Merge current main and harden paid-download delivery

- 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 +162,33 @@ 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. This request did not redeem an ecash payment."}"#,
+                ),
+            )),
+            Ok(content_server::ServeResult::RangeNotSatisfiable(total)) => Ok(Response::builder()
+                .status(StatusCode::RANGE_NOT_SATISFIABLE)
+                .header("Content-Range", format!("bytes */{total}"))
+                .body(hyper::Body::empty())
+                .unwrap()),
+            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

@@ -43,6 +43,25 @@ async fn reclaim_spent_ecash(data_dir: &std::path::Path, token: &str, backend: &
     }
 }
 
+/// Only pass through the peer's bounded, printable explanation; refund status
+/// is always determined locally and must never come from the peer's wording.
+fn seller_error_message(status: reqwest::StatusCode, body: &str) -> String {
+    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_owned));
+    match reason {
+        Some(reason) if !reason.trim().is_empty() => {
+            let clean: String = reason
+                .chars()
+                .filter(|c| !c.is_control())
+                .take(240)
+                .collect();
+            format!("Seller response ({status}): {clean}")
+        }
+        _ => format!("Peer returned an error ({status})."),
+    }
+}
+
 /// Keep first purchases and cached repeats compatible with both existing clients.
 fn paid_content_response(bytes: &[u8], mime: &str, paid_sats: u64) -> serde_json::Value {
     use base64::Engine;
@@ -623,13 +642,14 @@ impl RpcHandler {
 
         let path = format!("/content/{}", content_id);
         // Surface a real reason instead of the generic sanitized error (#30):
-        // the dial already tries FIPS/mesh then falls back to Tor, so a failure
-        // here means the peer is genuinely unreachable on both transports.
+        // A bearer token must not be replayed after an ambiguous delivery.
+        // A transport error can mean the seller received it without replying.
         let (response, transport) =
             match crate::fips::dial::PeerRequest::new(fips_npub.as_deref(), onion, &path)
                 .service(crate::settings::transport::PeerService::PeerFiles)
                 .header("X-Federation-DID", local_did)
                 .header("X-Payment-Token", token_str.clone())
+                .single_delivery()
                 .timeout(std::time::Duration::from_secs(900))
                 .send_get()
                 .await
@@ -642,7 +662,7 @@ impl RpcHandler {
                     let refund =
                         reclaim_spent_ecash(&self.config.data_dir, &token_str, used_backend).await;
                     return Ok(serde_json::json!({
-                        "error": format!("Could not reach the peer over mesh or Tor. {refund}")
+                        "error": format!("The purchase could not be completed. {refund}")
                     }));
                 }
             };
@@ -679,7 +699,7 @@ impl RpcHandler {
             tracing::warn!("paid download: seller {onion} returned {status}: {body}");
             let refund = reclaim_spent_ecash(&self.config.data_dir, &token_str, used_backend).await;
             return Ok(serde_json::json!({
-                "error": format!("Peer returned an error ({status}). {refund}")
+                "error": format!("{} {refund}", seller_error_message(status, &body))
             }));
         }
 
@@ -693,10 +713,17 @@ impl RpcHandler {
             .filter(|s| !s.is_empty())
             .unwrap_or_else(|| "application/octet-stream".to_string());
 
-        let bytes = response
-            .bytes()
-            .await
-            .context("Failed to read response body")?;
+        let bytes = match response.bytes().await {
+            Ok(bytes) => bytes,
+            Err(error) => {
+                tracing::warn!("paid download: response body failed: {error}");
+                let refund =
+                    reclaim_spent_ecash(&self.config.data_dir, &token_str, used_backend).await;
+                return Ok(serde_json::json!({
+                    "error": format!("The file transfer was interrupted after payment was sent. {refund}")
+                }));
+            }
+        };
 
         // Persist the purchase so it "stays unlocked" for this buyer: cache the
         // bytes + metadata keyed by (onion, content_id). The gallery then renders

