Cronometer MCP

edit_fast

Idempotent

Update a recorded fast by changing its start time, end time, or goal duration; also use it to close a fast that is still open.

Instructions

Change a recorded fast, including ending one that is still open.

Args: fast_id: The fast ID, from get_fasting_history. start: New start as YYYY-MM-DD HH:MM. end: New end as YYYY-MM-DD HH:MM. goal_hours: New 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": {
      "fast_id": "<fast_id>"
    },
    "name": "edit_fast"
  }
}

Input Schema

Name	Required	Description	Default
`end`	No
`start`	No
`fast_id`	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

A4.3/5.0

Behavior4/5

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

Beyond the annotations, the description reveals that the tool can close an open fast by setting an end value, and it clarifies that recorded fasts can be modified rather than just created/deleted. It does not contradict the idempotent/non-destructive hints, though it could have been clearer about whether omitted parameters leave existing values unchanged.

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 main behavior is front-loaded in the first sentence, followed by a compact Args list with no filler. Every sentence adds necessary information.

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 output schema exists and annotations cover safety hints, the description is mostly complete: it covers usage, all parameter formats, and special open-fast behavior. The only notable gap is that it does not state whether omitted optional fields are preserved or explicitly cleared, which would matter for an edit operation.

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

Parameters5/5

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

Schema description coverage is 0%, so the description carries the full burden. It explains every parameter: fast_id comes from get_fasting_history, start/end use a specific YYYY-MM-DD HH:MM format, and goal_hours is a target length in hours. This fully compensates for the sparse 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?

The description uses the specific verb 'Change' with the resource 'a recorded fast', and the phrase 'including ending one that is still open' adds a distinctive behavior that separates it from sibling tools like add_fast and delete_fast. It is immediately clear what operation the tool performs.

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 implies when to use the tool (modifying an existing fast) and even notes the edge case of ending an open fast, and it tells the agent to source fast_id from get_fasting_history. However, it does not explicitly state when not to use it or name the alternatives (e.g., add_fast for creation, delete_fast for removal).

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

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