feat(core): enhance shell command validation and add core tools allowlist (#25720)

Co-authored-by: David Pierce <davidapierce@google.com>
Co-authored-by: Keith Schaab <keithsc@google.com>
Co-authored-by: Keith Schaab <keith.schaab@gmail.com>
Co-authored-by: Emily Hedlund <ehedlund@google.com>

packages/cli/src/config/settingsSchema.ts

@@ -1667,6 +1667,19 @@ const SETTINGS_SCHEMA = {
         showInDialog: false,
         items: { type: 'string' },
       },
+      confirmationRequired: {
+        type: 'array',
+        label: 'Confirmation Required',
+        category: 'Advanced',
+        requiresRestart: true,
+        default: undefined as string[] | undefined,
+        description: oneLine`
+          Tool names that always require user confirmation.
+          Takes precedence over allowed tools and core tool allowlists.
+        `,
+        showInDialog: false,
+        items: { type: 'string' },
+      },
       exclude: {
         type: 'array',
         label: 'Exclude Tools',

packages/cli/src/ui/components/__snapshots__/InputPrompt.test.tsx.snap

@@ -168,6 +168,13 @@ exports[`InputPrompt > mouse interaction > should toggle paste expansion on doub
 "
 `;
 
+exports[`InputPrompt > mouse interaction > should toggle paste expansion on double-click 4`] = `
+"▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀
+ > [Pasted Text: 10 lines]                                                                          
+▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄
+"
+`;
+
 exports[`InputPrompt > multiline rendering > should correctly render multiline input including blank lines 1`] = `
 "────────────────────────────────────────────────────────────────────────────────────────────────────
  > hello

packages/core/src/policy/config.ts

@@ -74,7 +74,9 @@ export const ADMIN_POLICY_TIER = 5;
 
 export const MCP_EXCLUDED_PRIORITY = USER_POLICY_TIER + 0.9;
 export const EXCLUDE_TOOLS_FLAG_PRIORITY = USER_POLICY_TIER + 0.4;
+export const CONFIRMATION_REQUIRED_PRIORITY = USER_POLICY_TIER + 0.35;
 export const ALLOWED_TOOLS_FLAG_PRIORITY = USER_POLICY_TIER + 0.3;
+export const CORE_TOOLS_FLAG_PRIORITY = USER_POLICY_TIER + 0.25;
 export const TRUSTED_MCP_SERVER_PRIORITY = USER_POLICY_TIER + 0.2;
 export const ALLOWED_MCP_SERVER_PRIORITY = USER_POLICY_TIER + 0.1;
 
@@ -434,10 +436,21 @@ export async function createPolicyEngineConfig(
     }
   }
 
-  // Tools that are explicitly allowed in the settings.
-  // Priority: ALLOWED_TOOLS_FLAG_PRIORITY (user tier - explicit temporary allows)
-  if (settings.tools?.allowed) {
-    for (const tool of settings.tools.allowed) {
+  const nonPlanModes = [
+    ApprovalMode.DEFAULT,
+    ApprovalMode.AUTO_EDIT,
+    ApprovalMode.YOLO,
+  ];
+
+  const mapToolsToRules = (
+    tools: string[],
+    priority: number,
+    source: string,
+    modes?: ApprovalMode[],
+    addDefaultDenyForTools = false,
+  ) => {
+    const toolsWithNarrowing = new Set<string>();
+    for (const tool of tools) {
       // Check for legacy format: toolName(args)
       const match = tool.match(/^([a-zA-Z0-9_-]+)\((.*)\)$/);
       if (match) {
@@ -449,15 +462,17 @@ export async function createPolicyEngineConfig(
 
