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.
| 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
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.
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.
Every message passes through Harness’s optimization pipeline before it reaches Copilot, Cursor, Claude, Devin, or Kiro.
Efficiency
- Trim chat history to recent turns (default 24)
- Remove duplicate user messages and empty assistant placeholders
- Cap each context file (default 12 000 characters) while keeping head and tail
- Merge duplicate system guidance into one block
Quality
- Inject a response contract: answer the goal first, structured bullets, minimal code diffs
- Mode-aware hints for Ask, Agent, and Spec+Agent (plans, spec authority, no unsolicited edits)
- Same rules on every provider — switching pills does not drop your standards
On by default. Configure in Configuration → Workspace or read Prompt Optimization.
Use Spending to see estimated token impact over time.
- Specs — what the agent must follow (SDD)
- Context — which files matter (context engineering)
- Optimization — trim and structure prompts for efficiency and quality
- Provider — who answers (Copilot, Claude, … or Auto)
All four in the Harness sidebar — no IDE hopping.
- Getting Started
- Chat Interface
- User Guide
- Repository: docs/why-harness.md