diff --git a/.changes/v1.15/ENHANCEMENTS-20251123-224500.yaml b/.changes/v1.15/ENHANCEMENTS-20251123-224500.yaml
new file mode 100644
index 000000000000..68498dd5ab1b
--- /dev/null
+++ b/.changes/v1.15/ENHANCEMENTS-20251123-224500.yaml
@@ -0,0 +1,3 @@
+kind: ENHANCEMENTS
+body: 'core: Improved error message when an action trigger references an action expanded with count or for_each without specifying an instance'
+time: 2025-11-23T22:45:00.000000+00:00
diff --git a/internal/terraform/context_plan_actions_test.go b/internal/terraform/context_plan_actions_test.go
index 014a08489e1e..69befce568cf 100644
--- a/internal/terraform/context_plan_actions_test.go
+++ b/internal/terraform/context_plan_actions_test.go
@@ -967,7 +967,7 @@ resource "test_object" "a" {
 					return diags.Append(&hcl.Diagnostic{
 						Severity: hcl.DiagError,
 						Summary:  "Reference to non-existent action instance",
-						Detail:   "Action instance was not found in the current context.",
+						Detail:   "The referenced action could not be found. If the action uses 'count' or 'for_each', ensure you reference a specific instance, such as action.example[0] or action.example[\"key\"].",
 						Subject: &hcl.Range{
 							Filename: filepath.Join(m.Module.SourceDir, "main.tf"),
 							Start:    hcl.Pos{Line: 13, Column: 18, Byte: 224},
@@ -1002,7 +1002,7 @@ resource "test_object" "a" {
 					return diags.Append(&hcl.Diagnostic{
 						Severity: hcl.DiagError,
 						Summary:  "Reference to non-existent action instance",
-						Detail:   "Action instance was not found in the current context.",
+						Detail:   "The referenced action could not be found. If the action uses 'count' or 'for_each', ensure you reference a specific instance, such as action.example[0] or action.example[\"key\"].",
 						Subject: &hcl.Range{
 							Filename: filepath.Join(m.Module.SourceDir, "main.tf"),
 							Start:    hcl.Pos{Line: 13, Column: 18, Byte: 208},
diff --git a/internal/terraform/node_action_trigger_instance_plan.go b/internal/terraform/node_action_trigger_instance_plan.go
index a32b71f04f67..38982967a20f 100644
--- a/internal/terraform/node_action_trigger_instance_plan.go
+++ b/internal/terraform/node_action_trigger_instance_plan.go
@@ -107,7 +107,7 @@ func (n *nodeActionTriggerPlanInstance) Execute(ctx EvalContext, operation walkO
 		diags = diags.Append(&hcl.Diagnostic{
 			Severity: hcl.DiagError,
 			Summary:  "Reference to non-existent action instance",
-			Detail:   "Action instance was not found in the current context.",
+			Detail:   "The referenced action could not be found. If the action uses 'count' or 'for_each', ensure you reference a specific instance, such as action.example[0] or action.example[\"key\"].",
 			Subject:  n.lifecycleActionTrigger.invokingSubject,
 		})
 		return diags