diff --git a/server/src/main.rs b/server/src/main.rs
index a33a4e5..a215a7b 100644
--- a/server/src/main.rs
+++ b/server/src/main.rs
@@ -12,7 +12,9 @@ pub mod log_buffer;
 mod matrix;
 mod state;
 mod store;
+pub mod transport;
 mod workflow;
+pub mod whatsapp;
 mod worktree;
 
 use crate::agents::AgentPool;
diff --git a/server/src/matrix/bot.rs b/server/src/matrix/bot.rs
index 3c0b27d..3fee04d 100644
--- a/server/src/matrix/bot.rs
+++ b/server/src/matrix/bot.rs
@@ -2,6 +2,7 @@ use crate::agents::AgentPool;
 use crate::http::context::{PermissionDecision, PermissionForward};
 use crate::llm::providers::claude_code::{ClaudeCodeProvider, ClaudeCodeResult};
 use crate::slog;
+use crate::transport::ChatTransport;
 use matrix_sdk::{
     Client,
     config::SyncSettings,
@@ -10,7 +11,7 @@ use matrix_sdk::{
     ruma::{
         OwnedEventId, OwnedRoomId, OwnedUserId,
         events::room::message::{
-            MessageType, OriginalSyncRoomMessageEvent, Relation, RoomMessageEventContent,
+            MessageType, OriginalSyncRoomMessageEvent, Relation,
             RoomMessageEventContentWithoutRelation,
         },
     },
@@ -169,12 +170,18 @@ pub struct BotContext {
     /// Set of room IDs where ambient mode is active.  In ambient mode the bot
     /// responds to all messages rather than only addressed ones.
     /// Uses a sync mutex since locks are never held across await points.
-    pub ambient_rooms: Arc<std::sync::Mutex<HashSet<OwnedRoomId>>>,
+    /// Room IDs are stored as plain strings (platform-agnostic).
+    pub ambient_rooms: Arc<std::sync::Mutex<HashSet<String>>>,
     /// Agent pool for checking agent availability.
     pub agents: Arc<AgentPool>,
     /// Per-room htop monitoring sessions.  Keyed by room ID; each entry holds
     /// a stop-signal sender that the background task watches.
     pub htop_sessions: super::htop::HtopSessions,
+    /// Chat transport used for sending and editing messages.
+    ///
+    /// All message I/O goes through this abstraction so the bot logic works
+    /// with any platform, not just Matrix.
+    pub transport: Arc<dyn ChatTransport>,
 }
 
 // ---------------------------------------------------------------------------
@@ -344,12 +351,11 @@ pub async fn run_bot(
         persisted.len()
     );
 
-    // Restore persisted ambient rooms from config, ignoring any that are not
-    // in the configured target_room_ids to avoid stale entries.
-    let persisted_ambient: HashSet<OwnedRoomId> = config
+    // Restore persisted ambient rooms from config.
+    let persisted_ambient: HashSet<String> = config
         .ambient_rooms
         .iter()
-        .filter_map(|s| s.parse::<OwnedRoomId>().ok())
+        .cloned()
         .collect();
     if !persisted_ambient.is_empty() {
         slog!(
@@ -359,6 +365,18 @@ pub async fn run_bot(
         );
     }
 
+    // Create the transport abstraction based on the configured transport type.
+    let transport: Arc<dyn ChatTransport> = match config.transport.as_str() {
+        "whatsapp" => {
+            slog!("[matrix-bot] Using WhatsApp transport (stub)");
+            Arc::new(crate::whatsapp::WhatsAppTransport::new())
+        }
+        _ => {
+            slog!("[matrix-bot] Using Matrix transport");
+            Arc::new(super::transport_impl::MatrixTransport::new(client.clone()))
+        }
+    };
+
     let bot_name = config
         .display_name
         .clone()
@@ -380,6 +398,7 @@ pub async fn run_bot(
         ambient_rooms: Arc::new(std::sync::Mutex::new(persisted_ambient)),
         agents,
         htop_sessions: Arc::new(TokioMutex::new(HashMap::new())),
+        transport: Arc::clone(&transport),
     };
 
     slog!("[matrix-bot] Cryptographic identity verification is always ON — commands from unencrypted rooms or unverified devices are rejected");
@@ -391,9 +410,11 @@ pub async fn run_bot(
 
     // Spawn the stage-transition notification listener before entering the
     // sync loop so it starts receiving watcher events immediately.
+    let notif_room_id_strings: Vec<String> =
+        notif_room_ids.iter().map(|r| r.to_string()).collect();
     super::notifications::spawn_notification_listener(
-        client.clone(),
-        notif_room_ids,
+        Arc::clone(&transport),
+        notif_room_id_strings,
         watcher_rx,
         notif_project_root,
     );
@@ -403,15 +424,15 @@ pub async fn run_bot(
     // reconnects internally so this code is never reached again on a network
     // blip or sync resumption.
     let announce_msg = format_startup_announcement(&announce_bot_name);
+    let announce_html = markdown_to_html(&announce_msg);
     slog!("[matrix-bot] Sending startup announcement: {announce_msg}");
     for room_id in &announce_room_ids {
-        if let Some(room) = client.get_room(room_id) {
-            let content = RoomMessageEventContent::text_plain(announce_msg.clone());
-            if let Err(e) = room.send(content).await {
-                slog!("[matrix-bot] Failed to send startup announcement to {room_id}: {e}");
-            }
-        } else {
-            slog!("[matrix-bot] Room {room_id} not found in client state, skipping announcement");
+        let room_id_str = room_id.to_string();
+        if let Err(e) = transport
+            .send_message(&room_id_str, &announce_msg, &announce_html)
+            .await
+        {
+            slog!("[matrix-bot] Failed to send startup announcement to {room_id}: {e}");
         }
     }
 
