brain_session_start

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

Annotations already cover idempotency, destructiveness, and read-only hints. The description adds value by disclosing that this is a controlled lifecycle boundary and that the server only opens/closes agent tracer sessions on its own. This is behavioral context beyond what annotations provide, with no contradiction.

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?

Two sentences with no filler. The main action and trigger are front-loaded, and the second sentence earns its place by preventing incorrect automatic invocation.

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 output schema and annotations cover return values and safety traits, so those gaps are acceptable. However, the required project_key parameter is left unexplained, and the relationship between 'start/replay' and the sibling brain_session_resume is not clarified. This is a meaningful gap for an agent deciding which session tool to call.

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?

The description adds no parameter-level meaning. The schema describes client_key well, but project_key has no description and is required; the description does not compensate for that gap. With 50% schema coverage, the description should have provided at least some project/session context.

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 action ('Start or replay') and the resource ('a concurrent session'), plus the specific trigger ('from an explicit user command'). It does not explicitly distinguish itself from sibling brain_session_resume, and 'replay' vs 'resume' creates some potential overlap.

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 gives explicit when-to-use context: only from an explicit user command. It also gives a clear exclusion: no hook and no auto-close may invoke this lifecycle boundary. It does not name alternatives like brain_session_resume, but the trigger guidance is strong.

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