Design LINE Flex Message

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

Beyond annotations (readOnlyHint, idempotentHint), the description adds rich behavioral context: it validates, never calls the LINE API, returns a structured response with preview_url, size_bytes, and warnings[], and warns about the 30KB bubble limit. It also notes Thai localization of templates. These details go well beyond what annotations provide.

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 well-structured and front-loaded with purpose, then modes, usage, return format, and an example. Every sentence contributes information without fluff. The length is justified by the tool's complexity, and the example aids comprehension without redundant detail.

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?

With no output schema, the description explicitly lists the return fields (flex_json, alt_text, preview_url, size_bytes, warnings[]) and notes the 30KB limit. It explains the two modes, provides integration steps with line_send_message, and includes a Thai-language example. The only omission—detailed required fields per template—is addressed by the list_templates mode, making the description complete for practical use.

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%, providing a baseline of 3, but the description adds significant value by grouping parameters into modes (template+data, flex_json+alt_text, list_templates), reinforcing mutual exclusivity, and giving a concrete example with template data fields. This helps the agent understand parameter combinations beyond the schema's individual descriptions.

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 opens with a specific verb+resource: 'Compose a validated Flex Message for line_send_message.' It clearly distinguishes itself from siblings like line_design_imagemap and line_design_card by focusing on Flex Messages, and explicitly states 'DESIGN ONLY — never calls the LINE API,' differentiating it from line_send_message.

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 provides explicit guidance on when to use each mode (template mode recommended, raw mode advanced, list_templates for catalog), and explains how to integrate with line_send_message via message.flex_json or direct template/data passthrough. The 'DESIGN ONLY' note clarifies that this tool is not for sending, addressing when-not-to-use.

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