fix(ecash): resolve short v2 keyset ids before verifying a cashuB payment

The cashu crate's V4 (cashuB) encoder always writes a NUT-02 v2 keyset
id in its short 8-byte form (serialize_v4_keyset_id narrows to
ShortKeysetId unconditionally), which is spec-compliant: the receiver
must expand it against the mint's keyset list before spending. The
payment-receive loop in ecash.rs called MintClient::swap() directly
with the short id still attached, so mint.minibits.cash (whose active
keyset is v2) rejected every cashuB payment with
`422 inputs[0].id: NUT02: ID length invalid` — hence "seller doesn't
accept your Cashu mint" on any peer purchase.

MintClient::receive_token() already resolves this via
resolve_truncated_keyset_ids(); expose it pub(crate) and call it from
the ecash.rs loop too. The only other swap() call sites either run
after resolution or operate on our own full-id proofs.

Adds a test documenting that the short form is what crosses the wire,
so serialize_v4 is not "fixed" to defeat it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

core/archipelago/src/wallet/cashu.rs

@@ -489,6 +489,45 @@ pub fn amount_to_denominations(mut amount: u64) -> Vec<u64> {
 mod tests {
     use super::*;
 
+    /// A v4 (cashuB) token always carries a v2 keyset id in its short
+    /// (8-byte) form — confirmed against the real cashu 0.17.5 crate
+    /// (`TokenV4Token`'s `serialize_v4_keyset_id` unconditionally narrows to
+    /// `ShortKeysetId`) and live against mint.minibits.cash (2026-09-18).
+    /// That is spec-compliant, not a bug here: a receiver MUST resolve the
+    /// short id against the mint's keyset list before spending it (see
+    /// `MintClient::resolve_truncated_keyset_ids`, and its missing call site
+    /// that this exact round trip caught in `ecash.rs`'s payment-receive
+    /// path). This test documents that the short form is what actually
+    /// crosses the wire, so nobody re-"fixes" serialize_v4 to defeat it.
+    #[test]
+    fn v4_round_trip_shortens_a_v2_keyset_id_by_design() {
+        let real_v2_id = "01fc0ec0e59cd6fa01b7a88f8cd77fce81fd1e64bca67d752e984992b7a3c3a821";
+        assert_eq!(real_v2_id.len(), 66);
+        let token = CashuToken {
+            token: vec![TokenEntry {
+                mint: "https://mint.minibits.cash/Bitcoin".to_string(),
+                proofs: vec![Proof {
+                    amount: 2,
+                    id: real_v2_id.to_string(),
+                    secret: "abcdef1234567890".to_string(),
+                    // secp256k1 generator point G — a genuinely valid
+                    // compressed pubkey (the other tests' placeholder C
+                    // value is not, and serialize_v4 is the first path
+                    // here that actually parses it).
+                    c: "0279be667ef9dcbbac55a06295ce870b07029bfcdb2dce28d959f2815b16f81798"
+                        .to_string(),
+                }],
+            }],
+            memo: None,
+            unit: Some("sat".to_string()),
+        };
+        let v4 = token.serialize_v4().expect("serialize_v4 should accept a real v2 id");
+        let decoded = CashuToken::deserialize(&v4).unwrap();
+        let got_id = &decoded.token[0].proofs[0].id;
+        assert_eq!(got_id, "01fc0ec0e59cd6fa", "expected the short (8-byte) v2 form on the wire");
+        assert!(is_truncated_v2_keyset_id(got_id));
+    }
+
     #[test]
     fn test_serialize_deserialize_roundtrip() {
         let token = CashuToken {

core/archipelago/src/wallet/ecash.rs

@@ -1363,14 +1363,29 @@ pub async fn verify_and_receive_payment(
         let entry_total: u64 = entry.proofs.iter().map(|p| p.amount).sum();
         let target_amounts = amount_to_denominations(entry_total);
 
-        match client.swap(&entry.proofs, &target_amounts).await {
+        // The reference cashu crate's V4 (cashuB) encoder always writes a
+        // NUT-02 v2 keyset id in its short (8-byte) form — confirmed live
+        // against mint.minibits.cash (2026-09-18): every cashuB payment
+        // carrying that mint's active v2 keyset failed verification with a
+        // bare 422 "NUT02: ID length invalid" because this call skipped
+        // straight to swap() with the short id still attached. MintClient's
+        // own receive_token() already resolves this correctly; this is the
+        // same fix, just not routed through it (the loop here also tracks
+        // received_total/mint-scoped errors that receive_token() doesn't).
+        let proofs = client.resolve_truncated_keyset_ids(&entry.proofs).await;
+
+        match client.swap(&proofs, &target_amounts).await {
             Ok(result) => {
                 let amount: u64 = result.new_proofs.iter().map(|p| p.amount).sum();
                 wallet.add_proofs(&entry.mint, result.new_proofs);
                 received_total += amount;
             }
             Err(e) => {
-                warn!("Payment verification failed at mint {}: {}", entry.mint, e);
+                // {:#} walks the full anyhow context chain, including the raw
+                // mint response body `mint_error()` attaches as the cause —
+                // {} prints only the friendly top-level message and silently
+                // discards the one thing that would explain a bare 422.
+                warn!("Payment verification failed at mint {}: {:#}", entry.mint, e);
             }
         }
     }

core/archipelago/src/wallet/mint_client.rs

@@ -114,27 +114,68 @@ fn describe_mint_error_code(code: i64) -> Option<&'static str> {
     })
 }
 
