session_send_message

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

Adds context beyond annotations: states the operation is informational, does not disrupt sessions, is audited, and needs no confirm gate. This aligns with destructiveHint=false and readOnlyHint=false, and provides valuable operational details without contradicting the structured metadata.

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 well-structured: a purpose statement with an example, a rationale sentence, selection logic, and an Args list. Every sentence earns its place, and the format makes scanning easy without unnecessary verbosity.

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?

Provides a complete picture for a messaging tool: purpose, example, safety profile, selection criteria, and parameter meanings. No output schema exists, so no return-value documentation is required. Minor ambiguity remains about whether session_ids and user can be used simultaneously, but overall the description is sufficient for correct 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?

Schema description coverage is 0%, so the description carries full weight. It explains all 5 parameters in a dedicated Args block, adding semantics such as 'substring match' for user, 'INFO, WARNING, or ERROR' for message_type, and 'omit to use default' for target. This fully compensates for the missing schema descriptions.

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 sends a message to Horizon session(s), with a concrete example ('save your work, maintenance in 10 min'). This distinguishes it from sibling session tools like session_logoff or session_disconnect, as it is a messaging action rather than a control action.

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?

Provides clear context: 'Low blast radius (informational only, no session disruption), so no confirm gate.' This communicates ideal use cases and why no confirmation is needed. However, it does not explicitly name alternative tools or exclusions, so it stops short of a full when-to-use vs. when-not-to-use discussion.

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