Our coding agent was relearning our codebase every session

Our coding agent was burning through $40/day in API costs doing the same work over and over. Every session, it would re-read our entire codebase, re-discover our patterns, and re-learn our conventions. Like hiring a contractor who throws away all their notes at the end of each day.

The fix wasn't a bigger context window or smarter prompting. It was session persistence.

Here's the pattern that cut our costs 70% and made our agent actually accumulate knowledge:

# ~/.agent_sessions/project_name/
├── session.md          # Current work log
├── codebase_map.md     # Architecture overview
├── patterns.md         # Code patterns we use
├── decisions.md        # Why we built things this way
└── todos.md           # Outstanding work

Every session starts by reading these five files instead of scanning the entire codebase. The agent updates them as it learns. When it discovers a new pattern, it adds it to patterns.md. When it makes an architectural decision, it logs it in decisions.md.

The session file is the key. It's not a chat log — it's a work journal:

# Session: 2024-01-15

## Current Focus
Adding user authentication to the dashboard

## What I Learned Today
- Auth middleware is in /lib/auth.js
- We use JWT tokens, not sessions
- User model has soft deletes enabled

## Blockers
- Need clarification on password reset flow
- Waiting for design mockups

## Next Session
- Implement login form validation
- Add password strength requirements

This isn't just about cost savings. A persistent agent builds institutional knowledge. It remembers why you made decisions, what didn't work, and what patterns to follow. It becomes a team member instead of a expensive search engine.

The real win: our agent now catches inconsistencies across sessions. Last week it flagged that we were using two different validation patterns and suggested consolidating them.

Set up session persistence before your next coding session. Create the five files, give your agent write access, and watch it stop re-learning your codebase every conversation.