Semantic Context MCP

An MCP server that gives an LLM the meaning of a data warehouse, not just its schema.

A model with schema access sees a table called tbl_rev_fnl_v2 and a column called amt. It has no way to know whether that's gross or net, cents or dollars, or a table nobody has touched in 40 days — so it guesses, and the guess runs and returns a plausible, wrong number. This server closes that gap by exposing four tools backed by real derived metadata: ownership, freshness, column units, deprecation status, and column-level lineage.

Stack: Python · Model Context Protocol · dbt + DuckDB · SQLite (FTS5) · sentence-transformers · sqlglot · Anthropic API (tool use / eval harness) · pytest

Tool	Answers
search_tables	"Which table holds daily net revenue?"
describe_table	"What does one row mean, and what unit is this column in?"
trace_lineage	"Where did this number come from, and what breaks if I change it?"
check_health	"Is this table safe to rely on right now?"

time — see "Setup" below. Without it, search_tables still works, just as keyword-only BM25.

Where the metadata actually comes from

The credibility of a project like this rests entirely on whether the "meaning" it reports is real or hand-typed for the demo. It's mostly real:

Signal	Source	Human-authored?
Table purpose, grain, materialization	manifest.json	No
Test pass/fail	dbt build results	No
Model-level lineage	manifest.json's depends_on	No
Column-level lineage	sqlglot.lineage() over compiled SQL	No
Table search (keyword)	SQLite FTS5 / BM25 over name + description + columns	No
Table search (semantic)	Cosine similarity over MiniLM embeddings	No
Row counts, 7-day row deltas	Live DuckDB queries at ingestion time	No
Freshness (hours since latest data)	Recomputed live on every call	No
Null rates per column	Live DuckDB queries at ingestion time	No
Ownership	dbt meta.owner, else git blame on the model file	Partly
Column descriptions and units	dbt schema.yml	Yes — the same input any dbt project already has
Deprecation status + successor	dbt meta.status / meta.superseded_by	Yes

Only two rows are human-authored, and both are ordinary dbt metadata any real project already maintains — this isn't reading from a hand-curated demo file.

Column-level lineage resolves 35 of 36 columns (97%) in the fixture warehouse. The one exception sums a column with no table-qualifying prefix across a JOIN — genuinely ambiguous SQL that sqlglot correctly declines to guess on, falling back to model-level lineage instead. trace_lineage always names exactly where a fallback happened, in unresolved_notes — never silently.

The demo warehouse is dbt-labs/jaffle_shop_duckdb with four added "decoy" revenue models, since the stock fixture is too clean to show any ambiguity. The decoys are synthetic and labelled as such in fixture/ — the pipeline that derives metadata from them is not.

Setup

Requires Python 3.12+ and uv. No GPU needed.

uv sync

This installs only what the server itself needs and gives you a fully working server: describe_table, trace_lineage, and check_health are fully live, and search_tables works in keyword (BM25) mode. Heavier, optional pieces are separate extras so the base install stays fast:

uv sync --extra search   # sentence-transformers + numpy -- semantic ranking for search_tables
uv sync --extra ingest   # sqlglot + duckdb -- only needed to (re)build catalog.db
uv sync --extra dbt      # dbt-core + dbt-duckdb -- only needed to build the fixture warehouse
uv sync --extra eval     # anthropic + pyyaml -- only needed to run the eval harness

uv run <anything> re-syncs the environment to match whatever --extra flags that invocation passes — running a bare uv run python ... after syncing extras in can silently drop them again. If a tool that was working starts raising ModuleNotFoundError, re-run the uv sync --extra ... line below rather than debugging the code.

Build the fixture warehouse

Needed once before the server has data to serve — without it, every tool call fails with a clear error telling you to run this.

uv sync --extra dbt --extra ingest --extra search
uv run python fixture/build_warehouse.py       # seed -> dbt build -> dbt docs generate
uv run python -m semantic_context_mcp.ingest    # manifest + catalog + run_results -> catalog.db

Re-running this sequence is always safe — both scripts fully replace their outputs rather than merging into them, and build_warehouse.py regenerates seed dates so the "stale" decoy table always looks stale relative to today. dbt's very first invocation in a fresh venv can sit silent for 20-30 seconds while it compiles bytecode for its own dependency tree before printing anything — build_warehouse.py prints a heads-up before that step so it doesn't look hung; subsequent runs are fast.

