diff --git a/codex-rs/app-server/src/codex_message_processor.rs b/codex-rs/app-server/src/codex_message_processor.rs
index 346e200001..b32d2667c0 100644
--- a/codex-rs/app-server/src/codex_message_processor.rs
+++ b/codex-rs/app-server/src/codex_message_processor.rs
@@ -52,6 +52,8 @@ use codex_app_server_protocol::ServerRequestPayload;
 use codex_app_server_protocol::SessionConfiguredNotification;
 use codex_app_server_protocol::SetDefaultModelParams;
 use codex_app_server_protocol::SetDefaultModelResponse;
+use codex_app_server_protocol::UpdateConfigParams;
+use codex_app_server_protocol::UpdateConfigResponse;
 use codex_app_server_protocol::UserInfoResponse;
 use codex_app_server_protocol::UserSavedConfig;
 use codex_backend_client::Client as BackendClient;
@@ -74,6 +76,7 @@ use codex_core::config::load_config_as_toml;
 use codex_core::config_edit::CONFIG_KEY_EFFORT;
 use codex_core::config_edit::CONFIG_KEY_MODEL;
 use codex_core::config_edit::persist_overrides_and_clear_if_none;
+use codex_core::config_edit::persist_user_saved_config;
 use codex_core::default_client::get_codex_user_agent;
 use codex_core::exec::ExecParams;
 use codex_core::exec_env::create_env;
@@ -664,9 +667,43 @@ impl CodexMessageProcessor {
 
     async fn update_user_saved_config(&self, request_id: RequestId, params: UpdateConfigParams) {
         let UpdateConfigParams { config } = params;
-        let config: UserSavedConfig = config.into();
-        // TODO(owen): actually persist config
-        let response = UpdateConfigResponse { config };
+        if let Err(err) = persist_user_saved_config(&self.config.codex_home, &config).await {
+            let error = JSONRPCErrorError {
+                code: INTERNAL_ERROR_CODE,
+                message: format!("failed to persist config.toml: {err}"),
+                data: None,
+            };
+            self.outgoing.send_error(request_id, error).await;
+            return;
+        }
+
+        let toml_value = match load_config_as_toml(&self.config.codex_home).await {
+            Ok(val) => val,
+            Err(err) => {
+                let error = JSONRPCErrorError {
+                    code: INTERNAL_ERROR_CODE,
+                    message: format!("failed to load config.toml: {err}"),
+                    data: None,
+                };
+                self.outgoing.send_error(request_id, error).await;
+                return;
+            }
+        };
+
+        let cfg: ConfigToml = match toml_value.try_into() {
+            Ok(cfg) => cfg,
+            Err(err) => {
+                let error = JSONRPCErrorError {
+                    code: INTERNAL_ERROR_CODE,
+                    message: format!("failed to parse config.toml: {err}"),
+                    data: None,
+                };
+                self.outgoing.send_error(request_id, error).await;
+                return;
+            }
+        };
+
+        let response = UpdateConfigResponse { config: cfg.into() };
         self.outgoing.send_response(request_id, response).await;
     }
 
diff --git a/codex-rs/core/src/config_edit.rs b/codex-rs/core/src/config_edit.rs
index 6e68c08ca0..65c40b891b 100644
--- a/codex-rs/core/src/config_edit.rs
+++ b/codex-rs/core/src/config_edit.rs
@@ -1,8 +1,26 @@
 use crate::config::CONFIG_TOML_FILE;
+use crate::config_types::McpServerConfig;
+use crate::config_types::McpServerTransportConfig;
+use anyhow::Context;
 use anyhow::Result;
+use anyhow::anyhow;
+use codex_app_server_protocol::McpOAuthCredentialsStoreMode;
+use codex_app_server_protocol::McpServerConfig as ProtocolMcpServerConfig;
+use codex_app_server_protocol::McpServerTransportConfig as ProtocolMcpServerTransportConfig;
+use codex_app_server_protocol::Profile;
+use codex_app_server_protocol::SandboxSettings;
+use codex_app_server_protocol::Tools;
+use codex_app_server_protocol::UserSavedConfig;
+use std::collections::BTreeMap;
+use std::collections::HashMap;
+use std::io::ErrorKind;
 use std::path::Path;
+use std::time::Duration;
 use tempfile::NamedTempFile;
+use toml_edit::Array as TomlArray;
 use toml_edit::DocumentMut;
+use toml_edit::Item as TomlItem;
+use toml_edit::Table as TomlTable;
 
 pub const CONFIG_KEY_MODEL: &str = "model";
 pub const CONFIG_KEY_EFFORT: &str = "model_reasoning_effort";
@@ -49,6 +67,87 @@ pub async fn persist_overrides_and_clear_if_none(
     persist_overrides_with_behavior(codex_home, profile, overrides, NoneBehavior::Remove).await
 }
 
+pub async fn persist_user_saved_config(codex_home: &Path, config: &UserSavedConfig) -> Result<()> {
+    let servers = convert_mcp_servers(&config.mcp_servers)?;
+
+    let config_path = codex_home.join(CONFIG_TOML_FILE);
+    let existing = tokio::fs::read_to_string(&config_path).await;