@@ -704,7 +725,8 @@ async fn on_room_message(
     // ambient mode is enabled for this room.
     let is_addressed = mentions_bot(&body, formatted_body.as_deref(), &ctx.bot_user_id)
         || is_reply_to_bot(ev.content.relates_to.as_ref(), &ctx.bot_sent_event_ids).await;
-    let is_ambient = ctx.ambient_rooms.lock().unwrap().contains(&incoming_room_id);
+    let room_id_str = incoming_room_id.to_string();
+    let is_ambient = ctx.ambient_rooms.lock().unwrap().contains(&room_id_str);
 
     if !is_addressed && !is_ambient {
         slog!(
@@ -765,11 +787,10 @@ async fn on_room_message(
                 "Permission denied."
             };
             let html = markdown_to_html(confirmation);
-            if let Ok(resp) = room
-                .send(RoomMessageEventContent::text_html(confirmation, html))
-                .await
+            if let Ok(msg_id) = ctx.transport.send_message(&room_id_str, confirmation, &html).await
+                && let Ok(event_id) = msg_id.parse()
             {
-                ctx.bot_sent_event_ids.lock().await.insert(resp.event_id);
+                ctx.bot_sent_event_ids.lock().await.insert(event_id);
             }
             return;
         }
@@ -788,17 +809,16 @@ async fn on_room_message(
         project_root: &ctx.project_root,
         agents: &ctx.agents,
         ambient_rooms: &ctx.ambient_rooms,
-        room_id: &incoming_room_id,
+        room_id: &room_id_str,
         is_addressed,
     };
     if let Some(response) = super::commands::try_handle_command(&dispatch, &user_message) {
         slog!("[matrix-bot] Handled bot command from {sender}");
         let html = markdown_to_html(&response);
-        if let Ok(resp) = room
-            .send(RoomMessageEventContent::text_html(response, html))
-            .await
+        if let Ok(msg_id) = ctx.transport.send_message(&room_id_str, &response, &html).await
+            && let Ok(event_id) = msg_id.parse()
         {
-            ctx.bot_sent_event_ids.lock().await.insert(resp.event_id);
+            ctx.bot_sent_event_ids.lock().await.insert(event_id);
         }
         return;
     }
@@ -811,12 +831,13 @@ async fn on_room_message(
         slog!("[matrix-bot] Handling htop command from {sender}: {htop_cmd:?}");
         match htop_cmd {
             super::htop::HtopCommand::Stop => {
-                super::htop::handle_htop_stop(&room, &incoming_room_id, &ctx.htop_sessions).await;
+                super::htop::handle_htop_stop(&*ctx.transport, &room_id_str, &ctx.htop_sessions)
+                    .await;
             }
             super::htop::HtopCommand::Start { duration_secs } => {
                 super::htop::handle_htop_start(
-                    &room,
-                    &incoming_room_id,
+                    &ctx.transport,
+                    &room_id_str,
                     &ctx.htop_sessions,
                     Arc::clone(&ctx.agents),
                     duration_secs,
@@ -852,11 +873,10 @@ async fn on_room_message(
             }
         };
         let html = markdown_to_html(&response);
-        if let Ok(resp) = room
-            .send(RoomMessageEventContent::text_html(response, html))
-            .await
+        if let Ok(msg_id) = ctx.transport.send_message(&room_id_str, &response, &html).await
+            && let Ok(event_id) = msg_id.parse()
         {
-            ctx.bot_sent_event_ids.lock().await.insert(resp.event_id);
+            ctx.bot_sent_event_ids.lock().await.insert(event_id);
         }
         return;
     }
@@ -864,7 +884,7 @@ async fn on_room_message(
     // Spawn a separate task so the Matrix sync loop is not blocked while we
     // wait for the LLM response (which can take several seconds).
     tokio::spawn(async move {
-        handle_message(room, incoming_room_id, ctx, sender, user_message).await;
+        handle_message(room_id_str, incoming_room_id, ctx, sender, user_message).await;
     });
 }
 
@@ -879,7 +899,7 @@ fn format_user_prompt(sender: &str, message: &str) -> String {
 }
 
 async fn handle_message(
-    room: Room,
+    room_id_str: String,
     room_id: OwnedRoomId,
     ctx: BotContext,
     sender: String,
@@ -912,19 +932,19 @@ async fn handle_message(
     let (msg_tx, mut msg_rx) = tokio::sync::mpsc::unbounded_channel::<String>();
     let msg_tx_for_callback = msg_tx.clone();
 
-    // Spawn a task to post messages to Matrix as they arrive so we don't
-    // block the LLM stream while waiting for Matrix send round-trips.
-    let post_room = room.clone();
+    // Spawn a task to post messages via the transport as they arrive so we
+    // don't block the LLM stream while waiting for send round-trips.
+    let post_transport = Arc::clone(&ctx.transport);
+    let post_room_id = room_id_str.clone();
     let sent_ids = Arc::clone(&ctx.bot_sent_event_ids);
     let sent_ids_for_post = Arc::clone(&sent_ids);
     let post_task = tokio::spawn(async move {
         while let Some(chunk) = msg_rx.recv().await {
             let html = markdown_to_html(&chunk);
-            if let Ok(response) = post_room
-                .send(RoomMessageEventContent::text_html(chunk, html))
-                .await
+            if let Ok(msg_id) = post_transport.send_message(&post_room_id, &chunk, &html).await
+                && let Ok(event_id) = msg_id.parse()
             {
-                sent_ids_for_post.lock().await.insert(response.event_id);
+                sent_ids_for_post.lock().await.insert(event_id);
             }
         }
     });
@@ -966,7 +986,7 @@ async fn handle_message(
             r = &mut chat_fut => break r,
 
             Some(perm_fwd) = perm_rx_guard.recv() => {
-                // Post the permission prompt to the Matrix room.
+                // Post the permission prompt to the room via the transport.
                 let prompt_msg = format!(
                     "**Permission Request**\n\n\
                      Tool: `{}`\n```json\n{}\n```\n\n\
@@ -976,11 +996,10 @@ async fn handle_message(
                         .unwrap_or_else(|_| perm_fwd.tool_input.to_string()),
                 );
                 let html = markdown_to_html(&prompt_msg);
-                if let Ok(resp) = room
-                    .send(RoomMessageEventContent::text_html(&prompt_msg, html))
-                    .await
+                if let Ok(msg_id) = ctx.transport.send_message(&room_id_str, &prompt_msg, &html).await
+                    && let Ok(event_id) = msg_id.parse()
                 {
-                    sent_ids.lock().await.insert(resp.event_id);
+                    sent_ids.lock().await.insert(event_id);
                 }
 
