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

Annotations only declare openWorldHint and idempotentHint; the description adds substantial context beyond them: the specific gates enforced (do-not-call list, calling hours, wallet balance, concurrency), the refusal-code branching surface (suppressed, outside_hours, wallet_low), and the non-default 'dial' scope requirement. This is exactly the behavioral disclosure annotations cannot carry.

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?

Four sentences, each earning its place: what it does, the gating behavior, the error surface, and the routing alternative. The critical scope prerequisite and the cheaper sibling are front-loaded rather than buried.

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?

With no output schema, the description usefully explains the refusal-code return surface, and it covers gating and auth. The one gap is parameter meaning for leadId/agentId, which neither the schema nor the description explains.

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?

Schema coverage is only 33% — 'to' is documented as E.164, but leadId and agentId have no schema description. The description mentions none of the parameters, so it fails to compensate for the coverage gap; an agent gets no guidance on what agentId or leadId must reference.

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?

States a specific verb and resource ('Starts a real conversational call from this workspace's own number') and immediately distinguishes itself from the sibling send_announcement by contrasting listening conversation vs fixed script. An agent can differentiate this from send_announcement without opening either schema.

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?

Names the alternative explicitly ('For a fixed script with no listening... use send_announcement instead - it is far cheaper') with the condition that selects it. It also states the prerequisites (dial scope, gates) that gate a successful call, leaving nothing to inference.

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