AWS CloudFormation Validate

Validate AWS CloudFormation templates from JavaScript or TypeScript and catch schema violations, semantic errors, security risks, and best-practice findings before deployment - in your editor, build, service, or CI.

• Offline - all rules and CloudFormation resource schemas are bundled; nothing is fetched at runtime and no AWS credentials are needed.
• Fast - engines and schemas compile once and are reused across validations; typical templates validate in under a second.
• Self-contained - the WebAssembly module is bundled in the package; there are no native dependencies.

All types are exported from the @aws/cloudformation-validate package.

Installation

Available on npm as @aws/cloudformation-validate.

npm install @aws/cloudformation-validate

Requires Node.js 20 or later. The package has no runtime dependencies.

Quick start

Engines, models, and validators hold off-heap memory - call .free() when done with each object:

import { CompositeEngine, TemplateFile } from "@aws/cloudformation-validate";

const engine = new CompositeEngine();
try {
    const report = engine.validateTemplate(new TemplateFile("template.yaml"));
    for (const d of report.diagnostics) {
        console.log(`[${d.severity}] ${d.ruleId}: ${d.message}`);
    }
} finally {
    engine.free();
}

Each diagnostic identifies the rule, severity, affected entity and property, and source location - see Diagnostic. A complete, runnable project is in examples.

Engines are expensive to construct (rules compile once) and cheap to reuse - create one engine and validate many templates. Every fallible call throws on failure - the thrown value is the error message string from the validation core, so handle it with catch (error) { String(error) }; internal panics are caught at the WASM boundary and surface the same way, never a process abort. version() returns the version of the bundled validation core.

A template is passed as a Template - either a TemplateFile, which wraps a filesystem path the engine reads, or a TemplateContent, which carries template text or bytes already in memory so nothing is read from disk. The file path or the content name labels the report and its diagnostics; an omitted TemplateContent name defaults to "template" (DEFAULT_TEMPLATE_NAME).

type Template = TemplateFile | TemplateContent;

class TemplateFile {
    constructor(path: string);                                   // template read from disk
}

class TemplateContent {
    constructor(content: string | Uint8Array, name?: string);    // in-memory UTF-8 text or raw bytes
}

const fromDisk = engine.validateTemplate(new TemplateFile("template.yaml"));
const fromMemory = engine.validateTemplate(new TemplateContent("Resources: {}", "inline.yaml"));

TemplateModel and SchemaValidator.validate accept the same Template union.

Engine

RegoEngine and CelEngine both implement the Engine interface and are interchangeable - they produce identical diagnostics for the same template and config. CompositeEngine implements the same interface and layers custom Rego, CEL, and Guard rules on top of the built-in rules - see CompositeEngine.

Engine interface

interface Engine {
    validateTemplate(template: Template, config?: ValidateConfig): ValidationReport;
    validateAwsCliCommand(request: AwsCliCommand): AwsCliCommandValidation;
    listRules(): RuleInfo[];
    engineName(): string;
    free(): void;
}

| Method | Returns | Description | |---------------------------------------|---------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | validateTemplate(template, config?) | ValidationReport | Validates the template and returns a report. config.detailLevel (default DETAILED) selects how much per-diagnostic context is populated: DETAILED adds documentation URLs, rule descriptions, phase tags, and ViolationContext; STANDARD leaves those enrichment fields absent | | validateAwsCliCommand(request) | AwsCliCommandValidation | Models an AWS CLI command as CloudFormation resource state and validates it - see AWS CLI command validation | | listRules() | RuleInfo[] | Returns metadata for every built-in and loaded custom rule | | engineName() | string | "rego", "cel", or "composite" | | free() | void | Releases the engine's off-heap memory; the engine must not be used afterwards |

EngineConfig

Passed to the constructor. All fields are optional: the rule lists default to empty and an omitted schemaValidatorConfig uses only the bundled schemas.

