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

Annotations indicate a write operation (readOnlyHint false) and non-destructive (destructiveHint false). The description adds little beyond that: it says 'replace' but does not disclose error handling, permissions, side effects, or return value. The output schema exists but is not shown; without it, behavior on index out of bounds or invalid block is unclear.

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: the first states the core action and index, the second provides a rationale for efficiency. No redundant or irrelevant information, perfectly front-loaded.

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 complex block schema, the description sufficiently covers the basic operation and key parameter (index). However, it lacks mention of error cases (e.g., out-of-bound index) and does not explain that the block must match one of the defined types. The output schema likely covers return values, but its absence in the provided info leaves a minor gap.

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% with descriptions for all three parameters. The description adds no new semantic information beyond what the schema already provides (e.g., '0-based index' is repeated). No additional detail on constructing the complex block parameter is given.

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 'Replace a single block at the given index (0-based)', specifying the exact action and resource (block). It distinguishes from the sibling update_post by highlighting efficiency, avoiding fetching the full post.

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 mentions avoiding update_post, providing a reason to use this tool. However, it does not explicitly compare to other siblings like append_post_block, remove_post_block, or reorder_post_blocks, nor does it state when not to use it.

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