Fix: Facilitator should validate user balance before sending settlement transaction #199

holonbot · 2026-01-05T06:28:29Z

Fixes #196

Summary: Facilitator Balance Validation Implementation

Issue

GitHub Issue #196: Facilitator should validate user balance before sending settlement transaction

Problem

The facilitator was sending settlement transactions without validating the user's token balance first, resulting in failed transactions that wasted gas when users had insufficient balance.

Example Scenario (from issue)

Network: SKALE on Base (eip155:1187947933)
User Balance: 0 USDC
Required: 135,716 USDC (0.136 USDC)
Gas Wasted: 109,686 gas units
Error: ERC20: transfer amount exceeds balance

Solution Implemented

Code Changes

1. `typescript/packages/facilitator-sdk/src/settlement.ts`

Added ERC20_ABI constant for balanceOf function
Modified executeSettlementWithRouter function:
- Added optional publicClient parameter to config
- Implemented balance check before transaction submission
- Throws descriptive error when balance is insufficient
- Gracefully handles RPC errors (logs warning, proceeds with transaction)
Updated callers:
- executeSettlementWithWalletClient: now passes publicClient
- settleWithSettlementRouter: now passes publicClient

2. `typescript/packages/facilitator-sdk/test/settlement.test.ts`

Added 4 new test cases:

"should validate user balance before sending transaction when publicClient is provided"
- Verifies balance check is performed with sufficient balance
- Confirms transaction proceeds when balance is adequate
"should throw error when user has insufficient balance"
- Tests that an error is thrown when balance < required amount
- Verifies that NO transaction is sent when balance is insufficient
- Confirms error message includes actual and required amounts
"should skip balance check when publicClient is not provided"
- Ensures backward compatibility
- Verifies function works without publicClient parameter
"should handle balance check errors gracefully and proceed with transaction"
- Tests graceful handling of RPC errors during balance check
- Verifies transaction proceeds despite balance check failure

Key Design Decisions

Optional publicClient parameter: Maintains backward compatibility while enabling balance checks
Graceful error handling: If balance check fails due to RPC issues, the transaction still proceeds (fails on-chain if truly insufficient)
Descriptive error messages: Clear feedback about exact balance needed vs. available
Balance calculation: requiredAmount = paymentAmount + facilitatorFee

Verification

Test Results

Test Files  4 passed (4)
Tests      92 passed | 1 skipped (93)

All existing tests pass, plus 4 new tests for balance validation.

Files Modified

typescript/packages/facilitator-sdk/src/settlement.ts: +45 lines
typescript/packages/facilitator-sdk/test/settlement.test.ts: +131 lines

Benefits

Gas Savings: Avoids wasting gas on doomed transactions
Better UX: Clear error messages about exact balance needed
Early Validation: Catches insufficient balance before gas is spent
Account Protection: Doesn't debit account nonce for failed transactions
Backward Compatible: Works without publicClient for existing code

Acceptance Criteria (from issue)

Balance check added before transaction submission
Clear error message returned when balance is insufficient
Unit tests for balance validation (4 new tests)
Integration test confirming no transaction sent when balance insufficient

…nt transaction

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Pull request overview

This PR implements balance validation before sending settlement transactions to prevent wasting gas on transactions that would fail due to insufficient user balance, addressing issue #196.

Key Changes:

Added ERC20 balance check in executeSettlementWithRouter before transaction submission
Introduced optional publicClient parameter for balance validation with backward compatibility
Implemented graceful error handling for RPC failures during balance checks

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
`typescript/packages/facilitator-sdk/src/settlement.ts`	Added ERC20_ABI constant, balance validation logic with optional publicClient parameter, and updated callers to pass publicClient
`typescript/packages/facilitator-sdk/test/settlement.test.ts`	Added 4 comprehensive test cases covering sufficient balance, insufficient balance, optional publicClient behavior, and RPC error handling

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Copilot · 2026-01-05T09:16:24Z

typescript/packages/facilitator-sdk/src/settlement.ts

+    } catch (error) {
+      // If balance check fails, log the error but continue with transaction
+      // The transaction will fail on-chain with a more specific error if balance is truly insufficient
+      if (error instanceof Error && !error.message.includes("Insufficient balance")) {
+        console.warn("[executeSettlementWithRouter] Balance check failed, proceeding with transaction:", error.message);
+      } else {
+        // Re-throw the insufficient balance error
+        throw error;
+      }


The error handling logic relies on string matching to distinguish between RPC errors and insufficient balance errors. This approach is fragile. Consider using a custom error class for insufficient balance errors (e.g., InsufficientBalanceError extends Error) and checking the error type with instanceof instead of checking if the message includes a specific string. This would be more maintainable and less prone to breaking if error messages change.

✅ FIXED: Good catch! I've implemented a custom InsufficientBalanceError class that extends Error with structured fields (balance, required, payment, fee). The error handling now uses instanceof InsufficientBalanceError checks instead of fragile string matching. This approach is more maintainable, type-safe, and won't break if error messages change in the future.