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

@@ -162,3 +162,20 @@ async fn files_copy_fails_without_overwriting_or_claiming_success_on_errors() {
         assert_eq!(api.seen.lock().unwrap().len(), expected);
     }
 }
+
+#[test]
+fn seller_errors_are_bounded_printable_and_identified_as_peer_text() {
+    let status = reqwest::StatusCode::SERVICE_UNAVAILABLE;
+    let message = seller_error_message(status, r#"{"error":"Cannot read file\n\u0000"}"#);
+    assert!(message.starts_with("Seller response (503"));
+    assert!(message.ends_with("Cannot read file"));
+    assert!(!message.contains('\n') && !message.contains('\0'));
+    let body = serde_json::json!({"error": "é".repeat(1000)}).to_string();
+    assert!(seller_error_message(status, &body).chars().count() < 300);
+    for body in ["not JSON", r#"{"error": 7}"#, r#"{"error":"   "}"#] {
+        assert_eq!(
+            seller_error_message(status, body),
+            "Peer returned an error (503 Service Unavailable)."
+        );
+    }
+}

core/archipelago/src/content_server.rs

@@ -238,6 +238,11 @@ 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,
+    /// Requested byte range cannot be served; no payment was taken.
+    RangeNotSatisfiable(u64),
 }
 
 /// Serve a content item by ID with access control and optional range request.
@@ -252,6 +257,39 @@ pub async fn serve_content(
     range: Option<ByteRange>,
     owner_session: bool,
 ) -> Result<ServeResult> {
+    serve_content_with(
+        data_dir,
+        id,
+        payment_token,
+        invoice_hash,
+        peer_did,
+        range,
+        owner_session,
+        |path, range, mime| prepare_content(data_dir, path, range, mime),
+        |token, amount| async move { verify_payment_token(data_dir, &token, amount).await },
+    )
+    .await
+}
+
+// Inject only the read and payment boundaries, so tests can prove ordering
+// without mint access, file-permission assumptions or privileged commands.
+async fn serve_content_with<R, RF, V, VF>(
+    data_dir: &Path,
+    id: &str,
+    payment_token: Option<&str>,
+    invoice_hash: Option<&str>,
+    peer_did: Option<&str>,
+    range: Option<ByteRange>,
+    owner_session: bool,
+    read: R,
+    verify: V,
+) -> Result<ServeResult>
+where
+    R: FnOnce(PathBuf, Option<ByteRange>, String) -> RF,
+    RF: std::future::Future<Output = Result<ServeResult>>,
+    V: FnOnce(String, u64) -> VF,
+    VF: std::future::Future<Output = bool>,
+{
     let catalog = load_catalog(data_dir).await?;
     let item = match catalog.items.iter().find(|i| i.id == id) {
         Some(i) => i,
@@ -314,6 +352,29 @@ pub async fn serve_content(
         return Ok(ServeResult::NotFound);
     }
 
+    // Refuse unauthorized viewers before opening or reading any bytes.
+    if !owner_session && matches!(item.access, AccessControl::PeersOnly) && !is_known_peer {
+        return Ok(ServeResult::Forbidden);
+    }
+    if !owner_session {
+        if let AccessControl::Paid { price_sats, .. } = &item.access {
+            if payment_token.is_none() && invoice_hash.is_none() {
+                return Ok(ServeResult::PaymentRequired(*price_sats));
+            }
+        }
+    }
+
+    // Finish all file I/O before consuming bearer payment. Merely opening then
+    // reopening after charging still lost payments on read errors or deletion.
+    let prepared = match read(file_path, range, item.mime_type.clone()).await {
+        Ok(result @ (ServeResult::Ok(..) | ServeResult::Partial { .. })) => result,
+        Ok(other) => return Ok(other),
+        Err(error) => {
+            warn!(content_id = %id, "Cannot prepare shared content: {error:#}");
+            return Ok(ServeResult::Unavailable);
+        }
+    };
+
     // Check access control
     if !owner_session {
         match &item.access {
@@ -331,7 +392,7 @@ pub async fn serve_content(
                         "fedimint"
                     };
                     if method_accepted(&item.access, method)
-                        && verify_payment_token(data_dir, token, *price_sats).await
+                        && verify(token.to_owned(), *price_sats).await
                     {
                         authorized = true;
                     }
@@ -358,55 +419,127 @@ pub async fn serve_content(
         }
     }
 
