P7: thread backfill reads conversation containers and asks with If-None-Match

The FetchReplies job reads a thread's FEP-171b contextHistory before its FEP-7888 context: the posts each Add (or
Forte's plain Create) brought in, read from their own servers. A thread collection read whole keeps its ETag when the
document itself changes with every post (it counts them, or holds them all with no further page); the next read sends
it as If-None-Match and a 304 ends the job. The ETag is kept as sent, since NodeBB's has no quotes and the typed header
drops it. A context naming a post we hold (Forte's first post) is not fetched.

Checked in the pasture: Mastodon 4.7.3 answers the second read 304; NodeBB 4.16's unquoted ETag is kept (23/23 in its
scenario); a Forte thread completes through its replies.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012CzABvBkbcFqoHdmi8b9WB

FEDERATION.md

@@ -394,9 +394,11 @@ Posts with a location (shown to nearby users of this server) never leave the ser
 - **Fetching.** All fetches are signed by the instance actor. They go only to public addresses, follow at most three
   redirects and read at most 1 MB.
 - **Threads.** A reply's missing parents are fetched, up to 10 levels. When someone here opens a public remote thread,
-  its replies are read from their servers, at most once an hour: the thread's `context` collection (FEP-7888) if it has
+  its replies are read from their servers, at most once an hour: the thread's conversation container (FEP-171b's
-  one, else its `replies` (PeerTube's `comments`) and theirs, two levels down; 5 pages and 100 posts at most. Only
+  `contextHistory`: the posts its owner's `Add`s brought in) or its `context` collection (FEP-7888) if it has one, else
-  public and unlisted replies are kept, each fetched from its own origin.
+  its `replies` (PeerTube's `comments`) and theirs, two levels down; 5 pages and 100 posts at most. Only public and
+  unlisted replies are kept, each fetched from its own origin. A thread collection read whole, which counts its posts or
+  holds them all, is asked again with `If-None-Match` and its last ETag; a 304 ends the read.
 - **Reading our documents (SecureMode).** privapub.thepra.dev answers ActivityPub GETs only when they are signed, like
   Mastodon's authorized fetch; the server's own actors (the instance actor `/peasants/privapub` and the reporter
   `/peasants/privapub_reports`) are the exception, since their keys are needed first.

PrivaPub.Tests/Federation/JobHandlerTests.cs

@@ -216,6 +216,103 @@ namespace PrivaPub.Tests.Federation
 			Assert.All(_harness.Peer.Requests.Where(r => r.Path.EndsWith("/replies")), r => Assert.False(string.IsNullOrEmpty(r.Signature)));
 		}
 
