diff --git a/packages/core/src/sandbox/linux/LinuxSandboxManager.ts b/packages/core/src/sandbox/linux/LinuxSandboxManager.ts
index 000fea510f..f210138127 100644
--- a/packages/core/src/sandbox/linux/LinuxSandboxManager.ts
+++ b/packages/core/src/sandbox/linux/LinuxSandboxManager.ts
@@ -27,11 +27,16 @@ import {
   verifySandboxOverrides,
   getCommandName,
 } from '../utils/commandUtils.js';
+import { assertValidPathString } from '../../utils/paths.js';
 import {
   isKnownSafeCommand,
   isDangerousCommand,
 } from '../utils/commandSafety.js';
-import { parsePosixSandboxDenials } from '../utils/sandboxDenialUtils.js';
+import {
+  parsePosixSandboxDenials,
+  createSandboxDenialCache,
+  type SandboxDenialCache,
+} from '../utils/sandboxDenialUtils.js';
 import { handleReadWriteCommands } from '../utils/sandboxReadWriteUtils.js';
 import { buildBwrapArgs } from './bwrapArgsBuilder.js';
 
@@ -108,6 +113,7 @@ function getSeccompBpfPath(): string {
  * Ensures a file or directory exists.
  */
 function touch(filePath: string, isDirectory: boolean) {
+  assertValidPathString(filePath);
   try {
     // If it exists (even as a broken symlink), do nothing
     if (fs.lstatSync(filePath)) return;
@@ -129,6 +135,7 @@ function touch(filePath: string, isDirectory: boolean) {
 
 export class LinuxSandboxManager implements SandboxManager {
   private static maskFilePath: string | undefined;
+  private readonly denialCache: SandboxDenialCache = createSandboxDenialCache();
 
   constructor(private readonly options: GlobalSandboxOptions) {}
 
@@ -141,7 +148,7 @@ export class LinuxSandboxManager implements SandboxManager {
   }
 
   parseDenials(result: ShellExecutionResult): ParsedSandboxDenial | undefined {
-    return parsePosixSandboxDenials(result);
+    return parsePosixSandboxDenials(result, this.denialCache);
   }
 
   getWorkspace(): string {
diff --git a/packages/core/src/sandbox/macos/MacOsSandboxManager.ts b/packages/core/src/sandbox/macos/MacOsSandboxManager.ts
index 0fee35110a..44774e8e82 100644
--- a/packages/core/src/sandbox/macos/MacOsSandboxManager.ts
+++ b/packages/core/src/sandbox/macos/MacOsSandboxManager.ts
@@ -32,10 +32,16 @@ import {
   getCommandName as getFullCommandName,
   isStrictlyApproved,
 } from '../utils/commandUtils.js';
-import { parsePosixSandboxDenials } from '../utils/sandboxDenialUtils.js';
+import {
+  parsePosixSandboxDenials,
+  createSandboxDenialCache,
+  type SandboxDenialCache,
+} from '../utils/sandboxDenialUtils.js';
 import { handleReadWriteCommands } from '../utils/sandboxReadWriteUtils.js';
 
 export class MacOsSandboxManager implements SandboxManager {
+  private readonly denialCache: SandboxDenialCache = createSandboxDenialCache();
+
   constructor(private readonly options: GlobalSandboxOptions) {}
 
   isKnownSafeCommand(args: string[]): boolean {
@@ -52,7 +58,7 @@ export class MacOsSandboxManager implements SandboxManager {
   }
 
   parseDenials(result: ShellExecutionResult): ParsedSandboxDenial | undefined {
-    return parsePosixSandboxDenials(result);
+    return parsePosixSandboxDenials(result, this.denialCache);
   }
 
