diff --git a/App/lib/core/Gen/Expr/ExprGenFstr.vp b/App/lib/core/Gen/Expr/ExprGenFstr.vp
index 94e9340..6415216 100644
--- a/App/lib/core/Gen/Expr/ExprGenFstr.vp
+++ b/App/lib/core/Gen/Expr/ExprGenFstr.vp
@@ -183,6 +183,19 @@ novirtual class ExprGenFstr(ExprGenBase.ExprGenBase):
         zero_i32: llvmlite.Value* = llvmlite.const_int32(pool, 0)
         buf_ptr: llvmlite.Value* = llvmlite.build_gep_array(builder, buf_ty, i8_ty, buf_alloca, zero_i32)
         if buf_ptr is None: return None
+        # f-string 降级为 snprintf 调用，而 snprintf 的声明在 includes/stdio.vp:7
+        # （`extern def snprintf(buf, size, fmt, *args) -> int`）。源文件若未
+        # import 任何声明它的模块，当前模块就没有 @snprintf 的 declare，问题会
+        # 被拖到后端才炸（llc: "use of undefined value '@snprintf'"）——按"隐式
+        # 提供"原则在使用点补 declare（先例：AsyncGenMixin 的 GetTickCount64）。
+        # 签名三段固定参数（i8*, i64, i8*）+ 变参，与下面的 build_call 一致。
+        if self.Gen.CallGenBase.find_func_in_module(mod, "snprintf") is None:
+            snprintf_decl: llvmlite.Function* = llvmlite.create_declare(pool, mod, "snprintf", i32_ty)
+            if snprintf_decl is not None:
+                snprintf_decl.IsVarArg = 1
+                llvmlite.add_param(pool, snprintf_decl, i8_ty, "buf")
+                llvmlite.add_param(pool, snprintf_decl, llvmlite.Int64(pool), "size")
+                llvmlite.add_param(pool, snprintf_decl, i8_ty, "fmt")
         # snprintf(buf, FSTR_BUF_SIZE, fmt, args...)
         size_val: llvmlite.Value* = llvmlite.const_int64(pool, FSTR_BUF_SIZE)
         buf_ptr.Next = size_val