Convention Learner

Core Principles

1. Observe before enforcing — Never impose conventions without first analyzing the existing codebase. A project with 200 internal sealed class handlers should not get a new public class handler. Detect first, then match.
2. Project conventions override generic rules — If the project uses *Service instead of *Handler, follow the project's convention even if the kit default is different. Explicit .editorconfig and Directory.Build.props rules always win.
3. Use MCP tools for analysis — get_public_api reveals naming patterns, get_project_graph shows structure conventions, detect_antipatterns tracks quality trends. Tools provide objective data; file reads provide confirmation.
4. Document findings — After detecting conventions, suggest adding them to the project's CLAUDE.md. Undocumented conventions are lost when the original developers leave.
5. Consistency over perfection — A project with consistent snake_case database columns is better than a project with half snake_case and half PascalCase. Match the existing pattern, even if another convention is theoretically superior.

Patterns

Convention Detection Flow

Systematic analysis to understand a project's coding conventions. Run this when joining an existing project or before generating new code.

Step 1: Project Structure Analysis

→ get_project_graph
  Detect:
  - Project naming: PascalCase? Dots? (MyApp.Domain vs Domain)
  - Layer organization: by layer (Domain/Application/Infrastructure) or by feature?
  - Test project naming: *.Tests, *.UnitTests, *.IntegrationTests?
  - Shared project: Common/, Shared/, BuildingBlocks/?

Step 2: Type Naming Patterns

→ get_public_api (on 3-5 key types across different layers)
  Detect:
  - Class modifiers: sealed? internal? internal sealed?
  - Interface prefix: I* (standard) or no prefix?
  - Suffix conventions: Handler, Service, Repository, Validator, Endpoint?
  - Record usage: for DTOs? for value objects? for commands/queries?
  - Primary constructor usage: consistently? selectively?

Step 3: Folder Structure Patterns
Scan the file system for structural conventions:

• Feature folders: Features/{FeatureName}/ with all files together?
• Layer folders: Controllers/, Services/, Repositories/ separate?
• Shared patterns: Common/, Extensions/, Middleware/?
• Configuration location: root? Config/ folder? Infrastructure/?

Step 4: Configuration Detection
Check for explicit convention enforcers:

→ Look for Directory.Build.props
  - TreatWarningsAsErrors?
  - Nullable enabled globally?
  - ImplicitUsings?
  - AnalysisLevel?

→ Look for .editorconfig
  - Naming rules: camelCase fields? _prefixed privates?
  - Code style: var preferences, expression bodies, using placement

→ Look for global.json
  - SDK version pinned?
  - Roll-forward policy?

Step 5: Build Convention Summary
Compile findings into a structured summary:

## Detected Conventions

### Naming
- Classes: `internal sealed class` (95% of handlers/services)
- Suffixes: Handlers end in `Handler`, validators in `Validator`
- Records: Used for DTOs and commands/queries

### Structure
- Architecture: Vertical Slice Architecture
- Features: `Features/{Name}/` with command, handler, validator, endpoint in one file

### Code Style
- Primary constructors: Used consistently for DI injection
- Nullable: Enabled globally, no suppressions (`!`) used
- File-scoped namespaces: 100% consistent

Add categories as needed: EF Core (configurations, naming, migrations), Testing (framework, naming, fixtures), etc.

Convention Enforcement

Apply detected conventions when generating new code or reviewing existing code.