+    let mut doc = match existing {
+        Ok(contents) => contents.parse::<DocumentMut>()?,
+        Err(err) if err.kind() == ErrorKind::NotFound => DocumentMut::new(),
+        Err(err) => return Err(err.into()),
+    };
+
+    {
+        let root = doc.as_table_mut();
+        set_string_option(
+            root,
+            "approval_policy",
+            config.approval_policy.map(|value| value.to_string()),
+        );
+        set_string_option(
+            root,
+            "sandbox_mode",
+            config.sandbox_mode.map(|mode| mode.to_string()),
+        );
+        set_sandbox_workspace_write(root, config.sandbox_settings.as_ref())?;
+        set_string_option(
+            root,
+            "forced_chatgpt_workspace_id",
+            config.forced_chatgpt_workspace_id.clone(),
+        );
+        set_string_option(
+            root,
+            "forced_login_method",
+            config.forced_login_method.map(|mode| mode.to_string()),
+        );
+        set_string_option(root, "model", config.model.clone());
+        set_string_option(
+            root,
+            "model_reasoning_effort",
+            config
+                .model_reasoning_effort
+                .map(|effort| effort.to_string()),
+        );
+        set_string_option(
+            root,
+            "model_reasoning_summary",
+            config
+                .model_reasoning_summary
+                .map(|summary| summary.to_string()),
+        );
+        set_string_option(
+            root,
+            "model_verbosity",
+            config
+                .model_verbosity
+                .map(|verbosity| verbosity.to_string()),
+        );
+        set_tools(root, config.tools.as_ref())?;
+        set_string_option(
+            root,
+            "mcp_oauth_credentials_store",
+            config
+                .mcp_oauth_credentials_store
+                .map(protocol_oauth_mode_to_str)
+                .map(String::from),
+        );
+        set_string_option(root, "profile", config.profile.clone());
+        set_profiles(root, &config.profiles)?;
+        set_mcp_servers(root, &servers)?;
+    }
+
+    tokio::fs::create_dir_all(codex_home)
+        .await
+        .with_context(|| format!("failed to create Codex home at {}", codex_home.display()))?;
+
+    let tmp_file = NamedTempFile::new_in(codex_home)?;
+    tokio::fs::write(tmp_file.path(), doc.to_string()).await?;
+    tmp_file.persist(config_path)?;
+
+    Ok(())
+}
+
 /// Apply a single override onto a `toml_edit` document while preserving
 /// existing formatting/comments.
 /// The key is expressed as explicit segments to correctly handle keys that
@@ -212,6 +311,340 @@ fn remove_toml_edit_segments(doc: &mut DocumentMut, segments: &[&str]) -> bool {
     current.remove(segments[segments.len() - 1]).is_some()
 }
 
+fn set_string_option(table: &mut TomlTable, key: &str, value: Option<String>) {
+    match value {
+        Some(value) => {
+            table[key] = toml_edit::value(value);
+        }
+        None => {
+            table.remove(key);
+        }
+    }
+}
+
+fn set_sandbox_workspace_write(
+    table: &mut TomlTable,
+    settings: Option<&SandboxSettings>,
+) -> Result<()> {
+    table.remove("sandbox_workspace_write");
+
+    let Some(settings) = settings else {
+        return Ok(());
+    };
+
+    let mut sandbox = TomlTable::new();
+    sandbox.set_implicit(false);
+
+    if !settings.writable_roots.is_empty() {
+        let mut roots = TomlArray::new();
+        for root_path in &settings.writable_roots {
+            roots.push(root_path.to_string_lossy().to_string());
+        }
+        sandbox["writable_roots"] = TomlItem::Value(roots.into());
+    }
+
+    if let Some(network_access) = settings.network_access {
+        sandbox["network_access"] = toml_edit::value(network_access);
+    }
+
+    if let Some(exclude_tmpdir_env_var) = settings.exclude_tmpdir_env_var {
+        sandbox["exclude_tmpdir_env_var"] = toml_edit::value(exclude_tmpdir_env_var);
+    }
+
+    if let Some(exclude_slash_tmp) = settings.exclude_slash_tmp {
+        sandbox["exclude_slash_tmp"] = toml_edit::value(exclude_slash_tmp);
+    }
+
+    if sandbox.is_empty() {
+        return Ok(());
+    }
+
+    table.insert("sandbox_workspace_write", TomlItem::Table(sandbox));
+    Ok(())
+}
+
+fn set_tools(table: &mut TomlTable, tools: Option<&Tools>) -> Result<()> {
+    table.remove("tools");
+
+    let Some(tools) = tools else {
+        return Ok(());
+    };
+
+    let mut tools_table = TomlTable::new();
+    tools_table.set_implicit(false);
+
+    if let Some(web_search) = tools.web_search {
+        tools_table["web_search"] = toml_edit::value(web_search);
