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

The annotations already say this writes, is idempotent, not destructive, not open-world. So the description does not need to explain safety. It adds real value by explaining merge semantics: shallow merge with nested hashes merged one level. That implies fields it does not mention are preserved, which an agent cannot infer from the annotations. It does not cover permissions, validation failure behavior, or what happens when a property is unknown.

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 four short sentences. It states the action first, then merge semantics, then color convention, then the schema helper. Each sentence adds distinct, actionable detail, and nothing is redundant with the title or schema.

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?

Given five required parameters, a nested free-form object, an existing output schema, and annotations that cover safety, the description supplies the important missing semantics: the merge model, the color format, and the schema-discovery step. It does not say what an invalid property or unknown block type does, but for a tool of this weight it could do so.

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%, so the five parameters are documented fully in the schema, and the baseline is 3. The description still adds meaning about the free-form 'properties' object: it is merged rather than replaced, nested hashes merge one level down, and color values should use theme CSS variables. That goes beyond the schema's 'Properties to set on the block.'

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: update a block's content or styles. That is enough to separate it from add_site_block, remove_site_block, and reorder_site_blocks, and the word 'block' distinguishes it from update_site_section_styles. However, it does not name any sibling explicitly; the only tool it names is get_site_block_schema, a helper rather than an alternative.

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?

Tells the agent to call get_site_block_schema first to learn a block type's editable properties, and states a convention for color values. It does not say when NOT to use this tool or route to a section-level or block-creation sibling, but the usage context is otherwise clear.

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