Enforce user input length cap (#12823)

Currently there is no bound on the length of a user message submitted in
the TUI or through the app server interface. That means users can paste
many megabytes of text, which can lead to bad performance, hangs, and
crashes. In extreme cases, it can lead to a [kernel
panic](#12323).

This PR limits the length of a user input to 2**20 (about 1M)
characters. This value was chosen because it fills the entire context
window on the latest models, so accepting longer inputs wouldn't make
sense anyway.

Summary
- add a shared `MAX_USER_INPUT_TEXT_CHARS` constant in codex-protocol
and surface it in TUI and app server code
- block oversized submissions in the TUI submit flow and emit error
history cells when validation fails
- reject heavy app-server requests with JSON-RPC `-32602` and structured
`input_too_large` data, plus document the behavior

Testing
- ran the IDE extension with this change and verified that when I
attempt to paste a user message that's several MB long, it correctly
reports an error instead of crashing or making my computer hot.

Author: etraut-openai

codex-rs/app-server-protocol/src/protocol/v1.rs

@@ -531,6 +531,15 @@ impl From<V1TextElement> for CoreTextElement {
     }
 }
 
+impl InputItem {
+    pub fn text_char_count(&self) -> usize {
+        match self {
+            InputItem::Text { text, .. } => text.chars().count(),
+            InputItem::Image { .. } | InputItem::LocalImage { .. } => 0,
+        }
+    }
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 /// Deprecated in favor of AccountLoginCompletedNotification.

codex-rs/app-server-protocol/src/protocol/v2.rs

@@ -3052,6 +3052,18 @@ impl From<CoreUserInput> for UserInput {
     }
 }
 
+impl UserInput {
+    pub fn text_char_count(&self) -> usize {
+        match self {
+            UserInput::Text { text, .. } => text.chars().count(),
+            UserInput::Image { .. }
+            | UserInput::LocalImage { .. }
+            | UserInput::Skill { .. }
+            | UserInput::Mention { .. } => 0,
+        }
+    }
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
 #[serde(tag = "type", rename_all = "camelCase")]
 #[ts(tag = "type")]

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

@@ -1,5 +1,7 @@
 use crate::bespoke_event_handling::apply_bespoke_event_handling;
+use crate::error_code::INPUT_TOO_LARGE_ERROR_CODE;
 use crate::error_code::INTERNAL_ERROR_CODE;
+use crate::error_code::INVALID_PARAMS_ERROR_CODE;
 use crate::error_code::INVALID_REQUEST_ERROR_CODE;
 use crate::fuzzy_file_search::FuzzyFileSearchSession;
 use crate::fuzzy_file_search::run_fuzzy_file_search;
@@ -267,6 +269,7 @@ use codex_protocol::protocol::RolloutItem;
 use codex_protocol::protocol::SessionConfiguredEvent;
 use codex_protocol::protocol::SessionMetaLine;
 use codex_protocol::protocol::USER_MESSAGE_BEGIN;
+use codex_protocol::user_input::MAX_USER_INPUT_TEXT_CHARS;
 use codex_protocol::user_input::UserInput as CoreInputItem;
 use codex_rmcp_client::perform_oauth_login_return_url;
 use codex_utils_json_to_toml::json_to_toml;
@@ -4735,6 +4738,36 @@ impl CodexMessageProcessor {
         self.outgoing.send_error(request_id, error).await;
     }
 
