Improve Codebase Architecture

# Reference

## Dependency Categories

When assessing a candidate for deepening, classify its dependencies:

### 1. In-process

Pure computation, in-memory state, no I/O. Always deepenable — just merge the modules and test directly.

### 2. Local-substitutable

Dependencies that have local test stand-ins (e.g., PGLite for Postgres, in-memory filesystem). Deepenable if the test substitute exists. The deepened module is tested with the local stand-in running in the test suite.

### 3. Remote but owned (Ports & Adapters)

Your own services across a network boundary (microservices, internal APIs). Define a port (interface) at the module boundary. The deep module owns the logic; the transport is injected. Tests use an in-memory adapter. Production uses the real HTTP/gRPC/queue adapter.

Recommendation shape: "Define a shared interface (port), implement an HTTP adapter for production and an in-memory adapter for testing, so the logic can be tested as one deep module even though it's deployed across a network boundary."

### 4. True external (Mock)

Third-party services (Stripe, Twilio, etc.) you don't control. Mock at the boundary. The deepened module takes the external dependency as an injected port, and tests provide a mock implementation.

## Testing Strategy

The core principle: **replace, don't layer.**

- Old unit tests on shallow modules are waste once boundary tests exist — delete them
- Write new tests at the deepened module's interface boundary
- Tests assert on observable outcomes through the public interface, not internal state
- Tests should survive internal refactors — they describe behavior, not implementation

## Issue Template

<issue-template>

## Problem

Describe the architectural friction:

- Which modules are shallow and tightly coupled
- What integration risk exists in the seams between them
- Why this makes the codebase harder to navigate and maintain

## Proposed Interface

The chosen interface design:

- Interface signature (types, methods, params)
- Usage example showing how callers use it
- What complexity it hides internally

## Dependency Strategy

Which category applies and how dependencies are handled:

- **In-process**: merged directly
- **Local-substitutable**: tested with [specific stand-in]
- **Ports & adapters**: port definition, production adapter, test adapter
- **Mock**: mock boundary for external services

## Testing Strategy

- **New boundary tests to write**: describe the behaviors to verify at the interface
- **Old tests to delete**: list the shallow module tests that become redundant
- **Test environment needs**: any local stand-ins or adapters required

## Implementation Recommendations

Durable architectural guidance that is NOT coupled to current file paths:

- What the module should own (responsibilities)
- What it should hide (implementation details)
- What it should expose (the interface contract)
- How callers should migrate to the new interface

</issue-template>


---
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 under