CLI violates instructions by ignoring mandatory Microsoft cpp language server (LSP) usage

[victorseiculescugraitec-cmd]

Describe the bug

Copilot CLI used grep/glob/raw search even though:

LSP was available
LSP would clearly have been sufficient

The model did not check for LSP availability
The model acknowledged the rule but still violated it.

lsp_diagnostic_log_2026-05-25_16-06-45.zip

Note 1: I'm using the cpp language server (https://github.com/microsoft/cpp-language-server).
Note 2: I've tried with many models (GPT-5 mini, Claude Sonnet 4.6, Claude Opus 4.6), but the output was always the same

Affected version

GitHub Copilot CLI 1.0.54.

Steps to reproduce the behavior

lsp_diagnostic_log_2026-05-25_16-06-45.txt

Expected behavior

Follow instructions from .github\instructions.md

Additional context

No response

[juliusl]

I have been running into the same thing w/ rust. No matter how many guardrails I have up, even if I tell it explicitly to use the LSP, after several turns it will fall back to using grep and I'll have to remind it again. The agent explains it as a "hyper reflex".

Which is weird because it's the opposite of how claude-code behaves (with the same model and same instructions). Claude will aggressively choose to use LSP even if it isn't setup yet and then fall back to grep as a last resort. It feels like something in the system instructions is pushing it to use grep.