+    fn input_too_large_error(actual_chars: usize) -> JSONRPCErrorError {
+        JSONRPCErrorError {
+            code: INVALID_PARAMS_ERROR_CODE,
+            message: format!(
+                "Input exceeds the maximum length of {MAX_USER_INPUT_TEXT_CHARS} characters."
+            ),
+            data: Some(serde_json::json!({
+                "input_error_code": INPUT_TOO_LARGE_ERROR_CODE,
+                "max_chars": MAX_USER_INPUT_TEXT_CHARS,
+                "actual_chars": actual_chars,
+            })),
+        }
+    }
+
+    fn validate_v1_input_limit(items: &[WireInputItem]) -> Result<(), JSONRPCErrorError> {
+        let actual_chars: usize = items.iter().map(WireInputItem::text_char_count).sum();
+        if actual_chars > MAX_USER_INPUT_TEXT_CHARS {
+            return Err(Self::input_too_large_error(actual_chars));
+        }
+        Ok(())
+    }
+
+    fn validate_v2_input_limit(items: &[V2UserInput]) -> Result<(), JSONRPCErrorError> {
+        let actual_chars: usize = items.iter().map(V2UserInput::text_char_count).sum();
+        if actual_chars > MAX_USER_INPUT_TEXT_CHARS {
+            return Err(Self::input_too_large_error(actual_chars));
+        }
+        Ok(())
+    }
+
     async fn send_internal_error(&self, request_id: ConnectionRequestId, message: String) {
         let error = JSONRPCErrorError {
             code: INTERNAL_ERROR_CODE,
@@ -5034,6 +5067,10 @@ impl CodexMessageProcessor {
             conversation_id,
             items,
         } = params;
+        if let Err(error) = Self::validate_v1_input_limit(&items) {
+            self.outgoing.send_error(request_id, error).await;
+            return;
+        }
         let Ok(conversation) = self.thread_manager.get_thread(conversation_id).await else {
             let error = JSONRPCErrorError {
                 code: INVALID_REQUEST_ERROR_CODE,
@@ -5085,6 +5122,10 @@ impl CodexMessageProcessor {
             summary,
             output_schema,
         } = params;
+        if let Err(error) = Self::validate_v1_input_limit(&items) {
+            self.outgoing.send_error(request_id, error).await;
+            return;
+        }
 
         let Ok(conversation) = self.thread_manager.get_thread(conversation_id).await else {
             let error = JSONRPCErrorError {
@@ -5567,6 +5608,10 @@ impl CodexMessageProcessor {
     }
 
     async fn turn_start(&self, request_id: ConnectionRequestId, params: TurnStartParams) {
+        if let Err(error) = Self::validate_v2_input_limit(&params.input) {
+            self.outgoing.send_error(request_id, error).await;
+            return;
+        }
         let (_, thread) = match self.load_thread(&params.thread_id).await {
             Ok(v) => v,
             Err(error) => {
@@ -5672,6 +5717,10 @@ impl CodexMessageProcessor {
             .await;
             return;
         }
+        if let Err(error) = Self::validate_v2_input_limit(&params.input) {
+            self.outgoing.send_error(request_id, error).await;
+            return;
+        }
 
         let mapped_items: Vec<CoreInputItem> = params
             .input

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

@@ -1,3 +1,5 @@
 pub(crate) const INVALID_REQUEST_ERROR_CODE: i64 = -32600;
+pub const INVALID_PARAMS_ERROR_CODE: i64 = -32602;
 pub(crate) const INTERNAL_ERROR_CODE: i64 = -32603;
 pub(crate) const OVERLOADED_ERROR_CODE: i64 = -32001;
+pub const INPUT_TOO_LARGE_ERROR_CODE: &str = "input_too_large";

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

@@ -67,6 +67,8 @@ mod thread_state;
 mod thread_status;
 mod transport;
 
+pub use crate::error_code::INPUT_TOO_LARGE_ERROR_CODE;
+pub use crate::error_code::INVALID_PARAMS_ERROR_CODE;
 pub use crate::transport::AppServerTransport;
 
 const LOG_FORMAT_ENV_VAR: &str = "LOG_FORMAT";

