Share additional tool declarations across response item clones (#54945)

## Why

Cloning `ResponseItem::AdditionalTools` deep-copies its tool declarations for history and request copies.

## What changed

Store `tools` in an `Arc<[serde_json::Value]>` so clones share immutable declarations. Update constructors and existing test fixtures to use the shared representation.

GitOrigin-RevId: eb8831774afb45fe8431a796f34ae9d1243e884a

codex-rs/core/src/agent/control_tests.rs

@@ -2880,7 +2880,7 @@ async fn spawn_agent_can_fork_parent_thread_history_with_sanitized_items() {
     let tool_declarations = ResponseItem::AdditionalTools {
         id: None,
         role: "developer".to_string(),
-        tools: vec![serde_json::json!({"type": "function", "name": "parent_tool"})],
+        tools: Arc::from([serde_json::json!({"type": "function", "name": "parent_tool"})]),
     };
     let tool_state = serde_json::json!({"top_level_tools": {"function:parent_tool": "hash"}})
         .as_object()
@@ -3286,7 +3286,7 @@ async fn spawn_agent_fork_strips_parent_usage_hints_from_compacted_history(marke
     let tool_declarations = ResponseItem::AdditionalTools {
         id: None,
         role: "developer".to_string(),
-        tools: vec![serde_json::json!({"type": "function", "name": "compacted_tool"})],
+        tools: Arc::from([serde_json::json!({"type": "function", "name": "compacted_tool"})]),
     };
     let tools_update = ContextualUserFragment::into(IncrementalToolsHint(
         codex_prompts::ResolvedIncrementalToolMessages::default().tool_update_hint,
@@ -3865,7 +3865,7 @@ async fn spawn_agent_full_fork_legacy_compaction_rebuilds_child_instructions_onc
         rollout_items.push(rollout_response_item(ResponseItem::AdditionalTools {
             id: None,
             role: "developer".to_string(),
-            tools: vec![serde_json::json!({"type": "function", "name": "parent_tool"})],
+            tools: Arc::from([serde_json::json!({"type": "function", "name": "parent_tool"})]),
         }));
         rollout_items.push(RolloutItem::TurnContext(
             turn_context.to_turn_context_item(),

codex-rs/core/src/client.rs

@@ -986,7 +986,7 @@ impl ModelClient {
                         Uuid::new_v5(&prefix_namespace, &serde_json::to_vec(&tools)?),
                     )),
                     role: "developer".to_string(),
-                    tools,
+                    tools: Arc::from(tools),
                 });
             }
             None

codex-rs/core/src/context/world_state/top_level_tools.rs

@@ -213,7 +213,7 @@ impl WorldStateSection for TopLevelToolsState {
                 ResponseItem::AdditionalTools {
                     id: None,
                     role: "developer".to_string(),
-                    tools,
+                    tools: Arc::from(tools),
                 },
             ));
         }

codex-rs/core/src/context/world_state/top_level_tools_tests.rs

@@ -25,7 +25,7 @@ fn item(tools: Vec<Value>) -> ResponseItem {
     ResponseItem::AdditionalTools {
         id: None,
         role: "developer".to_string(),
-        tools,
+        tools: Arc::from(tools),
     }
 }
 

codex-rs/core/src/context/world_state/world_state_tests.rs

@@ -9,6 +9,7 @@ use pretty_assertions::assert_ne;
 use serde::Deserialize;
 use serde::Serialize;
 use serde_json::json;
+use std::sync::Arc;
 
 #[derive(Clone, Deserialize, Serialize)]
 struct TestSection {
@@ -443,9 +444,9 @@ impl WorldStateSection for MixedOutputSection {
                 content: WorldStateUpdateContent::Item(Box::new(ResponseItem::AdditionalTools {
                     id: None,
                     role: "developer".to_string(),
-                    tools: vec![
+                    tools: Arc::from([
                         json!({"type": "function", "name": "example", "description": self.0.to_string()}),
-                    ],
+                    ]),
                 })),
             },
             WorldStateUpdate {
@@ -457,7 +458,7 @@ impl WorldStateSection for MixedOutputSection {
                 content: WorldStateUpdateContent::Item(Box::new(ResponseItem::AdditionalTools {
                     id: None,
                     role: "developer".to_string(),
-                    tools: vec![json!({"type": "function", "name": "context_tool"})],
+                    tools: Arc::from([json!({"type": "function", "name": "context_tool"})]),
                 })),
             },
             WorldStateUpdate::fragment(TestFragment("after tools".to_string())),
@@ -481,13 +482,13 @@ fn mixed_output_preserves_order_and_restores_its_snapshot() {
         ResponseItem::AdditionalTools {
             id: None,
             role: "developer".to_string(),
-            tools: vec![json!({"type": "function", "name": "example", "description": "2"})],
+            tools: Arc::from([json!({"type": "function", "name": "example", "description": "2"})]),
         },
         ContextualUserFragment::into(TestFragment("prefix instructions".to_string())),
         ResponseItem::AdditionalTools {
             id: None,
             role: "developer".to_string(),
-            tools: vec![json!({"type": "function", "name": "context_tool"})],
+            tools: Arc::from([json!({"type": "function", "name": "context_tool"})]),
         },
         ContextualUserFragment::into(TestFragment("after tools".to_string())),
     ];

