STAC MCP Server

# ASR 1011: Comprehensive Test Coverage Initiative Status: Accepted Date: 2025-01-10 ## Context As the STAC MCP Server codebase has grown with new features (ADR 0003-0009, ASR 1001-1010), maintaining high code quality and confidence in changes has become critical. While the existing test suite provides good baseline coverage, systematic gaps exist in: 1. **Edge case testing**: Many tools and handlers lack tests for boundary conditions, empty results, malformed inputs, and error scenarios 2. **Branch coverage**: Conditional logic in observability, client retry mechanisms, and error handling lacks comprehensive testing 3. **Integration scenarios**: Limited tests for multi-tool workflows and component interactions 4. **Coverage visibility**: No enforced minimum coverage thresholds in CI to prevent regressions These gaps increase the risk of undetected bugs, make refactoring more dangerous, and reduce team confidence when making changes. ## Requirement Systematically increase test coverage to meet the following measurable targets: ### Coverage Targets - **Overall Project**: ≥ 85% statement coverage, ≥ 80% branch coverage (enforced in CI) - **Core Modules** (server.py, client.py, execution.py): ≥ 95% statement, ≥ 90% branch - **Tool Handlers**: ≥ 90% statement, ≥ 80% branch - **Observability**: ≥ 90% statement, ≥ 85% branch ### Test Categories Required 1. **Unit Tests**: Test individual functions and methods in isolation - Edge cases (empty inputs, boundary values, maximum limits) - Error conditions (network failures, timeouts, malformed data) - Type variations (different input types, None values) 2. **Integration Tests**: Test component interactions - Tool handler → client → external API flows - Observability instrumentation across tool calls - Retry logic with simulated failures 3. **Branch Coverage Tests**: Exercise all conditional paths - Environment variable configurations - Optional parameter combinations - Fallback mechanisms - Error recovery paths ### CI/CD Requirements - Automated coverage reporting on every PR and main branch push - Coverage threshold enforcement (fail build if < 85%) - Coverage badge generation and automatic updates - HTML coverage reports as build artifacts ### Documentation Requirements - Test coverage strategy document - Testing patterns and best practices guide - Coverage monitoring and maintenance procedures - Clear guidelines for new contributors ## Decision Implement comprehensive test coverage improvements across the following phases: ### Phase 1: Infrastructure & Documentation (Completed) ✅ Create test coverage strategy document (`docs/TEST_COVERAGE_STRATEGY.md`) ✅ Add coverage threshold enforcement to CI workflow (≥85%) ✅ Document testing patterns, best practices, and maintenance procedures ### Phase 2: Core Test Additions (Completed) ✅ **Tool Handler Edge Cases** (`tests/test_tool_handlers_edge_cases.py`) - 50+ tests covering all tool handlers (get_root, get_conformance, etc.) - Edge cases: empty results, minimal data, many items (>limit) - Output format variations (text/JSON) - Error conditions and malformed inputs ✅ **Tool Definitions Comprehensive** (`tests/test_definitions_comprehensive.py`) - 40+ tests validating tool definition structure - JSON Schema validation for all input schemas - Parameter consistency checks across tools - Transaction tool validation ✅ **Observability Branch Coverage** (`tests/test_observability_branches.py`) - 45+ tests for conditional branches - Environment variable handling (all permutations) - Metrics registry thread-safety - Log format variations (text/JSON) - Trace span enabled/disabled scenarios ✅ **Execution Module Comprehensive** (`tests/test_execution_comprehensive.py`) - 35+ tests for tool execution logic - Return type handling (dict, list, string, None) - Error propagation and logging - Performance edge cases (large results, deep nesting) ✅ **Client Edge Cases** (`tests/test_client_edge_cases.py`) - 40+ tests for HTTP client scenarios - URL construction and encoding - Header manipulation - Retry logic boundary conditions - Response parsing edge cases **Total New Tests**: ~250+ test cases across 5 new test files ### Phase 3: Monitoring & Maintenance (Ongoing) - Weekly CI coverage trend review - Quarterly coverage target reassessment - Test performance monitoring (keep < 2 minutes total) - Documentation updates as patterns evolve ## Consequences ### Positive 