test: regression tests for the paid-download fixes

- mint_client: a stub mint shows swap() sends the full v2 keyset id when
  given a cashuB short id, and leaves complete v1/v2 ids unchanged.
- fips::dial: the single-delivery decisions are now small functions
  (fips_answer_is_final, fips_retryable). Tests cover them and, against a
  silent local peer, check that a single-delivery request isn't resent
  after a timeout while an ordinary one still is.
- content_server: an unreadable paid file returns Unavailable before the
  payment gate runs, and a readable one still returns 402. Also covers
  ensure_readable's grant/reopen behaviour. The podman grant is replaced
  by a refusal under cfg(test) so results don't depend on the host.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

core/archipelago/src/api/handler/content.rs

@@ -162,11 +162,28 @@ impl ApiHandler {
                     r#"{"error":"This file is shared with the host's federation peers only. Federate with that node (exchange invites) so it recognizes you, then try again."}"#,
                 ),
             )),
-            Ok(content_server::ServeResult::NotFound) | Err(_) => Ok(build_response(
+            Ok(content_server::ServeResult::Unavailable) => Ok(build_response(
+                StatusCode::SERVICE_UNAVAILABLE,
+                "application/json",
+                hyper::Body::from(
+                    r#"{"error":"The seller's node can't read this file right now. No payment was taken."}"#,
+                ),
+            )),
+            Ok(content_server::ServeResult::NotFound) => Ok(build_response(
                 StatusCode::NOT_FOUND,
                 "text/plain",
                 hyper::Body::from("Content not found"),
             )),
+            // Not a 404: a paid request may already have been charged by the
+            // time this fails, and "not found" hid the real error entirely.
+            Err(e) => {
+                tracing::error!("Serving content {content_id} failed: {e:#}");
+                Ok(build_response(
+                    StatusCode::INTERNAL_SERVER_ERROR,
+                    "text/plain",
+                    hyper::Body::from("Failed to serve content"),
+                ))
+            }
         }
     }
 

core/archipelago/src/api/rpc/content.rs

@@ -555,6 +555,9 @@ impl RpcHandler {
         .service(crate::settings::transport::PeerService::PeerFiles)
         .header("X-Federation-DID", local_did)
         .header("X-Payment-Token", token_str.clone())
+        // The token is a bearer instrument the seller redeems on first sight:
+        // a Tor replay after FIPS delivered it can only arrive spent.
+        .single_delivery()
         .timeout(std::time::Duration::from_secs(900))
         .send_get()
         .await
@@ -610,8 +613,12 @@ impl RpcHandler {
             let body = response.text().await.unwrap_or_default();
             tracing::warn!("paid download: seller {onion} returned {status}: {body}");
             reclaim_spent_ecash(&self.config.data_dir, &token_str, used_backend).await;
+            let reason = serde_json::from_str::<serde_json::Value>(&body)
+                .ok()
+                .and_then(|v| v.get("error").and_then(|e| e.as_str()).map(str::to_string))
+                .unwrap_or_else(|| format!("Peer returned an error ({status})."));
             return Ok(serde_json::json!({
-                "error": format!("Peer returned an error ({status}). Your ecash was refunded to your wallet.")
+                "error": format!("{reason} Your ecash was refunded to your wallet.")
             }));
         }
 

core/archipelago/src/content_server.rs

@@ -7,7 +7,7 @@ use anyhow::{Context, Result};
 use serde::{Deserialize, Serialize};
 use std::path::{Path, PathBuf};
 use tokio::fs;
-use tracing::{debug, warn};
+use tracing::{debug, info, warn};
 
 const CATALOG_FILE: &str = "content/catalog.json";
 const CONTENT_DIR: &str = "content/files";
@@ -238,6 +238,9 @@ pub enum ServeResult {
     Forbidden,
     /// Content not found.
     NotFound,
+    /// The catalog entry and file exist but this node can't read the file.
+    /// Returned before any payment is taken.
+    Unavailable,
 }
 
