[app-server] make file_path for config optional (#7560)

When we are writing to config using `config/value/write` or
`config/batchWrite`, it always require a `config/read` before it right
now in order to get the correct file path to write to. make this
optional so we read from the default user config file if this is not
passed in.

codex-rs/app-server/src/config_api.rs

@@ -109,12 +109,17 @@ impl ConfigApi {
 
     async fn apply_edits(
         &self,
-        file_path: String,
+        file_path: Option<String>,
         expected_version: Option<String>,
         edits: Vec<(String, JsonValue, MergeStrategy)>,
     ) -> Result<ConfigWriteResponse, JSONRPCErrorError> {
         let allowed_path = self.codex_home.join(CONFIG_FILE_NAME);
-        if !paths_match(&allowed_path, &file_path) {
+        let provided_path = file_path
+            .as_ref()
+            .map(PathBuf::from)
+            .unwrap_or_else(|| allowed_path.clone());
+
+        if !paths_match(&allowed_path, &provided_path) {
             return Err(config_write_error(
                 ConfigWriteErrorCode::ConfigLayerReadonly,
                 "Only writes to the user config are allowed",
@@ -190,9 +195,16 @@ impl ConfigApi {
             .map(|_| WriteStatus::OkOverridden)
             .unwrap_or(WriteStatus::Ok);
 
+        let file_path = provided_path
+            .canonicalize()
+            .unwrap_or(provided_path.clone())
+            .display()
+            .to_string();
+
         Ok(ConfigWriteResponse {
             status,
             version: updated_layers.user.version.clone(),
+            file_path,
             overridden_metadata: overridden,
         })
     }
@@ -587,15 +599,14 @@ fn canonical_json(value: &JsonValue) -> JsonValue {
     }
 }
 
-fn paths_match(expected: &Path, provided: &str) -> bool {
-    let provided_path = PathBuf::from(provided);
+fn paths_match(expected: &Path, provided: &Path) -> bool {
     if let (Ok(expanded_expected), Ok(expanded_provided)) =
-        (expected.canonicalize(), provided_path.canonicalize())
+        (expected.canonicalize(), provided.canonicalize())
     {
         return expanded_expected == expanded_provided;
     }
 
-    expected == provided_path
+    expected == provided
 }
 
 fn value_at_path<'a>(root: &'a TomlValue, segments: &[String]) -> Option<&'a TomlValue> {
@@ -795,7 +806,7 @@ mod tests {
 
         let result = api
             .write_value(ConfigValueWriteParams {
-                file_path: tmp.path().join(CONFIG_FILE_NAME).display().to_string(),
+                file_path: Some(tmp.path().join(CONFIG_FILE_NAME).display().to_string()),
                 key_path: "approval_policy".to_string(),
                 value: json!("never"),
                 merge_strategy: MergeStrategy::Replace,
@@ -832,7 +843,7 @@ mod tests {
         let api = ConfigApi::new(tmp.path().to_path_buf(), vec![]);
         let error = api
             .write_value(ConfigValueWriteParams {
-                file_path: tmp.path().join(CONFIG_FILE_NAME).display().to_string(),
+                file_path: Some(tmp.path().join(CONFIG_FILE_NAME).display().to_string()),
                 key_path: "model".to_string(),
                 value: json!("gpt-5"),
                 merge_strategy: MergeStrategy::Replace,
@@ -852,6 +863,30 @@ mod tests {
         );
     }
 
+    #[tokio::test]
+    async fn write_value_defaults_to_user_config_path() {
+        let tmp = tempdir().expect("tempdir");
+        std::fs::write(tmp.path().join(CONFIG_FILE_NAME), "").unwrap();
+
+        let api = ConfigApi::new(tmp.path().to_path_buf(), vec![]);
+        api.write_value(ConfigValueWriteParams {
+            file_path: None,
+            key_path: "model".to_string(),
+            value: json!("gpt-new"),
+            merge_strategy: MergeStrategy::Replace,
+            expected_version: None,
+        })
+        .await
+        .expect("write succeeds");
+
+        let contents =
+            std::fs::read_to_string(tmp.path().join(CONFIG_FILE_NAME)).expect("read config");
+        assert!(
+            contents.contains("model = \"gpt-new\""),
+            "config.toml should be updated even when file_path is omitted"
+        );
+    }
+
     #[tokio::test]
     async fn invalid_user_value_rejected_even_if_overridden_by_managed() {
         let tmp = tempdir().expect("tempdir");
@@ -872,7 +907,7 @@ mod tests {
 
         let error = api
             .write_value(ConfigValueWriteParams {
-                file_path: tmp.path().join(CONFIG_FILE_NAME).display().to_string(),
+                file_path: Some(tmp.path().join(CONFIG_FILE_NAME).display().to_string()),
                 key_path: "approval_policy".to_string(),
                 value: json!("bogus"),
                 merge_strategy: MergeStrategy::Replace,
@@ -957,7 +992,7 @@ mod tests {
 
         let result = api
             .write_value(ConfigValueWriteParams {
-                file_path: tmp.path().join(CONFIG_FILE_NAME).display().to_string(),
+                file_path: Some(tmp.path().join(CONFIG_FILE_NAME).display().to_string()),
                 key_path: "approval_policy".to_string(),
                 value: json!("on-request"),
                 merge_strategy: MergeStrategy::Replace,