1. **Reduced Regression Risk**: Comprehensive tests catch bugs before they reach production 2. **Refactoring Confidence**: High coverage enables safe code restructuring 3. **Better Documentation**: Tests serve as executable documentation of expected behavior 4. **Quality Gate**: CI threshold prevents coverage regressions 5. **Onboarding Aid**: New contributors can understand behavior through tests ### Negative 1. **Initial Time Investment**: Creating 250+ tests requires significant upfront effort 2. **Maintenance Burden**: More tests to maintain when APIs change 3. **CI Time**: Longer test execution (though still < 2 minutes target) 4. **False Security**: High coverage doesn't guarantee correctness, just that code is exercised ### Mitigation Strategies - **For Maintenance**: Follow documented testing patterns to keep tests consistent and DRY - **For CI Time**: Run tests in parallel where possible, optimize slow tests - **For False Security**: Combine coverage with integration tests, manual QA, and production monitoring - **For Time Investment**: Tests written now prevent 10x time spent debugging later ## Implementation Details ### Test File Organization ``` tests/ ├── test_tool_handlers_edge_cases.py # Tool handler edge cases ├── test_definitions_comprehensive.py # Tool definition validation ├── test_observability_branches.py # Observability branch coverage ├── test_execution_comprehensive.py # Execution module tests ├── test_client_edge_cases.py # Client edge cases └── [existing test files...] # 10+ existing test files ``` ### Coverage Configuration `.coveragerc`: ```ini [run] source = stac_mcp omit = tests/*, scripts/version.py branch = True [report] show_missing = True skip_empty = True precision = 1 exclude_lines = pragma: no cover if __name__ == .__main__.: pass ``` ### CI Workflow Enhancement ```yaml - name: Tests + coverage run: | uv run coverage run -m pytest -q uv run coverage xml uv run python scripts/generate_coverage_badge.py coverage.xml coverage-badge.svg uv run coverage report - name: Check coverage threshold run: | uv run coverage report --fail-under=85 ``` ### Test Patterns Used 1. **Arrange-Act-Assert**: Clear test structure 2. **Parametrized Tests**: Reduce duplication for similar cases 3. **Mock External Dependencies**: Prevent network calls, ensure deterministic behavior 4. **Descriptive Test Names**: `test_<function>_<scenario>_<expected_behavior>` 5. **Docstrings**: Explain test purpose and edge case being verified ## Metrics & Success Criteria ### Before This Initiative - Total test files: 15 - Estimated coverage: ~70-80% (varies by module) - No CI coverage threshold - Limited edge case coverage ### After Phase 2 (Current) - Total test files: 20 (+5 comprehensive test files) - Total test cases: ~300+ tests - CI coverage threshold: 85% statement coverage - Comprehensive edge case coverage for: - All tool handlers - Tool definitions - Observability layer - Execution module - HTTP client ### Ongoing Success Metrics - Coverage stays ≥ 85% (enforced by CI) - Test execution time stays < 2 minutes - Zero coverage regressions in PRs - New code includes tests (coverage doesn't drop) ## Alternatives Considered 1. **Mutation Testing**: More rigorous than coverage, but much slower and complex - **Rejected**: Too expensive for current project size 2. **Property-Based Testing**: Hypothesis for generating test cases - **Deferred**: Consider for Phase 4 if needed 3. **Integration Tests Only**: Focus on end-to-end scenarios - **Rejected**: Misses edge cases in individual units 4. **Lower Coverage Threshold (70%)**: Less strict requirement - **Rejected**: 85% provides better safety net for critical infrastructure 5. **Manual Testing**: Rely on human QA - **Rejected**: Not scalable, not reproducible, can't run in CI ## References - pytest documentation: https://docs.pytest.org/ - coverage.py documentation: https://coverage.readthedocs.io/ - AGENTS.md: Project testing guidelines - docs/TEST_COVERAGE_STRATEGY.md: Detailed coverage strategy ## Addendums ### 2025-01-10: Phase 2 Completed - Added 5 new comprehensive test files - Created ~250+ new test cases - Coverage threshold enforcement added to CI - Documentation complete ### Future Enhancements (Phase 4) - Property-based testing for complex data structures - Performance benchmarking tests - Mutation testing for critical paths - Mock STAC server for integration tests (ref: ASR 1007)