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

Beyond the annotations (which only signal a non-readOnly, open-world, non-idempotent, non-destructive call), the description discloses that execution is synchronous, that billing is 2 credits charged by mcp-credit-gateway before forwarding, that embedded content is never echoed back, and that a server-held secret is used when text is omitted. Those are exactly the operational facts an agent cannot infer from structured fields.

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 dense but well-ordered paragraph: operation and input constraints first, then the secret/text branch, then algorithm options, then execution mode and cost. No filler sentences, and every clause carries decision-relevant 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?

With 5 parameters, 100% schema coverage, annotations present and no output schema, the description covers the remaining gaps an agent needs: synchronous behavior, credit cost and who charges it, secret-key fallback, and non-echo of embedded content. Nothing required to invoke it correctly is missing.

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 description coverage is 100%, so the schema already explains `model` (DWT vs DCT), `strength` 1-10, size limits and `watermark_text` semantics; the description largely restates these. It adds only marginal context (e.g., strength being optional, DWT as the robust default) beyond the schema, so baseline 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 names a specific verb and resource ("Embed an invisible/blind watermark into an image") plus the input contract (public URL, formats, size). The words "invisible/blind" implicitly distinguish it from the sibling add_watermark (visible overlay) and its inverse decode_blind_watermark, so an agent can route correctly without opening schemas.

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?

It gives clear operational guidance for the key branching decision: pass `watermark_text` or omit it to use the server-held BLIND_WATERMARK_KEY, with a concrete example (order id). However, it never states when to prefer this tool over add_watermark or that decode_blind_watermark is the verification counterpart, so sibling routing is left implicit.

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