   getWorkspace(): string {
diff --git a/packages/core/src/sandbox/utils/fsUtils.test.ts b/packages/core/src/sandbox/utils/fsUtils.test.ts
new file mode 100644
index 0000000000..9439050680
--- /dev/null
+++ b/packages/core/src/sandbox/utils/fsUtils.test.ts
@@ -0,0 +1,52 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, expect, beforeAll, afterAll } from 'vitest';
+import fs from 'node:fs';
+import path from 'node:path';
+import os from 'node:os';
+import { tryRealpath } from './fsUtils.js';
+
+describe('fsUtils', () => {
+  let tempDir: string;
+  let realTempDir: string;
+
+  beforeAll(() => {
+    tempDir = fs.mkdtempSync(path.join(os.tmpdir(), 'fs-utils-test-'));
+    realTempDir = fs.realpathSync(tempDir);
+  });
+
+  afterAll(() => {
+    fs.rmSync(tempDir, { recursive: true, force: true });
+  });
+
+  describe('tryRealpath', () => {
+    it('should throw error for paths with null bytes', () => {
+      expect(() => tryRealpath(path.join(tempDir, 'foo\0bar'))).toThrow(
+        'Invalid path',
+      );
+    });
+
+    it('should resolve existing paths', () => {
+      const resolved = tryRealpath(tempDir);
+      expect(resolved).toBe(realTempDir);
+    });
+
+    it('should handle non-existent paths by resolving parent', () => {
+      const nonExistentPath = path.join(tempDir, 'non-existent-file-12345');
+      const expected = path.join(realTempDir, 'non-existent-file-12345');
+      const resolved = tryRealpath(nonExistentPath);
+      expect(resolved).toBe(expected);
+    });
+
+    it('should handle nested non-existent paths', () => {
+      const nonExistentPath = path.join(tempDir, 'dir1', 'dir2', 'file');
+      const expected = path.join(realTempDir, 'dir1', 'dir2', 'file');
+      const resolved = tryRealpath(nonExistentPath);
+      expect(resolved).toBe(expected);
+    });
+  });
+});
diff --git a/packages/core/src/sandbox/utils/fsUtils.ts b/packages/core/src/sandbox/utils/fsUtils.ts
index e30d55c72d..2e3eda1342 100644
--- a/packages/core/src/sandbox/utils/fsUtils.ts
+++ b/packages/core/src/sandbox/utils/fsUtils.ts
@@ -6,12 +6,14 @@
 
 import fs from 'node:fs';
 import path from 'node:path';
+import { assertValidPathString } from '../../utils/paths.js';
 
 export function isErrnoException(e: unknown): e is NodeJS.ErrnoException {
   return e instanceof Error && 'code' in e;
 }
 
 export function tryRealpath(p: string): string {
+  assertValidPathString(p);
   try {
     return fs.realpathSync(p);
   } catch (e) {
diff --git a/packages/core/src/sandbox/utils/sandboxDenialUtils.test.ts b/packages/core/src/sandbox/utils/sandboxDenialUtils.test.ts
index 3d3380b057..f803cfa779 100644
--- a/packages/core/src/sandbox/utils/sandboxDenialUtils.test.ts
+++ b/packages/core/src/sandbox/utils/sandboxDenialUtils.test.ts
@@ -5,7 +5,10 @@
  */
 
 import { describe, it, expect } from 'vitest';
-import { parsePosixSandboxDenials } from './sandboxDenialUtils.js';
+import {
+  parsePosixSandboxDenials,
+  createSandboxDenialCache,
+} from './sandboxDenialUtils.js';
 import type { ShellExecutionResult } from '../../services/shellExecutionService.js';
 