Run it

uv run semantic-context-mcp

The server speaks MCP over stdio and sits silently waiting for a client — that's correct behavior, not a hang. To exercise it directly without a client:

uv run python scripts/smoke_stdio.py

That script spawns the real server over the real transport and drives every tool through both happy-path and error-path calls, printing each result.

Try it as a data analyst would

uv run python -c "
from semantic_context_mcp.core import search, describe, health

for m in search.run('daily net revenue'):
    print(m.name, '-', m.purpose)

print()
print(describe.run('fct_revenue_daily').columns)
print()
print(health.run('tbl_rev_fnl_v2').summary)
"

Connect to Claude Desktop

Add to claude_desktop_config.json (%APPDATA%\Claude\claude_desktop_config.json on Windows, ~/Library/Application Support/Claude/claude_desktop_config.json on macOS):

{
  "mcpServers": {
    "semantic-context": {
      "command": "uv",
      "args": ["--directory", "/absolute/path/to/semantic-context-mcp", "run", "semantic-context-mcp"]
    }
  }
}

Restart Claude Desktop, then ask "What's our daily revenue table?"

Testing

uv run pytest

The suite covers synthetic unit tests (fast, no fixture needed, always run) and real-data integration tests that build the actual dbt/DuckDB warehouse and ingest it into catalog.db — the latter are what caught every real bug during development (a dbt artifact-ordering issue, a DuckDB NULL-vs-zero aggregation bug, a naive/aware datetime crash, a missing test hiding a staleness signal, and a false "lineage unresolved" report on a fully-resolved column). Two additional scripts exercise the protocol layer directly rather than through pytest:

uv run python scripts/smoke_stdio.py                       # real server, real MCP stdio transport
uv run python -m semantic_context_mcp.eval.mcp_path_smoke  # proves the eval harness matches the real server

Evaluation

eval/ measures whether having this server's tools available actually changes whether Claude picks the right table and states the right caveats, compared to schema access alone. Three arms, the same 25 questions, the same model and system prompt — only the tool set differs:

Arm	Tools
schema_only	table + column names and SQL types only — no metadata (the realistic baseline)
schema_and_server	schema-only tools, plus the four semantic tools
server_only	the four semantic tools alone

uv sync --extra eval
export ANTHROPIC_API_KEY=...                                       # or `ant auth login`
uv run python -m semantic_context_mcp.eval                         # all 3 arms, 25 questions x 3 runs
uv run python -m semantic_context_mcp.eval --questions 5 --runs 1  # a cheap smoke run
uv run python -m semantic_context_mcp.eval --dry-run               # exercises the harness, no API calls

Each question/run/arm writes a transcript to eval/runs/<timestamp>/, so any surprising result is checkable rather than trusted. Grading (eval/grading.py) is deliberately simple keyword/table-name matching over the model's final answer — not an LLM judge — and is fully unit-tested without any network access.

Repository layout

fixture/
  jaffle_shop_duckdb/    Cloned dbt project + models/decoys/*.sql (4 synthetic ambiguous tables)
  pristine_seeds/        Untouched original seed CSVs -- prepare_seeds.py's source of truth
  prepare_seeds.py       Shifts seed dates to end "yesterday", idempotently
  build_warehouse.py     seed -> dbt build -> dbt docs generate
src/semantic_context_mcp/
  models.py        Pydantic contract for all four tool returns
  errors.py        Errors written to be read by the model
  server.py        MCP adapter -- no logic
  core/            The actual logic; also what eval/tools.py calls
  ingest/          dbt artifacts + git + SQLite -> catalog.db
  eval/            Three-arm measurement harness (see "Evaluation" above)
scripts/
  smoke_stdio.py   Drives the real server over the real MCP transport
tests/             Contract, consistency, ingestion, eval, and stdout-purity guards