sql-specialist-mcp

A small, fine-tuned open-weight model that answers natural-language questions about a SQLite database, served as an MCP tool any MCP client (Claude Desktop, Claude Code, custom agents) can call directly — backed by an execution-accuracy eval harness that benchmarks the specialist against prompting a frontier model on accuracy, latency, and cost.

The point of this project isn't "build a text-to-SQL demo" — it's to show the parts of LLM engineering that sit below prompting: taking a small model, adapting it to one task via LoRA, serving it efficiently, and proving with a real, execution-based eval that a cheap specialist is competitive with (or better than) prompting a frontier model for this narrow job.

Try the interactive demo — click through all 28 real eval questions and see the specialist's actual generated SQL, latency, and result rows next to the frontier baseline. No install required.

Why this exists

Most "AI portfolio" text-to-SQL projects are LangChain quickstarts. Two things here are meant to be different:

1. The eval is rigorous, not vibes. Every gold query is executed against the database at dataset-build time (139/139 validated), and scoring compares result sets, not query text — a semantically correct query with different column ordering still scores correct. A predictor that just echoes the gold SQL scores 100%; a predictor that always returns a trivially-wrong query scores 0%. Both are checked in as sanity tests (tests/test_harness_oracle.py) so the harness's own correctness isn't assumed.
2. It ships as something usable, not just a demo repo. The fine-tuned model is exposed as a real MCP tool (nl_to_sql) — point Claude Desktop or Claude Code at mcp_server/server.py and it can actually query the database as part of a conversation.

Results

The full pipeline has been run end-to-end on real hardware, both sides: real LoRA fine-tune, real merge, real GGUF quantization, real Ollama serving, real eval — and a real frontier baseline against the live Claude API. Base model: Qwen/Qwen2.5-Coder-0.5B-Instruct (chosen for a fast iteration loop on a laptop; see Fine-tuning below for the 1.5B path).

Predictor	Accuracy	n	p50 latency	p95 latency	Cost / 1k calls
frontier: Claude Haiku 4.5 (prompted)	53.6%	28	1055ms	1884ms	$1.06
sql-specialist (fine-tuned, quantized, local)	92.9%	28	207ms	371ms	$0.00

Read this with the caveat, not just the headline. I manually audited every one of Claude Haiku's 13 measured "failures" against this eval set: zero were SQL logic errors. All 13 were column-selection or row-order convention mismatches — e.g. returning (name, email) when the gold answer was just (name), or correct rows in a different order than an ORDER BY the original question never actually specified. The strict execution-accuracy metric (eval/execution.py compares result rows column-for-column) scores those identically to a genuinely wrong query, which the fine-tuned specialist never produces because it memorized this dataset's exact conventions from 111 training examples — something a frontier model prompted zero-shot has no way to know. Full failure-by-failure taxonomy in COMPARISON.md.

So: the accuracy gap is real but partly an artifact of what the eval rewards, not purely a reasoning gap. The latency and cost gap is not an artifact — 207ms/local/free vs. 1055ms/$1.06-per-1k-calls is the actual, unhedged result of running a quantized 0.5B model locally instead of calling an API, and it's the comparison this project's premise actually rests on.

The specialist's own 2 failures (out of 28) were genuine logic errors, not formatting mismatches — hallucinating a plausible orders.total column that doesn't exist in this schema, and dropping a table qualifier in one multi-table SELECT. Training converged cleanly over 3 epochs (eval loss 0.060 → 0.048 → 0.008), and the quantized model (988MB f16 → 373MB q4_k_m) serves through Ollama in ~200ms.

What's real here

Being upfront about this matters more than it looks — it's the difference between a project a recruiter can trust and one that reads like marketing.

• The synthetic database and dataset are provably correct. shopsphere.db is seeded deterministically (seed=42); every one of the 139 gold (question, SQL) pairs in data/*.jsonl is generated from parameterized templates and executed against the real database at build time — a template that produces invalid SQL fails the build, it doesn't silently ship a bad label.

• The eval harness's correctness is itself tested, not assumed — tests/test_harness_oracle.py asserts an oracle predictor (returns gold SQL verbatim) scores exactly 100% and a deliberately-wrong predictor scores ~0%, before any real predictor's number is trusted.

• Execution accuracy, not string match. eval/execution.py compares result sets (order-insensitive unless the gold query has ORDER BY), so a query that's differently written but semantically equivalent still scores correct.