interface EngineConfig {
    customRules?: RuleSource[];                      // engine-native rules (Rego for RegoEngine, CEL for CelEngine)
    guardRules?: RuleSource[];                       // CloudFormation Guard DSL rules - evaluated by the Guard evaluator
    schemaValidatorConfig?: SchemaValidatorConfig;   // additional resource provider schemas
}

interface SchemaValidatorConfig {
    additionalSchemas?: SchemaSource[];   // resource provider schemas merged over the bundled schemas
}

type RuleSource = ExternalRuleSource | RuleFile;
type SchemaSource = AdditionalSchemaSource | SchemaFile;

interface ExternalRuleSource {
    name: string;     // identifier shown in diagnostics (e.g. file path)
    content: string;  // full rule source text
}

interface AdditionalSchemaSource {
    typeName?: string; // omit to use the typeName inside the schema JSON
    schema: string;    // complete resource provider schema JSON
}

class RuleFile {
    constructor(path: string);                    // rule file read from disk; the path becomes the rule source name
}

class SchemaFile {
    constructor(path: string, typeName?: string); // schema file; typeName defaults to the value inside the JSON
}

| Field | Default | Description | |-------------------------|-------------|-----------------------------------------------------------------------------------------------| | customRules | [] | Engine-native rules: Rego source for RegoEngine, CEL JSON for CelEngine | | guardRules | [] | CloudFormation Guard DSL rules, evaluated by the Guard evaluator identically in every engine | | schemaValidatorConfig | undefined | Optional SchemaValidatorConfig whose additionalSchemas are merged over the bundled schemas |

Each rule is an ExternalRuleSource - name identifies the rule in diagnostics and content is the full rule source text. Pass a RuleFile to load one from disk (the same pattern as TemplateFile for templates), or an ExternalRuleSource when you already have the rule text in memory. Each additional schema is an AdditionalSchemaSource - a complete resource provider schema JSON plus an optional typeName that may be omitted when the schema JSON contains its own typeName; SchemaFile loads one from disk. Additional schemas extend the bundled schemas or register resource types CloudFormation has not published yet; a malformed, contradictory, or unsupported schema fails engine construction rather than silently weakening validation. Guard rules are evaluated by the CloudFormation Guard evaluator itself against the template as written, so every engine reports exactly what cfn-guard validate reports; a Guard file that does not parse also fails engine construction. The two forms can be mixed freely:

import { CelEngine, RuleFile, SchemaFile } from "@aws/cloudformation-validate";

const engine = new CelEngine({
    customRules: [new RuleFile("rules/s3_encryption.json")],
    guardRules: [new RuleFile("rules/compliance.guard")],
    schemaValidatorConfig: {
        additionalSchemas: [new SchemaFile("schemas/aws-lambda-function.json")],
    },
});

See Custom Rules for the Rego, CEL, and Guard rule formats and Additional Resource Provider Schemas for the schema merge model.

CompositeEngine

CompositeEngine implements the same Engine interface but takes a CompositeEngineConfig. It evaluates every built-in rule with a fixed built-in CEL evaluator and layers the caller-supplied custom rules on top: custom CEL and Guard rules run alongside that built-in engine, while custom Rego rules run in a separate external engine that is constructed only when Rego rules are supplied. With no custom rules it produces the same built-in diagnostics as RegoEngine and CelEngine, and engineName() returns "composite". Because the composite fixes which engine owns the built-ins, the config has no customRules field - it carries only the custom rules layered on top:

interface CompositeEngineConfig {
    regoRules?: RuleSource[];                        // custom Rego rules, run by the external engine
    celRules?: RuleSource[];                         // custom CEL rules, run by the built-in engine
    guardRules?: RuleSource[];                       // CloudFormation Guard DSL rules, evaluated alongside the built-in engine
    schemaValidatorConfig?: SchemaValidatorConfig;   // additional resource provider schemas, observed by both inner engines
}

| Field | Default | Description | |-------------------------|-------------|--------------------------------------------------------------------------------------------------| | regoRules | [] | Custom Rego rules layered on top of the built-in rules, run by the external engine | | celRules | [] | Custom CEL rules layered on top of the built-in rules, run by the built-in engine | | guardRules | [] | CloudFormation Guard DSL rules layered on top of the built-in rules, evaluated alongside the built-in engine | | schemaValidatorConfig | undefined | Optional SchemaValidatorConfig, observed by both inner engines |

import { CompositeEngine, RuleFile, TemplateFile } from "@aws/cloudformation-validate";

const engine = new CompositeEngine({
    regoRules: [new RuleFile("rules/s3_naming.rego")],
    guardRules: [new RuleFile("rules/compliance.guard")],
});
try {
    const report = engine.validateTemplate(new TemplateFile("template.yaml"));
} finally {
    engine.free();
}

ValidateConfig

Controls filtering, detail, severity, parameter overrides, and behavior for one validation call. All fields have defaults - omitting the config or passing {} uses them.