                 // Store the MCP oneshot sender so the event handler can
@@ -993,7 +1012,8 @@ async fn handle_message(
                 // Spawn a timeout task: auto-deny if the user does not respond.
                 let pending = Arc::clone(&ctx.pending_perm_replies);
                 let timeout_room_id = room_id.clone();
-                let timeout_room = room.clone();
+                let timeout_transport = Arc::clone(&ctx.transport);
+                let timeout_room_id_str = room_id_str.clone();
                 let timeout_sent_ids = Arc::clone(&ctx.bot_sent_event_ids);
                 let timeout_secs = ctx.permission_timeout_secs;
                 tokio::spawn(async move {
@@ -1002,11 +1022,12 @@ async fn handle_message(
                         let _ = tx.send(PermissionDecision::Deny);
                         let msg = "Permission request timed out — denied (fail-closed).";
                         let html = markdown_to_html(msg);
-                        if let Ok(resp) = timeout_room
-                            .send(RoomMessageEventContent::text_html(msg, html))
+                        if let Ok(msg_id) = timeout_transport
+                            .send_message(&timeout_room_id_str, msg, &html)
                             .await
+                            && let Ok(event_id) = msg_id.parse()
                         {
-                            timeout_sent_ids.lock().await.insert(resp.event_id);
+                            timeout_sent_ids.lock().await.insert(event_id);
                         }
                     }
                 });
@@ -1372,6 +1393,7 @@ mod tests {
             ambient_rooms: Arc::new(std::sync::Mutex::new(HashSet::new())),
             agents: Arc::new(AgentPool::new_test(3000)),
             htop_sessions: Arc::new(TokioMutex::new(HashMap::new())),
+            transport: Arc::new(crate::whatsapp::WhatsAppTransport::new()),
         };
         // Clone must work (required by Matrix SDK event handler injection).
         let _cloned = ctx.clone();
diff --git a/server/src/matrix/commands.rs b/server/src/matrix/commands.rs
index 3af2673..558a72f 100644
--- a/server/src/matrix/commands.rs
+++ b/server/src/matrix/commands.rs
@@ -7,7 +7,6 @@
 
 use crate::agents::{AgentPool, AgentStatus};
 use crate::config::ProjectConfig;
-use matrix_sdk::ruma::OwnedRoomId;
 use std::collections::{HashMap, HashSet};
 use std::path::Path;
 use std::sync::{Arc, Mutex};
@@ -31,19 +30,22 @@ pub struct BotCommand {
 /// Groups all the caller-supplied context needed to dispatch and execute bot
 /// commands.  Construct one per incoming message and pass it alongside the raw
 /// message body.
+///
+/// All identifiers are platform-agnostic strings so this struct works with
+/// any [`ChatTransport`](crate::transport::ChatTransport) implementation.
 pub struct CommandDispatch<'a> {
     /// The bot's display name (e.g., "Timmy").
     pub bot_name: &'a str,
-    /// The bot's full Matrix user ID (e.g., `"@timmy:homeserver.local"`).
+    /// The bot's full user ID (e.g., `"@timmy:homeserver.local"` on Matrix).
     pub bot_user_id: &'a str,
     /// Project root directory (needed by status, ambient).
     pub project_root: &'a Path,
     /// Agent pool (needed by status).
     pub agents: &'a AgentPool,
-    /// Set of rooms with ambient mode enabled (needed by ambient).
-    pub ambient_rooms: &'a Arc<Mutex<HashSet<OwnedRoomId>>>,
+    /// Set of room IDs with ambient mode enabled (needed by ambient).
+    pub ambient_rooms: &'a Arc<Mutex<HashSet<String>>>,
     /// The room this message came from (needed by ambient).
-    pub room_id: &'a OwnedRoomId,
+    pub room_id: &'a str,
     /// Whether the message directly addressed the bot (mention/reply).
     /// Some commands (e.g. ambient) only operate when directly addressed.
     pub is_addressed: bool,
@@ -59,10 +61,10 @@ pub struct CommandContext<'a> {
     pub project_root: &'a Path,
     /// Agent pool (needed by status).
     pub agents: &'a AgentPool,
-    /// Set of rooms with ambient mode enabled (needed by ambient).
-    pub ambient_rooms: &'a Arc<Mutex<HashSet<OwnedRoomId>>>,
+    /// Set of room IDs with ambient mode enabled (needed by ambient).
+    pub ambient_rooms: &'a Arc<Mutex<HashSet<String>>>,
     /// The room this message came from (needed by ambient).
-    pub room_id: &'a OwnedRoomId,
+    pub room_id: &'a str,
     /// Whether the message directly addressed the bot (mention/reply).
     /// Some commands (e.g. ambient) only operate when directly addressed.
     pub is_addressed: bool,
@@ -389,11 +391,11 @@ fn handle_ambient(ctx: &CommandContext) -> Option<String> {
     let room_ids: Vec<String> = {
         let mut ambient = ctx.ambient_rooms.lock().unwrap();
         if enable {
-            ambient.insert(ctx.room_id.clone());
+            ambient.insert(ctx.room_id.to_string());
         } else {
             ambient.remove(ctx.room_id);
         }
-        ambient.iter().map(|r| r.to_string()).collect()
+        ambient.iter().cloned().collect()
     };
     save_ambient_rooms(ctx.project_root, &room_ids);
     let msg = if enable {
@@ -665,11 +667,7 @@ mod tests {
 
