draftContent = "hello world",
+            )
+        assertNull(violation)
+    }
+
+    @Test
+    fun `kind not allowed yields KindNotAllowed`() {
+        val violation =
+            CommentPostViewModel.validateDraft(
+                rules = rules(allowedKind = 30023), // long-form, not kind:1111
+                author = author,
+                draftContent = "hello",
+            )
+        assertTrue(violation is CommunityRulesValidator.Violation.KindNotAllowed)
+        assertEquals(CommentEvent.KIND, (violation as CommunityRulesValidator.Violation.KindNotAllowed).kind)
+    }
+
+    @Test
+    fun `oversize draft yields KindSizeExceeded`() {
+        val draft = "x".repeat(120)
+        val violation =
+            CommentPostViewModel.validateDraft(
+                rules = rules(maxBytes = 100),
+                author = author,
+                draftContent = draft,
+            )
+        assertTrue(violation is CommunityRulesValidator.Violation.KindSizeExceeded)
+        val ks = violation as CommunityRulesValidator.Violation.KindSizeExceeded
+        assertEquals(120, ks.sizeBytes)
+        assertEquals(100, ks.maxBytes)
+    }
+
+    @Test
+    fun `denied author yields AuthorDenied regardless of content`() {
+        val violation =
+            CommentPostViewModel.validateDraft(
+                rules = rules(deny = listOf(denied)),
+                author = denied,
+                draftContent = "hello",
+            )
+        assertTrue(violation is CommunityRulesValidator.Violation.AuthorDenied)
+    }
+
+    @Test
+    fun `under-size boundary passes`() {
+        val draft = "x".repeat(100) // exactly the cap
+        val violation =
+            CommentPostViewModel.validateDraft(
+                rules = rules(maxBytes = 100),
+                author = author,
+                draftContent = draft,
+            )
+        assertNull(violation)
+    }
+
+    @Test
+    fun `multibyte chars count toward size cap by bytes not chars`() {
+        // "🚀" is 4 UTF-8 bytes, so 30 of them = 120 bytes > cap of 100.
+        val draft = "🚀".repeat(30)
+        val violation =
+            CommentPostViewModel.validateDraft(
+                rules = rules(maxBytes = 100),
+                author = author,
+                draftContent = draft,
+            )
+        assertTrue(violation is CommunityRulesValidator.Violation.KindSizeExceeded)
+        assertEquals(120, (violation as CommunityRulesValidator.Violation.KindSizeExceeded).sizeBytes)
+    }
+}