+		[Fact]
+		public async Task A_conversation_container_is_read_before_the_context_and_only_its_added_posts_taken()
+		{
+			var owner = new RemoteActor(_harness.Peer, "owner");
+			var answerer = new RemoteActor(_harness.Peer, "answerer");
+			var root = PublicNote(owner, "<p>a conversation</p>");
+			var history = NewId(owner, "conversations");
+			root["contextHistory"] = history;
+			root["context"] = NewId(owner, "never");
+			Served(root);
+			var embedded = Served(Reply(answerer, "added whole", root));
+			var referenced = Served(Reply(owner, "added by reference", embedded));
+			var liked = Served(Reply(answerer, "only liked", root));
+			var target = new JsonObject { ["type"] = "OrderedCollection", ["id"] = history, ["attributedTo"] = owner.Id };
+			JsonObject Add(JsonNode activity) => new()
+			{
+				["id"] = NewId(owner, "activities"), ["type"] = "Add", ["actor"] = owner.Id, ["object"] = activity, ["target"] = target.DeepClone()
+			};
+			var create = Served(Create(owner, referenced));
+			create["object"] = IdOf(referenced);
+			Served(create);
+			var byReference = Served(Add(IdOf(create)));
+			Served(new JsonObject
+			{
+				["id"] = history, ["type"] = "OrderedCollection", ["attributedTo"] = owner.Id, ["collectionOf"] = "Activity",
+				["orderedItems"] = new JsonArray(Create(owner, root), Add(Create(answerer, embedded)), IdOf(byReference), Add(Activity(answerer, "Like", JsonValue.Create(IdOf(liked)))))
+			});
+			var held = await Held(root);
+
+			Assert.Equal(1, await RunReplies(held));
+			var stored = await Stored(embedded, referenced, liked);
+			Assert.Equal(new[] { IdOf(embedded), IdOf(referenced) }.Order(), stored.Keys.Order());
+			Assert.Equal(held.ID, stored[IdOf(embedded)].AnsweringToPostId);
+			Assert.Equal(stored[IdOf(embedded)].ID, stored[IdOf(referenced)].AnsweringToPostId);
+			Assert.DoesNotContain(_harness.Peer.Requests, r => r.Path == new Uri(root["context"]!.GetValue<string>()).AbsolutePath);
+		}
+
+		[Fact]
+		public async Task An_unchanged_context_is_not_read_again_while_it_counts_the_thread()
+		{
+			var token = TestContext.Current.CancellationToken;
+			var poster = new RemoteActor(_harness.Peer, "poster");
+			var answerer = new RemoteActor(_harness.Peer, "answerer");
+			var root = PublicNote(poster, "<p>a topic</p>");
+			var context = NewId(poster, "topics");
+			var path = new Uri(context).AbsolutePath;
+			root["context"] = context;
+			Served(root);
+			var first = Served(Reply(answerer, "first", root));
+			var later = Served(Reply(poster, "later", root));
+			// NodeBB's: every post inline, counted, and an ETag without quotes
+			JsonObject Thread(params JsonObject[] notes) => new()
+			{
+				["id"] = context, ["type"] = "OrderedCollection", ["totalItems"] = notes.Length,
+				["orderedItems"] = new JsonArray(notes.Select(n => (JsonNode)IdOf(n)).ToArray())
+			};
+			_harness.Peer.ServeTagged(path, Thread(root, first).ToJsonString(), "4f930186eb");
+			var held = await Held(root);
+			var handler = new RepliesJobHandler(_harness.Db, _harness.Remote, _harness.RemotePosts, _harness.Queue);
+
+			await handler.Handle(RepliesJobHandler.For(held, 1), token);
+			Assert.Equal("4f930186eb", (await DB.Default.Find<ThreadCollection>().Match(c => c.URI == context).ExecuteFirstAsync(token)).ETag);
+			// the same version: a 304, and nothing read
+			_harness.Peer.ServeTagged(path, Thread(root, first, later).ToJsonString(), "4f930186eb");
+			await handler.Handle(RepliesJobHandler.For(held, 1), token);
+			Assert.Equal("4f930186eb", _harness.Peer.Requests.Last(r => r.Path == path).Headers["If-None-Match"]);
+			Assert.Single(await Stored(first, later));
+			// a new one
+			_harness.Peer.ServeTagged(path, Thread(root, first, later).ToJsonString(), "5a0b");
+			await handler.Handle(RepliesJobHandler.For(held, 1), token);
+			Assert.Equal(2, (await Stored(first, later)).Count);
+			Assert.Equal("5a0b", (await DB.Default.Find<ThreadCollection>().Match(c => c.URI == context).ExecuteFirstAsync(token)).ETag);
+		}
+
+		[Fact]
+		public async Task A_paged_contexts_etag_is_not_kept_since_its_first_page_hides_what_is_added_later()
+		{
+			var token = TestContext.Current.CancellationToken;
+			var poster = new RemoteActor(_harness.Peer, "poster");
+			var root = PublicNote(poster, "<p>a long thread</p>");
+			var context = NewId(poster, "contexts");
+			root["context"] = context;
+			Served(root);
+			var answer = Served(Reply(poster, "on the second page", root));
+			_harness.Peer.ServeTagged(new Uri(context).AbsolutePath, new JsonObject
+			{
+				["id"] = context, ["type"] = "Collection",
+				["first"] = new JsonObject { ["type"] = "CollectionPage", ["items"] = new JsonArray(IdOf(root)), ["next"] = context + "/2" }
+			}.ToJsonString(), "W/\"first\"");
+			Served(new JsonObject { ["id"] = context + "/2", ["type"] = "CollectionPage", ["items"] = new JsonArray(IdOf(answer)) });
+			var held = await Held(root);
+
+			await new RepliesJobHandler(_harness.Db, _harness.Remote, _harness.RemotePosts, _harness.Queue).Handle(RepliesJobHandler.For(held, 1), token);
+			Assert.Single(await Stored(answer));
+			Assert.False(await DB.Default.Find<ThreadCollection>().Match(c => c.URI == context && c.ETag != null).ExecuteAnyAsync(token));
+		}
+
 		[Fact]
 		public async Task A_threads_context_collection_brings_every_reply_at_once()
 		{