 describe('parsePosixSandboxDenials', () => {
@@ -116,4 +119,109 @@ EACCES: permission denied, mkdir '/Users/galzahavi/.pnpm-store/v3'
     expect(parsed).toBeDefined();
     expect(parsed?.filePaths).toContain('/Users/galzahavi/.pnpm-store/v3');
   });
+
+  it('should detect Python PermissionError and extract path accurately', () => {
+    const output = `Caught exception: [Errno 13] Permission denied: '/etc/test_sandbox_denial'
+Traceback (most recent call last):
+  File "/usr/local/google/home/davidapierce/gemini-cli/repro_sandbox.py", line 9, in <module>
+    raise e
+  File "/usr/local/google/home/davidapierce/gemini-cli/repro_sandbox.py", line 5, in <module>
+    with open('/etc/test_sandbox_denial', 'w') as f:
+         ~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+PermissionError: [Errno 13] Permission denied: '/etc/test_sandbox_denial'`;
+
+    const parsed = parsePosixSandboxDenials({
+      output,
+      exitCode: 1,
+      error: null,
+    } as unknown as ShellExecutionResult);
+
+    expect(parsed?.filePaths).toEqual(['/etc/test_sandbox_denial']);
+  });
+
+  it('should detect new keywords like "access denied" and "forbidden"', () => {
+    const parsed1 = parsePosixSandboxDenials({
+      output: 'Access denied to /var/log/syslog',
+      exitCode: 1,
+      error: null,
+    } as unknown as ShellExecutionResult);
+    expect(parsed1?.filePaths).toContain('/var/log/syslog');
+
+    const parsed2 = parsePosixSandboxDenials({
+      output: 'Forbidden: access to /root/secret is not allowed',
+      exitCode: 1,
+      error: null,
+    } as unknown as ShellExecutionResult);
+    expect(parsed2?.filePaths).toContain('/root/secret');
+  });
+
+  it('should detect read-only file system error', () => {
+    const parsed = parsePosixSandboxDenials({
+      output: 'rm: cannot remove /mnt/usb/test: Read-only file system',
+      exitCode: 1,
+      error: null,
+    } as unknown as ShellExecutionResult);
+    expect(parsed?.filePaths).toContain('/mnt/usb/test');
+  });
+
+  it('should reject paths with directory traversal', () => {
+    const output = 'ls: /etc/shadow/../../etc/passwd: Operation not permitted';
+    const parsed = parsePosixSandboxDenials({
+      output,
+    } as unknown as ShellExecutionResult);
+    expect(parsed?.filePaths || []).not.toContain(
+      '/etc/shadow/../../etc/passwd',
+    );
+  });
+
+  it('should reject home-relative paths with directory traversal', () => {
+    const output = "Operation not permitted, open '~/../../etc/shadow'";
+    const parsed = parsePosixSandboxDenials({
+      output,
+    } as unknown as ShellExecutionResult);
+    expect(parsed?.filePaths || []).not.toContain('~/../../etc/shadow');
+  });
+
+  it('should reject paths with null bytes', () => {
+    const output = "Operation not permitted, open '/etc/passwd\0/foo'";
+    const parsed = parsePosixSandboxDenials({
+      output,
+    } as unknown as ShellExecutionResult);
+    expect(parsed?.filePaths || []).not.toContain('/etc/passwd\0/foo');
+  });
+
+  it('should reject paths with internal tildes', () => {
+    const output = "Operation not permitted, open '/home/user/~/config'";
+    const parsed = parsePosixSandboxDenials({
+      output,
+    } as unknown as ShellExecutionResult);
+    expect(parsed?.filePaths || []).not.toContain('/home/user/~/config');
+  });
+
+  it('should suppress redundant denials if cache is provided', () => {
+    const cache = createSandboxDenialCache();
+    const result = {
+      output: 'ls: /root: Operation not permitted',
+    } as unknown as ShellExecutionResult;
+
+    // First call: should process
+    const parsed1 = parsePosixSandboxDenials(result, cache);
+    expect(parsed1).toBeDefined();
+
+    // Second call: should be suppressed
+    const parsed2 = parsePosixSandboxDenials(result, cache);
+    expect(parsed2).toBeUndefined();
+  });
+
+  it('should not suppress denials if no cache is provided', () => {
+    const result = {
+      output: 'ls: /root: Operation not permitted',
+    } as unknown as ShellExecutionResult;
+
+    const parsed1 = parsePosixSandboxDenials(result);
+    expect(parsed1).toBeDefined();
+
+    const parsed2 = parsePosixSandboxDenials(result);
+    expect(parsed2).toBeDefined();
+  });
 });
diff --git a/packages/core/src/sandbox/utils/sandboxDenialUtils.ts b/packages/core/src/sandbox/utils/sandboxDenialUtils.ts
index 96082767dd..e288be0ed0 100644
--- a/packages/core/src/sandbox/utils/sandboxDenialUtils.ts
+++ b/packages/core/src/sandbox/utils/sandboxDenialUtils.ts
@@ -4,8 +4,58 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+import { LRUCache } from 'mnemonist';
 import { type ParsedSandboxDenial } from '../../services/sandboxManager.js';
 import type { ShellExecutionResult } from '../../services/shellExecutionService.js';
+import { isValidPathString } from '../../utils/paths.js';
+
+/**
+ * Type for the sandbox denial error cache.
+ * Stores normalized error output to prevent redundant processing.
+ */
+export type SandboxDenialCache = LRUCache<string, boolean>;
+
+/**
+ * Creates a new sandbox denial cache with a standard LRU policy.
+ */
+export function createSandboxDenialCache(maxSize = 10): SandboxDenialCache {
+  return new LRUCache<string, boolean>(maxSize);
+}
+
+/**
+ * Sanitizes extracted paths to prevent path traversal vulnerabilities.
+ * Filters out paths containing '..' or null bytes.
+ */
+export function sanitizeExtractedPath(p: string): string | undefined {
+  if (!isValidPathString(p)) return undefined;
+
+  // Reject paths with directory traversal components
+  const parts = p.split(/[/\\]/);
+  if (parts.includes('..')) {
+    return undefined;
+  }
+
+  // Reject paths with internal tildes (tilde should only be at the beginning)
+  if (p.indexOf('~') > 0) {
+    return undefined;
+  }
+
+  // Basic normalization without resolving symlinks or accessing the file system
+  let normalized = p;
+
+  // Collapse multiple slashes
+  normalized = normalized.replace(/\/+/g, '/');
+
+  // Remove single dot segments
+  normalized = normalized.replace(/\/\.\//g, '/');
+
+  // Remove trailing slashes (unless it's exactly '/')
+  if (normalized.length > 1 && normalized.endsWith('/')) {
+    normalized = normalized.slice(0, -1);
+  }
+
+  return normalized;
+}
 
