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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint=false, so safety is covered. The description adds the cost tier (10 credits) and endpoint, which are genuinely useful behavioral facts not in the annotations, but says nothing about rate limits, error modes, or rendering latency.

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?

Two tight, front-loaded sentences followed by compact metadata lines. Nothing is padded, though the [Group]/[Endpoint]/[Cost] block is boilerplate rather than tool-specific guidance.

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?

An output schema exists so return values needn't be explained, and annotations carry the safety profile. The remaining gap is that the description says it "Returns SVG" while the format option defaults to json, a discrepancy an agent may trip over, and it omits any sibling differentiation.

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 100% and the nested body/options objects are thoroughly documented in-schema (timezone, ayanamsa, houseSystem letters, format default, etc.). The description adds nothing beyond "at the given moment," so the schema does all the work — baseline 3.

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 ("Render the moon disk with its illuminated fraction at the given moment") and names the outputs. However it never distinguishes itself from close siblings such as astroway_calendar_moon_phase (data) or other astroway_render_* tools, leaving the agent to infer that this is the image-producing variant.

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?

No when-to-use statement, no prerequisites, and no mention of alternatives despite a large sibling set that includes a moon-phase calendar tool. The agent gets no routing guidance beyond the name.

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