diff --git a/programs/fileio.c b/programs/fileio.c
index 051f94c29..29aa14ab5 100644
--- a/programs/fileio.c
+++ b/programs/fileio.c
@@ -3654,7 +3654,7 @@ typedef struct {
     const char* srcFileName;
     unsigned long long windowSize;
     size_t headerError;
-    unsigned windowLog;
+    unsigned windowLog; /* Rust derives this from windowSize; retain ABI layout. */
     unsigned memLimit;
     int errorCode;
     FIO_rust_zstdErrorHelpDisplayFn display;
@@ -3725,7 +3725,6 @@ FIO_zstdErrorHelp(const FIO_prefs_t* const prefs,
     int const errorCode = (int)ZSTD_getErrorCode(err);
     size_t headerError = 1;
     unsigned long long windowSize = 0;
-    unsigned windowLog = 0;
     unsigned memLimit = 0;
     FIO_rust_zstdErrorHelpState state = { 0 };
 
@@ -3735,8 +3734,6 @@ FIO_zstdErrorHelp(const FIO_prefs_t* const prefs,
             &header, ress->readCtx->srcBuffer, ress->readCtx->srcBufferLoaded);
         if (headerError == 0) {
             windowSize = header.windowSize;
-            windowLog = FIO_highbit64(windowSize)
-                      + ((windowSize & (windowSize - 1)) != 0);
             memLimit = prefs->memLimit;
         }
     }
@@ -3745,7 +3742,6 @@ FIO_zstdErrorHelp(const FIO_prefs_t* const prefs,
     state.srcFileName = srcFileName;
     state.windowSize = windowSize;
     state.headerError = headerError;
-    state.windowLog = windowLog;
     state.memLimit = memLimit;
     state.errorCode = errorCode;
     state.display = FIO_rust_zstdErrorHelp_display;
diff --git a/rust/src/fileio_prefs.rs b/rust/src/fileio_prefs.rs
index cee745c71..681db3100 100644
--- a/rust/src/fileio_prefs.rs
+++ b/rust/src/fileio_prefs.rs
@@ -390,9 +390,10 @@ type FIO_rust_zstdErrorHelpDisplayFn =
 
 /// Scalar projection for the zstd window-size error diagnostic.
 ///
-/// C keeps frame-header decoding, window-log extraction, and output formatting
-/// in C. Rust owns the error filter and selects the concrete or unsupported
-/// diagnostic through the display callback.
+/// C keeps frame-header decoding and output formatting in C. Rust derives the
+/// historical window log, owns the error filter, and selects the concrete or
+/// unsupported diagnostic through the display callback.  The `window_log`
+/// projection field remains part of the C ABI layout but is not consumed.
 #[cfg(feature = "decompression")]
 #[repr(C)]
 pub struct FIO_rust_zstdErrorHelpState {
@@ -406,6 +407,16 @@ pub struct FIO_rust_zstdErrorHelpState {
     display: Option<FIO_rust_zstdErrorHelpDisplayFn>,
 }
 
+#[cfg(feature = "decompression")]
+#[inline]
+fn ceil_log2_window_size(window_size: u64) -> c_uint {
+    if window_size == 0 {
+        return 0;
+    }
+    highbit64(window_size)
+        + c_uint::from((window_size & window_size.wrapping_sub(1)) != 0)
+}
+
 #[cfg(feature = "decompression")]
 const _: () = {
     let callback_offset = (offset_of!(FIO_rust_zstdErrorHelpState, error_code)
@@ -448,6 +459,7 @@ pub unsafe extern "C" fn FIO_rust_zstdErrorHelp(state: *const FIO_rust_zstdError
     let Some(display) = state.display else {
         return;
     };
+    let window_log = ceil_log2_window_size(state.window_size);
 
     if state.header_error == 0 {
         assert!(state.mem_limit > 0);
@@ -457,12 +469,12 @@ pub unsafe extern "C" fn FIO_rust_zstdErrorHelp(state: *const FIO_rust_zstdError
                 FIO_RUST_ZSTD_ERROR_HELP_WINDOW_SIZE,
                 state.src_file_name,
                 state.window_size,
-                state.window_log,
+                window_log,
                 0,
                 state.mem_limit,
             );
         }
-        if state.window_log <= ZSTD_WINDOWLOG_MAX {
+        if window_log <= ZSTD_WINDOWLOG_MAX {
             let window_mb = ((state.window_size >> 20)
                 + u64::from((state.window_size & ((1u64 << 20) - 1)) != 0))
                 as c_uint;
@@ -473,7 +485,7 @@ pub unsafe extern "C" fn FIO_rust_zstdErrorHelp(state: *const FIO_rust_zstdError
                     FIO_RUST_ZSTD_ERROR_HELP_WINDOW_GUIDANCE,
                     state.src_file_name,
                     state.window_size,
-                    state.window_log,
+                    window_log,
                     window_mb,
                     state.mem_limit,
                 );
@@ -488,7 +500,7 @@ pub unsafe extern "C" fn FIO_rust_zstdErrorHelp(state: *const FIO_rust_zstdError
             FIO_RUST_ZSTD_ERROR_HELP_UNSUPPORTED_WINDOW_LOG,
             state.src_file_name,
             state.window_size,
-            state.window_log,
+            window_log,
             0,
             state.mem_limit,
         );
@@ -2700,7 +2712,6 @@ mod tests {
         error_code: c_int,
         header_error: usize,
         window_size: u64,
-        window_log: c_uint,
         mem_limit: c_uint,
     ) -> FIO_rust_zstdErrorHelpState {
         FIO_rust_zstdErrorHelpState {
@@ -2708,18 +2719,44 @@ mod tests {
             src_file_name: ptr::null(),
             window_size,
             header_error,
-            window_log,
+            window_log: c_uint::MAX,
             mem_limit,
             error_code,
             display: Some(zstd_error_help_display_test),
         }
     }
 
+    #[cfg(feature = "decompression")]
+    #[test]
+    fn ceil_log2_window_size_handles_zero_powers_non_powers_and_boundaries() {
+        let cases = [
+            (0, 0),
+            (1, 0),
+            (2, 1),
+            (4, 2),
+            (3, 2),
+            (5, 3),
+            ((1u64 << 63) - 1, 63),
+            (1u64 << 63, 63),
+            ((1u64 << 63) + 1, 64),
+            (u64::MAX - 1, 64),
+            (u64::MAX, 64),
+        ];
+
+        for (window_size, expected_window_log) in cases {
+            assert_eq!(
+                ceil_log2_window_size(window_size),
+                expected_window_log,
+                "unexpected window log for window size {window_size}"
+            );
+        }
+    }
+
     #[cfg(feature = "decompression")]
     #[test]
     fn zstd_error_help_ignores_irrelevant_errors() {
         let mut context = ZstdErrorHelpTestState::default();
-        let state = zstd_error_help_test_state(&mut context, 0, 0, 8u64 << 20, 23, 64);
+        let state = zstd_error_help_test_state(&mut context, 0, 0, 8u64 << 20, 64);
 
         unsafe { FIO_rust_zstdErrorHelp(&state) };
 
@@ -2735,7 +2772,6 @@ mod tests {
             ZSTD_ERROR_FRAME_PARAMETER_WINDOW_TOO_LARGE,
             0,
             9u64 << 20,
-            24,
             64,
         );
 
@@ -2766,7 +2802,6 @@ mod tests {
             ZSTD_ERROR_FRAME_PARAMETER_WINDOW_TOO_LARGE,
             0,
             1u64 << window_log,
-            window_log,
             64,
         );