Copilot CLI Node.js v22 aborts on startup under gVisor runtime (exit 139, DecodeUTF8 assertion)

[lpcox]

Summary

The Copilot CLI's bundled Node.js runtime (v22.23.1) aborts with a native assertion failure on startup when the agent runs under the gVisor (runsc) runtime. Node crashes before any workflow logic executes (tokenCount=0, 0 bytes stdout), and after the harness exhausts its retries the wrapper itself segfaults, so the agent job exits with code 139.

Where observed

• Repo: github/gh-aw-mcpg
• Workflow: Read-Only Stress: gVisor runtime (.github/workflows/readonly-stress-gvisor.md), which sets sandbox.agent.runtime: gvisor
• Failing run: https://github.com/github/gh-aw-mcpg/actions/runs/30048808465/job/89354893639?pr=9941
• gh-aw version: v0.82.14
• Model: claude-sonnet-4.6

This is a runtime/sandbox failure, not a defect in the PR under test (#9941). The default and sbx runtime variants of the same stress test are unaffected.

Fatal error

#  node::StringBytes::Encode(v8::Isolate*, const char*, std::size_t, node::encoding)::<lambda(uint16_t*)> at ../src/string_bytes.cc:598
#  Assertion failed: (written) == (u16size)

----- Native stack trace -----
 1: node::Assert(node::AssertionInfo const&)
 2: 0xac3853
 3: node::encoding_binding::BindingData::DecodeUTF8(v8::FunctionCallbackInfo<v8::Value> const&)

----- JavaScript stack trace -----
1: decode (node:internal/encoding:494:28)
2: stringify (node:internal/modules/helpers:418:18)
3: moduleStrategy (node:internal/modules/esm/translators:86:12)
4: #translate (node:internal/modules/esm/loader:435:20)
5: afterLoad (node:internal/modules/esm/loader:491:29)
6: loadAndTranslate (node:internal/modules/esm/loader:496:12)
...

Node aborts (SIGABRT) inside its native UTF-8 decode path while loading an ES module. The copilot-harness retried 3 times; all attempts crashed identically within ~90ms:

[copilot-harness] attempt 1: process closed exitCode=1 signal=SIGABRT duration=0s stdout=0B stderr=1132B
[copilot-harness] attempt 1 failed: ... failureClass=partial_execution ... tokenCount=0
[copilot-harness] attempt 2: process closed exitCode=1 signal=SIGABRT
...
/tmp/awf-cmd-1.sh: line 29: 75 Segmentation fault  /bin/bash -c '... copilot_harness.cjs /usr/local/bin/copilot ...'
[WARN] Command completed with exit code: 139
##[error]Process completed with exit code 139.

Environment: nodeVersion=v22.23.1 platform=linux, network-isolation mode enforcing egress via Docker network topology.

Likely cause

A native assertion in Node's StringBytes::Encode / DecodeUTF8 firing during ESM module translation strongly suggests an incompatibility between the bundled Node runtime and gVisor's syscall/memory emulation (runsc), rather than a workflow or application bug.

Suggested fixes to investigate

• Bump or pin the Copilot CLI's bundled Node.js to a version that does not trip this assertion under gVisor.
• Review the runsc platform/syscall configuration used for the AWF agent container (e.g. platform mode, mmap/UTF-8 emulation paths).
• Confirm whether this reproduces deterministically or is a transient runner/sandbox flake.

Impact

Any agentic workflow using sandbox.agent.runtime: gvisor can fail at startup with exit 139 regardless of the PR content, producing false-negative stress-test results.