PrivaPub.Tests/Support/Peer.cs

@@ -21,6 +21,7 @@ namespace PrivaPub.Tests.Support
 		readonly ConcurrentDictionary<string, (string Text, string ContentType)> _documents = new();
 		readonly ConcurrentDictionary<string, (int Status, TimeSpan Delay)> _answers = new();
 		readonly ConcurrentDictionary<string, (byte[] Bytes, string ContentType)> _files = new();
+		readonly ConcurrentDictionary<string, string> _etags = new();
 
 		public int Port { get; }
 		public string A => $"http://127.0.0.1:{Port}";
@@ -64,6 +65,15 @@ namespace PrivaPub.Tests.Support
 					context.Response.StatusCode = StatusCodes.Status404NotFound;
 					return;
 				}
+				if (peer._etags.TryGetValue(key, out var etag))
+				{
+					if (context.Request.Headers.IfNoneMatch.ToString() == etag)
+					{
+						context.Response.StatusCode = StatusCodes.Status304NotModified;
+						return;
+					}
+					context.Response.Headers.ETag = etag;
+				}
 				context.Response.ContentType = document.ContentType;
 				await context.Response.WriteAsync(document.Text.Replace("{A}", peer.A).Replace("{B}", peer.B));
 			});
@@ -74,6 +84,13 @@ namespace PrivaPub.Tests.Support
 
 		public void Serve(string path, string json) => _documents[path] = (json, "application/activity+json");
 
+		// a document with an ETag: answered 304 to a request that names it in If-None-Match
+		public void ServeTagged(string path, string json, string etag)
+		{
+			_documents[path] = (json, "application/activity+json");
+			_etags[path] = etag;
+		}
+
 		public void ServeText(string path, string text, string contentType) => _documents[path] = (text, contentType);
 
 		public void ServeFile(string path, byte[] bytes, string contentType) => _files[path] = (bytes, contentType);

PrivaPub/Federation/Actors/RemoteActorService.cs

@@ -17,6 +17,8 @@ namespace PrivaPub.Federation.Actors
 	public interface IRemoteActorService
 	{
 		Task<FetchedJson> FetchObject(string uri, CancellationToken token);
+		// the same, or NotModified while the version `etag` names is still current
+		Task<FetchedJson> FetchObject(string uri, string etag, CancellationToken token) => FetchObject(uri, token);
 		Task<ForeignAvatar> GetActor(string actorUri, bool refresh, CancellationToken token);
 		Task<ForeignAvatar> GetActorByKeyId(string keyId, bool refresh, CancellationToken token);
 		bool KeyTemporarilyUnavailable(string keyId) => false;
@@ -50,13 +52,17 @@ namespace PrivaPub.Federation.Actors
 			_options = options;
 		}
 
-		public async Task<FetchedJson> FetchObject(string uri, CancellationToken token)
+		public Task<FetchedJson> FetchObject(string uri, CancellationToken token) => FetchObject(uri, default, token);
+
+		public async Task<FetchedJson> FetchObject(string uri, string etag, CancellationToken token)
 		{
 			using var scope = HttpScope.Default("object");
 			var signer = await _localActors.GetInstanceActor(token);
-			var fetched = await Get(uri, signer, token);
+			var fetched = string.IsNullOrEmpty(etag)
-			if (fetched == default)
+				? await Get(uri, signer, token)
-				return default;
+				: await _http.GetJsonIfChanged(uri, Accept, etag, request => HttpSignatures.Sign(request, signer, body: null), token);
+			if (fetched == default || fetched.NotModified)
+				return fetched;
 
 			var id = Text(fetched.Root, "id");
 			if (Origin.IsDocumentAt(id, fetched.FinalUri))

PrivaPub/Federation/Inbox/RemoteReplies.cs

@@ -20,9 +20,11 @@ namespace PrivaPub.Federation.Inbox
 	public sealed record RepliesPayload(string PostId, int Depth);
 
