fastmcp-creds

Pluggable credential management for FastMCP servers — supply API tokens or username/password credentials for the backend services they connect to, from environment variables, OS keychain, or HTTP headers, with automatic fallback through a priority chain.

• Installation
• Usage
  • Credential provider chain
  • Environment variables
  • HTTP Basic Auth header
  • Custom HTTP headers
  • OS keychain
• API Reference
  • CredentialsProvider
  • CredentialsProviderChain
  • EnvironmentCredentialsProvider
  • BasicAuthCredentialsProvider
  • CustomHeaderCredentialsProvider
  • KeyringCredentialsProvider
  • BasicAuthError
  • is_unresolved_placeholder_value
• Contributing
• License

Installation

pip install fastmcp-creds
# or, with uv:
uv add fastmcp-creds

The KeyringCredentialsProvider requires the optional keyring extra:

pip install fastmcp-creds[keyring]
# or, with uv:
uv add fastmcp-creds[keyring]

Requires Python 3.10+.

Usage

Credential provider chain

Compose providers in priority order. The chain tries each in sequence and returns the first result where both username and password (or token) are non-empty.

from fastmcp_creds import (
    CredentialsProviderChain,
    BasicAuthCredentialsProvider,
    EnvironmentCredentialsProvider,
)
from fastmcp_creds.keyring import KeyringCredentialsProvider  # optional dep

chain = CredentialsProviderChain([
    KeyringCredentialsProvider("my-service"),
    BasicAuthCredentialsProvider(),
    EnvironmentCredentialsProvider("MY_USERNAME", "MY_PASSWORD"),
])

username, password = chain.get_credentials()

Environment variables

Read username and password from two environment variables:

from fastmcp_creds import EnvironmentCredentialsProvider

provider = EnvironmentCredentialsProvider("MY_USERNAME", "MY_PASSWORD")
username, password = provider.get_credentials()

For a single token variable:

provider = EnvironmentCredentialsProvider.for_token("MY_API_TOKEN")
token, _ = provider.get_credentials()

HTTP Basic Auth header

Decode credentials from the standard Authorization: Basic <base64> header. Also accepts the non-compliant Bearer Basic <base64> variant emitted by MCP Inspector, and raw base64 without a scheme prefix.

from fastmcp_creds import BasicAuthCredentialsProvider

provider = BasicAuthCredentialsProvider()
username, password = provider.get_credentials()

Optionally check a custom header first before falling back to Authorization:

provider = BasicAuthCredentialsProvider(custom_header_name="X-My-Auth")

Custom HTTP headers

Read credentials from two plain-text HTTP headers:

from fastmcp_creds import CustomHeaderCredentialsProvider

provider = CustomHeaderCredentialsProvider("X-Username", "X-Password")
username, password = provider.get_credentials()

For a single token header:

provider = CustomHeaderCredentialsProvider.for_token("X-Api-Token")
token, _ = provider.get_credentials()

OS keychain

Read credentials stored in the OS keychain via keyring. Requires fastmcp-creds[keyring].

from fastmcp_creds.keyring import KeyringCredentialsProvider

# Two-key mode: reads "username" and "password" entries under the service name
provider = KeyringCredentialsProvider("my-service")
username, password = provider.get_credentials()

# Token mode: reads a single "token" entry
provider = KeyringCredentialsProvider.for_token("my-service")
token, _ = provider.get_credentials()

# Custom key names
provider = KeyringCredentialsProvider("my-service", username_key="user", password_key="secret")
provider = KeyringCredentialsProvider.for_token("my-service", token_key="jwt")

API Reference

CredentialsProvider

CredentialsProvider is a Protocol — any object with a get_credentials() -> tuple[str | None, str | None] method satisfies it.

CredentialsProviderChain

CredentialsProviderChain(providers) — tries each provider in order and returns the first result where both values are non-empty. Returns (None, None) if no provider succeeds.

EnvironmentCredentialsProvider

EnvironmentCredentialsProvider(username_env_var_name, password_env_var_name) — reads two environment variables.

EnvironmentCredentialsProvider.for_token(token_env_var_name) — reads a single environment variable; returns it as both elements of the tuple.

BasicAuthCredentialsProvider

BasicAuthCredentialsProvider(custom_header_name=None) — extracts credentials from an HTTP Basic Auth header. Checks custom_header_name first if provided, then falls back to the standard Authorization header. Silently ignores unresolved MCP client placeholders (e.g. ${MY_VAR}).

CustomHeaderCredentialsProvider

CustomHeaderCredentialsProvider(username_header_name, password_header_name) — reads credentials from two plain-text HTTP headers. Ignores unresolved placeholders.

CustomHeaderCredentialsProvider.for_token(token_header_name) — reads a single header; returns it as both elements of the tuple.

KeyringCredentialsProvider

KeyringCredentialsProvider(service, username_key="username", password_key="password") — reads credentials stored in the OS keychain under the given service name. Requires fastmcp-creds[keyring].

KeyringCredentialsProvider.for_token(service, token_key="token") — reads a single keychain entry.

BasicAuthError

BasicAuthError(message, header_name) — raised by BasicAuthCredentialsProvider when a header is present but malformed (bad base64, unsupported scheme, missing colon separator, empty username or password).

is_unresolved_placeholder_value

is_unresolved_placeholder_value(value: str) -> bool — returns True when a header value is an unresolved MCP client placeholder such as ${MY_VAR} or Basic ${MY_VAR}. Used internally by BasicAuthCredentialsProvider and CustomHeaderCredentialsProvider; exposed for custom provider implementations.

Contributing

See CONTRIBUTING.md for development setup and guidelines.

License

MIT — see LICENSE.