Send a message to all subscribers of a list using inline title and body content. API reference: https://www.courier.com/docs/api-reference/send/send-a-message.
| Name | Required | Description | Default |
|---|---|---|---|
| body | Yes | Message body | |
| data | No | Key-value data to include | |
| title | Yes | Message title | |
| method | No | Routing method | all |
| list_id | Yes | The list ID to send to | |
| channels | No | Channel names to route through. Omit to use default routing. |
Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already carry the readOnly/idempotent/destructive flags, and the description is consistent with them, so there is no contradiction. The description adds that this targets all list subscribers and uses inline content, but it does not disclose return shape or other side-effect details beyond what 'Send' implies.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.
Is the description appropriately sized, front-loaded, and free of redundancy?
A single direct sentence states the core purpose and content mode, followed by a useful API reference URL. There is no filler or repeated title.
Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.
Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?
Given no output schema and six parameters, the description is adequate but not rich: it identifies the target and content mode, while the schema covers field details. It leaves unaddressed the difference from template-based sends and does not describe the response, though the API reference mitigates this.
Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.
Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?
With 100% schema description coverage, the schema already documents all 6 parameters, including the method enum, data, and channels. The description adds no parameter semantics beyond mapping 'title' and 'body' to inline content, so the baseline of 3 applies.
Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.
Does the description clearly state what the tool does and how it differs from similar tools?
The description uses a specific verb ('Send') plus a resource ('message to all subscribers of a list'), and clarifies it uses inline title and body rather than a template. This distinguishes it from siblings like send_message_to_list_template and send_message, making the purpose unambiguous.
Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.
Does the description explain when to use this tool, when not to, or what alternatives exist?
The phrase 'to all subscribers of a list' and 'using inline title and body content' gives a clear context for choosing this tool over template-based or single-recipient senders. It does not explicitly name a sibling or provide when-not-to-use guidance, but the intended use case is evident.
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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Tools are mostly organized as distinct resource/action pairs, but several clusters are easy to confuse: list subscription tools (add_subscribers_to_list vs bulk_subscribe_to_list vs subscribe_user_to_list), message vs message-content vs message-history retrieval, and the many journey/journey-template list/get tools. Detailed descriptions rescue most selections, but the sheer number of near-identical verb/resource names creates real misselection risk.
Almost all tools follow a snake_case verb_noun pattern (create_, get_, list_, replace_, send_, publish_, archive_). Minor deviations keep it from a perfect score: courier_installation_guide is noun-first, and add_bulk_users sits awkwardly next to the bulk_add_* family, but the overall convention is predictable and readable.
144 tools is an extreme working-set size for an agent to hold and choose from, far beyond the reasonable 3–15 range. Even for a broad platform like Courier, this should be split into focused sub-servers (templates, journeys, users, lists, preferences, etc.) to remain usable.
The surface is remarkably comprehensive, covering sending, templates, journeys, automations, users, tenants, lists, preferences, providers, routing, brands, audiences, translations, digests, bulk jobs, and audit events. Notable gaps exist—automation template CRUD and digest schedule management are missing—but most workflows can still be completed with workarounds.
