diff --git a/lib/compress/zstd_compress.c b/lib/compress/zstd_compress.c
index d928b1d3e..9eabaa7a3 100644
--- a/lib/compress/zstd_compress.c
+++ b/lib/compress/zstd_compress.c
@@ -59,19 +59,9 @@
 /*-*************************************
 *  Helper functions
 ***************************************/
-/* ZSTD_compressBound()
- * Note that the result from this function is only valid for
- * the one-pass compression functions.
- * When employing the streaming mode,
- * if flushes are frequently altering the size of blocks,
- * the overhead from block headers can make the compressed data larger
- * than the return value of ZSTD_compressBound().
- */
-size_t ZSTD_compressBound(size_t srcSize) {
-    size_t const r = ZSTD_COMPRESSBOUND(srcSize);
-    if (r==0) return ERROR(srcSize_wrong);
-    return r;
-}
+/* ZSTD_compressBound() lives in rust/src/zstd_compress_api.rs.  Keep this
+ * translation unit for the context implementation, which still calls the
+ * exported ABI symbol. */
 
 
 /*-*************************************
diff --git a/rust/src/lib.rs b/rust/src/lib.rs
index bc7d8909d..54d299449 100644
--- a/rust/src/lib.rs
+++ b/rust/src/lib.rs
@@ -22,6 +22,8 @@ pub mod threading;
 pub mod xxhash;
 pub mod zstd_common;
 #[cfg(feature = "compression")]
+pub mod zstd_compress_api;
+#[cfg(feature = "compression")]
 pub mod zstd_compress_literals;
 #[cfg(feature = "compression")]
 pub mod zstd_compress_sequences;
diff --git a/rust/src/zstd_compress_api.rs b/rust/src/zstd_compress_api.rs
new file mode 100644
index 000000000..8f57cae8e
--- /dev/null
+++ b/rust/src/zstd_compress_api.rs
@@ -0,0 +1,45 @@
+//! Public, context-free compression API helpers.
+//!
+//! The stateful compression context remains in the C translation unit for
+//! now.  These helpers have no context layout dependency, so they can expose
+//! the public ABI directly from Rust.
+
+use crate::errors::{ZstdErrorCode, ERROR};
+
+#[cfg(target_pointer_width = "64")]
+const ZSTD_MAX_INPUT_SIZE: usize = 0xFF00_FF00_FF00_FF00;
+#[cfg(target_pointer_width = "32")]
+const ZSTD_MAX_INPUT_SIZE: usize = 0xFF00_FF00;
+
+const SMALL_INPUT_THRESHOLD: usize = 128 << 10;
+
+/// Returns the maximum one-pass compressed size, matching
+/// `ZSTD_COMPRESSBOUND()` and its public error contract.
+#[no_mangle]
+pub extern "C" fn ZSTD_compressBound(src_size: usize) -> usize {
+    if src_size >= ZSTD_MAX_INPUT_SIZE {
+        return ERROR(ZstdErrorCode::SrcSizeWrong);
+    }
+
+    let margin = if src_size < SMALL_INPUT_THRESHOLD {
+        (SMALL_INPUT_THRESHOLD - src_size) >> 11
+    } else {
+        0
+    };
+    src_size.wrapping_add(src_size >> 8).wrapping_add(margin)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::errors::ERR_isError;
+
+    #[test]
+    fn compress_bound_matches_the_public_macro_edges() {
+        assert_eq!(ZSTD_compressBound(0), 64);
+        assert_eq!(ZSTD_compressBound(1), 64);
+        assert_eq!(ZSTD_compressBound(SMALL_INPUT_THRESHOLD - 1), 131_582);
+        assert_eq!(ZSTD_compressBound(SMALL_INPUT_THRESHOLD), 131_584);
+        assert!(ERR_isError(ZSTD_compressBound(ZSTD_MAX_INPUT_SIZE)));
+    }
+}