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

Goes well beyond the annotations: it discloses the auth context (the user is already signed in), the side-effect on the user's tabs (URL goes to the site's existing tab, else a new one, so existing tabs survive), and what is returned ('the elements and text in view, with the same numbers'). The readOnlyHint=true is mildly at odds with launching a session and possibly a new tab, but since nothing is destroyed this is a nuance rather than a contradiction.

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, front-loaded with the action and the 'Call it first' rule, then tab behavior, then the return value — no filler. The final phrase 'with the same numbers' is terse to the point of being cryptic unless the agent already knows the numbering convention from sibling tools.

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?

With no output schema, the description correctly compensates by summarizing the returned page view (elements and text, shared numbering). It is nearly complete for a two-parameter session-start tool; only the opaque 'same numbers' reference and the absence of any failure/blocked-site note keep it from a 5.

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 100%, so both parameters are already documented and this is the baseline case. The description adds only tab-targeting behavior for `url` and says nothing extra about `app`, whose 'leave it out' guidance lives entirely in the 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?

States a specific verb and resource ('Starts the user's Cloud Browser ... and opens `url`') and pins down the operating environment ('a real Chrome on Cotlet, where the user is signed in'). The imperative 'Call it first' plus the tab-allocation rule separates it from siblings like cotlet_browser_navigate or cotlet_browser_tabs without requiring the agent to open those schemas.

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 tells the agent it must be called first, which is the key sequencing rule for the whole browser tool family. It stops short of stating when NOT to use it (e.g., that cotlet_browser_navigate or cotlet_browser_tabs should be used once a session already exists), leaving that inference to the agent.

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