session_evaluate

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

Annotations already declare destructiveHint=true and readOnlyHint=false, so the safety profile is covered. The description adds useful behavioral detail beyond that: return value formatting (JSON, strings, 'undefined'), and the critical warning that a trailing statement returns nothing useful. No contradiction with annotations.

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?

The description is compact, front-loads the core action, then provides usage context, return semantics, and a practical code-writing tip. Every sentence adds value without repetition or fluff.

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 an arbitrary JavaScript execution tool with no output schema, the description sufficiently covers what the tool does, when to use it, how to format code, and what return values look like. Combined with the annotation's destructive hint and the schema's tabId documentation, nothing essential is missing for correct invocation.

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 100%, so the baseline is 3. The description adds extra meaning by explaining completion-value behavior and advising the agent to end with the expression it wants back, reinforcing and extending the schema's parameter description.

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 opens with a specific verb and resource: 'Run JavaScript in the page's own context and return its completion value.' It also differentiates itself from siblings by giving concrete use cases (computed styles, walking data structures, conditions too specific for session_wait), which lets an agent distinguish it from other session tools.

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?

Explicitly says 'Use it for what the other tools do not cover' and lists example scenarios, directly naming session_wait as an alternative for certain checks. This gives clear when-to-use guidance, though it lacks an explicit 'when not to use' list for other siblings.

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