diff --git a/programs/fileio.c b/programs/fileio.c
index ad55b137e..7aad30953 100644
--- a/programs/fileio.c
+++ b/programs/fileio.c
@@ -1701,6 +1701,13 @@ int FIO_rust_compressZstdFrame(
         const char* srcFileName, U64 srcFileSize, int compressionLevel,
         U64* readsize, U64* compressedSize, size_t* zstdResult);
 
+/* Select the content size to pledge for the next zstd frame.  Rust owns the
+ * source-size precedence policy; C retains the private CCtx call and error
+ * handling in FIO_rust_compressZstdCallback(). */
+U64 FIO_rust_selectPledgedSrcSize(
+        U64 srcFileSize, U64 streamSrcSize,
+        U64 unknownSrcFileSize, U64 unknownPledgedSrcSize);
+
 enum {
     FIO_RUST_GZIP_OK = 0,
     FIO_RUST_GZIP_INIT_ERROR = 1,
@@ -3128,16 +3135,13 @@ FIO_rust_compressZstdCallback(void* fCtx, void* prefs, void* ress,
 
     DISPLAYLEVEL(6, "compression using zstd format \n");
 
-    /* Keep pledged-size and memory diagnostics in C while Rust owns the
-     * surrounding asynchronous stream loop. */
-    if (srcFileSize != UTIL_FILESIZE_UNKNOWN) {
-        pledgedSrcSize = srcFileSize;
-        CHECK(ZSTD_CCtx_setPledgedSrcSize(ressPtr->cctx, srcFileSize));
-    } else if (prefsPtr->streamSrcSize > 0) {
-        /* unknown source size; use the declared stream size */
-        pledgedSrcSize = prefsPtr->streamSrcSize;
-        CHECK(ZSTD_CCtx_setPledgedSrcSize(ressPtr->cctx, prefsPtr->streamSrcSize));
-    }
+    /* Rust selects the source-size precedence.  C retains the private CCtx
+     * operation and its exact diagnostic path. */
+    pledgedSrcSize = FIO_rust_selectPledgedSrcSize(
+        srcFileSize, (U64)prefsPtr->streamSrcSize,
+        UTIL_FILESIZE_UNKNOWN, ZSTD_CONTENTSIZE_UNKNOWN);
+    if (pledgedSrcSize != ZSTD_CONTENTSIZE_UNKNOWN)
+        CHECK(ZSTD_CCtx_setPledgedSrcSize(ressPtr->cctx, pledgedSrcSize));
 
     {   int windowLog;
         UTIL_HumanReadableSize_t windowSize;
diff --git a/rust/src/fileio_backend.rs b/rust/src/fileio_backend.rs
index 37f38c92d..603fa252c 100644
--- a/rust/src/fileio_backend.rs
+++ b/rust/src/fileio_backend.rs
@@ -1051,6 +1051,46 @@ const FIO_MMAP_DICT: c_int = 1;
 const FIO_MMAP_POLICY_ENABLE: c_int = 1;
 const FIO_MMAP_POLICY_DISABLE: c_int = 2;
 
+/// Chooses the source size to pledge for a zstd frame.
+///
+/// A statted source size has precedence over the user-declared stream size.
+/// The stream size is useful only when the source size is unknown and is
+/// positive; otherwise the caller's unknown-content sentinel is preserved.
+/// The sentinels are parameters so this policy does not assume that the CLI's
+/// file-size marker and zstd's content-size marker are represented by the
+/// same constant.
+#[inline]
+fn select_pledged_src_size(
+    src_file_size: u64,
+    stream_src_size: u64,
+    unknown_src_file_size: u64,
+    unknown_pledged_src_size: u64,
+) -> u64 {
+    if src_file_size != unknown_src_file_size {
+        src_file_size
+    } else if stream_src_size > 0 {
+        stream_src_size
+    } else {
+        unknown_pledged_src_size
+    }
+}
+
+/// Exposes the CLI source-size precedence policy to its C adapter.
+#[no_mangle]
+pub extern "C" fn FIO_rust_selectPledgedSrcSize(
+    src_file_size: u64,
+    stream_src_size: u64,
+    unknown_src_file_size: u64,
+    unknown_pledged_src_size: u64,
+) -> u64 {
+    select_pledged_src_size(
+        src_file_size,
+        stream_src_size,
+        unknown_src_file_size,
+        unknown_pledged_src_size,
+    )
+}
+
 /// Selects the dictionary backing store without performing any filesystem I/O.
 ///
 /// Explicit mmap enable wins for ordinary dictionaries, patch mode requests a
@@ -1170,6 +1210,33 @@ mod tests {
         );
     }
 
+    #[test]
+    fn pledged_source_size_prefers_known_source_then_positive_stream_size() {
+        let unknown_source = u64::MAX;
+        let unknown_pledged = u64::MAX - 1;
+
+        assert_eq!(
+            FIO_rust_selectPledgedSrcSize(123, 456, unknown_source, unknown_pledged),
+            123
+        );
+        assert_eq!(
+            FIO_rust_selectPledgedSrcSize(0, 456, unknown_source, unknown_pledged),
+            0
+        );
+        assert_eq!(
+            FIO_rust_selectPledgedSrcSize(unknown_source, 456, unknown_source, unknown_pledged,),
+            456
+        );
+        assert_eq!(
+            FIO_rust_selectPledgedSrcSize(unknown_source, 0, unknown_source, unknown_pledged,),
+            unknown_pledged
+        );
+        assert_eq!(
+            FIO_rust_selectPledgedSrcSize(unknown_source, 456, 123, unknown_pledged),
+            unknown_pledged
+        );
+    }
+
     fn open_source(path: &Path) -> (c_int, *mut c_void) {
         let path = c_path(path);
         let mut stat_buf = MaybeUninit::<libc::stat>::uninit();