     // -- test helpers -------------------------------------------------------
 
-    fn make_room_id(s: &str) -> OwnedRoomId {
-        s.parse().unwrap()
-    }
-
-    fn test_ambient_rooms() -> Arc<Mutex<HashSet<OwnedRoomId>>> {
+    fn test_ambient_rooms() -> Arc<Mutex<HashSet<String>>> {
         Arc::new(Mutex::new(HashSet::new()))
     }
 
@@ -681,11 +679,11 @@ mod tests {
         bot_name: &str,
         bot_user_id: &str,
         message: &str,
-        ambient_rooms: &Arc<Mutex<HashSet<OwnedRoomId>>>,
+        ambient_rooms: &Arc<Mutex<HashSet<String>>>,
         is_addressed: bool,
     ) -> Option<String> {
         let agents = test_agents();
-        let room_id = make_room_id("!test:example.com");
+        let room_id = "!test:example.com".to_string();
         let dispatch = CommandDispatch {
             bot_name,
             bot_user_id,
@@ -855,7 +853,7 @@ mod tests {
     #[test]
     fn ambient_on_requires_addressed() {
         let ambient_rooms = test_ambient_rooms();
-        let room_id = make_room_id("!myroom:example.com");
+        let room_id = "!myroom:example.com".to_string();
         let result = try_cmd(
             "Timmy",
             "@timmy:homeserver.local",
@@ -875,7 +873,7 @@ mod tests {
     fn ambient_on_enables_ambient_mode() {
         let ambient_rooms = test_ambient_rooms();
         let agents = test_agents();
-        let room_id = make_room_id("!myroom:example.com");
+        let room_id = "!myroom:example.com".to_string();
         let dispatch = CommandDispatch {
             bot_name: "Timmy",
             bot_user_id: "@timmy:homeserver.local",
@@ -902,7 +900,7 @@ mod tests {
     fn ambient_off_disables_ambient_mode() {
         let ambient_rooms = test_ambient_rooms();
         let agents = test_agents();
-        let room_id = make_room_id("!myroom:example.com");
+        let room_id = "!myroom:example.com".to_string();
         // Pre-insert the room
         ambient_rooms.lock().unwrap().insert(room_id.clone());
 
@@ -1211,7 +1209,7 @@ mod tests {
             .unwrap_or(std::path::Path::new("."));
         let agents = test_agents();
         let ambient_rooms = test_ambient_rooms();
-        let room_id = make_room_id("!test:example.com");
+        let room_id = "!test:example.com".to_string();
         let dispatch = CommandDispatch {
             bot_name: "Timmy",
             bot_user_id: "@timmy:homeserver.local",
@@ -1232,7 +1230,7 @@ mod tests {
             .unwrap_or(std::path::Path::new("."));
         let agents = test_agents();
         let ambient_rooms = test_ambient_rooms();
-        let room_id = make_room_id("!test:example.com");
+        let room_id = "!test:example.com".to_string();
         let dispatch = CommandDispatch {
             bot_name: "Timmy",
             bot_user_id: "@timmy:homeserver.local",
@@ -1256,7 +1254,7 @@ mod tests {
             .unwrap_or(std::path::Path::new("."));
         let agents = test_agents();
         let ambient_rooms = test_ambient_rooms();
-        let room_id = make_room_id("!test:example.com");
+        let room_id = "!test:example.com".to_string();
         let dispatch = CommandDispatch {
             bot_name: "Timmy",
             bot_user_id: "@timmy:homeserver.local",
@@ -1280,7 +1278,7 @@ mod tests {
             .unwrap_or(std::path::Path::new("."));
         let agents = test_agents();
         let ambient_rooms = test_ambient_rooms();
-        let room_id = make_room_id("!test:example.com");
+        let room_id = "!test:example.com".to_string();
         let dispatch = CommandDispatch {
             bot_name: "Timmy",
             bot_user_id: "@timmy:homeserver.local",
@@ -1335,7 +1333,7 @@ mod tests {
     fn cost_cmd_with_root(root: &std::path::Path) -> Option<String> {
         let agents = test_agents();
         let ambient_rooms = test_ambient_rooms();
-        let room_id = make_room_id("!test:example.com");
+        let room_id = "!test:example.com".to_string();
         let dispatch = CommandDispatch {
             bot_name: "Timmy",
             bot_user_id: "@timmy:homeserver.local",
@@ -1473,7 +1471,7 @@ mod tests {
     fn show_cmd_with_root(root: &std::path::Path, args: &str) -> Option<String> {
         let agents = test_agents();
         let ambient_rooms = test_ambient_rooms();
-        let room_id = make_room_id("!test:example.com");
+        let room_id = "!test:example.com".to_string();
         let dispatch = CommandDispatch {
             bot_name: "Timmy",
             bot_user_id: "@timmy:homeserver.local",
diff --git a/server/src/matrix/config.rs b/server/src/matrix/config.rs
index a7b14da..b475260 100644
--- a/server/src/matrix/config.rs
+++ b/server/src/matrix/config.rs
@@ -58,6 +58,18 @@ pub struct BotConfig {
     /// manually while the bot is running.
     #[serde(default)]
     pub ambient_rooms: Vec<String>,
+    /// Chat transport to use: `"matrix"` (default) or `"whatsapp"`.
+    ///
+    /// Selects which [`ChatTransport`] implementation the bot uses for
+    /// sending and editing messages.  Currently only read during bot
+    /// startup to select the transport; the field is kept for config
+    /// round-tripping.
+    #[serde(default = "default_transport")]
+    pub transport: String,
+}
+
+fn default_transport() -> String {
+    "matrix".to_string()
 }
 