 	// What was said under a remote post on other servers, read when a persona opens its thread, so the thread shows more
-	// than what happened to reach us. The thread's own collection comes first (FEP-7888's `context`, which Mastodon 4.5+
+	// than what happened to reach us. The thread's own collections come first: its conversation container (FEP-171b's
-	// serves with every reply at any depth); otherwise the post's `replies` (PeerTube's `comments`), and the replies'
+	// `contextHistory`, every activity its owner added to the thread), then its posts (FEP-7888's `context`, which
-	// own, two levels down. Read with the instance actor's signature, never a persona's, a few pages and at most
+	// Mastodon 4.5+, Mitra and NodeBB serve with every reply at any depth), sent the ETag of the last whole read so an
+	// unchanged thread costs one 304; otherwise the post's `replies` (PeerTube's `comments`, Forte's whole thread), and the
+	// replies' own, two levels down. Read with the instance actor's signature, never a persona's, a few pages and at most
 	// MaxItems posts a job; each one is fetched from its own server and stored like any post fetched for a thread
 	// (RemotePosts.StoreContext: public and unlisted only, its author checked against its origin).
 	public class RepliesJobHandler : IJobHandler
@@ -74,14 +76,35 @@ namespace PrivaPub.Federation.Inbox
 			if (await Document(post, token) is not { } document)
 				return JobOutcome.Done;
 
-			if (await Collection(document["context"], token) is { } thread)
+			foreach (var name in new[] { "contextHistory", "context" })
 			{
-				await StoreAll(thread, token);
+				var uri = Id(document[name]);
+				// Forte's `context` is the thread's first post
+				if (uri != default && await _dbEntities.Posts.Match(p => p.ObjectURI == uri).ExecuteAnyAsync(token))
+					continue;
+				var known = uri == default ? default : await DB.Default.Find<ThreadCollection>().Match(c => c.URI == uri).ExecuteFirstAsync(token);
+				var (thread, etag, unchanged) = await Collection(document[name], known?.ETag, token);
+				if (unchanged)
+				{
+					await DB.Default.Update<ThreadCollection>().MatchID(known.ID).Modify(c => c.ReadAt, DateTime.UtcNow).ExecuteAsync(token);
 					return JobOutcome.Done;
 				}
-			if (await Collection(document["replies"] ?? document["comments"], token) is not { } replies)
+				if (thread == default)
+					continue;
+				var (_, read) = await StoreAll(thread, name == "contextHistory", token);
+				var whole = read && Counted(thread);
+				if (uri != default && (known != default || whole && etag != default))
+					await DB.Default.Update<ThreadCollection>()
+						.Match(c => c.URI == uri)
+						.Modify(c => c.ETag, whole ? etag : default)
+						.Modify(c => c.ReadAt, DateTime.UtcNow)
+						.Option(o => o.IsUpsert = true)
+						.ExecuteAsync(token);
 				return JobOutcome.Done;
-			var answers = await StoreAll(replies, token);
+			}
+			if ((await Collection(document["replies"] ?? document["comments"], default, token)).Collection is not { } replies)
+				return JobOutcome.Done;
+			var (answers, _) = await StoreAll(replies, false, token);
 			if (payload.Depth < MaxDepth)
 				await _queue.EnqueueMany(answers.Where(a => a.IsFederatedCopy && a.AnsweringToPostId == post.ID).Take(MaxBranches)
 					.Select(a => For(a, payload.Depth + 1)), token);
@@ -100,35 +123,41 @@ namespace PrivaPub.Federation.Inbox
 			return fetched == default ? default : JsonNode.Parse(fetched.Root.GetRawText()) as JsonObject;
 		}
 
