Namespace multi-agent v2 tools under collaboration (#29067)

## Summary

Multi-agent v2 tools now use the fixed `collaboration` namespace when
namespace tools are available. This keeps the model-visible hint and the
actual tool surface aligned around `functions.collaboration.*`, without
exposing an unshipped namespace knob to users.

The PR also removes the old `features.multi_agent_v2.tool_namespace`
config/schema surface, updates the MAv2 test fixtures for namespaced
calls, and fixes stale `TurnContext.features` references that were
breaking `codex-core` builds.

## Changes

- Expose MAv2 tools under `collaboration` instead of relying on a
configurable namespace.
- Remove `tool_namespace` from MAv2 TOML config, resolved config,
validation, schema, and tests.
- Update tool-planning and integration fixtures to assert or emit
namespaced MAv2 tool calls.
- Read feature state through `TurnContext.config.features` in the
multi-agent mode context paths.

## Testing

- `just write-config-schema`
- `just test -p codex-features`

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

@@ -246,7 +246,8 @@ Payload:
 ```
 You may also see them addressed as to=/root/..., which indicates your identity is /root/...
 "#;
-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.spawn_agent` without a configured namespace or `to=functions.agents.spawn_agent` with `tool_namespace = "agents"`, 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.
+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.
 
 All agents share the same directory. In detail:
 - All agents have access to the same container and filesystem as you.
@@ -1157,7 +1158,7 @@ impl MultiAgentV2Config {
                 DEFAULT_MULTI_AGENT_V2_SUBAGENT_USAGE_HINT_TEXT,
                 max_concurrent_threads_per_session,
             )),
-            tool_namespace: None,
+            tool_namespace: Some(DEFAULT_MULTI_AGENT_V2_TOOL_NAMESPACE.to_string()),
             hide_spawn_agent_metadata: true,
             non_code_mode_only: true,
         }

codex-rs/core/src/tools/spec_plan_tests.rs

@@ -39,6 +39,8 @@ use crate::tools::router::ToolRouterParams;
 use crate::tools::router::ToolSuggestCandidates;
 use crate::tools::router::ToolSuggestPresentation;
 
+const MULTI_AGENT_V2_NAMESPACE: &str = "collaboration";
+
 #[derive(Default)]
 struct ToolPlanInputs {
     mcp_tools: Option<Vec<ToolInfo>>,
@@ -1151,19 +1153,48 @@ async fn multi_agent_feature_selects_one_agent_tool_family() {
         });
     })
     .await;
-    v2.assert_visible_contains(&[
+    v2.assert_visible_contains(&[MULTI_AGENT_V2_NAMESPACE]);
+    v2.assert_visible_lacks(&[
         "spawn_agent",
         "send_message",
         "followup_task",
         "wait_agent",
         "interrupt_agent",
         "list_agents",
+        "send_input",
+        "resume_agent",
+        "assign_task",
+        "close_agent",
     ]);
-    v2.assert_visible_lacks(&["send_input", "resume_agent", "assign_task", "close_agent"]);
-    let spawn_agent_description = match v2.visible_spec("spawn_agent") {
-        ToolSpec::Function(tool) => tool.description.as_str(),
-        other => panic!("expected spawn_agent function spec, got {other:?}"),
+    for tool_name in [
+        "spawn_agent",
+        "send_message",
+        "followup_task",
+        "wait_agent",
+        "interrupt_agent",
+        "list_agents",
+    ] {
+        assert!(
+            v2.namespace_function_names(MULTI_AGENT_V2_NAMESPACE)
+                .iter()
+                .any(|name| name == tool_name),
+            "expected {tool_name} in {MULTI_AGENT_V2_NAMESPACE} namespace"
+        );
+    }
+    let ToolSpec::Namespace(namespace) = v2.visible_spec(MULTI_AGENT_V2_NAMESPACE) else {
+        panic!("expected {MULTI_AGENT_V2_NAMESPACE} namespace");
     };
+    let Some(ResponsesApiNamespaceTool::Function(spawn_agent)) =
+        namespace.tools.iter().find(|tool| {
+            matches!(
+                tool,
+                ResponsesApiNamespaceTool::Function(tool) if tool.name == "spawn_agent"
+            )
+        })
+    else {
+        panic!("expected spawn_agent in {MULTI_AGENT_V2_NAMESPACE} namespace");
+    };
+    let spawn_agent_description = spawn_agent.description.as_str();
     assert!(!spawn_agent_description.contains("max_concurrent_threads_per_session"));
     assert!(spawn_agent_description.contains(
         "Note that passing `fork_turns=\"none\"` will not pass any surrounding context to the spawned subagent"
@@ -1183,9 +1214,11 @@ async fn multi_agent_feature_selects_one_agent_tool_family() {
         });
     })
     .await;
-    direct_model_only.assert_visible_contains(&["spawn_agent", "send_message", "wait_agent"]);
+    direct_model_only.assert_visible_contains(&[MULTI_AGENT_V2_NAMESPACE]);
+    direct_model_only.assert_visible_lacks(&["spawn_agent", "send_message", "wait_agent"]);
     assert_eq!(
-        direct_model_only.exposure("spawn_agent"),
+        direct_model_only
+            .exposure(&ToolName::namespaced(MULTI_AGENT_V2_NAMESPACE, "spawn_agent").to_string()),
         ToolExposure::DirectModelOnly
     );
 }
@@ -1196,9 +1229,17 @@ async fn multi_agent_v2_message_schemas_are_encrypted() {
         set_feature(turn, Feature::MultiAgentV2, /*enabled*/ true);
     })
     .await;
+    let ToolSpec::Namespace(namespace) = plan.visible_spec(MULTI_AGENT_V2_NAMESPACE) else {
+        panic!("expected {MULTI_AGENT_V2_NAMESPACE} namespace");
+    };
     for tool_name in ["spawn_agent", "send_message", "followup_task"] {
-        let ToolSpec::Function(tool) = plan.visible_spec(tool_name) else {
-            panic!("expected {tool_name} function spec");
+        let Some(ResponsesApiNamespaceTool::Function(tool)) = namespace.tools.iter().find(|tool| {
+            matches!(
+                tool,
+                ResponsesApiNamespaceTool::Function(tool) if tool.name == tool_name
+            )
+        }) else {
+            panic!("expected {tool_name} in {MULTI_AGENT_V2_NAMESPACE} namespace");
         };
         let properties = tool
             .parameters
@@ -1464,12 +1505,7 @@ async fn hosted_tools_follow_provider_auth_model_and_config_gates() {
             codex_code_mode::WAIT_TOOL_NAME,
             "request_user_input",
             // Multi-agent v2 tools.
-            "spawn_agent",
-            "send_message",
-            "followup_task",
-            "wait_agent",
-            "interrupt_agent",
-            "list_agents",
+            MULTI_AGENT_V2_NAMESPACE,
             // Hosted Responses tools.
             "web_search",
             "image_generation",