open

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

Annotations already declare not-read-only, open-world, non-destructive, so the bar is lower; the description still adds genuine context — profile defaults vs explicit overrides, the exactly-one-live-match constraint for selectors, workspace-relative replay paths, and the session_id return. It omits that the returned session is a stateful handle needing `close`, which keeps it from a 5.

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?

Front-loaded with the core action, then three dense sentences covering override semantics, selector matching, and replay. No filler, though the selector clause is packed enough to slow parsing slightly.

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?

With no output schema, the description correctly states the return value (session_id) and covers the three distinct invocation modes. Missing only failure behavior when no selector match exists and whether channel settings are mandatory without a device profile.

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 only 43% (baud, parity, data_bits, stop_bits are undocumented), so the description has to compensate. It does: the profile-supplies-defaults/arguments-override rule explains every optional parameter, and the selector and replay notations add meaning beyond the schema for `port`.

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?

States a specific verb and resource ('Open a serial port session'), which unambiguously separates it from close/send/list_ports. It never names a sibling or explicitly contrasts itself, so it lands at clear-but-undifferentiated rather than a 5.

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?

Gives real conditional guidance: `device` supplies defaults, explicit args override, `port` may be omitted only when the profile declares a selector, and `replay:<capture path>` triggers replay mode. It stops short of stating when-not-to-use or naming alternatives (e.g. close/read), so no 5.

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