invariance_session_attach_run

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

Annotations already indicate a non-read-only, non-destructive operation. The description adds value by specifying the PATCH semantics and the effect: linking the run's graph back to the session timeline for correlation. It does not contradict annotations and gives useful behavioral context beyond the structured data.

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, no fluff. The core action and HTTP method are front-loaded, followed by a precise usage trigger and outcome. Every sentence earns its place.

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 simple two-parameter attach operation, the description fully covers what the tool does, when to use it, and why it matters. Annotations cover safety and world-openness. No output schema exists, and the description provides enough context 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 100%, so the schema already documents both parameters as 'Run ID to attach' and 'Agent-session ID to update.' The description adds conceptual context (run's graph, session timeline) but does not provide additional format, syntax, or edge-case semantics. Baseline 3 is appropriate.

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?

Description uses a specific verb-resource pair ('Attach an existing run to an existing agent-session') and adds the HTTP method PATCH. It clearly distinguishes this from sibling session tools (like append_note) and run lifecycle tools by focusing on linking an existing run to an existing session.

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 states the use case: 'USE THIS when a Claude Code task that started a session later starts producing a run.' It provides clear context and rationale (linking the run's graph to the session timeline), though it does not name alternatives or explicitly state when not to use it. Since no sibling tool does this exact job, the exclusion is less critical.

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