 /**
  * Common POSIX-style sandbox denial detection.
@@ -13,10 +63,18 @@ import type { ShellExecutionResult } from '../../services/shellExecutionService.
  */
 export function parsePosixSandboxDenials(
   result: ShellExecutionResult,
+  cache?: SandboxDenialCache,
 ): ParsedSandboxDenial | undefined {
   const output = result.output || '';
   const errorOutput = result.error?.message;
-  const combined = (output + ' ' + (errorOutput || '')).toLowerCase();
+  const fullText = output + '\n' + (errorOutput || '');
+  const combined = fullText.toLowerCase();
+
+  // Cache by the first 200 characters of the error to handle variable data (timestamps, PIDs)
+  const cacheKey = combined.trim().slice(0, 200);
+  if (cacheKey && cache?.has(cacheKey)) {
+    return undefined;
+  }
 
   const isFileDenial = [
     'operation not permitted',
@@ -27,6 +85,12 @@ export function parsePosixSandboxDenials(
     'should be read/write',
     'sandbox_apply',
     'sandbox: ',
+    'access denied',
+    'read-only file system',
+    'permissionerror',
+    'fs.permissiondenied',
+    'forbidden',
+    'system.unauthorizedaccessexception',
   ].some((keyword) => combined.includes(keyword));
 
   const isNetworkDenial = [
@@ -46,6 +110,8 @@ export function parsePosixSandboxDenials(
     'err_pnpm_fetch',
     'err_pnpm_no_matching_version',
     "syscall: 'listen'",
+    'socketexception',
+    'networkaccessdenied',
   ].some((keyword) => combined.includes(keyword));
 
