diff --git a/packages/atxp-express/src/atxpExpress.ts b/packages/atxp-express/src/atxpExpress.ts
index d8252d5..06091ab 100644
--- a/packages/atxp-express/src/atxpExpress.ts
+++ b/packages/atxp-express/src/atxpExpress.ts
@@ -150,7 +150,7 @@ export function atxpExpress(args: ATXPArgs): Router {
               detected.credential,
               context as Parameters<typeof settlement.settle>[2],
             );
-            logger.info(`Settled ${detected.protocol} in middleware: txHash=${result.txHash}, amount=${result.settledAmount}`);
+            logger.info(`Settled ${detected.protocol} in middleware: txHash=${result.txHash ?? '<already-settled>'}, amount=${result.settledAmount}`);
           } catch (error) {
             logger.error(`Middleware settlement failed for ${detected.protocol}: ${error instanceof Error ? error.message : String(error)}`);
             // Don't store the credential — it's already consumed/invalid.
diff --git a/packages/atxp-server/src/protocol.test.ts b/packages/atxp-server/src/protocol.test.ts
index 30075eb..10c62cb 100644
--- a/packages/atxp-server/src/protocol.test.ts
+++ b/packages/atxp-server/src/protocol.test.ts
@@ -319,6 +319,31 @@ describe('ProtocolSettlement', () => {
       await expect(settlement.settle('x402', 'cred')).rejects.toThrow('Settlement failed for x402: 500');
     });
 
+    it('should pass through null txHash when auth reports already-settled', async () => {
+      // The auth server returns { txHash: null, alreadySettled: true, ... } on
+      // retries of an already-settled payload. The SDK should surface it as-is
+      // rather than crashing or forcing the type to string.
+      mockFetch.mockResolvedValue({
+        ok: true,
+        json: async () => ({
+          txHash: null,
+          settledAmount: '201000',
+          alreadySettled: true,
+          network: 'solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp',
+          payer: '3FnrCCfHhZhEyeQd5Q69B1faqLvdHoG3WxUesZBBJ7M2',
+          sourceAccountId: 'atxp:atxp_acct_6qB245zVIJeSiIHi8xPmY',
+        }),
+      });
+
+      const payload = { signature: '0xabc' };
+      const credential = Buffer.from(JSON.stringify(payload)).toString('base64');
+      const result = await settlement.settle('x402', credential, { paymentRequirements: { network: 'solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp' } });
+
+      expect(result.txHash).toBeNull();
+      expect(result.settledAmount).toBe('201000');
+      expect(result.alreadySettled).toBe(true);
+    });
+
     describe('X402 multi-chain accept routing', () => {
       const multiChainReqs = {
         x402Version: 2,
diff --git a/packages/atxp-server/src/protocol.ts b/packages/atxp-server/src/protocol.ts
index ff8edef..ae6a469 100644
--- a/packages/atxp-server/src/protocol.ts
+++ b/packages/atxp-server/src/protocol.ts
@@ -106,10 +106,16 @@ export type VerifyResult = {
 
 /**
  * Result of settling a payment.
+ *
+ * `txHash` is null when the auth server reports the payment was already settled
+ * by a prior call (HTTP retry after a successful settle). The original tx hash
+ * is not carried in that response; callers that need it must look it up by
+ * other means (payer / amount / destination).
  */
 export type SettleResult = {
-  txHash: string;
+  txHash: string | null;
   settledAmount: string;
+  alreadySettled?: boolean;
 };
 
 /**
@@ -230,7 +236,7 @@ export class ProtocolSettlement {
     }
 
     const result = await response.json() as SettleResult;
-    this.logger.info(`Settled ${protocol}: txHash=${result.txHash}, amount=${result.settledAmount}`);
+    this.logger.info(`Settled ${protocol}: txHash=${result.txHash ?? '<already-settled>'}, amount=${result.settledAmount}`);
     return result;
   }