diff --git a/modules/circe/src/main/scala/circemapping.scala b/modules/circe/src/main/scala/circemapping.scala
index 0346350e..c4439f00 100644
--- a/modules/circe/src/main/scala/circemapping.scala
+++ b/modules/circe/src/main/scala/circemapping.scala
@@ -20,7 +20,7 @@ import scala.collection.Factory
 import cats.MonadThrow
 import cats.implicits._
 import fs2.Stream
-import io.circe.{Encoder, Json}
+import io.circe.{Encoder, Json, JsonNumber}
 import org.tpolecat.sourcepos.SourcePos
 
 import grackle._
@@ -57,6 +57,37 @@ trait CirceMappingLike[F[_]] extends Mapping[F] {
       computeJson(fieldName)((p, e) => effect(p, e).map(_.map(enc(_))))
   }
 
+  // Coerces the value to a built-in scalar type, or None if it does not coerce
+  private def coerceBuiltin(tpe: Type, value: Json): Option[Json] = {
+    def numberFromString: Option[JsonNumber] = value.asString.flatMap(JsonNumber.fromString)
+
+    // No built-in scalar coerces from an object, array or null
+    if (value.isObject || value.isArray || value.isNull) None
+    else
+      tpe match {
+        // Strings "true" and "false" coerce to Boolean
+        case BooleanType =>
+          if (value.isBoolean) Some(value)
+          else value.asString.flatMap(_.toBooleanOption).map(Json.fromBoolean)
+        // Booleans and numbers coerce to their JSON text, e.g. "true" or "42"
+        case StringType =>
+          if (value.isString) Some(value)
+          else Some(Json.fromString(value.noSpaces))
+        // ID can come from String or Int, but coerces to String
+        case IDType =>
+          if (value.isString) Some(value)
+          else value.asNumber.flatMap(_.toBigInt).map(i => Json.fromString(i.toString))
+        // Strings such as "42" coerce to Int
+        case IntType =>
+          value.asNumber.orElse(numberFromString).flatMap(_.toLong).map(Json.fromLong)
+        // Strings such as "1.5" coerce to Float
+        case FloatType =>
+          if (value.isNumber) Some(value)
+          else numberFromString.map(Json.fromJsonNumber)
+        case _ => None
+      }
+  }
+
   def circeCursor(path: Path, env: Env, value: Json): Cursor =
     if (path.isRoot)
       CirceCursor(Context(path.rootTpe), value, None, env)
@@ -95,6 +126,17 @@ trait CirceMappingLike[F[_]] extends Mapping[F] {
       implicit val pos: SourcePos
   ) extends CirceFieldMapping
 
+  object CirceCursor {
+    // Coerce built-in scalars up-front, so predicates and `asLeaf` see the same value.
+    // Values that do not coerce stay unchanged. The `asLeaf` method reports them as errors.
+    def apply(context: Context, focus: Json, parent: Option[Cursor], env: Env): CirceCursor =
+      new CirceCursor(
+        context,
+        coerceBuiltin(context.tpe.dealias, focus).getOrElse(focus),
+        parent,
+        env)
+  }
+
   case class CirceCursor(
       context: Context,
       focus: Json,
@@ -117,14 +159,9 @@ trait CirceMappingLike[F[_]] extends Mapping[F] {
 
     def asLeaf: Result[Json] =
       tpe.dealias match {
-        case BooleanType if focus.isBoolean => focus.success
-        case StringType | IDType if focus.isString => focus.success
-        case IntType if focus.isNumber =>
-          focus
-            .asNumber
-            .flatMap(_.toLong.map(Json.fromLong))
-            .toResultOrError(s"Expected Int found ${focus.noSpaces}")
-        case FloatType if focus.isNumber => focus.success
+        case t: ScalarType if t.isBuiltIn =>
+          coerceBuiltin(t, focus).toResult(
+            s"Cannot coerce JSON ${focus.name} value '${focus.noSpaces}' to type ${t.name}")
         case e: EnumType if focus.isString =>
           if (focus.asString.exists(e.hasValue)) focus.success
           else Result.internalError(s"Expected Enum ${e.name}, found ${focus.noSpaces}")
diff --git a/modules/circe/src/test/scala/CirceData.scala b/modules/circe/src/test/scala/CirceData.scala
index 2ff09e28..41172deb 100644
--- a/modules/circe/src/test/scala/CirceData.scala
+++ b/modules/circe/src/test/scala/CirceData.scala
@@ -127,3 +127,50 @@ object TestCirceMapping extends CirceMapping[IO] {
       Elab.transformChild(_ => Count(Select("children")))
   }
 }
+
+object TestCirceScalarCoercionMapping extends CirceMapping[IO] {
+  val schema =
+    schema"""
+      type Query {
+        int: String!
+        float: String!
+        bool: String!
+        string: String!
+        array: String
+        object: String
+        bigInt: Int!
+        intFromString: Int!
+        floatFromString: Float!
+        boolFromString: Boolean!
+        idFromInt: ID!
+        badInt: Int
+        badFloat: Float
+        badBool: Boolean
+      }
+    """
+
+  val QueryType = schema.ref("Query")
+
+  val typeMappings =
+    List(
+      ObjectMapping(
+        tpe = QueryType,
+        fieldMappings = List(
+          CirceField("int", Json.fromInt(42)),
+          CirceField("float", Json.fromDoubleOrNull(1.5)),
+          CirceField("bool", Json.True),
+          CirceField("string", Json.fromString("foo")),
+          CirceField("array", Json.arr(Json.fromInt(1))),
+          CirceField("object", Json.obj("a" -> Json.fromInt(1))),
+          CirceField("bigInt", Json.fromLong(3000000000L)),
+          CirceField("intFromString", Json.fromString("42")),
+          CirceField("floatFromString", Json.fromString("1.5")),
+          CirceField("boolFromString", Json.fromString("true")),
+          CirceField("idFromInt", Json.fromInt(23)),
+          CirceField("badInt", Json.fromString("foo")),
+          CirceField("badFloat", Json.True),
+          CirceField("badBool", Json.fromInt(1))
+        )
+      )
+    )
+}
diff --git a/modules/circe/src/test/scala/CirceSuite.scala b/modules/circe/src/test/scala/CirceSuite.scala
index 3b681702..2a9d95ef 100644
--- a/modules/circe/src/test/scala/CirceSuite.scala
+++ b/modules/circe/src/test/scala/CirceSuite.scala
@@ -15,9 +15,14 @@
 
