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mssql-extension: Instructions file for Claude Code

CLAUDE.md

mssql-extension CLAUDE.md is an instructions file for Claude Code from hugr-lab/mssql-extension. It costs 11,888 tokens per session, scanned A, original, MIT.

Repository guidance for developing a DuckDB extension that connects to Microsoft SQL Server using C++ and the TDS network protocol. It covers the code structure, supported systems, security, versioning, and commands.

In plain words
What is it for?
Use it when adding features, fixing bugs, building, testing, or reviewing the DuckDB-to-SQL Server connection code.
Why use it?
It gives a coding agent the project rules it needs before changing this extension, reducing mistakes in its architecture, compatibility, and build setup.

Instructions file for Claude Code

Written for Claude Code: the file is CLAUDE.md.

This is hugr-lab/mssql-extension's own configuration. It tells Claude Code how to work on mssql-extension itself, so it is not a mod to install elsewhere. Copy it as a starting point and replace the rules that are about this project. Everything mssql-extension configures →

not rated 127repo changed today A scan 11,888 tokens original MIT
View source ↗ hugr-lab/mssql-extension hugr-lab.github.io
Reuse

Borrowing it

Nothing to install: this file belongs to hugr-lab/mssql-extension. Take a copy, put it at the same path in your own repository, and replace the rules that are about this project with yours.

Copy the file
curl -O https://raw.githubusercontent.com/hugr-lab/mssql-extension/main/CLAUDE.md
Clone the repo
git clone --depth 1 https://github.com/hugr-lab/mssql-extension

Made for: Claude Code.

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Per session 11,888 This file is loaded in full into every session.
When invoked 11,888 The same file — it is already loaded in full.
Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe.
Origin original No closer match found in the catalogue.
Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

ModelPer sessionOnce invoked
Fable 5.1 $0.11888 $0.11888
Opus 5 $0.05944 $0.05944
Sonnet 5 $0.02378 $0.02378
Haiku 4.5 $0.01189 $0.01189

Measured today against content hash f97f18e3470a, method: parsed. Prices are Anthropic first-party input rates as of 2026-09-08, from the pricing page.

Security

Grade A, and why

mssql-extension CLAUDE.md scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured today.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

CLAUDE.md · 399 lines

How it starts

The opening of the file, as written. The whole thing — 399 lines — stays where its author put it; the contents beside it link to each section on GitHub.

mssql-extension Development Guidelines

DuckDB extension for SQL Server via a custom TDS protocol implementation (no FreeTDS/ODBC).

Technology

  • Language: C++17 (DuckDB extension standard)
  • DuckDB: v2.0-cyanoptera, DuckDB's v2.0 pre-release stabilisation branch (2.0 API track, spec 069). Tracked instead of main since the branch was cut on 2026-09-03: it is where the 2.0 line is being stabilised, and main has already diverged from it. The 1.5.x world lives on branch duckdb-v1.5.5 for 0.2.x maintenance releases; main compiles against the pinned duckdb submodule SHA only (no dual-API shim since spec 069 retired mssql_compat.hpp)
  • TLS: OpenSSL via vcpkg (statically linked, symbol visibility controlled)
  • Platforms: Linux (GCC), macOS (Clang), Windows (MSVC, MinGW/Rtools 4.2)

Project Structure

src/
  azure/                # Azure AD authentication infrastructure
  catalog/              # DuckDB catalog integration + transaction manager
  codec/                # Per-type-family codec layer (spec 045): boolean/integer/
                        # float/decimal/money/string/binary/datetime/uuid. Each
                        # <family>_codec.cpp owns EncodeToBcp / DecodeFromTds /
                        # FormatSqlLiteral / FormatDdlTypeName for its types.
                        # literal_format.cpp + type_family.cpp are the dispatchers.
  connection/           # Connection pooling, provider, settings
  dml/                  # DML operations
    insert/             # INSERT (batched VALUES, OUTPUT INSERTED for RETURNING)
    update/             # UPDATE (rowid-based, VALUES JOIN pattern)
    delete/             # DELETE (rowid-based, VALUES JOIN pattern)
  include/              # Headers (mirrors src/ layout)
  query/                # Query execution and result streaming
  table_scan/           # Table scan with filter/projection pushdown
  tds/                  # TDS protocol implementation
    auth/               # Authentication strategies (SQL auth, FEDAUTH)
    encoding/           # Type encoding (datetime, decimal, GUID, UTF-16)
    tls/                # TLS via OpenSSL with custom BIO callbacks
test/
  sql/                  # SQLLogicTest files (require SQL Server for most)
    attach/             # ATTACH/DETACH tests
    azure/              # Azure AD authentication tests (no SQL Server required)
    catalog/            # Catalog, DDL, filter pushdown, statistics
    copy/               # COPY TO MSSQL (BulkLoadBCP) tests
    ctas/               # CREATE TABLE AS SELECT tests
    dml/                # UPDATE and DELETE tests
    insert/             # INSERT tests
    integration/        # Core integration (pool, TLS, large data)
    query/              # Query-level tests
    rowid/              # Rowid pseudo-column tests
    tds_connection/     # TDS protocol tests
    transaction/        # Transaction management tests
  cpp/                  # C++ unit tests (no SQL Server required)
docs/                   # Architecture documentation (see docs/architecture.md)
docker/                 # SQL Server container and Linux CI build

Read the full file on GitHub · 399 lines

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

  1. today Changed · +66 tokens per session f97f18e3470a
  2. 9d ago First seen · 399 lines · 11,822 tokens per session scan A 7ea8834eaeb6

Subscribe to this mod's changes

mssql-extension CLAUDE.md is an instructions file published in the GitHub repository hugr-lab/mssql-extension (127 stars, last pushed today), licensed MIT. It adds 11,888 tokens to every session, about $0.0594 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.

MSSQLduckdbduckdb-extensionhugrsqlsqlserver
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