-		// a collection as its server serves it now: what a post carries is read again by its id, since an embedded first
+		// a collection as its server serves it now, and its ETag: what a post carries is read again by its id, since an
-		// page is as old as the post; a value that is no web address (Pleroma's `context` is a tag: URI), or no collection,
+		// embedded first page is as old as the post; a value that is no web address (Pleroma's `context` is a tag: URI), or no
-		// gives nothing
+		// collection, gives nothing, and one whose server still has the version `etag` names is unchanged
-		async Task<JsonObject> Collection(JsonNode node, CancellationToken token)
+		async Task<(JsonObject Collection, string ETag, bool Unchanged)> Collection(JsonNode node, string etag, CancellationToken token)
 		{
 			var uri = Id(node);
+			string served = default;
 			if (uri != default && Uri.TryCreate(uri, UriKind.Absolute, out var parsed) && (parsed.Scheme == Uri.UriSchemeHttps || parsed.Scheme == Uri.UriSchemeHttp))
 			{
 				using var scope = HttpScope.For("replies");
-				using var fetched = await _remoteActors.FetchObject(uri, token);
+				using var fetched = await _remoteActors.FetchObject(uri, etag, token);
+				if (fetched?.NotModified == true)
+					return (default, etag, true);
 				node = fetched == default ? default : JsonNode.Parse(fetched.Root.GetRawText());
+				served = fetched?.ETag;
 			}
-			return node is JsonObject collection && Value(collection, "type") is { } type && CollectionTypes.Contains(type) ? collection : default;
+			return (node is JsonObject collection && Value(collection, "type") is { } type && CollectionTypes.Contains(type) ? collection : default, served, false);
 		}
 
-		// every post the collection lists, stored when we lack it: its pages in order, at most MaxPages and MaxItems
+		// every post the collection lists, stored when we lack it: its pages in order, at most MaxPages and MaxItems; whether
-		async Task<List<PostEntity>> StoreAll(JsonObject collection, CancellationToken token)
+		// it was read to its end. A conversation container's items are its owner's Adds of the thread's activities.
+		async Task<(List<PostEntity> Posts, bool Whole)> StoreAll(JsonObject collection, bool activities, CancellationToken token)
 		{
 			var posts = new List<PostEntity>();
 			var seen = 0;
 			var page = collection["orderedItems"] is JsonArray || collection["items"] is JsonArray ? collection : await Page(collection["first"], token);
-			for (var pages = 0; page != default && pages < MaxPages && seen < MaxItems; pages++)
+			for (var pages = 0; page != default; pages++)
 			{
+				if (pages == MaxPages)
+					return (posts, false);
 				foreach (var item in Items(page))
 				{
 					if (++seen > MaxItems)
-						break;
+						return (posts, false);
-					// a context may list the activities that made the thread (FEP-f228) rather than its posts
+					var id = activities ? await Added(item, token) : PostId(item);
-					var id = Value(item, "type") is "Create" or "Update" ? Id(item["object"]) : Id(item);
 					if (id == default)
 						continue;
 					var post = await _dbEntities.Posts.Match(p => p.ObjectURI == id && !p.DeletedAt.HasValue).ExecuteFirstAsync(token)
@@ -138,7 +167,32 @@ namespace PrivaPub.Federation.Inbox
 				}
 				page = page["next"] is { } next ? await Page(next, token) : default;
 			}
-			return posts;
+			return (posts, true);
+		}
+
+		// whether the document itself changes with every post added to the thread, so its ETag is the thread's: it counts
+		// them (NodeBB), or holds every one with no further page (a short Mastodon thread); a long Mastodon thread's first
+		// page stays as it was while its last one grows
+		static bool Counted(JsonObject collection)
+		{
+			if (collection["totalItems"] is JsonValue)
+				return true;
+			var page = collection["orderedItems"] is JsonArray || collection["items"] is JsonArray ? collection : collection["first"] as JsonObject;
+			return page != default && (page["orderedItems"] is JsonArray || page["items"] is JsonArray) && page["next"] == default;
+		}
+
+		// a context may list the activities that made the thread (FEP-f228) rather than its posts
+		static string PostId(JsonNode item) => Value(item, "type") is "Create" or "Update" ? Id(item["object"]) : Id(item);
+
+		// the post an activity in a conversation container brought into the thread, added by its owner or (Forte's) not: a
+		// Create's or an Update's object; a like, a removal or anything else adds no post. Only its id is taken, and the post
+		// is read from its own server.
+		async Task<string> Added(JsonNode item, CancellationToken token)
+		{
+			var activity = await Page(item, token);
+			if (activity != default && Value(activity, "type") == "Add")
+				activity = await Page(activity["object"], token);
+			return activity != default && Value(activity, "type") is "Create" or "Update" ? Id(activity["object"]) : default;
 		}
 
 		async Task<JsonObject> Page(JsonNode node, CancellationToken token)

