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

With no annotations at all, the description carries the full behavioral burden, and it delivers: it explains cloning, style/band/weld inheritance, word-gap text division, welded-line source-timeline behavior, the return shape, and the recommended follow-up operations. This is unusually transparent for a mutation tool.

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 tight and front-loaded with the core action, then layers necessary edge-case behavior, return value, and follow-up steps. Every sentence carries distinct information and the formatting keeps the complex rules readable.

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 tool with no output schema and no annotations, the description provides enough detail to invoke it correctly: input constraints, exact split behavior, clone properties, edge cases, return structure, and next steps. Nothing essential for correct invocation 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 100%, so the baseline is 3, but the description adds useful meaning beyond the schema: it maps atFrame to the editor playhead and explains the resulting split behavior for single-word lines and welded lines. The schema already covers 'strictly inside' and projectId semantics, so the added value is real but modest.

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 names a specific verb ('Split'), a precise resource ('one caption line'), and a distinctive condition ('at a COMPOSITION frame strictly inside its window'). It clearly separates this from siblings like merge_caption_lines by describing the resulting two halves and the follow-up tools.

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 gives clear operational context: the split point must be strictly inside the line's window, it describes the playhead mapping, and it covers edge cases like single-word lines and welded lines. However, it does not explicitly contrast this with alternatives or state when not to use it, so it stops short of full exclusion guidance.

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