add_image_overlay

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

With no annotations, the description carries the full burden. It adds meaningful behavioral context: default handling for opacity (uses image's alpha if None), start/end time behavior (full length if None), and position format options. However, it does not mention potential side effects like file overwriting or format constraints, but the return type and parameter defaults are well disclosed.

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: a front-loaded one-sentence summary, then a clean Args section with each parameter on its own line, followed by a Returns note. Every line adds necessary information without redundancy; the length is justified by the number of parameters.

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's moderate complexity (9 params, no annotations, minimal schema), the description is fairly complete. It covers all parameter behaviors and returns a status message, and an output schema exists for return details. Minor gaps include lack of explicit format support or overwrite behavior, but these are not critical for typical usage.

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 0%, so the description fully compensates. It explains each parameter in detail: position options including custom coordinate syntax, opacity range and default behavior, time formats, and width/height expression examples (e.g., 'iw*0.1'). This goes far beyond the basic types and defaults in the schema.

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 starts with a clear, specific verb phrase: 'Adds an image overlay (watermark/logo) to a video.' This identifies the action, the target resource (video), and the object (image overlay), distinguishing it from siblings like add_text_overlay and add_subtitles.

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 implies usage—use this when you need to overlay an image—but it does not explicitly compare with alternatives or state when not to use it. No exclusions or sibling references are provided, so the guidance is inferred rather than direct.

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