-    let metadata = fs::metadata(&file_path)
-        .await
-        .context("Failed to read file metadata")?;
-    let total_size = metadata.len();
-
-    // Handle range request for streaming
-    if let Some(range) = range {
-        let start = range.start.min(total_size.saturating_sub(1));
-        let end = range
-            .end
-            .map(|e| e.min(total_size - 1))
-            .unwrap_or(total_size - 1);
-
-        if start > end || start >= total_size {
-            return Ok(ServeResult::NotFound);
+    Ok(prepared)
 }
 
-        let len = (end - start + 1) as usize;
+async fn prepare_content(
+    data_dir: &Path,
+    path: PathBuf,
+    range: Option<ByteRange>,
+    mime: String,
+) -> Result<ServeResult> {
     use tokio::io::{AsyncReadExt, AsyncSeekExt};
-        let mut file = tokio::fs::File::open(&file_path)
+    let mut file = match fs::OpenOptions::new()
+        .read(true)
+        .custom_flags(libc::O_NONBLOCK)
+        .open(&path)
         .await
-            .context("Failed to open content file")?;
-        file.seek(std::io::SeekFrom::Start(start))
+    {
+        Ok(file) => file,
+        Err(error) if error.kind() == std::io::ErrorKind::PermissionDenied => {
+            let bytes = read_filebrowser_via_userns(data_dir, &path).await?;
+            return slice_prepared_content(bytes, range, mime);
+        }
+        Err(error) => return Err(error).context("Opening shared content"),
+    };
+    let metadata = file.metadata().await?;
+    anyhow::ensure!(metadata.is_file(), "Shared content is not a regular file");
+    let total = metadata.len();
+    if let Some(range) = range {
+        let Some((start, end)) = checked_range(&range, total) else {
+            return Ok(ServeResult::RangeNotSatisfiable(total));
+        };
+        file.seek(std::io::SeekFrom::Start(start)).await?;
+        let len = usize::try_from(end - start + 1).context("Content range is too large")?;
+        let mut bytes = vec![0; len];
+        file.read_exact(&mut bytes)
             .await
-            .context("Failed to seek")?;
-        let mut buf = vec![0u8; len];
-        file.read_exact(&mut buf)
-            .await
-            .context("Failed to read range")?;
-
-        debug!(
-            "Serving content '{}' range {}-{}/{} ({} bytes)",
-            id, start, end, total_size, len
-        );
+            .context("Reading shared content range")?;
         return Ok(ServeResult::Partial {
-            bytes: buf,
-            mime_type: item.mime_type.clone(),
+            bytes,
+            mime_type: mime,
             start,
             end,
-            total: total_size,
+            total,
         });
     }
-
-    let bytes = fs::read(&file_path)
+    let mut bytes = Vec::new();
+    file.read_to_end(&mut bytes)
         .await
-        .context("Failed to read content file")?;
+        .context("Reading shared content")?;
+    Ok(ServeResult::Ok(bytes, mime))
+}
 
-    debug!("Serving content '{}' ({} bytes)", id, bytes.len());
-    Ok(ServeResult::Ok(bytes, item.mime_type.clone()))
+fn checked_range(range: &ByteRange, total: u64) -> Option<(u64, u64)> {
+    let last = total.checked_sub(1)?;
+    let end = range.end.unwrap_or(last).min(last);
+    (range.start <= end && range.start < total).then_some((range.start, end))
+}
+
+fn slice_prepared_content(
+    bytes: Vec<u8>,
+    range: Option<ByteRange>,
+    mime: String,
+) -> Result<ServeResult> {
+    let total = bytes.len() as u64;
+    match range {
+        None => Ok(ServeResult::Ok(bytes, mime)),
+        Some(range) => match checked_range(&range, total) {
+            Some((start, end)) => Ok(ServeResult::Partial {
+                bytes: bytes[start as usize..=end as usize].to_vec(),
+                mime_type: mime,
+                start,
+                end,
+                total,
+            }),
+            None => Ok(ServeResult::RangeNotSatisfiable(total)),
+        },
+    }
+}
+
+/// Read only an explicitly shared, regular file within FileBrowser storage.
+/// Do not change its mode or grant world-readable access to paid/private data.
+async fn filebrowser_read_path(data_dir: &Path, path: &Path) -> Result<PathBuf> {
+    let root = fs::canonicalize(data_dir.join("filebrowser")).await?;
+    let target = fs::canonicalize(path).await?;
+    anyhow::ensure!(
+        target.starts_with(&root) && target != root,
+        "Shared file is outside Files storage"
+    );
+    anyhow::ensure!(
+        fs::metadata(&target).await?.is_file(),
+        "Shared content is not a regular file"
+    );
+    Ok(target)
+}
+
+async fn read_filebrowser_via_userns(data_dir: &Path, path: &Path) -> Result<Vec<u8>> {
+    let path = filebrowser_read_path(data_dir, path).await?;
+    // Tests exercise the boundary explicitly; they never launch the host Podman.
+    #[cfg(test)]
+    {
+        let _ = path;
+        anyhow::bail!("Files namespace read disabled in unit tests")
+    }
+    #[cfg(not(test))]
+    {
+        let output = tokio::time::timeout(
+            std::time::Duration::from_secs(900),
+            tokio::process::Command::new("podman")
+                .args(["unshare", "cat", "--"])
+                .arg(path)
+                .kill_on_drop(true)
+                .output(),
+        )
+        .await
+        .context("Files namespace read timed out")??;
+        anyhow::ensure!(
+            output.status.success(),
+            "Files namespace read failed: {}",
+            output.status
+        );
+        Ok(output.stdout)
+    }
 }
 
