diff --git a/agent-client-protocol-schema/src/v2/agent.rs b/agent-client-protocol-schema/src/v2/agent.rs
index 281b13590..8d4523af0 100644
--- a/agent-client-protocol-schema/src/v2/agent.rs
+++ b/agent-client-protocol-schema/src/v2/agent.rs
@@ -2778,8 +2778,8 @@ impl SetSessionConfigOptionRequest {
 #[non_exhaustive]
 pub struct SetSessionConfigOptionResponse {
     /// The full set of configuration options and their current values.
-    #[serde_as(deserialize_as = "DefaultOnError<VecSkipError<_>>")]
-    #[cfg_attr(feature = "schemars", schemars(extend("x-deserialize-default-on-error" = true, "x-deserialize-skip-invalid-items" = true)))]
+    #[serde_as(deserialize_as = "VecSkipError<_>")]
+    #[cfg_attr(feature = "schemars", schemars(extend("x-deserialize-skip-invalid-items" = true)))]
     pub config_options: Vec<SessionConfigOption>,
     /// The _meta property is reserved by ACP to allow clients and agents to attach additional
     /// metadata to their interactions. Omitted and `null` are equivalent and mean no metadata.
diff --git a/agent-client-protocol-schema/src/v2/client.rs b/agent-client-protocol-schema/src/v2/client.rs
index bd83dc7cd..9a00dde82 100644
--- a/agent-client-protocol-schema/src/v2/client.rs
+++ b/agent-client-protocol-schema/src/v2/client.rs
@@ -1338,8 +1338,8 @@ fn other_session_update_schema(schema: &mut Schema) {
 #[non_exhaustive]
 pub struct ConfigOptionUpdate {
     /// The full set of configuration options and their current values.
-    #[serde_as(deserialize_as = "DefaultOnError<VecSkipError<_>>")]
-    #[cfg_attr(feature = "schemars", schemars(extend("x-deserialize-default-on-error" = true, "x-deserialize-skip-invalid-items" = true)))]
+    #[serde_as(deserialize_as = "VecSkipError<_>")]
+    #[cfg_attr(feature = "schemars", schemars(extend("x-deserialize-skip-invalid-items" = true)))]
     pub config_options: Vec<SessionConfigOption>,
     /// The _meta property is reserved by ACP to allow clients and agents to attach additional
     /// metadata to their interactions. Omitted and `null` are equivalent and mean no metadata.
@@ -2271,8 +2271,8 @@ impl MessageId {
 #[non_exhaustive]
 pub struct AvailableCommandsUpdate {
     /// Commands the agent can execute.
-    #[serde_as(deserialize_as = "DefaultOnError<VecSkipError<_>>")]
-    #[cfg_attr(feature = "schemars", schemars(extend("x-deserialize-default-on-error" = true, "x-deserialize-skip-invalid-items" = true)))]
+    #[serde_as(deserialize_as = "VecSkipError<_>")]
+    #[cfg_attr(feature = "schemars", schemars(extend("x-deserialize-skip-invalid-items" = true)))]
     pub available_commands: Vec<AvailableCommand>,
     /// The _meta property is reserved by ACP to allow clients and agents to attach additional
     /// metadata to their interactions. Omitted and `null` are equivalent and mean no metadata.
@@ -3684,6 +3684,43 @@ impl AgentNotification {
 #[cfg(test)]
 mod tests {
 
+    #[test]
+    fn replacement_lists_reject_non_arrays_instead_of_clearing() {
+        use serde_json::json;
+        for (update, key) in [
+            ("config_option_update", "configOptions"),
+            ("available_commands_update", "availableCommands"),
+        ] {
+            let empty = json!({ "sessionUpdate": update, key: [] });
+            serde_json::from_value::<super::SessionUpdate>(empty).unwrap();
+            for bad in [json!("oops"), json!(null), json!({})] {
+                let wire = json!({ "sessionUpdate": update, key: bad });
+                assert!(
+                    serde_json::from_value::<super::SessionUpdate>(wire).is_err(),
+                    "{update} {bad}"
+                );
+            }
+        }
+        let plan = |entries: serde_json::Value| {
+            json!({ "sessionUpdate": "plan_update",
+                "plan": { "type": "items", "planId": "p", "entries": entries } })
+        };
+        serde_json::from_value::<super::SessionUpdate>(plan(json!([]))).unwrap();
+        assert!(serde_json::from_value::<super::SessionUpdate>(plan(json!("oops"))).is_err());
+        // Invalid items are still skipped.
