Potential settlement gating issue: paid MCP callback runs before settlement #43

Open
Open
Potential settlement gating issue: paid MCP callback runs before settlement#43

Description

@chenshj73
chenshj73
opened on May 28, 2026

Hi, I noticed a possible payment-flow ordering issue while reviewing the current source.

In packages/js-sdk/src/handler/server/plugins/with-x402.ts, withX402 builds a payment requirement for the MCP tool resource, verifies the submitted payment, executes the wrapped callback, and only then settles.

The requirement is scoped to the tool resource and payee:

172: reqs.push({173: scheme: "exact"asconst,174: network,175: maxAmountRequired,176: payTo: normalizedPayTo,177: asset: normalizedAsset,178: maxTimeoutSeconds: 300,179: resource: `mcp://${name}`,180: mimeType: "application/json",181: description,182: extra,183:});

The submitted payment is decoded and verified first:

272: // Decode & verify273: letdecoded: PaymentPayload;274: try{275: decoded=decodeX402Payment(token);276: decoded.x402Version=x402Version;277: }catch{278: returnpaymentRequired("INVALID_PAYMENT");279: }280:
281: constselected=findMatchingPaymentRequirements(accepts,decoded);286: constvr=awaitverify(decoded,selected);287: if(!vr.isValid){288: returnpaymentRequired(vr.invalidReason??"INVALID_PAYMENT",{289: payer: vr.payer,290:});291: }

But the paid tool callback runs before settlement:

293: // Execute tool294: letresult: CallToolResult;295: letfailed=false;296: try{297: result=awaitcb(args,extra);
...
316: // Settle only on success317: if(!failed){318: try{319: consts=awaitsettle(decoded,selected);320: if(s.success){

So the current order appears to be:

payment header -> decode/select requirement -> verify -> cb(args, extra) -> settle

Why this may matter:

  • Verification is not always the same as completed settlement.
  • A wrapped MCP tool callback can perform expensive computation, external calls, or state-changing work.
  • If settle(decoded, selected) fails after cb(args, extra) has already run, the wrapper can return a payment error but cannot undo the paid work.

A safer design would complete settlement before invoking externally effective callbacks, or clearly document that pre-settlement callbacks must be side-effect-free. A regression test where verify succeeds and settle fails after a callback would make the intended behavior explicit.

I am reporting this as a potential issue rather than a confirmed exploit, since the final impact depends on the callbacks users register with withX402.

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      Skip to content
      Potential settlement gating issue: paid MCP callback runs before settlement #43
      Open
      Open
      Potential settlement gating issue: paid MCP callback runs before settlement#43
      Description
      @chenshj73
      chenshj73
      opened on May 28, 2026
      Hi, I noticed a possible payment-flow ordering issue while reviewing the current source.
      In packages/js-sdk/src/handler/server/plugins/with-x402.ts, withX402 builds a payment requirement for the MCP tool resource, verifies the submitted payment, executes the wrapped callback, and only then settles.
      The requirement is scoped to the tool resource and payee:
      172: reqs.push({173: scheme: "exact"asconst,174: network,175: maxAmountRequired,176: payTo: normalizedPayTo,177: asset: normalizedAsset,178: maxTimeoutSeconds: 300,179: resource: `mcp://${name}`,180: mimeType: "application/json",181: description,182: extra,183:});
      The submitted payment is decoded and verified first:
      272: // Decode & verify273: letdecoded: PaymentPayload;274: try{275: decoded=decodeX402Payment(token);276: decoded.x402Version=x402Version;277: }catch{278: returnpaymentRequired("INVALID_PAYMENT");279: }280:
281: constselected=findMatchingPaymentRequirements(accepts,decoded);286: constvr=awaitverify(decoded,selected);287: if(!vr.isValid){288: returnpaymentRequired(vr.invalidReason??"INVALID_PAYMENT",{289: payer: vr.payer,290:});291: }
      But the paid tool callback runs before settlement:
      293: // Execute tool294: letresult: CallToolResult;295: letfailed=false;296: try{297: result=awaitcb(args,extra);
...
316: // Settle only on success317: if(!failed){318: try{319: consts=awaitsettle(decoded,selected);320: if(s.success){
      So the current order appears to be:
      payment header -> decode/select requirement -> verify -> cb(args, extra) -> settle

