First 90 days as a junior engineer on an AI-heavy team: what to learn first

You took the offer. The team uses Cursor or Copilot for almost every PR, runs an internal RAG bot over the docs, and at least one senior engineer is building agent workflows on the side. Your onboarding doc is half-written because the person who owned it left, and the codebase has roughly 40% more surface area than the org chart suggests, because LLMs have made it cheap to ship adapters, scripts, and one-off services nobody fully owns. This is the environment most junior engineers walk into in 2026. The traditional advice — read the codebase, ask questions, find a mentor — still applies, but it doesn't tell you what to prioritize when the senior engineers around you are visibly faster than you because they've internalized tools you've never touched. Below is a 90-day plan that assumes you have decent fundamentals (you can write a function, you understand HTTP, you've used git) but you're new to working in a codebase where AI is a first-class collaborator. Days 1-30: read more than you write, and read what the AI reads The biggest mistake juniors make in AI-heavy teams is opening Cursor on day three and trying to ship a feature. You will produce code that compiles, passes the obvious tests, and quietly violates three conventions nobody wrote down. Your PR will get approved by a tired senior because rejecting AI-generated junior code costs political capital. You will learn nothing. Instead, spend the first month doing three things in roughly equal proportion: 1. Read the codebase the way the AI reads it. Look at how the repo is structured for retrieval. Most AI-heavy teams have a CLAUDE.md , .cursorrules , AGENTS.md , or an internal RAG index. These files encode the conventions, the patterns the team wants reinforced, and — critically — the things the team has had to tell the AI not to do . Forbidden patterns are usually the result of an incident. Read them. Ask which incident produced each one. 2. Read closed PRs, not open ones. Open PRs are noisy. Closed PRs from th