         // Treat args as a command prefix for shell tool
         if (toolName === SHELL_TOOL_NAME) {
+          toolsWithNarrowing.add(toolName);
           const patterns = buildArgsPatterns(undefined, args);
           for (const pattern of patterns) {
             if (pattern) {
               rules.push({
                 toolName,
                 decision: PolicyDecision.ALLOW,
-                priority: ALLOWED_TOOLS_FLAG_PRIORITY,
+                priority,
                 argsPattern: new RegExp(pattern),
-                source: 'Settings (Tools Allowed)',
+                source,
+                modes,
               });
             }
           }
@@ -467,8 +482,9 @@ export async function createPolicyEngineConfig(
           rules.push({
             toolName,
             decision: PolicyDecision.ALLOW,
-            priority: ALLOWED_TOOLS_FLAG_PRIORITY,
-            source: 'Settings (Tools Allowed)',
+            priority,
+            source,
+            modes,
           });
         }
       } else {
@@ -479,11 +495,70 @@ export async function createPolicyEngineConfig(
         rules.push({
           toolName,
           decision: PolicyDecision.ALLOW,
-          priority: ALLOWED_TOOLS_FLAG_PRIORITY,
-          source: 'Settings (Tools Allowed)',
+          priority,
+          source,
+          modes,
         });
       }
     }
+
+    if (addDefaultDenyForTools) {
+      for (const toolName of toolsWithNarrowing) {
+        rules.push({
+          toolName,
+          decision: PolicyDecision.DENY,
+          priority: priority - 0.01,
+          source: `${source} (Narrowing Enforcement)`,
+          modes,
+        });
+      }
+    }
+  };
+
+  // Tools that are explicitly allowed in the settings.
+  // Priority: ALLOWED_TOOLS_FLAG_PRIORITY (user tier - explicit temporary allows)
+  if (settings.tools?.allowed) {
+    mapToolsToRules(
+      settings.tools.allowed,
+      ALLOWED_TOOLS_FLAG_PRIORITY,
+      'Settings (Tools Allowed)',
+      undefined,
+      true,
+    );
+  }
+
+  // Tools that explicitly require confirmation in the settings.
+  // Priority: CONFIRMATION_REQUIRED_PRIORITY (overrides allowed and core)
+  if (settings.tools?.confirmationRequired) {
+    for (const tool of settings.tools.confirmationRequired) {
+      rules.push({
+        toolName: SHELL_TOOL_NAMES.includes(tool) ? SHELL_TOOL_NAME : tool,
+        decision: PolicyDecision.ASK_USER,
+        priority: CONFIRMATION_REQUIRED_PRIORITY,
+        source: 'Settings (Confirmation Required)',
+      });
+    }
+  }
+
+  // Core tools that are restricted in the settings.
+  // Priority: CORE_TOOLS_FLAG_PRIORITY (user tier - core tool allowlist)
+  if (settings.tools?.core) {
+    mapToolsToRules(
+      settings.tools.core,
+      CORE_TOOLS_FLAG_PRIORITY,
+      'Settings (Core Tools)',
+      nonPlanModes,
+    );
+
+    // If core tools are restricted, we should add a default DENY rule for everything else
+    // at a slightly lower priority than the explicit allows.
+    rules.push({
+      toolName: '*',
+      decision: PolicyDecision.DENY,
+      priority: CORE_TOOLS_FLAG_PRIORITY - 0.01,
+      source: 'Settings (Core Tools Allowlist Enforcement)',
+      modes: nonPlanModes,
+    });
   }
 
   // MCP servers that are trusted in the settings.
@@ -501,6 +576,7 @@ export async function createPolicyEngineConfig(
           decision: PolicyDecision.ALLOW,
           priority: TRUSTED_MCP_SERVER_PRIORITY,
           source: 'Settings (MCP Trusted)',
+          modes: nonPlanModes,
         });
       }
     }
@@ -519,6 +595,7 @@ export async function createPolicyEngineConfig(
         decision: PolicyDecision.ALLOW,
         priority: ALLOWED_MCP_SERVER_PRIORITY,
         source: 'Settings (MCP Allowed)',
+        modes: nonPlanModes,
       });
     }
   }

