feat(serve): add --tools flag and CRG_TOOLS env var for MCP tool filtering

[sdeonvacation]

Summary

• Add --tools CLI flag to serve subcommand and CRG_TOOLS env var fallback for filtering which MCP tools are exposed
• When set, unlisted tools are removed via FastMCP.remove_tool() before the server starts
• When neither is set, all tools remain available (no breaking change)

Motivation

CRG exposes 28 MCP tools by default, adding ~8k description tokens per LLM turn. In token-constrained environments (e.g. agentic coding tools calling CRG via MCP), this overhead can exceed the savings from graph queries — especially when only 5-10 tools are actually needed.

Filtering to a working set reduces per-turn overhead by 70-85%, making CRG net-positive on tokens from the first few queries.

Usage

# Via CLI flag (highest precedence)
code-review-graph serve --tools query_graph_tool,semantic_search_nodes_tool,detect_changes_tool

# Via environment variable (fallback)
CRG_TOOLS=query_graph_tool,semantic_search_nodes_tool code-review-graph serve

MCP client config example:

{
  "mcpServers": {
    "code-review-graph": {
      "command": "code-review-graph",
      "args": ["serve", "--tools", "query_graph_tool,semantic_search_nodes_tool,detect_changes_tool"]
    }
  }
}

Changes

File	Change
code_review_graph/main.py	Add _apply_tool_filter() + update main() signature
code_review_graph/cli.py	Add --tools arg to serve subparser, pass to main()
tests/test_main.py	7 async tests: noop, arg filter, env var, precedence, empty string, whitespace
README.md	Tool Filtering section in Configuration

Testing

All 794 existing tests pass. 7 new tests added:

• test_no_filter_keeps_all_tools — noop when unset
• test_filter_via_argument — keeps only listed tools
• test_filter_via_env_var — CRG_TOOLS env var works
• test_argument_takes_precedence_over_env — CLI wins over env
• test_empty_string_is_noop — empty string doesn't remove all tools
• test_whitespace_handling — spaces around names are stripped

uv run pytest tests/test_main.py -v   # 13 passed
uv run pytest tests/ -q               # 794 passed, 1 skipped, 2 xpassed
uv run ruff check                     # All checks passed

[tirth8205]

This PR now has merge conflicts with main after recent merges. Could you rebase on the latest main? The feature looks good.

[tirth8205]

Closed: changes merged directly into main as commit a3a043b (feat(serve): add --tools flag and CRG_TOOLS env var for MCP tool filtering). Conflicts with --http/--host/--port HTTP transport flags were resolved by keeping both features.