eXcalibur MCP

Sword-swift posting of pretty tweets to X (Twitter) via AI agents, monetized with Bitcoin Lightning micropayments through the DPYC(TM) Tollbooth protocol.

eXcalibur is a FastMCP server that lets AI agents post to X (Twitter) with rich Unicode formatting and optional images. Patron authentication for X uses OAuth2 Authorization Code + PKCE -- no credentials appear in chat. Operator credentials (X app keys, BTCPay) arrive via human-in-the-loop Secure Courier and are stored in the operator's per-operator Neon vault schema. Tool calls are metered with Tollbooth DPYC(TM) pre-funded Lightning balances -- Don't Pester Your Customer.

Part of the DPYC(TM) Honor Chain.

Getting Started

Connect via FastMCP Cloud -- no local install needed:

https://www.fastmcp.cloud/mcp/lonniev/excalibur-mcp

Step 1: Check Session

Call excalibur_session_status to see your current session state. If you have an active session with funded credits, you are ready to post.

Step 2: X OAuth2 + PKCE (Patron Credentials for X)

1. Get your Nostr npub -- use the dpyc-oracle's how_to_join() tool, or any Nostr client.
2. Call excalibur_begin_oauth(npub=<npub>) -- returns an authorize_url. Open the primary authorize_url (not a shortlink) in your browser and authorize the app on X.
3. Call excalibur_check_oauth_status(npub=<npub>) -- completes the code exchange, vaults the tokens, and activates your session.

Sessions are keyed by npub. Tokens are stored in the Neon vault and auto-refresh on expiry.

Step 3: Secure Courier (Operator Credentials)

Operator credentials (X OAuth2 app keys, BTCPay connection) are delivered via Secure Courier (service="excalibur-operator"). This is a human-in-the-loop flow: the operator consciously approves each credential delivery via their Nostr client. On receive_credentials, the relay DM is destructively drained -- credentials exist only in the Neon vault after receipt.

Credits Model

Tool calls cost api_sats per call. Credits have a tranche_lifetime -- each purchase creates a tranche that expires after the configured lifetime. Auth and balance tools are always free. Use excalibur_check_balance to see your balance and tranche expiry. Top up via excalibur_purchase_credits.

Available Tools

Standard Tools (from the wheel)

Standard DPYC(TM) tools are registered by register_standard_tools() from tollbooth-dpyc. These include session management, credit operations, Secure Courier, pricing, notarization, and Oracle delegation. Each tool is identified by a deterministic UUID v5.

Domain Tools (eXcalibur-specific)

All tools that take an npub also accept a proof: str parameter for kind-27235 Schnorr proof attestation.

Security

• Npub identity -- Patrons are identified by a Nostr public key (npub), not an email or password. One keypair per role, managed by the user.
• Kind-27235 Schnorr proof -- Tool calls carry an NIP-98-style Schnorr signature proving the caller controls the claimed npub. Proof is cached for ~1 hour; renew via request_npub_proof / receive_npub_proof.
• Human-in-the-loop Secure Courier -- Credential delivery requires conscious operator approval via Nostr DM. On receipt, the relay message is destructively drained so secrets exist only in the encrypted Neon vault.
• Per-operator Neon schema -- Each operator's credentials are stored in an isolated Postgres schema with a dedicated LOGIN role. No cross-operator access.

Architecture

src/excalibur_mcp/
  server.py        FastMCP server -- domain tools + register_standard_tools()
  config.py        Pydantic settings from environment variables
  oauth_flow.py    X-specific OAuth2 Authorization Code + PKCE wrapper
  vault.py         In-memory Bearer token session cache (keyed by npub)
  x_client.py      X API v2 client with OAuth 2.0 Bearer token auth
  formatter.py     Markdown -> Unicode rich text (bold, italic, headers)
  actor.py         OperatorProtocol implementation for tool catalog

Key design choices:

• OAuth2 Authorization Code + PKCE -- Patron tokens acquired via browser flow. begin_oauth returns the primary authorize_url (not a shortlink); check_oauth_status completes the exchange. Tokens stored in Neon vault, auto-refreshed on expiry. No OAuth 1.0a.
• UUID v5 tool identity -- Every tool (standard and domain) has a deterministic UUID derived from its capability name. Pricing, constraints, and billing all key on UUID.
• Standard tools from the wheel -- register_standard_tools() provides all DPYC(TM) infrastructure tools. Only domain-specific X/Twitter tools are defined in server.py.
• Markdown to Unicode -- Converts **bold**, *italic*, and # headers to Unicode characters that render in tweets without markup syntax.

Self-Hosting

Environment Variables

DPYC(TM) Identity (required to boot)

This is the only env var required to start. All other secrets (X app keys, BTCPay credentials) arrive via Secure Courier credential templates and are stored in the per-operator Neon vault schema.

Operator Credentials (via Secure Courier)

These are delivered via Secure Courier (service="excalibur-operator"), not set as environment variables:

Certified operators bootstrap their Neon database URL from the Authority via encrypted Nostr DM -- NEON_DATABASE_URL is not read from the environment.

Optional Tuning

Troubleshooting

Cold start

On first request after deploy, the service may take a few seconds to initialize Neon connections and load the pricing model. The runtime handles this with inline retry -- the caller receives the result after the warm-up completes, not a "service unavailable" error.

Credential lifecycle states

Credential-related responses describe the current lifecycle state with clear guidance, not error messages:

Common tool call situations

• "proof is required" -- Call request_npub_proof then receive_npub_proof to prove npub ownership. The cache expires after ~1 hour; renew with a fresh request/receive cycle.
• "Insufficient credit balance" -- Call purchase_credits to top up. Check tranche expiry with check_balance.
• "Operator credentials not configured" -- Operator setup issue, not the patron's problem.

Don't Pester Your Customer: Do NOT ask the patron to re-authenticate, re-authorize, or re-do OAuth unless the response specifically says the upstream token is expired or invalid.

Development

# Install in development mode
cd excalibur-mcp
python -m venv .venv
source .venv/bin/activate
pip install -e ".[dev]"

# Run tests
.venv/bin/pytest tests/

# Type checking
mypy src/excalibur_mcp/

# Formatting
black src/ tests/
ruff check src/ tests/

DPYC(TM) Ecosystem

• dpyc-community -- Registry + governance
• tollbooth-dpyc -- Python SDK for Tollbooth monetization
• tollbooth-authority -- Authority MCP service
• thebrain-mcp -- Personal Brain MCP service
• excalibur-mcp -- Twitter MCP service
• dpyc-oracle -- Community concierge

Trademarks

DPYC, Tollbooth DPYC, and Don't Pester Your Customer are trademarks of Lonnie VanZandt. See the TRADEMARKS.md in the dpyc-community repository for usage guidelines.

License

Apache License 2.0 -- see LICENSE for details.