const report = engine.validateTemplate(new TemplateFile("template.yaml"), {
    exclude: { ids: ["I1002"] },
    severityLevel: "WARN",
});

interface ValidateConfig {
    include?: RuleFilterConfig;
    exclude?: RuleFilterConfig;
    detailLevel?: DetailLevel;
    severityLevel?: Severity;
    parameterOverrides?: Record<string, string>;
    pseudoParameterOverrides?: PseudoParameterOverrides;
    strict?: boolean;
    disableBuiltinRules?: boolean;
}

| Field | Default | Description | |----------------------------|-------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | include | {} (all rules) | When set, only matching rules produce diagnostics. Empty means include everything. | | exclude | {} (nothing excluded) | Matching rules are suppressed. Applied after include. | | detailLevel | "DETAILED" | Per-diagnostic context. "DETAILED" populates documentation URLs, rule descriptions, phase tags, and ViolationContext; "STANDARD" leaves those enrichment fields absent. | | severityLevel | "INFO" | Minimum severity threshold. Diagnostics below this level are dropped. Values: DEBUG, INFO, WARN, ERROR, FATAL. | | parameterOverrides | {} | Override template parameter values during resolution. Keys are parameter logical IDs. | | pseudoParameterOverrides | all undefined | Override CloudFormation pseudo-parameters (AWS::AccountId, AWS::Region, etc.). | | strict | false | When true, WARN-severity diagnostics are upgraded to ERROR. | | disableBuiltinRules | false | When true, all built-in rules (schema validation, Step Functions, engine rules) are skipped; only custom and Guard rules are evaluated. |

RuleFilterConfig

Both include and exclude use this structure. All fields are additive - a rule matches if it hits any criterion.

interface RuleFilterConfig {
    ids?: string[];                       // exact rule IDs, e.g. ["E3012", "W3010"]
    categories?: string[];                // category names, e.g. ["security", "best_practices"]
    idRanges?: IdRange[];                 // numeric ranges, e.g. { prefix: "E", start: 3000, end: 3099 }
    idPatterns?: string[];                // regex patterns matched against rule IDs
    resourceIds?: ResourceIdFilter[];     // a rule (or every rule) on a logical resource ID
    logicalIds?: LogicalIdFilter[];       // a rule (or every rule) on a named template entity
    resourceTypes?: ResourceTypeFilter[]; // a rule (or every rule) on a resource type
    services?: ServiceFilter[];           // a rule (or every rule) on a service, e.g. "AWS::AutoScaling"
}

// resourceIds / logicalIds / resourceTypes / services each carry an optional ruleId:
// set it to scope the filter to one rule, or omit it for every rule on the target.
interface ResourceIdFilter   { ruleId?: string; resourceId: string; }
interface LogicalIdFilter    { ruleId?: string; logicalId: string; entityType?: EntityType; }
interface ResourceTypeFilter { ruleId?: string; resourceType: string; }
interface ServiceFilter      { ruleId?: string; service: string; }

The service is matched verbatim against the service-provider::service-name prefix of the resource type - its first two ::-delimited segments (e.g. AWS::AutoScaling in AWS::AutoScaling::LaunchConfiguration).

The resourceIds dimension matches only diagnostics attributed to a resource; logicalIds additionally matches diagnostics on parameters, outputs, mappings, conditions, and template rules (for resource diagnostics the two carry the same value). An optional entityType scopes a LogicalIdFilter to entities of one type, so MyThing as a "Parameter" is matched without touching a same-named entity of another type.

PseudoParameterOverrides

Override CloudFormation pseudo-parameters used during intrinsic function resolution. All fields are optional - when undefined, the engine uses built-in defaults (e.g. region defaults to us-east-1).

interface PseudoParameterOverrides {
    accountId?: string;         // AWS::AccountId
    notificationArns?: string;  // AWS::NotificationARNs
    partition?: string;         // AWS::Partition
    region?: string;            // AWS::Region (default: "us-east-1")
    stackId?: string;           // AWS::StackId
    stackName?: string;         // AWS::StackName
    urlSuffix?: string;         // AWS::URLSuffix
}

TemplateModel

Parses a template into the resolved SemanticModel for direct inspection - the same model the engines evaluate rules against.

