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) Social Contract.

Getting Started

Connect via Horizon -- no local install needed:

https://excalibur-mcp.fastmcp.app/mcp

The web editor at https://excalibur.tollbooth-dpyc.com drives the same tools from a browser.

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: Prove Your Npub on the Connection

Identity rides on the connection, not on tool arguments. Connecting from an AI assistant is an OAuth holder connection: call excalibur_request_npub_proof(patron_npub=<npub>), reply to the Nostr DM it sends, then call excalibur_receive_npub_proof(patron_npub=<npub>, nonce=<nonce>) once. The connection then holds your npub for the lifetime you chose in the reply (up to 30 days), and every tool call passes only npub. A caller that holds the nsec may instead sign each HTTP request (NIP-98).

Step 3: 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 4: 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.

Related MCP server: X-MCP 2.0

Credits Model

Tool calls cost api_sats per call. Each purchase creates a tranche; it expires only if the operator's pricing model sets a tranche_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)

Domain tools are defined in server.py (with storage handlers under tools/). Prices are set by the operator's pricing model -- preview any call with check_price. Read and auth tools are free.

Posting to X

Stored posts (drafts & scheduling)

Snippets & Voice

AI editorial & dynamic blocks

Conversation leads

Post metrics

Scheduler

OAuth flow tools (begin_oauth, check_oauth_status) are now standard tools provided by the wheel (see the Standard Tools table above).

Tools that take an npub take only the npub: the connection proves it (see Security).

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.

• Identity on the connection -- A connection proves its npub in one of two ways: each HTTP request is signed with the npub's key (NIP-98, kind 27235), or an OAuth holder connection carries an Operator-signed grant earned through request_npub_proof / receive_npub_proof. The patron chooses the grant's lifetime, up to 30 days; held_identities and check_proof_status show what a connection holds.

• 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
  x_client.py      X API v2 client with OAuth 2.0 Bearer token auth
  formatter.py     Markdown -> Unicode rich text (bold, italic, headers)
  refine.py        Server-side editorial refinement for the post editor
  resolve.py       Server-side resolution of dynamic (prompt-backed) post blocks
  resolver.py      Builds one post's finished body (the slow half: dynamic blocks)
  publisher.py     Carries a finished body to X (the fast half: bill, post, record)
  scheduler.py     Scheduled-post firing (publishes due posts on the owner's behalf)
  conversation_scoring.py  Pure scoring + query composition for conversation leads
  metrics_harvest.py       Post-metrics harvest cadence and derived reach scores
  nostr_note.py    Public Nostr notes signed by an ephemeral scribe key
  tools/           Domain storage handlers (posts, snippets, voices, conversations, metrics)
  db/              eXcalibur persistence on the wheel's NeonVault (posts, snippets, voices,
                   conversations, metrics, scheduler runs, migrations)
modal_app.py       Detached runner for long dynamic-block resolves (Modal)
scheduler-worker/  Cloudflare Worker cron that fires due scheduled posts
frontend/          The web editor (excalibur.tollbooth-dpyc.com)

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.

Common tool call situations

Responses describe the current lifecycle state with guidance on the next step, not bare error messages.

• proof_required -- The connection does not prove that npub (or its grant expired). Call request_npub_proof, reply to the DM, then receive_npub_proof once. check_proof_status shows how long a grant has left.

• "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

• tollbooth-dpyc -- Python SDK for Tollbooth monetization (vault, auth, pricing, Lightning payments, Nostr identity)

• dpyc-community -- Governance registry: membership, advisories, threat model

• dpyc-oracle -- Community concierge (free onboarding + member lookup)

• tollbooth-authority -- Certification backbone (Schnorr-signed purchase-order certificates)

• tollbooth-sample -- Sample Operator (canonical template)

• tollbooth-pricing-studio -- iOS pricing-model editor / operator console

• cypher-mcp -- Monetized graph answers: named Cypher templates over Neo4j/AuraDB

• schwab-mcp -- Charles Schwab brokerage data

• thebrain-mcp -- TheBrain personal knowledge graph

• excalibur-mcp -- X/Twitter posting

• taxsort-mcp -- Tax classification + Cloudflare Pages UI

• optionality-mcp -- Options-trading practice game

• goodearth-mcp -- Region-scoped climate analytics for small farms

• beesknees-mcp -- The Bee's Knees race game

• roastify-mcp -- Roastify coffee catalog and packaging artwork

• ChartRemotely-mcp -- Puts a security on a desktop chart remotely

• tollbooth-web -- Shared browser SDK for operator front ends

• tollbooth-oauth2-collector -- OAuth2 callback handler (advocate service)

• tollbooth-shortlinks -- URL shortener utility

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.