 impl BotConfig {
@@ -509,4 +521,45 @@ ambient_rooms = ["!abc:example.com"]
         let config = BotConfig::load(tmp.path()).unwrap();
         assert!(config.ambient_rooms.is_empty());
     }
+
+    #[test]
+    fn load_transport_defaults_to_matrix() {
+        let tmp = tempfile::tempdir().unwrap();
+        let sk = tmp.path().join(".story_kit");
+        fs::create_dir_all(&sk).unwrap();
+        fs::write(
+            sk.join("bot.toml"),
+            r#"
+homeserver = "https://matrix.example.com"
+username = "@bot:example.com"
+password = "secret"
+room_ids = ["!abc:example.com"]
+enabled = true
+"#,
+        )
+        .unwrap();
+        let config = BotConfig::load(tmp.path()).unwrap();
+        assert_eq!(config.transport, "matrix");
+    }
+
+    #[test]
+    fn load_transport_reads_custom_value() {
+        let tmp = tempfile::tempdir().unwrap();
+        let sk = tmp.path().join(".story_kit");
+        fs::create_dir_all(&sk).unwrap();
+        fs::write(
+            sk.join("bot.toml"),
+            r#"
+homeserver = "https://matrix.example.com"
+username = "@bot:example.com"
+password = "secret"
+room_ids = ["!abc:example.com"]
+enabled = true
+transport = "whatsapp"
+"#,
+        )
+        .unwrap();
+        let config = BotConfig::load(tmp.path()).unwrap();
+        assert_eq!(config.transport, "whatsapp");
+    }
 }
diff --git a/server/src/matrix/htop.rs b/server/src/matrix/htop.rs
index 28b5099..045ade3 100644
--- a/server/src/matrix/htop.rs
+++ b/server/src/matrix/htop.rs
@@ -10,15 +10,11 @@ use std::collections::HashMap;
 use std::sync::Arc;
 use std::time::Duration;
 
-use matrix_sdk::room::Room;
-use matrix_sdk::ruma::OwnedEventId;
-use matrix_sdk::ruma::events::room::message::{
-    ReplacementMetadata, RoomMessageEventContent, RoomMessageEventContentWithoutRelation,
-};
 use tokio::sync::{Mutex as TokioMutex, watch};
 
 use crate::agents::{AgentPool, AgentStatus};
 use crate::slog;
+use crate::transport::ChatTransport;
 
 use super::bot::markdown_to_html;
 
@@ -39,7 +35,10 @@ pub struct HtopSession {
 }
 
 /// Per-room htop session map type alias.
-pub type HtopSessions = Arc<TokioMutex<HashMap<matrix_sdk::ruma::OwnedRoomId, HtopSession>>>;
+///
+/// Keys are platform-agnostic room ID strings (e.g. `"!abc:example.com"` on
+/// Matrix) so this type works with any [`ChatTransport`] implementation.
+pub type HtopSessions = Arc<TokioMutex<HashMap<String, HtopSession>>>;
 
 /// Parse an htop command from a raw Matrix message body.
 ///
@@ -253,40 +252,19 @@ pub fn build_htop_message(agents: &AgentPool, tick: u32, total_duration_secs: u6
     lines.join("\n")
 }
 
-// ---------------------------------------------------------------------------
-// Matrix replacement helper
-// ---------------------------------------------------------------------------
-
-/// Edit an existing Matrix message by sending a replacement event.
-///
-/// Uses `RoomMessageEventContentWithoutRelation::make_replacement` with
-/// `ReplacementMetadata` so the replacement carries the original event ID.
-async fn send_replacement(
-    room: &Room,
-    original_event_id: &OwnedEventId,
-    plain: &str,
-    html: &str,
-) -> Result<(), String> {
-    let new_content =
-        RoomMessageEventContentWithoutRelation::text_html(plain.to_string(), html.to_string());
-    let metadata = ReplacementMetadata::new(original_event_id.clone(), None);
-    let content = new_content.make_replacement(metadata);
-
-    room.send(content)
-        .await
-        .map(|_| ())
-        .map_err(|e| format!("Matrix send error: {e}"))
-}
-
 // ---------------------------------------------------------------------------
 // Background monitoring loop
 // ---------------------------------------------------------------------------
 
 /// Run the htop background loop: update the message every 5 seconds until
 /// the stop signal is received or the timeout expires.
+///
+/// Uses the [`ChatTransport`] abstraction so the loop works with any chat
+/// platform, not just Matrix.
 pub async fn run_htop_loop(
-    room: Room,
-    initial_event_id: OwnedEventId,
+    transport: Arc<dyn ChatTransport>,
+    room_id: String,
+    initial_message_id: String,
     agents: Arc<AgentPool>,
     mut stop_rx: watch::Receiver<bool>,
     duration_secs: u64,
@@ -303,7 +281,7 @@ pub async fn run_htop_loop(
             _ = &mut sleep => {}
             Ok(()) = stop_rx.changed() => {
                 if *stop_rx.borrow() {
-                    send_stopped_message(&room, &initial_event_id).await;
+                    send_stopped_message(&*transport, &room_id, &initial_message_id).await;
                     return;
                 }
             }
@@ -311,27 +289,27 @@ pub async fn run_htop_loop(
 
         // Re-check after waking — the sender might have signalled while we slept.
         if *stop_rx.borrow() {
-            send_stopped_message(&room, &initial_event_id).await;
+            send_stopped_message(&*transport, &room_id, &initial_message_id).await;
             return;
         }
 
         let text = build_htop_message(&agents, tick as u32, duration_secs);
         let html = markdown_to_html(&text);
 
-        if let Err(e) = send_replacement(&room, &initial_event_id, &text, &html).await {
+        if let Err(e) = transport.edit_message(&room_id, &initial_message_id, &text, &html).await {
             slog!("[htop] Failed to update message: {e}");
             return;
         }
     }
 
     // Auto-stop: timeout reached.
-    send_stopped_message(&room, &initial_event_id).await;
+    send_stopped_message(&*transport, &room_id, &initial_message_id).await;
 }
 
