FYN MCP server

Most assistants only need the URL. Add https://api.fyn.fyi/v1/mcp as a connector, sign in to FYN when asked, and select Allow. Setup steps for ChatGPT, Claude, Cursor, and Claude Code are on fyn.fyi/agents.

FYN implements the MCP authorization spec:

1. A request without a valid token gets HTTP 401 with WWW-Authenticate: Bearer resource_metadata="https://api.fyn.fyi/.well-known/oauth-protected-resource".
2. Protected resource metadata (RFC 9728): https://api.fyn.fyi/.well-known/oauth-protected-resource lists the resource https://api.fyn.fyi/v1/mcp, the authorization server https://api.fyn.fyi, and scope fyn.read.
3. Authorization server metadata (RFC 8414): https://api.fyn.fyi/.well-known/oauth-authorization-server

Token lifetimes: access tokens last 30 days. Refresh tokens last 180 days and rotate on every use (the old one stops working). Authorization codes last 10 minutes.

Account scope: a token is bound to one FYN household. The household is taken from the token and never from request parameters.

Notice: FYN emails the account holder whenever a new app connects.

Alternative for developer tools: a bearer key from FYN Settings → Agent access (Authorization: Bearer fyn_ak_…), with the same read-only access.

• Free plan ($0): connects and can use the tools listed below as Free + Pro. Limit: 500 tool calls per day, and 50 AI-assisted calls per day. A free account has receipts and receipt-created transactions. It does not sync bank accounts, so balance, transaction, budget, cash-flow, net-worth, subscription, and investment tools return little or no data.
• FYN Pro ($9.99/month): live bank sync through Plaid, plus four tools offered only to Pro households (marked Pro). Limit: unlimited tool calls per day, and 500 AI-assisted calls per day. For other plans these four tools are not listed in tools/list, and calling one returns Unknown tool.
• AI-assisted tools: search_finances and draft_claim use FYN's AI, which processes data within FYN's own AWS environment under zero data retention: model providers do not store your prompts or data, and your data is never used to train AI models. They count against the AI-assisted daily limit above, separate from the general tool-call limit.

All 27 tools are classified as Read: they retrieve information without changing data, account state, or permissions. None is a Write or a Sensitive write. Every tool carries the MCP annotations readOnlyHint: true, destructiveHint: false, and openWorldHint: false, plus a title. Dates use YYYY-MM-DD. Amounts are in USD.

Side effects

• check_affordability adds one record of the check (amount, merchant, category, and FYN's assessment) to the household's agent activity log, which the household sees in Settings → Agent access. It does not change balances, budgets, or transactions.
• Every tool call is counted toward the household's daily limit and monthly usage counters (tool name only).
• No other tool writes anything. The in-app write tools (update_transaction_category, exclude_from_budget) are never offered over MCP, and calls to them are refused.

Results

Results come back as JSON text in content. search_finances and search_receipt_items also return structuredContent, which a client can render as a receipt card. Internal transaction and category IDs are removed. A household with no linked bank gets empty results, and the server's initialize instructions say why.

Limits apply per FYN household per UTC day, reset at midnight UTC, and count only calls that would run. Listing tools and connecting never count.

When a limit is reached, the call returns a tool error (not HTTP 429), for example: "FYN's daily limit for connected-app tool calls is reached for this household. It resets at midnight UTC." FYN does not publish a separate per-second limit. Calls can still be slowed by shared infrastructure throttling.

HTTP level

JSON-RPC errors

HTTP 200: -32700 parse error · -32600 invalid JSON-RPC request · -32601 method not found · -32602 unknown tool or unknown resource · -32603 internal server error.

Tool errors

HTTP 200, result.isError: true, with a readable message in content[0].text:

• Missing required argument(s): <names>
• A daily limit was reached (see above). Other tools keep working where allowed.
• "<tool>" is not available to connected apps: FYN access is read-only. (write tools)
• AI_CONSENT_REQUIRED: … Applies only when FYN's AI provider is an external model. It does not apply while AI processes data within FYN's own AWS environment under zero data retention.
• Tool "<tool>" failed: <reason>

Results that report a problem

A successful call whose JSON says why there is no answer. For example, {"error":"Receipt not found."} for draft_claim and split_bill, or status: "category_not_found" / "ambiguous_category" for get_budget_status.

OAuth errors

OAuth errors follow RFC 6749 / 7591, as JSON {"error","error_description"}: invalid_request, invalid_grant (including "Access was revoked"), unsupported_grant_type, server_error, invalid_client_metadata, invalid_redirect_uri. The authorize endpoint answers bad requests with HTTP 400 OAuth error: <reason> (plain text).

• What the server reads: only the connected household's FYN records, for the tool being called.
• What it receives from the assistant: the tool name and arguments (for example a search phrase or a dollar amount). FYN does not receive your conversation.
• Where it runs: AWS in the United States (us-east-1). Traffic is encrypted with TLS, and data is encrypted at rest with AES-256. FYN's AI for the AI-assisted tools processes data within FYN's own AWS environment under zero data retention: model providers do not store your prompts or data, and your data is never used to train AI models.
• What FYN keeps from connector use: usage counts by tool name. Operational logs (tool name, timing, and errors) are kept for 90 days. Traces of AI-assisted calls are kept for 90 days. The check_affordability activity record stays in your household's log. FYN does not sell data, use it for advertising, or train models on it.
• Bank data comes from your bank through Plaid with read-only access. The MCP server never calls Plaid or your bank; it serves FYN's stored copy of your records.

Full details: Privacy Policy · Security

• In FYN: Settings → Agent access. Find the connected app and revoke it. Its tokens stop working, and its refresh token cannot mint new ones.
• Removing the connector inside your assistant stops the assistant from using it. To end FYN-side access too, revoke it in FYN as above.
• Delete your FYN account and data: fyn.fyi/remove-account.

[email protected] · fyn.fyi/support. Security reports: [email protected].