Design LINE Imagemap (Rich Message)

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

Annotations already mark this as read-only and idempotent, but the description adds substantial behavioral detail beyond that: 'never calls the LINE API,' returns a { message, usage_hint } shape, validates the output, forces canvas width to 1040, and explains the hosting behavior with LINE's lack of upload API. It fully discloses the tool's side-effect-free, design-only nature and its integration with image preparation.

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 moderately long but well-structured, opening with the core purpose, then hosting paths, constraints, return value, and examples. It front-loads the most critical info (design-only, never calls API) and includes two examples that are compact and illustrative. Some repetition with schema details exists (e.g., the /1040 size variants), but the examples and flow explanation justify 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?

This is a complex tool with two hosting modes, coordinate constraints, optional video, and integration with other tools. The description covers all of these, including the return value shape (not provided by any output schema), the recommended workflow via line_prepare_image, and the fact that LINE has no upload API. It is complete enough for an agent to select and invoke the tool correctly without missing context.

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 description coverage is 100%, so the baseline is 3. The description adds meaningful semantics beyond individual field docs by explaining the relationship between prepared_key and base_url/base_height, the two mutually exclusive hosting paths, and the coordinate system (all px on a 1040-wide canvas). The illustrative example shows how to combine alt_text, areas, and either prepared_key or base_url, giving the agent a concrete usage template.

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 states a specific verb and resource: 'Build a validated Imagemap message' and 'return it as JSON to hand to line_send_message via message.message_json.' It clearly distinguishes itself from siblings like line_send_message and line_design_flex by noting it only designs and never sends. This makes the tool's purpose unambiguous and distinct.

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 offers explicit guidance on when to use this tool vs alternatives: it says 'DESIGN ONLY — never calls the LINE API,' implying sending should go through line_send_message. It also gives a recommended vs self-hosted path, explicitly telling the agent to run line_prepare_image first when possible, and when to choose base_url instead. This is clear, actionable usage context.

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