• The fine-tune is real, on this machine, verified converging. LoRA (8.8M trainable params, 1.75% of the model) over 3 epochs, eval loss dropping monotonically each epoch. See Engineering notes below for two real bugs hit and fixed along the way.

• SQL execution is genuinely sandboxed, not just prompted to behave: read queries are validated against a regex allowlist and executed against a true read-only SQLite connection (mode=ro at the OS level) — a bug in the regex guard still can't result in a write. This matters beyond the eval harness, because the same guard runs in the MCP server, where the SQL comes from a model responding to an agent's question, not a curated eval set.

• The MCP server is a real, callable tool serving the real fine-tuned model, verified end-to-end: nl_to_sql("Which employees have no manager assigned?") → generates SQL via the quantized model over Ollama → executes it read-only → returns real rows → logs latency/cost to observability.

• Observability is self-built and dependency-free — observability/logger.py logs every call (latency, tokens, estimated cost, success/failure) to a local SQLite file, no external account needed, same pattern as pr-review-agent.

• The frontier baseline is real too — eval/baseline_frontier.py ran against the live Claude API (Claude Haiku 4.5), not just imported cleanly. Its "failures" turned out to reveal a real eval-methodology finding — see Results above and COMPARISON.md for the full manual failure audit.

Engineering notes: two real bugs found running this for real

Actually executing the fine-tune (rather than leaving it as "should work in theory") surfaced two genuine PyTorch memory bugs, both fixed in the current code:

1. MPS caching-allocator runaway. Training on Apple Silicon's MPS backend via transformers.Trainer caused the process to balloon to 23GB RSS and hang, on dynamic per-batch padding — each distinct (batch, seq_len) shape gets its own memory pool in PyTorch's MPS allocator, which doesn't return freed memory to the OS. Fix: --device cpu override in finetune.py, and more fundamentally, fixed-length padding (below) so this class of bug can't recur on any backend.
2. Trainer/DataLoader overhead, not the model. A direct forward+backward pass timed at 1.6s/example; the same computation through transformers.Trainer left the process idle for minutes between logged steps with no corresponding compute. Root-caused by isolating the actual model+LoRA forward/backward with manual timing before assuming the bug was in model code. Fix: replaced Trainer with a ~40-line manual training loop (training/finetune.py) — same LoRA setup, direct control over the batch loop, no unexplained overhead. Also switched batch collation from dynamic-per-batch to fixed-length padding (every batch shaped identically), which independently fixed the allocator-fragmentation pattern from bug #1.

Neither fix is a workaround bolted on top — both are visible in training/finetune.py as the only implementation, not an alternate path.

Architecture

data/build_dataset.py ──▶ data/{train,eval}.jsonl   (139 examples, template-generated,
                                                       every gold SQL executed at build time)
                              │
        ┌─────────────────────┼─────────────────────┐
        ▼                     ▼                      ▼
training/finetune.py   eval/baseline_frontier.py   tests/test_harness_oracle.py
  (LoRA on a small        (prompt Claude Haiku/       (sanity-checks the harness
   open model)             Sonnet as the baseline)     itself before trusting scores)
        │                     │
        ▼                     │
training/merge_and_quantize.py
        │                     │
        ▼                     ▼
serving/ollama_predictor.py ──┴──▶ eval/harness.py ──▶ eval/report.py ──▶ COMPARISON.md
        │                          (execution-accuracy scoring,
        │                           same logic for every predictor)
        ▼
mcp_server/server.py  (nl_to_sql tool -- installable in Claude Desktop/Code)
        │
        ▼
observability/logger.py  (latency, tokens, cost -- local SQLite, no external account)

Project structure

schema/           synthetic "ShopSphere" e-commerce DB (7 tables) + seeded generator
data/              templated gold (question, SQL) dataset -- every query build-time validated
eval/              execution-accuracy harness, frontier baseline, comparison report
training/          LoRA fine-tuning pipeline + LoRA-merge/quantize script
serving/           Ollama-backed and in-process HF predictors, Ollama Modelfile template
mcp_server/        the installable MCP tool (nl_to_sql)
observability/     self-built call logging (latency/tokens/cost), no external account
tests/             harness sanity checks (oracle predictor must score 100%)

Setup

python3.11 -m venv .venv && source .venv/bin/activate
pip install -r requirements.txt          # base: anthropic, mcp, requests
python schema/generate_data.py           # build the seeded database
python data/build_dataset.py             # build + validate the gold dataset
python tests/test_harness_oracle.py      # confirm the eval harness itself is sound

requirements-train.txt adds torch/transformers/peft/trl for the fine-tuning path — heavier, kept separate so the eval/serving/MCP path installs fast.

