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

Annotations already declare readOnlyHint=true and openWorldHint=false, so the safety profile is covered. The description adds that the data is the publicly visible shared state and points to `agent_notes` for key semantics, which is useful context, but says nothing about auth, rate limits, or what happens with an unknown key.

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, zero filler, with the core action and the mode-selection rule front-loaded and the agent_notes cross-reference last. Every clause earns its place.

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 3-parameter, read-only tool with no output schema, the description covers what is read, how to select keys, and where to learn key meaning. It stops short of describing the shape of returned values (single value vs. key list), which matters slightly more given the absence of an output schema.

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 meaning beyond the terse schema strings: it establishes that `key` returns one value while `prefix` (or omission) triggers listing, and that omitting both is a valid listing call. That mode-selection semantics is not encoded anywhere in the schema.

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 uses a specific verb+resource ('Read the shared key/value data of a hosted charm') and clarifies scope with 'the same state its human visitors see'. It implicitly contrasts with the write_app_data sibling via the read/write pairing, but never names or distinguishes itself from get_app, which also concerns an app.

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 explains the three invocation modes (key for a single value, prefix to list a subset, nothing to list all), which is real operational guidance. However, it gives no when-to-use vs. alternatives guidance (e.g. when to read here versus get_app, or that writes must go through write_app_data).

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