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

With annotations only declaring the safety profile (not read-only, not idempotent, not destructive), the description adds real context: the call is PROJECT-SCOPED, acts only on the explicit project_id (never the active-project pointer), returns the project identity with the result, and leaves everything outside the region untouched. This bounded, non-guessing mutation behavior is valuable beyond annotations. It still omits overlap/interaction behavior with existing effects.

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 scoping constraint is front-loaded in caps, and the sentence is dense with parameter-to-effect mappings rather than filler. It is somewhat run-on in the effect list, but every clause carries information.

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 non-idempotent mutation with 11 parameters, 18% schema coverage, and no output schema, the description covers scope, affected area, and effect-parameter pairings but omits defaults for non-required parameters, behavior on repeated/invalid regions, and interaction with existing region effects. Adequate but with clear gaps.

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 only 18% over 11 parameters, so the description must carry more weight. It does map key parameters to effects (colour for solarise/tint, ev for exposure, scale for heat) and gives the stage default plus the glyph:<id> region form, but strength, id, and the exact start/end semantics are left unexplained. Partial compensation, not full.

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: it adds exactly one effect inside a region over a start-end span, and it enumerates the supported effect values, which tells an agent precisely what this tool produces. It does not explicitly differentiate itself from near-neighbours like blur_region, isolate_colour or add_stylize, so it falls short of a 5.

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?

There is no when-to-use guidance and no named alternative. The only conditional content is scoping behavior ('outside the region the picture is untouched', 'stage defaults to pre_captions'), which is a behavioral trait, not a routing rule. An agent must infer when this beats siblings like blur_region or add_stylize.

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