+    }
+
+    if let Some(view_image) = tools.view_image {
+        tools_table["view_image"] = toml_edit::value(view_image);
+    }
+
+    if tools_table.is_empty() {
+        return Ok(());
+    }
+
+    table.insert("tools", TomlItem::Table(tools_table));
+    Ok(())
+}
+
+fn set_profiles(table: &mut TomlTable, profiles: &HashMap<String, Profile>) -> Result<()> {
+    table.remove("profiles");
+
+    if profiles.is_empty() {
+        return Ok(());
+    }
+
+    let mut profiles_table = TomlTable::new();
+    profiles_table.set_implicit(true);
+
+    let mut keys: Vec<_> = profiles.keys().cloned().collect();
+    keys.sort();
+
+    for key in keys {
+        let profile = profiles.get(&key).expect("profile key should exist");
+        let mut profile_table = TomlTable::new();
+        profile_table.set_implicit(false);
+
+        if let Some(model) = profile.model.clone() {
+            profile_table["model"] = toml_edit::value(model);
+        }
+
+        if let Some(model_provider) = profile.model_provider.clone() {
+            profile_table["model_provider"] = toml_edit::value(model_provider);
+        }
+
+        if let Some(approval_policy) = profile.approval_policy {
+            profile_table["approval_policy"] = toml_edit::value(approval_policy.to_string());
+        }
+
+        if let Some(effort) = profile.model_reasoning_effort {
+            profile_table["model_reasoning_effort"] = toml_edit::value(effort.to_string());
+        }
+
+        if let Some(summary) = profile.model_reasoning_summary {
+            profile_table["model_reasoning_summary"] = toml_edit::value(summary.to_string());
+        }
+
+        if let Some(verbosity) = profile.model_verbosity {
+            profile_table["model_verbosity"] = toml_edit::value(verbosity.to_string());
+        }
+
+        if let Some(chatgpt_base_url) = profile.chatgpt_base_url.clone() {
+            profile_table["chatgpt_base_url"] = toml_edit::value(chatgpt_base_url);
+        }
+
+        profiles_table.insert(&key, TomlItem::Table(profile_table));
+    }
+
+    table.insert("profiles", TomlItem::Table(profiles_table));
+    Ok(())
+}
+
+fn set_mcp_servers(
+    table: &mut TomlTable,
+    servers: &BTreeMap<String, McpServerConfig>,
+) -> Result<()> {
+    table.remove("mcp_servers");
+
+    if servers.is_empty() {
+        return Ok(());
+    }
+
+    let mut servers_table = TomlTable::new();
+    servers_table.set_implicit(true);
+
+    for (name, config) in servers {
+        let mut entry = TomlTable::new();
+        entry.set_implicit(false);
+
+        match &config.transport {
+            McpServerTransportConfig::Stdio {
+                command,
+                args,
+                env,
+                env_vars,
+                cwd,
+            } => {
+                entry["command"] = toml_edit::value(command.clone());
+
+                if !args.is_empty() {
+                    let mut args_array = TomlArray::new();
+                    for arg in args {
+                        args_array.push(arg.clone());
+                    }
+                    entry["args"] = TomlItem::Value(args_array.into());
+                }
+
+                if let Some(env) = env
+                    && !env.is_empty()
+                {
+                    let mut env_table = TomlTable::new();
+                    env_table.set_implicit(false);
+                    let mut pairs: Vec<_> = env.iter().collect();
+                    pairs.sort_by(|(a, _), (b, _)| a.cmp(b));
+                    for (key, value) in pairs {
+                        env_table.insert(key, toml_edit::value(value.clone()));
+                    }
+                    entry["env"] = TomlItem::Table(env_table);
+                }
+
+                if !env_vars.is_empty() {
+                    let mut vars = TomlArray::new();
+                    for var in env_vars {
+                        vars.push(var.clone());
+                    }
+                    entry["env_vars"] = TomlItem::Value(vars.into());
+                }
+
+                if let Some(cwd) = cwd {
+                    entry["cwd"] = toml_edit::value(cwd.to_string_lossy().to_string());
+                }
+            }
+            McpServerTransportConfig::StreamableHttp {
+                url,
+                bearer_token_env_var,
+                http_headers,
+                env_http_headers,
+            } => {
+                entry["url"] = toml_edit::value(url.clone());
+
+                if let Some(env_var) = bearer_token_env_var {
+                    entry["bearer_token_env_var"] = toml_edit::value(env_var.clone());
+                }
+
