Subscribes once to a private message bus topic, invoking the callback for the next matching message in EasyEDA Pro. Use it to handle a single event without repeated listening.
sys_MessageBus.subscribeOnce(topic: string, callbackFn: (message: any) => void) -> ISYS_MessageBusTask 私有消息总线：订阅单次消息 returns: 消息总线任务
| Name | Required | Description | Default |
|---|---|---|---|
| args | No | 按官方签名顺序排列的JSON参数数组 | |
| windowId | No | 目标EDA窗口ID；省略时使用当前活动窗口 |
Changes observed during successful MCP inspections.
v2.0.0Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
With no annotations provided, the description carries the full burden. It does disclose the once-only delivery semantics, the private scope, and the ISYS_MessageBusTask return type. However, it never explains what the returned task is for (e.g., cancellation), how callbacks are delivered, or lifecycle implications of 'once'.
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 lines with zero padding, but the English signature, Chinese purpose line, and 'returns' line partially restate one another. The bilingual mixing ('returns: 消息总线任务') adds noise, though the overall length is appropriate.
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 tool with four message-bus subscribe siblings, no output schema, and no annotations, the description covers the core (purpose, args mapping, return type) but omits the meaning of the returned task, windowId usage guidance, and any behavior that would let an agent distinguish persistent vs once-only subscriptions beyond the name itself.
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?
Although schema coverage is 100%, the schema's args field is a generic array with empty item schemas ('按官方签名顺序排列的JSON参数数组'), giving no per-parameter meaning. The description's official signature — topic: string, callbackFn: (message: any) => void — is the only place an agent learns what belongs in args, adding real value beyond 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?
The description states a specific verb and resource: subscribing once on the private message bus ('订阅单次消息'), and the included signature clarifies topic + callback semantics. The '私有' (private) and '单次' (once) qualifiers differentiate it from sibling variants like subscribe_public and persistent subscribe, though not by name.
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?
No guidance is given on when to choose this tool over its near-siblings eda_sys_message_bus_subscribe, subscribe_public, or subscribe_once_public. The windowsId parameter's role in targeting a specific EDA window is also left entirely to the generic schema text, and no exclusions or prerequisites are mentioned.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.