sqlglass

An MCP server that gives GitHub Copilot (VS Code agent mode) — or any MCP client — what it lacks when writing SQL for SQL Server / Azure SQL: the real schema, a safe way to try a query, the optimizer's opinion of it, and a managed library of the queries you keep.

Read-only, in layers

1. Guard – the statement is tokenised; anything but SELECT / WITH … SELECT (plus DECLARE / SET @var) is refused before a connection is opened. Because T-SQL needs no semicolons, any write/DDL/EXEC keyword anywhere outside a string, comment or [quoted name] rejects the batch — including SELECT … INTO, OPENROWSET, sp_*/xp_*, WAITFOR.
2. Transaction – every query runs with autocommit off and is always rolled back. The ODBC connection is opened read-only with ApplicationIntent=ReadOnly.
3. Limits – rows are capped (max_rows, default 200) and queries time out (timeout_seconds, default 30). explain_query uses SET SHOWPLAN_XML ON: the server compiles the query and executes nothing.
4. Yours to add – point the connection at a login that only has db_datareader (+ GRANT SHOWPLAN for plans). That is the layer the server itself enforces; use it for anything that matters.

Values go in as bound parameters (params={"@Start": "2026-01-01"}), not pasted into SQL text. Passwords are never stored: SQL-auth connections name an environment variable (password_env).

Writes and DDL: previewed and scripted, never executed

The server has no write mode. Instead:

preview_write turns one write statement into read-only SQL (plus a COUNT(*) of affected rows) and can run it:

UPDATE h SET h.Status = 'CLOSED' FROM dbo.PoHeader h JOIN dbo.Vendor v ON ... WHERE v.Country = 'US'
-- becomes
SELECT [h].[PoId], [h].[Status] AS [Status (current)], 'CLOSED' AS [Status (new)]
FROM dbo.PoHeader AS h JOIN dbo.Vendor AS v ON ... WHERE v.Country = 'US'

UPDATE → key columns + a current/new pair per SET column (changed_only=true hides no-op rows, NULL-safe via EXCEPT); DELETE → the rows that would go, plus which child tables reference them; INSERT → the rows that would be added under the target's column names, plus NOT NULL columns left unsupplied. MERGE is refused with advice to split it. The generated text must itself pass the read-only guard before it is returned. Triggers, cascades and constraint failures are not simulated.

build_create_table / build_procedure / build_view return DDL text (sql, undo_sql, notes) for a person to review and run in SSMS. Checked against the cached schema: the name must be free, foreign keys must reference a real primary/unique key with matching types (and get an index), constraints get conventional names, CREATE TABLE is wrapped in IF OBJECT_ID(...) IS NULL. build_procedure(query="open-po-value-by-vendor") wraps a library query and turns its header params into typed procedure parameters; every @variable must be declared. save_query(kind="script") keeps a script in the library — versioned with the queries, never linted as a query, never runnable through the server.

Install

pip install sqlglass          # or run it without installing: uvx sqlglass

Needs Python 3.11+ and a SQL Server ODBC driver. The legacy SQL Server driver that ships with Windows works for Windows/SQL authentication; Azure SQL / Entra ID sign-in needs "ODBC Driver 18 for SQL Server". On Linux/macOS, install unixODBC plus Microsoft's ODBC driver.

Copy sqlglass.example.toml to sqlglass.toml in your workspace (or in %LOCALAPPDATA%\sqlglass\) and define your connections. Passwords never go in the file.

VS Code / Copilot. Add the server to the user-level %APPDATA%\Code\User\mcp.json so it works from every window (a workspace .vscode/mcp.json only loads once you trust that workspace's MCP servers):

"sqlglass": {
  "type": "stdio",
  "command": "uvx",
  "args": ["sqlglass"],
  "env": { "SQLGLASS_WORKSPACE": "C:\\path\\to\\your\\project", "REPORTING_SQL_PASSWORD": "${input:reporting-sql-pwd}" }
}

with a matching "inputs": [{ "id": "reporting-sql-pwd", "type": "promptString", "password": true, "description": "..." }]. VS Code asks for the password once and keeps it in its secret storage — never put the value in the file, and do not rely on a user environment variable: a VS Code process started before the variable existed will never see it. Verified 2026-09-21 with Copilot agent mode (GPT-5.6): list_tables → describe_table ×3 → build_select → lint_sql → explain_query → run_query, 19 steps, results identical to a direct run.

Config lookup order: $SQLGLASS_CONFIG → $SQLGLASS_WORKSPACE\sqlglass.toml → .\sqlglass.toml → %LOCALAPPDATA%\sqlglass\sqlglass.toml. Schema cache and snapshots live under %LOCALAPPDATA%\sqlglass\ (override with SQLGLASS_HOME).

The query library

A folder of plain .sql files; the id is the path without .sql. Each opens with a header:

-- name: Open POs by vendor
-- description: Open purchase-order value per vendor since a start date.
-- connection: erp
-- tags: purchasing, monthly
-- param: @StartDate date = '2026-01-01' | first order date to include

SELECT v.Name, SUM(l.Amount) AS OpenValue
FROM dbo.PoHeader AS h
JOIN dbo.Vendor AS v ON v.VendorId = h.VendorId
...
WHERE h.OrderDate >= @StartDate

The body does not declare its parameters — the server does that when running it (in SSMS, add the DECLAREs yourself). Git is the history; on top of that every write through the server takes a snapshot first (restore_snapshot undoes it), returns a diff, and supports dry_run.

• find_usage("dbo.Vendor", "Name") – which saved queries break if this changes.
• rename_in_library – follow a table/column rename through every query, token-aware (strings/comments untouched).
• lint_library after refresh_schema – finds queries that reference tables/columns that no longer exist.

Lint rules

Correctness: join-without-on comma-join not-in-subquery left-join-filtered-in-where top-without-order-by between-date-end undeclared-parameter unused-parameter parameter-declared-twice unused-cte · Performance: non-sargable-predicate leading-wildcard-like select-star nolock distinct-over-join union-distinct · Style: order-by-ordinal missing-schema-prefix unqualified-column · With a cached schema: unknown-table unknown-column (with did-you-mean).

Layout

src/sqlglass/ — tsql/ (lexer, read-only guard, sqlglot analysis) → pure lint / refactor / builder / preview / ddl / plan (SHOWPLAN XML summariser) / schema (model + cache + FK join paths) → engines/ (mssql over pyodbc, sqlite for tests and local files) → library + snapshots → server.py (the tools).

Development

git clone https://github.com/S-CurveLabs/sqlglass; cd sqlglass
python -m venv .venv
.venv\Scripts\pip install -e .[dev]
.venv\Scripts\pytest

Everything except the live SQL Server path runs against a SQLite fixture. To exercise engines/mssql.py, define a connection and run set SQLGLASS_TEST_CONNECTION=<name> then pytest -m mssql.

Not built yet

A write mode (by design), reading existing procedure / view definitions from the database, MERGE previews, actual (post-execution) plans and STATISTICS IO, Postgres/MySQL engines, SQL embedded in Power Query (Value.NativeQuery) — the bridge to letin.