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Open an isolated session

browser_session_open

Open an independent browsing session and get its sessionId and environment, with epoch 0, URL about:blank and runtime version/build belonging to that new session.

Instructions

Open an independent browsing session and get its sessionId and environment, with epoch 0, URL about:blank and runtime version/build belonging to that new session. Optional viewport, colorScheme and reducedMotion configure responsive/media QA before navigation; defaults are 1280×800, light, no-preference. Each session owns a private BrowserContext, cookie/storage jar, popup tree, observation counter and ids, so several sweeps run AT THE SAME TIME without invalidating or authenticating each other. Pass the returned sessionId on every call belonging to that sweep. Close it with browser_session_close when done.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
viewportNoCSS-pixel viewport. Width and height must be integers from 1 to 8192.
colorSchemeNoPreferred color scheme exposed to CSS and matchMedia.
reducedMotionNoPreferred reduced-motion setting exposed to CSS and matchMedia.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed3 schema fields changedv0.1.3
    • addedInput schema / properties / colorScheme
      Added value: +{
+  "description": "Preferred color scheme exposed to CSS and matchMedia.",
+  "enum": [
+    "light",
+    "dark",
+    "no-preference"
+  ],
+  "type": "string"
+}
    • addedInput schema / properties / reducedMotion
      Added value: +{
+  "description": "Preferred reduced-motion setting exposed to CSS and matchMedia.",
+  "enum": [
+    "reduce",
+    "no-preference"
+  ],
+  "type": "string"
+}
    • addedInput schema / properties / viewport
      Added value: +{
+  "description": "CSS-pixel viewport. Width and height must be integers from 1 to 8192.",
+  "properties": {
+    "height": {
+      "maximum": 8192,
+      "minimum": 1,
+      "type": "number"
+    },
+    "width": {
+      "maximum": 8192,
+      "minimum": 1,
+      "type": "number"
+    }
+  },
+  "required": [
+    "width",
+    "height"
+  ],
+  "type": "object"
+}
  2. First observedv0.1.1

TDQS

A4.6/5.0
Behavior5/5

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

Annotations only declare the safety profile (non-read-only, non-destructive, non-idempotent, closed-world); the description adds substantial state semantics the annotations cannot convey: each session owns a private BrowserContext, cookie/storage jar, popup tree, observation counter and ids, so concurrent sweeps do not invalidate or re-authenticate each other. It also documents the initial state (epoch 0, about:blank) and the parameter defaults, which is real value beyond structured fields.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Front-loaded with purpose and return contract, then lifecycle guidance, with no filler sentences. The middle sentence is dense with parallel state details, but each clause carries distinct behavioral information rather than padding.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

No output schema exists, yet the description fully characterizes the return payload (sessionId, environment, epoch, initial URL, runtime build). Combined with parameter defaults, concurrency semantics, and the explicit close path, an agent has everything needed to open, use, and tear down a session.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters4/5

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, but the description goes further by supplying the defaults (1280×800, light, no-preference) and the intent behind the settings (pre-navigation responsive/media QA). It does not restate the 1–8192 bounds or enum values, but the added defaults and rationale exceed the schema.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb and resource ("Open an independent browsing session") and immediately declares the return contract (sessionId, environment, epoch 0, about:blank, runtime version/build). The emphasis on independent/isolated context clearly separates it from browser_open, browser_session_close and browser_session_list.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines4/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

Gives explicit downstream instructions: pass the returned sessionId on every call in the sweep, and close with browser_session_close when done, which names the terminal sibling. It also scopes the optional viewport/colorScheme/reducedMotion params to pre-navigation responsive/media QA. It does not explicitly contrast when to prefer an isolated session over browser_open, which keeps it out of 5 territory.

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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