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

Annotations already declare destructiveHint=true, idempotentHint=false and openWorldHint=false, so the safety profile is covered. The description does add genuinely new context: the insertion lands at a 1-based position (default end) and the plan's photo limit applies. It does not say what happens to existing shots at that position or what error occurs when the plan limit is exceeded — the two behaviors an agent most needs for a destructive mutation.

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?

Two short sentences, front-loaded with the action and the constraint that defines eligibility; the default-position detail is placed where it is needed. Nothing is padded, though the parenthetical partially duplicates the schema's photoIds description.

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?

An output schema exists, so return values need no explanation, and all three parameters are fully described in the schema. Combined with annotations covering the destructive/non-idempotent profile, the description supplies the remaining tool-specific constraints (source eligibility, default position, plan limit) an agent needs to call it.

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 both positional semantics (1-based index, default end) are already documented in the schema and the description largely restates them. The one piece of added meaning is the plan-level photo limit, which the schema's maxItems=75 does not convey. Baseline 3 is correct when the schema carries the parameter burden.

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 uses a concrete verb and resource ("put library photos ... into the video") and narrows scope with "listing photos not in the video," which separates it from generic photo-adding siblings. It stops short of naming the nearest alternative (add_listing_photos/add_photos), so the agent must infer the boundary.

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?

Usage is only implied: the parenthetical tells the agent the photos must come from the library and must not already be in the video, which is a real selection condition. There is no explicit when-to-use/when-not guidance and no pointer to a sibling when the source photos are not already in the listing library.

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