diff --git a/DESCRIPTION b/DESCRIPTION
index 8df026c..05ae834 100644
--- a/DESCRIPTION
+++ b/DESCRIPTION
@@ -1,7 +1,7 @@
 Package: mx.client
 Type: Package
 Title: Stateful Matrix Client Helpers
-Version: 0.2.0.4
+Version: 0.2.0.5
 Date: 2026-08-04
 Authors@R: c(
     person("Troy", "Hernandez", role = c("aut", "cre"),
diff --git a/NEWS.md b/NEWS.md
index 51536a7..7129d04 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -1,3 +1,14 @@
+# mx.client 0.2.0.5
+
+## New
+
+* `mx_send_text(thread =)` sends into a Matrix thread. Given a root
+  event id it attaches `m.relates_to` with `rel_type` `m.thread`, plus
+  the reply fallback (`is_falling_back` and an `m.in_reply_to` pointing
+  at the root) so thread-unaware clients render the message as a reply
+  instead of as loose chatter. `mx_extract_text_events()` has carried
+  the inbound `relates_to` since 0.2.0.4, so the round trip now closes.
+
 # mx.client 0.2.0.4
 
 ## New
diff --git a/R/messages.R b/R/messages.R
index acf4d30..b46e67a 100644
--- a/R/messages.R
+++ b/R/messages.R
@@ -16,6 +16,16 @@
 #'   \code{@localpart} in the body becomes a \code{matrix.to} pill in the
 #'   HTML. Implies an HTML formatted body even when \code{markdown} is
 #'   FALSE -- pills only render from HTML.
+#' @param thread Event id of a thread root, or NULL for an ordinary
+#'   message. The event is sent as a threaded reply
+#'   (\code{m.relates_to} with \code{rel_type} \code{"m.thread"}).
+#'
+#'   It also carries the reply fallback the spec asks for --
+#'   \code{is_falling_back} with an \code{m.in_reply_to} pointing at the
+#'   root -- so a client that does not implement threads renders the
+#'   message as a reply to the root rather than as a loose message in
+#'   the room. Without it those clients show a threaded conversation
+#'   as unattached chatter.
 #' @return Event id, or NULL on dry-run.
 #' @examples
 #' client <- list(room_id = "!default:example.org")
@@ -29,7 +39,7 @@
 #' @export
 mx_send_text <- function(client, text, room = NULL, msgtype = "m.text",
                          room_cache = NULL, dry_run = FALSE,
-                         markdown = FALSE, mentions = NULL) {
+                         markdown = FALSE, mentions = NULL, thread = NULL) {
     rid <- mx_resolve_room(client, room, room_cache = room_cache)
     if (isTRUE(dry_run)) {
         message("=== mx_send_text (dry-run) [", room %||% "default",
@@ -48,6 +58,16 @@ mx_send_text <- function(client, text, room = NULL, msgtype = "m.text",
         extra <- c(extra,
                    list("m.mentions" = list(user_ids = as.list(mentions))))
     }
+    if (!is.null(thread) && length(thread) && nzchar(thread[[1L]])) {
+        extra <- c(extra, list("m.relates_to" = list(
+                    rel_type = "m.thread",
+                    event_id = as.character(thread)[[1L]],
+                    # The reply fallback: thread-unaware clients read
+                    # m.in_reply_to and render this as a reply to the root.
+                    is_falling_back = TRUE,
+                    "m.in_reply_to" = list(
+                        event_id = as.character(thread)[[1L]]))))
+    }
     mx.api::mx_send(mx_client_session(client), rid, text, msgtype = msgtype,
                     extra = extra)
 }
diff --git a/inst/tinytest/test_mx.client.R b/inst/tinytest/test_mx.client.R
index 724c468..b8d80b7 100644
--- a/inst/tinytest/test_mx.client.R
+++ b/inst/tinytest/test_mx.client.R
@@ -147,6 +147,57 @@ expect_equal(rel[[3]]$relates_to$`m.in_reply_to`$event_id, "$orig")
 expect_equal(rel[[2]]$body, "in a thread")
 expect_equal(rel[[2]]$event_id, "$thread")
 
+# --- Threaded sends ---
+# The outbound half of the relation the extraction tests above read.