+        let skipped =
+            plan(json!([42, { "content": "a", "priority": "high", "status": "pending" }]));
+        let super::SessionUpdate::PlanUpdate(update) =
+            serde_json::from_value::<super::SessionUpdate>(skipped).unwrap()
+        else {
+            panic!("expected plan_update");
+        };
+        let crate::v2::PlanUpdateContent::Items(items) = update.plan else {
+            panic!("expected items");
+        };
+        assert_eq!(items.entries.len(), 1);
+    }
+
     #[test]
     fn other_available_command_preserves_meta_verbatim() {
         for meta in [serde_json::json!({ "k": 1 }), serde_json::json!("bad")] {
diff --git a/agent-client-protocol-schema/src/v2/plan.rs b/agent-client-protocol-schema/src/v2/plan.rs
index d172502a2..56a195700 100644
--- a/agent-client-protocol-schema/src/v2/plan.rs
+++ b/agent-client-protocol-schema/src/v2/plan.rs
@@ -238,8 +238,8 @@ pub struct PlanItems {
     ///
     /// When updating an item-based plan, the agent must send a complete list of all entries
     /// with their current status. The client replaces that plan with each update.
-    #[serde_as(deserialize_as = "DefaultOnError<VecSkipError<_>>")]
-    #[cfg_attr(feature = "schemars", schemars(extend("x-deserialize-default-on-error" = true, "x-deserialize-skip-invalid-items" = true)))]
+    #[serde_as(deserialize_as = "VecSkipError<_>")]
+    #[cfg_attr(feature = "schemars", schemars(extend("x-deserialize-skip-invalid-items" = true)))]
     pub entries: Vec<PlanEntry>,
     /// The _meta property is reserved by ACP to allow clients and agents to attach additional
     /// metadata to their interactions. Omitted and `null` are equivalent and mean no metadata.
diff --git a/schema/v2/schema.json b/schema/v2/schema.json
index e5a7412ef..2009df5b6 100644
--- a/schema/v2/schema.json
+++ b/schema/v2/schema.json
@@ -4274,7 +4274,6 @@
           "items": {
             "$ref": "#/$defs/SessionConfigOption"
           },
-          "x-deserialize-default-on-error": true,
           "x-deserialize-skip-invalid-items": true
         },
         "_meta": {
@@ -5815,7 +5814,6 @@
           "items": {
             "$ref": "#/$defs/PlanEntry"
           },
-          "x-deserialize-default-on-error": true,
           "x-deserialize-skip-invalid-items": true
         },
         "_meta": {
@@ -5858,7 +5856,6 @@
           "items": {
             "$ref": "#/$defs/AvailableCommand"
           },
-          "x-deserialize-default-on-error": true,
           "x-deserialize-skip-invalid-items": true
         },
         "_meta": {
@@ -5880,7 +5877,6 @@
           "items": {
             "$ref": "#/$defs/SessionConfigOption"
           },
-          "x-deserialize-default-on-error": true,
           "x-deserialize-skip-invalid-items": true
         },
         "_meta": {
diff --git a/schema/v2/schema.unstable.json b/schema/v2/schema.unstable.json
index c2a9e691b..2c8abe970 100644
--- a/schema/v2/schema.unstable.json
+++ b/schema/v2/schema.unstable.json
@@ -4757,7 +4757,6 @@
           "items": {
             "$ref": "#/$defs/SessionConfigOption"
           },
-          "x-deserialize-default-on-error": true,
           "x-deserialize-skip-invalid-items": true
         },
         "_meta": {
@@ -6556,7 +6555,6 @@
           "items": {
             "$ref": "#/$defs/PlanEntry"
           },
-          "x-deserialize-default-on-error": true,
           "x-deserialize-skip-invalid-items": true
         },
         "_meta": {
@@ -6671,7 +6669,6 @@
           "items": {
             "$ref": "#/$defs/AvailableCommand"
           },
-          "x-deserialize-default-on-error": true,
           "x-deserialize-skip-invalid-items": true
         },
         "_meta": {
@@ -6693,7 +6690,6 @@
           "items": {
             "$ref": "#/$defs/SessionConfigOption"
           },
-          "x-deserialize-default-on-error": true,
           "x-deserialize-skip-invalid-items": true
         },
         "_meta": {