packages/core/src/policy/core-tools-mapping.test.ts

@@ -0,0 +1,76 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, expect } from 'vitest';
+import { createPolicyEngineConfig } from './config.js';
+import { PolicyEngine } from './policy-engine.js';
+import { PolicyDecision, ApprovalMode } from './types.js';
+
+describe('PolicyEngine - Core Tools Mapping', () => {
+  it('should allow tools explicitly listed in settings.tools.core', async () => {
+    const settings = {
+      tools: {
+        core: ['run_shell_command(ls)', 'run_shell_command(git status)'],
+      },
+    };
+
+    const config = await createPolicyEngineConfig(
+      settings,
+      ApprovalMode.DEFAULT,
+      undefined,
+      true, // interactive
+    );
+
+    const engine = new PolicyEngine(config);
+
+    // Test simple tool name
+    const result1 = await engine.check(
+      { name: 'run_shell_command', args: { command: 'ls' } },
+      undefined,
+    );
+    expect(result1.decision).toBe(PolicyDecision.ALLOW);
+
+    // Test tool name with args
+    const result2 = await engine.check(
+      { name: 'run_shell_command', args: { command: 'git status' } },
+      undefined,
+    );
+    expect(result2.decision).toBe(PolicyDecision.ALLOW);
+
+    // Test tool not in core list
+    const result3 = await engine.check(
+      { name: 'run_shell_command', args: { command: 'npm test' } },
+      undefined,
+    );
+    // Should be DENIED because of strict allowlist
+    expect(result3.decision).toBe(PolicyDecision.DENY);
+  });
+
+  it('should allow tools in tools.core even if they are restricted by default policies', async () => {
+    // By default run_shell_command is ASK_USER.
+    // Putting it in tools.core should make it ALLOW.
+    const settings = {
+      tools: {
+        core: ['run_shell_command'],
+      },
+    };
+
+    const config = await createPolicyEngineConfig(
+      settings,
+      ApprovalMode.DEFAULT,
+      undefined,
+      true,
+    );
+
+    const engine = new PolicyEngine(config);
+
+    const result = await engine.check(
+      { name: 'run_shell_command', args: { command: 'any command' } },
+      undefined,
+    );
+    expect(result.decision).toBe(PolicyDecision.ALLOW);
+  });
+});

packages/core/src/policy/policy-engine.test.ts

@@ -43,6 +43,35 @@ vi.mock('../utils/shell-utils.js', async (importOriginal) => {
       }
       return [command];
     }),
+    parseCommandDetails: vi.fn().mockImplementation((command: string) => {
+      // Basic mock implementation for PolicyEngine test needs
+      const commands = command.includes('&&')
+        ? command.split('&&').map((c) => c.trim())
+        : [command.trim()];
+
+      // Detect $(...) or `...` and add as sub-commands for recursion tests
+      const subCommands = [...commands];
+      for (const cmd of commands) {
+        const subMatch = cmd.match(/\$\((.*)\)/) || cmd.match(/`(.*)`/);
+        if (subMatch?.[1]) {
+          subCommands.push(subMatch[1].trim());
+        }
+      }
+
+      return {
+        details: subCommands.map((c, i) => ({
+          name: c.split(' ')[0],
+          text: c,
+          startIndex: i === 0 ? 0 : -1, // Simple root indication
+        })),
+        hasError: false,
+      };
+    }),
+    stripShellWrapper: vi.fn().mockImplementation((command: string) => {
+      // Simple mock for stripping wrappers
+      const match = command.match(/^(?:bash|sh|zsh)\s+-c\s+["'](.*)["']$/i);
+      return match ? match[1] : command;
+    }),
     hasRedirection: vi.fn().mockImplementation(
       (command: string) =>
         // Simple mock: true if '>' is present, unless it looks like "-> arrow"
@@ -1862,7 +1891,6 @@ describe('PolicyEngine', () => {
     });
 
     it('should return ASK_USER in non-YOLO mode if shell command parsing fails', async () => {
-      const { splitCommands } = await import('../utils/shell-utils.js');
       const rules: PolicyRule[] = [
         {
           toolName: 'run_shell_command',
@@ -1877,7 +1905,11 @@ describe('PolicyEngine', () => {
       });
 
       // Simulate parsing failure
-      vi.mocked(splitCommands).mockReturnValueOnce([]);
+      const { parseCommandDetails } = await import('../utils/shell-utils.js');
+      vi.mocked(parseCommandDetails).mockReturnValueOnce({
+        details: [],
+        hasError: true,
+      });
 
       const result = await engine.check(
         { name: 'run_shell_command', args: { command: 'complex command' } },

