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

Annotations indicate readOnlyHint=false and openWorldHint=true, and the description adds meaningful behavioral context: it resolves declared placeholders, renders through Fabric/SVG, and does not call an internal language or design model. This goes beyond the annotations by explaining the rendering pipeline and the absence of a design model, which helps the agent understand what happens when the tool is invoked.

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 description is two sentences with no wasted words. It front-loads the critical constraint (CODEX-ONLY) and the core behavior (resolves placeholders, renders through Fabric/SVG), then clarifies what it does not do. Every sentence earns its place.

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 tool with 4 parameters, 1 required, and no output schema, the description covers the essential behavioral context: what input is expected (complete SVG), what happens (placeholder resolution, rendering), and what is not involved (no internal design model). It doesn't describe return values or error conditions, but the absence of an output schema and the simplicity of the tool make this acceptable. The description is complete enough for an agent to select and 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?

Schema description coverage is 50%, with the 'svg' and 'preview' parameters described in the schema. The description adds context about the SVG being complete and Codex-authored, which reinforces the 'svg' parameter's meaning. However, it doesn't add detail about 'width' or 'height' beyond the schema's numeric constraints, and the description doesn't compensate for the undocumented parameters. Baseline 3 is appropriate.

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 clearly states the tool's purpose: it renders a Codex-authored finished design by resolving placeholders and rendering through Fabric/SVG. It explicitly distinguishes itself from other tools by noting it is CODEX-ONLY and that no internal language or design model is called, which differentiates it from siblings like picorn_edit_image or picorn_generate_image.

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 description provides clear context for when to use this tool: when Codex supplies the complete SVG. It doesn't explicitly name alternatives or exclusions, but the CODEX-ONLY marker and the contrast with 'no internal language or design model is called' imply it should be used instead of generation or editing tools. It could be stronger with explicit when-not-to-use guidance, but the context is clear.

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