diff --git a/codex-rs/core/src/client_tool_metadata.rs b/codex-rs/core/src/client_tool_metadata.rs
index 5fecb42a1b..f5193109de 100644
--- a/codex-rs/core/src/client_tool_metadata.rs
+++ b/codex-rs/core/src/client_tool_metadata.rs
@@ -2,9 +2,9 @@
 //! Only the wire copy changes; model-visible input and WebSocket continuation history stay intact.
 
 use crate::tools::metadata_metrics;
-use crate::utils::json::serialized_json_bytes;
 use codex_protocol::models::ResponseItem;
 use codex_protocol::models::bound_executed_tool_calls_for_message;
+use codex_utils_string::serialized_json_bytes;
 use serde::Serialize;
 
 const MAX_RESPONSE_MESSAGE_BYTES: usize = 15 * 1024 * 1024;
diff --git a/codex-rs/core/src/context_manager/history.rs b/codex-rs/core/src/context_manager/history.rs
index 42c894f294..9e2624bf03 100644
--- a/codex-rs/core/src/context_manager/history.rs
+++ b/codex-rs/core/src/context_manager/history.rs
@@ -33,7 +33,6 @@ use crate::guardian::GUARDIAN_MAX_ROOT_MESSAGE_TOKENS;
 use crate::guardian::guardian_truncate_text;
 use crate::session::FileAudioDurations;
 use crate::session::turn_context::TurnContext;
-use crate::utils::json::serialized_json_bytes;
 use base64::Engine;
 use base64::engine::general_purpose::STANDARD as BASE64_STANDARD;
 use codex_attachment_store::AttachmentStore;
@@ -79,6 +78,7 @@ use codex_utils_output_truncation::approx_token_count;
 use codex_utils_output_truncation::approx_tokens_from_byte_count_i64;
 use codex_utils_output_truncation::truncate_function_output_payload;
 use codex_utils_output_truncation::with_serialization_allowance;
+use codex_utils_string::serialized_json_bytes;
 use image::ImageReader;
 use std::collections::BTreeSet;
 use std::io::Cursor;
diff --git a/codex-rs/core/src/tools/executed_tool_calls.rs b/codex-rs/core/src/tools/executed_tool_calls.rs
index 52df442d30..563acf59d8 100644
--- a/codex-rs/core/src/tools/executed_tool_calls.rs
+++ b/codex-rs/core/src/tools/executed_tool_calls.rs
@@ -39,7 +39,7 @@ use crate::tools::context::ToolOutput;
 use crate::tools::context::ToolPayload;
 use crate::tools::metadata_metrics;
 use crate::tools::router::ToolCall;
-use crate::utils::json::serialized_json_bytes;
+use codex_utils_string::serialized_json_bytes;
 
 mod seen_ids;
 
diff --git a/codex-rs/core/src/utils/json.rs b/codex-rs/core/src/utils/json.rs
deleted file mode 100644
index 54b2303f1f..0000000000
--- a/codex-rs/core/src/utils/json.rs
+++ /dev/null
@@ -1,22 +0,0 @@
-#[derive(Default)]
-struct JsonByteCounter(usize);
-
-impl std::io::Write for JsonByteCounter {
-    fn write(&mut self, bytes: &[u8]) -> std::io::Result<usize> {
-        self.0 = self.0.saturating_add(bytes.len());
-        Ok(bytes.len())
-    }
-
-    fn flush(&mut self) -> std::io::Result<()> {
-        Ok(())
-    }
-}
-
-/// Counts serialized JSON bytes without retaining the serialized output.
-pub(crate) fn serialized_json_bytes<T: serde::Serialize + ?Sized>(
-    value: &T,
-) -> serde_json::Result<usize> {
-    let mut counter = JsonByteCounter::default();
-    serde_json::to_writer(&mut counter, value)?;
-    Ok(counter.0)
-}
diff --git a/codex-rs/core/src/utils/mod.rs b/codex-rs/core/src/utils/mod.rs
index b4bc359f3a..adbdcef12b 100644
--- a/codex-rs/core/src/utils/mod.rs
+++ b/codex-rs/core/src/utils/mod.rs
@@ -1,2 +1 @@
-pub(crate) mod json;
 pub mod path_utils;
diff --git a/codex-rs/utils/string/src/json.rs b/codex-rs/utils/string/src/json.rs
index da69ed6318..84a209e912 100644
--- a/codex-rs/utils/string/src/json.rs
+++ b/codex-rs/utils/string/src/json.rs
@@ -1,9 +1,30 @@
-//! JSON serialization helpers for ASCII-only transports and bounded output.
+//! JSON serialization helpers for counting bytes, bounding output, and ASCII-only transports.
 
 use std::io;
 
 use serde::Serialize;
 
+#[derive(Default)]
+struct JsonByteCounter(usize);
+
+impl io::Write for JsonByteCounter {
+    fn write(&mut self, bytes: &[u8]) -> io::Result<usize> {
+        self.0 = self.0.saturating_add(bytes.len());
+        Ok(bytes.len())
+    }
+
+    fn flush(&mut self) -> io::Result<()> {
+        Ok(())
+    }
+}
+
+/// Counts serialized JSON bytes without retaining the serialized output.
+pub fn serialized_json_bytes<T: Serialize + ?Sized>(value: &T) -> serde_json::Result<usize> {
+    let mut counter = JsonByteCounter::default();
+    serde_json::to_writer(&mut counter, value)?;
+    Ok(counter.0)
+}
+
 struct AsciiJsonFormatter;
 
 impl serde_json::ser::Formatter for AsciiJsonFormatter {
diff --git a/codex-rs/utils/string/src/lib.rs b/codex-rs/utils/string/src/lib.rs
index cbc755829c..6c3b70d4df 100644
--- a/codex-rs/utils/string/src/lib.rs
+++ b/codex-rs/utils/string/src/lib.rs
@@ -1,6 +1,7 @@
 mod json;
 mod truncate;
 
+pub use json::serialized_json_bytes;
 pub use json::to_ascii_json_string;
 pub use json::to_json_string_bounded;
 pub use truncate::TextTruncation;