# Chroma Cache Path Styleguide

## The Rule

The ChromaDB persistent vector cache lives at:

```
<project_root>/tests/artifacts/.slop_cache/chroma_<collection_name>/
```

**NOT** at the per-run `tests/artifacts/live_gui_workspace_<timestamp>/` subdir.

Tests that interact with RAG **MUST** pre-clean the cache to avoid persistent state from prior tests in the batched run.

## Why This Rule Exists

The chroma cache path is auto-derived from `RAGEngine._init_vector_store()` (`src/rag_engine.py:108-125`):

```python
db_path = os.path.abspath(os.path.join(
    self.base_dir, ".slop_cache", f"chroma_{vs_config.collection_name}"
))
```

`self.base_dir` is computed as `Path(active_project_path).parent`. **The trailing-slash bug**: when the test config produces a project path ending in `/` (e.g., from `os.path.join` with a trailing `/`), `Path(p).parent` returns the directory ONE LEVEL HIGHER than expected. So the chroma cache lands at `tests/artifacts/.slop_cache/` (the parent of the per-run `live_gui_workspace_<timestamp>/` subdir) instead of inside the per-run subdir.

This was the dominant cause of `tier-3-live_gui` failures in the 2026-06-08 to 2026-06-10 window. A prior batched run with a different embedding provider (e.g., Gemini 3072-dim vs local 384-dim) leaves a corrupt collection on disk. The next test's `search()` raises `chromadb.errors.InvalidDimensionError: Collection expecting embedding with dimension of X, got Y`, the AI request never reaches `'done'` status, and the live_gui test polls timeout at 50×0.5s = 25s.

## The Pre-Cleanup Pattern

RAG tests should wipe the chroma cache BEFORE pushing RAG config. The pattern is in `tests/test_rag_phase4_final_verify.py`:

```python
from pathlib import Path
import shutil

def test_phase4_final_verify(live_gui):
    # Wipe any stale chroma from prior batched runs
    cache = Path("tests/artifacts/.slop_cache/chroma_test_final_verify")
    if cache.exists():
        shutil.rmtree(cache, ignore_errors=True)
    # ... rest of test
```

`ignore_errors=True` is required because:
- On Windows, the chroma client may still hold file handles; `rmtree` may fail with `WinError 32` (sharing violation).
- If a parallel xdist worker is mid-write, the rmtree can race; `ignore_errors` lets the next worker's write retry.

The `_validate_collection_dim()` mechanism in `RAGEngine` (`src/rag_engine.py:127-213`) also auto-recovers by wiping the dim-mismatched collection (see [docs/guide_rag.md](../docs/guide_rag.md#dimension-mismatch-protection)). But pre-cleaning is faster and avoids the stderr warning.

## Anti-Patterns

❌ **Assuming the cache is per-run:**
```python
def test_rag(live_gui, live_gui_workspace):
    # WRONG: live_gui_workspace is a per-run subdir, but the chroma
    # cache is at tests/artifacts/.slop_cache/, NOT under live_gui_workspace
    cache = live_gui_workspace / ".slop_cache" / "chroma_test"
    if cache.exists():
        shutil.rmtree(cache)  # Doesn't find the actual cache
```

❌ **Not pre-cleaning at all:**
```python
def test_rag(live_gui):
    # WRONG: no pre-cleanup. If a prior batched run with a different
    # embedding provider is on disk, this test will hit dim-mismatch
    client = ApiHookClient()
    client.push_event("set_value", {"field": "rag_enabled", "value": True})
    # ... eventually hangs polling for 'done' status
```

❌ **Asserting on the FIRST retrieved chunk:**
```python
assert "Manual Slop RAG is great" in entry.get("content")
# WRONG: in batched context, the chroma ordering may rank a .py
# file first instead of the .txt file. Either file's content
# proves RAG worked; the assertion must accept either.
```

## When in Doubt

If a RAG test is flaky in batched runs but passes in isolation, the chroma cache is the #1 suspect. The test's actual chroma path is `Path("tests/artifacts/.slop_cache") / f"chroma_{collection_name}"`. Wipe it before the test starts.

## Related

- [docs/guide_testing.md §Chroma Cache Path and Cross-Test Pollution](../docs/guide_testing.md) — broader context in the testing guide
- [docs/guide_rag.md §Dimension Mismatch Protection](../docs/guide_rag.md) — the auto-recovery mechanism
- [conductor/code_styleguides/workspace_paths.md](./workspace_paths.md) — sibling styleguide for test workspace paths
- [docs/reports/test_infrastructure_hardening_batch_green_20260610.md](../docs/reports/test_infrastructure_hardening_batch_green_20260610.md) — the 6-lesson summary this styleguide is sourced from