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

Annotations declare readOnlyHint=false and destructiveHint=false, which are consistent with the description calling it a WRITE operation. The description adds valuable behavioral nuance: the preview gate (confirm=false returns a preview without sending) is not derivable from annotations or schema. It clarifies the exact sequence an agent must follow to actually send, which is beyond what structured fields convey.

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, front-loaded with the core purpose, then efficiently explains the preview gate and key parameter usage. There is zero redundancy; every clause adds distinct value. It is highly concise without sacrificing necessary detail.

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 send operation with four parameters (all documented in schema), the description covers the essential usage context: the preview behavior, the source of the thread ID, and the group type flag. It does not describe the return format (success vs. preview content), but the annotations and low complexity make that a minor gap. Overall, it provides enough to call the tool correctly in most workflows.

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 each parameter, so the baseline is 3. The description adds meaningful semantics: it explains that `to` is a thread ID from a specific tool, that `type` should be set to 'group' for groups, and that `confirm` controls the preview vs. send behavior. These clarifications go beyond the schema's individual field descriptions and help an agent use parameters correctly.

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 states a specific action ('Sends a Zalo message to a conversation') and clearly distinguishes this tool from other messaging tools by naming the platform. It also explains the preview gate mechanism, which further clarifies the tool's unique behavior. This is unambiguous and context-rich.

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 explicitly instructs when to use the confirm parameter to preview versus send, and points to zalo_list_chats as the source for the `to` thread ID. It also explains how to specify group type. While it does not explicitly contrast with sibling send_message tools, the Zalo-specific context implicitly differentiates it, and the practical guidance on parameter usage is strong.

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