ignition-mcp-server

write_tag

Writes a value to a tag on a live Ignition gateway so AI agents can update tag data; gateway writes must be enabled.

Instructions

Write a value to a tag on a live Ignition gateway.

DISABLED by default — requires the server to be started with --gateway-url AND --enable-writes. Writing to a live gateway can actuate real equipment. The value is sent as-is; the gateway handles type coercion.

Args: tag_path: Full tag path (e.g. "[default]Conveyors/Line1/Speed"). value: Value to write (string representation — gateway coerces to tag data type).

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>",
      "value": "<value>"
    },
    "name": "write_tag"
  }
}

Input Schema

Name	Required	Description	Default
`value`	Yes
`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

A4.5/5.0

Behavior4/5

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

With no annotations, the description carries the full burden and does well: it warns that writes can actuate real equipment, discloses the default-disabled state, and explains that the value is sent as-is with the gateway performing type coercion. It omits return/confirmation behavior, but an output schema exists to cover that.

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 safety warning is front-loaded, followed by the enablement condition, then the parameter notes. Every sentence adds information and there is no filler.

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?

For a two-parameter mutation tool with an output schema, the description covers the risky behaviors (live actuation, disabled by default), the value coercion semantics, and the path format. Nothing an agent needs to invoke it safely is missing.

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 the description must compensate, and it does: tag_path is documented with a concrete example ('[default]Conveyors/Line1/Speed') and value is explained as a string representation the gateway coerces to the tag's data type. Both parameters gain meaning beyond the bare string types in 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 ('Write a value to a tag on a live Ignition gateway') and scopes it to a live gateway, which cleanly distinguishes it from the read_tag sibling. An agent can identify the operation without opening the schema.

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 a clear precondition for use: the tool is DISABLED by default and requires --gateway-url and --enable-writes, so the agent knows when the call will fail. It does not explicitly route to alternatives (e.g., read_tag for inspection), but the gating condition is strong, actionable guidance.

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

Deploy Server