-async fn send_stopped_message(room: &Room, event_id: &OwnedEventId) {
+async fn send_stopped_message(transport: &dyn ChatTransport, room_id: &str, message_id: &str) {
     let text = "**htop** — monitoring stopped.";
     let html = markdown_to_html(text);
-    if let Err(e) = send_replacement(room, event_id, text, &html).await {
+    if let Err(e) = transport.edit_message(room_id, message_id, text, &html).await {
         slog!("[htop] Failed to send stop message: {e}");
     }
 }
@@ -344,9 +322,11 @@ async fn send_stopped_message(room: &Room, event_id: &OwnedEventId) {
 ///
 /// Stops any existing session for the room, sends the initial dashboard
 /// message, and spawns a background task that edits it every 5 seconds.
+///
+/// Uses the [`ChatTransport`] abstraction so htop works with any platform.
 pub async fn handle_htop_start(
-    room: &Room,
-    room_id: &matrix_sdk::ruma::OwnedRoomId,
+    transport: &Arc<dyn ChatTransport>,
+    room_id: &str,
     htop_sessions: &HtopSessions,
     agents: Arc<AgentPool>,
     duration_secs: u64,
@@ -357,15 +337,8 @@ pub async fn handle_htop_start(
     // Send the initial message.
     let initial_text = build_htop_message(&agents, 0, duration_secs);
     let initial_html = markdown_to_html(&initial_text);
-    let send_result = room
-        .send(RoomMessageEventContent::text_html(
-            initial_text,
-            initial_html,
-        ))
-        .await;
-
-    let event_id = match send_result {
-        Ok(r) => r.event_id,
+    let message_id = match transport.send_message(room_id, &initial_text, &initial_html).await {
+        Ok(id) => id,
         Err(e) => {
             slog!("[htop] Failed to send initial message: {e}");
             return;
@@ -376,18 +349,26 @@ pub async fn handle_htop_start(
     let (stop_tx, stop_rx) = watch::channel(false);
     {
         let mut sessions = htop_sessions.lock().await;
-        sessions.insert(room_id.clone(), HtopSession { stop_tx });
+        sessions.insert(room_id.to_string(), HtopSession { stop_tx });
     }
 
     // Spawn the background update loop.
-    let room_clone = room.clone();
+    let transport_clone = Arc::clone(transport);
     let sessions_clone = Arc::clone(htop_sessions);
-    let room_id_clone = room_id.clone();
+    let room_id_owned = room_id.to_string();
     tokio::spawn(async move {
-        run_htop_loop(room_clone, event_id, agents, stop_rx, duration_secs).await;
+        run_htop_loop(
+            transport_clone,
+            room_id_owned.clone(),
+            message_id,
+            agents,
+            stop_rx,
+            duration_secs,
+        )
+        .await;
         // Clean up the session entry when the loop exits naturally.
         let mut sessions = sessions_clone.lock().await;
-        sessions.remove(&room_id_clone);
+        sessions.remove(&room_id_owned);
     });
 }
 
@@ -396,18 +377,15 @@ pub async fn handle_htop_start(
 /// When there is no active session, sends a "no active session" reply
 /// to the room so the user knows the command was received.
 pub async fn handle_htop_stop(
-    room: &Room,
-    room_id: &matrix_sdk::ruma::OwnedRoomId,
+    transport: &dyn ChatTransport,
+    room_id: &str,
     htop_sessions: &HtopSessions,
 ) {
     let had_session = stop_existing_session(htop_sessions, room_id).await;
     if !had_session {
         let msg = "No active htop session in this room.";
         let html = markdown_to_html(msg);
-        if let Err(e) = room
-            .send(RoomMessageEventContent::text_html(msg, html))
-            .await
-        {
+        if let Err(e) = transport.send_message(room_id, msg, &html).await {
             slog!("[htop] Failed to send no-session reply: {e}");
         }
     }
@@ -417,10 +395,7 @@ pub async fn handle_htop_stop(
 /// Signal and remove the existing session for `room_id`.
 ///
 /// Returns `true` if a session was found and stopped.
-async fn stop_existing_session(
-    htop_sessions: &HtopSessions,
-    room_id: &matrix_sdk::ruma::OwnedRoomId,
-) -> bool {
+async fn stop_existing_session(htop_sessions: &HtopSessions, room_id: &str) -> bool {
     let mut sessions = htop_sessions.lock().await;
     if let Some(session) = sessions.remove(room_id) {
         // Signal the background task to stop (ignore error — task may be done).
diff --git a/server/src/matrix/mod.rs b/server/src/matrix/mod.rs
index d2725a9..6213ef1 100644
--- a/server/src/matrix/mod.rs
+++ b/server/src/matrix/mod.rs
@@ -21,6 +21,7 @@ mod config;
 pub mod delete;
 pub mod htop;
 pub mod notifications;
+pub mod transport_impl;
 
 pub use config::BotConfig;
 
diff --git a/server/src/matrix/notifications.rs b/server/src/matrix/notifications.rs
index 7b32134..3ed17dc 100644
--- a/server/src/matrix/notifications.rs
+++ b/server/src/matrix/notifications.rs
@@ -6,10 +6,9 @@
 use crate::io::story_metadata::parse_front_matter;
 use crate::io::watcher::WatcherEvent;
 use crate::slog;
-use matrix_sdk::ruma::events::room::message::RoomMessageEventContent;
-use matrix_sdk::ruma::OwnedRoomId;
-use matrix_sdk::Client;
+use crate::transport::ChatTransport;
 use std::path::{Path, PathBuf};
+use std::sync::Arc;
 use tokio::sync::broadcast;
 
 /// Human-readable display name for a pipeline stage directory.
@@ -100,10 +99,11 @@ pub fn format_error_notification(
 }
 