codex-rs/core/src/context_manager/updates_tests.rs

@@ -8,6 +8,7 @@ use codex_protocol::models::InternalChatMessageMetadataPassthrough;
 use codex_protocol::models::input_text;
 use pretty_assertions::assert_eq;
 use serde_json::json;
+use std::sync::Arc;
 
 struct TestFragment {
     role: &'static str,
@@ -76,7 +77,7 @@ fn additional_tools() -> ResponseItem {
     ResponseItem::AdditionalTools {
         id: None,
         role: "developer".to_string(),
-        tools: vec![json!({"type": "function", "name": "lookup", "parameters": {}})],
+        tools: Arc::from([json!({"type": "function", "name": "lookup", "parameters": {}})]),
     }
 }
 

codex-rs/core/src/session/tests.rs

@@ -365,7 +365,7 @@ fn assign_missing_response_item_ids_assigns_additional_tools_ids() {
     let items = Cow::Owned(vec![ResponseItem::AdditionalTools {
         id: None,
         role: "developer".to_string(),
-        tools: Vec::new(),
+        tools: Arc::from([]),
     }]);
 
     let items = Session::assign_missing_response_item_ids(items);

codex-rs/core/src/session/world_state_item_tests.rs

@@ -77,7 +77,7 @@ fn additional_tools(name: &str) -> ResponseItem {
     ResponseItem::AdditionalTools {
         id: None,
         role: "developer".to_string(),
-        tools: vec![json!({"type": "function", "name": name})],
+        tools: Arc::from([json!({"type": "function", "name": name})]),
     }
 }
 

codex-rs/ext/guardian-v2/src/async_scorer/sampler.rs

@@ -189,7 +189,7 @@ impl LunaSampler {
                 ResponseItemEnvelope::new(ResponseItem::AdditionalTools {
                     id: None,
                     role: "developer".to_owned(),
-                    tools: Vec::new(),
+                    tools: Arc::from([]),
                 }),
                 ResponseItemEnvelope::new(ResponseItem::from(request.instructions.clone())),
             ];

codex-rs/protocol/src/models.rs

@@ -2,6 +2,7 @@ use std::collections::HashMap;
 use std::io;
 use std::num::NonZeroUsize;
 use std::path::Path;
+use std::sync::Arc;
 
 use codex_utils_image::PromptImageMode;
 use codex_utils_image::data_url_from_bytes;
@@ -1055,7 +1056,8 @@ pub enum ResponseItem {
         id: Option<ResponseItemId>,
         role: String,
         #[ts(type = "unknown[]")]
-        tools: Vec<serde_json::Value>,
+        // Share immutable declarations across history and request copies.
+        tools: Arc<[serde_json::Value]>,
     },
     Message {
         #[serde(default, skip_serializing_if = "Option::is_none")]
@@ -2807,7 +2809,7 @@ mod tests {
         let mut additional_tools = ResponseItem::AdditionalTools {
             id: None,
             role: "developer".to_string(),
-            tools: Vec::new(),
+            tools: Arc::from([]),
         };
         additional_tools.set_id(Some(ResponseItemId::with_suffix("at", "test")));
         assert_eq!(

codex-rs/rollout/src/recorder_tests.rs

@@ -638,14 +638,14 @@ async fn recorder_preserves_additional_tools_in_both_history_modes() -> std::io:
             ResponseItem::AdditionalTools {
                 id: None,
                 role: "developer".to_string(),
-                tools: vec![serde_json::json!({
+                tools: Arc::from([serde_json::json!({
                     "type": "namespace",
                     "name": "functions",
                     "description": "Available tools.",
                     "tools": [{"type": "function", "name": "lookup",
                         "description": "Look up a value.",
                         "parameters": {"type": "object", "properties": {}}}]
-                })],
+                })]),
             }
             .into(),
         );