retell_register_phone_call

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

With no annotations, the description must carry the transparency burden. It discloses the outcome (returns call_id and access_token) and hints at the audio websocket connection, which is a behavioral trait. However, it does not explain what happens after registration (e.g., that the agent must connect to the websocket and whether there are any prerequisites or side effects). This is a moderate level of disclosure, sufficient for understanding the core action but lacking operational depth.

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?

The description is a single, front-loaded sentence that states the action, context, and key return values without filler. Every clause adds useful information, and it is appropriately sized for a tool with a clearly named purpose and well-documented schema.

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?

The tool has 6 parameters, nested objects, no output schema, and no annotations. The description names the returns but does not outline the required follow-up steps (e.g., using the access_token for the audio websocket) or any post-registration behavior. While the purpose is clear, an agent might benefit from knowing that this is part of a two-step flow. The description is adequate but leaves operational context to be inferred.

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?

The input schema has 100% description coverage for all six parameters, including nested objects and enums. The description itself does not add parameter-level details beyond what the schema already states. Since the schema is fully self-explanatory, a baseline score of 3 is appropriate; there is no gap for the description to compensate for.

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 'Register' and identifies the resource 'phone call' with the key qualifier 'for custom telephony (bring-your-own telephony)'. This clearly distinguishes it from siblings like retell_create_phone_call (which likely uses Retell's infrastructure). It also states the return values (call_id and access_token), leaving no ambiguity about the tool's purpose.

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 'bring-your-own telephony' provides a clear use case: when a caller has their own telephony setup rather than using Retell's phone numbers. While it doesn't explicitly name alternative tools, the sibling context (e.g., retell_create_phone_call, retell_create_web_call) makes it evident when this tool applies. No exclusions or when-not guidance, but the context is specific enough for an agent to select appropriately.

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