+# mx_send_text() hands mx.api::mx_send() an `extra` block, so what is
+# asserted here is the content the homeserver would receive.
+local({
+    seen <- NULL
+    orig_send <- mx.api::mx_send
+    orig_sess <- mx.client::mx_client_session
+    assignInNamespace("mx_send", function(session, room_id, body,
+                                          msgtype = "m.text",
+                                          extra = NULL) {
+        seen <<- list(room_id = room_id, body = body, msgtype = msgtype,
+                      extra = extra)
+        "$sent"
+    }, ns = "mx.api")
+    assignInNamespace("mx_client_session", function(client, ...) list(),
+                      ns = "mx.client")
+    on.exit({
+        assignInNamespace("mx_send", orig_send, ns = "mx.api")
+        assignInNamespace("mx_client_session", orig_sess, ns = "mx.client")
+    }, add = TRUE)
+    client <- list(room_id = "!r:ex")
+
+    # No thread: no relation at all, rather than an empty one a
+    # homeserver would have to interpret.
+    mx.client::mx_send_text(client, "plain")
+    expect_null(seen$extra$`m.relates_to`)
+
+    expect_equal(mx.client::mx_send_text(client, "in a thread",
+                                         thread = "$root"), "$sent")
+    rel <- seen$extra$`m.relates_to`
+    expect_equal(rel$rel_type, "m.thread")
+    expect_equal(rel$event_id, "$root")
+    # The reply fallback, so a thread-unaware client renders this as a
+    # reply to the root instead of as loose chatter in the room.
+    expect_true(rel$is_falling_back)
+    expect_equal(rel$`m.in_reply_to`$event_id, "$root")
+
+    # A thread rides alongside markdown and mentions rather than
+    # replacing them: all three land in one content block.
+    mx.client::mx_send_text(client, "**bold**", markdown = TRUE,
+                            mentions = "@troy:ex", thread = "$root")
+    expect_equal(seen$extra$format, "org.matrix.custom.html")
+    expect_equal(seen$extra$`m.mentions`$user_ids[[1]], "@troy:ex")
+    expect_equal(seen$extra$`m.relates_to`$rel_type, "m.thread")
+
+    # An empty thread id is "no thread", not a relation pointing nowhere.
+    mx.client::mx_send_text(client, "x", thread = "")
+    expect_null(seen$extra$`m.relates_to`)
+})
+
 # --- General reaction extraction ---
 # mx_extract_reaction_verdict() answers one approve/deny question about
 # one event, with the key semantics baked in. This reports every reaction
diff --git a/man/mx_send_text.Rd b/man/mx_send_text.Rd
index 17b627e..12d063e 100644
--- a/man/mx_send_text.Rd
+++ b/man/mx_send_text.Rd
@@ -11,7 +11,8 @@ mx_send_text(
   room_cache = NULL,
   dry_run = FALSE,
   markdown = FALSE,
-  mentions = NULL
+  mentions = NULL,
+  thread = NULL
 )
 }
 \arguments{
@@ -36,6 +37,17 @@ from a conservative markdown subset.}
 \code{@localpart} in the body becomes a \code{matrix.to} pill in the
 HTML. Implies an HTML formatted body even when \code{markdown} is
 FALSE -- pills only render from HTML.}
+
+\item{thread}{Event id of a thread root, or NULL for an ordinary
+message. The event is sent as a threaded reply
+(\code{m.relates_to} with \code{rel_type} \code{"m.thread"}).
+
+It also carries the reply fallback the spec asks for --
+\code{is_falling_back} with an \code{m.in_reply_to} pointing at the
+root -- so a client that does not implement threads renders the
+message as a reply to the root rather than as a loose message in
+the room. Without it those clients show a threaded conversation
+as unattached chatter.}
 }
 \value{
 Event id, or NULL on dry-run.