 /// Spawn a background task that listens for watcher events and posts
-/// stage-transition notifications to all configured Matrix rooms.
+/// stage-transition notifications to all configured rooms via the
+/// [`ChatTransport`] abstraction.
 pub fn spawn_notification_listener(
-    client: Client,
-    room_ids: Vec<OwnedRoomId>,
+    transport: Arc<dyn ChatTransport>,
+    room_ids: Vec<String>,
     watcher_rx: broadcast::Receiver<WatcherEvent>,
     project_root: PathBuf,
 ) {
@@ -133,14 +133,10 @@ pub fn spawn_notification_listener(
                     slog!("[matrix-bot] Sending stage notification: {plain}");
 
                     for room_id in &room_ids {
-                        if let Some(room) = client.get_room(room_id) {
-                            let content =
-                                RoomMessageEventContent::text_html(plain.clone(), html.clone());
-                            if let Err(e) = room.send(content).await {
-                                slog!(
-                                    "[matrix-bot] Failed to send notification to {room_id}: {e}"
-                                );
-                            }
+                        if let Err(e) = transport.send_message(room_id, &plain, &html).await {
+                            slog!(
+                                "[matrix-bot] Failed to send notification to {room_id}: {e}"
+                            );
                         }
                     }
                 }
@@ -159,14 +155,10 @@ pub fn spawn_notification_listener(
                     slog!("[matrix-bot] Sending error notification: {plain}");
 