 /// Serve a content item by ID with access control and optional range request.
@@ -296,6 +299,37 @@ pub async fn serve_content(
         }
     }
 
+    let file_path = content_file_path(data_dir, item);
+    if !file_path.exists() {
+        // The catalog entry survived (it's a separate JSON file) but its
+        // backing file is gone — most likely lost in an unrelated data-dir
+        // reset (a shared filebrowser file, 2026-07-01: two catalog entries
+        // outlived a filebrowser reinstall that wiped the files themselves).
+        // Leaving the entry in place would keep advertising it as available
+        // to every peer forever, each hitting the exact same dead end this
+        // one just did. Prune it so it stops being offered.
+        warn!(
+            content_id = %id,
+            filename = %item.filename,
+            "content catalog entry's file is missing on disk — pruning the stale entry"
+        );
+        prune_missing_content_entry(data_dir, id).await;
+        return Ok(ServeResult::NotFound);
+    }
+
+    // Confirm the file is readable BEFORE the paid gate below redeems the
+    // buyer's token. Reading it only afterwards meant a permission error
+    // surfaced after the sale: the buyer was charged and got an error
+    // instead of the file (2026-09-29, a FileBrowser upload left 0640).
+    if let Err(e) = ensure_readable(&file_path).await {
+        warn!(
+            content_id = %id,
+            path = %file_path.display(),
+            "shared content file is not readable by this node: {e:#}"
+        );
+        return Ok(ServeResult::Unavailable);
+    }
+
     // Check access control
     if !owner_session {
         match &item.access {
@@ -336,23 +370,6 @@ pub async fn serve_content(
         }
     }
 
-    let file_path = content_file_path(data_dir, item);
-    if !file_path.exists() {
-        // The catalog entry survived (it's a separate JSON file) but its
-        // backing file is gone — most likely lost in an unrelated data-dir
-        // reset (a shared filebrowser file, 2026-07-01: two catalog entries
-        // outlived a filebrowser reinstall that wiped the files themselves).
-        // Leaving the entry in place would keep advertising it as available
-        // to every peer forever, each hitting the exact same dead end this
-        // one just did. Prune it so it stops being offered.
-        warn!(
-            content_id = %id,
-            filename = %item.filename,
-            "content catalog entry's file is missing on disk — pruning the stale entry"
-        );
-        prune_missing_content_entry(data_dir, id).await;
-        return Ok(ServeResult::NotFound);
-    }
 
     let metadata = fs::metadata(&file_path)
         .await
@@ -572,6 +589,62 @@ pub async fn serve_content_preview(data_dir: &Path, id: &str) -> Result<PreviewR
     }
 }
 
