diff --git a/packages/expressions/lib/constant.js b/packages/expressions/lib/constant.js
index c42ce6474..8c2fd35de 100644
--- a/packages/expressions/lib/constant.js
+++ b/packages/expressions/lib/constant.js
@@ -5,23 +5,20 @@ import { Schema } from './schemas'
 //
 // Implements the same interface as Expression
 export class Constant {
-  constructor (value, id = uuidv4()) {
+  constructor (value, { id = uuidv4(), schema = Schema.resolve('#/definitions/constant') } = {}) {
     this.value = value
     this.id = id
+    this.schema = schema
   }
 
-  clone (value, id = this.id) {
-    return new Constant(value, id)
+  clone (value, { id = this.id, schema = this.schema } = {}) {
+    return new Constant(value, { id, schema })
   }
 
   get args () {
     return [this.value]
   }
 
-  get schema () {
-    return Schema.resolve('#/definitions/constant')
-  }
-
   validate (schema = this.schema) {
     return schema.validate(this.value)
   }
diff --git a/packages/expressions/lib/expression.js b/packages/expressions/lib/expression.js
index baba64b1b..3d9b80e09 100644
--- a/packages/expressions/lib/expression.js
+++ b/packages/expressions/lib/expression.js
@@ -15,7 +15,7 @@ function toArray (arg) {
 // Simple model to transform this: `{ All: [{ Boolean: [true] }]`
 // into this: `{ id: uuidv4(), name: 'All', args: [{ id: uuidv4(), name: 'Boolean', args: [true] }] }`
 export class Expression {
-  static build (expression) {
+  static build (expression, schema = undefined) {
     if (expression instanceof Expression || expression instanceof Constant) {
       return expression
     }
@@ -25,37 +25,34 @@ export class Expression {
         throw new TypeError(`Invalid expression: ${JSON.stringify(expression)}`)
       }
       const name = Object.keys(expression)[0]
-      const args = toArray(expression[name]).map(Expression.build)
-
-      return new Expression({ name, args })
+      return new Expression({ name, args: expression[name] })
     } else if (['number', 'string', 'boolean'].includes(typeof expression)) {
-      return new Constant(expression)
+      return new Constant(expression, { schema })
     } else {
       throw new TypeError(`Invalid expression: ${JSON.stringify(expression)}`)
     }
   }
 
   constructor ({ name, args, id = uuidv4() }) {
-    Object.assign(this, { name, args, id })
+    this.id = id
+    this.name = name
+    this.schema = Schema.resolve(`${name}.schema.json`)
+    this.args = toArray(args).map((arg, i) => Expression.build(arg, this.schema.arrayItem(i)))
   }
 
   clone ({ id = this.id, name = this.name, args = this.args } = {}) {
-    return new Expression({ id, name, args: args.map(Expression.build) })
+    return new Expression({ id, name, args })
   }
 
   get value () {
     return { [this.name]: this.args.map(arg => arg.value) }
   }
 
-  get schema () {
-    return Schema.resolve('#')
-  }
-
   validate (schema = this.schema) {
-    return schema.validate(this.value)
+    return schema.validate(this.args.map(arg => arg.value))
   }
 
-  matches (localSchema) {
-    return localSchema.validate(this.value).valid
+  matches (schema) {
+    return this.validate(schema).valid
   }
 }
diff --git a/packages/expressions/lib/schemas.js b/packages/expressions/lib/schemas.js
index b642f9d81..2b9127265 100644
--- a/packages/expressions/lib/schemas.js
+++ b/packages/expressions/lib/schemas.js
@@ -15,35 +15,6 @@ const ajv = new Ajv({
 })
 addFormats(ajv)
 
