Fill an input or textarea with the given value. ref is either an aria-ref token from browser.snapshot ('e7') OR a CSS selector ('input[name=email]'). Prefer the aria-ref token — it's stable and matches exactly what snapshot returned.
| Name | Required | Description | Default |
|---|---|---|---|
| ref | Yes | ||
| value | Yes | ||
| page_id | Yes | ||
| in_workspace | No | Run this one call in this workspace id instead of the session's. Nothing is stored; other sessions are not affected. |
Changes observed during successful MCP inspections.
Input schema / properties / in_workspaceAdded value: +{
+ "description": "Run this one call in this workspace id instead of the session's. Nothing is stored; other sessions are not affected.",
+ "type": "integer"
+}Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnly=true, idempotent=true, destructive=false and openWorld=false, so the safety profile is covered. The description adds useful evidence about ref stability and snapshot matching, but omits key fill behavior: whether an existing value is replaced or appended, and what happens on an invalid/stale ref.
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 tight sentences with the action front-loaded, followed by parameter disambiguation and a concrete recommendation. No filler, though the final preference clause slightly restates the format point already made.
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 4-parameter tool with no output schema and thin schema descriptions, the description covers the ambiguous parameter (ref) and the action clearly. Remaining gaps -- page_id meaning and value-replacement semantics -- are minor but real.
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% (only in_workspace is documented), so the description must compensate, and it does well for the hardest parameter: ref is explained as either an aria-ref token ('e7') or a CSS selector ('input[name=email]'). value is self-evident and page_id is left unexplained, keeping this below a 5.
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?
States a specific verb and resource: "Fill an input or textarea with the given value." An agent immediately knows this writes text into a form field. However, it never distinguishes itself from the very similar siblings browser_type and browser_fill_form, so the differentiation half of the criterion is unmet.
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?
The description gives a real decision rule, but only for parameter format ("Prefer the aria-ref token — it's stable"), not for tool selection. There is no guidance on when to choose browser_fill over browser_type or browser_fill_form, so usage is only implied by the stated purpose.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.
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