 /// Result of attempting to serve a preview.
@@ -729,3 +862,301 @@ mod prune_missing_content_tests {
         assert_eq!(reloaded.items[0].id, "present-item");
     }
 }
+
+#[cfg(test)]
+mod paid_read_order_tests {
+    use super::*;
+    use std::sync::atomic::{AtomicUsize, Ordering};
+
+    async fn fixture(bytes: &[u8]) -> tempfile::TempDir {
+        let dir = tempfile::tempdir().unwrap();
+        fs::create_dir_all(dir.path().join("content/files"))
+            .await
+            .unwrap();
+        fs::write(dir.path().join("content/files/test.bin"), bytes)
+            .await
+            .unwrap();
+        save_catalog(
+            dir.path(),
+            &ContentCatalog {
+                items: vec![ContentItem {
+                    id: "paid".into(),
+                    filename: "test.bin".into(),
+                    mime_type: "application/octet-stream".into(),
+                    size_bytes: bytes.len() as u64,
+                    description: String::new(),
+                    access: AccessControl::Paid {
+                        price_sats: 10,
+                        accepted: vec!["ecash".into()],
+                    },
+                    availability: Availability::AllPeers,
+                    added_at: "2026-09-30".into(),
+                }],
+            },
+        )
+        .await
+        .unwrap();
+        dir
+    }
+
+    #[tokio::test]
+    async fn all_read_failures_precede_redemption_even_as_root() {
+        for kind in [
+            std::io::ErrorKind::PermissionDenied,
+            std::io::ErrorKind::UnexpectedEof,
+            std::io::ErrorKind::NotFound,
+            std::io::ErrorKind::Other,
+        ] {
+            let dir = fixture(b"abc").await;
+            let charged = AtomicUsize::new(0);
+            let result = serve_content_with(
+                dir.path(),
+                "paid",
+                Some("cashuBtest"),
+                None,
+                None,
+                None,
+                false,
+                |_, _, _| async move { Err(std::io::Error::from(kind).into()) },
+                |_, _| async {
+                    charged.fetch_add(1, Ordering::SeqCst);
+                    true
+                },
+            )
+            .await
+            .unwrap();
+            assert!(matches!(result, ServeResult::Unavailable));
+            assert_eq!(charged.load(Ordering::SeqCst), 0);
+            assert_eq!(load_catalog(dir.path()).await.unwrap().items.len(), 1);
+        }
+    }
+
+    #[tokio::test]
+    async fn deletion_during_payment_cannot_lose_prepared_bytes() {
+        let dir = fixture(b"original").await;
+        let result = serve_content_with(
+            dir.path(),
+            "paid",
+            Some("cashuBtest"),
+            None,
+            None,
+            None,
+            false,
+            |path, range, mime| prepare_content(dir.path(), path, range, mime),
+            |_, amount| {
+                assert_eq!(amount, 10);
+                async {
+                    fs::remove_file(dir.path().join("content/files/test.bin"))
+                        .await
+                        .unwrap();
+                    true
+                }
+            },
+        )
+        .await
+        .unwrap();
+        assert!(matches!(result, ServeResult::Ok(bytes, _) if bytes == b"original"));
+    }
+
+    #[tokio::test]
+    async fn empty_out_of_bounds_and_reversed_ranges_never_charge() {
+        for (bytes, start, end) in [
+            (b"".as_slice(), 0, None),
+            (b"abc".as_slice(), 3, None),
+            (b"abc".as_slice(), 2, Some(1)),
+        ] {
+            let dir = fixture(bytes).await;
+            let result = serve_content_with(
+                dir.path(),
+                "paid",
+                Some("cashuBtest"),
+                None,
+                None,
+                Some(ByteRange { start, end }),
+                false,
+                |path, range, mime| prepare_content(dir.path(), path, range, mime),
+                |_, _| async { panic!("invalid range reached payment") },
+            )
+            .await
+            .unwrap();
+            assert!(
+                matches!(result, ServeResult::RangeNotSatisfiable(n) if n == bytes.len() as u64)
+            );
+        }
+    }
+
+    #[tokio::test]
+    async fn prepared_range_survives_file_change_while_payment_is_verified() {
+        let dir = fixture(b"abcdef").await;
+        let result = serve_content_with(
+            dir.path(),
+            "paid",
+            Some("cashuBtest"),
+            None,
+            None,
+            Some(ByteRange {
+                start: 2,
+                end: Some(999),
+            }),
+            false,
+            |path, range, mime| prepare_content(dir.path(), path, range, mime),
+            |_, _| async {
+                fs::write(dir.path().join("content/files/test.bin"), b"x")
+                    .await
+                    .unwrap();
+                true
+            },
+        )
+        .await
+        .unwrap();
+        assert!(
