diff --git a/src/jmap/http.rs b/src/jmap/http.rs
index 4623c46..3c9e614 100644
--- a/src/jmap/http.rs
+++ b/src/jmap/http.rs
@@ -97,6 +97,10 @@ impl Inner {
 #[derive(Debug, Clone, Copy)]
 enum Kind {
     Api,
+    /// An API call that must not be sent twice: a write the server may
+    /// already have applied when the connection failed. A transport error is
+    /// returned to the caller, which checks the target instead of resending.
+    ApiOnce,
     Upload,
 }
 
@@ -220,6 +224,23 @@ impl HttpClient {
             .map_err(|e| JmapError::Malformed(format!("response is not valid json: {e}")))
     }
 
+    /// As `post_json`, but a transport failure is not retried: the request
+    /// may have reached the server, and sending it again could apply it twice.
+    /// Throttling and `503` answers, which mean it was not processed, are still
+    /// retried.
+    pub fn post_json_once(&self, url: &str, body: &Value) -> Result<Value, JmapError> {
+        let payload = serde_json::to_vec(body)?;
+        let raw = self.execute(
+            Kind::ApiOnce,
+            "POST",
+            url,
+            Some(&payload),
+            Some("application/json"),
+        )?;
+        serde_json::from_slice(&raw)
+            .map_err(|e| JmapError::Malformed(format!("response is not valid json: {e}")))
+    }
+
     pub fn upload(
         &self,
         upload_url: &str,
@@ -298,6 +319,16 @@ impl HttpClient {
                                 body: truncate(&body),
                             });
                         }
+                        // A gateway error may come back after the server
+                        // behind it applied the write: not safe to resend.
+                        StatusOutcome::Retryable
+                            if matches!(kind, Kind::ApiOnce) && matches!(status, 502 | 504) =>
+                        {
+                            return Err(JmapError::HttpStatus {
+                                status,
+                                body: truncate(&body),
+                            });
+                        }
                         StatusOutcome::Retryable => {
                             attempt += 1;
                             self.inner.retries_total.fetch_add(1, Ordering::Relaxed);
@@ -344,6 +375,7 @@ impl HttpClient {
                         }
                     }
                 }
+                Attempt::Transport(err) if matches!(kind, Kind::ApiOnce) => return Err(err),
                 Attempt::Transport(err) => match transport_disposition(&err) {
                     Disposition::Fatal => return Err(err),
                     Disposition::Retryable => {
@@ -659,6 +691,90 @@ mod tests {
     use super::*;
     use crate::net::CertOverride;
 
+    /// A server that reads each request and hangs up without answering: a
+    /// transport failure after the request has been sent. Returns its URL and
+    /// the count of requests it has seen.
+    fn hang_up_server() -> (String, Arc<AtomicU64>) {
+        use std::io::Read;
+        let listener = std::net::TcpListener::bind("127.0.0.1:0").unwrap();
+        let url = format!("http://{}/jmap/api", listener.local_addr().unwrap());
+        let seen = Arc::new(AtomicU64::new(0));
+        let counter = seen.clone();
+        std::thread::spawn(move || {
+            for stream in listener.incoming() {
+                let Ok(mut stream) = stream else { break };
+                let mut buf = [0u8; 4096];
+                let _ = stream.read(&mut buf);
+                counter.fetch_add(1, Ordering::SeqCst);
+            }
+        });
+        (url, seen)
+    }
+
+    fn quick_retries(max_retries: u32) -> RetryPolicy {
+        RetryPolicy {
+            max_retries,
+            base: Duration::from_millis(1),
+            cap: Duration::from_millis(2),
+        }
+    }
+
+    #[test]
+    fn a_write_sent_once_is_not_resent_after_a_transport_failure() {
+        let (url, seen) = hang_up_server();
+        let client = HttpClient::new(
+            Auth::Bearer { token: "t".into() },
+            quick_retries(2),
+            CertOverride::none(),
+        );
+        let err = client
+            .post_json_once(&url, &serde_json::json!({}))
+            .unwrap_err();
+        assert!(matches!(err, JmapError::Transport(_)), "{err}");
+        assert_eq!(seen.load(Ordering::SeqCst), 1, "sent exactly once");
+
+        let (url, seen) = hang_up_server();
+        let _ = client.post_json(&url, &serde_json::json!({}));
+        assert_eq!(
+            seen.load(Ordering::SeqCst),
+            3,
+            "an ordinary call retries twice"
+        );
+    }
+
+    #[test]
+    fn a_write_sent_once_is_not_resent_after_a_gateway_timeout_but_is_after_503() {
+        let mut server = mockito::Server::new();
+        let url = format!("{}/jmap/api", server.url());
+        let client = HttpClient::new(
+            Auth::Bearer { token: "t".into() },
+            quick_retries(2),
+            CertOverride::none(),
+        );
+        let gateway = server
+            .mock("POST", "/jmap/api")
+            .with_status(504)
+            .expect(1)
+            .create();
+        let err = client
+            .post_json_once(&url, &serde_json::json!({}))
+            .unwrap_err();
+        assert!(
+            matches!(err, JmapError::HttpStatus { status: 504, .. }),
+            "{err}"
+        );
+        gateway.assert();
+        gateway.remove();
+
+        let busy = server
+            .mock("POST", "/jmap/api")
+            .with_status(503)
+            .expect(3)
+            .create();
+        let _ = client.post_json_once(&url, &serde_json::json!({}));
+        busy.assert();
+    }
+
     #[test]
     fn invalid_certificates_are_accepted_only_for_the_named_host() {
         let client = HttpClient::new(
diff --git a/src/jmap/request.rs b/src/jmap/request.rs
index 7a6b4cc..9c1265f 100644
--- a/src/jmap/request.rs
+++ b/src/jmap/request.rs
@@ -131,6 +131,14 @@ impl Request {
         let value = client.post_json(api_url, &self.envelope()?)?;
         Response::parse(value)
     }
+
+    /// As `send`, for a write that must not be applied twice: a transport
+    /// failure comes back as an error instead of being resent (see
+    /// `HttpClient::post_json_once`).
+    pub fn send_once(&self, client: &HttpClient, api_url: &str) -> Result<Response, JmapError> {
+        let value = client.post_json_once(api_url, &self.envelope()?)?;
+        Response::parse(value)
+    }
 }
 
 #[derive(Debug)]