dcert (Python)

A Python MCP wrapper for the dcert Rust server.

The package uses FastMCP to run a transparent proxy around the dcert-mcp Rust binary and exposes all TLS certificate tools over the Model Context Protocol. Tools added to the Rust binary are discovered at runtime, so the Python layer never needs to change.

Requirements

• Python 3.12 or later
• The dcert-mcp Rust binary. Every published wheel bundles it, together with dcert.

Installation

pip install dcert

Wheels exist for Linux x86_64 (glibc 2.34 or later: RHEL 9, Ubuntu 22.04, Debian 12 and later), macOS Intel, macOS Apple Silicon and Windows x86_64. Only these platform wheels are published, to PyPI and to each GitHub release, so an install from PyPI, a private mirror or a downloaded wheel never fetches anything from GitHub. On any other platform pip reports "no matching distribution"; build the binaries from source and point DCERT_MCP_BINARY and DCERT_PATH at them.

Quick start

As a server

from dcert import create_server

create_server().run()  # stdio transport

As a client

import asyncio
from dcert import create_client

async def main() -> None:
    async with create_client() as client:
        tools = await client.list_tools()
        result = await client.call_tool("analyze_certificate", {"target": "example.com"})
        print(len(tools), result)

asyncio.run(main())

Typed async tool functions

The functional API opens a session (a subprocess plus its resilience stack) and passes it to plain async functions:

import asyncio
from dcert import analyze_certificate, check_expiry, create_session, export_pem

async def main() -> None:
    async with create_session(timeout=60.0) as session:
        cert = await analyze_certificate(target="example.com", session=session)
        expiry = await check_expiry(target="example.com", days=90, session=session)
        pem = await export_pem(target="example.com", output_path="chain.pem", session=session)
        raw = await session.call("tls_connection_info", {"target": "example.com"})

asyncio.run(main())

Leaving out session uses a shared default session that is created on first use and reused afterwards; close_default_session() shuts it down. The shared session belongs to the event loop that created it: a later asyncio.run() gets a fresh one, and calling close_default_session() before a loop ends releases the subprocess cleanly.

The network facing functions accept the shared mTLS and connection options (client_cert, client_key, pkcs12, cert_password, ca_cert, connect_to, resolve, proxy, noproxy) as keyword arguments.

Error handling

from dcert import (
    DcertError,            # base class
    DcertTimeoutError,     # the call exceeded its timeout
    DcertConnectionError,  # the subprocess died or the circuit breaker is open
    DcertToolError,        # the tool returned an error result (has .tool)
)

Only transport failures are retried, with exponential backoff and full jitter, and only for tools whose MCP annotations mark them read-only or idempotent (the typed client reads the backend's tool list once per session to decide). TypeError and ValueError surface immediately and cancellation is never swallowed. A call that times out closes the subprocess (it is reconnected on the next call) and counts against the circuit breaker; concurrent calls that fail together share one reconnect and count as one failure.

Command line

dcert-python                                   # stdio (for MCP clients such as Claude Code)
dcert-python --transport http --port 8080      # HTTP; needs DCERT_MCP_AUTH_TOKEN or OIDC
dcert-python --setup                           # download the binary and exit
dcert-python --binary /usr/local/bin/dcert-mcp # explicit binary
dcert-python --rate-limit 20 --bulkhead-max 4  # resilience flags
dcert-python --help                            # every option

HTTP and SSE mode refuse to start without authentication: a static bearer token of at least 32 characters (--auth-token or, better, DCERT_MCP_AUTH_TOKEN, compared in constant time) or an OIDC configuration (--oidc-issuer, --oidc-audience, --oidc-jwks-uri or DCERT_MCP_OIDC_ISSUER, DCERT_MCP_OIDC_AUDIENCE, DCERT_MCP_OIDC_JWKS_URL, all https). Requests whose Host header is not a loopback name, the bind address or an entry from --allowed-hosts are answered 400, which defeats DNS rebinding from a browser.

The proxy forwards only the environment variables listed under subprocess.passthrough_env in config.yaml to dcert-mcp: proxy and CA settings, TMPDIR, XDG_CACHE_HOME, DCERT_PROXY, DCERT_NOPROXY, DCERT_KB_FILE, every DCERT_MCP_* policy variable and the VAULT_ADDR, VAULT_TOKEN, VAULT_CACERT, VAULT_CAPATH and VAULT_SKIP_VERIFY settings. An entry ending in * matches by prefix.

dcert and dcert-mcp are thin wrappers that exec the Rust binaries with all arguments. A wrapper refuses to run when the only dcert-mcp it can find is itself (for example the pip console script on PATH), instead of exec-ing in a loop.

Configuration

Defaults live in the packaged dcert/config.yaml: the release URL, download limits, the install directory, the platform table, the environment variables forwarded to the subprocess, the server bind address, the transports and exporters offered by dcert-python, and every resilience value. dcert.load_config() returns them as an immutable record.

Resilience values can be overridden with DCERT_MCP_* environment variables (listed in dcert/resilience.py) and the dcert-python flags override those. A value outside its valid range (for example DCERT_MCP_CB_THRESHOLD=0 or DCERT_MCP_BULKHEAD_MAX=0) is rejected with a message naming the variable rather than crashing or hanging later. DCERT_MCP_NO_RETRY disables retries in both the proxy and the typed client, and DCERT_MCP_CONNECT_TIMEOUT bounds how long starting the subprocess and reading its tool list may take.

The proxy reads each tool's MCP annotations before applying caching or retries:

• Caching (--cache) replays results only for tools marked read-only and idempotent. Tools that generate keys, issue or sign certificates, or change Vault state (create_csr, vault_issue, vault_sign, vault_renew, …) are always run.
• Retry after a transport failure applies only to read-only or idempotent tools. Any other call surfaces the error instead of running a second time, because the first attempt may already have issued a certificate.

Binary discovery

1. DCERT_MCP_BINARY (or DCERT_PATH for the dcert CLI); a relative path is resolved against the working directory, never through PATH
2. The bundled binary in the package bin/ directory
3. A compiled dcert-mcp on PATH (pip console script wrappers and the running launcher are skipped)
4. Download from GitHub Releases, only when none of the above found a binary (a source checkout or a locally built wheel; never with a published wheel): https only, redirects restricted to GitHub hosts, size and time capped, SHA256 verified before extraction, installed atomically under a lock

An install record beside the binaries (.install-record.json: package version, archive digest and each binary's digest) makes a later run download again when the package version changes, the embedded checksum changes or an installed binary was modified. The install directory is created with mode 0700.

Downloaded binaries are installed into a directory owned by dcert, never into pip's scripts directory, so a download can never replace a console script: ~/.local/share/dcert/bin on Linux and macOS, %LOCALAPPDATA%\dcert\bin on Windows. Set DCERT_INSTALL_DIR to use another directory. dcert-python --setup downloads the binary ahead of time and prints where it was installed.

Environment variables forwarded to the binary

Development

cd python
pip install -e ".[dev]"
ruff check src tests ../scripts
ruff format --check src tests ../scripts
mypy
pytest

License

MIT