Expose model overrides for multi-agent v2 spawns (#32749)

## What changed

- Add the `features.multi_agent_v2.expose_spawn_agent_model_overrides` setting, enabled by default, to expose `model` and `reasoning_effort` on the v2 `spawn_agent` tool.
- Keep these controls available when other spawn metadata is hidden, while allowing them to be disabled independently.
- Add root-agent and subagent guidance that overrides require a partial or context-free fork and should only be used when explicitly authorized.

## Testing

- Cover configuration parsing and defaults, usage-hint preservation, and tool-schema behavior with override exposure enabled and disabled.

GitOrigin-RevId: 92370498108c96fbd51f32965624ce531e991d9a

codex-rs/core/src/config/mod.rs

@@ -248,6 +248,7 @@ Payload:
 ```
 You may also see them addressed as to=/root/..., which indicates your identity is /root/...
 "#;
+const DEFAULT_MULTI_AGENT_V2_MODEL_OVERRIDE_USAGE_HINT_TEXT: &str = "Full-history forks (`fork_turns` omitted or `\"all\"`) inherit the parent model and reasoning effort and do not accept overrides. Only set `model` or `reasoning_effort` when explicitly requested by the user, applicable `AGENTS.md` instructions, or skill instructions; when doing so, set `fork_turns` to `\"none\"` or a positive integer string.";
 const DEFAULT_MULTI_AGENT_V2_TOOL_NAMESPACE: &str = "collaboration";
 const DEFAULT_MULTI_AGENT_V2_SHARED_USAGE_HINT_TEXT: &str = r#"Note that collaboration tools cannot be called from inside `functions.exec`. Call `spawn_agent`, `send_message`, `followup_task`, `wait_agent`, `interrupt_agent`, and `list_agents` only as direct tool calls using the recipient shown in their tool definitions, such as `to=functions.collaboration.spawn_agent`, since they are intentionally absent from the `functions.exec` `tools.*` namespace. Available tools in `functions.exec` are explicitly described with a `tools` namespace in the developer message.
 
@@ -1150,6 +1151,7 @@ pub struct MultiAgentV2Config {
     pub multi_agent_mode_hint_text: Option<String>,
     pub tool_namespace: Option<String>,
     pub hide_spawn_agent_metadata: bool,
+    pub expose_spawn_agent_model_overrides: bool,
     pub non_code_mode_only: bool,
 }
 
@@ -1172,6 +1174,7 @@ impl MultiAgentV2Config {
             multi_agent_mode_hint_text: None,
             tool_namespace: Some(DEFAULT_MULTI_AGENT_V2_TOOL_NAMESPACE.to_string()),
             hide_spawn_agent_metadata: true,
+            expose_spawn_agent_model_overrides: true,
             non_code_mode_only: true,
         }
     }
@@ -2516,13 +2519,31 @@ fn resolve_multi_agent_v2_config(config_toml: &ConfigToml) -> MultiAgentV2Config
         .and_then(|config| config.usage_hint_text.as_ref())
         .cloned()
         .or(default.usage_hint_text);
+    let hide_spawn_agent_metadata = base
+        .and_then(|config| config.hide_spawn_agent_metadata)
+        .unwrap_or(default.hide_spawn_agent_metadata);
+    let expose_spawn_agent_model_overrides = base
+        .and_then(|config| config.expose_spawn_agent_model_overrides)
+        .unwrap_or(default.expose_spawn_agent_model_overrides);
+    let mut default_root_agent_usage_hint_text = default.root_agent_usage_hint_text;
+    let mut default_subagent_usage_hint_text = default.subagent_usage_hint_text;
+    if expose_spawn_agent_model_overrides {
+        default_root_agent_usage_hint_text = Some(append_usage_hint_text(
+            default_root_agent_usage_hint_text.as_deref(),
+            DEFAULT_MULTI_AGENT_V2_MODEL_OVERRIDE_USAGE_HINT_TEXT,
+        ));
+        default_subagent_usage_hint_text = Some(append_usage_hint_text(
+            default_subagent_usage_hint_text.as_deref(),
+            DEFAULT_MULTI_AGENT_V2_MODEL_OVERRIDE_USAGE_HINT_TEXT,
+        ));
+    }
     let root_agent_usage_hint_text = resolve_optional_prompt_text(
         base.map(|config| &config.root_agent_usage_hint_text),
-        default.root_agent_usage_hint_text,
+        default_root_agent_usage_hint_text,
     );
     let subagent_usage_hint_text = resolve_optional_prompt_text(
         base.map(|config| &config.subagent_usage_hint_text),
-        default.subagent_usage_hint_text,
+        default_subagent_usage_hint_text,
     );
     let multi_agent_mode_hint_text = base
         .and_then(|config| config.multi_agent_mode_hint_text.as_ref())
@@ -2532,9 +2553,6 @@ fn resolve_multi_agent_v2_config(config_toml: &ConfigToml) -> MultiAgentV2Config
         .and_then(|config| config.tool_namespace.as_ref())
         .cloned()
         .or(default.tool_namespace);
-    let hide_spawn_agent_metadata = base
-        .and_then(|config| config.hide_spawn_agent_metadata)
-        .unwrap_or(default.hide_spawn_agent_metadata);
     let non_code_mode_only = base
         .and_then(|config| config.non_code_mode_only)
         .unwrap_or(default.non_code_mode_only);
@@ -2550,6 +2568,7 @@ fn resolve_multi_agent_v2_config(config_toml: &ConfigToml) -> MultiAgentV2Config
         multi_agent_mode_hint_text,
         tool_namespace,
         hide_spawn_agent_metadata,
+        expose_spawn_agent_model_overrides,
         non_code_mode_only,
     }
 }
@@ -2734,6 +2753,13 @@ fn resolve_optional_prompt_text(
     }
 }
 
+fn append_usage_hint_text(usage_hint_text: Option<&str>, additional_text: &str) -> String {
+    match usage_hint_text {
+        Some(usage_hint_text) => format!("{usage_hint_text}\n\n{additional_text}"),
+        None => additional_text.to_string(),
+    }
+}
+
 fn code_mode_toml_config(features: Option<&FeaturesToml>) -> Option<&CodeModeConfigToml> {
     match features?.code_mode.as_ref()? {
         FeatureToml::Enabled(_) => None,