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

Annotations already declare readOnly/idempotent/non-destructive, and the description adds real value on top: the response shape {codes, addonAvailable}, the meaning of addonAvailable=false (set_api_source would be refused), and an instruction to relay that to the user. That is behavioral context the annotations cannot convey.

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?

Purpose is front-loaded in the first clause, then usage form, return shape, and caveat follow in a logical order. The nested parentheticals make it dense, but no sentence is 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 one-parameter read tool with no output schema, the description covers the return shape, the addon-availability edge case, and the optional-parameter default. Nothing an agent needs to call it correctly 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?

Schema coverage is 0% and the schema only says integer > 0, so the description must compensate — it does, stating project_id is OPTIONAL and is inferred for a single-project customer. Minor gap: it doesn't say what happens for multi-project customers beyond the inference.

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?

Names a specific verb and resource ("List the API source attribute codes registered on a project") and adds the exact usage form ("api_source.<code>") plus the sibling that populates it (set_api_source). An agent can distinguish this from list_ai_sources and list_source_attributes from the wording alone.

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?

"Use this to see which API source attributes a project has" gives clear context, and the addonAvailable note tells the agent what to do when the feature is unavailable. It stops short of explicitly contrasting with the nearest siblings (list_ai_sources, list_source_attributes), so no exclusion guidance.

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