_substitute_vars

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

The description adds a genuinely useful behavioral detail beyond the annotation: unresolved {{key}} placeholders are left as-is rather than erroring or being blanked. Since the only annotation is destructiveHint=true, the description's disclosure of missing-key handling is valuable, but it says nothing about mutation semantics, error modes, or the destructive implication.

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 tight sentences, front-loaded with the core transformation and followed by the single most important edge case. Every clause earns its place with no padding.

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 two-parameter pure helper, the description covers the transformation and the fallback behavior, and an output schema exists so return format needn't be explained. Minor gaps around value types and quoting are tolerable given the tool's simplicity.

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?

With 0% schema description coverage, the description must carry the parameter burden, and it does explain the key relationship: {{key}} in text maps to variables[key]. This clarifies the nested variables object's role as a string-to-string lookup, though it doesn't spell out edge cases like non-string values.

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 gives a specific verb ('Replace') and precisely states the transformation: substituting {{key}} placeholders with values from variables. This is unambiguous about what the tool does. It does not, however, differentiate itself from the sibling tools, which are all file/config/test operations.

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?

There is no guidance on when to use this tool versus alternatives, no preconditions, and no mention of the surrounding workflow (e.g., template rendering for configs or files). The behavior is described, but the context for invoking it is not.

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