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

@@ -7,8 +7,10 @@
 import { type FunctionCall } from '@google/genai';
 import {
   SHELL_TOOL_NAMES,
+  REDIRECTION_NAMES,
   initializeShellParsers,
-  splitCommands,
+  parseCommandDetails,
+  stripShellWrapper,
   hasRedirection,
   extractStringFromParseEntry,
 } from '../utils/shell-utils.js';
@@ -359,7 +361,8 @@ export class PolicyEngine {
     }
 
     await initializeShellParsers();
-    const subCommands = splitCommands(command);
+    const parsed = parseCommandDetails(command);
+    const subCommands = parsed?.details ?? [];
 
     if (subCommands.length === 0) {
       // If the matched rule says DENY, we should respect it immediately even if parsing fails.
@@ -380,115 +383,109 @@ export class PolicyEngine {
       );
 
       // Parsing logic failed, we can't trust it. Use default decision ASK_USER (or DENY in non-interactive).
-      // We return the rule that matched so the evaluation loop terminates.
       return {
         decision: this.defaultDecision,
         rule,
       };
     }
 
-    // If there are multiple parts, or if we just want to validate the single part against DENY rules
-    if (subCommands.length > 0) {
-      debugLogger.debug(
-        `[PolicyEngine.check] Validating shell command: ${subCommands.length} parts`,
-      );
+    debugLogger.debug(
+      `[PolicyEngine.check] Validating shell command: ${subCommands.length} parts`,
+    );
 
-      if (ruleDecision === PolicyDecision.DENY) {
-        return { decision: PolicyDecision.DENY, rule };
-      }
+    if (ruleDecision === PolicyDecision.DENY) {
+      return { decision: PolicyDecision.DENY, rule };
+    }
 
-      // Start optimistically. If all parts are ALLOW, the whole is ALLOW.
-      // We will downgrade if any part is ASK_USER or DENY.
-      let aggregateDecision = PolicyDecision.ALLOW;
-      let responsibleRule: PolicyRule | undefined;
+    // Start with the decision from the rule or heuristics.
+    // If the tool call was already downgraded (e.g. by heuristics), we start there.
+    let aggregateDecision = ruleDecision;
 
-      // Check for redirection on the full command string
-      if (this.shouldDowngradeForRedirection(command, allowRedirection)) {
+    // If heuristics downgraded the decision, we don't blame the rule.
+    let responsibleRule: PolicyRule | undefined =
+      rule && ruleDecision === rule.decision ? rule : undefined;
+
+    // Check for redirection on the full command string.
+    // Redirection always downgrades ALLOW to ASK_USER (it never upgrades).
+    if (this.shouldDowngradeForRedirection(command, allowRedirection)) {
+      if (aggregateDecision === PolicyDecision.ALLOW) {
         debugLogger.debug(
           `[PolicyEngine.check] Downgrading ALLOW to ASK_USER for redirected command: ${command}`,
         );
         aggregateDecision = PolicyDecision.ASK_USER;
         responsibleRule = undefined; // Inherent policy
       }
+    }
 
