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/tests/suite/compact_resume_fork.rs

@@ -17,6 +17,7 @@ use codex_core::spawn::CODEX_SANDBOX_NETWORK_DISABLED_ENV_VAR;
 use codex_protocol::config_types::CollaborationMode;
 use codex_protocol::config_types::ModeKind;
 use codex_protocol::config_types::Settings;
+use codex_protocol::mcp::ClientMcpExtensions;
 use codex_protocol::protocol::EventMsg;
 use codex_protocol::protocol::Op;
 use codex_protocol::protocol::WarningEvent;
@@ -830,7 +831,7 @@ async fn resume_conversation(
         path,
         auth_manager,
         /*parent_trace*/ None,
-        /*supports_openai_form_elicitation*/ false,
+        ClientMcpExtensions::default(),
     ))
     .await
     .expect("resume conversation")

codex-rs/core/tests/suite/fork_thread.rs

@@ -5,6 +5,7 @@ use codex_core::NewThread;
 use codex_core::ThreadConfigSnapshot;
 use codex_core::parse_turn_item;
 use codex_protocol::items::TurnItem;
+use codex_protocol::mcp::ClientMcpExtensions;
 use codex_protocol::protocol::EventMsg;
 use codex_protocol::protocol::InitialHistory;
 use codex_protocol::protocol::Op;
@@ -238,7 +239,7 @@ async fn assert_copied_fork_persists_inherited_history(history_mode: ThreadHisto
             }),
             /*thread_source*/ None,
             /*parent_trace*/ None,
-            /*supports_openai_form_elicitation*/ false,
+            ClientMcpExtensions::default(),
         )
         .await
         .expect("fork from stored history");
@@ -274,7 +275,7 @@ async fn assert_copied_fork_persists_inherited_history(history_mode: ThreadHisto
                     codex_login::CodexAuth::from_api_key("dummy"),
                 ),
                 /*parent_trace*/ None,
-                /*supports_openai_form_elicitation*/ false,
+                ClientMcpExtensions::default(),
             )
             .await
             .expect("resume copied paginated fork")

codex-rs/core/tests/suite/mcp_auth_refresh.rs

@@ -17,6 +17,7 @@ use codex_mcp::McpRuntime;
 use codex_mcp::McpRuntimeContext;
 use codex_mcp::McpRuntimeInput;
 use codex_mcp::McpToolCatalogCache;
+use codex_protocol::mcp::ClientMcpExtensions;
 use codex_protocol::protocol::AskForApproval;
 use core_test_support::apps_test_server::AppsTestServer;
 use core_test_support::responses::start_mock_server;
@@ -102,7 +103,7 @@ async fn hosted_plugin_runtime_ps_mcp_tool_calls_use_current_auth_manager_token(
         codex_apps_tools_cache: CodexAppsToolsCache::default(),
         tool_catalog_cache: McpToolCatalogCache::default(),
         codex_apps_tools_cache_key: codex_mcp::codex_apps_tools_cache_key(Some(&expected_auth)),
-        supports_openai_form_elicitation: false,
+        client_mcp_extensions: ClientMcpExtensions::default(),
         auth: Some(expected_auth.clone()),
         codex_apps_auth_manager: Some(Arc::clone(&auth_manager)),
         elicitation_reviewer: None,

codex-rs/core/tests/suite/mcp_refresh_cleanup.rs

@@ -6,6 +6,7 @@ use std::time::Duration;
 use codex_config::DEFAULT_MCP_SERVER_ENVIRONMENT_ID;
 use codex_config::types::McpServerConfig;
 use codex_config::types::McpServerTransportConfig;
+use codex_protocol::protocol::Op;
 use core_test_support::process::process_is_alive;
 use core_test_support::process::wait_for_pid_file;
 use core_test_support::process::wait_for_process_exit;
@@ -112,10 +113,7 @@ async fn refresh_keeps_superseded_mcp_server_alive_for_in_flight_calls() -> anyh
         ]),
     )
     .await;
-    fixture
-        .codex
-        .set_openai_form_elicitation_support(/*supported*/ true)
-        .await?;
+    fixture.codex.submit(Op::RefreshMcpServers).await?;
     fixture.submit_turn("refresh MCP servers").await?;
 
     let replacement_pid = wait_for_pid_file(&pid_file).await?;

