Why Harness

Why Harness?

The main advantage of Harness: stay in one IDE (VS Code) and reuse several AI providers — Copilot, Claude, Cursor, Devin, Kiro — without switching editors. At the same time, use Spec-Driven Development and context engineering in the same chat and workspace.

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One IDE, many agents

Problem today	Harness approach
Copilot only inside VS Code	Copilot plus other providers in the same sidebar
Cursor or Claude need their own IDE/app	Select Cursor or Claude as a provider pill — stay in VS Code
Different chat history per tool	One Harness chat; switch provider for the next message
Re-learn UI for each vendor	One composer, one context bar, one spec manager

Configure keys in Configuration. Use Auto Routing to pick the best provider per prompt.

Providers in one panel: Copilot · Claude · Cursor · Devin · Kiro · Auto

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Spec-Driven Development (SDD)

Define how agents should behave in .harness/specs/ (skills, tools, workflows) — versioned with your repo.

• Spec Manager — edit specs in the sidebar (SDD Specs)
• Spec+Agent mode — injects active specs as system context before Copilot runs (Copilot Modes)
• Reuse — same spec for chat, CLI, and CI-style runs

SDD is not a separate product; it lives inside Harness next to your code.

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Context engineering

Give the model the right files every time:

• Right-click → Add to Harness Context
• Chips above the composer show what is attached
• The CLI reads files and sends them with every provider

Important: context survives when you change provider. Add src/ once; ask Copilot, then switch to Claude — same context, no re-upload.

Details: Context and Specs.

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Together in one workflow

1. Specs — what the agent must follow (SDD)
2. Context — which files matter (context engineering)
3. Provider — who answers (Copilot, Claude, … or Auto)

All three in the Harness sidebar — no IDE hopping.

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Learn more

• Getting Started
• Chat Interface
• User Guide
• Repository: docs/why-harness.md