+/// Make sure this service can open `path`, granting read access if it can't.
+///
+/// FileBrowser writes uploads as its container user (a rootless subuid such
+/// as 100999), and some arrive 0640 — unreadable by this service, although
+/// most shared files are already 0644. Inside the rootless user namespace
+/// that subuid is ours, so `podman unshare chmod a+r` grants the same read
+/// access the other shared files have, without sudo.
+async fn ensure_readable(path: &Path) -> Result<()> {
+    ensure_readable_with(path, grant_read_access).await
+}
+
+async fn ensure_readable_with<F, Fut>(path: &Path, grant: F) -> Result<()>
+where
+    F: FnOnce(PathBuf) -> Fut,
+    Fut: std::future::Future<Output = Result<()>>,
+{
+    match fs::File::open(path).await {
+        Ok(_) => return Ok(()),
+        Err(e) if e.kind() == std::io::ErrorKind::PermissionDenied => {}
+        Err(e) => return Err(e).context("Failed to open content file"),
+    }
+    grant(path.to_path_buf()).await?;
+    info!("Granted read access to shared content file {}", path.display());
+    fs::File::open(path)
+        .await
+        .context("Content file still unreadable after chmod")?;
+    Ok(())
+}
+
+// Tests must not shell out to podman: whether it exists (and can chmod a
+// file the test user owns) would decide the outcome.
+#[cfg(not(test))]
+use grant_read_via_podman as grant_read_access;
+
+#[cfg(test)]
+async fn grant_read_access(_path: PathBuf) -> Result<()> {
+    anyhow::bail!("granting read access is disabled in tests")
+}
+
+#[cfg_attr(test, allow(dead_code))]
+async fn grant_read_via_podman(path: PathBuf) -> Result<()> {
+    let out = tokio::process::Command::new("podman")
+        .args(["unshare", "chmod", "a+r"])
+        .arg(&path)
+        .output()
+        .await
+        .context("Failed to run podman unshare chmod")?;
+    if !out.status.success() {
+        anyhow::bail!(
+            "podman unshare chmod a+r failed: {}",
+            String::from_utf8_lossy(&out.stderr).trim()
+        );
+    }
+    Ok(())
+}
+
 /// Verify a payment token covers the required amount.
 /// Accepts both cashuA tokens (real Cashu) and legacy cashuSend_ format.
 /// Swaps proofs at the mint to verify they're unspent before accepting.
@@ -725,3 +798,137 @@ mod prune_missing_content_tests {
         assert_eq!(reloaded.items[0].id, "present-item");
     }
 }
+
+#[cfg(test)]
+mod unreadable_content_tests {
+    use super::*;
+    use std::os::unix::fs::PermissionsExt;
+
+    /// Writes `bytes` to the FileBrowser area and makes it unreadable, the
+    /// way a 0640 upload owned by a container subuid looks to this service.
+    /// `None` when the test runs as root, where mode bits don't stop reads.
+    fn unreadable_file(data_dir: &Path, name: &str) -> Option<PathBuf> {
+        let dir = data_dir.join("filebrowser").join("Music");
+        std::fs::create_dir_all(&dir).unwrap();
+        let path = dir.join(name);
+        std::fs::write(&path, b"audio").unwrap();
+        std::fs::set_permissions(&path, std::fs::Permissions::from_mode(0o000)).unwrap();
+        std::fs::File::open(&path).is_err().then_some(path)
+    }
+
+    fn paid_item(filename: &str) -> ContentItem {
+        ContentItem {
+            id: "paid-item".to_string(),
+            filename: filename.to_string(),
+            mime_type: "audio/mpeg".to_string(),
+            size_bytes: 5,
+            description: String::new(),
+            access: AccessControl::Paid {
+                price_sats: 10,
+                accepted: vec!["ecash".to_string()],
+            },
+            availability: Availability::AllPeers,
