Ogen

Discover a source's fields

discover_fields

See what a bound source exposes, so you can name keys against real fields. Reads the source once, live, and returns field names and types — never the values. Record; free on the demo keys.

Input Schema

TableJSON Schema

Name	Required	Description	Default
`key`	Yes	A bound key — its live source is read once to discover its fields.
`field`	No	For a plain JSON endpoint: a dotted path to look under, e.g. "data" — lists the fields at that path instead of the whole body. Defaults to the field the key was bound with, if any.

Schema Changelog

Changes observed during successful MCP inspections.

Changed1 schema field changedSep 25, 2026

addedInput schema / properties / field

Added value: +{
+  "description": "For a plain JSON endpoint: a dotted path to look under, e.g. \"data\" — lists the fields at that path instead of the whole body. Defaults to the field the key was bound with, if any.",
+  "type": "string"
+}

First observedSep 24, 2026

TDQS

A4.1/5.0

Behavior4/5

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

With no annotations, the description carries the behavioral burden and covers it well: the operation is a one-time live read, returns schema information, and 'never the values,' implying a safe introspection behavior. It does not elaborate on error conditions or authentication, but the core read-only profile is disclosed.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is compact and front-loaded, opening with the purpose and then the behavioral guarantee. 'Record; free on the demo keys' is terse and somewhat cryptic, which prevents a 5.

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?

For a simple two-parameter introspection tool with no output schema, the description adequately explains the output shape ('field names and types') and the privacy guarantee ('never the values'). It could add failure behavior or a concrete example, but nothing essential for correct invocation is missing.

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 schema fully documents 'key' and 'field'. The description adds the live-read and field-names-only context, but it does not add meaningful semantics beyond what the parameter descriptions already state.

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 names a specific verb ('see'), a specific resource ('a bound source'), and the outcome ('field names and types'), while explicitly scoping the tool to discovery rather than values. This distinguishes it from siblings like recall_live_fact or bind_source, which either return values or establish bindings.

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?

It frames when to use the tool: 'so you can name keys against real fields,' and clarifies that it reads the source 'live' once. It does not explicitly name alternatives or exclusion conditions, so it stops short of a full when/when-not statement.

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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