From 6b0c486861ffcf6e0bbab65697d181985e692ded Mon Sep 17 00:00:00 2001
From: Gabriel Peal <gpeal@users.noreply.github.com>
Date: Fri, 17 Oct 2025 14:47:50 -0700
Subject: [PATCH] [MCP] Render full MCP errors to the model (#5298)
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Previously, the model couldn't see why MCP tool calls failed, many of
which were the model using the parameters incorrectly. A common failure
is the model stringifying the json for the notion-update-page tool which
it then couldn't correct.

I want to do some system prompt massaging around this as well. However,
it is crucial that the model sees the error so it can fix it.

Before:
<img width="2984" height="832" alt="CleanShot 2025-10-17 at 13 02 36"
src="https://github.com/user-attachments/assets/709a3d27-b71b-4d8d-87b6-9b2d7fe4e6f2"
/>

After:
<img width="2488" height="1550" alt="CleanShot 2025-10-17 at 13 01 18"
src="https://github.com/user-attachments/assets/13a0b7dc-fdad-4996-bf2d-0772872c34fc"
/>

🎉
<img width="1078" height="568" alt="CleanShot 2025-10-17 at 13 09 30"
src="https://github.com/user-attachments/assets/64cde8be-9e6c-4e61-b971-c2ba22504292"
/>


Fixes #4707
---
 codex-rs/core/src/mcp_tool_call.rs | 5 ++++-
 codex-rs/tui/src/history_cell.rs   | 7 ++++++-
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/codex-rs/core/src/mcp_tool_call.rs b/codex-rs/core/src/mcp_tool_call.rs
index 7091a056..21fdaf00 100644
--- a/codex-rs/core/src/mcp_tool_call.rs
+++ b/codex-rs/core/src/mcp_tool_call.rs
@@ -58,7 +58,10 @@ pub(crate) async fn handle_mcp_tool_call(
     let result = sess
         .call_tool(&server, &tool_name, arguments_value.clone())
         .await
-        .map_err(|e| format!("tool call error: {e}"));
+        .map_err(|e| format!("tool call error: {e:?}"));
+    if let Err(e) = &result {
+        tracing::warn!("MCP tool call error: {e:?}");
+    }
     let tool_call_end_event = EventMsg::McpToolCallEnd(McpToolCallEndEvent {
         call_id: call_id.clone(),
         invocation,
diff --git a/codex-rs/tui/src/history_cell.rs b/codex-rs/tui/src/history_cell.rs
index 85acaf9b..0a358257 100644
--- a/codex-rs/tui/src/history_cell.rs
+++ b/codex-rs/tui/src/history_cell.rs
@@ -854,7 +854,12 @@ impl HistoryCell for McpToolCallCell {
                     }
                 }
                 Err(err) => {
-                    let err_line = Line::from(format!("Error: {err}").dim());
+                    let err_text = format_and_truncate_tool_result(
+                        &format!("Error: {err}"),
+                        TOOL_CALL_MAX_LINES,
+                        width as usize,
+                    );
+                    let err_line = Line::from(err_text.dim());
                     let wrapped = word_wrap_line(
                         &err_line,
                         RtOptions::new((width as usize).saturating_sub(4))