mirror Discord replies as replies on X and bluesky

Replying to a message on the watched channel now posts the reply as a
reply to the posts that message produced, instead of as a standalone
post.

Discord tells us the parent through MessageReference, and the store maps
it back to the Ref each output returned; that Ref rides along on the new
Post.ReplyTo. X takes it as in_reply_to_tweet_id. Bluesky needs the
thread root as well as the parent, so a Ref now carries the root it was
posted under and a post that is not itself a reply acts as its own root.

Replying to something we never distributed, an error notice from the bot
for instance, still posts normally.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

output/bluesky.go

@@ -67,6 +67,28 @@ func (bo *blueskyoutput) Write(post Post) (Ref, error) {
 		Langs:     []string{"ja"},
 	}
 
+	root := Ref{}
+	if post.ReplyTo != nil && post.ReplyTo.URI != "" {
+		parent := &atproto.RepoStrongRef{
+			Uri: post.ReplyTo.URI,
+			Cid: post.ReplyTo.CID,
+		}
+		// A post that is itself a reply carries the thread root; one that is
+		// not is the root of its own thread.
+		rootref := &atproto.RepoStrongRef{
+			Uri: post.ReplyTo.RootURI,
+			Cid: post.ReplyTo.RootCID,
+		}
+		if rootref.Uri == "" {
+			rootref = parent
+		}
+		feedpost.Reply = &bsky.FeedPost_ReplyRef{
+			Parent: parent,
+			Root:   rootref,
+		}
+		root = Ref{RootURI: rootref.Uri, RootCID: rootref.Cid}
+	}
+
 	if len(post.Images) > 0 {
 		embedimages := make([]*bsky.EmbedImages_Image, 0, len(post.Images))
 		for _, img := range post.Images {
@@ -96,7 +118,12 @@ func (bo *blueskyoutput) Write(post Post) (Ref, error) {
 		return Ref{}, recerr
 	}
 
-	return Ref{URI: record.Uri, CID: record.Cid}, nil
+	return Ref{
+		URI:     record.Uri,
+		CID:     record.Cid,
+		RootURI: root.RootURI,
+		RootCID: root.RootCID,
+	}, nil
 }
 
 func (bo *blueskyoutput) Delete(ref Ref) error {

output/output.go

@@ -7,18 +7,28 @@ type Image struct {
 }
 
 // Ref identifies a post an output has already published so it can later be
-// deleted. The fields are output specific; only the ones the publishing
-// output filled in are meaningful to it.
+// replied to or deleted. The fields are output specific; only the ones the
+// publishing output filled in are meaningful to it.
 type Ref struct {
-	ID  string `json:"id,omitempty"`  // twitter: tweet ID
-	URI string `json:"uri,omitempty"` // bluesky: at:// URI of the record
-	CID string `json:"cid,omitempty"` // bluesky: record CID
+	ID      string `json:"id,omitempty"`  // twitter: tweet ID
+	URI     string `json:"uri,omitempty"` // bluesky: at:// URI of the record
+	CID     string `json:"cid,omitempty"` // bluesky: record CID
+	RootURI string `json:"rooturi,omitempty"`
+	RootCID string `json:"rootcid,omitempty"`
+}
+
+// IsZero reports whether the ref points at nothing.
+func (r Ref) IsZero() bool {
+	return r == Ref{}
 }
 
 // Post is a single message to distribute.
 type Post struct {
 	Text   string
 	Images []Image
+	// ReplyTo is the Ref this same output returned for the post being
+	// replied to, or nil for a top level post.
+	ReplyTo *Ref
 }
 
 type OutputInterface interface {

output/stdout.go

@@ -12,6 +12,9 @@ func StdOutput() *stdoutput {
 }
 
 func (so *stdoutput) Write(post Post) (Ref, error) {
+	if post.ReplyTo != nil {
+		fmt.Printf("[reply to: %s]\n", post.ReplyTo.ID)
+	}
 	fmt.Println(post.Text)
 	for _, img := range post.Images {
 		fmt.Printf("[image: %s (%s, %d bytes)]\n", img.Filename, img.ContentType, len(img.Data))

output/twitter.go

@@ -63,6 +63,12 @@ func (to *twitteroutput) Write(post Post) (Ref, error) {
 			MediaIDs: mediaIDs,
 		}
 	}
+	if post.ReplyTo != nil && post.ReplyTo.ID != "" {
+		p.Reply = &types.CreateInputReply{
+			InReplyToTweetID: post.ReplyTo.ID,
+		}
+	}
+
 	out, err := managetweet.Create(context.Background(), to.client, p)
 	if err != nil {
 		return Ref{}, err