+            added_at: "2026-01-01T00:00:00Z".to_string(),
+        }
+    }
+
+    /// Regression (2026-09-29): the seller redeemed the buyer's token and
+    /// only then failed to read the file, so the buyer paid for nothing.
+    /// An unreadable file must be refused before the payment gate runs,
+    /// which is why a token that would never verify still gets Unavailable
+    /// rather than PaymentRequired.
+    #[tokio::test]
+    async fn an_unreadable_paid_file_is_refused_before_any_payment_is_taken() {
+        let dir = tempfile::tempdir().unwrap();
+        let data_dir = dir.path();
+        let Some(_path) = unreadable_file(data_dir, "song.mp3") else {
+            return; // running as root
+        };
+        save_catalog(
+            data_dir,
+            &ContentCatalog {
+                items: vec![paid_item("Music/song.mp3")],
+            },
+        )
+        .await
+        .unwrap();
+
+        let result = serve_content(
+            data_dir,
+            "paid-item",
+            Some("cashuBnot-a-real-token"),
+            None,
+            None,
+            None,
+            false,
+        )
+        .await
+        .unwrap();
+        assert!(matches!(result, ServeResult::Unavailable));
+        // An unreadable file is not a missing one: keep the catalog entry.
+        assert_eq!(load_catalog(data_dir).await.unwrap().items.len(), 1);
+    }
+
+    #[tokio::test]
+    async fn a_readable_paid_file_still_demands_payment() {
+        let dir = tempfile::tempdir().unwrap();
+        let data_dir = dir.path();
+        let music = data_dir.join("filebrowser").join("Music");
+        std::fs::create_dir_all(&music).unwrap();
+        std::fs::write(music.join("song.mp3"), b"audio").unwrap();
+        save_catalog(
+            data_dir,
+            &ContentCatalog {
+                items: vec![paid_item("Music/song.mp3")],
+            },
+        )
+        .await
+        .unwrap();
+
+        let result = serve_content(data_dir, "paid-item", None, None, None, None, false)
+            .await
+            .unwrap();
+        assert!(matches!(result, ServeResult::PaymentRequired(10)));
+    }
+
+    #[tokio::test]
+    async fn ensure_readable_grants_access_once_then_reopens() {
+        let dir = tempfile::tempdir().unwrap();
+        let Some(path) = unreadable_file(dir.path(), "a.mp3") else {
+            return;
+        };
+        let calls = std::sync::atomic::AtomicUsize::new(0);
+        ensure_readable_with(&path, |p| {
+            calls.fetch_add(1, std::sync::atomic::Ordering::SeqCst);
+            async move {
+                std::fs::set_permissions(&p, std::fs::Permissions::from_mode(0o644))?;
+                Ok(())
+            }
+        })
+        .await
+        .unwrap();
+        assert_eq!(calls.load(std::sync::atomic::Ordering::SeqCst), 1);
+    }
+
+    #[tokio::test]
+    async fn ensure_readable_leaves_a_readable_file_alone() {
+        let dir = tempfile::tempdir().unwrap();
+        let path = dir.path().join("ok.mp3");
+        std::fs::write(&path, b"x").unwrap();
+        ensure_readable_with(&path, |_| async { anyhow::bail!("must not grant") })
+            .await
+            .unwrap();
+    }
+
+    #[tokio::test]
+    async fn ensure_readable_reports_a_failed_grant() {
+        let dir = tempfile::tempdir().unwrap();
+        let Some(path) = unreadable_file(dir.path(), "b.mp3") else {
+            return;
+        };
+        let err = ensure_readable_with(&path, |_| async { anyhow::bail!("no podman") })
+            .await
+            .unwrap_err();
+        assert!(err.to_string().contains("no podman"));
+    }
+}

