Support standalone tool outputs in turn/start (#41002)

## What changed

- Add an optional `toolOutput` to `turn/start` for starting or steering a turn with a named function-call output instead of user input.
- Emit standalone outputs as `functionCallOutput` thread items, persist them in durable history, and include them in resumed threads.
- Validate that tool outputs have a name, are not combined with nonempty `input`, satisfy the text-size limit, and use supported image URLs.

## Testing

- Cover standalone tool-output notifications, model input, and resumed history in both legacy and paginated history modes.
- Cover the text-size limit for structured tool output.

GitOrigin-RevId: a1eeec7b9b7de67b1a1c5e829a54552b85ec118b

codex-rs/app-server-protocol/src/protocol/thread_history.rs

@@ -604,6 +604,7 @@ impl ThreadHistoryBuilder {
         let should_upsert = match item {
             codex_protocol::items::TurnItem::Plan(plan) => !plan.text.is_empty(),
             codex_protocol::items::TurnItem::HookPrompt(_)
+            | codex_protocol::items::TurnItem::FunctionCallOutput(_)
             | codex_protocol::items::TurnItem::CommandExecution(_)
             | codex_protocol::items::TurnItem::DynamicToolCall(_)
             | codex_protocol::items::TurnItem::CollabAgentToolCall(_)

codex-rs/app-server-protocol/src/protocol/v2/item.rs

@@ -34,6 +34,7 @@ use codex_protocol::items::McpToolCallStatus as CoreMcpToolCallStatus;
 use codex_protocol::items::TurnItem as CoreTurnItem;
 use codex_protocol::memory_citation::MemoryCitation as CoreMemoryCitation;
 use codex_protocol::memory_citation::MemoryCitationEntry as CoreMemoryCitationEntry;
+use codex_protocol::models::FunctionCallOutputBody;
 use codex_protocol::models::MessagePhase;
 use codex_protocol::models::ResponseItem;
 use codex_protocol::openai_models::ReasoningEffort;
@@ -257,6 +258,14 @@ pub enum ThreadItem {
     },
     #[serde(rename_all = "camelCase")]
     #[ts(rename_all = "camelCase")]
+    FunctionCallOutput {
+        id: String,
+        name: String,
+        namespace: Option<String>,
+        output: FunctionCallOutputBody,
+    },
+    #[serde(rename_all = "camelCase")]
+    #[ts(rename_all = "camelCase")]
     /// EXPERIMENTAL - proposed plan item content. The completed plan item is
     /// authoritative and may not match the concatenation of `PlanDelta` text.
     Plan {
@@ -429,6 +438,7 @@ impl ThreadItem {
             ThreadItem::UserMessage { id, .. }
             | ThreadItem::HookPrompt { id, .. }
             | ThreadItem::AgentMessage { id, .. }
+            | ThreadItem::FunctionCallOutput { id, .. }
             | ThreadItem::Plan { id, .. }
             | ThreadItem::Reasoning { id, .. }
             | ThreadItem::CommandExecution { id, .. }
@@ -873,6 +883,12 @@ impl From<CoreTurnItem> for ThreadItem {
                     delivery: agent.delivery,
                 }
             }
+            CoreTurnItem::FunctionCallOutput(output) => ThreadItem::FunctionCallOutput {
+                id: output.id,
+                name: output.name,
+                namespace: output.namespace,
+                output: output.output,
+            },
             CoreTurnItem::Plan(plan) => ThreadItem::Plan {
                 id: plan.id,
                 text: plan.text,

codex-rs/app-server-protocol/src/protocol/v2/tests.rs

@@ -4724,6 +4724,7 @@ fn turn_start_params_preserve_explicit_null_service_tier() {
         client_user_message_id: None,
         input: vec![],
         turn_trigger: None,
+        tool_output: None,
         responsesapi_client_metadata: None,
         additional_context: None,
         environments: None,

codex-rs/app-server-protocol/src/protocol/v2/turn.rs

@@ -9,6 +9,7 @@ use codex_protocol::config_types::CollaborationMode;
 use codex_protocol::config_types::MultiAgentMode;
 use codex_protocol::config_types::Personality;
 use codex_protocol::config_types::ReasoningSummary;
+use codex_protocol::models::FunctionCallOutputBody;
 use codex_protocol::models::ImageDetail;
 use codex_protocol::openai_models::ReasoningEffort;
 use codex_protocol::plan_tool::PlanItemArg as CorePlanItemArg;
@@ -134,6 +135,15 @@ impl From<CyberAccessProgram> for CoreCyberAccessProgram {
     }
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct TurnToolOutput {
+    pub name: String,
+    pub namespace: Option<String>,
+    pub output: FunctionCallOutputBody,
+}
+
 #[derive(
     Serialize, Deserialize, Debug, Default, Clone, PartialEq, JsonSchema, TS, ExperimentalApi,
 )]
@@ -148,6 +158,8 @@ pub struct TurnStartParams {
     /// Ignored when this request steers an already-active turn.
     #[ts(optional = nullable)]
     pub turn_trigger: Option<String>,
+    #[ts(optional = nullable)]
+    pub tool_output: Option<Box<TurnToolOutput>>,
     /// Optional metadata to enrich Codex's ResponsesAPI turn metadata.
     ///
     /// Entries are flattened into the JSON string sent as