Preserve complete MCP namespace descriptions (#36882)

## What changed

- Keep complete MCP namespace descriptions in tool-search source metadata.
- Raise the namespace tool-spec description limit from 1,000 bytes to 512 KiB,
  truncating at a UTF-8 character boundary only when the new limit is exceeded.

## Testing

- Cover descriptions beyond the former limit and multibyte truncation at 512 KiB.
- Update SSE and stdio MCP tests to verify that complete server instructions are
  preserved without hiding tools.

GitOrigin-RevId: 000bfcafb3df348065ae451685bfbf978a0e3248

codex-rs/core/src/tools/handlers/mcp.rs

@@ -31,7 +31,7 @@ use serde_json::Value;
 
 const LEGACY_MCP_TOOL_NAME_PREFIX: &str = "mcp__";
 const MCP_TOOL_NAME_DELIMITER: &str = "__";
-const MAX_MCP_NAMESPACE_DESCRIPTION_BYTES: usize = 1_000;
+const MAX_MCP_NAMESPACE_DESCRIPTION_BYTES: usize = 512 * 1024;
 
 pub struct McpHandler {
     tool_info: ToolInfo,
@@ -106,7 +106,6 @@ impl ToolExecutor<ToolInvocation> for McpHandler {
                 .as_deref()
                 .map(str::trim)
                 .filter(|description| !description.is_empty())
-                .map(bounded_mcp_namespace_description)
                 .map(str::to_string),
         });
 
@@ -258,15 +257,12 @@ fn create_tool_spec(tool_info: &ToolInfo) -> Result<ToolSpec, serde_json::Error>
 
     Ok(ToolSpec::Namespace(ResponsesApiNamespace {
         name: tool_info.callable_namespace.clone(),
-        description: bounded_mcp_namespace_description(&description).to_string(),
+        description: take_bytes_at_char_boundary(&description, MAX_MCP_NAMESPACE_DESCRIPTION_BYTES)
+            .to_string(),
         tools: vec![ResponsesApiNamespaceTool::Function(tool)],
     }))
 }
 
-fn bounded_mcp_namespace_description(description: &str) -> &str {
-    take_bytes_at_char_boundary(description, MAX_MCP_NAMESPACE_DESCRIPTION_BYTES)
-}
-
 fn mcp_hook_tool_input(raw_arguments: &str) -> Value {
     if raw_arguments.trim().is_empty() {
         return Value::Object(Map::new());

codex-rs/core/src/tools/handlers/mcp_search_tests.rs

@@ -43,9 +43,8 @@ fn search_info_uses_connector_name_for_output_namespace_description() {
 }
 
 #[test]
-fn mcp_namespace_descriptions_are_bounded_without_mutating_metadata() {
-    let expected_description = "é".repeat(499);
-    let full_description = format!("{expected_description}🦀keep the complete app metadata");
+fn mcp_namespace_descriptions_preserve_complete_metadata() {
+    let full_description = format!("{}🦀keep the complete app metadata", "é".repeat(499));
     let mut info = tool_info();
     info.namespace_description = Some(full_description.clone());
     let handler = McpHandler::new(info).expect("MCP tool spec should build");
@@ -55,17 +54,39 @@ fn mcp_namespace_descriptions_are_bounded_without_mutating_metadata() {
         search_info.source_info,
         Some(ToolSearchSourceInfo {
             name: "Calendar".to_string(),
-            description: Some(expected_description.clone()),
+            description: Some(full_description.clone()),
+        })
+    );
+    let LoadableToolSpec::Namespace(namespace) = search_info.entry.output else {
+        panic!("expected namespace search output");
+    };
+    assert_eq!(namespace.description, full_description);
+    assert_eq!(
+        handler.tool_info.namespace_description,
+        Some(full_description)
+    );
+}
+
+#[test]
+fn mcp_namespace_descriptions_are_bounded_at_512_kib() {
+    let expected_description = "é".repeat(MAX_MCP_NAMESPACE_DESCRIPTION_BYTES / 2 - 1);
+    let full_description = format!("{expected_description}🦀overflow");
+    let mut info = tool_info();
+    info.namespace_description = Some(full_description.clone());
+    let handler = McpHandler::new(info).expect("MCP tool spec should build");
+    let search_info = handler.search_info().expect("MCP search info");
+
+    assert_eq!(
+        search_info.source_info,
+        Some(ToolSearchSourceInfo {
+            name: "Calendar".to_string(),
+            description: Some(full_description),
         })
     );
     let LoadableToolSpec::Namespace(namespace) = search_info.entry.output else {
         panic!("expected namespace search output");
     };
     assert_eq!(namespace.description, expected_description);
-    assert_eq!(
-        handler.tool_info.namespace_description,
-        Some(full_description)
-    );
 }
 
 fn tool_info() -> ToolInfo {