      Why this may matter:
      • Verification is not always the same as completed settlement.
      • A wrapped MCP tool callback can perform expensive computation, external calls, or state-changing work.
      • If settle(decoded, selected) fails after cb(args, extra) has already run, the wrapper can return a payment error but cannot undo the paid work.
      A safer design would complete settlement before invoking externally effective callbacks, or clearly document that pre-settlement callbacks must be side-effect-free. A regression test where verify succeeds and settle fails after a callback would make the intended behavior explicit.
      I am reporting this as a potential issue rather than a confirmed exploit, since the final impact depends on the callbacks users register with withX402.
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          Skip to content
          Potential settlement gating issue: paid MCP callback runs before settlement #43
          Open
          Open
          Potential settlement gating issue: paid MCP callback runs before settlement#43
          Description
          @chenshj73
          chenshj73
          opened on May 28, 2026
          Hi, I noticed a possible payment-flow ordering issue while reviewing the current source.
          In packages/js-sdk/src/handler/server/plugins/with-x402.ts, withX402 builds a payment requirement for the MCP tool resource, verifies the submitted payment, executes the wrapped callback, and only then settles.
          The requirement is scoped to the tool resource and payee:
          172: reqs.push({173: scheme: "exact"asconst,174: network,175: maxAmountRequired,176: payTo: normalizedPayTo,177: asset: normalizedAsset,178: maxTimeoutSeconds: 300,179: resource: `mcp://${name}`,180: mimeType: "application/json",181: description,182: extra,183:});
          The submitted payment is decoded and verified first:
          272: // Decode & verify273: letdecoded: PaymentPayload;274: try{275: decoded=decodeX402Payment(token);276: decoded.x402Version=x402Version;277: }catch{278: returnpaymentRequired("INVALID_PAYMENT");279: }280:
281: constselected=findMatchingPaymentRequirements(accepts,decoded);286: constvr=awaitverify(decoded,selected);287: if(!vr.isValid){288: returnpaymentRequired(vr.invalidReason??"INVALID_PAYMENT",{289: payer: vr.payer,290:});291: }
          But the paid tool callback runs before settlement:
          293: // Execute tool294: letresult: CallToolResult;295: letfailed=false;296: try{297: result=awaitcb(args,extra);
...
316: // Settle only on success317: if(!failed){318: try{319: consts=awaitsettle(decoded,selected);320: if(s.success){
          So the current order appears to be:
          payment header -> decode/select requirement -> verify -> cb(args, extra) -> settle

          Why this may matter:
          • Verification is not always the same as completed settlement.
          • A wrapped MCP tool callback can perform expensive computation, external calls, or state-changing work.
          • If settle(decoded, selected) fails after cb(args, extra) has already run, the wrapper can return a payment error but cannot undo the paid work.
          A safer design would complete settlement before invoking externally effective callbacks, or clearly document that pre-settlement callbacks must be side-effect-free. A regression test where verify succeeds and settle fails after a callback would make the intended behavior explicit.
          I am reporting this as a potential issue rather than a confirmed exploit, since the final impact depends on the callbacks users register with withX402.
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(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

