> ## Documentation Index > Fetch the complete documentation index at: https://docs.prelude.so/llms.txt > Use this file to discover all available pages before exploring further. # List the signals a rule can read > **Beta.** The request and response shapes may still change. List the Prelude signals a rule expression may reference. A signal is a name Prelude resolves for the evaluation being scored, and it means the same thing for every customer. Write it in an expression and compare it like any other value of its type. Prelude provides these names. The rest of a rule's vocabulary is yours - the `attr.` names a recipe declares and the `param.` names it is configured with - and those do not appear here. A signal may be absent from an evaluation, whatever its type. A rule that reads a signal that did not arrive reports `NOT_EVALUATED` rather than treating the missing value as a zero. Prelude publishes the catalog progressively, so this listing grows over time. Write rules against what it returns: a signal it does not list is one Prelude has not committed to, and it can change or be removed without notice. A signal is a name Prelude resolves for the evaluation being scored: a fact about the request, the device, or the number that Prelude determines for you. Every customer gets the same signals, and each one means the same thing for all of them. ## Your own values are separate A rule can also read the `attr.` names a recipe declares and the `param.` names it is configured with. Those belong to you, they exist only in the recipe that declares them, and they are not listed here. ## An absent signal is not a zero Any signal can be missing from a given evaluation. A rule that reads a signal that did not arrive reports `NOT_EVALUATED` instead of scoring. It does not read the absence as `false`, as `0`, or as an empty string. Write the condition you mean, and let the engine report the absence. ## The listing grows Prelude publishes the catalog progressively, so this listing grows over time. Treat it as the supported vocabulary: a signal it does not list is one Prelude has not committed to, so read it rather than keeping a copy. ## OpenAPI ````yaml get /v2/watch/management/rules/catalog openapi: 3.1.0 info: title: Prelude API version: 2.0.0 description: The Prelude API allows you to send messages to your users. contact: email: support@prelude.so servers: - url: https://api.prelude.dev description: Production server security: - apiToken: [] tags: - name: Notify description: Send transactional and marketing messages with compliance enforcement. - name: Transactional description: Send transactional messages (deprecated - use Notify API instead). - name: Verify description: Verify phone numbers. - name: Watch description: Evaluate email addresses and phone numbers for trustworthiness. - name: Lookup description: >- Retrieve detailed information about a phone number including carrier data, line type, and portability status. paths: /v2/watch/management/rules/catalog: get: tags: - Watch summary: List signals description: >- **Beta.** The request and response shapes may still change. List the Prelude signals a rule expression may reference. A signal is a name Prelude resolves for the evaluation being scored, and it means the same thing for every customer. Write it in an expression and compare it like any other value of its type. Prelude provides these names. The rest of a rule's vocabulary is yours - the `attr.` names a recipe declares and the `param.` names it is configured with - and those do not appear here. A signal may be absent from an evaluation, whatever its type. A rule that reads a signal that did not arrive reports `NOT_EVALUATED` rather than treating the missing value as a zero. Prelude publishes the catalog progressively, so this listing grows over time. Write rules against what it returns: a signal it does not list is one Prelude has not committed to, and it can change or be removed without notice. operationId: listWatchSignals parameters: - name: limit in: query required: false schema: type: integer minimum: 1 maximum: 200 default: 50 description: >- Maximum number of signals to return per page. A larger value is clamped to 200 rather than refused. - name: cursor in: query required: false schema: type: string description: Pagination cursor from the previous response. responses: '200': description: OK content: application/json: schema: $ref: '#/components/schemas/WatchSignalListResponse' '400': description: KO content: application/json: schema: $ref: '#/components/schemas/Error' components: schemas: WatchSignalListResponse: type: object properties: signals: type: array items: $ref: '#/components/schemas/WatchSignal' next_cursor: type: string description: >- Pass this back as `cursor` for the next page. Absent on the last page. examples: - hardware_manufacturer required: - signals Error: type: object properties: code: type: string description: The error code. examples: - invalid_phone_number message: type: string examples: - >- The provided phone number is invalid. Provide a valid E.164 phone number. description: A human-readable message describing the error. type: type: string examples: - bad_request description: The error type. request_id: type: string examples: - 3d19215e-2991-4a05-a41a-527314e6ff6a description: >- A string that identifies this specific request. Report it back to us to help us diagnose your issues. param: type: string examples: - phone_number description: The parameter the error refers to, when it applies to a single one. required: - code - message - type WatchSignal: type: object properties: name: type: string description: The identifier to write in a rule expression. examples: - ip type: type: string enum: - bool - int - double - string description: The CEL type the signal compares as. examples: - bool description: type: string description: What the signal means. examples: - >- The end user's IP address: the one sent with the evaluation request, or the one the Prelude SDK collected. required: - name - type - description securitySchemes: apiToken: type: http scheme: bearer ````