+                if let Some(headers) = http_headers
+                    && !headers.is_empty()
+                {
+                    let mut headers_table = TomlTable::new();
+                    headers_table.set_implicit(false);
+                    let mut pairs: Vec<_> = headers.iter().collect();
+                    pairs.sort_by(|(a, _), (b, _)| a.cmp(b));
+                    for (key, value) in pairs {
+                        headers_table.insert(key, toml_edit::value(value.clone()));
+                    }
+                    entry["http_headers"] = TomlItem::Table(headers_table);
+                }
+
+                if let Some(headers) = env_http_headers
+                    && !headers.is_empty()
+                {
+                    let mut headers_table = TomlTable::new();
+                    headers_table.set_implicit(false);
+                    let mut pairs: Vec<_> = headers.iter().collect();
+                    pairs.sort_by(|(a, _), (b, _)| a.cmp(b));
+                    for (key, value) in pairs {
+                        headers_table.insert(key, toml_edit::value(value.clone()));
+                    }
+                    entry["env_http_headers"] = TomlItem::Table(headers_table);
+                }
+            }
+        }
+
+        entry["enabled"] = toml_edit::value(config.enabled);
+
+        if let Some(startup) = config.startup_timeout_sec {
+            entry["startup_timeout_sec"] = toml_edit::value(startup.as_secs_f64());
+        }
+
+        if let Some(tool_timeout) = config.tool_timeout_sec {
+            entry["tool_timeout_sec"] = toml_edit::value(tool_timeout.as_secs_f64());
+        }
+
+        if let Some(enabled_tools) = config.enabled_tools.as_ref()
+            && !enabled_tools.is_empty()
+        {
+            let mut tools = TomlArray::new();
+            for tool in enabled_tools {
+                tools.push(tool.clone());
+            }
+            entry["enabled_tools"] = TomlItem::Value(tools.into());
+        }
+
+        if let Some(disabled_tools) = config.disabled_tools.as_ref()
+            && !disabled_tools.is_empty()
+        {
+            let mut tools = TomlArray::new();
+            for tool in disabled_tools {
+                tools.push(tool.clone());
+            }
+            entry["disabled_tools"] = TomlItem::Value(tools.into());
+        }
+
+        servers_table.insert(name, TomlItem::Table(entry));
+    }
+
+    table.insert("mcp_servers", TomlItem::Table(servers_table));
+    Ok(())
+}
+
+fn convert_mcp_servers(
+    servers: &HashMap<String, ProtocolMcpServerConfig>,
+) -> Result<BTreeMap<String, McpServerConfig>> {
+    let mut result = BTreeMap::new();
+    for (name, config) in servers {
+        result.insert(name.clone(), convert_mcp_server_config(config)?);
+    }
+    Ok(result)
+}
+
+fn convert_mcp_server_config(config: &ProtocolMcpServerConfig) -> Result<McpServerConfig> {
+    let transport = match &config.transport {
+        ProtocolMcpServerTransportConfig::Stdio {
+            command,
+            args,
+            env,
+            env_vars,
+            cwd,
+        } => McpServerTransportConfig::Stdio {
+            command: command.clone(),
+            args: args.clone(),
+            env: env.clone(),
+            env_vars: env_vars.clone(),
+            cwd: cwd.clone(),
+        },
+        ProtocolMcpServerTransportConfig::StreamableHttp {
+            url,
+            bearer_token_env_var,
+            http_headers,
+            env_http_headers,
+        } => McpServerTransportConfig::StreamableHttp {
+            url: url.clone(),
+            bearer_token_env_var: bearer_token_env_var.clone(),
+            http_headers: http_headers.clone(),
+            env_http_headers: env_http_headers.clone(),
+        },
+    };
+
+    Ok(McpServerConfig {
+        transport,
+        enabled: config.enabled,
+        startup_timeout_sec: config
+            .startup_timeout_sec
+            .map(duration_from_secs)
+            .transpose()?,
+        tool_timeout_sec: config
+            .tool_timeout_sec
+            .map(duration_from_secs)
+            .transpose()?,
+        enabled_tools: config.enabled_tools.clone(),
+        disabled_tools: config.disabled_tools.clone(),
+    })
+}
+
+fn duration_from_secs(value: f64) -> Result<Duration> {
+    Duration::try_from_secs_f64(value).map_err(|err| anyhow!(err))
+}
+
+fn protocol_oauth_mode_to_str(mode: McpOAuthCredentialsStoreMode) -> &'static str {
+    match mode {
+        McpOAuthCredentialsStoreMode::Auto => "auto",
+        McpOAuthCredentialsStoreMode::File => "file",
+        McpOAuthCredentialsStoreMode::Keyring => "keyring",
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;