Cronometer MCP

add_fast

Idempotent

Record a fasting period in Cronometer by entering its start time, optional end time, and target duration; omit the end time to log an ongoing fast.

Instructions

Record a fast.

Args: start: When the fast started, as YYYY-MM-DD HH:MM. end: When it ended, same format. Omit for an ongoing fast. goal_hours: Target length in hours.

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": {
      "start": "<start>"
    },
    "name": "add_fast"
  }
}

Input Schema

Name	Required	Description	Default
`end`	No
`start`	Yes
`goal_hours`	No

Output Schema

Name	Required	Description	Default
`result`	Yes

Schema Changelog

Changes observed during successful MCP inspections.

First observedv2.1.0Nov 14, 2026

TDQS

A3.5/5.0

Behavior3/5

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

Annotations already indicate readOnlyHint=false, idempotentHint=true, and destructiveHint=false. The description adds the ongoing-fast behavior by stating that omitting 'end' records an ongoing fast, which is useful context beyond the schema's null default. It does not contradict the annotations.

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 compact and well-organized, leading with the action and following with a clear parameter list. Every sentence adds necessary information and there is no filler.

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?

The description covers parameter formats and the key ongoing-fast nuance, and an output schema exists to describe return values. Minor gaps remain, such as timezone handling and behavior if an ongoing fast already exists, but it is adequate for a simple creation tool.

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?

With 0% schema description coverage, the Args section fully compensates by explaining all three parameters: the date-time format for start/end, the omission semantics for an ongoing fast, and goal_hours as target length. This adds real meaning beyond the raw schema, though goal_hours remains somewhat terse.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose4/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description opens with 'Record a fast,' which is a specific verb and resource. It clearly identifies this as creating/adding a fast entry, though it does not explicitly differentiate from siblings like edit_fast or delete_fast.

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

Usage Guidelines2/5

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

No guidance is given about when to use this tool versus edit_fast, delete_fast, or get_fasting_history. The intended use is only implied by the phrase 'Record a fast,' with no explicit when-to-use or alternative conditions.

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

Deploy Server