MCP Federation

How it works

┌─────────────────────────┐
│  Your AI agent          │
│  .mcp.json: your hub    │
└────────────┬────────────┘
             │
┌────────────▼────────────┐
│  Your trip2g hub        │
│  /_system/mcp           │
│  • your own notes       │
│  • KB-notes pointing at │
│    peers and adapters   │
└─┬──────────┬───────────┬┘
  │          │           │
  ▼          ▼           ▼
Public     Partner    External
base       peer       adapter
(no auth)  (HMAC)     (HMAC)

The hub stores no remote content. It holds your notes plus routing metadata — KB-notes in your vault and federation secrets in the admin.

Core concepts

KB-note — a regular Obsidian note with mcp_federation_kb_url in its frontmatter. That single field registers a peer base. The note body is free text describing when to use this base; the agent reads it during search and uses it as context.

Federation secret — a shared HMAC key that authenticates your hub to a private peer (and vice versa). Public bases need no secret at all.

kb_id — a short slug identifying a peer, used when you want to target a specific base instead of fanning out across all of them. Defaults to the URL hostname; you can override it with mcp_federation_kb_id.

Adding a public peer

Create a note anywhere in your vault with the following frontmatter:

---
mcp_federation_kb_url: https://philosophers.example.com/_system/mcp
mcp_federation_kb_id: philosophy
---
Use when: finding philosophical references for engineering decisions.
Don't use when: anything time-sensitive or domain-specific.

Sync. Done. The agent can now reach that base via federated_search with kb_id="philosophy" or automatically when it fans out across all configured bases.

mcp_federation_kb_id is optional. If you omit it, the kb_id defaults to the URL hostname (philosophers.example.com).

Adding a private peer (two-step exchange)

Private peers authenticate with a shared HMAC secret. The setup is a one-time key exchange — each side generates a secret for the other.

Step 1 — Bob generates a secret for you.

Bob opens Admin → Federation → Add inbound secret. He picks a short key ID (alice-2026), optionally selects which of his subgraphs you may access, and clicks Generate. The admin shows the secret hex once. Bob sends you the kid and secretHex over a trusted channel (a direct Telegram message is fine).

Step 2 — You paste it as an outbound secret.

In your admin → Federation → Add outbound secret, enter:

• kid: the key ID Bob gave you (e.g. alice-2026)
• secretHex: the hex bytes Bob gave you
• kb_url: Bob's MCP endpoint (https://bob.team.io/_system/mcp)

Step 3 — Create the KB-note.

---
mcp_federation_kb_url: https://bob.team.io/_system/mcp
mcp_federation_kb_id: bob
---
Use when: Bob's work-status updates, our shared design notes.

Step 4 — Wire the reverse direction (optional).

If Bob also wants to query your base, repeat the process the other way: you generate an inbound secret, send Bob the kid and secretHex, and Bob pastes it as outbound on his side with your /_system/mcp URL.

Each direction is independent. You can give Bob read access to your team-status subgraph without giving him anything else.

Federated tools

Your hub always exposes six MCP methods regardless of how many peers are connected:

federated_search without kb_id fans out to all accessible peers in parallel and merges results. With kb_id="bob" it targets Bob's base only. With kb_ids=["bob","philosophy"] it hits exactly those two.

When the agent's local search surfaces a KB-note, it returns a kind: "federation_kb" marker with an inline instruction telling the agent to call federated_search with that kb_id. The agent assembles context as it queries — no upfront dump.

Permissions

Federation reuses your existing subgraph access control.

Outbound (what peers see from you). When Bob's hub calls yours with a valid JWT, your instance checks which subgraphs his kid is scoped to and filters results accordingly. Notes outside that scope are invisible to him, the same as they would be to any other reader without access.

Inbound (what you see from peers). KB-notes are regular notes — you can assign them to a subgraph. If you later open your hub to colleagues, they only see KB-notes whose subgraph they have local access to.

KB-note visibility tiers. Because a KB-note is just a note, the same visibility rules apply:

A KB-note with no free: and no subgraphs: is not admin-only — it is visible to any authenticated subscriber. To make a KB-note visible only to admins, put it in a dedicated subgraph that no non-admin users have access to (e.g. subgraphs: admin-only). The federated_search response for an inaccessible kb_id is always "not configured" — identical to a kb_id that does not exist, so the peer's existence is not disclosed.

Revoking access

Admin → Federation → find the row → Revoke. The row goes grey. Any future request from that kid gets a 401 response. No coordination with the peer is needed — their calls simply start failing immediately.

To reduce scope without full revocation, remove individual subgraphs from the kid in the Manage scope panel.

Limits and known constraints

• Auth: HMAC-SHA256 only in the current version. No mTLS, no OAuth.
• Fan-out timeout: 2 seconds per peer call. Slow or unreachable peers are skipped; you get results from the rest.
• Depth cap: fan-out recursion stops at depth 3 (configurable via MCP_FEDERATION_MAX_DEPTH on self-hosted). This prevents loops when peers also have peers.
• TLS: not enforced by the hub. Use HTTPS peer URLs in production.
• No replay cache: JWT expiry is 30 seconds. Acceptable for personal-hub scale.

Troubleshooting

"federation_not_configured" in the agent response — no KB-notes exist in your vault, or the KB-note's mcp_federation_kb_url frontmatter field is missing or misspelled.

401 errors in your hub logs — the outbound secret does not match what the peer expects, or the peer has revoked your kid. Re-exchange secrets with the peer.

Peer returns no results but no error — the peer's subgraph scope for your kid is empty. Bob needs to add at least one subgraph to your kid in his Admin → Federation → Manage scope.

Fan-out takes the full 2 seconds — one or more peers are unreachable. Check your hub logs (mcp:federation prefix) for per-peer timeout warnings. Remove or fix the unreachable KB-note.

Related

• Hub — knowledge bases federated into this hub
• MCP Server — how the local MCP server works
• Self-hosted — MCP_FEDERATION_MAX_DEPTH and other environment variables
• Advanced — subgraphs and access control