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

Annotations already declare readOnlyHint=true, so the agent knows this is safe. The description adds valuable behavioral context beyond that: it states 'UTC only — no timezone conversion' and warns that 'An unsupported strftime format silently falls back to the default format instead of erroring.' This discloses failure modes and constraints, which is exactly the kind of context that helps an agent avoid surprises. No contradiction with annotations.

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 concise and well-structured. It front-loads the core purpose in the first sentence, then provides 'Use when' guidance, 'Limitations,' and 'Alternatives' in separate labeled sections. Every sentence earns its place; there is no fluff or redundancy. The structure makes it easy for an agent to quickly extract the key decision points.

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?

Given the tool's simplicity (one optional parameter, no nested objects) and the presence of an output schema (indicated but not shown), the description is fully complete. It covers purpose, usage conditions, exclusions, limitations, and alternatives. There is no missing information an agent would need to correctly invoke the tool.

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% (the format parameter has a description), so the baseline is 3. The description adds semantic value by explaining the parameter is a strftime format string, reiterating the default behavior, and specifically noting the silent fallback on unsupported formats. This goes beyond the schema's terse description and helps the agent choose an appropriate format.

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 function: 'Return the current UTC date and time as a formatted string and ISO 8601 timestamp.' This is a specific verb (return) with a precise resource (current UTC datetime), and it distinguishes itself from siblings by focusing on datetime retrieval. The added purpose of 'grounding time-relative reasoning' clarifies its intent without ambiguity.

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 'Use when' section explicitly enumerates scenarios (computing ages, windows, deadlines, 'as of today') and also provides a critical exclusion: 'The runtime context block already states the date, so call this only when a specific strftime format or a precise timestamp is needed.' This tells the agent exactly when NOT to call it, and it lists alternatives (calculate, get_block) for comparison. The guidance is actionable and prevents unnecessary calls.

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