I was curious to understand how Copilot implements its harness, and also how I was exhausting my quota so quickly. End up going down a rabbit hole of intercepting its network traffic with mitmproxy.

A few interesting things I found along the way:

- watched model/capability discovery and routing happen in real time - looked at what gets injected into context and sent with ghost completions - found that recent edits can pull in context from files other than the one you're currently editing (including infamous .env) - found the SQLite session store behind Chronicle, including previous prompts/responses - watched the model query that history through tool calls

I then went through the VS Code source to reconcile some of what I was seeing on the wire with the actual implementation.

Overall some interesting lessons around how their harness is implemented.

Nice one! Really shows why we should run those in sandboxes without env access. I like the proxy swap approach

Nice deep dive, I always wondered how copilot worked compared to similar tools. I'm shocked at the lack of of a rule for env files, I at least thought with a tool more integrated with github as a whole that would be a default but alas.

Minor factual correction: The Codex client is open source. https://github.com/openai/codex