codex-rs/app-server/tests/suite/output_schema.rs

@@ -1,8 +1,11 @@
 use anyhow::Result;
 use app_test_support::McpProcess;
 use app_test_support::to_response;
+use codex_app_server::INPUT_TOO_LARGE_ERROR_CODE;
+use codex_app_server::INVALID_PARAMS_ERROR_CODE;
 use codex_app_server_protocol::AddConversationListenerParams;
 use codex_app_server_protocol::InputItem;
+use codex_app_server_protocol::JSONRPCError;
 use codex_app_server_protocol::JSONRPCResponse;
 use codex_app_server_protocol::NewConversationParams;
 use codex_app_server_protocol::NewConversationResponse;
@@ -13,6 +16,7 @@ use codex_protocol::config_types::ReasoningSummary;
 use codex_protocol::openai_models::ReasoningEffort;
 use codex_protocol::protocol::AskForApproval;
 use codex_protocol::protocol::SandboxPolicy;
+use codex_protocol::user_input::MAX_USER_INPUT_TEXT_CHARS;
 use core_test_support::responses;
 use core_test_support::skip_if_no_network;
 use pretty_assertions::assert_eq;
@@ -124,6 +128,85 @@ async fn send_user_turn_accepts_output_schema_v1() -> Result<()> {
     Ok(())
 }
 
+#[tokio::test]
+async fn send_user_turn_rejects_oversized_input_v1() -> Result<()> {
+    let server = responses::start_mock_server().await;
+    let body = responses::sse(vec![
+        responses::ev_response_created("resp-1"),
+        responses::ev_assistant_message("msg-1", "Done"),
+        responses::ev_completed("resp-1"),
+    ]);
+    let _response_mock = responses::mount_sse_once(&server, body).await;
+
+    let codex_home = TempDir::new()?;
+    create_config_toml(codex_home.path(), &server.uri())?;
+
+    let mut mcp = McpProcess::new(codex_home.path()).await?;
+    timeout(DEFAULT_READ_TIMEOUT, mcp.initialize()).await??;
+
+    let new_conv_id = mcp
+        .send_new_conversation_request(NewConversationParams {
+            ..Default::default()
+        })
+        .await?;
+    let new_conv_resp: JSONRPCResponse = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_response_message(RequestId::Integer(new_conv_id)),
+    )
+    .await??;
+    let NewConversationResponse {
+        conversation_id, ..
+    } = to_response::<NewConversationResponse>(new_conv_resp)?;
+
+    let listener_id = mcp
+        .send_add_conversation_listener_request(AddConversationListenerParams {
+            conversation_id,
+            experimental_raw_events: false,
+        })
+        .await?;
+    timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_response_message(RequestId::Integer(listener_id)),
+    )
+    .await??;
+
+    let oversized_input = "x".repeat(MAX_USER_INPUT_TEXT_CHARS + 1);
+    let send_turn_id = mcp
+        .send_send_user_turn_request(SendUserTurnParams {
+            conversation_id,
+            items: vec![InputItem::Text {
+                text: oversized_input.clone(),
+                text_elements: Vec::new(),
+            }],
+            cwd: codex_home.path().to_path_buf(),
+            approval_policy: AskForApproval::Never,
+            sandbox_policy: SandboxPolicy::DangerFullAccess,
+            model: "mock-model".to_string(),
+            effort: Some(ReasoningEffort::Low),
+            summary: ReasoningSummary::Auto,
+            output_schema: None,
+        })
+        .await?;
+
+    let err: JSONRPCError = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_error_message(RequestId::Integer(send_turn_id)),
+    )
+    .await??;
+
+    assert_eq!(err.error.code, INVALID_PARAMS_ERROR_CODE);
+    assert_eq!(
+        err.error.message,
+        format!("Input exceeds the maximum length of {MAX_USER_INPUT_TEXT_CHARS} characters.")
+    );
+    let data = err.error.data.expect("expected structured error data");
+    assert_eq!(data["input_error_code"], INPUT_TOO_LARGE_ERROR_CODE);
+    assert_eq!(data["max_chars"], MAX_USER_INPUT_TEXT_CHARS);
+    assert_eq!(data["actual_chars"], oversized_input.chars().count());
+
+    Ok(())
+}
+
 #[tokio::test]
 async fn send_user_turn_output_schema_is_per_turn_v1() -> Result<()> {
     skip_if_no_network!(Ok(()));

