session_send_message

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

The description adds significant behavioral context beyond the annotations: it explicitly notes 'Low blast radius (informational only, no session disruption)' and 'no confirm gate', which is not visible in the annotations. It also discloses that the action is audited. This enriches understanding beyond the readOnlyHint/destructiveHint flags.

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: a lead sentence with an example, a short safety note, targeting guidance, and a clean args list. Every sentence adds value without redundancy.

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 description provides sufficient context for a tool with 5 parameters and no output schema. It covers purpose, targeting, safety, and parameter meanings. Minor gaps exist (e.g., behavior when both session_ids and user are provided, or error handling), but these are not critical for basic invocation.

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?

All five parameters are explicitly explained in the description: message content, session_ids targeting, user substring match, message_type options, and target with default behavior. Since the schema has no descriptions (0% coverage), the description fully compensates by providing clear semantics for every parameter.

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 purpose: 'Send a message to Horizon session(s)' with a concrete example. It distinguishes itself from sibling tools like session_logoff or session_disconnect by specifying it's informational only and non-disruptive.

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 context for when to use the tool: it's for informational messages with low blast radius and no confirm gate. It also explains how to target sessions (by session_ids OR user), which implies it's the appropriate choice for communication rather than disruptive actions. However, it doesn't explicitly name alternative tools to avoid or contrast with.

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