-// Proxy to resolve $refs in schema definitions
-const Dereference = {
-  get (target, property) {
-    const value = target[property]
-
-    if (Array.isArray(value)) {
-      // Schema definition returns an array for this property, return the array with all refs resolved
-      return value.map((item, i) => {
-        const $ref = item.$ref || this.join(target.$id, `${property}/${i}`)
-        return Schema.resolve($ref, target.$id)
-      })
-    } else if (value !== null && typeof value === 'object') {
-      // Schema definition returns an object for this property, return the subschema and proxy it
-      return Schema.proxy(this.join(target.$id, property), value)
-    } else if (value !== undefined) {
-      // Schema definition returns a value for this property, just return it
-      return value
-    } else if (target.$ref) {
-      // Schema includes a ref, so delegate to it
-      return Schema.resolve(target.$ref, target.$id)[property]
-    }
-  },
-
-  join ($id, path) {
-    const url = new URL($id)
-    url.hash = [url.hash, path].join('/')
-    return url.toString()
-  }
-}
 
 // Delegate property access to the schema definition
 const DelegateToDefinition = {
@@ -62,7 +33,12 @@ export class Schema {
     const { href } = new URL($ref, $id)
     const validator = ajv.getSchema(href)
 
-    if (!validator) throw new TypeError('Schema not found: ' + href)
+    if (validator === undefined) throw new TypeError('Schema not found: ' + href)
+
+    // Schema definition is a primitive, just return it
+    if (typeof validator.schema !== 'object') return validator.schema
+
+    // Create a new proxy to the schema definition
     if (!validator.proxy) validator.proxy = Schema.proxy(href, validator.schema)
 
     return validator.proxy
@@ -73,10 +49,11 @@ export class Schema {
   }
 
   constructor ($id, definition) {
-    this.definition = new Proxy({ $id, ...definition }, Dereference)
+    this.$id = $id
+    this.definition = new Proxy(definition, this)
   }
 
-  resolve ($ref = this.definition.$ref, $id = this.definition.$id) {
+  resolve ($ref = this.definition.$ref, $id = this.$id) {
     return Schema.resolve($ref, $id)
   }
 
@@ -90,11 +67,42 @@ export class Schema {
   }
 
   validate (data) {
-    const validator = ajv.getSchema(this.definition.$id)
+    const validator = ajv.getSchema(this.$id)
     const valid = validator(data)
     const errors = validator.errors
     return { valid, errors }
   }
+
+  // This instance acts as a Proxy to resolve $refs in the schema definition
+  get (target, property) {
+    const value = target[property]
+
+    if (Array.isArray(value)) {
+      // Schema definition returns an array for this property, return the array with all refs resolved
+      return value.map((item, i) => {
+        if (typeof item === 'object') {
+          return Schema.resolve(item.$ref || this.join(`${property}/${i}`), this.$id)
+        } else {
+          return item
+        }
+      })
+    } else if (value !== null && typeof value === 'object') {
+      // Schema definition returns an object for this property, return the subschema and proxy it
+      return Schema.proxy(this.join(property), value)
+    } else if (value !== undefined) {
+      // Schema definition returns a value for this property, just return it
+      return value
+    } else if (target.$ref) {
+      // Schema includes a ref, so delegate to it
+      return Schema.resolve(target.$ref, this.$id)[property]
+    }
+  }
+
+  join (path, $id = this.$id) {
+    const url = new URL($id)
+    url.hash = [url.hash, path].join('/')
+    return url.toString()
+  }
 }
 
 export const schema = Schema.resolve('#')
diff --git a/packages/expressions/test/constant.test.js b/packages/expressions/test/constant.test.js
index 715ed4f12..1981f5c1d 100644
--- a/packages/expressions/test/constant.test.js
+++ b/packages/expressions/test/constant.test.js
@@ -3,9 +3,16 @@ import { Constant, Schema } from '../lib'
 
 describe('Constant', () => {
   describe('schema', () => {
-    test('returns Constant schema', () => {
+    test('defaults to Constant schema', () => {
       expect(new Constant('string').schema.title).toEqual('Constant')
     })
+
+    test('uses provided schema', () => {
+      const schema = Schema.resolve('#/definitions/number')
+      const number = new Constant(42, { schema })
+      expect(number.schema).toEqual(schema)
+      expect(number.clone(99).schema).toEqual(schema)
+    })
   })
 
