AssertionError [ERR_ASSERTION] in Copilot CLI when generating plan with claude-opus-4.6

[mcthesw]

Describe the bug

I'm encountering a consistent AssertionError when using the GitHub Copilot CLI. The crash specifically occurs when the agent attempts to generate a plan using the claude-opus-4.6 model, usually after some research and multiple rounds of ask user interactions.When it happens, ctrl+c and switching to GPT-5.3-codex xhigh works normally. I had it create a plan and then let Opus execute it, and that seems to work.

The CLI throws the following error, followed by a massive block of minified/obfuscated code: Error: (system_err_assertion) AssertionError [ERR_ASSERTION]: The expression evaluated to a falsy value:

Affected version

GitHub Copilot CLI 0.0.421.

Steps to reproduce the behavior

1. Open the Copilot CLI within a project directory. (Using Claude Opus 4.6)
2. Switch to plan mode using Shift+Tab.
3. Engage in a session that involves code research and multiple interactions.
4. When the agent tries to finalize and output the plan, it crashes.

Alternative trigger: Starting a new session using /new, pasting in previously generated documentation and context, and asking it to generate a plan with Opus also consistently reproduces the same error.

Expected behavior

Write plan to file.

Additional context

• OS: Windows 11 (24H2)
• Terminal: Windows Terminal with Nushell
• Node.js: v24.12.0
• Package Manager: pnpm 10.30.3
• Model: claude-opus-4.6 (high) (3x)

[srlynch1]

This is happening currently with any use of subagents. We have built out workflows using copilot that we are now demoing to multiple customers instead using Claude Code due to the reliability and Instability copilot cli is currently not useable due to this issue