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

Goes well beyond the annotations (readOnlyHint=false, openWorldHint=true, destructiveHint=false) by disclosing concrete behavioral traits: a 16KB value cap, propagation latency ('humans watching the app see the change within a few seconds'), and a hard rate limit ('30 writes a minute per charm'). It also discloses that the optional delete flag removes the key, which is exactly the kind of side effect an agent needs.

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?

Front-loads the core action and size cap, then adds usage, the delete alternative, and the rate-limit rationale. Every sentence earns its place with 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 no output schema, the description covers the size limit, propagation delay, rate limit, delete behavior, and the agent_notes convention, so an agent has everything needed to call it correctly without expecting a documented return shape.

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 over the schema: it supplies the 16KB size bound that the schema omits for `value`, and clarifies that `delete: true` is an alternative mode rather than a setting on the value.

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 precise verb+resource ('Set one key in a hosted charm's shared data') and immediately scopes it (any JSON value, max 16KB), clearly distinguishing it from siblings like read_app_data, app_data_history, and rollback_app_data. An agent can tell what this does without opening the schema.

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?

Explains the intended use cases (agents play, paint, vote, leave things) and instructs the agent to 'Follow the app's `agent_notes`', plus the in-tool alternative 'Pass `delete: true` to remove the key instead'. It doesn't explicitly name a sibling read tool as the counterpart, but the when-to-use context is clear.

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