fix(openfeature): strict type checking for flag evaluations

Check the requested evaluation type against the flag's declared
variationType before calling mapValue(). Return TYPE_MISMATCH when
they don't match (e.g., STRING flag evaluated as Boolean).

Separately, variant values that don't match their declared type
(e.g., INTEGER flag with string variant value) now return PARSE_ERROR
instead of TYPE_MISMATCH, aligning with Go SDK (dd-trace-go#4590)
and Python's libdatadog behavior.

Type compatibility mapping:
  BOOLEAN  → Boolean.class
  STRING   → String.class
  INTEGER  → Integer.class
  NUMERIC  → Double.class
  JSON     → Value.class

Author: typotter

products/feature-flagging/feature-flagging-api/src/main/java/datadog/trace/api/openfeature/DDEvaluator.java

@@ -14,6 +14,7 @@
 import datadog.trace.api.featureflag.ufc.v1.Shard;
 import datadog.trace.api.featureflag.ufc.v1.ShardRange;
 import datadog.trace.api.featureflag.ufc.v1.Split;
+import datadog.trace.api.featureflag.ufc.v1.ValueType;
 import datadog.trace.api.featureflag.ufc.v1.Variant;
 import dev.openfeature.sdk.ErrorCode;
 import dev.openfeature.sdk.EvaluationContext;
@@ -343,14 +344,39 @@ private static <T> ProviderEvaluation<T> resolveVariant(
           .build();
     }
 
+    if (!isTypeCompatible(target, flag.variationType)) {
+      return error(
+          defaultValue,
+          ErrorCode.TYPE_MISMATCH,
+          "Requested type "
+              + target.getSimpleName()
+              + " does not match flag variationType "
+              + flag.variationType.name());
+    }
+
+    final T mappedValue;
+    try {
+      mappedValue = mapValue(target, variant.value);
+    } catch (final NumberFormatException e) {
+      return error(
+          defaultValue,
+          ErrorCode.PARSE_ERROR,
+          "Variant '"
+              + variant.key
+              + "' value does not match declared type "
+              + flag.variationType.name()
+              + ": "
+              + e.getMessage());
+    }
+
     final ImmutableMetadata.ImmutableMetadataBuilder metadataBuilder =
         ImmutableMetadata.builder()
             .addString("flagKey", flag.key)
             .addString("variationType", flag.variationType.name())
             .addString("allocationKey", allocation.key);
     final ProviderEvaluation<T> result =
         ProviderEvaluation.<T>builder()
-            .value(mapValue(target, variant.value))
+            .value(mappedValue)
             .reason(Reason.TARGETING_MATCH.name())
             .variant(variant.key)
             .flagMetadata(metadataBuilder.build())
@@ -371,6 +397,26 @@ private static Object resolveAttribute(final String name, final EvaluationContex
     return context.convertValue(resolved);
   }
 
+  private static boolean isTypeCompatible(final Class<?> target, final ValueType variationType) {
+    if (variationType == null) {
+      return true; // No type info — allow any
+    }
+    switch (variationType) {
+      case BOOLEAN:
+        return target == Boolean.class;
+      case STRING:
+        return target == String.class;
+      case INTEGER:
+        return target == Integer.class;
+      case NUMERIC:
+        return target == Double.class;
+      case JSON:
+        return target == Value.class;
+      default:
+        return true; // Unknown types pass through — mapValue errors caught as GENERAL
+    }
+  }
+
   @SuppressWarnings("unchecked")
   static <T> T mapValue(final Class<T> target, final Object value) {
     if (value == null) {

products/feature-flagging/feature-flagging-api/src/test/java/datadog/trace/api/openfeature/DDEvaluatorTest.java

@@ -288,10 +288,37 @@ private static List<TestCase<?>> evaluateTestCases() {
                 new Result<>("default")
                     // Result depends on shard calculation - either match or default
                     .reason(TARGETING_MATCH.name(), DEFAULT.name())),
+        // Type mismatch: STRING flag evaluated as Integer
         new TestCase<>(0)
             .flag("string-number-flag")
             .targetingKey("user-123")
-            .result(new Result<>(123).reason(TARGETING_MATCH.name()).variant("string-num")),
+            .result(new Result<>(0).reason(ERROR.name()).errorCode(ErrorCode.TYPE_MISMATCH)),
+        // Type mismatch: STRING flag evaluated as Boolean
+        new TestCase<>(false)
+            .flag("simple-string")
+            .targetingKey("user-123")
+            .result(new Result<>(false).reason(ERROR.name()).errorCode(ErrorCode.TYPE_MISMATCH)),
+        // Type mismatch: BOOLEAN flag evaluated as String
+        new TestCase<>("default")
+            .flag("boolean-flag")
+            .targetingKey("user-123")
+            .result(
+                new Result<>("default").reason(ERROR.name()).errorCode(ErrorCode.TYPE_MISMATCH)),
+        // Type mismatch: NUMERIC flag evaluated as Integer
+        new TestCase<>(0)
+            .flag("double-flag")
+            .targetingKey("user-123")
+            .result(new Result<>(0).reason(ERROR.name()).errorCode(ErrorCode.TYPE_MISMATCH)),
+        // Type mismatch: INTEGER flag evaluated as Double
+        new TestCase<>(0.0)
+            .flag("integer-flag")
+            .targetingKey("user-123")
+            .result(new Result<>(0.0).reason(ERROR.name()).errorCode(ErrorCode.TYPE_MISMATCH)),
+        // Variant value type mismatch: INTEGER flag with string variant value
+        new TestCase<>(0)
+            .flag("integer-string-variant-flag")
+            .targetingKey("user-123")
+            .result(new Result<>(0).reason(ERROR.name()).errorCode(ErrorCode.PARSE_ERROR)),
         new TestCase<>("default")
             .flag("broken-flag")
             .targetingKey("user-123")
@@ -551,6 +578,9 @@ private ServerConfiguration createTestConfiguration() {
     flags.put("not-one-of-false-flag", createNotOneOfFalseFlag());
     flags.put("null-context-values-flag", createNullContextValuesFlag());
     flags.put("country-rule-flag", createCountryRuleFlag());
+    flags.put(
+        "integer-string-variant-flag",
+        createSimpleFlag("integer-string-variant-flag", ValueType.INTEGER, "not-a-number", "bad"));
     return new ServerConfiguration(null, null, null, flags);
   }