-      for (const rawSubCmd of subCommands) {
-        const subCmd = rawSubCmd.trim();
-        // Prevent infinite recursion for the root command
-        if (subCmd === command) {
-          if (this.shouldDowngradeForRedirection(subCmd, allowRedirection)) {
-            debugLogger.debug(
-              `[PolicyEngine.check] Downgrading ALLOW to ASK_USER for redirected command: ${subCmd}`,
-            );
-            // Redirection always downgrades ALLOW to ASK_USER
-            if (aggregateDecision === PolicyDecision.ALLOW) {
-              aggregateDecision = PolicyDecision.ASK_USER;
-              responsibleRule = undefined; // Inherent policy
-            }
+    for (const detail of subCommands) {
+      if (REDIRECTION_NAMES.has(detail.name)) {
+        continue;
+      }
+
+      const subCmd = detail.text.trim();
+      const isAtomic =
+        subCmd === command ||
+        (detail.startIndex === 0 && detail.text.length === command.length);
+
+      // Recursive check for shell wrappers (bash -c, etc.)
+      const stripped = stripShellWrapper(subCmd);
+      if (stripped !== subCmd) {
+        const wrapperResult = await this.check(
+          { name: toolName, args: { command: stripped, dir_path } },
+          serverName,
+          toolAnnotations,
+          subagent,
+          true,
+        );
+
+        if (wrapperResult.decision === PolicyDecision.DENY)
+          return wrapperResult;
+        if (wrapperResult.decision === PolicyDecision.ASK_USER) {
+          if (aggregateDecision === PolicyDecision.ALLOW) {
+            responsibleRule = wrapperResult.rule;
           } else {
-            // Atomic command matching the rule.
-            if (
-              ruleDecision === PolicyDecision.ASK_USER &&
-              aggregateDecision === PolicyDecision.ALLOW
-            ) {
-              aggregateDecision = PolicyDecision.ASK_USER;
-              responsibleRule = rule;
-            }
+            responsibleRule ??= wrapperResult.rule;
           }
-          continue;
+          aggregateDecision = PolicyDecision.ASK_USER;
         }
+      }
 
+      if (!isAtomic) {
         const subResult = await this.check(
           { name: toolName, args: { command: subCmd, dir_path } },
           serverName,
           toolAnnotations,
           subagent,
+          true,
         );
 
-        // subResult.decision is already filtered through applyNonInteractiveMode by this.check()
-        const subDecision = subResult.decision;
+        if (subResult.decision === PolicyDecision.DENY) return subResult;
 
-        // If any part is DENIED, the whole command is DENY
-        if (subDecision === PolicyDecision.DENY) {
-          return {
-            decision: PolicyDecision.DENY,
-            rule: subResult.rule,
-          };
-        }
-
-        // If any part requires ASK_USER, the whole command requires ASK_USER
-        if (subDecision === PolicyDecision.ASK_USER) {
-          aggregateDecision = PolicyDecision.ASK_USER;
-          if (!responsibleRule) {
+        if (subResult.decision === PolicyDecision.ASK_USER) {
+          if (aggregateDecision === PolicyDecision.ALLOW) {
             responsibleRule = subResult.rule;
+          } else {
+            responsibleRule ??= subResult.rule;
           }
+          aggregateDecision = PolicyDecision.ASK_USER;
         }
 
-        // Check for redirection in allowed sub-commands
+        // Downgrade if sub-command has redirection
         if (
-          subDecision === PolicyDecision.ALLOW &&
+          subResult.decision === PolicyDecision.ALLOW &&
           this.shouldDowngradeForRedirection(subCmd, allowRedirection)
         ) {
-          debugLogger.debug(
-            `[PolicyEngine.check] Downgrading ALLOW to ASK_USER for redirected command: ${subCmd}`,
-          );
           if (aggregateDecision === PolicyDecision.ALLOW) {
             aggregateDecision = PolicyDecision.ASK_USER;
             responsibleRule = undefined;
           }
         }
       }
-
-      return {
-        decision: aggregateDecision,
-        // If we stayed at ALLOW, we return the original rule (if any).
-        // If we downgraded, we return the responsible rule (or undefined if implicit).
-        rule: aggregateDecision === ruleDecision ? rule : responsibleRule,
-      };
     }
 
     return {
-      decision: ruleDecision,
-      rule,
+      decision: aggregateDecision,
+      rule: aggregateDecision === ruleDecision ? rule : responsibleRule,
     };
   }
 
@@ -501,6 +498,7 @@ export class PolicyEngine {
     serverName: string | undefined,
     toolAnnotations?: Record<string, unknown>,
     subagent?: string,
+    skipHeuristics = false,
   ): Promise<CheckResult> {
     // Case 1: Metadata injection is the primary and safest way to identify an MCP server.
     // If we have explicit `_serverName` metadata (usually injected by tool-registry for active tools), use it.
@@ -594,6 +592,7 @@ export class PolicyEngine {
 
