diff --git a/internal/cuetools/testdata/assertionmodes/tests/incorrect_unification.cue b/internal/cuetools/testdata/assertionmodes/tests/incorrect_unification.cue
new file mode 100644
index 0000000..83b9f44
--- /dev/null
+++ b/internal/cuetools/testdata/assertionmodes/tests/incorrect_unification.cue
@@ -0,0 +1,13 @@
+
+@if(incorrect_unification)
+package tests
+
+#request: observed: composite: resource: {
+	foo: "bar"
+}
+
+response: {
+    desired: resources: main: resource: {
+		foo: "baz"
+	}
+} @assertionMode(unification)
diff --git a/internal/cuetools/testdata/assertionmodes/tests/incorrect_unification_extra.cue b/internal/cuetools/testdata/assertionmodes/tests/incorrect_unification_extra.cue
new file mode 100644
index 0000000..3a8c224
--- /dev/null
+++ b/internal/cuetools/testdata/assertionmodes/tests/incorrect_unification_extra.cue
@@ -0,0 +1,15 @@
+
+@if(incorrect_unification_extra)
+package tests
+
+#request: observed: composite: resource: {
+	foo: "bar"
+}
+
+#mode: "unification"
+
+response: {
+    desired: resources: main: resource: {
+		extra: "value"
+	}
+} @assertionMode(unification)
diff --git a/internal/cuetools/testdata/assertionmodes/tests/unification.cue b/internal/cuetools/testdata/assertionmodes/tests/unification.cue
new file mode 100644
index 0000000..7db962b
--- /dev/null
+++ b/internal/cuetools/testdata/assertionmodes/tests/unification.cue
@@ -0,0 +1,12 @@
+@if(unification)
+package tests
+
+#request: observed: composite: resource: {
+	foo: "bar"
+}
+
+response: {
+    desired: resources: main: resource: {
+		foo: "bar"
+	}
+} @assertionMode(unification)
diff --git a/internal/cuetools/tester.go b/internal/cuetools/tester.go
index ef3427a..b50689c 100644
--- a/internal/cuetools/tester.go
+++ b/internal/cuetools/tester.go
@@ -27,6 +27,7 @@ import (
 	"strings"
 
 	"cuelang.org/go/cue"
+	"cuelang.org/go/cue/cuecontext"
 	"cuelang.org/go/cue/load"
 	"cuelang.org/go/cue/parser"
 	"github.com/crossplane-contrib/function-cue/internal/fn"
@@ -53,6 +54,32 @@ type Tester struct {
 	config *TestConfig
 }
 
+type assertionMode string
+
+const (
+	AssertionModeDiff        assertionMode = "diff"
+	AssertionModeUnification assertionMode = "unification"
+)
+
+type ErrUnknownAssertionMode struct {
+	Mode string
+}
+
+func (e *ErrUnknownAssertionMode) Error() string {
+	return fmt.Sprintf("unknown assertion mode: %s", e.Mode)
+}
+
+func assertionModeFromString(mode string) (assertionMode, error) {
+	switch mode {
+	case "diff":
+		return AssertionModeDiff, nil
+	case "unification":
+		return AssertionModeUnification, nil
+	default:
+		return "", &ErrUnknownAssertionMode{Mode: mode}
+	}
+}
+
 // NewTester returns a test for the supplied configuration. It auto-discovers tags from test file names if needed.
 func NewTester(config TestConfig) (*Tester, error) {
 	ret := &Tester{config: &config}
@@ -102,16 +129,18 @@ func (t *Tester) discoverTags() error {
 	return nil
 }
 
-func evalPackage(pkg string, tag string, expr string, into proto.Message) (finalErr error) {
+// evalPackage evaluates a CUE package with a specific tag and returns the value of the given expression.
+func evalPackage(pkg string, tag string, expr string) (cue.Value, error) {
 	iv, err := loadSingleInstanceValue(pkg, &load.Config{Tags: []string{tag}})
 	if err != nil {
-		return err
+		return cue.Value{}, err
 	}
+
 	val := iv.value
 	if expr != "" {
 		e, err := parser.ParseExpr("expression", expr)
 		if err != nil {
-			return errors.Wrap(err, "parse expression")
+			return val, errors.Wrap(err, "parse expression")
 		}
 		val = iv.value.Context().BuildExpr(e,
 			cue.Scope(iv.value),
@@ -119,9 +148,14 @@ func evalPackage(pkg string, tag string, expr string, into proto.Message) (final
 			cue.InferBuiltins(true),
 		)
 		if val.Err() != nil {
-			return errors.Wrap(val.Err(), "build expression")
+			return val, errors.Wrap(val.Err(), "build expression")
 		}
 	}
+	return val, nil
+}
+
+// marshalValueIntoProtoMessage marshals a CUE value into a proto message.
+func marshalValueIntoProtoMessage(val cue.Value, into proto.Message) error {
 	b, err := val.MarshalJSON()
 	if err != nil {
 		return errors.Wrap(err, "marshal json")
@@ -199,22 +233,29 @@ func (t *Tester) runTest(f *fn.Cue, codeBytes []byte, tag string) (finalErr erro
 	}
 
