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

Annotations already set readOnlyHint=true, openWorldHint=false, idempotentHint=true, destructiveHint=false. The description adds that the fetch is access-scoped when modelId is provided)Skip-duplicate of annotation. It also warns about result size limitsatch, which is not in annotations. This is valuable behavioral context beyond the structured fields.

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 sentences, front-loaded with the core operation and output format, then the parameter nuance and a practical size warning. No filler or repetition.

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?

The description covers what the tool returns (base64-encoded bytes), when to use each parameter, and the size limitation. With both input and output schemas present, plus clear annotations, nothing essential is missing for an agent to invoke it correctly.

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?

While the schema already documents both parameters with descriptions, the description adds meaning by explaining the distinction between direct store fetch and access-scoped fetch (via modelId), and hints at the size constraint. This goes beyond the schema's parameter descriptions, which are brief. Even with full schema coverage, the added context justifies a score above baseline.

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 exactly what the tool does ('Fetch a blob's bytes by id'), how it returns data ('base64-encoded in the result'), and clearly distinguishes the two fetch modes ('Pass 'modelId' to fetch a blob referenced by a specific model (access-scoped); omit it for a direct store fetch'). No ambiguity for an agent to select and use it.

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?

Explicitly explains when to include modelId vs. omit it, tying the parameter to access scoping. The size warning ('Large blobs may exceed the result-size limit — this channel suits small binaries only') gives practical usage constraint. Even though no alternative tool is named for large blobs, the guidance is actionable.

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