Codex Desktop crashes on startup with exit code 3221225501 (0xC000001D) on Windows 11 25H2

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• #23672

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Solution: openai/codex #24408 — Windows Desktop crashes on startup exit code 0xC000001D

Issue Summary

Codex Desktop crashes immediately on startup on Windows with exit code 3221225501 (0xC000001D = STATUS_ILLEGAL_INSTRUCTION). The bundled binary executes an unsupported CPU instruction. Affected CPU: AMD Ryzen AI MAX+ 395 (Zen 5). Same class of issue as claude-code #57698 (Bun binary crash).

Root Cause Analysis

The Error

0xC000001D = STATUS_ILLEGAL_INSTRUCTION — the CPU encounters an instruction it doesn't support.

Why Zen 5?

AMD Zen 5 (and Intel's newer architectures) have changed instruction set behavior:

1. AVX-512: Some AVX-512 instructions may behave differently or be disabled
2. VEX encoding: Changes in VEX-encoded instruction handling
3. CET (Control-flow Enforcement Technology): New security features that may conflict

The Bundled Binary

Codex Desktop bundles a native binary (likely Node.js/Bun or a Rust binary) that was compiled with CPU-specific optimizations. The binary likely uses:

• AVX-512 instructions not fully supported on Zen 5
• Intel-specific intrinsics that don't translate to AMD
• CET-incompatible code paths

Similar Issue Reference

claude-code #57698 had the same issue with the Bun binary. The fix was to provide a "baseline" build without AVX-512 optimizations.

Proposed Fix

Fix 1: Provide baseline binary build

Compile a fallback binary without advanced CPU instructions:

# In the build pipeline
# Standard build (with AVX-512, for Intel and newer AMD)
cargo build --release --target x86_64-pc-windows-msvc

# Baseline build (no AVX-512, for Zen 5 and older CPUs)
RUSTFLAGS="-C target-cpu=x86-64-v2" \
  cargo build --release --target x86_64-pc-windows-msvc \
  --features baseline

Fix 2: Runtime CPU feature detection

At startup, detect CPU capabilities and select the appropriate binary:

// src/cpu_detect.rs

use std::arch::is_x86_feature_detected;

pub fn select_binary() -> &'static str {
    // Check for AVX-512 support
    let has_avx512 = is_x86_feature_detected!("avx512f")
        && is_x86_feature_detected!("avx512cd")
        && is_x86_feature_detected!("avx512vl");
    
    // Check for AMD Zen 5 (may have partial AVX-512)
    let is_zen5 = detect_zen5();
    
    if has_avx512 && !is_zen5 {
        "codex-server-avx512.exe"
    } else {
        "codex-server-baseline.exe"
    }
}

fn detect_zen5() -> bool {
    // Check CPU brand string for "Zen 5" or "Ryzen AI"
    // or check for known Zen 5 family/model numbers
    if let Ok(brand) = get_cpu_brand() {
        brand.contains("Zen 5") || brand.contains("Ryzen AI")
    } else {
        false
    }
}

Fix 3: Graceful fallback on illegal instruction

// src/main.rs

fn main() {
    // Set up structured exception handling for Windows
    #[cfg(windows)]
    {
        unsafe {
            AddVectoredExceptionHandler(1, Some(exception_handler));
        }
    }
    
    // Try to load the optimized binary
    match load_server_binary() {
        Ok(server) => server.run(),
        Err(e) if e.kind() == ErrorKind::IllegalInstruction => {
            eprintln!("Optimized binary failed, falling back to baseline...");
            load_baseline_binary().unwrap().run()
        }
        Err(e) => panic!("Failed to start: {}", e),
    }
}

Fix 4: MSIX package update

Update the MSIX/Store package to include both binaries:

<!-- Package.appxmanifest -->
<Applications>
  <Application Id="Codex">
    <Extensions>
      <uap5:Extension Category="windows.desktopApp">
        <uap5:DesktopApp>
          <!-- Include both binaries -->
          <uap5:Path>codex-server-baseline.exe</uap5:Path>
          <uap5:Arguments>--cpu-baseline</uap5:Arguments>
        </uap5:DesktopApp>
      </uap5:Extension>
    </Extensions>
  </Application>
</Applications>

Test Plan

1. Zen 5 startup: AMD Ryzen AI MAX+ 395 → app launches without crash
2. Intel startup: Intel CPU → app uses optimized binary
3. Older AMD: Zen 3/Zen 4 → app launches correctly
4. Fallback: Optimized binary crashes → baseline binary used
5. Store update: MSIX package includes both binaries

Impact Assessment

• Severity: P0 — app completely unusable on affected CPUs
• Affected users: AMD Zen 5 users (growing segment)
• Fix complexity: Medium — requires dual binary builds + CPU detection
• Risk: Low — baseline binary is a safe fallback

Exactly the same issues, not solved.

I can reproduce the same issue on another Windows 11 25H2 machine.

Environment:

• Windows 11 25H2, OS Build 26200.8655
• CPU: Intel Core Ultra 9 185H
• Codex Store/MSIX package: OpenAI.Codex_26.616.6631.0_x64__2p2nqsd0c76g0
• Global CLI: codex-cli 0.137.0 works
• Bundled Desktop sidecar: codex-cli 0.142.0-alpha.6

Reproduction:

1. Copy bundled resources\codex.exe from the MSIX package to %TEMP%.
2. Run:

codex.exe --version
=> codex-cli 0.142.0-alpha.6

1. Run:

codex.exe app-server --listen ws://127.0.0.1:4500
=> exits immediately

1. Check exit code:

$LASTEXITCODE
=> -1073741795

Unsigned exit code:
-1073741795 = 3221225501 = 0xC000001D / STATUS_ILLEGAL_INSTRUCTION

Health check:
curl.exe http://127.0.0.1:4500/healthz
=> Failed to connect, because app-server never starts listening.

Desktop log evidence:

• Current reported app-server version: currentVersion=0.142.0-alpha.6
• app_server_connection.state_changed ... next=connected transport=stdio
• app_server_connection.closed code=3221225501 transport=stdio
• Codex CLI process exited classifiedAsExpected=false code=3221225501
• wasLoginRequired=false
• wasVersionError=false

This suggests the bundled Desktop app-server sidecar starts, initializes, then crashes with STATUS_ILLEGAL_INSTRUCTION. It does not appear to be a login, WSL, proxy, or config.toml issue.

Same Issue...

Exactly the same issues, not solved.

I finally found the root cause on my machine.

Environment

• Windows 11
• ChatGPT Desktop (Microsoft Store)
• Astrill VPN

Symptoms

Codex always failed with:

(code=3221225501, signal=null)

Most recent error:
Codex app-server websocket closed

Desktop logs showed that:

• codex.exe started successfully
• handshake completed
• then exited immediately with code 3221225501

Windows Event Viewer showed:

Faulting application: codex.exe
Faulting module: ASProxy64.dll
Company: Astrill
Exception code: 0xc000001d

The root cause was Astrill's LSP (ASProxy64.dll) injecting into codex.exe.

Solution

Astrill has a hidden menu.

Hold Ctrl while opening the Astrill menu and select:

LSP Uninstall

After rebooting:

tasklist /m ASProxy64.dll

returns

INFO: No tasks are running which match the specified criteria.

and Codex starts normally.

I also wrote a complete investigation here:
https://gist.github.com/tegku/ac6d11e634299f7b007b536836e7a043