From 2f3b5ed81ac939db908fb8ca2aeb80c42e8e7600 Mon Sep 17 00:00:00 2001
From: colby-oai <228809017+colby-oai@users.noreply.github.com>
Date: Mon, 27 Apr 2026 20:42:10 -0400
Subject: [PATCH] disallow fileparams metadata for custom mcps (#19836)

## Summary
Disallow fileParams metadata for custom MCPs

Restricts Codex openai/fileParams handling to the first-party codex_apps
MCP server. Custom MCP servers may still advertise the metadata, but
Codex now ignores it for upload rewriting, preventing non-Apps tools
from receiving signed OpenAI file refs for local paths. Added a
regression test for the allowed and denied cases.
---
 codex-rs/core/src/mcp_tool_call.rs       | 16 +++++++++++++---
 codex-rs/core/src/mcp_tool_call_tests.rs | 17 +++++++++++++++++
 2 files changed, 30 insertions(+), 3 deletions(-)

diff --git a/codex-rs/core/src/mcp_tool_call.rs b/codex-rs/core/src/mcp_tool_call.rs
index b6646a338..efa50b2b4 100644
--- a/codex-rs/core/src/mcp_tool_call.rs
+++ b/codex-rs/core/src/mcp_tool_call.rs
@@ -1194,13 +1194,23 @@ pub(crate) async fn lookup_mcp_tool_metadata(
             .and_then(|meta| meta.get(MCP_TOOL_CODEX_APPS_META_KEY))
             .and_then(serde_json::Value::as_object)
             .cloned(),
-        openai_file_input_params: Some(declared_openai_file_input_param_names(
+        // Disallow custom MCPs from uploading files via fileParams.
+        openai_file_input_params: openai_file_input_params_for_server(
+            server,
             tool_info.tool.meta.as_deref(),
-        ))
-        .filter(|params| !params.is_empty()),
+        ),
     })
 }
 
+fn openai_file_input_params_for_server(
+    server: &str,
+    meta: Option<&serde_json::Map<String, serde_json::Value>>,
+) -> Option<Vec<String>> {
+    (server == CODEX_APPS_MCP_SERVER_NAME)
+        .then_some(declared_openai_file_input_param_names(meta))
+        .filter(|params| !params.is_empty())
+}
+
 fn get_mcp_app_resource_uri(
     meta: Option<&serde_json::Map<String, serde_json::Value>>,
 ) -> Option<String> {
diff --git a/codex-rs/core/src/mcp_tool_call_tests.rs b/codex-rs/core/src/mcp_tool_call_tests.rs
index 8b3f22770..5ab2c6c4e 100644
--- a/codex-rs/core/src/mcp_tool_call_tests.rs
+++ b/codex-rs/core/src/mcp_tool_call_tests.rs
@@ -183,6 +183,23 @@ fn mcp_app_resource_uri_reads_known_tool_meta_keys() {
     );
 }
 
+#[test]
+fn openai_file_params_are_only_honored_for_codex_apps() {
+    let meta = serde_json::json!({
+        "openai/fileParams": ["file"],
+    });
+    let meta = meta.as_object();
+
+    assert_eq!(
+        openai_file_input_params_for_server(CODEX_APPS_MCP_SERVER_NAME, meta),
+        Some(vec!["file".to_string()])
+    );
+    assert_eq!(
+        openai_file_input_params_for_server("minimaltest", meta),
+        None
+    );
+}
+
 #[test]
 fn approval_required_when_read_only_false_and_destructive() {
     let annotations = annotations(Some(false), Some(true), /*open_world*/ None);