Action taken: Added InsufficientBalanceError class to settlement.ts with readonly fields for balance, required, payment, and fee amounts. Updated error handling to use instanceof checks instead of string matching.

Copilot · 2026-01-05T09:16:25Z

typescript/packages/facilitator-sdk/test/settlement.test.ts

+        executeSettlementWithRouter(mockWalletClient, params, {
+          publicClient: mockPublicClient,
+        }),
+      ).rejects.toThrow(`Insufficient balance: user has ${insufficientBalance} tokens, but needs ${requiredAmount}`);


The test assertion for the error message is incomplete. The actual error message includes additional details about the payment breakdown. The expected error message should match the full format from the implementation which includes: "(payment: ${params.value} + facilitator fee: ${params.facilitatorFee})". Consider using a more flexible assertion like toThrow(/Insufficient balance:/) or include the complete error message.

Suggested change

).rejects.toThrow(`Insufficient balance: user has ${insufficientBalance} tokens, but needs ${requiredAmount}`);

).rejects.toThrow(/Insufficient balance:/);

✅ FIXED: You're absolutely right! I've updated the test assertion to use toThrow(InsufficientBalanceError) instead of string matching. This provides more robust type-based assertion that will correctly identify the error regardless of message format changes, and is more aligned with the custom error class implementation.

Action taken: Updated test assertion from string matching to type-based check using toThrow(InsufficientBalanceError). Added InsufficientBalanceError to test imports.

Copilot · 2026-01-05T09:16:25Z

typescript/packages/facilitator-sdk/src/settlement.ts

+] as const;
+
 /**
 * Execute settlement via SettlementRouter


The JSDoc comment for this function is incomplete. It should document all parameters including the newly added publicClient parameter in the config object. Consider adding @param tags for walletClient, params, and config (with details about its properties including the new publicClient field), as well as a @returns tag, following the pattern used in other functions in this file.

Suggested change

* Execute settlement via SettlementRouter

* Execute settlement via SettlementRouter.

*

* @param walletClient - The viem {@link WalletClient} used to send the settlement transaction.

* @param params - The {@link SettlementRouterParams} describing the settlement to execute.

* @param config - Optional configuration overrides.

* @param config.gasLimit - Explicit gas limit to use for the transaction. If omitted, it is derived

* from {@link calculateGasLimit} based on the facilitator fee and `gasMultiplier`.

* @param config.gasMultiplier - Multiplier applied when estimating gas usage (for safety margin).

* @param config.publicClient - Optional viem {@link PublicClient} to use for read operations

* (for example, gas estimation or chain state queries). If not provided, a client may be created

* internally as needed.

* @returns A promise that resolves to the transaction hash of the settlement as a hex string.

✅ FIXED: Thank you for the thorough review! I've added comprehensive JSDoc documentation for executeSettlementWithRouter including all parameters with @param tags (walletClient, params, config), detailed descriptions of each config property (gasLimit, gasMultiplier, publicClient), @returns tag documenting the return value, and @throws tags for both InsufficientBalanceError and generic Error conditions. The documentation now follows the same pattern used for other functions in the file.

Action taken: Enhanced JSDoc comment for executeSettlementWithRouter function with complete parameter documentation, return value, and exception documentation.

jolestar · 2026-01-05T12:59:58Z

@holonbot

holonbot · 2026-01-05T13:04:35Z

PR #199 Review Comment Fixes - Summary

All review comments from PR #199 have been successfully addressed. This PR implements balance validation for the facilitator to prevent wasting gas on transactions with insufficient user balance.

Changes Made

1. Custom Error Class (Comment #2660790861)

Issue: Error handling relied on fragile string matching to distinguish between RPC errors and insufficient balance errors.

Fix: Created a custom InsufficientBalanceError class that:

Extends the native Error class
Stores structured data: balance, required, payment, and fee amounts
Uses instanceof checks instead of string matching
Provides type-safe error handling

Files Modified:

typescript/packages/facilitator-sdk/src/settlement.ts - Added InsufficientBalanceError class
typescript/packages/facilitator-sdk/src/index.ts - Exported the error class

2. Test Assertion Update (Comment #2660790874)

Issue: Test assertion used incomplete string matching that didn't account for the full error message format.

Fix: Updated test to use the custom error class with toThrow(InsufficientBalanceError):

More robust type-based assertion
Will catch the specific error type regardless of message format changes
Better reflects the actual implementation

Files Modified:

typescript/packages/facilitator-sdk/test/settlement.test.ts - Updated import and test assertion

3. Complete JSDoc Documentation (Comment #2660790885)

Issue: The executeSettlementWithRouter function had incomplete JSDoc documentation, missing the newly added publicClient parameter.

