start_session

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

With no annotations provided, the description carries full burden for behavioral disclosure. It discloses that the tool captures a snapshot of plan credits and wall-clock time, and returns session_id and starting credit balance. It does not mention error conditions or idempotency, but for a simple init tool, this is adequate.

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 at three sentences. The first states purpose, the second provides usage guidance, and the third explains behavior and return values. Every sentence earns its place with no redundancy or fluff.

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?

Given the tool's simplicity (no parameters, output schema exists), the description adequately covers purpose, usage, behavior, and return values. It might benefit from noting idempotency or error handling, but overall it is sufficiently complete for an agent.

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 tool has zero parameters, and schema coverage is 100% (trivially). The description adds value by explaining why the tool exists (to enable delta calculations for get_session_stats), which goes beyond the empty schema.

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: 'Start tracking a new chat session.' It uses a specific verb and resource, and explicitly distinguishes the tool from siblings by noting it should be called 'ONCE at the beginning of every chat session,' setting it apart from query tools like get_session_stats.

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: 'Call this ONCE at the beginning of every chat session.' It explains why (to capture snapshots for later deltas) but does not explicitly clarify when not to use it or compare directly to siblings. However, the context is sufficient for an agent to understand its role.

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