Stop Prompt-Engineering Copilot. Write Three Rules in a File Instead.
Most advice about getting better output from Copilot is advice about phrasing. Be specific. Give it context. Ask it to think step by step. That plateaus quickly, and it plateaus for a structural reason: better phrasing asks the model to try harder. It does not tell the model anything it did not already know. The thing that actually moves output is a file. .github/copilot-instructions.md ,…
Three essential guidelines are crucial to ensure consistent and error-free outputs from Copilot, transcending mere phrasing. To harness the full potential of this tool, a dedicated .github/copilot-instructions.md file must be committed to the repository. This file, located precisely within the .github directory, should be plain Markdown without any frontmatter.
One rule that has proven effective is to specify the data types and handling methods explicitly. For instance, instead of a general instruction like "Use proper error handling," a more precise rule would be "Use proper error handling with `Decimal` data type, rounding half-up to two places only at the response boundary." This level of specificity provides the model with the necessary context to produce accurate and well-structured outputs, avoiding the plateau effect that often occurs with more general phrasing.
Another crucial rule is to ensure timezone awareness in all datetime operations. The model should be instructed to use timezone-aware UTC datetimes, such as `datetime.utcnow()`, as opposed to naive datetimes. This detail is vital because a naive datetime can lead to incorrect comparisons against other datetime values, resulting in errors that may not be immediately apparent.
By explicitly requiring timezone-aware datetimes, the rule ensures that the model understands the importance of handling date and time information correctly, thereby producing reliable outputs.
Lastly, prohibitions should be emphasized over aspirations. Instead of just stating "Do not use datetime.utcnow() since it returns a naive datetime," a more effective approach is to articulate the reason behind this prohibition: "Do not use datetime.utcnow() because it returns a naive datetime, which can lead to incorrect comparisons against timezone-aware datetimes within this codebase.
Use timezone-aware datetimes instead." This prohibition, coupled with a clear explanation, provides the model with a concrete directive that is more likely to result in consistent and accurate outputs. By incorporating these three rules into the .github/copilot-instructions.md file, the team can ensure that Copilot's outputs adhere to established conventions and standards, minimizing errors and enhancing overall code quality.
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