PrivaPub/Infrastructure/Data/Indexes.cs

@@ -15,6 +15,7 @@ namespace PrivaPub.Infrastructure.Data
 	public static class Indexes
 	{
 		public static readonly TimeSpan TombstoneLifetime = TimeSpan.FromDays(90);
+		public static readonly TimeSpan ThreadCollectionLifetime = TimeSpan.FromDays(30);
 
 		public static async Task Create(CancellationToken token = default)
 		{
@@ -166,6 +167,8 @@ namespace PrivaPub.Infrastructure.Data
 			await Plain<ObjectRecord>(token, r => r.PostId);
 			await Unique<DeletedObject>(d => d.ObjectURI, Builders<DeletedObject>.Filter.Gt(d => d.ObjectURI, ""), token);
 			await DB.Default.Index<DeletedObject>().Key(d => d.DeletedAt, KeyType.Ascending).Option(o => o.ExpireAfter = TombstoneLifetime).CreateAsync(token);
+			await Unique<ThreadCollection>(c => c.URI, Builders<ThreadCollection>.Filter.Gt(c => c.URI, ""), token);
+			await DB.Default.Index<ThreadCollection>().Key(c => c.ReadAt, KeyType.Ascending).Option(o => o.ExpireAfter = ThreadCollectionLifetime).CreateAsync(token);
 			await DB.Default.Index<Downvote>().Key(d => d.PostId, KeyType.Ascending).Key(d => d.ActorURI, KeyType.Ascending).Option(o => o.Unique = true).CreateAsync(token);
 			await Plain<Downvote>(token, d => d.ActivityURI);
 			await DB.Default.Index<PollVote>().Key(v => v.PostId, KeyType.Ascending).Key(v => v.ActorURI, KeyType.Ascending).Key(v => v.Choice, KeyType.Ascending)

PrivaPub/Infrastructure/Http/FederationHttp.cs

@@ -15,6 +15,9 @@ namespace PrivaPub.Infrastructure.Http
 		public Uri FinalUri { get; init; }
 		public JsonDocument Document { get; init; }
 		public JsonElement Root => Document.RootElement;
+		public string ETag { get; init; }
+		// the origin answered 304 to an If-None-Match: no document, the one read before still stands
+		public bool NotModified { get; init; }
 
 		public void Dispose() => Document?.Dispose();
 	}
@@ -24,6 +27,9 @@ namespace PrivaPub.Infrastructure.Http
 		bool IsAllowed(Uri target);
 		Task<FetchedJson> GetJson(string url, string accept, Action<HttpRequestMessage> sign, CancellationToken token);
 		Task<(int Status, FetchedJson Json)> GetJsonStatus(string url, string accept, Action<HttpRequestMessage> sign, CancellationToken token);
+		/// <summary>The document, or one that is <see cref="FetchedJson.NotModified"/> when its origin still has the version
+		/// <paramref name="etag"/> names.</summary>
+		Task<FetchedJson> GetJsonIfChanged(string url, string accept, string etag, Action<HttpRequestMessage> sign, CancellationToken token);
 		bool FailedTemporarily(string url);
 		Task<(Uri FinalUri, string Html)> GetPage(string url, CancellationToken token);
 		Task<HttpResponseMessage> OpenMedia(string url, System.Net.Http.Headers.RangeHeaderValue range, CancellationToken token);
@@ -291,7 +297,20 @@ namespace PrivaPub.Infrastructure.Http
 			var exchange = new Exchange(url, HttpScope.Purpose ?? "object");
 			try
 			{
-				return await GetJson(url, accept, sign, exchange, token);
+				return await GetJson(url, accept, sign, default, exchange, token);
+			}
+			finally
+			{
+				Record(exchange);
+			}
+		}
+
+		public async Task<FetchedJson> GetJsonIfChanged(string url, string accept, string etag, Action<HttpRequestMessage> sign, CancellationToken token)
+		{
+			var exchange = new Exchange(url, HttpScope.Purpose ?? "object");
+			try
+			{
+				return await GetJson(url, accept, sign, etag, exchange, token);
 			}
 			finally
 			{
@@ -308,7 +327,7 @@ namespace PrivaPub.Infrastructure.Http
 			var exchange = new Exchange(url, HttpScope.Purpose ?? "object");
 			try
 			{
-				var json = await GetJson(url, accept, sign, exchange, token);
+				var json = await GetJson(url, accept, sign, default, exchange, token);
 				return (exchange.Status ?? 0, json);
 			}
 			finally
@@ -317,7 +336,7 @@ namespace PrivaPub.Infrastructure.Http
 			}
 		}
 