         let ruleDecision = rule.decision;
         if (
+          !skipHeuristics &&
           isShellCommand &&
           command &&
           !('commandPrefix' in rule) &&
@@ -615,12 +614,10 @@ export class PolicyEngine {
             subagent,
           );
           decision = shellResult.decision;
-          if (shellResult.rule) {
-            matchedRule = shellResult.rule;
-            break;
-          }
+          matchedRule = shellResult.rule;
+          break;
         } else {
-          decision = rule.decision;
+          decision = ruleDecision;
           matchedRule = rule;
           break;
         }
@@ -643,7 +640,7 @@ export class PolicyEngine {
       );
       if (toolName && SHELL_TOOL_NAMES.includes(toolName)) {
         let heuristicDecision = this.defaultDecision;
-        if (command) {
+        if (!skipHeuristics && command) {
           heuristicDecision = await this.applyShellHeuristics(
             command,
             heuristicDecision,

packages/core/src/policy/shell-safety-regression.test.ts

@@ -0,0 +1,134 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, expect, beforeAll } from 'vitest';
+import { PolicyEngine } from './policy-engine.js';
+import { PolicyDecision, ApprovalMode } from './types.js';
+import { initializeShellParsers } from '../utils/shell-utils.js';
+import { buildArgsPatterns } from './utils.js';
+
+describe('PolicyEngine - Shell Safety Regression Suite', () => {
+  let engine: PolicyEngine;
+
+  beforeAll(async () => {
+    await initializeShellParsers();
+  });
+
+  const setupEngine = (allowedCommands: string[]) => {
+    const rules = allowedCommands.map((cmd) => ({
+      toolName: 'run_shell_command',
+      decision: PolicyDecision.ALLOW,
+      argsPattern: new RegExp(buildArgsPatterns(undefined, cmd)[0]!),
+      priority: 10,
+    }));
+
+    return new PolicyEngine({
+      rules,
+      approvalMode: ApprovalMode.DEFAULT,
+      defaultDecision: PolicyDecision.ASK_USER,
+    });
+  };
+
+  it('should block unauthorized chained command with &&', async () => {
+    engine = setupEngine(['echo']);
+    const result = await engine.check(
+      { name: 'run_shell_command', args: { command: 'echo hi && ls' } },
+      undefined,
+    );
+    expect(result.decision).toBe(PolicyDecision.ASK_USER);
+  });
+
+  it('should allow authorized chained command with &&', async () => {
+    engine = setupEngine(['echo', 'ls']);
+    const result = await engine.check(
+      { name: 'run_shell_command', args: { command: 'echo hi && ls' } },
+      undefined,
+    );
+    expect(result.decision).toBe(PolicyDecision.ALLOW);
+  });
+
+  it('should block unauthorized chained command with ||', async () => {
+    engine = setupEngine(['false']);
+    const result = await engine.check(
+      { name: 'run_shell_command', args: { command: 'false || ls' } },
+      undefined,
+    );
+    expect(result.decision).toBe(PolicyDecision.ASK_USER);
+  });
+
+  it('should block unauthorized chained command with ;', async () => {
+    engine = setupEngine(['echo']);
+    const result = await engine.check(
+      { name: 'run_shell_command', args: { command: 'echo hi; ls' } },
+      undefined,
+    );
+    expect(result.decision).toBe(PolicyDecision.ASK_USER);
+  });
+
+  it('should block unauthorized command in pipe |', async () => {
+    engine = setupEngine(['echo']);
+    const result = await engine.check(
+      { name: 'run_shell_command', args: { command: 'echo hi | grep "hi"' } },
+      undefined,
+    );
+    expect(result.decision).toBe(PolicyDecision.ASK_USER);
+  });
+
+  it('should allow authorized command in pipe |', async () => {
+    engine = setupEngine(['echo', 'grep']);
+    const result = await engine.check(
+      { name: 'run_shell_command', args: { command: 'echo hi | grep "hi"' } },
+      undefined,
+    );
+    expect(result.decision).toBe(PolicyDecision.ALLOW);
+  });
+
+  it('should block unauthorized chained command with &', async () => {
+    engine = setupEngine(['echo']);
+    const result = await engine.check(
+      { name: 'run_shell_command', args: { command: 'echo hi & ls' } },
+      undefined,
+    );
+    expect(result.decision).toBe(PolicyDecision.ASK_USER);
+  });
+
+  it('should allow authorized chained command with &', async () => {
+    engine = setupEngine(['echo', 'ls']);
+    const result = await engine.check(
+      { name: 'run_shell_command', args: { command: 'echo hi & ls' } },
+      undefined,
+    );
+    expect(result.decision).toBe(PolicyDecision.ALLOW);
+  });
+
+  it('should block unauthorized command in nested substitution', async () => {
+    engine = setupEngine(['echo', 'cat']);
+    const result = await engine.check(
+      { name: 'run_shell_command', args: { command: 'echo $(cat $(ls))' } },
+      undefined,
+    );
+    expect(result.decision).toBe(PolicyDecision.ASK_USER);
+  });
+
+  it('should allow authorized command in nested substitution', async () => {
+    engine = setupEngine(['echo', 'cat', 'ls']);
+    const result = await engine.check(
+      { name: 'run_shell_command', args: { command: 'echo $(cat $(ls))' } },
+      undefined,
+    );
+    expect(result.decision).toBe(PolicyDecision.ALLOW);
+  });
+
+  it('should block command redirection if not explicitly allowed', async () => {
+    engine = setupEngine(['echo']);
+    const result = await engine.check(
+      { name: 'run_shell_command', args: { command: 'echo hi > /tmp/test' } },
+      undefined,
+    );
+    // Inherent policy: redirection downgrades to ASK_USER
+    expect(result.decision).toBe(PolicyDecision.ASK_USER);
+  });
+});

