pier_update_env_var

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

No annotations are provided, so the description carries the full behavioral burden. It does add useful behavioral context: 'Add or update' implies mutation and idempotent overwrite, and 'without affecting other variables' is a strong guarantee. However, it does not disclose side effects (e.g., whether changes trigger a restart), permissions, or what the response is, which leaves notable gaps for a write operation.

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, zero filler. The action and scope are front-loaded in the first sentence, and the usage routing is in the second. Every word 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 mutation tool with fully documented parameters, an explicit behavioral guarantee, and clear sibling differentiation, the description is nearly complete. It lacks an output schema and does not mention return values or prerequisites, but given the tool's simplicity, those omissions are less critical than for more complex tools.

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 schema already describes all three parameters at 100% coverage, so the description does not need to add parameter details. The tool description adds almost no parameter-specific meaning beyond the schema, so the baseline of 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?

The description states the specific action 'Add or update a single environment variable' with a clear resource and scope. It explicitly differentiates from the sibling pier_set_env_vars by emphasizing 'single' and 'without affecting other variables', so an agent can distinguish it immediately.

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?

It explicitly states 'Preferred over set_env_vars for individual changes', naming the alternative and the condition for choosing this tool. This is rare and very helpful.

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