core/archipelago/src/fips/dial.rs

@@ -46,6 +46,25 @@ fn fips_should_fall_back(status: reqwest::StatusCode) -> bool {
     status == reqwest::StatusCode::NOT_FOUND || status.is_server_error()
 }
 
+/// Is this FIPS answer the final one, or should the request go again over
+/// Tor? A single-delivery request already reached the peer, so any answer
+/// is final: a Tor replay would carry the same (possibly spent) payload.
+fn fips_answer_is_final(
+    pref: crate::settings::transport::TransportPref,
+    single_delivery: bool,
+    status: reqwest::StatusCode,
+) -> bool {
+    pref == crate::settings::transport::TransportPref::Fips
+        || single_delivery
+        || !fips_should_fall_back(status)
+}
+
+/// May a failed FIPS attempt be sent again? Only a failed connect proves the
+/// peer never saw it; a timeout can land after the request was delivered.
+fn fips_retryable(single_delivery: bool, e: &reqwest::Error) -> bool {
+    e.is_connect() || (!single_delivery && e.is_timeout())
+}
+
 /// DNS suffix appended to a peer's bech32 npub.
 pub const FIPS_DNS_SUFFIX: &str = "fips";
 
@@ -130,10 +149,18 @@ pub fn client_with_timeout(timeout: Duration) -> reqwest::Client {
 /// robust". Only connect/timeout errors are retried (a real HTTP response,
 /// including 4xx/5xx, is returned as-is for the caller to interpret).
 async fn send_with_retry(rb: reqwest::RequestBuilder) -> Result<reqwest::Response, reqwest::Error> {
+    send_with_retry_if(rb, |e| e.is_connect() || e.is_timeout()).await
+}
+
+/// [`send_with_retry`], retrying only on errors `retryable` accepts.
+async fn send_with_retry_if(
+    rb: reqwest::RequestBuilder,
+    retryable: impl Fn(&reqwest::Error) -> bool,
+) -> Result<reqwest::Response, reqwest::Error> {
     let retry = rb.try_clone();
     match rb.send().await {
         Ok(resp) => Ok(resp),
-        Err(e) if (e.is_connect() || e.is_timeout()) && retry.is_some() => {
+        Err(e) if retryable(&e) && retry.is_some() => {
             // Brief pause so the hole-punch packets from the first attempt can
             // traverse before we re-dial onto the warmed path.
             tokio::time::sleep(Duration::from_millis(600)).await;
@@ -350,6 +377,9 @@ pub struct PeerRequest<'a> {
     /// the per-peer FIPS/Tor badge reflects reality. Opt-in because not
     /// every caller has a data dir in scope.
     pub record_data_dir: Option<std::path::PathBuf>,
+    /// The request carries something that must reach the peer at most once
+    /// (a bearer ecash token). See [`PeerRequest::single_delivery`].
+    pub single_delivery: bool,
 }
 
 impl<'a> PeerRequest<'a> {
@@ -363,9 +393,25 @@ impl<'a> PeerRequest<'a> {
             fips_timeout: None,
             service: None,
             record_data_dir: None,
+            single_delivery: false,
         }
     }
 
