assert

Assert a natural language statement against the current page/screen.

SERVERAndroid SOURCE@midscene/android-mcp

Medium RISK CLASS

Category Write

Parameters 111 required

Recommended Rate-limitedsee the rule below

Registry record Grade F, identity unverified Pull the record →

This record as markdown: /tools/android/assert.md

WHAT IT DOES

What assert does on Android

AI agents use assert to create or update resources in Android, usually the action step of a workflow, after the agent has gathered context. Every call changes real data in your Android environment.

Parameter	Type	Required	Description
`image`	object	—	Reference image URL/path. Repeat --image for multiple images.
`prompt`	string	Yes	Natural language assertion to verify, e.g. "there is a login button visible"
`message`	string	—	Custom error message to throw when the assertion fails, e.g. "the login button should be visible".
`imageName`	object	—	Reference image name. Repeat --image-name; must align with --image order.
`android.deviceId`	string	—	Android device ID (from adb devices)
`android.useScrcpy`	boolean	—	Enable scrcpy accelerated screenshots
`android.aiActContext`	string	—	Background knowledge passed to aiAct. Default: no extra context.
`android.waitAfterAction`	number	—	Wait time in milliseconds after each action execution. Default: 300ms.
`convertHttpImage2Base64`	boolean \| string	—	If true, convert http(s) image URLs to base64 before sending to the model.
`android.replanningCycleLimit`	integer	—	Maximum number of replanning cycles for aiAct. Default: model adapter default.
`android.screenshotShrinkFactor`	number	—	Screenshot shrink factor before sending images to AI. Default: 1; high values may reduce recognition quality, especially on mobile.

Parameters from the server's own tool schema.

RISK

Why assert is rated Medium

An AI agent can call assert faster than any human can review: one bad instruction and it creates or modifies resources in Android by the hundred, each call as confident as the last.

Risk signalsHigh parameter count (11 properties)

Attacks that exploit this kind of access

RECOMMENDED RULE

The rule that runs assert safely

PolicyLayer is an MCP gateway: it sits between your AI agents and Android, and checks every tool call against a rule you set before the call runs. Nothing changes on the server itself. For assert, this is the rule to start with:

assert Rate-limited

assert stays usable, but capped: an agent stuck in a loop can't make hundreds of changes a minute. Everything else on the server is denied unless you say otherwise.

View as policy code

policy.json

{
  "version": "1",
  "default": "deny",
  "tools": {
    "assert": {
      "limits": [
        {
          "counter": "assert_rate",
          "window": "minute",
          "max": 30,
          "scope": "grant"
        }
      ]
    }
  }
}

LIMIT THIS TOOL → Instant setup, no code required.

The button opens the PolicyLayer dashboard: create your workspace, connect Android, apply this rule, and every assert call is checked against it from then on.

FAQ

Questions about assert

What does the assert tool do? +

Assert a natural language statement against the current page/screen. It is categorised as a Write tool in the Android MCP Server, which means it can create or modify data. Consider rate limits to prevent runaway writes.

What parameters does assert accept? +

assert accepts 11 parameters: image, prompt, message, imageName, android.deviceId, android.useScrcpy, android.aiActContext, android.waitAfterAction, convertHttpImage2Base64, android.replanningCycleLimit, android.screenshotShrinkFactor. Required: prompt. The full parameter table on this page comes from the server's own tool schema.

How do I enforce a policy on assert? +

Register the Android MCP server in PolicyLayer and add a rule for assert: allow, deny, rate-limit, or require approval. Point your MCP client at the PolicyLayer proxy URL and the rule is enforced on every call, before it reaches Android. Nothing to install.

What risk level is assert? +

assert is a Write tool with medium risk. Write tools should be rate-limited to prevent accidental bulk modifications.

Can I rate-limit assert? +

Yes. Add a rate_limit block to the assert rule in your PolicyLayer policy. For example, setting max: 10 and window: 60 limits the tool to 10 calls per minute. Rate limits are tracked per agent session and reset automatically.

How do I block assert completely? +

Set action: deny in the PolicyLayer policy for assert. The AI agent will receive a policy violation error and cannot call the tool. You can also include a reason field to explain why the tool is blocked.

What MCP server provides assert? +

assert is provided by the Android MCP server (@midscene/android-mcp). PolicyLayer sits as a proxy in front of this server to enforce policies before tool calls reach the server.

KEEP EXPLORING

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All 23 Android tools →

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