diff --git a/src/uplink/webrtc.rs b/src/uplink/webrtc.rs
index 396f891..583c082 100644
--- a/src/uplink/webrtc.rs
+++ b/src/uplink/webrtc.rs
@@ -62,6 +62,29 @@ fn checked_stream_name(name: &str) -> anyhow::Result<&str> {
     Ok(name)
 }
 
+/// Flattens an error's `source` chain into one line.
+///
+/// Needed because reqwest's own `Display` is just "error sending request for
+/// url (...)" - the reason lives one or two sources down, and "connection
+/// refused" versus "timed out" is the whole content of the message. This
+/// error travels to the cloud and then to a viewer, so it is the only
+/// explanation anyone gets.
+fn with_causes(err: &(dyn std::error::Error + 'static)) -> String {
+    let mut out = err.to_string();
+    let mut source = err.source();
+    while let Some(cause) = source {
+        let text = cause.to_string();
+        // Skip a cause that only repeats what we already said, which hyper's
+        // wrappers do.
+        if !out.contains(&text) {
+            out.push_str(": ");
+            out.push_str(&text);
+        }
+        source = cause.source();
+    }
+    out
+}
+
 /// Posts the browser's offer to go2rtc and returns its answer SDP.
 pub async fn negotiate(api_listen: &str, camera_stream: &str, offer_sdp: &str) -> anyhow::Result<String> {
     let base = local_api_base(api_listen)?;
@@ -73,7 +96,13 @@ pub async fn negotiate(api_listen: &str, camera_stream: &str, offer_sdp: &str) -
         .body(offer_sdp.to_string())
         .timeout(NEGOTIATION_TIMEOUT)
         .send()
-        .await?;
+        .await
+        .map_err(|err| {
+            // Names go2rtc explicitly. The most likely cause by far is that
+            // it is not running, and an error mentioning only a port sends
+            // whoever reads it looking at the network.
+            anyhow::anyhow!("could not reach go2rtc at {base}: {}", with_causes(&err))
+        })?;
 
     // go2rtc answers 201 rather than 200 here, so this checks the class and
     // not a single code.