diff --git a/codex-rs/code-mode/src/runtime/value.rs b/codex-rs/code-mode/src/runtime/value.rs
index cba9b4efef0b..661179963197 100644
--- a/codex-rs/code-mode/src/runtime/value.rs
+++ b/codex-rs/code-mode/src/runtime/value.rs
@@ -6,6 +6,8 @@ use codex_code_mode_protocol::ImageDetail;
 
 const IMAGE_HELPER_EXPECTS_MESSAGE: &str = "image expects a non-empty image URL string, an object with image_url and optional detail, or a raw MCP image block";
 const REMOTE_IMAGE_URL_ERROR: &str = "Tool call failed: remote image URLs are not supported in tool outputs. Pass a base64 data URI instead";
+const INVALID_IMAGE_URL_ERROR: &str =
+    "Tool call failed: invalid image output. Pass a base64 data URI instead";
 const CODEX_IMAGE_DETAIL_META_KEY: &str = "codex/imageDetail";
 
 pub(super) fn serialize_output_text(
@@ -59,10 +61,15 @@ pub(super) fn normalize_output_image(
         if image_url.is_empty() {
             return Err(IMAGE_HELPER_EXPECTS_MESSAGE.to_string());
         }
-        let lower = image_url.to_ascii_lowercase();
-        if lower.starts_with("http://") || lower.starts_with("https://") {
+        let Some((scheme, _)) = image_url.split_once(':') else {
+            return Err(INVALID_IMAGE_URL_ERROR.to_string());
+        };
+        if scheme.eq_ignore_ascii_case("http") || scheme.eq_ignore_ascii_case("https") {
             return Err(REMOTE_IMAGE_URL_ERROR.to_string());
         }
+        if !scheme.eq_ignore_ascii_case("data") {
+            return Err(INVALID_IMAGE_URL_ERROR.to_string());
+        }
 
         let detail = detail_override.or(detail);
         let detail = match detail {
diff --git a/codex-rs/code-mode/src/service_tests.rs b/codex-rs/code-mode/src/service_tests.rs
index 49cfc53d9912..707540e3f07b 100644
--- a/codex-rs/code-mode/src/service_tests.rs
+++ b/codex-rs/code-mode/src/service_tests.rs
@@ -873,6 +873,40 @@ async fn image_helpers_reject_remote_urls() {
     }
 }
 
+#[tokio::test]
+async fn image_helpers_reject_invalid_image_outputs() {
+    let image_url =
+        "Error executing tool exec: Expected at least one message to convert to CallToolResult";
+    for source in [
+        format!("image({image_url:?}, \"original\");"),
+        format!("generatedImage({{ image_url: {image_url:?} }});"),
+    ] {
+        let service = InProcessCodeModeSession::new();
+
+        let response = execute(
+            &service,
+            ExecuteRequest {
+                source,
+                yield_time_ms: None,
+                ..execute_request("")
+            },
+        )
+        .await;
+
+        assert_eq!(
+            response,
+            RuntimeResponse::Result {
+                cell_id: cell_id("1"),
+                content_items: Vec::new(),
+                error_text: Some(
+                    "Tool call failed: invalid image output. Pass a base64 data URI instead"
+                        .to_string(),
+                ),
+            }
+        );
+    }
+}
+
 #[tokio::test]
 async fn image_helper_rejects_unsupported_detail() {
     let service = InProcessCodeModeSession::new();
diff --git a/codex-rs/core/tests/suite/code_mode.rs b/codex-rs/core/tests/suite/code_mode.rs
index a20eaa267a8d..7d2f1e3205e9 100644
--- a/codex-rs/core/tests/suite/code_mode.rs
+++ b/codex-rs/core/tests/suite/code_mode.rs
@@ -2983,6 +2983,49 @@ async fn code_mode_image_helper_rejects_remote_url() -> Result<()> {
     Ok(())
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn code_mode_image_helper_rejects_invalid_image_output() -> Result<()> {
+    skip_if_no_network!(Ok(()));
+
+    let server = responses::start_mock_server().await;
+    let (_test, second_mock) = run_code_mode_turn(
+        &server,
+        "use exec to return an image",
+        r#"
+const s = "Error executing tool exec: Expected at least one message to convert to CallToolResult";
+image(s.trim(), "original");
+"#,
+    )
+    .await?;
+
+    let req = second_mock.single_request();
+    let items = custom_tool_output_items(&req, "call-1");
+    let (_, success) = custom_tool_output_body_and_success(&req, "call-1");
+    assert_ne!(
+        success,
+        Some(true),
+        "code_mode invalid image output unexpectedly succeeded"
+    );
+    assert_eq!(items.len(), 2);
+    assert_regex_match(
+        concat!(
+            r"(?s)\A",
+            r"Script failed\nWall time \d+\.\d seconds\nOutput:\n\z"
+        ),
+        text_item(&items, /*index*/ 0),
+    );
+    assert_eq!(
+        text_item(&items, /*index*/ 1),
+        concat!(
+            "Script error:\n",
+            "Tool call failed: invalid image output. ",
+            "Pass a base64 data URI instead"
+        )
+    );
+
+    Ok(())
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn code_mode_can_use_view_image_result_with_image_helper() -> Result<()> {
     skip_if_no_network!(Ok(()));