+    /// Never send this request twice. A paid download carries a bearer ecash
+    /// token that the seller redeems on first sight; replaying it over Tor
+    /// after FIPS already delivered it hands the seller a spent token, so the
+    /// buyer is charged and gets a 402 instead of the file (2026-09-29: FIPS
+    /// answered 404 after the seller redeemed, the Tor retry got 402).
+    ///
+    /// With this set, whatever FIPS answers is final, the FIPS retry fires
+    /// only when the first attempt never connected, and Tor is used only when
+    /// FIPS could not have delivered the request. An attempt that may have
+    /// been delivered but timed out is an error, not a fallback.
+    pub fn single_delivery(mut self) -> Self {
+        self.single_delivery = true;
+        self
+    }
+
     /// Record the transport that serves this request into federation storage
     /// (matched by this request's onion host). Best-effort, off the hot path.
     pub fn record_transport(mut self, data_dir: impl Into<std::path::PathBuf>) -> Self {
@@ -481,7 +527,7 @@ impl<'a> PeerRequest<'a> {
         if matches!(pref, TransportPref::Auto | TransportPref::Fips) {
             match self.try_fips_get().await? {
                 Some(resp) => {
-                    if pref == TransportPref::Fips || !fips_should_fall_back(resp.status()) {
+                    if fips_answer_is_final(pref, self.single_delivery, resp.status()) {
                         telemetry::record_fips_ok();
                         self.spawn_record(crate::transport::TransportKind::Fips);
                         return Ok((resp, crate::transport::TransportKind::Fips));
@@ -617,8 +663,23 @@ impl<'a> PeerRequest<'a> {
         for (k, v) in &self.headers {
             rb = rb.header(*k, v);
         }
-        match tokio::time::timeout(budget, send_with_retry(rb)).await {
+        let single = self.single_delivery;
+        let attempt = send_with_retry_if(rb, |e| fips_retryable(single, e));
+        match tokio::time::timeout(budget, attempt).await {
             Ok(Ok(r)) => Ok(Some(r)),
+            // Anything but a failed connect may have reached the peer.
+            Ok(Err(e)) if single && !e.is_connect() => Err(anyhow::anyhow!(
+                "FIPS GET {} failed after the request may have been delivered \
+                 (not retrying over Tor): {}",
+                self.path,
+                e
+            )),
+            Err(_) if single => Err(anyhow::anyhow!(
+                "FIPS GET {} exceeded its {:?} budget after the request may have \
+                 been delivered (not retrying over Tor)",
+                self.path,
+                budget
+            )),
             Ok(Err(e)) => {
                 telemetry::record_fallback(FallbackReason::ConnectFail);
                 tracing::info!(
@@ -759,4 +820,92 @@ mod tests {
         let err = decode_response(0xAABB, &r, "x").unwrap_err();
         assert!(err.to_string().contains("no AAAA"));
     }
+
+    #[test]
+    fn a_single_delivery_answer_is_final_whatever_its_status() {
+        use crate::settings::transport::TransportPref;
+        use reqwest::StatusCode;
+        // Regression (2026-09-29): the seller redeemed a paid download's
+        // token, answered 404, and the Tor fallback replayed the spent token.
+        for status in [
+            StatusCode::NOT_FOUND,
+            StatusCode::INTERNAL_SERVER_ERROR,
+            StatusCode::SERVICE_UNAVAILABLE,
+            StatusCode::OK,
+        ] {
+            assert!(fips_answer_is_final(TransportPref::Auto, true, status));
+        }
+        // Everything else keeps the existing fallback rules.
+        assert!(!fips_answer_is_final(
+            TransportPref::Auto,
+            false,
+            StatusCode::NOT_FOUND
+        ));
+        assert!(!fips_answer_is_final(
+            TransportPref::Auto,
+            false,
+            StatusCode::BAD_GATEWAY
+        ));
+        assert!(fips_answer_is_final(
+            TransportPref::Auto,
+            false,
+            StatusCode::PAYMENT_REQUIRED
+        ));
+        assert!(fips_answer_is_final(
+            TransportPref::Fips,
+            false,
+            StatusCode::NOT_FOUND
+        ));
+    }
+
+    /// A listener that accepts connections and never answers, counting them.
+    async fn silent_peer() -> (String, std::sync::Arc<std::sync::atomic::AtomicUsize>) {
+        let listener = tokio::net::TcpListener::bind("127.0.0.1:0").await.unwrap();
+        let addr = listener.local_addr().unwrap();
+        let seen = std::sync::Arc::new(std::sync::atomic::AtomicUsize::new(0));
+        let counter = seen.clone();
+        tokio::spawn(async move {
+            let mut held = Vec::new();
+            while let Ok((stream, _)) = listener.accept().await {
+                counter.fetch_add(1, std::sync::atomic::Ordering::SeqCst);
+                held.push(stream); // keep it open, never reply
+            }
+        });
+        (format!("http://{addr}/content/x"), seen)
+    }
+
+    #[tokio::test]
+    async fn a_single_delivery_request_is_not_resent_after_a_timeout() {
+        let (url, seen) = silent_peer().await;
+        let c = client_with_timeout(Duration::from_millis(300));
+        let err = send_with_retry_if(c.get(&url), |e| fips_retryable(true, e))
+            .await
+            .expect_err("peer never answers");
+        assert!(err.is_timeout());
+        assert_eq!(seen.load(std::sync::atomic::Ordering::SeqCst), 1);
+    }
+
+    #[tokio::test]
+    async fn an_ordinary_request_is_still_retried_once_after_a_timeout() {
+        let (url, seen) = silent_peer().await;
+        let c = client_with_timeout(Duration::from_millis(300));
+        let _ = send_with_retry_if(c.get(&url), |e| fips_retryable(false, e)).await;
+        assert_eq!(seen.load(std::sync::atomic::Ordering::SeqCst), 2);
+    }
+
+    #[tokio::test]
+    async fn a_single_delivery_request_still_retries_a_refused_connect() {
+        // Nothing listening: the peer provably never saw the request.
+        let listener = std::net::TcpListener::bind("127.0.0.1:0").unwrap();
+        let addr = listener.local_addr().unwrap();
+        drop(listener);
+        let c = client_with_timeout(Duration::from_millis(500));
+        let err = send_with_retry_if(c.get(format!("http://{addr}/")), |e| {
+            fips_retryable(true, e)
+        })
+        .await
+        .expect_err("nothing listening");
+        assert!(err.is_connect());
+        assert!(fips_retryable(true, &err));
+    }
 }

