AI Developer Guide - Technical Decision Criteria and Anti-pattern Collection

Technical Anti-patterns (Red Flag Patterns)

Immediately stop and reconsider design when detecting the following patterns:

Code Quality Anti-patterns

1. Writing similar code 3 or more times - Violates Rule of Three
2. Multiple responsibilities mixed in a single file - Violates Single Responsibility Principle (SRP)
3. Defining same content in multiple files - Violates DRY principle
4. Making changes without checking dependencies - Potential for unexpected impacts
5. Disabling code with comments - Should use version control
6. Error suppression - Hiding problems creates technical debt
7. Bypassing safety mechanisms (type systems, validation, contracts) - Circumventing language's correctness guarantees

Design Anti-patterns

• "Make it work for now" thinking - Accumulation of technical debt
• Patchwork implementation - Unplanned additions to existing code
• Optimistic implementation of uncertain technology - Designing unknown elements assuming "it'll probably work"
• Symptomatic fixes - Surface-level fixes that don't solve root causes
• Unplanned large-scale changes - Lack of incremental approach

Fail-Fast Fallback Design Principles

Core Principle

Make all errors visible and traceable with full context. Prioritize primary code reliability over fallback implementations. Excessive fallback mechanisms mask errors and make debugging difficult.

Implementation Guidelines

Default Approach

• Propagate all errors explicitly unless a Design Doc specifies a fallback
• Make failures explicit: Errors should be visible and traceable
• Preserve error context: Include original error information when re-throwing

When Fallbacks Are Acceptable

• Only with explicit Design Doc approval: Document why fallback is necessary
• Business-critical continuity: When partial functionality is better than none
• Graceful degradation paths: Clearly defined degraded service levels

Layer Responsibilities

• Infrastructure Layer:

  • Always throw errors upward
  • No business logic decisions
  • Provide detailed error context

• Application Layer:

  • Make business-driven error handling decisions
  • Implement fallbacks only when specified in requirements
  • Log all fallback activations for monitoring

Error Masking Detection

Review Triggers (require design review):

• Writing 3rd error handler in the same feature
• Multiple error handling blocks in single function/method
• Nested error handling structures
• Error handlers that return default values without logging

Before Implementing Any Fallback:

1. Verify Design Doc explicitly defines this fallback
2. Document the business justification
3. Ensure error is logged with full context
4. Add monitoring/alerting for fallback activation

Implementation Pattern

AVOID: Silent fallback that hides errors
    <handle error>:
        return DEFAULT_VALUE  // Error hidden, debugging impossible

PREFERRED: Explicit failure with context
    <handle error>:
        log_error('Operation failed', context, error)
        <propagate error>  // Re-throw exception, return Error, return error tuple

Adaptation: Use language-appropriate error handling (exceptions, Result types, error tuples, etc.)

Rule of Three - Criteria for Code Duplication

How to handle duplicate code based on Martin Fowler's "Refactoring":

Criteria for Commonalization

Cases for Commonalization

• Business logic duplication
• Complex processing algorithms
• Areas likely requiring bulk changes
• Validation rules

Cases to Avoid Commonalization

• Accidental matches (coincidentally same code)
• Possibility of evolving in different directions
• Significant readability decrease from commonalization
• Simple helpers in test code

Common Failure Patterns and Avoidance Methods

Pattern 1: Error Fix Chain

Symptom: Fixing one error causes new errors Cause: Surface-level fixes without understanding root cause Avoidance: Identify root cause with 5 Whys before fixing

Pattern 2: Circumventing Correctness Guarantees

Symptom: Bypassing safety mechanisms (type systems, validation, contracts) Cause: Impulse to avoid correctness errors Avoidance: Use language-appropriate safety mechanisms (static checking, runtime validation, contracts, assertions)

Pattern 3: Implementation Without Sufficient Testing

Symptom: Many bugs after implementation Cause: Ignoring Red-Green-Refactor process Avoidance: Always start with failing tests

Pattern 4: Ignoring Technical Uncertainty

Symptom: Frequent unexpected errors when introducing new technology Cause: Assuming "it should work according to official documentation" without prior investigation Avoidance:

• Record certainty evaluation at the beginning of task files

  Certainty: low (Reason: no working examples found for this integration)
  Exploratory implementation: true
  Fallback: use established alternative approach
  

• For low certainty cases, create minimal verification code first

Pattern 5: Insufficient Existing Code Investigation

Symptom: Duplicate implementations, architecture inconsistency, integration failures, adopting outdated patterns Cause: Insufficient understanding of existing code before implementation; referencing only nearby files without verifying representativeness Avoidance Methods:

• Before implementation, always search for similar functionality (using domain, responsibility, configuration patterns as keywords)
• Similar functionality found → Use that implementation (do not create new implementation)
• Similar functionality is technical debt → Create ADR improvement proposal before implementation
• No similar functionality exists → Implement new functionality following existing design philosophy
• Record all decisions and rationale in "Existing Codebase Analysis" section of Design Doc
• Reference representativeness check: When adopting a pattern or dependency from nearby code, verify it is representative across the repository before adopting — nearby files alone are an insufficient basis

Debugging Techniques

1. Error Analysis Procedure

1. Read error message (first line) accurately
2. Focus on first and last of stack trace
3. Identify first line where your code appears

2. 5 Whys - Root Cause Analysis

Trace the failure through repeated "why" questions until the root cause is actionable.

3. Minimal Reproduction Code

To isolate problems, attempt reproduction with minimal code:

• Remove unrelated parts
• Replace external dependencies with mocks
• Create minimal configuration that reproduces problem

4. Debug Log Output

Include operation context, relevant input data, current state, and timestamp.

Quality Assurance Mechanism Awareness

Before executing quality checks, identify what quality mechanisms exist for the change area:

• Primary detection: inspect the change area's file types, project manifest, and configuration to identify applicable quality tools
  • Check CI pipeline definitions for checks that cover the affected paths
  • Check for domain-specific linter or validator configurations (e.g., schema validators, API spec validators, configuration file linters)
  • Check for domain-specific constraints in project configuration (naming rules, length limits, format requirements)
• Supplementary hint: IF task file specifies Quality Assurance Mechanisms → use them as additional hints for which domain-specific checks to look for
• Include discovered domain-specific checks alongside standard quality phase