Stacks/Coding agent

The coding harness behind GitHub Copilot in VS Code

The VS Code team's account of Copilot's editor-side coding harness treats the model as the engine and the harness as the car. The post walks through context assembly, tool exposure (including MCP and custom agents), the tool-calling loop, and a dedicated eval pipeline (`vscode-evals` and `github/evald`) that benchmarks harness changes on every relevant PR.

Use case

The agentic coding experience inside Visual Studio Code: assembling context, exposing and executing tools, and running the agent loop that turns model output into editor actions

Category

Coding agent

Team size

not stated

Orchestrator / framework

A tool-calling agent loop (think, act, observe, think again). The harness builds the prompt (system instructions, user query, workspace structure, conversation history, tool results, custom instructions, and memory from earlier sessions), sends it to the model, executes any tool calls (read_file, replace_string_in_file or apply_patch, run_in_terminal, semantic_search, and more), and repeats until the model returns no tool calls. The available tool set can change per request, including MCP servers, extensions, user toggles, and custom agents defined in `.agent.md`.

Memory

The context assembly step includes memory from earlier sessions, along with conversation history and custom instructions.

Evals

A CI path for harness changes: a successful build publishes a versioned eval agent to a `vscode-evals` npm feed, which opens a model-evaluation issue on `github/evald`. evald runs the benchmark and reports analysis back to the original VS Code PR via an Azure Logic App (comment URLs only; the analysis body stays private).

Observability

A Chat Debug View in VS Code lets developers inspect prompts, tool calls, and results behind an agent run. A Tools UI shows which tools are available for a request.

Monthly cost

not stated

Models

not stated

Observability tools

VS Code Chat Debug View

Other tools

MCPCustom agents (`.agent.md`)github/evaldAzure Logic App

Source

https://code.visualstudio.com/blogs/2026/05/15/agent-harnesses-github-copilot-vscode

Julia Kasper, Megan Rogge, and Aaron Munger · May 14, 2026