fcpkit_timeline_diff

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

Annotations already establish readOnly=true, destructive=false, closed-world, so the safety profile is covered. The description still adds real behavioral content beyond them: it discloses that items are paired by unique key (so both timelines must carry stable identifiers) and that deltas are reported in frames for start/end/source. It does not touch cost or output shape, but for a read-only comparison that gap is minor.

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?

A single front-loaded sentence opens with the core question ('What changed between two timelines') and then packs the comparison semantics and reported fields. Every clause carries information; nothing is padding.

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 must carry return-value expectations, and it does list the categories of changes reported. What is missing is the result's shape (a match/changed/added/removed grouping, units of the frame deltas), which an agent would have to discover by calling. For a 4-parameter read-only tool this is close to sufficient but not fully closed.

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 schema already documents 'a' as baseline, 'b' as changed, the allowed `kinds` values, and the `caption_pattern` regex. The description reinforces the asymmetric baseline-vs-changed framing via the build-vs-export example but adds no syntax or format detail beyond the schema. Baseline 3 applies.

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 (diff / 'what changed') and a specific resource (two timelines), then enumerates exactly which aspects are compared: unique-key matching, frame deltas of start/end/source, lane and text changes, added/removed items. No sibling tool (handles, filmstrip, effects_check, relink_preview) shares this operation, so an agent can route unambiguously.

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 parenthetical 'e.g. a generated build vs the editor's FCP export' gives a concrete scenario that tells the agent when this tool is the right pick. There is no explicit when-not guidance or named alternative, but the siblings are functionally distant enough that no exclusion is needed.

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