              Skip to content
              Potential settlement gating issue: paid MCP callback runs before settlement #43
              Open
              Open
              Potential settlement gating issue: paid MCP callback runs before settlement#43
              Description
              @chenshj73
              chenshj73
              opened on May 28, 2026
              Hi, I noticed a possible payment-flow ordering issue while reviewing the current source.
              In packages/js-sdk/src/handler/server/plugins/with-x402.ts, withX402 builds a payment requirement for the MCP tool resource, verifies the submitted payment, executes the wrapped callback, and only then settles.
              The requirement is scoped to the tool resource and payee:
              172: reqs.push({173: scheme: "exact"asconst,174: network,175: maxAmountRequired,176: payTo: normalizedPayTo,177: asset: normalizedAsset,178: maxTimeoutSeconds: 300,179: resource: `mcp://${name}`,180: mimeType: "application/json",181: description,182: extra,183:});
              The submitted payment is decoded and verified first:
              272: // Decode & verify273: letdecoded: PaymentPayload;274: try{275: decoded=decodeX402Payment(token);276: decoded.x402Version=x402Version;277: }catch{278: returnpaymentRequired("INVALID_PAYMENT");279: }280:
281: constselected=findMatchingPaymentRequirements(accepts,decoded);286: constvr=awaitverify(decoded,selected);287: if(!vr.isValid){288: returnpaymentRequired(vr.invalidReason??"INVALID_PAYMENT",{289: payer: vr.payer,290:});291: }
              But the paid tool callback runs before settlement:
              293: // Execute tool294: letresult: CallToolResult;295: letfailed=false;296: try{297: result=awaitcb(args,extra);
...
316: // Settle only on success317: if(!failed){318: try{319: consts=awaitsettle(decoded,selected);320: if(s.success){
              So the current order appears to be:
              payment header -> decode/select requirement -> verify -> cb(args, extra) -> settle

              Why this may matter:
              • Verification is not always the same as completed settlement.
              • A wrapped MCP tool callback can perform expensive computation, external calls, or state-changing work.
              • If settle(decoded, selected) fails after cb(args, extra) has already run, the wrapper can return a payment error but cannot undo the paid work.
              A safer design would complete settlement before invoking externally effective callbacks, or clearly document that pre-settlement callbacks must be side-effect-free. A regression test where verify succeeds and settle fails after a callback would make the intended behavior explicit.
              I am reporting this as a potential issue rather than a confirmed exploit, since the final impact depends on the callbacks users register with withX402.
              Metadata
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                  Skip to content
                  Potential settlement gating issue: paid MCP callback runs before settlement #43
                  Open
                  Open
                  Potential settlement gating issue: paid MCP callback runs before settlement#43
                  Description
                  @chenshj73
                  chenshj73
                  opened on May 28, 2026
                  Hi, I noticed a possible payment-flow ordering issue while reviewing the current source.
                  In packages/js-sdk/src/handler/server/plugins/with-x402.ts, withX402 builds a payment requirement for the MCP tool resource, verifies the submitted payment, executes the wrapped callback, and only then settles.
                  The requirement is scoped to the tool resource and payee:
                  172: reqs.push({173: scheme: "exact"asconst,174: network,175: maxAmountRequired,176: payTo: normalizedPayTo,177: asset: normalizedAsset,178: maxTimeoutSeconds: 300,179: resource: `mcp://${name}`,180: mimeType: "application/json",181: description,182: extra,183:});
                  The submitted payment is decoded and verified first:
                  272: // Decode & verify273: letdecoded: PaymentPayload;274: try{275: decoded=decodeX402Payment(token);276: decoded.x402Version=x402Version;277: }catch{278: returnpaymentRequired("INVALID_PAYMENT");279: }280:
281: constselected=findMatchingPaymentRequirements(accepts,decoded);286: constvr=awaitverify(decoded,selected);287: if(!vr.isValid){288: returnpaymentRequired(vr.invalidReason??"INVALID_PAYMENT",{289: payer: vr.payer,290:});291: }
                  But the paid tool callback runs before settlement:
                  293: // Execute tool294: letresult: CallToolResult;295: letfailed=false;296: try{297: result=awaitcb(args,extra);
...
316: // Settle only on success317: if(!failed){318: try{319: consts=awaitsettle(decoded,selected);320: if(s.success){
                  So the current order appears to be:
                  payment header -> decode/select requirement -> verify -> cb(args, extra) -> settle

                  Why this may matter:
                  • Verification is not always the same as completed settlement.
                  • A wrapped MCP tool callback can perform expensive computation, external calls, or state-changing work.
                  • If settle(decoded, selected) fails after cb(args, extra) has already run, the wrapper can return a payment error but cannot undo the paid work.
                  A safer design would complete settlement before invoking externally effective callbacks, or clearly document that pre-settlement callbacks must be side-effect-free. A regression test where verify succeeds and settle fails after a callback would make the intended behavior explicit.
                  I am reporting this as a potential issue rather than a confirmed exploit, since the final impact depends on the callbacks users register with withX402.
                  Metadata
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(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

