gemini-cli/packages/core/src/policy/policy-engine.ts

/**
 * @license
 * Copyright 2025 Google LLC
 * SPDX-License-Identifier: Apache-2.0
 */

import { type FunctionCall } from '@google/genai';
import {
  PolicyDecision,
  type PolicyEngineConfig,
  type PolicyRule,
  type SafetyCheckerRule,
} from './types.js';
import { stableStringify } from './stable-stringify.js';
import { debugLogger } from '../utils/debugLogger.js';
import type { CheckerRunner } from '../safety/checker-runner.js';
import { SafetyCheckDecision } from '../safety/protocol.js';

function ruleMatches(
  rule: PolicyRule | SafetyCheckerRule,
  toolCall: FunctionCall,
  stringifiedArgs: string | undefined,
  serverName: string | undefined,
): boolean {
  // Check tool name if specified
  if (rule.toolName) {
    // Support wildcard patterns: "serverName__*" matches "serverName__anyTool"
    if (rule.toolName.endsWith('__*')) {
      const prefix = rule.toolName.slice(0, -3); // Remove "__*"
      if (serverName !== undefined) {
        // Robust check: if serverName is provided, it MUST match the prefix exactly.
        // This prevents "malicious-server" from spoofing "trusted-server" by naming itself "trusted-server__malicious".
        if (serverName !== prefix) {
          return false;
        }
      }
      // Always verify the prefix, even if serverName matched
      if (!toolCall.name || !toolCall.name.startsWith(prefix + '__')) {
        return false;
      }
    } else if (toolCall.name !== rule.toolName) {
      return false;
    }
  }

  // Check args pattern if specified
  if (rule.argsPattern) {
    // If rule has an args pattern but tool has no args, no match
    if (!toolCall.args) {
      return false;
    }
    // Use stable JSON stringification with sorted keys to ensure consistent matching
    if (
      stringifiedArgs === undefined ||
      !rule.argsPattern.test(stringifiedArgs)
    ) {
      return false;
    }
  }

  return true;
}

export class PolicyEngine {
  private rules: PolicyRule[];
  private checkers: SafetyCheckerRule[];
  private readonly defaultDecision: PolicyDecision;
  private readonly nonInteractive: boolean;
  private readonly checkerRunner?: CheckerRunner;

  constructor(config: PolicyEngineConfig = {}, checkerRunner?: CheckerRunner) {
    this.rules = (config.rules ?? []).sort(
      (a, b) => (b.priority ?? 0) - (a.priority ?? 0),
    );
    this.checkers = (config.checkers ?? []).sort(
      (a, b) => (b.priority ?? 0) - (a.priority ?? 0),
    );
    this.defaultDecision = config.defaultDecision ?? PolicyDecision.ASK_USER;
    this.nonInteractive = config.nonInteractive ?? false;
    this.checkerRunner = checkerRunner;
  }

  /**
   * Check if a tool call is allowed based on the configured policies.
   * Returns the decision and the matching rule (if any).
   */
  async check(
    toolCall: FunctionCall,
    serverName: string | undefined,
  ): Promise<{
    decision: PolicyDecision;
    rule?: PolicyRule;
  }> {
    let stringifiedArgs: string | undefined;
    // Compute stringified args once before the loop
    if (
      toolCall.args &&
      (this.rules.some((rule) => rule.argsPattern) ||
        this.checkers.some((checker) => checker.argsPattern))
    ) {
      stringifiedArgs = stableStringify(toolCall.args);
    }

    debugLogger.debug(
      `[PolicyEngine.check] toolCall.name: ${toolCall.name}, stringifiedArgs: ${stringifiedArgs}`,
    );

    // Find the first matching rule (already sorted by priority)
    let matchedRule: PolicyRule | undefined;
    let decision: PolicyDecision | undefined;

    for (const rule of this.rules) {
      if (ruleMatches(rule, toolCall, stringifiedArgs, serverName)) {
        debugLogger.debug(
          `[PolicyEngine.check] MATCHED rule: toolName=${rule.toolName}, decision=${rule.decision}, priority=${rule.priority}, argsPattern=${rule.argsPattern?.source || 'none'}`,
        );
        matchedRule = rule;
        decision = this.applyNonInteractiveMode(rule.decision);
        break;
      }
    }

    if (!decision) {
      // No matching rule found, use default decision
      debugLogger.debug(
        `[PolicyEngine.check] NO MATCH - using default decision: ${this.defaultDecision}`,
      );
      decision = this.applyNonInteractiveMode(this.defaultDecision);
    }

    // If decision is not DENY, run safety checkers
    if (decision !== PolicyDecision.DENY && this.checkerRunner) {
      for (const checkerRule of this.checkers) {
        if (ruleMatches(checkerRule, toolCall, stringifiedArgs, serverName)) {
          debugLogger.debug(
            `[PolicyEngine.check] Running safety checker: ${checkerRule.checker.name}`,
          );
          try {
            const result = await this.checkerRunner.runChecker(
              toolCall,
              checkerRule.checker,
            );

            if (result.decision === SafetyCheckDecision.DENY) {
              debugLogger.debug(
                `[PolicyEngine.check] Safety checker denied: ${result.reason}`,
              );
              return {
                decision: PolicyDecision.DENY,
                rule: matchedRule,
              };
            } else if (result.decision === SafetyCheckDecision.ASK_USER) {
              debugLogger.debug(
                `[PolicyEngine.check] Safety checker requested ASK_USER: ${result.reason}`,
              );
              decision = PolicyDecision.ASK_USER;
            }
          } catch (error) {
            debugLogger.debug(
              `[PolicyEngine.check] Safety checker failed: ${error}`,
            );
            return {
              decision: PolicyDecision.DENY,
              rule: matchedRule,
            };
          }
        }
      }
    }

    return {
      decision: this.applyNonInteractiveMode(decision),
      rule: matchedRule,
    };
  }

  /**
   * Add a new rule to the policy engine.
   */
  addRule(rule: PolicyRule): void {
    this.rules.push(rule);
    // Re-sort rules by priority
    this.rules.sort((a, b) => (b.priority ?? 0) - (a.priority ?? 0));
  }

  addChecker(checker: SafetyCheckerRule): void {
    this.checkers.push(checker);
    this.checkers.sort((a, b) => (b.priority ?? 0) - (a.priority ?? 0));
  }

  /**
   * Remove rules for a specific tool.
   */
  removeRulesForTool(toolName: string): void {
    this.rules = this.rules.filter((rule) => rule.toolName !== toolName);
  }

  /**
   * Get all current rules.
   */
  getRules(): readonly PolicyRule[] {
    return this.rules;
  }

  getCheckers(): readonly SafetyCheckerRule[] {
    return this.checkers;
  }

  private applyNonInteractiveMode(decision: PolicyDecision): PolicyDecision {
    // In non-interactive mode, ASK_USER becomes DENY
    if (this.nonInteractive && decision === PolicyDecision.ASK_USER) {
      return PolicyDecision.DENY;
    }
    return decision;
  }
}