codex-rs/app-server/tests/suite/send_message.rs

@@ -3,9 +3,12 @@ use app_test_support::McpProcess;
 use app_test_support::create_fake_rollout;
 use app_test_support::rollout_path;
 use app_test_support::to_response;
+use codex_app_server::INPUT_TOO_LARGE_ERROR_CODE;
+use codex_app_server::INVALID_PARAMS_ERROR_CODE;
 use codex_app_server_protocol::AddConversationListenerParams;
 use codex_app_server_protocol::AddConversationSubscriptionResponse;
 use codex_app_server_protocol::InputItem;
+use codex_app_server_protocol::JSONRPCError;
 use codex_app_server_protocol::JSONRPCNotification;
 use codex_app_server_protocol::JSONRPCResponse;
 use codex_app_server_protocol::NewConversationParams;
@@ -27,6 +30,7 @@ use codex_protocol::protocol::RolloutItem;
 use codex_protocol::protocol::RolloutLine;
 use codex_protocol::protocol::SandboxPolicy;
 use codex_protocol::protocol::TurnContextItem;
+use codex_protocol::user_input::MAX_USER_INPUT_TEXT_CHARS;
 use core_test_support::responses;
 use pretty_assertions::assert_eq;
 use std::io::Write;
@@ -272,6 +276,66 @@ async fn test_send_message_session_not_found() -> Result<()> {
     Ok(())
 }
 
+#[tokio::test]
+async fn test_send_message_rejects_oversized_input() -> Result<()> {
+    let server = responses::start_mock_server().await;
+    let body = responses::sse(vec![
+        responses::ev_response_created("resp-1"),
+        responses::ev_assistant_message("msg-1", "Done"),
+        responses::ev_completed("resp-1"),
+    ]);
+    let _response_mock = responses::mount_sse_once(&server, body).await;
+
+    let codex_home = TempDir::new()?;
+    create_config_toml(codex_home.path(), &server.uri())?;
+
+    let mut mcp = McpProcess::new(codex_home.path()).await?;
+    timeout(DEFAULT_READ_TIMEOUT, mcp.initialize()).await??;
+
+    let new_conv_id = mcp
+        .send_new_conversation_request(NewConversationParams {
+            ..Default::default()
+        })
+        .await?;
+    let new_conv_resp: JSONRPCResponse = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_response_message(RequestId::Integer(new_conv_id)),
+    )
+    .await??;
+    let NewConversationResponse {
+        conversation_id, ..
+    } = to_response::<_>(new_conv_resp)?;
+
+    let oversized_input = "x".repeat(MAX_USER_INPUT_TEXT_CHARS + 1);
+    let req_id = mcp
+        .send_send_user_message_request(SendUserMessageParams {
+            conversation_id,
+            items: vec![InputItem::Text {
+                text: oversized_input.clone(),
+                text_elements: Vec::new(),
+            }],
+        })
+        .await?;
+
+    let err: JSONRPCError = timeout(
+        DEFAULT_READ_TIMEOUT,
+        mcp.read_stream_until_error_message(RequestId::Integer(req_id)),
+    )
+    .await??;
+
+    assert_eq!(err.error.code, INVALID_PARAMS_ERROR_CODE);
+    assert_eq!(
+        err.error.message,
+        format!("Input exceeds the maximum length of {MAX_USER_INPUT_TEXT_CHARS} characters.")
+    );
+    let data = err.error.data.expect("expected structured error data");
+    assert_eq!(data["input_error_code"], INPUT_TOO_LARGE_ERROR_CODE);
+    assert_eq!(data["max_chars"], MAX_USER_INPUT_TEXT_CHARS);
+    assert_eq!(data["actual_chars"], oversized_input.chars().count());
+
+    Ok(())
+}
+
 #[tokio::test]
 async fn resume_with_model_mismatch_appends_model_switch_once() -> Result<()> {
     let server = responses::start_mock_server().await;