   if (!isFileDenial && !isNetworkDenial) {
@@ -57,27 +123,28 @@ export function parsePosixSandboxDenials(
   // Extract denied paths (POSIX absolute paths or home-relative paths starting with ~)
   const regexes = [
     // format: /path: operation not permitted
-    /(?:^|\s)['"]?((?:\/|~)[\w.\-/:~]+)['"]?:\s*[Oo]peration not permitted/gi,
+    /(?:^|\s)['"]?((?:\/|~)(?:[\w.\-/:~]*[\w.\-/~])?)['"]?[\s:,'"[\]]*operation not permitted/gi,
     // format: operation not permitted, open '/path'
-    /[Oo]peration not permitted,\s*open\s*['"]?((?:\/|~)[\w.\-/:~]+)['"]?/gi,
+    /operation not permitted[\s:,'"[\]]*open[\s:,'"[\]]*['"]?((?:\/|~)(?:[\w.\-/:~]*[\w.\-/~])?)['"]?/gi,
     // format: permission denied, open '/path'
-    /[Pp]ermission denied,\s*open\s*['"]?((?:\/|~)[\w.\-/:~]+)['"]?/gi,
+    /permission denied[\s:,'"[\]]*open[\s:,'"[\]]*['"]?((?:\/|~)(?:[\w.\-/:~]*[\w.\-/~])?)['"]?/gi,
     // format: npm error path /path or npm ERR! path /path
-    /npm\s+(?:error|ERR!)\s+path\s+((?:\/|~)[\w.\-/:~]+)/gi,
-    // format: EACCES: permission denied, mkdir '/path'
-    /EACCES:\s*permission denied,\s*\w+\s*['"]?((?:\/|~)[\w.\-/:~]+)['"]?/gi,
+    /npm[\s!]*[A-Za-z]*err[A-Za-z!]*[\s!]+path[\s!]*((?:\/|~)(?:[\w.\-/:~]*[\w.\-/~])?)/gi,
+    // format: eacces: permission denied, mkdir '/path'
+    /eacces[\s:,'"[\]]*permission denied[\s:,'"[\]]*\w+[\s:,'"[\]]*['"]?((?:\/|~)[\w.\-/:~]*[\w.\-/~])?/gi,
+    // format: PermissionError: [Errno 13] Permission denied: '/path'
+    /permissionerror[\s:,'"[\]]*(?:[^'"]*)['"]((?:\/|~)[\w.\-/:~]*[\w.\-/~])?['"]/gi,
+    // format: FileNotFoundError: [Errno 2] No such file or directory: '/path' (sometimes returned in sandbox denials if directory is hidden)
+    /filenotfounderror[\s:,'"[\]]*(?:[^'"]*)['"]((?:\/|~)[\w.\-/:~]*[\w.\-/~])?['"]/gi,
+    // format: Error: EACCES: permission denied, open '/path'
+    /error[\s:,'"[\]]*eacces[\s:,'"[\]]*permission denied[\s:,'"[\]]*(?:[^'"]*)['"]((?:\/|~)[\w.\-/:~]*[\w.\-/~])?['"]/gi,
   ];
 
   for (const regex of regexes) {
     let match;
-    while ((match = regex.exec(output)) !== null) {
-      filePaths.add(match[1]);
-    }
-    if (errorOutput) {
-      regex.lastIndex = 0; // Reset for next use
-      while ((match = regex.exec(errorOutput)) !== null) {
-        filePaths.add(match[1]);
-      }
+    while ((match = regex.exec(fullText)) !== null) {
+      const sanitized = sanitizeExtractedPath(match[1]);
+      if (sanitized) filePaths.add(sanitized);
     }
   }
 
@@ -86,22 +153,16 @@ export function parsePosixSandboxDenials(
     const fallbackRegex =
       /(?:^|[\s"'[\]])(\/[a-zA-Z0-9_.-]+(?:\/[a-zA-Z0-9_.-]+)+)(?:$|[\s"'[\]:])/gi;
     let m;
-    while ((m = fallbackRegex.exec(output)) !== null) {
-      const p = m[1];
-      if (p && !p.startsWith('/bin/') && !p.startsWith('/usr/bin/')) {
-        filePaths.add(p);
-      }
-    }
-    if (errorOutput) {
-      while ((m = fallbackRegex.exec(errorOutput)) !== null) {
-        const p = m[1];
-        if (p && !p.startsWith('/bin/') && !p.startsWith('/usr/bin/')) {
-          filePaths.add(p);
-        }
-      }
+    while ((m = fallbackRegex.exec(fullText)) !== null) {
+      const sanitized = sanitizeExtractedPath(m[1]);
+      if (sanitized) filePaths.add(sanitized);
     }
   }
 
