render_composite_chart

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

Annotations only declare it as a safe, idempotent read; the description adds substantial context beyond that: no prior calculation call is required, stdio yields no public image URL, the client must support image content, and the three outcome paths (needs_clarification without image, error without image, inline PNG on success) are each spelled out.

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?

Front-loaded with the core action, then alternatives, then requirements, then return/error behavior — a sensible ordering. It is a dense paragraph with some stacking of caveats, but nearly every clause carries actionable information.

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?

For a two-parameter rendering tool with no output schema, the description covers everything an agent needs: input prerequisites, whether a prior calculation call is needed, the return medium, transport limitations, and both failure modes.

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 already 100%, so the baseline is 3; the description goes further by insisting on complete BirthInput objects with exact local civil times and separately confirmed birthplace IDs, and explicitly warns not to copy person A's location to person B. The only minor gap is that the required/optional status is not restated (all fields default to null in the schema).

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?

Opens with a specific verb and resource: 'Render the midpoint composite chart of two people as a 720px PNG wheel.' It immediately distinguishes itself from the two nearest siblings (calculate_relationship_charts for structured data, render_davison_chart for the time-space midpoint), so an agent can pick it without opening a 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?

Explicit positive trigger ('use for geometric midpoint positions') plus two named alternatives with their selection conditions. It also sets a negative boundary ('does not score compatibility or predict relationship outcomes'), which prevents a plausible misuse.

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