Writes a single localStorage key-value pair in the current browser page, replacing any existing value for that key to persist session data or test app state.
Write one localStorage value in the current page context. An existing value under the same key is replaced.
| Name | Required | Description | Default |
|---|---|---|---|
| key | Yes | Storage key to write | |
| value | Yes | Value to store | |
| session_id | Yes | Session ID returned by browser_navigate |
Changes observed during successful MCP inspections.
v2.15.1Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare readOnlyHint=false, destructiveHint=true, and openWorldHint=true. The description adds genuinely new context beyond those flags by stating that an existing value under the same key is replaced, which clarifies the overwrite semantics an agent needs before calling.
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 short sentences, front-loaded with the operation and followed by the one behavioral caveat. No filler or redundancy.
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 three-parameter write tool with full schema coverage and annotations carrying the safety profile, the description covers purpose and overwrite behavior. Only the session/prerequisite context is left implicit, which is minor.
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%, with all three parameters documented in the schema itself. The description adds no syntax, format, or constraint detail beyond what the schema provides, so the baseline of 3 applies.
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 (write) and resource (localStorage value) with scope (current page context). This cleanly separates it from browser_get_local_storage, browser_clear_local_storage, and browser_set_cookies without needing to name them.
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?
Usage is implied by 'current page context' and the session_id parameter, but the description never states when to prefer this over alternatives like browser_evaluate or browser_set_cookies, nor any prerequisite such as requiring an active navigated session.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.