packages/core/src/policy/shell-safety.test.ts

@@ -59,6 +59,30 @@ vi.mock('../utils/shell-utils.js', async (importOriginal) => {
   return {
     ...actual,
     initializeShellParsers: vi.fn(),
+    parseCommandDetails: (command: string) => {
+      if (Object.prototype.hasOwnProperty.call(commandMap, command)) {
+        const subcommands = commandMap[command];
+        return {
+          details: subcommands.map((text) => ({
+            name: text.split(' ')[0],
+            text,
+            startIndex: command.indexOf(text),
+          })),
+          hasError: subcommands.length === 0 && command.includes('&&&'),
+        };
+      }
+      return {
+        details: [
+          {
+            name: command.split(' ')[0],
+            text: command,
+            startIndex: 0,
+          },
+        ],
+        hasError: false,
+      };
+    },
+    stripShellWrapper: (command: string) => command,
     splitCommands: (command: string) => {
       if (Object.prototype.hasOwnProperty.call(commandMap, command)) {
         return commandMap[command];

packages/core/src/policy/shell-substitution.test.ts

@@ -0,0 +1,97 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { expect, describe, it, beforeAll, vi } from 'vitest';
+import { PolicyEngine } from './policy-engine.js';
+import { PolicyDecision } from './types.js';
+import { initializeShellParsers } from '../utils/shell-utils.js';
+
+// Mock node:os to ensure shell-utils logic always thinks it's on a POSIX-like system.
+// This ensures that internal calls to getShellConfiguration() and isWindows()
+// within the shell-utils module return 'bash' configuration, even on Windows CI.
+vi.mock('node:os', async (importOriginal) => {
+  const actual = await importOriginal<typeof import('node:os')>();
+  return {
+    ...actual,
+    default: {
+      ...actual,
+      platform: () => 'linux',
+    },
+    platform: () => 'linux',
+  };
+});
+
+// Mock shell-utils to ensure consistent behavior across platforms (especially Windows CI)
+// We want to test PolicyEngine logic with Bash syntax rules.
+vi.mock('../utils/shell-utils.js', async (importOriginal) => {
+  const actual =
+    await importOriginal<typeof import('../utils/shell-utils.js')>();
+  return {
+    ...actual,
+    getShellConfiguration: () => ({
+      executable: 'bash',
+      argsPrefix: ['-c'],
+      shell: 'bash',
+    }),
+  };
+});
+
+describe('PolicyEngine Command Substitution Validation', () => {
+  beforeAll(async () => {
+    await initializeShellParsers();
+  });
+
+  const setupEngine = (blockedCmd: string) =>
+    new PolicyEngine({
+      defaultDecision: PolicyDecision.ALLOW,
+      rules: [
+        {
+          toolName: 'run_shell_command',
+          argsPattern: new RegExp(`"command":"${blockedCmd}"`),
+          decision: PolicyDecision.DENY,
+        },
+      ],
+    });
+
+  it('should block echo $(dangerous_cmd) when dangerous_cmd is explicitly blocked', async () => {
+    const engine = setupEngine('dangerous_cmd');
+    const result = await engine.check(
+      { name: 'run_shell_command', args: { command: 'echo $(dangerous_cmd)' } },
+      'test-server',
+    );
+    expect(result.decision).toBe(PolicyDecision.DENY);
