posthog-mcp-server

evaluate_feature_flag

Check if a feature flag is enabled for a specific user and retrieve the variant value.

Instructions

Evaluate a feature flag for a specific user. Returns whether the flag is enabled and its variant value.

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": {
      "flag_key": "<flag_key>",
      "distinct_id": "<distinct_id>"
    },
    "name": "evaluate_feature_flag"
  }
}

Input Schema

Name	Required	Description	Default
`flag_key`	Yes	The feature flag key to evaluate
`distinct_id`	Yes	The user's distinct_id to evaluate the flag for

Schema Changelog

Changes observed during successful MCP inspections.

First observedv1.0.0Aug 6, 2026

TDQS

A4.2/5.0

Behavior4/5

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

With no annotations provided, the description carries the transparency burden. It clearly states what the tool returns (enabled state and variant value), which is the core behavioral expectation. It implies a read-only operation via 'evaluate' and 'returns', though it does not explicitly discuss side effects, error conditions, or permissions. This is adequate for a simple evaluation tool.

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 and front-loaded: two sentences totaling about 20 words. It states the action and the return value without any redundancy or extraneous detail.

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 tool's simplicity, the description is complete enough. It explains the return value, and the schema fully documents both parameters. The context of sibling tools makes the tool's role clear. It lacks mention of edge cases (e.g., missing flag) but that is not critical for a basic evaluation function.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, so the baseline is 3. The description does not add any additional meaning beyond the schema, but the parameter names (flag_key, distinct_id) are self-explanatory, making the schema sufficient.

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 ('Evaluate') and resource ('feature flag for a specific user'), clearly distinguishing it from sibling tools like list_feature_flags which list flags without user context. It precisely states the scope and action.

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?

The phrase 'for a specific user' implies this tool is for evaluating a flag against a single user, contrasting with list_feature_flags. It provides clear contextual usage without explicitly naming alternatives or exclusions, but the purpose is evident and unambiguous.

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

Deploy Server