/platform-custom-metadata-type-generate

name: platform-custom-metadata-type-generate
description: "Use this skill when users need to create, generate, or validate Salesforce Custom Metadata Type metadata — the __mdt object, its fields, and its deployable records. Trigger when users mention custom metadata types, CMDT, __mdt objects, custom metadata records, .md-meta.xml files, or reference/config data that must ship between orgs. Also trigger for admin-maintained mapping, lookup, or crosswalk tables and field mappings (\"map fields from A to B\", code lookups) — config admins change without a code deploy belongs in a CMDT, never hardcoded in Apex or Flow — and when troubleshooting CMDT record/type deploy errors. DO NOT TRIGGER for Custom Settings — route to platform-custom-setting-generate; for regular custom objects holding business records — use platform-custom-object-generate; or for secrets and API keys — recommend a Named Credential, which this skill never generates."
metadata:
  version: "1.0"
  minApiVersion: "60.0"
  domains: ["Platform"]
  relatedSkills:
    - "platform-custom-field-generate"
    - "platform-custom-object-generate"
    - "platform-custom-setting-generate"
  cliTools:
    - tool: ["sf"]
      semver: ">=2.136.8"

Salesforce Custom Metadata Type Generator and Validator

When to Use This Skill

Use this skill when you need to:

• Create a Custom Metadata Type (`__mdt`)
• Generate or validate CMDT fields, including `MetadataRelationship` fields
• Generate CMDT **records** as deployable `.md-meta.xml` files
• Troubleshoot CMDT deployment errors

---

1. Overview and Purpose

A Custom Metadata Type produces **two separate artifact families**, and most requests need both:

| Artifact | Path | Metadata type | |---|---|---| | Type definition | `objects/<Name>__mdt/<Name>__mdt.object-meta.xml` | `CustomObject` | | Fields | `objects/<Name>__mdt/fields/<Field>__c.field-meta.xml` | `CustomField` | | Records | `customMetadata/<Name>.<Record>.md-meta.xml` | `CustomMetadata` |

**API name suffix:** `__mdt` on the type; fields still end in `__c`.

**The defining advantage over a custom setting: CMDT records are metadata and therefore deploy between orgs.** When a user says configuration should "ship with the package" or "be the same in every org," CMDT is the right answer.

> **The root element is `<CustomObject>`, but almost none of a custom object's rules apply.** `sharingModel`, > `nameField`, and `deploymentStatus` are required or normal on a regular custom object and are **hard > errors** here. Do not carry assumptions across from `platform-custom-object-generate`.

---

2. Syntactic Essentials — Type Definition (Tier 1)

Required and Allowed Elements

| Element | Requirement | Notes | |---------|-------------|-------| | `<label>` | **Required** | Singular UI name | | `<pluralLabel>` | **Required** | Omitting it gives `Must specify a non-empty plural label for the CustomObject` | | `<visibility>` | Always include | `Public`, or `Protected`/`PackageProtected` only in dev/sandbox/scratch (Section 6) | | `<description>` | Always include | What this type configures and who owns it |

`<pluralLabel>` being **required here but forbidden on a custom setting** is the most commonly inverted rule between the two families. Neither failure mentions the other family's rule.

Forbidden Elements

Every element below produces `Cannot specify: <element> for Custom Metadata Type` — reusing a regular custom object's skeleton (with `sharingModel`, `deploymentStatus`, a `nameField` block, or `enableSearch`) is the usual cause:

`sharingModel`, `nameField`, `deploymentStatus`, `enableActivities`, `enableReports`, `enableHistory`, `enableSearch`

**CORRECT** — minimum valid `__mdt` type:

<?xml version="1.0" encoding="UTF-8"?>
<CustomObject xmlns="http://soap.sforce.com/2006/04/metadata">
  <label>Partner Tier</label>
  <pluralLabel>Partner Tiers</pluralLabel>
  <description>Discount and threshold configuration per partner tier. Ships with the package; edited by the revenue ops team.</description>
  <visibility>Public</visibility>
</CustomObject>

---

3. Field Rules

**This skill owns the CMDT-specific deltas only** — the type allowlist, `fieldManageability`, and `MetadataRelationship` below. For generic field mechanics (`<fullName>` derivation, `<label>`, `<description>`, `<inlineHelpText>`, precision/scale, `<length>`, `visibleLines`), follow `platform-custom-field-generate`.

Supported Field Types

`Checkbox`, `Date`, `DateTime`, `Email`, `Number`, `Percent`, `Phone`, `Picklist`, `Text`, `TextArea`, `LongTextArea`, `Url`, plus `MetadataRelationship`.

Unsupported Field Types

`Currency`, `AutoNumber`, `MasterDetail`, `Summary`, `Location`, `Time`, `EncryptedText`, `Html`, `MultiselectPicklist`. Each fails with a precise, well-formed error:

Type {TypeName} of CustomMetadataField {Object}__mdt.{Field}__c is not supported for the Entity {Object}__mdt

(There is no trailing period.) Example: `Type Currency of CustomMetadataField Partner_Tier__mdt.Discount__c is not supported for the Entity Partner_Tier__mdt`

**Currency is the common trap** — it works on a custom setting but not here. Use `Number` with `<precision>`/`<scale>` and put the currency in the label or help text.

**Formula fields are unsupported**, but they break the pattern above. A `<formula>` element on an otherwise legal type gives only:

Invalid data type.

`Lookup` is silently coerced — CRITICAL

`<type>Lookup</type>` on a `__mdt` **does not fail** when its `referenceTo` resolves to a real sObject: the deploy is green and the platform silently rewrites the field to `MetadataRelationship`. A later retrieve shows a field the user never wrote:

<type>MetadataRelationship</type>   <!-- was deployed as Lookup -->

**Always write `MetadataRelationship` explicitly.** Emitting `Lookup` produces a green deploy and a source-vs-org mismatch that churns in git the first time anyone retrieves.