diff --git a/testing/testrunner/BUILD b/testing/testrunner/BUILD
index 536adaf4d..10b906028 100644
--- a/testing/testrunner/BUILD
+++ b/testing/testrunner/BUILD
@@ -13,6 +13,7 @@ cc_library(
     hdrs = ["cel_test_context.h"],
     deps = [
         ":cel_expression_source",
+        "//common:value",
         "//compiler",
         "//eval/public:cel_expression",
         "//runtime",
@@ -20,7 +21,6 @@ cc_library(
         "@com_google_absl//absl/base:nullability",
         "@com_google_absl//absl/container:flat_hash_map",
         "@com_google_absl//absl/memory",
-        "@com_google_absl//absl/status",
         "@com_google_absl//absl/status:statusor",
         "@com_google_cel_spec//proto/cel/expr:checked_cc_proto",
         "@com_google_cel_spec//proto/cel/expr:value_cc_proto",
diff --git a/testing/testrunner/cel_test_context.h b/testing/testrunner/cel_test_context.h
index 176cc18b1..aa6aab3ac 100644
--- a/testing/testrunner/cel_test_context.h
+++ b/testing/testrunner/cel_test_context.h
@@ -26,6 +26,7 @@
 #include "absl/container/flat_hash_map.h"
 #include "absl/memory/memory.h"
 #include "absl/status/statusor.h"
+#include "common/value.h"
 #include "compiler/compiler.h"
 #include "eval/public/cel_expression.h"
 #include "runtime/activation.h"
@@ -42,6 +43,10 @@ class CelTestContext {
   using CelActivationFactoryFn = std::function<absl::StatusOr<cel::Activation>(
       const cel::expr::conformance::test::TestCase& test_case,
       google::protobuf::Arena* arena)>;
+  using AssertFn = std::function<void(
+      const cel::Value& computed,
+      const cel::expr::conformance::test::TestCase& test_case,
+      google::protobuf::Arena* arena)>;
 
   // Creates a CelTestContext using a `CelExpressionBuilder`.
   //
@@ -127,6 +132,12 @@ class CelTestContext {
     return activation_factory_;
   }
 
+  // Allows the runner to inject a custom assertion function. If not set, the
+  // default assertion logic in TestRunner will be used.
+  void SetAssertFn(AssertFn assert_fn) { assert_fn_ = std::move(assert_fn); }
+
+  const AssertFn& assert_fn() const { return assert_fn_; }
+
  private:
   // Delete copy and move constructors.
   CelTestContext(const CelTestContext&) = delete;
@@ -173,6 +184,7 @@ class CelTestContext {
   std::unique_ptr<const cel::Runtime> runtime_;
 
   CelActivationFactoryFn activation_factory_;
+  AssertFn assert_fn_;
 };
 
 }  // namespace cel::test
diff --git a/testing/testrunner/runner_lib.cc b/testing/testrunner/runner_lib.cc
index b6279ab3c..ae09de255 100644
--- a/testing/testrunner/runner_lib.cc
+++ b/testing/testrunner/runner_lib.cc
@@ -340,6 +340,10 @@ void TestRunner::AssertError(const cel::Value& computed,
 
 void TestRunner::Assert(const cel::Value& computed, const TestCase& test_case,
                         google::protobuf::Arena* arena) {
+  if (test_context_->assert_fn()) {
+    test_context_->assert_fn()(computed, test_case, arena);
+    return;
+  }
   TestOutput output = test_case.output();
   if (output.has_result_value() || output.has_result_expr()) {
     AssertValue(computed, output, arena);
diff --git a/testing/testrunner/runner_lib_test.cc b/testing/testrunner/runner_lib_test.cc
index 0ddb62ca7..804826b6c 100644
--- a/testing/testrunner/runner_lib_test.cc
+++ b/testing/testrunner/runner_lib_test.cc
@@ -652,6 +652,35 @@ TEST(TestRunnerStandaloneTest, BasicTestWithActivationFactorySucceeds) {
   EXPECT_NO_FATAL_FAILURE(test_runner.RunTest(test_case));
 }
 
+TEST(TestRunnerStandaloneTest, CustomAssertFnIsUsed) {
+  // Compile the expression.
+  ASSERT_OK_AND_ASSIGN(cel::ValidationResult validation_result,
+                       DefaultCompiler().Compile("1 + 1"));
+  CheckedExpr checked_expr;
+  ASSERT_THAT(cel::AstToCheckedExpr(*validation_result.GetAst(), &checked_expr),
+              absl_testing::IsOk());
+  // Create a runtime.
+  ASSERT_OK_AND_ASSIGN(std::unique_ptr<const cel::Runtime> runtime,
+                       CreateTestRuntime());
+  // Set the output to a value that would fail the default assertion.
+  TestCase test_case = ParseTextProtoOrDie<TestCase>(R"pb(
+    output { result_value { int64_value: 102 } }
+  )pb");
+  std::unique_ptr<CelTestContext> context =
+      CelTestContext::CreateFromRuntime(std::move(runtime));
+
+  context->SetAssertFn([&](const cel::Value& computed,
+                           const TestCase& test_case, google::protobuf::Arena* arena) {
+    ASSERT_TRUE(computed.Is<cel::IntValue>());
+    EXPECT_EQ(computed.As<cel::IntValue>().value(), 2);
+  });
+
+  context->SetExpressionSource(
+      CelExpressionSource::FromCheckedExpr(std::move(checked_expr)));
+  TestRunner test_runner(std::move(context));
+  EXPECT_NO_FATAL_FAILURE(test_runner.RunTest(test_case));
+}
+
 TEST(CoverageTest, RuntimeCoverage) {
   ASSERT_OK_AND_ASSIGN(
       std::unique_ptr<cel::CompilerBuilder> compiler_builder,