ignition-mcp-server

read_tag

Fetch the current value of one or more live Ignition gateway tags by tag path; pass comma-separated paths to query multiple tags at once.

Instructions

Read the current value of one or more tags from a live Ignition gateway.

Requires the server to be started with --gateway-url pointing to an Ignition gateway with the WebDev module installed.

Args: tag_path: Tag path(s), comma-separated for multiple (e.g. "[default]Conveyors/Line1/Speed").

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": {
      "tag_path": "<tag_path>"
    },
    "name": "read_tag"
  }
}

Input Schema

Name	Required	Description	Default
`tag_path`	Yes

Output Schema

Name	Required	Description	Default
`result`	Yes

Schema Changelog

Changes observed during successful MCP inspections.

First observedv0.5.0Oct 20, 2026

TDQS

A3.8/5.0

Behavior2/5

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

With no annotations provided, the description carries full burden. It implies read-only via the verb 'read' but does not disclose behavioral traits such as error handling (e.g., if tag not found), authentication requirements, rate limits, or side effects. The description is minimal in covering expected behaviors beyond the basic operation.

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 concise: two sentences plus the parameter description. Every sentence serves a purpose (purpose, prerequisite, arg details). No wasteful words.

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?

Given the presence of an output schema (which likely describes return values), the description does not need to detail outputs. It covers the input and prerequisite adequately. However, it could briefly mention what the tool returns (e.g., current value), but that is reasonable to leave to the output schema.

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 coverage is 0%, so description must compensate. It explains the one parameter 'tag_path' clearly: it is a string, can include multiple tags comma-separated, and provides an example format. This adds significant value beyond the schema's type definition.

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 clearly states the action ('Read the current value') and the resource ('tags from a live Ignition gateway'). The verb 'read' is specific, and the resource 'tags' distinguishes it from sibling tools like write_tag (write), get_alarm (alarms), and get_history (history).

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

Usage Guidelines3/5

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

The description provides a necessary prerequisite (server started with --gateway-url and WebDev module). However, it does not explicitly guide when to use this tool versus alternatives (e.g., get_tags, write_tag). The sibling list exists but the description itself lacks explicit usage context or exclusions.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

Deploy Server