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klodr

mercury-invoicing-mcp

mercury_create_webhook

Register a webhook to receive real-time Mercury events (e.g., invoice.paid, transaction.posted) via HTTPS POST. Enables external systems to react without polling.

Instructions

Register a new webhook endpoint. Mercury will POST events as JSON to the provided URL.

USE WHEN: subscribing an external system to Mercury events (transaction.posted, invoice.paid, etc.) so you can react in near real-time without polling.

DO NOT USE: with non-HTTPS URLs, loopback / RFC 1918 / link-local / cloud-metadata IPs — the MCP enforces a defense-in-depth gate on top of Mercury's own validation to block accidental SSRF or exfiltration via prompt injection.

SIDE EFFECTS: writes a new webhook subscription to Mercury. Persistent. Once registered, Mercury immediately starts delivering matching events to your URL — make sure the endpoint is ready to receive (and ideally verify Mercury's signature). NOT idempotent at the API level — calling twice creates two subscriptions firing duplicate events.

RETURNS: { id, url, status, events, ... } — keep id for mercury_update_webhook / mercury_delete_webhook.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
urlYesPublicly reachable HTTPS URL that will receive webhook events (POST). Must not be a loopback, private, or link-local address.
eventsYesEvent types to subscribe to. Common values: - transaction.created / transaction.updated - transaction.posted / transaction.pending / transaction.failed - invoice.created / invoice.sent / invoice.paid / invoice.overdue / invoice.cancelled - customer.created / customer.updated - recipient.created / recipient.updated Check https://docs.mercury.com/reference/webhooks for the full list.
Behavior5/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Despite no annotations, the description fully discloses side effects: writes a persistent subscription, immediate event delivery, non-idempotent, and reminders to verify signatures. This comprehensive behavioral coverage compensates for missing annotations.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Description is front-loaded with purpose, well-structured with labeled sections (USE WHEN, DO NOT USE, SIDE EFFECTS, RETURNS), and every sentence adds value without redundancy. Length is justified by the richness of content.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Given no output schema, the description includes a return format example. It covers all relevant aspects: when to use, security constraints, side effects, and return value pointers. No gaps remain for this 2-parameter creation tool.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100%, so description adds limited parameter-level detail. It reinforces safety constraints (public HTTPS) and provides example events, but these are partially covered by schema descriptions. Baseline of 3 is appropriate.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description clearly states 'Register a new webhook endpoint' as the action, specifies the resource (webhook), and distinguishes from sibling tools like update/delete by focusing on creation and event subscription.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines5/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

Provides explicit 'USE WHEN' (subscribing to events) and 'DO NOT USE' (non-HTTPS, private IPs) guidelines, including security warnings about SSRF/exfiltration. This gives clear context for when to invoke vs. avoid the tool.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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