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

The description discloses merge semantics (additive, partial update) and excludes theme-related fields, which goes beyond the annotations. It does not fully address the destructiveHint annotation or openWorldHint implications, but the merge behavior is sufficiently communicated to set expectations for an update operation.

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 two sentences, front-loaded with the operation, followed by a usage tip and a routing note. Every sentence serves a purpose with no fluff or repetition of schema details.

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 tool with a rich input schema and an output schema, the description covers the essential usage guidance, merge behavior, and theme-tool routing. It does not explain null-value semantics or potential side effects hinted by openWorldHint, but these are secondary for correct invocation.

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 both parameters are described. The additional instruction 'Pass only the keys you want to add or update' adds practical meaning to the metadata parameter beyond the schema, clarifying that omitted keys are preserved. This modest extra context justifies moving above the 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?

Description uses a specific verb ('Merge') and resource ('custom metadata into a blog'), clearly defining the operation. It also distinguishes itself from theme-related tools by explicitly pointing them to dedicated tools, which prevents confusion with siblings like update_theme_settings.

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 description provides clear usage instructions ('Pass only the keys you want to add or update') and explicitly states that theme changes belong to other tools. It does not explicitly mention alternatives like update_blog, but 'custom metadata' makes the boundary reasonably clear without additional explanation.

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