const model = new TemplateModel(new TemplateFile("template.yaml"));
const inMemory = new TemplateModel(new TemplateContent("Resources: {}"));

| Method | Returns | Description | |------------------------|------------------------------------|-------------------------------------------------------------------------------------------------| | resources() | Record<string, ResolvedResource> | All resources with resolved property values | | parameters() | Record<string, ParameterInfo> | Parameter definitions with types, defaults, constraints | | outputs() | Record<string, ResolvedOutput> | Outputs with resolved values and export names | | conditions() | string[] | Condition names defined in the template | | transforms() | string[] | Transform declarations (e.g. AWS::Serverless-2016-10-31) | | formatVersion() | string \| undefined | AWSTemplateFormatVersion value | | description() | string \| undefined | Template description | | toDiagnosticModel() | DiagnosticModel | Full diagnostic model including reference graph, condition implications, and resolution sources | | sourceLocation(path) | SourceSpan \| null | Source line/column span for a JSON path (e.g. Resources/MyBucket/Properties/BucketName) | | free() | void | Releases the model's off-heap memory; the model must not be used afterwards |

SchemaValidator

Runs schema validation independently from the rule engines. Checks each resource against the compiled CloudFormation provider schemas and produces FATAL-severity diagnostics for structural violations. The optional constructor argument is the same SchemaValidatorConfig accepted by EngineConfig; omitting it uses only the bundled schemas.

const validator = new SchemaValidator();
try {
    const diagnostics = validator.validate(new TemplateFile("template.yaml"), "us-east-1");
} finally {
    validator.free();
}

| Method | Returns | Description | |--------------------------------|-------------------|------------------------------------------------------------------------------------------------------------------| | new SchemaValidator(config?) | SchemaValidator | Constructs a validator; an omitted SchemaValidatorConfig uses only the bundled schemas | | validate(template, region?) | Diagnostic[] | Schema diagnostics for a Template at STANDARD detail - the enrichment fields are absent. region defaults to "us-east-1". | | listRules() | RuleInfo[] | Schema rule metadata | | schemaCount() | number | Number of compiled provider schemas | | free() | void | Releases the validator's off-heap memory; the validator must not be used afterwards |

AWS CLI command validation

validateAwsCliCommand models an AWS CLI (or SDK) API call as CloudFormation resource state and validates it offline before it is sent. It classifies the operation, maps it to a CloudFormation resource type through a closed, generated adapter catalog, synthesizes a template from the supplied parameters, and runs the normal template pipeline on it. A TemplateBody parameter of a CloudFormation operation is validated as-is. Any request that cannot be modeled exactly - an unregistered operation, a parameter without a lossless property mapping, or a value outside a CloudFormation constraint the API itself does not enforce - is skipped with a reason, never guessed.

import { AwsCliCommand, CompositeEngine } from "@aws/cloudformation-validate";

const engine = new CompositeEngine();
try {
    const request = new AwsCliCommand("s3", "CreateBucket", { Bucket: "example-bucket" });
    const validation = engine.validateAwsCliCommand(request);
    if (validation.status === "VALIDATED") {
        for (const d of validation.report!.diagnostics) {
            console.log(`[${d.severity}] ${d.ruleId}: ${d.message}`);
        }
    } else {
        console.log(`skipped (${validation.operationKind}): ${validation.reason}`);
    }
} finally {
    engine.free();
}

class AwsCliCommand {
    constructor(
        serviceName: string,                 // canonical botocore service name, e.g. "s3" or "cloudformation"
        operationName: string,               // API operation name, e.g. "CreateBucket"
        parameters: Record<string, unknown>, // request parameters (a plain object with string keys)
        options?: AwsCliCommandOptions,
    );
}

interface AwsCliCommandOptions {
    servicePrefix?: string; // signing prefix; context only
    httpMethod?: string;    // classification hint ("GET"/"HEAD"/"DELETE") for unrecognized verbs
    isReadOnly?: boolean;   // true classifies the operation as READ_ONLY
}

• serviceName is matched case-insensitively. Signing names, endpoint aliases, and ARN prefixes are never resolved; translate an SDK's service identity first.
• parameters accepts nested plain objects and arrays, strings, numbers, bigint, booleans, null, Uint8Array, and Date (serialized as ISO 8601). Any other value is carried as an explicit unsupported marker, and because synthesis is all-or-nothing the request is then skipped with a reason naming the offending parameter - no parameter is ever silently dropped.

