## What Was Built

Phase 8 created a comprehensive test suite for the LLM integration layer (Phase 6) and mission generation pipeline (Phase 7). The test suite includes:

- **298 new tests** across 12 test files
- **6 conftest.py helper functions** for LLM test factories
- **Property-based tests** using Hypothesis for invariants on retry backoff, token budgets, line clamping, and prerequisite validation
- **Full mock coverage** for LangChain interactions — no real API calls

## Key Decisions

1. **LangChain mocking via prompt patching**: LangChain's pipe operator (`|`) doesn't work with plain MagicMocks. Tests patch `BRIEFING_PROMPT` and `MISSION_PROMPT` at the module level so that `|` returns a mock chain with controllable `invoke()` behavior.

2. **Property-based testing for invariants**: Used Hypothesis to verify mathematical invariants (backoff timing, token budgets, line clamping) that would be tedious to test with explicit examples.

3. **Test isolation from implementation**: Tests written from interface contracts in Phase 6/7 plan.md files, not by reading implementation code. This ensured tests catch bugs rather than mirror them.

## Major Logic Flows In This Phase

1. **Exception hierarchy tests**: Verified inheritance chain, attribute storage, and polymorphic catching for all 5 LLM exception types.

2. **Retry mechanism tests**: Covered happy path, auth errors (immediate raise), transient errors (retry), backoff formula verification, and helper function behavior.

3. **Context builder tests**: Tested all 7 sections of briefing context, token budget enforcement, and completeness guarantee.

4. **Validation tests**: Comprehensive coverage of file reference validation (path traversal, line clamping) and prerequisite validation (cycle detection, depth enforcement, root guarantee).

5. **Property-based invariants**:
   - Backoff delay: `base * 2^(attempt-1) <= delay <= base * 2^(attempt-1) + 1.0`
   - Line clamping: `1 <= line_start <= line_end <= file_line_count`
   - Prerequisite graph: acyclic, bounded depth, at least one root, all reachable

## Bugs Found During Testing

The property-based tests found a bug in `validate_file_refs()`: when both `line_start=0` and `line_end=0`, the code fixed `line_start` to 1, then swapped with `line_end`, resulting in `line_start=0` again. Fixed by also fixing `line_end` when < 1.

## Connection To Previous Phases

- **Phase 6** — LLM provider abstraction, briefing generation, retry mechanism
- **Phase 7** — Mission generation, validation, and LangGraph pipeline
- This phase validates all public interfaces from those phases with mocked LLM calls