-		async Task<FetchedJson> GetJson(string url, string accept, Action<HttpRequestMessage> sign, Exchange exchange, CancellationToken token)
+		async Task<FetchedJson> GetJson(string url, string accept, Action<HttpRequestMessage> sign, string etag, Exchange exchange, CancellationToken token)
 		{
 			if (!Uri.TryCreate(url, UriKind.Absolute, out var target) || !IsAllowed(target))
 			{
@@ -338,11 +357,15 @@ namespace PrivaPub.Infrastructure.Http
 				for (var hop = 0; hop <= MaxRedirects; hop++)
 				{
 					using var request = Request(target, accept);
+					if (!string.IsNullOrEmpty(etag))
+						request.Headers.TryAddWithoutValidation("If-None-Match", etag);
 					sign?.Invoke(request);
 
 					using var response = await _httpClientFactory.CreateClient(ClientName)
 						.SendAsync(request, HttpCompletionOption.ResponseHeadersRead, timeout.Token);
 					exchange.Answered(response);
+					if (response.StatusCode == HttpStatusCode.NotModified && !string.IsNullOrEmpty(etag))
+						return new FetchedJson { FinalUri = target, ETag = etag, NotModified = true };
 
 					if (IsRedirect(response.StatusCode))
 					{
@@ -370,7 +393,7 @@ namespace PrivaPub.Infrastructure.Http
 						return Refuse(negativeKey, url, "a body over the size limit", exchange, "too-large");
 					exchange.Bytes = body.Length;
 
-					return new FetchedJson { FinalUri = target, Document = JsonDocument.Parse(body) };
+					return new FetchedJson { FinalUri = target, Document = JsonDocument.Parse(body), ETag = RawETag(response) };
 				}
 				return Refuse(negativeKey, url, "too many redirects", exchange, "too-many-redirects");
 			}
@@ -658,6 +681,10 @@ namespace PrivaPub.Infrastructure.Http
 			return buffer.ToArray();
 		}
 
+		// as sent, since it is only ever sent back: NodeBB's carries no quotes, which the typed header rejects
+		static string RawETag(HttpResponseMessage response) =>
+			response.Headers.TryGetValues("ETag", out var values) ? values.FirstOrDefault() : default;
+
 		static bool IsRedirect(HttpStatusCode status) =>
 			status is HttpStatusCode.MovedPermanently or HttpStatusCode.Found or HttpStatusCode.SeeOther
 				or HttpStatusCode.TemporaryRedirect or HttpStatusCode.PermanentRedirect;

PrivaPub/Models/Federation/ThreadCollection.cs

@@ -0,0 +1,13 @@
+using MongoDB.Entities;
+
+namespace PrivaPub.Models.Federation
+{
+	// a thread's collection on another server as we last read it whole: its ETag, sent back as If-None-Match so an unchanged
+	// thread costs its server one 304 (NodeBB's context carries a digest of its posts)
+	public class ThreadCollection : Entity
+	{
+		public string URI { get; set; }
+		public string ETag { get; set; }
+		public DateTime ReadAt { get; set; } = DateTime.UtcNow;//expires after Indexes.ThreadCollectionLifetime
+	}
+}

