feat: expose provider capability bounds to app server clients (#20049)

follow up of #19442. The app server now exposes provider-derived bounds
through a new v2 `modelProvider/read` method. The response reports the
configured provider map key as `modelProvider` and returns the effective
capability booleans so clients can align their UI with the same
provider-owned limits used by core.

codex-rs/app-server/tests/common/mcp_process.rs

@@ -54,6 +54,7 @@ use codex_app_server_protocol::McpResourceReadParams;
 use codex_app_server_protocol::McpServerToolCallParams;
 use codex_app_server_protocol::MockExperimentalMethodParams;
 use codex_app_server_protocol::ModelListParams;
+use codex_app_server_protocol::ModelProviderCapabilitiesReadParams;
 use codex_app_server_protocol::PluginInstallParams;
 use codex_app_server_protocol::PluginListParams;
 use codex_app_server_protocol::PluginReadParams;
@@ -517,6 +518,16 @@ impl McpProcess {
         self.send_request("model/list", params).await
     }
 
+    /// Send a `modelProvider/capabilities/read` JSON-RPC request.
+    pub async fn send_model_provider_capabilities_read_request(
+        &mut self,
+        params: ModelProviderCapabilitiesReadParams,
+    ) -> anyhow::Result<i64> {
+        let params = Some(serde_json::to_value(params)?);
+        self.send_request("modelProvider/capabilities/read", params)
+            .await
+    }
+
     /// Send an `experimentalFeature/list` JSON-RPC request.
     pub async fn send_experimental_feature_list_request(
         &mut self,

codex-rs/app-server/tests/suite/v2/mod.rs

@@ -26,6 +26,7 @@ mod mcp_server_status;
 mod mcp_tool;
 mod memory_reset;
 mod model_list;
+mod model_provider_capabilities_read;
 mod output_schema;
 mod plan_item;
 mod plugin_install;

codex-rs/app-server/tests/suite/v2/model_provider_capabilities_read.rs

@@ -0,0 +1,69 @@
+use std::time::Duration;
+
+use anyhow::Result;
+use app_test_support::McpProcess;
+use app_test_support::to_response;
+use codex_app_server_protocol::JSONRPCResponse;
+use codex_app_server_protocol::ModelProviderCapabilitiesReadParams;
+use codex_app_server_protocol::ModelProviderCapabilitiesReadResponse;
+use codex_app_server_protocol::RequestId;
+use pretty_assertions::assert_eq;
+use tempfile::TempDir;
+use tokio::time::timeout;
+
+const DEFAULT_TIMEOUT: Duration = Duration::from_secs(30);
+
+#[tokio::test]
+async fn read_default_provider_capabilities() -> Result<()> {
+    let codex_home = TempDir::new()?;
+    let mut mcp = McpProcess::new(codex_home.path()).await?;
+    timeout(DEFAULT_TIMEOUT, mcp.initialize()).await??;
+
+    let request_id = mcp
+        .send_model_provider_capabilities_read_request(ModelProviderCapabilitiesReadParams {})
+        .await?;
+    let response: JSONRPCResponse = timeout(
+        DEFAULT_TIMEOUT,
+        mcp.read_stream_until_response_message(RequestId::Integer(request_id)),
+    )
+    .await??;
+    let received: ModelProviderCapabilitiesReadResponse = to_response(response)?;
+
+    let expected = ModelProviderCapabilitiesReadResponse {
+        namespace_tools: true,
+        image_generation: true,
+        web_search: true,
+    };
+    assert_eq!(received, expected);
+    Ok(())
+}
+
+#[tokio::test]
+async fn read_amazon_bedrock_provider_capabilities() -> Result<()> {
+    let codex_home = TempDir::new()?;
+    std::fs::write(
+        codex_home.path().join("config.toml"),
+        r#"model_provider = "amazon-bedrock"
+"#,
+    )?;
+    let mut mcp = McpProcess::new(codex_home.path()).await?;
+    timeout(DEFAULT_TIMEOUT, mcp.initialize()).await??;
+
+    let request_id = mcp
+        .send_model_provider_capabilities_read_request(ModelProviderCapabilitiesReadParams {})
+        .await?;
+    let response: JSONRPCResponse = timeout(
+        DEFAULT_TIMEOUT,
+        mcp.read_stream_until_response_message(RequestId::Integer(request_id)),
+    )
+    .await??;
+    let received: ModelProviderCapabilitiesReadResponse = to_response(response)?;
+
+    let expected = ModelProviderCapabilitiesReadResponse {
+        namespace_tools: false,
+        image_generation: false,
+        web_search: false,
+    };
+    assert_eq!(received, expected);
+    Ok(())
+}