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

Annotations indicate readOnlyHint=false and destructiveHint=false, so the mutation nature is already known. The description adds the behavioral detail that this tool avoids fetching the entire post, providing some insight into its efficiency. However, it does not disclose what happens with out-of-bounds indexes, whether the block is fully replaced or merged, or if any side effects occur, leaving moderate gaps for an AI agent.

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, with two sentences that front-load the core purpose and then add a valuable rationale for using this tool. Every word earns its place, and the structure is clear and scannable.

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 tool is simple, with only 3 parameters and a rich schema that fully documents the block types and constraints. An output schema exists, so return values need not be described. The description covers the primary use case and differentiates from update_post, though it could mention handling of invalid indexes or prerequisites, but these are minor for a well-specified replacement operation.

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 100%, with each parameter (postId, index, block) having a description in the schema. The tool description does not add meaning beyond the schema—it only reiterates the 0-based index detail. Since the schema carries the full burden, the baseline score of 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 action: 'Replace a single block at the given index (0-based).' This specifies the verb, resource, and scope, and it explicitly contrasts with update_post, making it distinguishable from sibling tools like append_post_block and remove_post_block.

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 usage context by noting that this tool 'Avoids fetching the full post and re-sending all blocks via update_post,' which tells the agent when to prefer it over update_post. However, it does not explicitly mention when not to use it or provide exclusions for other sibling tools, though the distinct operation names make this clear.

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