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

Annotations already declare the mutation safety profile (readOnlyHint=false, destructiveHint=true, idempotentHint=true, openWorldHint=true). The description adds a genuinely non-obvious behavioral constraint: sending a sprite while a custom icon image is set returns an error. It stops short of stating whether omitted fields are preserved or cleared on update.

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 with no filler, and the error constraint is stated compactly. It opens by restating the title 'Update Source', which is wasted space, but the remainder is tightly written.

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 an output schema present, return values need not be explained, and all five parameters are fully documented in the schema. The description covers purpose plus one edge-case error; the remaining gap is partial-update/null-clearing behavior and any permission requirements, which are partially covered by the schema's 'omit, send null, or whitespace-only to clear' note.

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 id, name, slug, color, and sprite with formats and length limits. The description only echoes the field list ('display name, slug, color, or sprite class') without adding format, null, or clearing semantics beyond what the schema provides, so the baseline of 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?

States a specific verb (update) and resource (an existing review source) and enumerates the mutable fields (display name, slug, color, sprite class). It does not explicitly differentiate itself from the similarly named sibling updateReviewSource, which changes a review's source rather than the source record itself, so the agent must infer the distinction.

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?

'For an existing review source' implies the precondition that the record already exists and must be updated rather than created, and the sprite/icon error note gives a concrete misuse case. There is no explicit when-not guidance or pointer to alternatives such as createSource, deleteSource, or updateReviewSource.

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