 package grackle.circetests
 
+import io.circe.Json
 import io.circe.literal._
 import munit.CatsEffectSuite
 
+import grackle.{Env, Path, Predicate}
+import grackle.PathTerm.UniquePath
+import grackle.Predicate.{Const, Eql}
+
 final class CirceSuite extends CatsEffectSuite {
   test("scalars") {
     val query = """
@@ -339,4 +344,153 @@ final class CirceSuite extends CatsEffectSuite {
 
     assertIO(res, expected)
   }
+
+  test("booleans and numbers are coerced to String") {
+    val query = """
+      query {
+        int
+        float
+        bool
+        string
+      }
+    """
+
+    val expected = json"""
+      {
+        "data" : {
+          "int" : "42",
+          "float" : "1.5",
+          "bool" : "true",
+          "string" : "foo"
+        }
+      }
+    """
+
+    val res = TestCirceScalarCoercionMapping.compileAndRun(query)
+
+    assertIO(res, expected)
+  }
+
+  test("lists and objects are not coerced to String") {
+    val query = """
+      query {
+        array
+        object
+      }
+    """
+
+    val expected = json"""
+      {
+        "errors" : [
+          {
+            "message" : "Cannot coerce JSON Array value '[1]' to type String",
+            "locations" : [ { "line" : 3, "column" : 9 } ],
+            "path" : [ "array" ]
+          },
+          {
+            "message" : "Cannot coerce JSON Object value '{\"a\":1}' to type String",
+            "locations" : [ { "line" : 4, "column" : 9 } ],
+            "path" : [ "object" ]
+          }
+        ],
+        "data" : {
+          "array" : null,
+          "object" : null
+        }
+      }
+    """
+
+    val res = TestCirceScalarCoercionMapping.compileAndRun(query)
+
+    assertIO(res, expected)
+  }
+
+  test("large Int values and strings are coerced to Int, Float, Boolean and ID") {
+    val query = """
+      query {
+        bigInt
+        intFromString
+        floatFromString
+        boolFromString
+        idFromInt
+      }
+    """
+
+    val expected = json"""
+      {
+        "data" : {
+          "bigInt" : 3000000000,
+          "intFromString" : 42,
+          "floatFromString" : 1.5,
+          "boolFromString" : true,
+          "idFromInt" : "23"
+        }
+      }
+    """
+
+    val res = TestCirceScalarCoercionMapping.compileAndRun(query)
+
+    assertIO(res, expected)
+  }
+
+  test("values that do not coerce to Int, Float or Boolean are errors") {
+    val query = """
+      query {
+        badInt
+        badFloat
+        badBool
+      }
+    """
+
+    val expected = json"""
+      {
+        "errors" : [
+          {
+            "message" : "Cannot coerce JSON String value '\"foo\"' to type Int",
+            "locations" : [ { "line" : 3, "column" : 9 } ],
+            "path" : [ "badInt" ]
+          },
+          {
+            "message" : "Cannot coerce JSON Boolean value 'true' to type Float",
+            "locations" : [ { "line" : 4, "column" : 9 } ],
+            "path" : [ "badFloat" ]
+          },
+          {
+            "message" : "Cannot coerce JSON Number value '1' to type Boolean",
+            "locations" : [ { "line" : 5, "column" : 9 } ],
+            "path" : [ "badBool" ]
+          }
+        ],
+        "data" : {
+          "badInt" : null,
+          "badFloat" : null,
+          "badBool" : null
+        }
+      }
+    """
+
+    val res = TestCirceScalarCoercionMapping.compileAndRun(query)
+
+    assertIO(res, expected)
+  }
+
+  test("filters compare coerced values") {
+    import TestCirceScalarCoercionMapping._
+
+    val cursor =
+      circeCursor(
+        Path.from(QueryType),
+        Env.empty,
+        json"""{ "bigInt": 3000000000, "intFromString": "42", "idFromInt": 23, "boolFromString": "true" }"""
+      )
+
+    def eql(field: String, value: Json): Predicate =
+      Eql(UniquePath[Json](List(field)), Const(value))
+
+    assertEquals(eql("bigInt", Json.fromLong(3000000000L))(cursor).toOption, Some(true))
+    assertEquals(eql("intFromString", Json.fromInt(42))(cursor).toOption, Some(true))
+    assertEquals(eql("intFromString", Json.fromString("42"))(cursor).toOption, Some(false))
+    assertEquals(eql("idFromInt", Json.fromString("23"))(cursor).toOption, Some(true))
+    assertEquals(eql("boolFromString", Json.True)(cursor).toOption, Some(true))
+  }
 }