fenix-skills/skills/improve-codebase-architecture/SKILL.md

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---
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 propose module-deepening refactors as GitHub issue RFCs.
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 the Agent tool with subagent_type=Explore to navigate the codebase naturally. 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+ sub-agents in parallel using the Agent tool. Each must produce a **radically different** interface for the deepened module.
Prompt each sub-agent with 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. Give each agent 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. Create issue
Ask the user where to file the RFC:
- **GitHub/Gitea**: Use `gh issue create` or `tea issue create` with the template in [REFERENCE.md](REFERENCE.md)
- **Org file**: Write to the project's planning file (TASKS.org, TODO.org, etc.)
Do NOT ask the user to review before creating — just create it and share the URL or file path.