When Generating Code:
Match every detected pattern:

// If existing handlers are: internal sealed class + primary constructor
// Generate matching:
internal sealed class CreateProductHandler(AppDbContext db, TimeProvider clock)
{
    // Not: public class CreateProductHandler
    // Not: internal class CreateProductHandler (missing sealed)
}

// If existing DTOs are records with init properties
// Generate matching:
public record ProductResponse(Guid Id, string Name, decimal Price);
// Not: public class ProductResponse { public Guid Id { get; set; } }

When Reviewing Code:
Flag deviations from detected conventions:

⚠️ Convention violation: CreateOrderHandler is `public class` but project convention
   is `internal sealed class` (detected in 12/12 existing handlers).
   Change to: internal sealed class CreateOrderHandler

Suggesting Enforcement Rules:
After detecting conventions, suggest .editorconfig rules to enforce them automatically:

# Key .editorconfig rules to suggest based on detected conventions
dotnet_diagnostic.CA1852.severity = warning           # Seal internal types
csharp_style_namespace_declarations = file_scoped:warning
csharp_style_prefer_primary_constructors = true:suggestion
# Add dotnet_naming_rule entries for private field prefix (_camelCase) if detected

Anti-pattern Tracking

Use detect_antipatterns to track recurring quality issues across sessions.

Periodic Check:

→ detect_antipatterns (scope: solution)
  Track over time:
  - Are the same patterns recurring? (DateTime.Now keeps appearing)
  - Are new patterns emerging? (new HttpClient() in a new module)
  - Is the count trending up or down?

Prioritization:

| Anti-pattern | Count | Trend | Priority |
|-------------|-------|-------|----------|
| DateTime.Now | 12 | ↑ +3 | High — add to CLAUDE.md conventions |
| async void | 1 | → same | Medium — one-off fix |
| new HttpClient | 0 | ↓ -2 | Low — already fixing |

When patterns recur, add explicit rules to CLAUDE.md:

## Conventions
- **NEVER use DateTime.Now** — Use TimeProvider.GetUtcNow() (12 violations found, fixing)

Anti-patterns

Enforcing Without Detecting

# BAD — Imposing kit defaults on a project with its own conventions
"All handlers should be internal sealed class"
# But this project uses public class with interfaces for testing

# GOOD — Detect first, then follow what exists
→ get_public_api reveals: 8/8 handlers are `public class` implementing `IHandler<T>`
"This project uses public handlers with interfaces. Matching that convention."

Overriding Explicit Project Rules

# BAD — Ignoring .editorconfig because kit says otherwise
# .editorconfig says: csharp_style_expression_bodied_methods = false
# But generating expression-bodied methods anyway

# GOOD — .editorconfig and Directory.Build.props always win
"Your .editorconfig disables expression-bodied methods.
I'll use block-bodied methods to match your project settings."

Applying Generic Conventions to Unconventional Projects

# BAD — Forcing Clean Architecture naming on a VSA project
"You need a Services/ folder and a Repositories/ folder"
# But this project uses feature folders with everything co-located

# GOOD — Match the project's organizational convention
"This project uses feature folders. I'll add the new feature
at Features/Shipping/ with all related files together."

Documenting Conventions Without Evidence

# BAD — "Conventions" based on reading one file
"Convention: Use var everywhere" (based on seeing var in one method)

# GOOD — Document only patterns confirmed across multiple files
→ get_public_api on 5 types: 100% use explicit types for non-obvious cases
"Convention: Use explicit types for non-obvious cases (e.g., method returns),
var for obvious cases (e.g., new MyClass()). Confirmed across 5 files."

Decision Guide

Scenario	Action	Tool
Joining existing project	Run full convention detection flow	get_project_graph, get_public_api
Generating new code	Check detected conventions first	Previous detection results
Reviewing code	Flag convention deviations	get_public_api + comparison
Convention conflict (kit vs project)	Project wins	—
Convention conflict (team disagreement)	Document both, suggest .editorconfig	—
No conventions detected	Use kit defaults, document them	architecture-advisor skill
Recurring anti-pattern	Add to CLAUDE.md conventions	detect_antipatterns
New team member onboarding	Run detection, generate convention doc	Full detection flow
.editorconfig exists	Trust it, don't override	Read .editorconfig
No .editorconfig	Suggest creating one based on detected patterns	Detection + generation
Pattern seen once	Create instinct at 0.3 confidence via instinct-system skill	instinct-system
Pattern confirmed 3+ times	Instinct auto-promotes to 0.7, suggest adding to CLAUDE.md	instinct-system