Negotiate MCP extensions per app-server session (#36910)

## Why

App-server clients need to advertise structured MCP extension settings, including supported MCP App UI MIME types, rather than only opting into OpenAI form elicitation with a boolean.

## What changed

- Add an `extensions` map to initialize capabilities and preserve `mcpServerOpenaiFormElicitation` as a legacy alias for `openai/form`.
- Capture the declared extension profile when a thread is started, resumed, or forked, propagate it to subagents, and advertise it to downstream MCP servers during initialization.
- Keep the profile stable for the lifetime of the loaded session instead of changing it on later turns or direct tool calls.

## Testing

- Cover extension conversion, downstream MCP initialization, session isolation, legacy form support, and subagent inheritance.

GitOrigin-RevId: fbcedbb74ce788e574b0f884a4c45c4cedb9de54

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

@@ -558,26 +558,6 @@ impl Session {
             .await
     }
 
-    pub(crate) async fn set_openai_form_elicitation_support(
-        &self,
-        supported: bool,
-    ) -> anyhow::Result<()> {
-        if self
-            .services
-            .supports_openai_form_elicitation
-            .load(std::sync::atomic::Ordering::Relaxed)
-            == supported
-        {
-            return Ok(());
-        }
-
-        self.services
-            .supports_openai_form_elicitation
-            .store(supported, std::sync::atomic::Ordering::Relaxed);
-        self.request_mcp_runtime_refresh();
-        Ok(())
-    }
-
     pub(crate) async fn refresh_mcp_servers_now(
         &self,
         turn_context: &TurnContext,

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

@@ -138,10 +138,6 @@ impl Session {
         elicitation_reviewer: Option<ElicitationReviewerHandle>,
     ) -> McpRuntimeInput {
         let auth = desired.auth.clone();
-        let supports_openai_form_elicitation = self
-            .services
-            .supports_openai_form_elicitation
-            .load(std::sync::atomic::Ordering::Acquire);
         let McpRuntimeProjection {
             mut config,
             plugins_available,
@@ -186,7 +182,7 @@ impl Session {
             codex_apps_tools_cache: self.services.mcp_manager.codex_apps_tools_cache(),
             tool_catalog_cache: self.services.mcp_manager.tool_catalog_cache(),
             codex_apps_tools_cache_key: connector_runtime_context_key(auth.as_ref()),
-            supports_openai_form_elicitation,
+            client_mcp_extensions: self.services.client_mcp_extensions.clone(),
             auth,
             codex_apps_auth_manager,
             elicitation_reviewer,

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

@@ -340,6 +340,7 @@ use codex_protocol::config_types::CollaborationMode;
 use codex_protocol::config_types::Personality;
 use codex_protocol::config_types::ReasoningSummary as ReasoningSummaryConfig;
 use codex_protocol::config_types::WindowsSandboxLevel;
+use codex_protocol::mcp::ClientMcpExtensions;
 use codex_protocol::models::LocalImagePreparation;
 use codex_protocol::models::ResponseInputItem;
 use codex_protocol::models::ResponseItem;
@@ -448,7 +449,7 @@ pub(crate) struct SessionSpawnArgs {
     pub(crate) parent_trace: Option<W3cTraceContext>,
     pub(crate) environment_selections: Vec<TurnEnvironmentSelection>,
     pub(crate) thread_extension_init: ExtensionDataInit,
-    pub(crate) supports_openai_form_elicitation: bool,
+    pub(crate) client_mcp_extensions: ClientMcpExtensions,
     pub(crate) analytics_events_client: Option<AnalyticsEventsClient>,
     pub(crate) thread_store: Arc<dyn ThreadStore>,
     pub(crate) attestation_provider: Option<Arc<dyn AttestationProvider>>,
@@ -540,7 +541,7 @@ impl Session {
             parent_trace: _,
             environment_selections,
             thread_extension_init,
-            supports_openai_form_elicitation,
+            client_mcp_extensions,
             analytics_events_client,
             thread_store,
             attestation_provider,
@@ -731,7 +732,7 @@ impl Session {
             code_mode_session_provider,
             extensions,
             thread_extension_init,
-            supports_openai_form_elicitation,
+            client_mcp_extensions,
             agent_control,
             environment_manager,
             inherited_environments,

