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The precheck function provides fast, local validation before committing to expensive Lit Action execution. It’s your Policy’s first line of defense against invalid requests.
The precheck function is executed locally by the Vincent Ability executor to provide a best-effort check that the policy shouldn’t fail when the policy’s evaluate function is called.

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Function Parameters

The precheck function receives two parameters:
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params
object
required
Object containing the ability and user parameters
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policyContext
object
required
Context object provided by the SDK with helpers and metadata

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Response Schemas

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Allow Response

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precheckAllowResultSchema
ZodSchema
required
A Zod schema that defines the structure of successful precheck results. Include details about why the precheck passed, such as current spending limits and remaining quotas.

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Deny Response

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precheckDenyResultSchema
ZodSchema
required
A Zod schema that defines the structure of failed precheck results. Include details about why the precheck failed, such as exceeded limits or disallowed tokens.

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Example Implementation

import { createVincentPolicy } from '@lit-protocol/vincent-ability-sdk';
import { z } from 'zod';

const vincentPolicy = createVincentPolicy({
  // ... other policy definitions

  precheck: async ({ abilityParams, userParams }, policyContext) => {
    const { amount, tokenAddress } = abilityParams;
    const { dailySpendingLimit, allowedTokens } = userParams;

    const isTokenAllowed = allowedTokens.includes(tokenAddress);
    const { isSpendingLimitExceeded, currentDailySpending } = await checkSpendingLimit(
      tokenAddress,
      amount,
      dailySpendingLimit,
    );

    if (!isTokenAllowed) {
      return policyContext.deny({
        reason: 'Token not allowed',
        maxDailySpendingLimit: dailySpendingLimit,
        currentDailySpending,
        allowedTokens: allowedTokens,
      });
    }

    if (isSpendingLimitExceeded) {
      return policyContext.deny({
        reason: 'Spending limit exceeded',
        maxDailySpendingLimit: dailySpendingLimit,
        currentDailySpending,
        allowedTokens: allowedTokens,
      });
    }

    return policyContext.allow({
      maxDailySpendingLimit: dailySpendingLimit,
      currentDailySpending,
      allowedTokens: allowedTokens,
    });
  },
});

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Best Practices

  1. Validate Early - Check user parameters first before expensive operations
  2. Return Helpful Context - Include current state information in responses
  3. Handle Errors Gracefully - Provide clear error messages for debugging
  4. Keep It Fast - Avoid expensive blockchain calls in precheck

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Next Steps

Evaluate Function

Implement final validation with blockchain access

Commit Function

Update state after successful execution
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