BCD — Bridge Control Delegate

app_decrement

Step a slider, stepper or other adjustable element of the latest app_snapshot down by one step. Returns the app's new state with the next snapshotId, so no extra snapshot is needed. If the window changed, or a person used the keyboard or mouse in the app since the snapshot, nothing is done and the error comes with the current state to retry from. A sent action is not proof the task succeeded. Needs ui and process permission (operating apps can run programs).

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "snapshotId": "<snapshotId>",
      "elementId": "<elementId>"
    },
    "name": "app_decrement"
  }
}

Input Schema

Name	Required	Description
`deviceId`	No	Computer name or id from list_devices. Optional with one computer online or a default set.
`takeOver`	No	Only when the user says so: act even though another conversation is operating this screen, which then holds it no longer.
`elementId`	Yes
`snapshotId`	Yes
`workSessionId`	No	Work session from create_work_session. Pass the same ID on every call in this conversation; omit for legacy unscoped work.

Schema Changelog

Changes observed during successful MCP inspections.

Changed1 schema field changedOct 23, 2026

addedInput schema / properties / takeOver

Added value: +{
+  "description": "Only when the user says so: act even though another conversation is operating this screen, which then holds it no longer.",
+  "type": "boolean"
+}

AddedOct 8, 2026

TDQS

A4.6/5.0

Behavior5/5

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

The description adds substantial behavior beyond the annotations: it explains that the return includes the app's new state and next snapshotId, that stale state causes no action and an error with current state for retry, that a sent action is not proof of success, and that ui and process permissions are required.

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

Conciseness5/5

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

The description is front-loaded with the core action, then efficiently layers return behavior, failure conditions, reliability caveat, and permissions. Every sentence contributes operational information without padding.

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

Completeness4/5

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

For a mutation tool with no output schema, the description covers purpose, return state, snapshot chaining, failure handling, and permission requirements well. It leaves optional parameters like takeOver and workSessionId to the schema, which is reasonable, though a brief mention of snapshot-bound element IDs could further complete the picture.

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?

The description clarifies the two required parameters by identifying elementId as the adjustable element (slider/stepper) and snapshotId as coming from the latest app_snapshot. The remaining optional parameters are documented in the schema itself, so overall semantic coverage is strong but not exhaustive in the description.

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?

The description uses a specific verb and resource: 'Step a slider, stepper or other adjustable element ... down by one step.' It also scopes the action to the latest app_snapshot, which clearly distinguishes it from app_increment, app_press, and app_select.

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?

It gives clear operating context by saying the tool acts on the latest app_snapshot and steps an adjustable element down by one. It does not explicitly name app_increment as the alternative for stepping up, so the when-to-use guidance is clear but not fully comparative.

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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