+  if (cacheKey && cache) {
+    cache.set(cacheKey, true);
+  }
+
   return {
     network: isNetworkDenial || undefined,
     filePaths: filePaths.size > 0 ? Array.from(filePaths) : undefined,
diff --git a/packages/core/src/sandbox/utils/sandboxReadWriteUtils.ts b/packages/core/src/sandbox/utils/sandboxReadWriteUtils.ts
index 21f8c1f7c3..c1a611716b 100644
--- a/packages/core/src/sandbox/utils/sandboxReadWriteUtils.ts
+++ b/packages/core/src/sandbox/utils/sandboxReadWriteUtils.ts
@@ -8,6 +8,7 @@ import {
   type SandboxPermissions,
   type SandboxRequest,
 } from '../../services/sandboxManager.js';
+import { isValidPathString } from '../../utils/paths.js';
 
 /**
  * Validates if the requested paths are within the allowed workspace or allowed paths.
@@ -18,6 +19,9 @@ function validatePaths(
   allowedPaths: string[],
 ): boolean {
   for (const p of paths) {
+    if (!isValidPathString(p)) {
+      return false; // Reject malicious paths
+    }
     const resolvedPath = path.resolve(p);
     const resolvedWorkspace = path.resolve(workspace);
     const isInsideWorkspace =
diff --git a/packages/core/src/sandbox/windows/WindowsSandboxManager.ts b/packages/core/src/sandbox/windows/WindowsSandboxManager.ts
index 943a339960..a2d6428906 100644
--- a/packages/core/src/sandbox/windows/WindowsSandboxManager.ts
+++ b/packages/core/src/sandbox/windows/WindowsSandboxManager.ts
@@ -35,7 +35,15 @@ import {
 } from './commandSafety.js';
 import { verifySandboxOverrides } from '../utils/commandUtils.js';
 import { parseWindowsSandboxDenials } from './windowsSandboxDenialUtils.js';
-import { isSubpath, resolveToRealPath } from '../../utils/paths.js';
+import {
+  isSubpath,
+  resolveToRealPath,
+  assertValidPathString,
+} from '../../utils/paths.js';
+import {
+  type SandboxDenialCache,
+  createSandboxDenialCache,
+} from '../utils/sandboxDenialUtils.js';
 
 const __filename = fileURLToPath(import.meta.url);
 const __dirname = path.dirname(__filename);
@@ -54,6 +62,7 @@ export class WindowsSandboxManager implements SandboxManager {
   private initialized = false;
   private readonly allowedCache = new Set<string>();
   private readonly deniedCache = new Set<string>();
+  private readonly denialCache: SandboxDenialCache = createSandboxDenialCache();
 
   constructor(private readonly options: GlobalSandboxOptions) {
     this.helperPath = path.resolve(__dirname, WindowsSandboxManager.HELPER_EXE);
@@ -73,7 +82,7 @@ export class WindowsSandboxManager implements SandboxManager {
   }
 
   parseDenials(result: ShellExecutionResult): ParsedSandboxDenial | undefined {
-    return parseWindowsSandboxDenials(result);
+    return parseWindowsSandboxDenials(result, this.denialCache);
   }
 
   getWorkspace(): string {
@@ -88,6 +97,7 @@ export class WindowsSandboxManager implements SandboxManager {
    * Ensures a file or directory exists.
    */
   private touch(filePath: string, isDirectory: boolean): void {
+    assertValidPathString(filePath);
     try {
       // If it exists (even as a broken symlink), do nothing
       if (fs.lstatSync(filePath)) return;
diff --git a/packages/core/src/sandbox/windows/windowsSandboxDenialUtils.ts b/packages/core/src/sandbox/windows/windowsSandboxDenialUtils.ts
index a2b12b0336..288f2dc309 100644
--- a/packages/core/src/sandbox/windows/windowsSandboxDenialUtils.ts
+++ b/packages/core/src/sandbox/windows/windowsSandboxDenialUtils.ts
@@ -6,6 +6,10 @@
 
 import { type ParsedSandboxDenial } from '../../services/sandboxManager.js';
 import type { ShellExecutionResult } from '../../services/shellExecutionService.js';
+import {
+  type SandboxDenialCache,
+  sanitizeExtractedPath,
+} from '../utils/sandboxDenialUtils.js';
 
