> ## Documentation Index > Fetch the complete documentation index at: https://docs.mangrovesystems.com/llms.txt > Use this file to discover all available pages before exploring further. # Create a rule > Creates a data rule on the project. A rule either flags matching records with an alert, or corrects the matching value by working through a chain of substitution methods. Evaluation over the rule's historical range is queued as soon as the rule is saved, so results and corrections land shortly after this call returns rather than during it. Creating a substitute rule requires corrections to be enabled on the account. ## OpenAPI ````yaml POST /projects/{project_id}/rules openapi: 3.1.0 info: title: Mangrove - API version: 1.1.0 servers: - url: https://app.gomangrove.com/api/v1 security: - sec0: [] paths: /projects/{project_id}/rules: post: summary: Create a rule description: >- Creates a data rule on the project. A rule either flags matching records with an alert, or corrects the matching value by working through a chain of substitution methods. Evaluation over the rule's historical range is queued as soon as the rule is saved, so results and corrections land shortly after this call returns rather than during it. Creating a substitute rule requires corrections to be enabled on the account. operationId: create-project-rule parameters: - name: project_id in: path schema: type: string required: true requestBody: content: application/json: schema: type: object required: - rule properties: rule: $ref: '#/components/schemas/RuleInput' examples: Alert rule: value: rule: name: Negative Temp Alert target_type: data_point rule_text: '{{data-point.ambient-temperature-c}} < 0' alert_message: Ambient temperature below zero Substitute rule: value: rule: name: Fill ambient temperature dropouts target_type: data_point action: substitute rule_text: '{{data-point.ambient-temperature-c}} = 0' subject_slugs: - ambient-temperature-c method_chain: - method: trailing_average params: window: kind: rolling value: 7 unit: day - method: forward_fill responses: '201': description: '201' content: application/json: schema: $ref: '#/components/schemas/Rule' '400': description: '400' content: application/json: examples: Result: value: errors: - message: >- param is missing or the value is empty or invalid: rule '422': description: >- A duplicate rule repeats its detail as a `duplicate_rule` sibling of `errors` as well as inside the entry. Its `message` ends with the existing rule's effective period in parentheses, which the example below leaves off: read the period from `duplicate_rule` rather than parsing the text. content: application/json: examples: Missing method chain: value: errors: - message: >- Method chain must list at least one method for substitute rules field: method_chain code: must list at least one method for substitute rules Duplicate rule: value: errors: - message: >- A rule with the same expression already exists: 'Fill ambient temperature dropouts' field: rule_text code: duplicate_rule duplicate_rule: friendly_id: rule_Qb3k9TzdL0pWnXyz name: Fill ambient temperature dropouts effective_from: '2026-01-01' effective_to: null duplicate_rule: friendly_id: rule_Qb3k9TzdL0pWnXyz name: Fill ambient temperature dropouts effective_from: '2026-01-01' effective_to: null deprecated: false components: schemas: RuleInput: type: object description: >- The rule body on a create or update. On an update, send only the fields you are changing. properties: name: type: string target_type: type: string enum: - data_point - model rule_text: type: string action: type: string enum: - alert - substitute default: alert alert_message: type: string subject_slugs: type: array items: type: string description: >- A substitute rule takes exactly one slug. It has to appear in `rule_text`, and its data point type has to be numeric. method_chain: type: array description: >- Required on a substitute rule. Mangrove rejects a chain that repeats a method with the same parameters, or that places a method after a `constant`. items: $ref: '#/components/schemas/SubstitutionMethod' modeling_model_id: type: string description: Required on a model rule. Pass the model's friendly ID. effective_from: type: string format: date nullable: true effective_to: type: string format: date nullable: true Rule: type: object description: >- A data rule on a project. The rule's identity is stable across edits; the condition lives on its latest version. properties: id: type: string description: Rule friendly ID. example: rule_Qb3k9TzdL0pWnXyz name: type: string rule_type: type: string enum: - alert - substitution description: Derived from `action`. Read-only. action: type: string enum: - alert - substitute description: >- `alert` flags the matching record and changes nothing. `substitute` replaces the matching value with one derived from `method_chain`. Only data point rules may substitute. method_chain: type: array description: >- Ordered substitution candidates. The first method that resolves wins. Empty on alert rules. items: $ref: '#/components/schemas/SubstitutionMethod' target_type: type: string enum: - data_point - model description: >- `data_point` evaluates event data as it arrives. `model` evaluates model outputs after a batch calculation. status: type: string enum: - active enabled: type: boolean rule_text: type: string description: The condition, in the rule expression language. example: '{{data-point.ambient-temperature-c}} < 0' subject_slugs: type: array items: type: string description: >- On an alert rule, narrows which of the values the condition reads may be flagged. On a substitute rule, names the single value the rule corrects. alert_message: type: string nullable: true description: >- Shown on the records this rule flags. Required on an alert rule. Optional on a substitute rule, where it covers the records whose method chain resolved nothing, and Mangrove supplies a default. version: type: integer effective_from: type: string format: date nullable: true effective_to: type: string format: date nullable: true model_id: type: string nullable: true description: Model rules only. model_name: type: string nullable: true created_by: type: string created_at: type: string format: date-time updated_at: type: string format: date-time SubstitutionMethod: type: object description: One candidate in a substitute rule's method chain. properties: method: type: string enum: - constant - fallback_to_dpt - forward_fill - backward_fill - trailing_average - trailing_min - trailing_max description: >- `constant` uses a fixed number and always resolves, so no method may follow it. `fallback_to_dpt` copies another data point type on the same event. `forward_fill` and `backward_fill` borrow the nearest earlier or later valid reading. The three trailing methods take the average, lowest or highest reading across a window. params: type: object description: >- `constant` takes `value`. `fallback_to_dpt` takes `slug`. The trailing methods take `window`. The two fills take no parameters. properties: value: type: number description: >- `constant` only. Rejected when the value would itself match the rule's condition. slug: type: string description: >- `fallback_to_dpt` only. A data point type slug on the same event. window: type: object description: Trailing methods only. properties: kind: type: string enum: - rolling - calendar value: type: integer description: Required on a rolling window. Not used on a calendar window. unit: type: string description: >- Rolling windows take `second`, `minute`, `hour`, `day`, `week`, `month` or `year`. Calendar windows take `day`, `week`, `month`, `quarter` or `year`. securitySchemes: sec0: type: apiKey in: header name: Authorization x-bearer-format: '' ````