GitHub Copilot customization architecture

Choose the smallest customization that supplies the missing behavior. Inspect the current workspace and active VS Code/Copilot version before writing files.

Selection model

Need	Choose	Activation
Broad convention	Instructions	Automatic
File/task-specific convention	.instructions.md	Conditional
Focused reusable task	Prompt file	Manual slash command
Multi-step capability/resources	Agent Skill	Relevant task or slash command
Specialist persona/tool boundary	Custom agent	Selected/invoked role
Guaranteed lifecycle action	Hook	Deterministic event; preview
External live tools/data	MCP	Tool call after trust
Installable bundle	Plugin	Install/enable; preview

Subagents are runtime delegation, not another file to create.

Workflow

1. Inventory current instructions, prompts, skills, agents, hooks, MCP configs, plugins, VS Code settings, and parent-repository discovery.
2. State the repeated job, activation mode, workspace/user scope, required capabilities, deterministic enforcement, and trust boundary.
3. Route each requirement using the table. Split requirements whose activation, authority, or verification differs.
4. Build a collision map. Remove duplicate or contradictory rules; never rely on instruction ordering.
5. Assign least-privilege tools and explicit trust steps. Do not silently enable executable hooks, servers, or plugins.
6. Author through the matching specialized skill, then parse and test every artifact before a VS Code smoke test.

Decision rules

• If guidance belongs in nearly every request, use repository instructions.
• If a person deliberately starts one task, use a prompt file; if Copilot must discover a multi-step capability with resources, use a skill.
• If behavior must happen even when the model forgets, use a hook, not prose.
• If a role only needs read access, exclude edit and terminal tools.
• If a capability needs live external state, use a trusted tool/MCP server; do not paste credentials or pretend durable instructions are current data.
• If several artifacts ship together, use a plugin only when installation and trust are worth the packaging cost.
• Default to workspace scope. User scope requires an explicit cross-project job.

Read the format and scope map before choosing locations. Use architecture recipes for a multi-artifact design and verification before declaring completion.

Completion

Deliver an artifact map with one owner per rule, explicit activation and trust boundaries, no unresolved collisions, parsed files, offline executable checks, and a recorded VS Code discovery plan. Mark hooks and plugins as preview and host/model/tool identifiers as locally verified or unverified.