Merge 38f9b9c676 into 8bc9fe7c72

DiscordChatExporter.Core/Discord/Data/MessageKind.cs

@@ -13,4 +13,5 @@ public enum MessageKind
     GuildMemberJoin = 7,
     ThreadCreated = 18,
     Reply = 19,
+    ThreadStarterMessage = 21,
 }

DiscordChatExporter.Core/Discord/DiscordClient.cs

@@ -629,6 +629,33 @@ public class DiscordClient(
         return response.EnumerateArray().Select(Message.Parse).LastOrDefault();
     }
 
+    public async ValueTask<Message?> TryGetMessageAsync(
+        Snowflake channelId,
+        Snowflake messageId,
+        CancellationToken cancellationToken = default
+    )
+    {
+        // Use the regular message listing endpoint with the 'around' parameter instead of the
+        // dedicated single-message endpoint, because the latter is not accessible to user tokens.
+        var url = new UrlBuilder()
+            .SetPath($"channels/{channelId}/messages")
+            .SetQueryParameter("around", messageId.ToString())
+            .SetQueryParameter("limit", "1")
+            .Build();
+
+        // Can be null on channels that the user cannot access
+        var response = await TryGetJsonResponseAsync(url, cancellationToken);
+        if (response is null)
+            return null;
+
+        // The endpoint returns messages around the requested ID, so make sure to only return
+        // the message that exactly matches it (it may be absent if it has been deleted).
+        return response
+            .Value.EnumerateArray()
+            .Select(Message.Parse)
+            .FirstOrDefault(m => m.Id == messageId);
+    }
+
     public async IAsyncEnumerable<Message> GetMessagesAsync(
         Snowflake channelId,
         Snowflake? after = null,

DiscordChatExporter.Core/Exporting/ChannelExporter.cs

@@ -1,4 +1,6 @@
 using System;
+using System.Collections.Generic;
+using System.Runtime.CompilerServices;
 using System.Threading;
 using System.Threading.Tasks;
 using DiscordChatExporter.Core.Discord;
@@ -80,7 +82,63 @@ public class ChannelExporter(DiscordClient discord)
                 cancellationToken
             );
 
-        await foreach (var message in messages)
+        // Threads created from a message don't include that message in their own history, even
+        // though it logically belongs to the thread as its very first message. Because the thread
+        // shares its ID with that starter message, we can fetch it from the parent channel and
+        // export it together with the rest of the thread's messages.
+        // This doesn't apply to forum/media posts, whose starter message is already part of the
+        // thread's own history.
+        // https://github.com/Tyrrrz/DiscordChatExporter/issues/1265
+        var starterMessage =
+            request.Channel.IsThread
+            && request.Channel.Parent is { Kind: not ChannelKind.GuildForum } parent
+                ? await discord.TryGetMessageAsync(parent.Id, request.Channel.Id, cancellationToken)
+                : null;
+
+        // Only include the starter message if it falls within the requested range
+        if (
+            starterMessage is not null
+            && (
+                (request.After is not null && starterMessage.Id < request.After.Value)
+                || (request.Before is not null && starterMessage.Id > request.Before.Value)
+            )
+        )
+        {
+            starterMessage = null;
+        }
+
+        // Prepend the starter message to the thread's history, or append it when exporting in
+        // reverse order, so that it remains the oldest message in the output either way.
+        async IAsyncEnumerable<Message> GetMessagesWithStarter(
+            [EnumeratorCancellation] CancellationToken innerCancellationToken
+        )
+        {
+            // If the message order is not reversed, the starter message should be the first one in the output, so we yield it before fetching the rest of the thread's history.
+            if (starterMessage is not null && !request.IsReverseMessageOrder)
+                yield return starterMessage;
+
+            // Fetch the rest of the thread's history and yield messages one by one
+            await foreach (var message in messages.WithCancellation(innerCancellationToken))
+            {
+                // A thread created from a message has an empty THREAD_STARTER_MESSAGE placeholder
+                // at the top of its history that merely points back to the originating message.
+                // It never carries any renderable content of its own and is superseded by the
+                // actual starter message fetched above, so skip it.
+                if (message.Kind == MessageKind.ThreadStarterMessage)
+                    continue;
+
+                // Avoid exporting the starter message twice in case it's also returned as part
+                // of the thread's own history.
+                if (message.Id != starterMessage?.Id)
+                    yield return message;
+            }
+
+            // If the message order is reversed, the starter message should be the last one in the output, so we yield it after fetching the rest of the thread's history.
+            if (starterMessage is not null && request.IsReverseMessageOrder)
+                yield return starterMessage;
+        }
+
+        await foreach (var message in GetMessagesWithStarter(cancellationToken))
         {
             try
             {