+            matches!(result, ServeResult::Partial { bytes, start: 2, end: 5, total: 6, .. } if bytes == b"cdef")
+        );
+    }
+
+    #[tokio::test]
+    async fn payment_denial_never_returns_prepared_content() {
+        let dir = fixture(b"secret").await;
+        let charged = AtomicUsize::new(0);
+        let result = serve_content_with(
+            dir.path(),
+            "paid",
+            Some("cashuBtest"),
+            None,
+            None,
+            None,
+            false,
+            |path, range, mime| prepare_content(dir.path(), path, range, mime),
+            |_, _| async {
+                charged.fetch_add(1, Ordering::SeqCst);
+                false
+            },
+        )
+        .await
+        .unwrap();
+        assert!(matches!(result, ServeResult::PaymentRequired(10)));
+        assert_eq!(charged.load(Ordering::SeqCst), 1);
+    }
+
+    #[tokio::test]
+    async fn missing_payment_and_peer_restrictions_precede_file_reads() {
+        let dir = fixture(b"secret").await;
+        let result = serve_content_with(
+            dir.path(),
+            "paid",
+            None,
+            None,
+            None,
+            None,
+            false,
+            |_, _, _| async { panic!("unauthorized file read") },
+            |_, _| async { panic!("unexpected payment") },
+        )
+        .await
+        .unwrap();
+        assert!(matches!(result, ServeResult::PaymentRequired(10)));
+        let mut catalog = load_catalog(dir.path()).await.unwrap();
+        catalog.items[0].access = AccessControl::PeersOnly;
+        save_catalog(dir.path(), &catalog).await.unwrap();
+        let result = serve_content_with(
+            dir.path(),
+            "paid",
+            None,
+            None,
+            None,
+            None,
+            false,
+            |_, _, _| async { panic!("unauthorized file read") },
+            |_, _| async { panic!("unexpected payment") },
+        )
+        .await
+        .unwrap();
+        assert!(matches!(result, ServeResult::Forbidden));
+    }
+
+    #[tokio::test]
+    async fn owner_reads_paid_content_without_redemption() {
+        let dir = fixture(b"own file").await;
+        let result = serve_content_with(
+            dir.path(),
+            "paid",
+            None,
+            None,
+            None,
+            None,
+            true,
+            |path, range, mime| prepare_content(dir.path(), path, range, mime),
+            |_, _| async { panic!("owner charged") },
+        )
+        .await
+        .unwrap();
+        assert!(matches!(result, ServeResult::Ok(bytes, _) if bytes == b"own file"));
+    }
+
+    #[tokio::test]
+    async fn directory_in_place_of_file_does_not_charge() {
+        let dir = fixture(b"abc").await;
+        let path = dir.path().join("content/files/test.bin");
+        fs::remove_file(&path).await.unwrap();
+        fs::create_dir(&path).await.unwrap();
+        let result = serve_content_with(
+            dir.path(),
+            "paid",
+            Some("cashuBtest"),
+            None,
+            None,
+            None,
+            false,
+            |path, range, mime| prepare_content(dir.path(), path, range, mime),
+            |_, _| async { panic!("directory charged") },
+        )
+        .await
+        .unwrap();
+        assert!(matches!(result, ServeResult::Unavailable));
+    }
+
+    #[tokio::test]
+    async fn files_namespace_read_is_scoped_to_regular_files_and_keeps_mode() {
+        use std::os::unix::fs::{symlink, PermissionsExt};
+        let dir = fixture(b"outside").await;
+        let root = dir.path().join("filebrowser");
+        fs::create_dir(&root).await.unwrap();
+        let inside = root.join("song");
+        fs::write(&inside, b"song").await.unwrap();
+        fs::set_permissions(&inside, std::fs::Permissions::from_mode(0o640))
+            .await
+            .unwrap();
+        assert_eq!(
+            filebrowser_read_path(dir.path(), &inside).await.unwrap(),
+            inside
+        );
+        assert_eq!(
+            fs::metadata(&inside).await.unwrap().permissions().mode() & 0o777,
+            0o640
+        );
+        let outside = dir.path().join("content/files/test.bin");
+        symlink(&outside, root.join("escape")).unwrap();
+        for path in [outside, root.join("escape"), root.clone()] {
+            assert!(filebrowser_read_path(dir.path(), &path).await.is_err());
+        }
+    }
+
+    #[test]
+    fn user_namespace_bytes_use_the_same_range_rules() {
+        assert!(matches!(
+            slice_prepared_content(
+                vec![],
+                Some(ByteRange {
+                    start: 0,
+                    end: None
+                }),
+                "x".into()
+            )
+            .unwrap(),
+            ServeResult::RangeNotSatisfiable(0)
+        ));
+        assert!(
+            matches!(slice_prepared_content(b"abc".to_vec(), Some(ByteRange { start: 1, end: None }), "x".into()).unwrap(), ServeResult::Partial { bytes, start: 1, end: 2, total: 3, .. } if bytes == b"bc")
+        );
+    }
+}

