diff --git a/internal/stacks/stackruntime/internal/stackeval/input_variable.go b/internal/stacks/stackruntime/internal/stackeval/input_variable.go
index ebcdc5037f..48a883f71c 100644
--- a/internal/stacks/stackruntime/internal/stackeval/input_variable.go
+++ b/internal/stacks/stackruntime/internal/stackeval/input_variable.go
@@ -477,12 +477,8 @@ func evalVariableValidation(validation *stackconfig.CheckRule, hclCtx *hcl.EvalC
 	const errInvalidValue = "Invalid value for variable"
 	var diags tfdiags.Diagnostics
 
-	// Evaluate both condition and error message up front. The error message is
-	// always inspected for structural problems (sensitive/ephemeral marks), not only
-	// when the condition fails.
 	result, moreDiags := validation.Condition.Value(hclCtx)
 	diags = diags.Append(moreDiags)
-	errorValue, errorDiags := validation.ErrorMessage.Value(hclCtx)
 
 	if moreDiags.HasErrors() {
 		// If we couldn't evaluate the condition at all (syntax error, etc.),
@@ -529,27 +525,24 @@ func evalVariableValidation(validation *stackconfig.CheckRule, hclCtx *hcl.EvalC
 	// The marks don't affect the validation result, only how we handle the error message.
 	result, _ = result.Unmark()
 
-	// Always process and validate the error_message expression, even when the condition
-	// passes — an invalid error message (sensitive, ephemeral, non-string, etc.) should
-	// be flagged regardless of whether the check succeeds or fails.
+	// Always evaluate the error_message expression, even when the condition passes —
+	// unknown, sensitive, or ephemeral values in the message are structural problems
+	// regardless of whether the check succeeds or fails.
+	errorValue, errorDiags := validation.ErrorMessage.Value(hclCtx)
 	diags = diags.Append(errorDiags)
 
 	var errorMessage string
 	if !errorDiags.HasErrors() {
 		if !errorValue.IsKnown() {
-			if !result.True() {
-				// An unknown error message is only a problem when the condition actually
-				// fails, since we need to display it to the user.
-				diags = diags.Append(&hcl.Diagnostic{
-					Severity:    hcl.DiagError,
-					Summary:     "Invalid error message",
-					Detail:      "Unsuitable value for error message: expression refers to values that won't be known until the apply phase.",
-					Subject:     validation.ErrorMessage.Range().Ptr(),
-					Expression:  validation.ErrorMessage,
-					EvalContext: hclCtx,
-				})
-				return diags
-			}
+			diags = diags.Append(&hcl.Diagnostic{
+				Severity:    hcl.DiagError,
+				Summary:     "Invalid error message",
+				Detail:      "Unsuitable value for error message: expression refers to values that won't be known until the apply phase.",
+				Subject:     validation.ErrorMessage.Range().Ptr(),
+				Expression:  validation.ErrorMessage,
+				EvalContext: hclCtx,
+			})
+			return diags
 		} else if !errorValue.IsNull() {
 			errorValue, err = convert.Convert(errorValue, cty.String)
 			if err != nil {
diff --git a/internal/stacks/stackruntime/internal/stackeval/input_variable_test.go b/internal/stacks/stackruntime/internal/stackeval/input_variable_test.go
index 2288c62b24..e8c3f27457 100644
--- a/internal/stacks/stackruntime/internal/stackeval/input_variable_test.go
+++ b/internal/stacks/stackruntime/internal/stackeval/input_variable_test.go
@@ -665,9 +665,12 @@ func TestEvalVariableValidation(t *testing.T) {
 
 	// --- Unknown error message ---
 
-	t.Run("error message unknown, condition fails → Invalid error message (no failure diag)", func(t *testing.T) {
-		// We must flag the unknown error message but must NOT also emit
-		// "Invalid value for variable" because we return early.
+	t.Run("error message unknown, condition fails → Invalid error message only", func(t *testing.T) {
+		// An unknown error_message is always a structural problem: the validation
+		// block is invalid regardless of whether the condition passes or fails,
+		// because Terraform can never safely display the message.
+		// We return early on the unknown message, so "Invalid value for variable"
+		// must NOT also be emitted.
 		rule := makeFakeRule(
 			hcltest.MockExprLiteral(cty.False),
 			hcltest.MockExprLiteral(cty.UnknownVal(cty.String)),
@@ -684,16 +687,26 @@ func TestEvalVariableValidation(t *testing.T) {
 		}
 	})
 
-	t.Run("error message unknown, condition passes → no diagnostic about error message", func(t *testing.T) {
-		// Unknown error message is only a problem when the condition actually
-		// fails (because we'd need to display it).  When the condition passes
-		// the unknown error message must be silently ignored.
+	t.Run("error message unknown, condition passes → Invalid error message only", func(t *testing.T) {
+		// An unknown error_message is always a structural problem: the validation
+		// block is invalid regardless of whether the condition passes or fails,
+		// because Terraform can never safely display the message.
+		// We return early on the unknown message, so "Invalid value for variable"
+		// must NOT be emitted even though the condition passed.
 		rule := makeFakeRule(
 			hcltest.MockExprLiteral(cty.True),
 			hcltest.MockExprLiteral(cty.UnknownVal(cty.String)),
 		)
 		diags := evalVariableValidation(rule, makeVarCtx(cty.StringVal("good")), valueRange)
-		assertNoDiags(t, diags)
+		assertMatchingDiag(t, diags, func(d tfdiags.Diagnostic) bool {
+			return d.Severity() == tfdiags.Error &&
+				d.Description().Summary == "Invalid error message"
+		})
+		for _, d := range diags {
+			if d.Description().Summary == "Invalid value for variable" {
+				t.Errorf("unexpected 'Invalid value for variable' when error message is unknown")
+			}
+		}
 	})
 
 	// --- Sensitive variable value in condition expression ---