core/archipelago/src/wallet/mint_client.rs

@@ -514,6 +514,13 @@ impl MintClient {
     pub async fn swap(&self, inputs: &[Proof], target_amounts: &[u64]) -> Result<SwapResult> {
         let keyset = self.get_active_sat_keyset().await?;
 
+        // cashuB tokens carry NUT-02 v2 keyset ids in their 8-byte short form,
+        // which the mint rejects (bare 422). Repair here, not at each caller:
+        // the paid-download seller path swapped directly and every Minibits
+        // payment failed once the mint rotated to a v2 keyset.
+        let repaired = self.resolve_truncated_keyset_ids(inputs).await;
+        let inputs: &[Proof] = &repaired;
+
         // NUT-02: a mint may charge a per-input fee, and it rejects the swap
         // outright unless outputs == inputs - fee (`11005 Transaction inputs
         // should equal outputs less fee`). Applied here rather than at each
@@ -813,8 +820,7 @@ impl MintClient {
             let total: u64 = entry.proofs.iter().map(|p| p.amount).sum();
             let target_amounts = amount_to_denominations(total);
 
-            let proofs = self.resolve_truncated_keyset_ids(&entry.proofs).await;
-            let result = self.swap(&proofs, &target_amounts).await?;
+            let result = self.swap(&entry.proofs, &target_amounts).await?;
             all_new_proofs.extend(result.new_proofs);
         }
 
