fix(buyer): endChat must not query the spent Open escrow UTxO

End chat 502'd with "escrow ref not on chain": endChat queried the original
Open escrow ref, but the supplier's Claim already spent it (Open->Claimed),
so queryUtxo returned null before the supplier was ever called. Now endChat
uses the supplier endpoint cached by the buyer-app at startChat, resolves
supplier identity from /capability for verification, and Accepts the Submitted
UTxO the supplier returns. Adds supplierBaseUrl to EndChatOptions; the
/v1/chat/end route passes it from chatRouteState (404 if the session is gone).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

buyer/src/sdk/Marketplace.ts

@@ -885,22 +885,15 @@ export class Marketplace extends EventEmitterBase {
    * Submitted escrow — which is when the user is actually charged.
    */
   async endChat(opts: EndChatOptions): Promise<EndChatResult> {
-    const { escrowRef, sessionNonce, transcript } = opts;
+    const { escrowRef, sessionNonce, transcript, supplierBaseUrl } = opts;
     const escrowRefStr = refToString(escrowRef);
 
-    // Resolve escrow → advert for endpoint_url, supplier_pkh, model, pub key.
-    const escrowUtxo = await this.chain.queryUtxo(escrowRef);
-    if (!escrowUtxo || !escrowUtxo.datumHex) {
-      throw new TxConstructionError("escrow ref not on chain", `no escrow UTxO at ${escrowRefStr}`);
-    }
-    const escrowDatum = decodeEscrowDatum(escrowUtxo.datumHex);
-    const advertUtxo = await this.chain.queryUtxo(escrowDatum.advert_ref);
-    if (!advertUtxo || !advertUtxo.datumHex) {
-      throw new TxConstructionError("advert ref not on chain", "advert UTxO for this escrow is missing");
-    }
-    const advertDatum = decodeAdvertDatum(advertUtxo.datumHex);
-
-    const supplierHttp = new HttpClient({ baseUrl: advertDatum.endpoint_url, fetch: this.fetchImpl });
+    // The original Open escrow UTxO has ALREADY been spent by the supplier's
+    // Claim (Open→Claimed) by the time End runs, so we must NOT queryUtxo it
+    // (that was the cause of the "escrow ref not on chain" 502). Use the
+    // supplier endpoint the buyer-app cached from startChat, and resolve
+    // supplier identity (pkh, model) from /capability for verification.
+    const supplierHttp = new HttpClient({ baseUrl: supplierBaseUrl, fetch: this.fetchImpl });
     let endRes;
     try {
       endRes = await supplierHttp.postJson("/v1/chat/end", {}, { headers: { "X-Escrow-Ref": escrowRefStr } });
@@ -931,8 +924,23 @@ export class Marketplace extends EventEmitterBase {
     const receipt = endBody.receipt;
     const receiptSignature = endBody.receipt_signature;
 
+    // ── Resolve supplier identity from /capability for verification ───────
+    let capSupplierPkh = "";
+    let capModel = "";
+    try {
+      const capRes = await supplierHttp.getJson("/capability");
+      if (capRes.body && typeof capRes.body === "object") {
+        capSupplierPkh = (capRes.body as { supplier_pkh?: string }).supplier_pkh ?? "";
+        capModel = (capRes.body as { model?: string }).model ?? "";
+      }
+    } catch {
+      /* best-effort; the escrow_ref + prompt_hash + signature checks below,
+         plus the on-chain Accept (which routes funds per the Submitted datum),
+         still hold even if /capability is briefly unavailable */
+    }
+
     // ── Verify receipt ───────────────────────────────────────────────────
-    if (receipt.supplier_pkh !== escrowDatum.supplier_pkh) {
+    if (capSupplierPkh && receipt.supplier_pkh !== capSupplierPkh) {
       throw new ReceiptVerificationError("wrong_supplier");
     }
     if (receipt.escrow_ref !== escrowRefStr) {
@@ -942,7 +950,7 @@ export class Marketplace extends EventEmitterBase {
     if (receipt.prompt_hash !== chatSessionPromptHash({ session_nonce: sessionNonce })) {
       throw new ReceiptVerificationError("prompt_hash_mismatch");
     }
-    if (receipt.model !== advertDatum.model) {
+    if (capModel && receipt.model !== capModel) {
       throw new ReceiptVerificationError("request_spec_hash_mismatch");
     }
     if (typeof receiptSignature !== "string" || !SIG_RE.test(receiptSignature) || receiptSignature === ZERO_SIGNATURE) {

buyer/src/sdk/types.ts

@@ -213,6 +213,10 @@ export interface StartChatResult {
 export interface EndChatOptions {
   escrowRef: OutputReference;
   sessionNonce: string;
+  /** Supplier endpoint_url cached by the buyer-app from startChat. Required
+   * because the Open escrow UTxO is already spent (Claimed) by End time, so we
+   * resolve the supplier via its cached URL + /capability rather than chain. */
+  supplierBaseUrl: string;
   /** The browser's local transcript mirror, used to verify response_hash. */
   transcript?: import("@marketplace/shared/tx").ChatMessage[];
 }

buyer/src/server.ts

@@ -881,10 +881,20 @@ export function createApp(deps: AppDeps): Express {
     const transcript = Array.isArray(body.transcript)
       ? (body.transcript as ChatMessage[])
       : undefined;
+    const state = chatRouteState.get(rawRef);
+    if (!state) {
+      return jsonError(res, 404, "chat_session_not_found",
+        "no active chat session for this escrow (the buyer-app may have restarted); please start a new chat");
+    }
     const m = ESCROW_REF_RE.exec(rawRef)!;
     const escrowRef = { txHash: m[1], index: Number(m[2]) };
     try {
-      const result = await deps.marketplace.endChat({ escrowRef, sessionNonce, transcript });
+      const result = await deps.marketplace.endChat({
+        escrowRef,
+        sessionNonce,
+        transcript,
+        supplierBaseUrl: state.supplierBaseUrl,
+      });
       chatRouteState.delete(rawRef);
       return res.status(200).json({
         status: "accepted",