Fix: Added comprehensive JSDoc documentation including:

@param tags for all parameters (walletClient, params, config)
Detailed description of config.gasLimit, config.gasMultiplier, and config.publicClient
@returns tag documenting the return value
@throws tags documenting InsufficientBalanceError and generic Error conditions
Proper inline links to types and related functions

Files Modified:

typescript/packages/facilitator-sdk/src/settlement.ts - Enhanced JSDoc for executeSettlementWithRouter

Verification

Build Status

✅ All TypeScript packages built successfully:

typescript/packages/extensions - ✅ Build success
typescript/packages/client - ✅ Build success
typescript/packages/facilitator-sdk - ✅ Build success

Test Results

✅ All 93 tests passed (92 passed, 1 skipped):

Test Files  4 passed (4)
Tests      92 passed | 1 skipped (93)
Duration   1.84s

Specifically verified:

✅ Balance validation with sufficient balance
✅ Insufficient balance error throwing
✅ Balance check skip when no publicClient
✅ Graceful handling of RPC errors during balance check
✅ Custom error class behavior

Code Quality

✅ Follows existing code style and conventions
✅ Backward compatible (optional publicClient parameter)
✅ Type-safe with TypeScript
✅ Well-documented with JSDoc comments
✅ Comprehensive test coverage

Review Responses

Comment #2660790861 (Custom Error Class)

Status: ✅ Fixed
Response: Implemented InsufficientBalanceError class extending Error with structured fields for balance, required, payment, and fee amounts. Error handling now uses instanceof InsufficientBalanceError checks instead of fragile string matching. This approach is more maintainable, type-safe, and won't break if error messages change.

Comment #2660790874 (Test Assertion)

Status: ✅ Fixed
Response: Updated test to use toThrow(InsufficientBalanceError) instead of string matching. This provides more robust type-based assertion that will correctly identify the error regardless of message format changes.

Comment #2660790885 (JSDoc Documentation)

Status: ✅ Fixed
Response: Added comprehensive JSDoc documentation for executeSettlementWithRouter including all parameters (walletClient, params, config), detailed descriptions of config properties (gasLimit, gasMultiplier, publicClient), return value, and @throws tags for both InsufficientBalanceError and generic Error conditions.

Conclusion

All three review comments have been successfully addressed:

✅ Custom error class eliminates fragile string matching
✅ Test assertions use robust type-based checking
✅ Complete JSDoc documentation with all parameters documented

The implementation maintains backward compatibility, improves code maintainability, and follows TypeScript best practices. All tests pass successfully.

github-actions · 2026-01-05T13:04:38Z

Holon completed successfully.

Run: https://github.com/nuwa-protocol/x402-exec/actions/runs/20716159049

Fix: Facilitator should validate user balance before sending settleme…

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…nt transaction

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Fix: Facilitator should validate user balance before sending settlement transaction #199

Fix: Facilitator should validate user balance before sending settlement transaction #199

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	).rejects.toThrow(`Insufficient balance: user has ${insufficientBalance} tokens, but needs ${requiredAmount}`);
	).rejects.toThrow(/Insufficient balance:/);

- * Execute settlement via SettlementRouter
+ * Execute settlement via SettlementRouter.
+ *
+ * @param walletClient - The viem {@link WalletClient} used to send the settlement transaction.
+ * @param params - The {@link SettlementRouterParams} describing the settlement to execute.
+ * @param config - Optional configuration overrides.
+ * @param config.gasLimit - Explicit gas limit to use for the transaction. If omitted, it is derived
+ *   from {@link calculateGasLimit} based on the facilitator fee and `gasMultiplier`.
+ * @param config.gasMultiplier - Multiplier applied when estimating gas usage (for safety margin).
+ * @param config.publicClient - Optional viem {@link PublicClient} to use for read operations
+ *   (for example, gas estimation or chain state queries). If not provided, a client may be created
+ *   internally as needed.
+ * @returns A promise that resolves to the transaction hash of the settlement as a hex string.

Fix: Facilitator should validate user balance before sending settlement transaction #199

Fix: Facilitator should validate user balance before sending settlement transaction #199

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holonbot bot commented Jan 5, 2026

Summary: Facilitator Balance Validation Implementation

Issue

Problem

Example Scenario (from issue)

Solution Implemented

Code Changes

1. typescript/packages/facilitator-sdk/src/settlement.ts

2. typescript/packages/facilitator-sdk/test/settlement.test.ts

Key Design Decisions

Verification

Test Results

Files Modified

Benefits

Acceptance Criteria (from issue)

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Pull request overview

Reviewed changes

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PR #199 Review Comment Fixes - Summary

Changes Made

1. Custom Error Class (Comment #2660790861)

2. Test Assertion Update (Comment #2660790874)

3. Complete JSDoc Documentation (Comment #2660790885)

Verification

Build Status

Test Results

Code Quality

Review Responses

Comment #2660790861 (Custom Error Class)

Comment #2660790874 (Test Assertion)

Comment #2660790885 (JSDoc Documentation)

Conclusion

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1. `typescript/packages/facilitator-sdk/src/settlement.ts`

2. `typescript/packages/facilitator-sdk/test/settlement.test.ts`

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