+  });
+
+  it('should block backtick substitution `dangerous_cmd`', async () => {
+    const engine = setupEngine('dangerous_cmd');
+    const result = await engine.check(
+      { name: 'run_shell_command', args: { command: 'echo `dangerous_cmd`' } },
+      'test-server',
+    );
+    expect(result.decision).toBe(PolicyDecision.DENY);
+  });
+
+  it('should block commands inside subshells (dangerous_cmd)', async () => {
+    const engine = setupEngine('dangerous_cmd');
+    const result = await engine.check(
+      { name: 'run_shell_command', args: { command: '(dangerous_cmd)' } },
+      'test-server',
+    );
+    expect(result.decision).toBe(PolicyDecision.DENY);
+  });
+
+  it('should handle nested substitutions deeply', async () => {
+    const engine = setupEngine('deep_danger');
+    const result = await engine.check(
+      {
+        name: 'run_shell_command',
+        args: { command: 'echo $(ls $(deep_danger))' },
+      },
+      'test-server',
+    );
+    expect(result.decision).toBe(PolicyDecision.DENY);
+  });
+});

packages/core/src/policy/types.ts

@@ -335,8 +335,10 @@ export interface PolicySettings {
     allowed?: string[];
   };
   tools?: {
+    core?: string[];
     exclude?: string[];
     allowed?: string[];
+    confirmationRequired?: string[];
   };
   mcpServers?: Record<string, { trust?: boolean }>;
   // User provided policies that will replace the USER level policies in ~/.gemini/policies

packages/core/src/tools/ripGrep.test.ts

@@ -1993,13 +1993,15 @@ describe('getRipgrepPath', () => {
       vi.mocked(fileExists).mockImplementation(
         async (checkPath) =>
           checkPath.includes(path.normalize('core/vendor/ripgrep')) &&
-          !checkPath.includes('tools'),
+          !checkPath.includes(path.join(path.sep, 'tools', path.sep)),
       );
 
       const resolvedPath = await getRipgrepPath();
       expect(resolvedPath).not.toBeNull();
       expect(resolvedPath).toContain(path.normalize('core/vendor/ripgrep'));
-      expect(resolvedPath).not.toContain('tools');
+      expect(resolvedPath).not.toContain(
+        path.join(path.sep, 'tools', path.sep),
+      );
     });
 
     it('should return null if binary is missing from both paths', async () => {

packages/core/src/utils/shell-utils.ts

@@ -240,11 +240,15 @@ foreach ($commandAst in $commandAsts) {
   'utf16le',
 ).toString('base64');
 
-const REDIRECTION_NAMES = new Set([
+export const REDIRECTION_NAMES = new Set([
   'redirection (<)',
   'redirection (>)',
   'heredoc (<<)',
   'herestring (<<<)',
+  'command substitution',
+  'backtick substitution',
+  'process substitution',
+  'subshell',
 ]);
 
 function createParser(): Parser | null {
@@ -360,6 +364,14 @@ function extractNameFromNode(node: Node): string | null {
       return 'heredoc (<<)';
     case 'herestring_redirect':
       return 'herestring (<<<)';
+    case 'command_substitution':
+      return 'command substitution';
+    case 'backtick_substitution':
+      return 'backtick substitution';
+    case 'process_substitution':
+      return 'process substitution';
+    case 'subshell':
+      return 'subshell';
     default:
       return null;
   }