+/// Render a FastAPI-style validation error list — `detail` as an array of
+/// `{"loc": [...], "msg": "...", "type": "..."}` objects — into one line per
+/// entry. This is the shape FastAPI (and therefore most Cashu mint
+/// implementations, including Nutshell) actually sends for a 422, not the
+/// plain string the rest of this file otherwise expects; without this a
+/// mint's real reason (e.g. `body -> inputs -> 0 -> id: NUT02: ID length
+/// invalid`) was silently replaced with "no further detail".
+fn describe_validation_errors(detail: &serde_json::Value) -> Option<String> {
+    let items = detail.as_array()?;
+    if items.is_empty() {
+        return None;
+    }
+    let lines: Vec<String> = items
+        .iter()
+        .filter_map(|item| {
+            let msg = item.get("msg").and_then(|m| m.as_str())?;
+            let loc = item
+                .get("loc")
+                .and_then(|l| l.as_array())
+                .map(|parts| {
+                    parts
+                        .iter()
+                        .map(|p| p.as_str().map(str::to_string).unwrap_or_else(|| p.to_string()))
+                        .collect::<Vec<_>>()
+                        .join(" -> ")
+                })
+                .unwrap_or_default();
+            Some(if loc.is_empty() {
+                msg.to_string()
+            } else {
+                format!("{loc}: {msg}")
+            })
+        })
+        .collect();
+    (!lines.is_empty()).then(|| lines.join("; "))
+}
+
 /// Parse a mint's error body (`{"code": N, "detail": "..."}`) and pick the
 /// best user-facing message: the plain-language translation when we know the
-/// code, otherwise the mint's own `detail` text, otherwise the raw body.
+/// code, otherwise the mint's own `detail` text (a plain string, or a
+/// FastAPI-style validation-error array), otherwise the raw body.
 fn describe_mint_error_body(status: reqwest::StatusCode, body: &str) -> String {
     let parsed: Option<serde_json::Value> = serde_json::from_str(body).ok();
     let code = parsed
         .as_ref()
         .and_then(|v| v.get("code"))
         .and_then(|c| c.as_i64());
-    let detail = parsed
-        .as_ref()
-        .and_then(|v| v.get("detail"))
-        .and_then(|d| d.as_str());
+    let detail = parsed.as_ref().and_then(|v| v.get("detail"));
 
     if let Some(friendly) = code.and_then(describe_mint_error_code) {
         return friendly.to_string();
     }
-    match detail {
-        Some(d) if !d.is_empty() => d.to_string(),
-        _ => format!("mint returned {} with no further detail", status),
+    if let Some(d) = detail {
+        if let Some(s) = d.as_str() {
+            if !s.is_empty() {
+                return s.to_string();
+            }
+        } else if let Some(rendered) = describe_validation_errors(d) {
+            return rendered;
+        }
     }
+    format!("mint returned {} with no further detail", status)
 }
 
 /// Build the error for a failed mint HTTP call: `op` + status + raw body as
@@ -742,7 +783,7 @@ impl MintClient {
     /// verification at the mint and no coins move. Anything already valid, or
     /// with no unambiguous match, is passed through untouched so the mint's
     /// own error is what the operator sees.
-    async fn resolve_truncated_keyset_ids(&self, proofs: &[Proof]) -> Vec<Proof> {
+    pub(crate) async fn resolve_truncated_keyset_ids(&self, proofs: &[Proof]) -> Vec<Proof> {
         let needs_repair = proofs.iter().any(|p| is_truncated_v2_keyset_id(&p.id));
         if !needs_repair {
             return proofs.to_vec();
@@ -850,6 +891,31 @@ mod tests {
         );
     }
 
+    #[test]
+    fn a_fastapi_validation_error_array_is_rendered_not_swallowed() {
+        // FastAPI's actual 422 shape — `detail` is a list of
+        // {loc, msg, type}, not the plain string the rest of this file
+        // otherwise expects. Confirmed live against mint.minibits.cash
+        // (2026-09-18): this used to collapse to "mint returned 422
+        // Unprocessable Entity with no further detail", discarding the one
+        // piece of text that actually explains the failure.
+        let body = serde_json::json!({
+            "detail": [
+                {"loc": ["body", "inputs", 0, "id"], "msg": "NUT02: ID length invalid", "type": "value_error"}
+            ]
+        })
+        .to_string();
+        let msg = super::describe_mint_error_body(reqwest::StatusCode::UNPROCESSABLE_ENTITY, &body);
+        assert_eq!(msg, "body -> inputs -> 0 -> id: NUT02: ID length invalid");
+    }
+
+    #[test]
+    fn an_empty_validation_error_array_falls_back_to_the_generic_message() {
+        let body = serde_json::json!({"detail": []}).to_string();
+        let msg = super::describe_mint_error_body(reqwest::StatusCode::UNPROCESSABLE_ENTITY, &body);
+        assert_eq!(msg, "mint returned 422 Unprocessable Entity with no further detail");
+    }
+
     use super::*;
 
     #[test]