fix(codex): preserve custom tool metadata in chat routing (#3644)

* fix(codex): preserve custom tool metadata in chat routing

* fix(codex): reduce custom tool metadata description overhead

* fix(codex): stabilize custom tool metadata descriptions

* refactor(codex): remove unreachable custom tool description fallback

src-tauri/src/proxy/providers/transform_codex_chat.rs

@@ -40,6 +40,8 @@ const EXTRA_CHAT_PASSTHROUGH_FIELDS: &[&str] = &[
 const TOOL_SEARCH_PROXY_NAME: &str = "tool_search";
 const CUSTOM_TOOL_INPUT_FIELD: &str = "input";
 const CHAT_TOOL_NAME_MAX_LEN: usize = 64;
+const CUSTOM_TOOL_INPUT_DESCRIPTION: &str = "Raw string input for the original custom tool. Preserve formatting exactly and follow the original tool definition embedded in the description.";
+const CUSTOM_TOOL_PRESERVED_METADATA_HEADING: &str = "Original tool definition:";
 
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub(crate) enum CodexToolKind {
@@ -132,10 +134,7 @@ impl CodexToolContext {
         let Some(name) = responses_tool_name(tool) else {
             return;
         };
-        let description = tool
+        let description = json!(responses_custom_tool_description(tool));
-            .get("description")
-            .cloned()
-            .unwrap_or_else(|| json!("Custom Codex tool."));
         let chat_tool = json!({
             "type": "function",
             "function": {
@@ -146,7 +145,7 @@ impl CodexToolContext {
                     "properties": {
                         CUSTOM_TOOL_INPUT_FIELD: {
                             "type": "string",
-                            "description": "Input to pass to the custom Codex tool."
+                            "description": CUSTOM_TOOL_INPUT_DESCRIPTION
                         }
                     },
                     "required": [CUSTOM_TOOL_INPUT_FIELD]
@@ -1016,6 +1015,22 @@ fn responses_tool_name(tool: &Value) -> Option<String> {
         .map(ToString::to_string)
 }
 
+fn responses_custom_tool_description(tool: &Value) -> String {
+    let mut description = String::new();
+    description.push_str(CUSTOM_TOOL_PRESERVED_METADATA_HEADING);
+    description.push_str("\n```json\n");
+    description.push_str(&serialize_tool_definition_for_description(tool));
+    description.push_str("\n```");
+    description
+}
+
+fn serialize_tool_definition_for_description(tool: &Value) -> String {
+    // Keep the embedded definition compact to reduce tool-description token
+    // overhead for chat-only upstreams, while remaining stable across map
+    // storage order.
+    canonical_json_string(tool)
+}
+
 fn responses_function_tool_to_chat_tool(tool: &Value, chat_name: &str) -> Option<Value> {
     if tool.get("type").and_then(|v| v.as_str()) != Some("function") {
         return None;
@@ -1857,6 +1872,34 @@ mod tests {
         );
     }
 
+    #[test]
+    fn responses_request_to_chat_preserves_custom_tool_metadata_in_description() {
+        let input = json!({
+            "model": "gpt-5.4",
+            "tools": [{
+                "type": "custom",
+                "name": "apply_patch",
+                "description": "Use the `apply_patch` tool to edit files. This is a FREEFORM tool, so do not wrap the patch in JSON.",
+                "format": {
+                    "type": "grammar",
+                    "syntax": "lark",
+                    "definition": "start: begin_patch hunk+ end_patch"
+                }
+            }]
+        });
+
+        let result = responses_to_chat_completions(input).unwrap();
+        let description = result["tools"][0]["function"]["description"]
+            .as_str()
+            .unwrap();
+
+        assert!(description.starts_with("Original tool definition:"));
+        assert!(!description.contains("Original Codex tool definition"));
+        assert!(description.contains("\"type\":\"custom\""));
+        assert!(description.contains("\"format\":"));
+        assert!(description.contains("\"syntax\":\"lark\""));
+    }
+
     #[test]
     fn responses_request_to_chat_uses_provider_reasoning_effort_for_deepseek_model() {
         let input = json!({