plc_update_variable

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

Beyond the annotations (readOnly=false, destructive=false, idempotent=false), the description discloses that changes are 'staged' rather than applied immediately and that a bound variable must be unbound first — non-obvious operational constraints. It does not explain the transaction/revision lifecycle (that a transaction_id and matching expected_revision are required and a commit step follows), which is the remaining behavioral gap.

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?

Three short sentences, each carrying distinct information (what can be staged, the unbind prerequisite, the sibling routing), with no filler. The opening is a fragment that reads awkwardly ('Stage data_type, initial_value, retained or comment fields'), but it is front-loaded and efficient.

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?

An output schema exists, so return values need no explanation, and the destructive/read-only profile is covered by annotations. Still, for a tool with four required parameters in a transactional workflow, the description never mentions that a transaction must be begun and committed or how expected_revision guard failures behave, leaving real gaps.

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 25%, so the description must carry weight; it does add meaning for the nested 'changes' object by enumerating four legal keys (data_type, initial_value, retained, comment), which a free-form additionalProperties object would not otherwise reveal. However, transaction_id and variable_id are unexplained, and the optimistic-concurrency meaning of expected_revision is left to the schema's one line.

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 pairs a specific verb ('Stage') with a concrete set of updatable fields (data_type, initial_value, retained, comment) and explicitly distinguishes itself from plc_rename_variable and bind_io/unbind_io. The resource itself ('variable') is only implied via the tool name, so it stops short of a fully self-contained statement of what is being modified.

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 names two concrete alternatives with the conditions that select them (renames go to plc_rename_variable, addresses go to bind_io/unbind_io) and states a prerequisite ordering: unbind a variable before changing its type. An agent has enough to route correctly without inspecting sibling schemas.

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