Improve Codebase Architecture

---
name: improve-codebase-architecture
description: Explore a codebase to find opportunities for architectural improvement, focusing on making the codebase more testable by deepening shallow modules. Use when user wants to improve architecture, find refactoring opportunities, consolidate tightly-coupled modules, or make a codebase more AI-navigable.
---

# Improve Codebase Architecture

Explore a codebase like an AI would, surface architectural friction, discover opportunities for improving testability, and write module-deepening refactor RFCs into a local `.rfcs/` folder.

A **deep module** (John Ousterhout, "A Philosophy of Software Design") has a small interface hiding a large implementation. Deep modules are more testable, more AI-navigable, and let you test at the boundary instead of inside.

## Process

### 1. Explore the codebase

Use Pi's `subagent` tool to navigate the codebase naturally. Prefer builtin `scout` first for fast recon; if needed, follow with `worker` for deeper analysis. Do NOT follow rigid heuristics — explore organically and note where you experience friction:

- Where does understanding one concept require bouncing between many small files?
- Where are modules so shallow that the interface is nearly as complex as the implementation?
- Where have pure functions been extracted just for testability, but the real bugs hide in how they're called?
- Where do tightly-coupled modules create integration risk in the seams between them?
- Which parts of the codebase are untested, or hard to test?

The friction you encounter IS the signal.

### 2. Present candidates

Present a numbered list of deepening opportunities. For each candidate, show:

- **Cluster**: Which modules/concepts are involved
- **Why they're coupled**: Shared types, call patterns, co-ownership of a concept
- **Dependency category**: See [REFERENCE.md](REFERENCE.md) for the four categories
- **Test impact**: What existing tests would be replaced by boundary tests

Do NOT propose interfaces yet. Ask the user: "Which of these would you like to explore?"

### 3. User picks a candidate

### 4. Frame the problem space

Before spawning sub-agents, write a user-facing explanation of the problem space for the chosen candidate:

- The constraints any new interface would need to satisfy
- The dependencies it would need to rely on
- A rough illustrative code sketch to make the constraints concrete — this is not a proposal, just a way to ground the constraints

Show this to the user, then immediately proceed to Step 5. The user reads and thinks about the problem while the sub-agents work in parallel.

### 5. Design multiple interfaces

Spawn 3+ subagents in parallel using Pi's `subagent` tool with the `tasks` parameter. Each must produce a **radically different** interface for the deepened module.

Give each subagent a separate technical brief (file paths, coupling details, dependency category, what's being hidden). This brief is independent of the user-facing explanation in Step 4. Use builtin `worker` for each parallel task, and give each one a different design constraint:

- Agent 1: "Minimize the interface — aim for 1-3 entry points max"
- Agent 2: "Maximize flexibility — support many use cases and extension"
- Agent 3: "Optimize for the most common caller — make the default case trivial"
- Agent 4 (if applicable): "Design around the ports & adapters pattern for cross-boundary dependencies"

Each sub-agent outputs:

1. Interface signature (types, methods, params)
2. Usage example showing how callers use it
3. What complexity it hides internally
4. Dependency strategy (how deps are handled — see [REFERENCE.md](REFERENCE.md))
5. Trade-offs

Present designs sequentially, then compare them in prose.

After comparing, give your own recommendation: which design you think is strongest and why. If elements from different designs would combine well, propose a hybrid. Be opinionated — the user wants a strong read, not just a menu.

### 6. User picks an interface (or accepts recommendation)

### 7. Write RFC file

Create a refactor RFC as a markdown file in a local `.rfcs/` folder using the template in [REFERENCE.md](REFERENCE.md).

Requirements:
- Create `.rfcs/` if it does not exist
- Use a durable kebab-case filename based on the problem, e.g. `.rfcs/deepen-checkout-pricing-module.md`
- Do NOT ask the user to review before writing it
- After writing, share the file path and a one-line summary of the recommendation

## Communication
Honor active caveman mode for user-facing replies per `../../references/communication-mode.md`. Keep durable artifacts normal unless the human asks otherwise. Drop caveman for safety/clarity when needed, then resume.