build(graphrag): make graphrag an eval-only optional extra (keep prod image lean)

F28 concluded GraphRAG is not worth adopting, so the retriever never runs in
prod. Its deps (graphrag + lancedb/litellm/pandas) are imported lazily and only
by app/graph_rag.py, and nothing else in the app uses them — so move graphrag
out of the default dependencies into [project.optional-dependencies].graphrag.

- Prod / CI `uv sync` no longer installs graphrag/lancedb/litellm (verified
absent); the Spaces image stays lean. Default runtime behavior is unchanged
(the import path is never hit unless retriever=graphrag is selected).
- tests/test_graph_rag.py guards with pytest.importorskip("graphrag") so the
default suite collects clean (5 tests skip without the extra, run + pass with
`uv sync --extra graphrag`).
- evals_graphrag.md documents the `uv sync --extra graphrag` reproduce step.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

evals_graphrag.md

@@ -49,6 +49,20 @@ question). Both arms run on those 41 cases, both restricted to that source
 Measured course-content indexing rate: **~$88 / 1M corpus tokens** (vs ~$255 for
 dense API-reference docs). full_stack = 486k tokens -> **~$43** to index.
 
+## Setup (optional extra)
+
+`graphrag` (and its `lancedb`/`litellm`/`pandas` deps) is an **eval-only optional
+extra** — it is imported lazily and only by the opt-in `graphrag` retriever, never
+on the production path, so it is kept out of the default install to keep the prod
+image lean. Install it to build/run this experiment:
+
+```bash
+uv sync --extra graphrag
+```
+
+A default `uv sync` (prod, CI) omits it; `tests/test_graph_rag.py` skips itself
+when the extra is absent.
+
 ## Build the graph DB
 
 Local-only; never uploaded to the HF vector-db bundle (the `data/graphrag/`

pyproject.toml

@@ -10,7 +10,6 @@ dependencies = [
     "cohere",
     "fastapi",
     "google-genai",
-    "graphrag>=3.1.0",
     "hf-xet",
     "huggingface-hub",
     "langchain",
@@ -28,6 +27,13 @@ dependencies = [
     "uvicorn",
 ]
 
+[project.optional-dependencies]
+# Eval-only: the GraphRAG-vs-RAG experiment (F28). Pulls graphrag + lancedb/litellm/
+# pandas, all imported lazily and only by the opt-in `graphrag` retriever — never on
+# the production path. Kept out of the default install so the Spaces image stays lean.
+# Reproduce the experiment with: uv sync --extra graphrag  (see evals_graphrag.md).
+graphrag = ["graphrag>=3.1.0"]
+
 [dependency-groups]
 dev = [
     "httpx",

tests/test_graph_rag.py

@@ -4,6 +4,13 @@ import tempfile
 import unittest
 from pathlib import Path
 
+import pytest
+
+# The GraphRAG retriever is an optional eval-only extra (see pyproject
+# [project.optional-dependencies].graphrag). Skip this module unless it is
+# installed, so a default/prod `uv sync` (which omits it) still collects clean.
+pytest.importorskip("graphrag", reason="install the optional 'graphrag' extra to run")
+
 import pandas as pd
 
 from app.chat_types import ChatRequest