                      Skip to content
                      Potential settlement gating issue: paid MCP callback runs before settlement #43
                      Open
                      Open
                      Potential settlement gating issue: paid MCP callback runs before settlement#43
                      Description
                      @chenshj73
                      chenshj73
                      opened on May 28, 2026
                      Hi, I noticed a possible payment-flow ordering issue while reviewing the current source.
                      In packages/js-sdk/src/handler/server/plugins/with-x402.ts, withX402 builds a payment requirement for the MCP tool resource, verifies the submitted payment, executes the wrapped callback, and only then settles.
                      The requirement is scoped to the tool resource and payee:
                      172: reqs.push({173: scheme: "exact"asconst,174: network,175: maxAmountRequired,176: payTo: normalizedPayTo,177: asset: normalizedAsset,178: maxTimeoutSeconds: 300,179: resource: `mcp://${name}`,180: mimeType: "application/json",181: description,182: extra,183:});
                      The submitted payment is decoded and verified first:
                      272: // Decode & verify273: letdecoded: PaymentPayload;274: try{275: decoded=decodeX402Payment(token);276: decoded.x402Version=x402Version;277: }catch{278: returnpaymentRequired("INVALID_PAYMENT");279: }280:
281: constselected=findMatchingPaymentRequirements(accepts,decoded);286: constvr=awaitverify(decoded,selected);287: if(!vr.isValid){288: returnpaymentRequired(vr.invalidReason??"INVALID_PAYMENT",{289: payer: vr.payer,290:});291: }
                      But the paid tool callback runs before settlement:
                      293: // Execute tool294: letresult: CallToolResult;295: letfailed=false;296: try{297: result=awaitcb(args,extra);
...
316: // Settle only on success317: if(!failed){318: try{319: consts=awaitsettle(decoded,selected);320: if(s.success){
                      So the current order appears to be:
                      payment header -> decode/select requirement -> verify -> cb(args, extra) -> settle

                      Why this may matter:
                      • Verification is not always the same as completed settlement.
                      • A wrapped MCP tool callback can perform expensive computation, external calls, or state-changing work.
                      • If settle(decoded, selected) fails after cb(args, extra) has already run, the wrapper can return a payment error but cannot undo the paid work.
                      A safer design would complete settlement before invoking externally effective callbacks, or clearly document that pre-settlement callbacks must be side-effect-free. A regression test where verify succeeds and settle fails after a callback would make the intended behavior explicit.
                      I am reporting this as a potential issue rather than a confirmed exploit, since the final impact depends on the callbacks users register with withX402.
                      Metadata
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                      No one assigned
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                          No branches or pull requests
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(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

                          Skip to content
                          Potential settlement gating issue: paid MCP callback runs before settlement #43
                          Open
                          Open
                          Potential settlement gating issue: paid MCP callback runs before settlement#43
                          Description
                          @chenshj73
                          chenshj73
                          opened on May 28, 2026
                          Hi, I noticed a possible payment-flow ordering issue while reviewing the current source.
                          In packages/js-sdk/src/handler/server/plugins/with-x402.ts, withX402 builds a payment requirement for the MCP tool resource, verifies the submitted payment, executes the wrapped callback, and only then settles.
                          The requirement is scoped to the tool resource and payee:
                          172: reqs.push({173: scheme: "exact"asconst,174: network,175: maxAmountRequired,176: payTo: normalizedPayTo,177: asset: normalizedAsset,178: maxTimeoutSeconds: 300,179: resource: `mcp://${name}`,180: mimeType: "application/json",181: description,182: extra,183:});
                          The submitted payment is decoded and verified first:
                          272: // Decode & verify273: letdecoded: PaymentPayload;274: try{275: decoded=decodeX402Payment(token);276: decoded.x402Version=x402Version;277: }catch{278: returnpaymentRequired("INVALID_PAYMENT");279: }280:
281: constselected=findMatchingPaymentRequirements(accepts,decoded);286: constvr=awaitverify(decoded,selected);287: if(!vr.isValid){288: returnpaymentRequired(vr.invalidReason??"INVALID_PAYMENT",{289: payer: vr.payer,290:});291: }
                          But the paid tool callback runs before settlement:
                          293: // Execute tool294: letresult: CallToolResult;295: letfailed=false;296: try{297: result=awaitcb(args,extra);
...
316: // Settle only on success317: if(!failed){318: try{319: consts=awaitsettle(decoded,selected);320: if(s.success){
                          So the current order appears to be:
                          payment header -> decode/select requirement -> verify -> cb(args, extra) -> settle

