Copilot Subagents: Parallel Orchestration Demo

Summary

A VS Code Insiders user demonstrates a multi-agent orchestration system using GitHub Copilot's new custom subagent features. The setup uses different models for different roles—Sonnet 4.5 for orchestration, Gemini 3 Flash for exploration, GPT 5.2 for research—running in parallel while preserving main context.

Key Points from OP

Parallel subagent execution now works in VS Code Insiders (Jan 2026 build)
Tasks that consumed 80K tokens in a single agent now use under 4K via delegation
Main orchestrator (Atlas) only delegates and synthesizes—no direct work
Subagents are defined via [agent-name].agent.md files with model assignments

Agent Architecture

Agent	Model	Role
Atlas	Sonnet 4.5	Orchestrator—delegates and synthesizes
Prometheus	GPT 5.2 High	Deep research and planning, writes to plan directories
Oracle	GPT 5.2	Research and analysis
Sisyphus	Sonnet 4.5	Task execution from plans
Explorer	Gemini 3 Flash	File discovery, web fetches
Code Review	GPT 5.2	Autonomous code review
Frontend Engineer	Gemini 3 Pro	UI/UX specialist

Notable Comments

"If anyone is wondering on the technicalities, they are just documentation files, in markdown (.md) format, which Copilot recognizes and auto-loads them on every message, if you name them agent-name.agent.md. The sub-agents can take Copilot to the next level in regards to its context-window."
— u/SourceCodeplz (2 points)

"What's interesting to me is not the 'parallel execution' aspect but more the context size optimization. Big functional prompts or zero-to-POC prompts are probably going to benefit the most from the system. But what about 'fix that error' prompts?"
— u/WSATX (1 point)

Model Assignment Caveat

Several commenters report that model assignments in subagent definitions may be ignored—subagents appear to use the main chat's model regardless of what's defined in their .agent.md file. One commenter tested with reasoningEffort: xhigh (only supported by GPT 5.2 Codex) and found subagents ran without errors even when assigned GPT 5-mini, suggesting the model setting was discarded.

Discussion Takeaways

Context conservation is the primary value—not just parallelism
Subagent cost structure unclear: some claim subagents don't consume additional premium requests
Enabling requires: "chat.customAgentInSubagent.enabled": true in VS Code settings
Nested subagents (sub-sub-agents) not yet supported
Features currently Insiders-only, expected in public VS Code within weeks

Resources

Github-Copilot-Atlas repo — OP's agent configuration
copilot-orchestra — Original inspiration
oh-my-opencode — Naming conventions source

Connections

mastering-subagents-in-vs-code-copilot — Covers the same #runSubagent feature with isolation and worktree concepts
the-context-window-problem — Explains why context conservation matters: enterprise codebases exceed model limits
using-agents-in-vscode — Official VS Code agent documentation covering the four execution modes