register_api_key

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

The description goes far beyond annotations by disclosing a security warning (plaintext transmission), encryption at rest, plan auto-detection, and prerequisites (OAuth 2.1, MCP_ENCRYPTION_KEY). It also notes that calling again updates the key (idempotent) and that rate limits and date-range restrictions are applied. This adds significant behavioral context beyond the annotations without any contradiction.

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?

The description is long but structured: purpose, security warning, behavior, requirements, and parameter definition. Each section adds necessary detail, and the most important information (purpose and security) is front-loaded. It could be slightly more concise, but the complexity justifies the length.

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?

The description covers all essential context: purpose, side effects (storage, plan detection), prerequisites (OAuth, encryption), and behavioral consequences (affected rate limits). The output schema is present, so return values need no description. For a setup tool with security implications, this is comprehensive.

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?

The description explains that api_key is 'Your J-Quants API key (refresh token from the J-Quants portal)', providing essential semantics beyond the schema's minimal title 'Api Key'. It clarifies what the parameter is and where to obtain it, fully covering the only parameter despite schema coverage of 0%.

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 clearly states the tool's purpose: 'Register or update your J-Quants API key'. This is a specific verb+resource combination, and it distinguishes from siblings like delete_api_key. The multi-user mode and OAuth association add context, making the tool's role unambiguous.

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?

The description explains that this tool is required for subsequent tool calls to work ('Subsequent tool calls will automatically use this key'), which implies when to use it. However, it does not explicitly mention alternatives or when not to use it, though the sibling delete_api_key exists. The context is clear, but explicit exclusions are missing.

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