@@ -863,4 +869,136 @@ mod tests {
         let client = MintClient::new("http://mint.example.com").unwrap();
         assert_eq!(client.url(), "http://mint.example.com");
     }
+
+    /// A minimal mint on 127.0.0.1 answering `/v1/keys` and `/v1/keysets`
+    /// with one v2 keyset, and rejecting every `/v1/swap` as already spent.
+    /// Each swap request body is sent back on the returned channel.
+    async fn stub_mint(
+        full_id: &'static str,
+    ) -> (String, tokio::sync::mpsc::UnboundedReceiver<serde_json::Value>) {
+        use tokio::io::{AsyncReadExt, AsyncWriteExt};
+        let listener = tokio::net::TcpListener::bind("127.0.0.1:0").await.unwrap();
+        let addr = listener.local_addr().unwrap();
+        let (tx, rx) = tokio::sync::mpsc::unbounded_channel();
+        tokio::spawn(async move {
+            loop {
+                let Ok((mut stream, _)) = listener.accept().await else {
+                    return;
+                };
+                let mut buf = Vec::new();
+                let mut chunk = [0u8; 4096];
+                // Read headers, then as much body as Content-Length says.
+                let (head, body) = loop {
+                    let n = stream.read(&mut chunk).await.unwrap_or(0);
+                    if n == 0 {
+                        break (String::new(), Vec::new());
+                    }
+                    buf.extend_from_slice(&chunk[..n]);
+                    let Some(pos) = buf.windows(4).position(|w| w == b"\r\n\r\n") else {
+                        continue;
+                    };
+                    let head = String::from_utf8_lossy(&buf[..pos]).to_string();
+                    let len = head
+                        .lines()
+                        .find_map(|l| {
+                            let (k, v) = l.split_once(':')?;
+                            k.eq_ignore_ascii_case("content-length")
+                                .then(|| v.trim().parse::<usize>().ok())?
+                        })
+                        .unwrap_or(0);
+                    while buf.len() < pos + 4 + len {
+                        let n = stream.read(&mut chunk).await.unwrap_or(0);
+                        if n == 0 {
+                            break;
+                        }
+                        buf.extend_from_slice(&chunk[..n]);
+                    }
+                    break (head, buf[pos + 4..].to_vec());
+                };
+                let request_line = head.lines().next().unwrap_or_default().to_string();
+                let (status, reply) = if request_line.starts_with("GET /v1/keys ") {
+                    let key = "0279be667ef9dcbbac55a06295ce870b07029bfcdb2dce28d959f2815b16f81798";
+                    let keys: serde_json::Map<String, serde_json::Value> = (0..16)
+                        .map(|i| ((1u64 << i).to_string(), serde_json::json!(key)))
+                        .collect();
+                    (
+                        "200 OK",
+                        serde_json::json!({"keysets": [
+                            {"id": full_id, "unit": "sat", "active": true, "keys": keys}
+                        ]}),
+                    )
+                } else if request_line.starts_with("GET /v1/keysets ") {
+                    (
+                        "200 OK",
+                        serde_json::json!({"keysets": [
+                            {"id": full_id, "unit": "sat", "active": true, "input_fee_ppk": 0}
+                        ]}),
+                    )
+                } else if request_line.starts_with("POST /v1/swap ") {
+                    let _ = tx.send(serde_json::from_slice(&body).unwrap_or_default());
+                    (
+                        "400 Bad Request",
+                        serde_json::json!({"code": 11001, "detail": "Token Already Spent"}),
+                    )
+                } else {
+                    ("404 Not Found", serde_json::json!({}))
+                };
+                let reply = reply.to_string();
+                let _ = stream
+                    .write_all(
+                        format!(
+                            "HTTP/1.1 {status}\r\nContent-Type: application/json\r\n\
+                             Content-Length: {}\r\nConnection: close\r\n\r\n{reply}",
+                            reply.len()
+                        )
+                        .as_bytes(),
+                    )
+                    .await;
+            }
+        });
+        (format!("http://{addr}"), rx)
+    }
+
+    fn proof_with_id(id: &str) -> Proof {
+        Proof {
+            amount: 8,
+            id: id.to_string(),
+            secret: "test-secret".to_string(),
+            c: "02".to_string() + &"11".repeat(32),
+        }
+    }
+
+    /// Regression (2026-09-29): the paid-download seller called `swap`
+    /// directly with a cashuB token's short v2 keyset id and the mint
+    /// answered 422. `swap` itself must send the full id.
+    #[tokio::test]
+    async fn swap_expands_a_short_v2_keyset_id_before_calling_the_mint() {
+        const FULL: &str = "01fc0ec0e59cd6fa01b7a88f8cd77fce81fd1e64bca67d752e984992b7a3c3a821";
+        let (url, mut swaps) = stub_mint(FULL).await;
+        let client = MintClient::new(&url).unwrap();
+
+        let Err(err) = client
+            .swap(&[proof_with_id("01fc0ec0e59cd6fa")], &[8])
+            .await
+        else {
+            panic!("stub mint rejects every swap");
+        };
+        assert!(err.is::<AlreadyRedeemed>(), "unexpected error: {err:#}");
+
+        let body = swaps.recv().await.expect("swap reached the mint");
+        assert_eq!(body["inputs"][0]["id"], FULL);
+    }
+
+    #[tokio::test]
+    async fn swap_passes_complete_keyset_ids_through_unchanged() {
+        const FULL: &str = "01fc0ec0e59cd6fa01b7a88f8cd77fce81fd1e64bca67d752e984992b7a3c3a821";
+        let (url, mut swaps) = stub_mint(FULL).await;
+        let client = MintClient::new(&url).unwrap();
+
+        for id in [FULL, "009a1f293253e41e"] {
+            let _ = client.swap(&[proof_with_id(id)], &[8]).await;
+            let body = swaps.recv().await.expect("swap reached the mint");
+            assert_eq!(body["inputs"][0]["id"], id);
+        }
+    }
 }