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";
 
@@ -113,7 +132,21 @@ pub fn client() -> reqwest::Client {
 /// before the Tor fallback ever gets a chance. The generous `connect_timeout`
 /// is preserved so a cold hole-punched path still gets time to establish.
 pub fn client_with_timeout(timeout: Duration) -> reqwest::Client {
+    client_with_delivery_policy(timeout, false)
+}
+
+fn delivery_redirect_policy(single: bool) -> reqwest::redirect::Policy {
+    if single {
+        reqwest::redirect::Policy::none()
+    } else {
+        reqwest::redirect::Policy::default()
+    }
+}
+
+fn client_with_delivery_policy(timeout: Duration, single: bool) -> reqwest::Client {
     reqwest::Client::builder()
+        .no_proxy()
+        .redirect(delivery_redirect_policy(single))
         .timeout(timeout)
         .connect_timeout(Duration::from_secs(8))
         .user_agent("archipelago-fips/1")
@@ -130,10 +163,18 @@ 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 +391,9 @@ 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 +407,25 @@ 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 {
@@ -442,7 +502,7 @@ impl<'a> PeerRequest<'a> {
                     // Use the FIPS reply unless it's one a Tor retry could
                     // fix (404 path-not-served / 5xx) and we're allowed to
                     // fall back. FIPS-only never falls back.
-                    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));
@@ -481,7 +541,7 @@ 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));
@@ -551,13 +611,21 @@ impl<'a> PeerRequest<'a> {
         } else {
             budget
         };
-        let c = client_with_timeout(per_attempt);
+        let c = client_with_delivery_policy(per_attempt, self.single_delivery);
         let mut rb = c.post(&url).json(body);
         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)),
+            Ok(Err(e)) if single && !e.is_connect() => Err(anyhow::anyhow!(
+                "FIPS POST failed after possible delivery; not replaying: {e}"
+            )),
+            Err(_) if single => Err(anyhow::anyhow!(
+                "FIPS POST exceeded its budget after possible delivery; not replaying"
+            )),
             Ok(Err(e)) => {
                 telemetry::record_fallback(FallbackReason::ConnectFail);
                 tracing::info!(
@@ -612,13 +680,28 @@ impl<'a> PeerRequest<'a> {
         } else {
             budget
         };
-        let c = client_with_timeout(per_attempt);
+        let c = client_with_delivery_policy(per_attempt, self.single_delivery);
         let mut rb = c.get(&url);
         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!(
@@ -676,6 +759,7 @@ impl<'a> PeerRequest<'a> {
             .context("Invalid Tor SOCKS proxy URL")?;
         reqwest::Client::builder()
             .proxy(proxy)
+            .redirect(delivery_redirect_policy(self.single_delivery))
             .timeout(self.timeout)
             .build()
             .context("Build Tor HTTP client")
@@ -759,4 +843,181 @@ 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));
+    }
+}
+
+#[cfg(test)]
+mod delivery_redirect_tests {
+    use super::*;
+    use hyper::{
+        service::{make_service_fn, service_fn},
+        Body, Response, Server,