                     for room_id in &room_ids {
-                        if let Some(room) = client.get_room(room_id) {
-                            let content =
-                                RoomMessageEventContent::text_html(plain.clone(), html.clone());
-                            if let Err(e) = room.send(content).await {
-                                slog!(
-                                    "[matrix-bot] Failed to send error notification to {room_id}: {e}"
-                                );
-                            }
+                        if let Err(e) = transport.send_message(room_id, &plain, &html).await {
+                            slog!(
+                                "[matrix-bot] Failed to send error notification to {room_id}: {e}"
+                            );
                         }
                     }
                 }
diff --git a/server/src/matrix/transport_impl.rs b/server/src/matrix/transport_impl.rs
new file mode 100644
index 0000000..8e30ed1
--- /dev/null
+++ b/server/src/matrix/transport_impl.rs
@@ -0,0 +1,96 @@
+//! Matrix implementation of [`ChatTransport`].
+//!
+//! Wraps a [`matrix_sdk::Client`] and delegates message sending / editing
+//! to the Matrix SDK.
+
+use async_trait::async_trait;
+use matrix_sdk::Client;
+use matrix_sdk::ruma::OwnedRoomId;
+use matrix_sdk::ruma::events::room::message::{
+    ReplacementMetadata, RoomMessageEventContent, RoomMessageEventContentWithoutRelation,
+};
+
+use crate::transport::{ChatTransport, MessageId};
+
+/// Matrix-backed [`ChatTransport`] implementation.
+///
+/// Holds a [`Client`] and resolves room IDs at send time.
+pub struct MatrixTransport {
+    client: Client,
+}
+
+impl MatrixTransport {
+    pub fn new(client: Client) -> Self {
+        Self { client }
+    }
+}
+
+#[async_trait]
+impl ChatTransport for MatrixTransport {
+    async fn send_message(
+        &self,
+        room_id: &str,
+        plain: &str,
+        html: &str,
+    ) -> Result<MessageId, String> {
+        let room_id: OwnedRoomId = room_id
+            .parse()
+            .map_err(|e| format!("Invalid room ID '{room_id}': {e}"))?;
+        let room = self
+            .client
+            .get_room(&room_id)
+            .ok_or_else(|| format!("Room {room_id} not found in client state"))?;
+
+        let content = RoomMessageEventContent::text_html(plain.to_string(), html.to_string());
+        let resp = room
+            .send(content)
+            .await
+            .map_err(|e| format!("Matrix send error: {e}"))?;
+
+        Ok(resp.event_id.to_string())
+    }
+
+    async fn edit_message(
+        &self,
+        room_id: &str,
+        original_message_id: &str,
+        plain: &str,
+        html: &str,
+    ) -> Result<(), String> {
+        let room_id: OwnedRoomId = room_id
+            .parse()
+            .map_err(|e| format!("Invalid room ID '{room_id}': {e}"))?;
+        let room = self
+            .client
+            .get_room(&room_id)
+            .ok_or_else(|| format!("Room {room_id} not found in client state"))?;
+
+        let original_event_id = original_message_id
+            .parse()
+            .map_err(|e| format!("Invalid event ID '{original_message_id}': {e}"))?;
+
+        let new_content =
+            RoomMessageEventContentWithoutRelation::text_html(plain.to_string(), html.to_string());
+        let metadata = ReplacementMetadata::new(original_event_id, None);
+        let content = new_content.make_replacement(metadata);
+
+        room.send(content)
+            .await
+            .map(|_| ())
+            .map_err(|e| format!("Matrix edit error: {e}"))
+    }
+
+    async fn send_typing(&self, room_id: &str, typing: bool) -> Result<(), String> {
+        let room_id: OwnedRoomId = room_id
+            .parse()
+            .map_err(|e| format!("Invalid room ID '{room_id}': {e}"))?;
+        let room = self
+            .client
+            .get_room(&room_id)
+            .ok_or_else(|| format!("Room {room_id} not found in client state"))?;
+
+        room.typing_notice(typing)
+            .await
+            .map_err(|e| format!("Matrix typing indicator error: {e}"))
+    }
+}
diff --git a/server/src/transport.rs b/server/src/transport.rs
new file mode 100644
index 0000000..169035b
--- /dev/null
+++ b/server/src/transport.rs
@@ -0,0 +1,91 @@
+//! Transport abstraction for chat platforms.
+//!
+//! The [`ChatTransport`] trait defines a platform-agnostic interface for
+//! sending and editing messages, allowing the bot logic (commands, htop,
+//! notifications) to work against any chat platform — Matrix, WhatsApp, etc.
+
+use async_trait::async_trait;
+
+/// A platform-agnostic identifier for a sent message.
+///
+/// On Matrix this is the event ID; on other platforms it may be a message ID
+/// or similar opaque string.  The transport implementation is responsible for
+/// producing and consuming these identifiers.
+pub type MessageId = String;
+
+/// A platform-agnostic identifier for a chat room / channel / conversation.
+pub type RoomId = String;
+
+/// Abstraction over a chat platform's message-sending capabilities.
+///
+/// Implementations must be `Send + Sync` so they can be shared across
+/// async tasks via `Arc<dyn ChatTransport>`.
+#[async_trait]
+pub trait ChatTransport: Send + Sync {
+    /// Send a plain-text + HTML message to a room.
+    ///
+    /// Returns the platform-specific message ID on success so it can be
+    /// referenced later (e.g. for edits or reply detection).
+    async fn send_message(
+        &self,
+        room_id: &str,
+        plain: &str,
+        html: &str,
+    ) -> Result<MessageId, String>;
+
+    /// Edit a previously sent message.
+    ///
+    /// `original_message_id` is the [`MessageId`] returned by a prior
+    /// [`send_message`](ChatTransport::send_message) call.
+    ///
+    /// Platforms that do not support editing (e.g. WhatsApp) should send a
+    /// new message instead.
+    async fn edit_message(
+        &self,
+        room_id: &str,
+        original_message_id: &str,
+        plain: &str,
+        html: &str,
+    ) -> Result<(), String>;
+
+    /// Signal that the bot is typing (or has stopped typing) in a room.
+    ///
+    /// Platforms that do not support typing indicators should no-op.
+    async fn send_typing(&self, room_id: &str, typing: bool) -> Result<(), String>;
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::sync::Arc;
+
+    /// Verify that the WhatsApp stub satisfies the ChatTransport trait and
+    /// can be used as `Arc<dyn ChatTransport>`.
+    #[tokio::test]
+    async fn whatsapp_transport_satisfies_trait() {
+        let transport: Arc<dyn ChatTransport> =
+            Arc::new(crate::whatsapp::WhatsAppTransport::new());
+
+        let msg_id = transport
+            .send_message("room-1", "hello", "<p>hello</p>")
+            .await
+            .unwrap();
+        assert!(!msg_id.is_empty());
+
+        transport
+            .edit_message("room-1", &msg_id, "edited", "<p>edited</p>")
+            .await
+            .unwrap();
+
+        transport.send_typing("room-1", true).await.unwrap();
+        transport.send_typing("room-1", false).await.unwrap();
+    }
+
+    /// MatrixTransport cannot be tested without a live homeserver, but we
+    /// can verify the type implements the trait at compile time.
+    #[test]
+    fn matrix_transport_is_send_sync() {
+        fn assert_send_sync<T: Send + Sync>() {}
+        assert_send_sync::<crate::matrix::transport_impl::MatrixTransport>();
+    }
+}
diff --git a/server/src/whatsapp.rs b/server/src/whatsapp.rs
new file mode 100644
index 0000000..b535a48
--- /dev/null
+++ b/server/src/whatsapp.rs
@@ -0,0 +1,117 @@
+//! WhatsApp stub implementation of [`ChatTransport`].
+//!
+//! This is a placeholder transport that logs operations and returns stub
+//! values.  It exists to prove the transport abstraction works with a
+//! second platform and will be replaced with a real WhatsApp Business API
+//! integration in the future.
+
+use async_trait::async_trait;
+
+use crate::slog;
+use crate::transport::{ChatTransport, MessageId};
+
+/// Stub WhatsApp transport.
+///
+/// All methods log the operation and return success with placeholder values.
+/// Message editing is not supported by WhatsApp — `edit_message` sends a
+/// new message instead (TODO: implement via WhatsApp Business API).
+pub struct WhatsAppTransport {
+    /// Counter for generating unique stub message IDs.
+    next_id: std::sync::atomic::AtomicU64,
+}
+
+impl WhatsAppTransport {
+    pub fn new() -> Self {
+        Self {
+            next_id: std::sync::atomic::AtomicU64::new(1),
+        }
+    }
+
+    fn next_message_id(&self) -> String {
+        let id = self
+            .next_id
+            .fetch_add(1, std::sync::atomic::Ordering::Relaxed);
+        format!("whatsapp-stub-{id}")
+    }
+}
+
+impl Default for WhatsAppTransport {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+#[async_trait]
+impl ChatTransport for WhatsAppTransport {
+    async fn send_message(
+        &self,
+        room_id: &str,
+        plain: &str,
+        _html: &str,
+    ) -> Result<MessageId, String> {
+        // TODO: Send via WhatsApp Business API
+        let msg_id = self.next_message_id();
+        slog!(
+            "[whatsapp-stub] send_message to {room_id}: {plain:.80} (id={msg_id})"
+        );
+        Ok(msg_id)
+    }
+
+    async fn edit_message(
+        &self,
+        room_id: &str,
+        original_message_id: &str,
+        plain: &str,
+        html: &str,
+    ) -> Result<(), String> {
+        // WhatsApp does not support message editing.
+        // Send a new message instead.
+        slog!(
+            "[whatsapp-stub] edit_message (original={original_message_id}) — \
+             WhatsApp does not support edits, sending new message"
+        );
+        self.send_message(room_id, plain, html).await.map(|_| ())
+    }
+
+    async fn send_typing(&self, room_id: &str, typing: bool) -> Result<(), String> {
+        // TODO: Send typing indicator via WhatsApp Business API
+        slog!("[whatsapp-stub] send_typing to {room_id}: typing={typing}");
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[tokio::test]
+    async fn send_message_returns_unique_ids() {
+        let transport = WhatsAppTransport::new();
+        let id1 = transport
+            .send_message("room1", "hello", "<p>hello</p>")
+            .await
+            .unwrap();
+        let id2 = transport
+            .send_message("room1", "world", "<p>world</p>")
+            .await
+            .unwrap();
+        assert_ne!(id1, id2, "each message should get a unique ID");
+        assert!(id1.starts_with("whatsapp-stub-"));
+    }
+
+    #[tokio::test]
+    async fn edit_message_succeeds() {
+        let transport = WhatsAppTransport::new();
+        let result = transport
+            .edit_message("room1", "msg-1", "updated", "<p>updated</p>")
+            .await;
+        assert!(result.is_ok(), "edit should succeed (sends new message)");
+    }
+
+    #[tokio::test]
+    async fn send_typing_succeeds() {
+        let transport = WhatsAppTransport::new();
+        assert!(transport.send_typing("room1", true).await.is_ok());
+        assert!(transport.send_typing("room1", false).await.is_ok());
+    }
+}