diff --git a/src/sync/uploader.rs b/src/sync/uploader.rs
index cfbac09..bc110da 100644
--- a/src/sync/uploader.rs
+++ b/src/sync/uploader.rs
@@ -22,6 +22,11 @@ fn convex_site_url() -> String {
         .unwrap_or_else(|_| "https://ceaseless-shepherd-756.convex.site".to_string())
 }
 
+/// Optional staging Convex URL for dual-upload mirroring.
+fn staging_convex_site_url() -> Option<String> {
+    std::env::var("THREADER_STAGING_CONVEX_SITE_URL").ok()
+}
+
 /// Background worker that processes the upload queue.
 pub struct BackgroundUploader {
     queue: UploadQueue,
@@ -80,12 +85,22 @@ impl BackgroundUploader {
                 continue;
             }
 
-            match self.upload(&entry, &token).await {
+            let prod_url = convex_site_url();
+            match self.upload(&entry, &token, &prod_url).await {
                 Ok(()) => {
                     debug!(
                         "Uploaded {:?} for session {}",
                         entry.action, entry.session_id
                     );
+                    // Best-effort staging mirror
+                    if let Some(ref staging_url) = staging_convex_site_url() {
+                        if let Err(e) = self.upload(&entry, &token, staging_url).await {
+                            warn!(
+                                "Staging mirror failed for session {} (non-blocking): {}",
+                                entry.session_id, e
+                            );
+                        }
+                    }
                     self.queue.remove(&path)?;
                 }
                 Err(e) => {
@@ -124,23 +139,23 @@ impl BackgroundUploader {
     }
 
     /// Upload a single queue entry to the Convex HTTP endpoint.
-    async fn upload(&self, entry: &QueueEntry, token: &str) -> Result<()> {
+    async fn upload(&self, entry: &QueueEntry, token: &str, base_url: &str) -> Result<()> {
         match entry.action {
-            QueueAction::Create => self.upload_create(entry, token).await,
+            QueueAction::Create => self.upload_create(entry, token, base_url).await,
             QueueAction::Append => {
                 // Ensure session exists on server (idempotent create)
-                self.upload_create(entry, token).await?;
-                self.upload_append(entry, token).await
+                self.upload_create(entry, token, base_url).await?;
+                self.upload_append(entry, token, base_url).await
             }
             QueueAction::Finalize => {
                 // Ensure session exists on server (idempotent create)
-                self.upload_create(entry, token).await?;
-                self.upload_finalize(entry, token).await
+                self.upload_create(entry, token, base_url).await?;
+                self.upload_finalize(entry, token, base_url).await
             }
         }
     }
 
-    async fn upload_create(&self, entry: &QueueEntry, token: &str) -> Result<()> {
+    async fn upload_create(&self, entry: &QueueEntry, token: &str, base_url: &str) -> Result<()> {
         let meta = self.storage.read_meta(&entry.session_id)?;
 
         // Build body, omitting None fields (Convex rejects null for optional string fields)
@@ -161,7 +176,7 @@ impl BackgroundUploader {
             body["repo"] = serde_json::json!(repo);
         }
 
-        let url = format!("{}/api/sessions", convex_site_url());
+        let url = format!("{}/api/sessions", base_url);
         info!("Creating session {} at {}", entry.session_id, url);
 
         let resp = self
@@ -182,7 +197,7 @@ impl BackgroundUploader {
         Ok(())
     }
 
-    async fn upload_append(&self, entry: &QueueEntry, token: &str) -> Result<()> {
+    async fn upload_append(&self, entry: &QueueEntry, token: &str, base_url: &str) -> Result<()> {
         // Read from the source transcript (Claude Code's file), not the local copy.
         // The local copy can diverge due to concurrent poller + hook appends.
         let meta = self.storage.read_meta(&entry.session_id)?;
@@ -204,7 +219,7 @@ impl BackgroundUploader {
 
         // Process lines through image processor — rewrite image blocks with URLs
         let image_processor =
-            ImageProcessor::new(self.client.clone(), convex_site_url());
+            ImageProcessor::new(self.client.clone(), base_url.to_string());
         let mut processed_lines: Vec<String> = Vec::with_capacity(lines_to_send.len());
         for line in &lines_to_send {
             if line_has_images(line) {
@@ -244,7 +259,7 @@ impl BackgroundUploader {
             .client
             .post(format!(
                 "{}/api/sessions/{}/append",
-                convex_site_url(), entry.session_id
+                base_url, entry.session_id
             ))
             .bearer_auth(token)
             .json(&body)
@@ -264,7 +279,7 @@ impl BackgroundUploader {
         Ok(())
     }
 
-    async fn upload_finalize(&self, entry: &QueueEntry, token: &str) -> Result<()> {
+    async fn upload_finalize(&self, entry: &QueueEntry, token: &str, base_url: &str) -> Result<()> {
         let meta = self.storage.read_meta(&entry.session_id)?;
 
         let mut body = serde_json::json!({});
@@ -294,7 +309,7 @@ impl BackgroundUploader {
             .client
             .post(format!(
                 "{}/api/sessions/{}/finalize",
-                convex_site_url(), entry.session_id
+                base_url, entry.session_id
             ))
             .bearer_auth(token)
             .json(&body)