+    };
+    use std::{
+        convert::Infallible,
+        sync::{
+            atomic::{AtomicUsize, Ordering},
+            Arc,
+        },
+    };
+
+    #[tokio::test]
+    async fn paid_bearer_request_does_not_follow_redirects_but_normal_get_does() {
+        let seen = Arc::new(AtomicUsize::new(0));
+        let counter = seen.clone();
+        let server = Server::bind(&([127, 0, 0, 1], 0).into());
+        let address = server.local_addr();
+        let service = make_service_fn(move |_| {
+            let counter = counter.clone();
+            async move {
+                Ok::<_, Infallible>(service_fn(move |request: hyper::Request<Body>| {
+                    let counter = counter.clone();
+                    async move {
+                        counter.fetch_add(1, Ordering::SeqCst);
+                        let response = if request.uri().path() == "/first" {
+                            Response::builder()
+                                .status(302)
+                                .header("Location", "/replay")
+                                .body(Body::empty())
+                                .unwrap()
+                        } else {
+                            Response::new(Body::from("replayed"))
+                        };
+                        Ok::<_, Infallible>(response)
+                    }
+                }))
+            }
+        });
+        let task = tokio::spawn(server.serve(service));
+        let url = format!("http://{address}/first");
+        let response = client_with_delivery_policy(Duration::from_secs(2), true)
+            .get(&url)
+            .header("X-Payment-Token", "dummy-test-token")
+            .send()
+            .await
+            .unwrap();
+        assert_eq!(response.status(), reqwest::StatusCode::FOUND);
+        assert_eq!(seen.load(Ordering::SeqCst), 1);
+        let response = client_with_delivery_policy(Duration::from_secs(2), false)
+            .get(url)
+            .send()
+            .await
+            .unwrap();
+        assert_eq!(response.status(), reqwest::StatusCode::OK);
+        assert_eq!(seen.load(Ordering::SeqCst), 3);
+        task.abort();
+    }
+
+    #[tokio::test]
+    async fn paid_request_is_not_resent_when_peer_disconnects_after_reading_it() {
+        use tokio::io::AsyncReadExt;
+        let listener = tokio::net::TcpListener::bind("127.0.0.1:0").await.unwrap();
+        let address = listener.local_addr().unwrap();
+        let seen = Arc::new(AtomicUsize::new(0));
+        let counter = seen.clone();
+        let task = tokio::spawn(async move {
+            while let Ok((mut stream, _)) = listener.accept().await {
+                let mut buf = [0; 4096];
+                let _ = stream.read(&mut buf).await;
+                counter.fetch_add(1, Ordering::SeqCst);
+                drop(stream);
+            }
+        });
+        let c = client_with_delivery_policy(Duration::from_secs(2), true);
+        let error = send_with_retry_if(c.get(format!("http://{address}/")), |e| {
+            fips_retryable(true, e)
+        })
+        .await
+        .unwrap_err();
+        assert!(!error.is_connect());
+        assert_eq!(seen.load(Ordering::SeqCst), 1);
+        task.abort();
+    }
 }