 	var expected fnv1.RunFunctionResponse
-	var err error
+	expectedVal, err := evalPackage(t.config.TestPackage, tag, responseVar)
+	if err != nil {
+		return errors.Wrap(err, "evaluate expected")
+	}
 	if t.config.LegacyDesiredOnlyResponse {
 		expected.Desired = &fnv1.State{}
-		err = evalPackage(t.config.TestPackage, tag, responseVar, expected.Desired)
+		if err := marshalValueIntoProtoMessage(expectedVal, expected.Desired); err != nil {
+			return errors.Wrap(err, "marshal expected")
+		}
 	} else {
-		err = evalPackage(t.config.TestPackage, tag, responseVar, &expected)
-	}
-	if err != nil {
-		return errors.Wrap(err, "evaluate expected")
+		if err := marshalValueIntoProtoMessage(expectedVal, &expected); err != nil {
+			return errors.Wrap(err, "marshal expected")
+		}
 	}
 
 	var req fnv1.RunFunctionRequest
-	err = evalPackage(t.config.TestPackage, tag, requestVar, &req)
+	requestVal, err := evalPackage(t.config.TestPackage, tag, requestVar)
 	if err != nil {
 		return errors.Wrap(err, "evaluate request")
 	}
+	if err := marshalValueIntoProtoMessage(requestVal, &req); err != nil {
+		return errors.Wrap(err, "marshal request")
+	}
 
 	opts := fn.EvalOptions{
 		RequestVar:          requestVar,
@@ -227,21 +268,54 @@ func (t *Tester) runTest(f *fn.Cue, codeBytes []byte, tag string) (finalErr erro
 		return errors.Wrap(err, "evaluate package with test request")
 	}
 
-	expectedString, err := canonicalYAML(&expected)
-	if err != nil {
-		return errors.Wrap(err, "serialize expected")
-	}
-	actualString, err := canonicalYAML(actual)
-	if err != nil {
-		return errors.Wrap(err, "serialize actual")
-	}
-	if expectedString == actualString {
-		return nil
+	assertionMode := AssertionModeDiff
+
+	attr := expectedVal.Attribute("assertionMode")
+	if attr.Err() == nil {
+		assertionMode, err = assertionModeFromString(attr.Contents())
+		if err != nil {
+			return err
+		}
 	}
 
-	err = printDiffs(expectedString, actualString)
-	if err != nil {
-		_, _ = fmt.Fprintln(TestOutput, "error in running diff:", err)
+	switch assertionMode {
+	case AssertionModeUnification:
+		// in unification mode, we check if the expected and actual values are unifiable
+		// by compiling a cue script that unifies the two values
+
+		actualBytes, err := protojson.MarshalOptions{Indent: "  "}.Marshal(actual)
+		if err != nil {
+			return errors.Wrap(err, "proto json marshal")
+		}
+
+		assertionScript := fmt.Sprintf("expected: %s\n#Actual: %s\nunified: #Actual & expected\n", expectedVal, actualBytes)
+
+		runtime := cuecontext.New()
+		assertVal := runtime.CompileString(assertionScript)
+		if assertVal.Err() != nil {
+			return errors.Wrap(assertVal.Err(), "compile cue code")
+		}
+
+		if _, err := assertVal.MarshalJSON(); err != nil {
+			return errors.Wrap(err, "marshal cue output")
+		}
+		// script compiles and marshals, so actual and expected are unifiable.
+	case AssertionModeDiff:
+		expectedString, err := canonicalYAML(&expected)
+		if err != nil {
+			return errors.Wrap(err, "serialize expected")
+		}
+		actualString, err := canonicalYAML(actual)
+		if err != nil {
+			return errors.Wrap(err, "serialize actual")
+		}
+		if expectedString != actualString {
+			err = printDiffs(expectedString, actualString)
+			if err != nil {
+				_, _ = fmt.Fprintln(TestOutput, "error in running diff:", err)
+			}
+			return fmt.Errorf("expected did not match actual")
+		}
 	}
-	return fmt.Errorf("expected did not match actual")
+	return nil
 }
diff --git a/internal/cuetools/tester_test.go b/internal/cuetools/tester_test.go
index 66f64d5..a537624 100644
--- a/internal/cuetools/tester_test.go
+++ b/internal/cuetools/tester_test.go
@@ -85,6 +85,38 @@ FAIL incorrect: expected did not match actual
 	assert.Equal(t, strings.TrimSpace(expected), strings.TrimSpace(buf.String()))
 }
 
+func TestAssertionModes(t *testing.T) {
+	fn := chdirCueRoot(t)
+	defer fn()
+	buf, reset := getOutput()
+	defer reset()
+	tester, err := NewTester(TestConfig{
+		Package:     "./runtime",
+		TestPackage: "./assertionmodes/tests",
+	})
+	require.NoError(t, err)
+	envDiff := ExternalDiffEnvVar
+	diffProgram := os.Getenv(envDiff)
+	if diffProgram != "" {
+		err = os.Unsetenv(envDiff) // we expect a specific diff format
+		require.NoError(t, err)
+		defer func() { _ = os.Setenv(envDiff, diffProgram) }()
+	}
+	err = tester.Run()
+	expected := `
+running test tags: incorrect_unification, incorrect_unification_extra, unification
+> run test "incorrect_unification"
+FAIL incorrect_unification: compile cue code: unified.desired.resources.main.resource.foo: conflicting values "baz" and "bar"
+> run test "incorrect_unification_extra"
+FAIL incorrect_unification_extra: compile cue code: unified.desired.resources.main.resource.extra: field not allowed
+> run test "unification"
+PASS unification
+`
+	require.Error(t, err)
+	assert.Equal(t, "2 of 3 tests had errors", err.Error())
+	assert.Equal(t, strings.TrimSpace(expected), strings.TrimSpace(buf.String()))
+}
+
 func TestTesterLegacyOptions(t *testing.T) {
 	fn := chdirCueRoot(t)
 	defer fn()