list_capabilities

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

Annotations already declare readOnlyHint=true, and the description adds a key behavioral detail: 'Costs no Marvin API call.' This is beyond what annotations provide and informs the agent about resource usage. Does not contradict annotations. Slightly stronger than baseline because it discloses a practical cost aspect, though no further behavioral details are needed given the simple read-only nature.

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?

Three short sentences with no fluff. The primary purpose is front-loaded in the first sentence, usage guidance follows, and the caveat about other tools is concise. Every sentence earns its place and the description is highly scannable.

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 the tool has no parameters and an output schema exists, the description covers all essential aspects: what it lists, when to use it, its cost (free), and a pointer to per-tool details. Nothing an agent needs to decide to invoke it is missing.

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?

Tool has zero parameters, so schema coverage is trivially 100%. Baseline is 4 for no parameters. The description doesn't need to explain parameters; it does clarify the output nature ('every Marvin tool by area'), which adds value beyond the empty schema. No gaps.

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 ('List') and resource ('every Marvin tool by area') plus content details ('what it can and cannot do'). Clearly distinguishes itself from all sibling tools, which are operational tools, not a meta-listing tool. No ambiguity about its function.

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?

Explicitly prescribes when to call it: 'Call it before assuming something is impossible via MCP or requires manual work in the app.' Also gives a caveat that detailed live conditions are in each tool's own description, guiding the agent to consult the appropriate tool for specifics. Provides clear context and exclusions.

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