   describe('validate', () => {
@@ -16,6 +23,20 @@ describe('Constant', () => {
       expect(new Constant(42).validate().valid).toBe(true)
       expect(new Constant(3.14).validate().valid).toBe(true)
     })
+
+    test('returns false for invalid value', () => {
+      expect(new Constant(['array']).validate().valid).toBe(false)
+      expect(new Constant({Now: []}).validate().valid).toBe(false)
+    })
+
+    test('uses provided schema', () => {
+      const schema = Schema.resolve('#/definitions/number')
+      expect(new Constant(42, { schema }).validate().valid).toBe(true)
+      expect(new Constant(42).validate(schema).valid).toBe(true)
+
+      expect(new Constant('nope', { schema }).validate().valid).toBe(false)
+      expect(new Constant('nope').validate(schema).valid).toBe(false)
+    })
   })
 
   describe('matches', () => {
diff --git a/packages/expressions/test/expression.test.js b/packages/expressions/test/expression.test.js
index 37e7ed06f..bd96cde76 100644
--- a/packages/expressions/test/expression.test.js
+++ b/packages/expressions/test/expression.test.js
@@ -1,5 +1,5 @@
 import { describe, test, expect } from 'vitest'
-import { Expression, Constant } from '../lib'
+import { Expression, Constant, Schema } from '../lib'
 
 describe('Expression', () => {
   describe('build', () => {
@@ -16,6 +16,21 @@ describe('Expression', () => {
       expect(() => Expression.build(new Date())).toThrowError(TypeError)
       expect(() => Expression.build({ All: [], Any: [] })).toThrowError(TypeError)
     })
+
+    test('sets schema for constant args', () => {
+      const expression = Expression.build({ Duration: [5, 'minutes'] })
+      const schema = Schema.resolve('Duration.schema.json')
+      expect(expression.schema).toEqual(schema)
+      expect(expression.args[0].schema).toEqual(schema.items[0])
+      expect(expression.args[1].schema).toEqual(schema.items[1])
+    })
+
+    test('each subexpression uses its own schema', () => {
+      const expression = Expression.build({ GreaterThan: [ { Now: [] }, { Property: ['released_at'] } ] })
+      expect(expression.schema).toEqual(Schema.resolve('GreaterThan.schema.json'))
+      expect(expression.args[0].schema).toEqual(Schema.resolve('Now.schema.json'))
+      expect(expression.args[1].schema).toEqual(Schema.resolve('Property.schema.json'))
+    })
   })
 
   describe('clone', () => {
diff --git a/packages/expressions/test/schemas.test.js b/packages/expressions/test/schemas.test.js
index d40c979ec..cff727813 100644
--- a/packages/expressions/test/schemas.test.js
+++ b/packages/expressions/test/schemas.test.js
@@ -45,6 +45,11 @@ describe('schema.json', () => {
       expect(Schema.resolve('#/definitions/function/properties/Any').title).toEqual('Any')
       expect(Schema.resolve('#').definitions.function.properties.Any.title).toEqual('Any')
     })
+
+    test('returns array values', () => {
+      const expected = ['seconds', 'minutes', 'hours', 'days', 'weeks', 'months', 'years']
+      expect(Schema.resolve('Duration.schema.json#/items/1/anyOf/0').enum).toEqual(expected)
+    })
   })
 
   describe('resolveAnyOf', () => {
@@ -69,8 +74,8 @@ describe('schema.json', () => {
 
     test('returns schema for tuple', () => {
       const duration = Schema.resolve('Duration.schema.json')
-      expect(duration.arrayItem(0).title).toEqual('Number')
-      expect(duration.arrayItem(1).title).toEqual('Unit')
+      expect(duration.arrayItem(0).$id).toMatch('schema.json#/definitions/number')
+      expect(duration.arrayItem(1).$id).toMatch('Duration.schema.json#/items/1')
       expect(duration.arrayItem(2)).toBe(undefined)
     })
   })