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

Annotations already declare destructiveHint=true and openWorldHint=false, so the safety profile is covered. The description adds the genuinely useful behavioral constraint that content must conform to a type-specific schema sourced from create_activity or get_activity_schema, which is not derivable from the 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?

Two sentences, tightly front-loaded: the action first, then the critical schema-source guidance. No filler.

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 mutation tool with a nested object parameter, the description covers the key pitfall (unsourced content schema) and annotations handle safety and openness. Return behavior is not described, but no output schema exists and the call itself is fully specified; a quick note on append-vs-replace consequences would complete it.

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 baseline is 3, but the description adds real meaning for the nested 'content' property by telling the agent where to obtain its required schema. The mode enum semantics are left to the schema, which already documents them.

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 pair ('Add or replace') and the exact resource ('the content of an activity'), making it immediately distinguishable from the read counterpart get_activity_content. An agent can tell what this does without opening anything else.

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?

Gives strong prerequisite guidance: the content object must follow the activity type schema obtained from create_activity's content_schema or get_activity_schema, with the emphatic 'Never guess it.' However, it does not contrast with update_activity or clarify when to prefer append vs replace beyond the schema enum.

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