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

@@ -17,6 +17,7 @@ use codex_protocol::SessionId;
 use codex_protocol::capabilities::SelectedCapabilityRoot;
 use codex_protocol::config_types::SERVICE_TIER_DEFAULT_REQUEST_VALUE;
 use codex_protocol::config_types::ServiceTier;
+use codex_protocol::mcp::ClientMcpExtensions;
 use codex_protocol::permissions::FileSystemPath;
 use codex_protocol::permissions::FileSystemSpecialPath;
 use codex_protocol::protocol::MultiAgentVersion;
@@ -515,7 +516,7 @@ impl Session {
         code_mode_session_provider: Arc<dyn codex_code_mode::CodeModeSessionProvider>,
         extensions: Arc<codex_extension_api::ExtensionRegistry<crate::config::Config>>,
         mut thread_extension_init: ExtensionDataInit,
-        supports_openai_form_elicitation: bool,
+        client_mcp_extensions: ClientMcpExtensions,
         agent_control: AgentControl,
         environment_manager: Arc<EnvironmentManager>,
         inherited_environments: Option<TurnEnvironmentSnapshot>,
@@ -1126,9 +1127,7 @@ impl Session {
                 thread_extension_data,
                 selected_capability_roots,
                 mcp_thread_init,
-                supports_openai_form_elicitation: std::sync::atomic::AtomicBool::new(
-                    supports_openai_form_elicitation,
-                ),
+                client_mcp_extensions,
                 agent_control,
                 network_proxy: arc_swap::ArcSwapOption::from(network_proxy.map(Arc::new)),
                 network_proxy_audit_metadata,

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

@@ -5546,7 +5546,7 @@ async fn session_new_fails_when_zsh_fork_enabled_without_packaged_zsh() {
         Arc::new(codex_code_mode::DisabledCodeModeSessionProvider),
         Arc::new(codex_extension_api::ExtensionRegistryBuilder::new().build()),
         codex_extension_api::ExtensionDataInit::default(),
-        /*supports_openai_form_elicitation*/ false,
+        ClientMcpExtensions::default(),
         AgentControl::default(),
         environment_manager,
         /*inherited_environments*/ None,
@@ -5742,7 +5742,7 @@ pub(crate) async fn make_session_and_context() -> (Session, TurnContext) {
         thread_extension_data: codex_extension_api::ExtensionData::new(thread_id.to_string()),
         selected_capability_roots: Vec::new(),
         mcp_thread_init: codex_extension_api::ExtensionDataInit::default(),
-        supports_openai_form_elicitation: std::sync::atomic::AtomicBool::new(false),
+        client_mcp_extensions: ClientMcpExtensions::default(),
         agent_control,
         network_proxy: arc_swap::ArcSwapOption::from(None),
         network_proxy_audit_metadata: crate::config::NetworkProxyAuditMetadata::default(),
@@ -5956,7 +5956,7 @@ async fn make_session_with_config_and_rx(
         Arc::new(codex_code_mode::DisabledCodeModeSessionProvider),
         Arc::new(codex_extension_api::ExtensionRegistryBuilder::new().build()),
         codex_extension_api::ExtensionDataInit::default(),
-        /*supports_openai_form_elicitation*/ false,
+        ClientMcpExtensions::default(),
         AgentControl::default(),
         environment_manager,
         /*inherited_environments*/ None,
@@ -6066,7 +6066,7 @@ async fn make_session_with_history_source_and_agent_control_and_rx(
         Arc::new(codex_code_mode::DisabledCodeModeSessionProvider),
         Arc::new(codex_extension_api::ExtensionRegistryBuilder::new().build()),
         codex_extension_api::ExtensionDataInit::default(),
-        /*supports_openai_form_elicitation*/ false,
+        ClientMcpExtensions::default(),
         agent_control,
         environment_manager,
         /*inherited_environments*/ None,
@@ -7936,7 +7936,7 @@ where
         thread_extension_data: codex_extension_api::ExtensionData::new(thread_id.to_string()),
         selected_capability_roots: Vec::new(),
         mcp_thread_init: codex_extension_api::ExtensionDataInit::default(),
-        supports_openai_form_elicitation: std::sync::atomic::AtomicBool::new(false),
+        client_mcp_extensions: ClientMcpExtensions::default(),
         agent_control,
         network_proxy: arc_swap::ArcSwapOption::from(None),
         network_proxy_audit_metadata: crate::config::NetworkProxyAuditMetadata::default(),

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

@@ -765,7 +765,7 @@ async fn guardian_subagent_does_not_inherit_parent_exec_policy_rules() {
         parent_trace: None,
         environment_selections: Vec::new(),
         thread_extension_init: codex_extension_api::ExtensionDataInit::default(),
-        supports_openai_form_elicitation: false,
+        client_mcp_extensions: ClientMcpExtensions::default(),
         analytics_events_client: None,
         thread_store,
         attestation_provider: None,