Why This Matters

Most beginners think a prompt is just the text they type. It isn't. A prompt includes your cursor position, the file you're editing, attached files, recent edits, and Copilot's system instructions. Understanding this invisible interface is the foundation of all later prompt engineering.

Concept Explained

A prompt is the complete input bundle sent to the model. It has explicit parts (what you type, what you attach) and implicit parts (your cursor, recent edits, the active file, terminal output if @terminal is used). Copilot assembles all of these into one context bundle before invoking the model.

How It Works

When you press Enter in Chat, Copilot runs a context-assembly function: gather explicit attachments, gather implicit context (recent edits, current selection), prepend system instructions, append your message, and send to the model. The model never sees your editor — it sees the assembled bundle. That's why attaching the right file matters.

Step-by-Step Tutorial

1. Decompose a real prompt

2. Identify implicit context

3. Recognize the invisible interface

4. Reason about what Copilot sees

5. Improve future prompts

Real-World Example

A developer was frustrated that Copilot kept suggesting deprecated APIs. They decomposed their prompt and realized: the file open was a 3-year-old module using the old API. Copilot was faithfully matching the file's style. Fix: open a file using the new API, or attach the new API docs as a file. The 'bug' was a context issue, not a model issue.

Example Prompts / Commands / Code

Explicit:
  - Text typed:       'add error handling'
  - Attached files:   none
  - Slash command:    none
  - Participant:      none

Implicit:
  - Current file:     auth.js
  - Selection:        lines 12–28 (the login function)
  - Recent edits:     added try/catch to register() 5 min ago
  - Terminal output:  not used

Assembled context sent to model:
  [system instructions]
  [contents of auth.js]
  [your selection, lines 12-28]
  [your message: 'add error handling']

Common Mistakes

• Thinking the typed text is the whole prompt.
• Forgetting that Copilot sees your recent edits.
• Not attaching relevant files and assuming Copilot knows.
• Switching files mid-conversation without realizing context changed.

Best Practices

• Decompose prompts when results surprise you.
• Attach files explicitly when they matter.
• Close irrelevant tabs — open files leak into context.
• Before sending, ask: 'what is Copilot about to see?'

Troubleshooting

Practical Exercise

Frequently Asked Questions

Further Reading

• Copilot Chat reference

Official References

• Copilot Chat reference

Related lessons: BE-16, BE-20