Why It Matters

GitHub Copilot repository and path-specific instruction files are easier to reason about when they stay short and focused. Once instruction markdown grows past the documented 4000-character guidance limit, part of the intended behavior can become easier to ignore, harder to review, and less portable across review-oriented Copilot workflows.

What Triggers

SEC353 applies only to GitHub Copilot instruction paths:

• .github/copilot-instructions.md
• .github/instructions/*.instructions.md

It triggers when the file content exceeds 4000 characters.

Examples that trigger:

• a repository-level .github/copilot-instructions.md packed with multi-page guidance
• a path-specific .github/instructions/review.instructions.md that grows beyond the same limit

Examples that stay clean:

• short repository-level Copilot instructions
• path-specific instruction files that stay within the limit
• fixture-like paths such as tests/fixtures/.github/copilot-instructions.md

False Positives

This rule now lives in the explicit guidance lane because some projects may intentionally keep large instruction files while they evaluate how much Copilot actually consumes in their workflows. The rule is guidance about reviewability and effective application, not a claim that every long file is broken.

Remediation

Split long Copilot guidance into:

• a shorter shared .github/copilot-instructions.md
• narrower .github/instructions/*.instructions.md files for path-specific behavior

Prefer small, focused instruction files over one large catch-all document.