docs/INTEROP.md

@@ -96,7 +96,9 @@ Priorities, used throughout:
   - `Move`.
   - `QuoteRequest`, its `Accept`/`Reject` (with `result`), and `Delete{QuoteAuthorization}`.
   - `FeatureRequest`/`FeatureAuthorization` (4.6).
-- **Threads:** `context` is a dereferenceable collection of the thread (FEP-7888, threads started on 4.5+).
+- **Threads:** `context` is a dereferenceable collection of the thread (FEP-7888, threads started on 4.5+). It
+  answers `If-None-Match` with a 304 (a weak ETag over the body, 4.7.3), but the body embeds only the first page, so its
+  ETag stands for the thread only while the thread fits on that page.
 - **Interaction counts:** `likes` and `shares` carry `totalItems`.
 - **Quotes:** `quote`, plus `quoteUri` and `_misskey_quote`, `quoteAuthorization`, and `interactionPolicy.canQuote`.
 - **Link previews:** from 4.7, a `{type: Link, href}` **attachment** names the link a card is made from (FEP-8967).
@@ -659,7 +661,9 @@ What it showed:
 - **NodeBB:**
   - A topic's first post is an `Article` with `name`, **`summary` = an excerpt** and `preview`; replies are Notes.
   - Categories are `Group`s **without `followers`**.
-  - `context` is a paged collection with an **ETag digest**; NodeBB refetches with `If-None-Match`.
+  - `context` is a collection with an **ETag digest** of its posts, sent **without quotes** (not an RFC 9110
+    entity-tag: .NET's typed header drops it, so PrivaPub keeps the raw value); it carries `totalItems` and its items
+    inline, and answers `If-None-Match` with a 304. NodeBB refetches with it too.
   - Since 4.15, an Announce of anything but a Create or a plain object is accepted only from Group actors.
   - It sends `Move`/`Remove` of a whole context (FEP-f15d) and `Add{post → context}` (FEP-11dd).
   - Chats are private Notes, threaded by `inReplyTo` into the room they answer.
@@ -855,6 +859,8 @@ without a port, before it gives out its OAuth client.
 - Its Mastodon API resolves an account elsewhere only when the search is not limited to a type
   (`/api/v2/search?resolve=true`, no `type=accounts`); reactions go through Pleroma's route
   (`PUT /api/v1/pleroma/statuses/:id/reactions/:emoji`).
+- **Threads:** a post's `context` is `/collections/conversations/<id>`, a collection whose first page lists the
+  thread's post ids (FEP-7888), with no ETag; no `contextHistory`.
 - **Pasture evidence (2026-10-05, `tools/pasture/scenarios/mitra.sh`):** 28 checks pass, with no change to PrivaPub:
   follows both ways, posts, replies both ways, likes and reposts both ways, an emoji reaction, a poll and alice's vote,
   edits and deletions both ways, direct messages both ways, the unfollow, statistics.
@@ -918,6 +924,11 @@ without a port, before it gives out its OAuth client.
 - **Everything in a thread is added to its conversation** (FEP-171b): its edits come only as `Add{Update}`, under the
   same activity id as the post's `Create`, and likes as `Add{Like}`. PrivaPub unwraps Create, Update and Delete, telling
   an Update from the Create whose id it reuses by what it carries.
+- **Its conversation container is not named by its posts:** `contextHistory` (and `context`) pointing at
+  `…/conversation/<uuid>` ride on the activities only; a Note's `context` is the thread's first post and it has no
+  `contextHistory`, though the FEP asks the first post for one. The container (public threads: unsigned GETs work)
+  lists the first post's plain `Create`, then an `Add{Create}` for every post. A post's `replies` lists every comment of
+  the thread, embedded, and is what PrivaPub reads to complete a Forte thread (checked live 2026-10-07).
 - **Its own collections name themselves wrongly:** a channel's followers, following and outbox give an `id` with the
   gateway path twice and a trailing `?`, which 404s, so their counts are not read.
 - **Running it:** no image is published; `images/forte` builds its tag with composer on Hubzilla's PHP image, behind an

docs/ROADMAP.md

@@ -677,8 +677,11 @@ it, raw where it doesn't.
 
 #### P7 Threads, communities and the social graph
 - **Thread backfill:**
-  - read in order: `contextHistory`, then `context` (paged, with ETag), then `replies`. **Done (2026-10-05), but for
+  - read in order: `contextHistory`, then `context` (paged, with ETag), then `replies`. **Done (2026-10-05; `contextHistory`
-    `contextHistory` and ETags:** a persona opening a public remote thread queues `FetchReplies` for the post and its
+    and ETags 2026-10-07: a container's `Add`ed Creates and Updates are read for their posts; a whole read of a collection
+    that counts its posts or holds them all keeps its ETag, sent back as `If-None-Match`, a 304 checked live on Mastodon
+    4.7.3 and NodeBB 4.16, whose ETag has no quotes; neither Forte nor Mitra puts `contextHistory` on a post, and Forte's
+    `replies` already holds its whole thread):** a persona opening a public remote thread queues `FetchReplies` for the post and its
     root, at most hourly each. The job reads the thread's FEP-7888 `context` collection (posts or, as FEP-f228 allows,
     the activities that made them), else the post's `replies` (PeerTube's `comments`) and theirs, two levels down. It
     reads at most 5 pages and 100 posts per job, signed by the instance actor, and stores what it lacks through