diff --git a/lib/compress/zstdmt_compress.c b/lib/compress/zstdmt_compress.c
index d146456ec..d0e951630 100644
--- a/lib/compress/zstdmt_compress.c
+++ b/lib/compress/zstdmt_compress.c
@@ -2723,6 +2723,11 @@ static size_t ZSTDMT_createCompressionJob(void* opaque, size_t srcSize, unsigned
 {
     ZSTDMT_CCtx* const mtctx = (ZSTDMT_CCtx*)opaque;
     ZSTD_EndDirective const endOp = (ZSTD_EndDirective)end;
+    /* Rust fills the reusable input range in its local projection before
+     * calling this C-owned job-construction callback.  Publish that count
+     * before the callback prepares a job and checks the C context. */
+    assert(srcSize <= mtctx->inBuff.buffer.capacity);
+    mtctx->inBuff.filled = srcSize;
     ZSTDMT_RustCreateJobProjection const projection = {
         mtctx->doneJobID,
         mtctx->nextJobID,