diff --git a/MODULE.bazel b/MODULE.bazel
index 66f6054c12..1d00dc338f 100644
--- a/MODULE.bazel
+++ b/MODULE.bazel
@@ -679,6 +679,7 @@ crate.annotation(
 # The musl CLI's allocator needs declared native tools and consistent header flags.
 crate.annotation(
     build_script_env = {
+        "JEMALLOC_SYS_WITH_MALLOC_CONF": "narenas_ratio:0.25",
         "MAKE": "$${pwd}/$(MAKE)",
         "NM": "$${pwd}/$(execpath @llvm//tools:llvm-nm)",
         "RANLIB": "$${pwd}/$(execpath @llvm//tools:llvm-ranlib)",
diff --git a/codex-rs/.cargo/config.toml b/codex-rs/.cargo/config.toml
index 652e09e7fc..2de645c21e 100644
--- a/codex-rs/.cargo/config.toml
+++ b/codex-rs/.cargo/config.toml
@@ -1,4 +1,7 @@
 [env]
+# Roughly one arena per four CPUs reduces idle RSS after Linux musl conversations unload.
+JEMALLOC_SYS_WITH_MALLOC_CONF = "narenas_ratio:0.25"
+
 # curl-sys enables its NTLM sources but does not set the flag required by libcurl 8.21.
 CFLAGS_aarch64_pc_windows_gnullvm = "-DCURL_ENABLE_NTLM"
 CFLAGS_aarch64_pc_windows_msvc = "-DCURL_ENABLE_NTLM"