The result is an AwsCliCommandValidation:

| Field | Description | |------------------|-------------------------------------------------------------------------------------------------------------------------------------------------------------------| | operationKind | AwsCliOperationKind: "READ_ONLY", "CLOUD_FORMATION_CREATE", "CLOUD_FORMATION_UPDATE", "CLOUD_FORMATION_DELETE", "DATA_PLANE_MUTATION", or "UNMAPPED_MUTATION" | | status | AwsCliCommandValidationStatus: "VALIDATED" when the modeled template ran through the pipeline, "SKIPPED" otherwise | | templateSource | AwsCliTemplateSource \| null: "TEMPLATE_BODY", "CLOUD_CONTROL_DESIRED_STATE", "SYNTHESIZED_CREATE", or "SYNTHESIZED_UPDATE"; null when skipped | | resourceTypes | string[] - CloudFormation resource types the operation maps to | | reason | string - why the request was validated or skipped | | report | ValidationReport \| null - present when "VALIDATED". The configuration is fixed: STANDARD detail level and a WARN severity floor | | template | Uint8Array \| null - the exact template bytes that were validated (the caller's TemplateBody unchanged, or the synthesized JSON); null when skipped |

The full contract - the adapter catalog, all-or-nothing mapping, and which rules are dropped for synthesized state - is documented in validation-engine/API.md.

Report Types

ValidationReport

validateTemplate always returns a ValidationReport - a template syntax failure is returned as a report with status "ERROR" and an F1101 diagnostic; only infrastructure or engine failures throw:

interface ValidationReport {
    filePath: string;
    status: "OK" | "ANALYSIS_INCOMPLETE" | "ERROR"; // ERROR is a pipeline failure; ANALYSIS_INCOMPLETE may omit findings
    version: string;
    metadata: ReportMetadata;
    performance: PerformanceMetrics;
    diagnostics: Diagnostic[];
}

Every finding is a Diagnostic (see Diagnostic). Its enrichment fields - documentationUrl, ruleDescription, phase (PARSE | SCHEMA | LINT), and context (ViolationContext with actualValue, expectedConstraint, resolutionSource, etc.) - are populated only at detailLevel "DETAILED" (the default); validating at "STANDARD" leaves them absent, keeping the base diagnostic fields.

metadata carries the summary counts, the number of suppressed diagnostics, the resources scanned and rules evaluated, the strict flag and severity threshold used, and optional budget-exhaustion records. Each budget-exhaustion record retains a stable machine-readable kind and also includes a human-readable description sentence, the numeric limit, and whether that specific exhaustion makes analysis incomplete. requiredPropertyCombinations is context-only, so its analysisIncomplete value is false and the report can remain "OK".

Diagnostic

interface Diagnostic {
    ruleId: string;                    // e.g. "E3012", "F1001", "W3010"
    severity: Severity;                // "FATAL" | "ERROR" | "WARN" | "INFO" | "DEBUG"
    message: string;
    source: RuleOrigin;                // "SCHEMA" | "CFN_LINT" | "ENGINE" | "CUSTOM" | "GUARD"
    entity?: Entity;                   // the named template entity the finding targets, if any
    propertyPath?: string;             // e.g. "Properties.BucketName", or section-absolute like "Parameters/MyParam/Type"
    suggestedFix?: string;
    category?: string;
    startLine?: number;
    startColumn?: number;
    endLine?: number;
    endColumn?: number;
    relatedResources?: RelatedResource[];
    conditionScenario?: Record<string, boolean>;  // condition truth assignment that triggers this diagnostic
    // Enrichment fields: populated at detailLevel "DETAILED" (the default), absent at "STANDARD".
    documentationUrl?: string;
    ruleDescription?: string;
    phase?: "PARSE" | "SCHEMA" | "LINT";           // pipeline stage that produced the finding
    context?: ViolationContext;                    // actualValue, expectedConstraint, resolutionSource, etc.
}

// The named template entity a diagnostic is attributed to. The entity type is the
// singular form of the top-level template section the entity is declared in.
interface Entity {
    logicalId: string;                 // logical ID as declared in the template
    entityType: EntityType;
    resourceType?: string;             // CloudFormation type, when the entity is a resource whose type is known
}

type EntityType = "Resource" | "Parameter" | "Output" | "Mapping" | "Metadata"
                | "Rule" | "Condition" | "Transform" | "FormatVersion" | "Description";

Severity, RuleOrigin, and DetailLevel are string-literal union types ("WARN", "GUARD", "STANDARD", ...), as is the report status ("OK", "ANALYSIS_INCOMPLETE", "ERROR").