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

Beyond annotations (readOnlyHint, idempotentHint, destructiveHint), the description discloses error behavior: 'never raises a protocol error — it returns {"error": ...}'. It also explains the underlying mechanism ('nothing is decoded', 'bytes_inspected') and retry safety. No contradiction exists with annotations; in fact, the description reinforces them.

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 well structured: a one-line summary, a compact explanation with an example, usage guidance, error behavior, and safety note. Every sentence carries information; there is no fluff or repetition. The length is justified by the richness of the content, and it is front-loaded with the purpose.

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 one parameter, strong annotations, and an output schema, the description is fully complete. It covers input (base64 prefix), output (format, dimensions, derived values), error semantics, limitations (no pixel/EXIF), and retry behavior. There are no unanswered questions an agent would need to resolve before invoking.

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?

The input schema already provides 100% coverage for data_base64, including the prefix strategy and data-URL handling, so the description doesn't need to add much. It does add a concrete example ('{"data_base64": "iVBORw0KG..."}') and restates the prefix guidance in context, which slightly enhances usability without redundancy. A 4 is appropriate given the schema's strength.

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 opens with a specific verb+resource: 'Read an image's format and pixel size from its header alone.' This clearly distinguishes it from sibling tools like resize_plan (planning) and detection_metrics (metrics), and the 'header alone' scoping prevents overreach. The FREE note and supported formats further pin down its exact function.

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?

It explicitly states when to use: 'Use to find out what you are dealing with before planning a resize.' It also gives clear exclusions: 'Not for pixel content - nothing here reads pixels - and not for EXIF.' This tells the agent precisely when to invoke and when to avoid, and it hints at the resize_plan sibling without needing to name it.

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