Running the full pipeline

1. Frontier baseline (needs ANTHROPIC_API_KEY):

export ANTHROPIC_API_KEY="..."
python -m eval.baseline_frontier --model claude-haiku-4-5
# writes eval/results_claude-haiku-4-5.json

2. Fine-tune the specialist (this is what was actually run to produce the results above — takes ~15 min of active compute on a laptop CPU, though wall clock varies a lot with system load; a GPU is much faster, see below):

pip install -r requirements-train.txt
python -m training.finetune --base-model Qwen/Qwen2.5-Coder-0.5B-Instruct --device cpu
python -m training.merge_and_quantize --base-model Qwen/Qwen2.5-Coder-0.5B-Instruct
# then follow the printed llama.cpp + ollama create instructions

3. Score the specialist the same way as the baseline:

python -c "
from eval.harness import run_eval, report_to_dict
from serving.ollama_predictor import OllamaPredictor
import json
report = run_eval(OllamaPredictor())
json.dump(report_to_dict(report), open('eval/results_specialist.json', 'w'), indent=2)
"

4. Generate the comparison report:

python -m eval.report eval/results_claude-haiku-4-5.json eval/results_specialist.json

Fine-tuning: scaling up

The results above use Qwen2.5-Coder-0.5B-Instruct on CPU, for a fast local iteration loop. training/finetune.py --base-model accepts any HF causal-LM repo (or a local directory) — Qwen2.5-Coder-1.5B-Instruct is a straightforward swap for better quality, and a single cloud GPU (a T4 is enough for this dataset size) trains either size in a couple of minutes instead of ~15:

pip install -r requirements-train.txt
python -m training.finetune \
  --base-model Qwen/Qwen2.5-Coder-1.5B-Instruct \
  --epochs 3

--device {cuda,mps,cpu} overrides auto-detection. MPS is auto-detected on Apple Silicon but not recommended for this task yet — see Engineering notes above.

MCP server

# Backend defaults to a locally-served model via Ollama:
python -m mcp_server.server

# Or run against a prompted frontier model instead (no fine-tune needed --
# useful for trying the tool before training anything):
SQL_SPECIALIST_BACKEND=frontier SQL_SPECIALIST_MODEL=claude-haiku-4-5 \
  ANTHROPIC_API_KEY=... python -m mcp_server.server

Add to Claude Desktop's MCP config (claude_desktop_config.json):

{
  "mcpServers": {
    "sql-specialist": {
      "command": "/absolute/path/to/sql-specialist-mcp/.venv/bin/python",
      "args": ["-m", "mcp_server.server"],
      "cwd": "/absolute/path/to/sql-specialist-mcp"
    }
  }
}

Then ask Claude something like "Using the sql-specialist tool, which customers have never placed an order?" — it calls nl_to_sql, gets back real rows from the database, and answers grounded in the actual data.

Security notes

• SQL execution is read-only at two independent layers: a regex guard rejecting anything but SELECT/WITH, and a true OS-level read-only SQLite connection (file:...?mode=ro) as the backstop.
• The MCP server never executes anything the guard rejects, regardless of what the model or the calling agent asked for.
• No secrets are stored in this repo. ANTHROPIC_API_KEY is read from the environment only.

What I'd build next

• Normalize the eval for column supersets — score a prediction correct if the gold-requested columns' values are present, rather than requiring an exact column-for-column match. This is the fix implied by the failure taxonomy in COMPARISON.md; it would very likely close most of the measured 53.6%→92.9% gap and produce a comparison that isolates actual reasoning ability from convention-matching.
• Run eval/baseline_frontier.py against Claude Sonnet too, for a stronger-model comparison point (Haiku is the cheap/fast tier; Sonnet is the "how much does model strength alone close the gap" question).
• Fine-tune Qwen2.5-Coder-1.5B-Instruct on a GPU and compare accuracy against the 0.5B result (92.9%) to quantify the size/quality tradeoff directly.
• DPO targeting the specialist's two known failure modes (hallucinated columns, dropped table qualifiers in multi-joins) now that real failure data exists.
• vLLM serving path for throughput comparison against the Ollama/GGUF path.