codex-rs/core/tests/suite/realtime_conversation.rs

@@ -6,6 +6,7 @@ use codex_core::test_support::auth_manager_from_auth;
 use codex_login::CodexAuth;
 use codex_login::OPENAI_API_KEY_ENV_VAR;
 use codex_protocol::ThreadId;
+use codex_protocol::mcp::ClientMcpExtensions;
 use codex_protocol::models::ContentItem;
 use codex_protocol::models::ResponseItem;
 use codex_protocol::protocol::CodexErrorInfo;
@@ -2726,7 +2727,7 @@ async fn conversation_startup_context_current_thread_selects_many_turns_by_budge
             InitialHistory::Forked(history),
             auth_manager_from_auth(CodexAuth::from_api_key("dummy")),
             /*parent_trace*/ None,
-            /*supports_openai_form_elicitation*/ false,
+            ClientMcpExtensions::default(),
         )
         .await?;
     let codex = resumed_thread.thread;

codex-rs/core/tests/suite/resume_warning.rs

@@ -7,6 +7,7 @@ use codex_login::CodexAuth;
 use codex_protocol::ThreadId;
 use codex_protocol::config_types::ModeKind;
 use codex_protocol::config_types::ReasoningSummary;
+use codex_protocol::mcp::ClientMcpExtensions;
 use codex_protocol::protocol::EventMsg;
 use codex_protocol::protocol::InitialHistory;
 use codex_protocol::protocol::ResumedHistory;
@@ -116,7 +117,7 @@ async fn emits_warning_when_resumed_model_differs() {
             initial_history,
             auth_manager,
             /*parent_trace*/ None,
-            /*supports_openai_form_elicitation*/ false,
+            ClientMcpExtensions::default(),
         )
         .await
         .expect("resume conversation");

codex-rs/core/tests/suite/rmcp_client.rs

@@ -426,58 +426,6 @@ async fn openai_form_capability_is_not_advertised_by_default() -> anyhow::Result
     assert_openai_form_capability_advertisement(/*expected*/ false).await
 }
 
-#[tokio::test(flavor = "multi_thread", worker_threads = 1)]
-async fn openai_form_capability_updates_for_loaded_thread() -> anyhow::Result<()> {
-    skip_if_wine_exec!(
-        Ok(()),
-        "requires a Windows test_stdio_server in the Wine-exec environment"
-    );
-
-    let server = start_mock_server().await;
-    let server_name = "capabilities";
-    let command = stdio_server_bin()?;
-    let fixture = test_codex()
-        .with_config(move |config| {
-            insert_mcp_server(
-                config,
-                server_name,
-                stdio_transport(command, /*env*/ None, Vec::new()),
-                TestMcpServerOptions::default(),
-            );
-        })
-        .build(&server)
-        .await?;
-    wait_for_mcp_server(&fixture.codex, server_name).await?;
-
-    let unsupported = call_structured_tool(
-        &server,
-        &fixture,
-        server_name,
-        "client_capabilities",
-        "call-client-capabilities-unsupported",
-    )
-    .await?;
-    assert_eq!(
-        unsupported,
-        json!({ "supportsOpenaiFormElicitation": false })
-    );
-
-    fixture
-        .codex
-        .set_openai_form_elicitation_support(/*supported*/ true)
-        .await?;
-    let supported = call_structured_tool(
-        &server,
-        &fixture,
-        server_name,
-        "client_capabilities",
-        "call-client-capabilities-supported",
-    )
-    .await?;
-    assert_eq!(supported, json!({ "supportsOpenaiFormElicitation": true }));
-    Ok(())
-}
-
 async fn assert_openai_form_capability_advertisement(expected: bool) -> anyhow::Result<()> {
     skip_if_wine_exec!(
         Ok(()),

codex-rs/core/tests/suite/unstable_features_warning.rs

@@ -4,6 +4,7 @@ use codex_config::CONFIG_TOML_FILE;
 use codex_core::NewThread;
 use codex_features::Feature;
 use codex_login::CodexAuth;
+use codex_protocol::mcp::ClientMcpExtensions;
 use codex_protocol::protocol::EventMsg;
 use codex_protocol::protocol::InitialHistory;
 use codex_protocol::protocol::WarningEvent;
@@ -50,7 +51,7 @@ async fn emits_warning_when_unstable_features_enabled_via_config() {
             InitialHistory::New,
             auth_manager,
             /*parent_trace*/ None,
-            /*supports_openai_form_elicitation*/ false,
+            ClientMcpExtensions::default(),
         )
         .await
         .expect("spawn conversation");
@@ -100,7 +101,7 @@ async fn suppresses_warning_when_configured() {
             InitialHistory::New,
             auth_manager,
             /*parent_trace*/ None,
-            /*supports_openai_form_elicitation*/ false,
+            ClientMcpExtensions::default(),
         )
         .await
         .expect("spawn conversation");