variables_update

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

No annotations provided, so description carries full burden. It discloses read-modify-write semantics, field preservation, and parameter merging by key, plus required flags. No contradictions.

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: first packs core behavior (write, read-modify-write, merge), second adds prerequisites. Extremely efficient with no wasted words.

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?

The description covers the key behavior but omits details like return values (no output schema) and error handling. For a 10-param tool with nested schema, it is adequate but not comprehensive.

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 coverage is low (10%). Description only mentions parameter merging behavior. No explanation of accountId, containerId, workspaceId, variableId, or other fields. The schema is complex with nested parameters, but description adds minimal value beyond 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 starts with "[WRITE] Update a GTM variable" which clearly states the verb and resource. It also explains the read-modify-write behavior and parameter merging, distinguishing it from sibling tools like variables_create.

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?

Explicit prerequisites are given: "Requires GTM_MCP_ENABLE_WRITES=true and confirm=true." It implies use for updating existing variables, but does not explicitly contrast with creation (variables_create). Still clear enough.

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