                          Why this may matter:
                          • Verification is not always the same as completed settlement.
                          • A wrapped MCP tool callback can perform expensive computation, external calls, or state-changing work.
                          • If settle(decoded, selected) fails after cb(args, extra) has already run, the wrapper can return a payment error but cannot undo the paid work.
                          A safer design would complete settlement before invoking externally effective callbacks, or clearly document that pre-settlement callbacks must be side-effect-free. A regression test where verify succeeds and settle fails after a callback would make the intended behavior explicit.
                          I am reporting this as a potential issue rather than a confirmed exploit, since the final impact depends on the callbacks users register with withX402.
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, 'i');
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filter: invert(1) hue-rotate(180deg) !important;
background: #1a1a2e !important;
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filter: invert(1) hue-rotate(180deg) !important;
}
/* Preserve specific elements that should not be inverted */
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[data-theme="light"], [data-theme="light"],
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filter: none !important;
}
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filter: invert(1) hue-rotate(180deg) !important;
background: #2d2d44 !important;
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} catch(__e) { console.warn('[Userscript:Universal Dark Mode]', __e); }
})();
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                              Potential settlement gating issue: paid MCP callback runs before settlement #43
                              Open
                              Open
                              Potential settlement gating issue: paid MCP callback runs before settlement#43
                              Description
                              @chenshj73
                              chenshj73
                              opened on May 28, 2026
                              Hi, I noticed a possible payment-flow ordering issue while reviewing the current source.
                              In packages/js-sdk/src/handler/server/plugins/with-x402.ts, withX402 builds a payment requirement for the MCP tool resource, verifies the submitted payment, executes the wrapped callback, and only then settles.
                              The requirement is scoped to the tool resource and payee:
                              172: reqs.push({173: scheme: "exact"asconst,174: network,175: maxAmountRequired,176: payTo: normalizedPayTo,177: asset: normalizedAsset,178: maxTimeoutSeconds: 300,179: resource: `mcp://${name}`,180: mimeType: "application/json",181: description,182: extra,183:});
                              The submitted payment is decoded and verified first:
                              272: // Decode & verify273: letdecoded: PaymentPayload;274: try{275: decoded=decodeX402Payment(token);276: decoded.x402Version=x402Version;277: }catch{278: returnpaymentRequired("INVALID_PAYMENT");279: }280:
281: constselected=findMatchingPaymentRequirements(accepts,decoded);286: constvr=awaitverify(decoded,selected);287: if(!vr.isValid){288: returnpaymentRequired(vr.invalidReason??"INVALID_PAYMENT",{289: payer: vr.payer,290:});291: }
                              But the paid tool callback runs before settlement:
                              293: // Execute tool294: letresult: CallToolResult;295: letfailed=false;296: try{297: result=awaitcb(args,extra);
...
316: // Settle only on success317: if(!failed){318: try{319: consts=awaitsettle(decoded,selected);320: if(s.success){
                              So the current order appears to be:
                              payment header -> decode/select requirement -> verify -> cb(args, extra) -> settle

                              Why this may matter:
                              • Verification is not always the same as completed settlement.
                              • A wrapped MCP tool callback can perform expensive computation, external calls, or state-changing work.
                              • If settle(decoded, selected) fails after cb(args, extra) has already run, the wrapper can return a payment error but cannot undo the paid work.
                              A safer design would complete settlement before invoking externally effective callbacks, or clearly document that pre-settlement callbacks must be side-effect-free. A regression test where verify succeeds and settle fails after a callback would make the intended behavior explicit.
                              I am reporting this as a potential issue rather than a confirmed exploit, since the final impact depends on the callbacks users register with withX402.
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