 /**
  * Windows-specific sandbox denial detection.
@@ -13,10 +17,18 @@ import type { ShellExecutionResult } from '../../services/shellExecutionService.
  */
 export function parseWindowsSandboxDenials(
   result: ShellExecutionResult,
+  cache?: SandboxDenialCache,
 ): ParsedSandboxDenial | undefined {
   const output = result.output || '';
   const errorOutput = result.error?.message;
-  const combined = (output + ' ' + (errorOutput || '')).toLowerCase();
+  const fullText = output + '\n' + (errorOutput || '');
+  const combined = fullText.toLowerCase();
+
+  // Cache by the first 200 characters of the error to handle variable data (timestamps, PIDs)
+  const cacheKey = combined.trim().slice(0, 200);
+  if (cacheKey && cache?.has(cacheKey)) {
+    return undefined;
+  }
 
   const isFileDenial = [
     'access is denied',
@@ -46,30 +58,24 @@ export function parseWindowsSandboxDenials(
 
   // 1. Quoted paths: 'C:\Foo Bar' or "C:\Foo Bar"
   const quotedRegex = /['"]((?:\\\\(?:\?|\.)\\)?[a-zA-Z]:[\\/][^'"]+)['"]/g;
-  for (const match of output.matchAll(quotedRegex)) {
-    filePaths.add(match[1]);
-  }
-  if (errorOutput) {
-    for (const match of errorOutput.matchAll(quotedRegex)) {
-      filePaths.add(match[1]);
-    }
+  for (const match of fullText.matchAll(quotedRegex)) {
+    const sanitized = sanitizeExtractedPath(match[1]);
+    if (sanitized) filePaths.add(sanitized);
   }
 
   // 2. Unquoted paths or paths in PowerShell error format: PermissionDenied: (C:\path:String)
   const generalRegex =
     /(?:^|[\s(])((?:\\\\(?:\?|\.)\\)?[a-zA-Z]:[\\/][^"'\s()<>|?*]+)/g;
-  for (const match of output.matchAll(generalRegex)) {
+  for (const match of fullText.matchAll(generalRegex)) {
     // Clean up trailing colon which might be part of the error message rather than the path
     let p = match[1];
     if (p.endsWith(':')) p = p.slice(0, -1);
-    filePaths.add(p);
+    const sanitized = sanitizeExtractedPath(p);
+    if (sanitized) filePaths.add(sanitized);
   }
-  if (errorOutput) {
-    for (const match of errorOutput.matchAll(generalRegex)) {
-      let p = match[1];
-      if (p.endsWith(':')) p = p.slice(0, -1);
-      filePaths.add(p);
-    }
+
+  if (cacheKey && cache) {
+    cache.set(cacheKey, true);
   }
 
   return {
diff --git a/packages/core/src/utils/paths.ts b/packages/core/src/utils/paths.ts
index 135e047530..b83860eadb 100644
--- a/packages/core/src/utils/paths.ts
+++ b/packages/core/src/utils/paths.ts
@@ -369,6 +369,22 @@ export function isSubpath(parentPath: string, childPath: string): boolean {
   );
 }
 
+/**
+ * Type guard to verify a value is a string and does not contain null bytes.
+ */
+export function isValidPathString(p: unknown): p is string {
+  return typeof p === 'string' && !p.includes('\0');
+}
+
+/**
+ * Asserts that a value is a valid path string, throwing an Error otherwise.
+ */
+export function assertValidPathString(p: unknown): asserts p is string {
+  if (!isValidPathString(p)) {
+    throw new Error(`Invalid path: ${String(p)}`);
+  }
+}
+
 /**
  * Resolves a path to its real path, sanitizing it first.
  * - Removes 'file://' protocol if present.
@@ -379,6 +395,7 @@ export function isSubpath(parentPath: string, childPath: string): boolean {
  * @returns The resolved real path.
  */
 export function resolveToRealPath(pathStr: string): string {
+  assertValidPathString(pathStr);
   let resolvedPath = pathStr;
 
   try {