diff --git a/resources/tester/helper_est_native.cjs b/resources/tester/helper_est_native.cjs
index 23ea0e740..b0a2350e5 100644
--- a/resources/tester/helper_est_native.cjs
+++ b/resources/tester/helper_est_native.cjs
@@ -64,7 +64,16 @@ else if (command === "repeat")
   );
 else if (command === "import")
   result = require(path.join(base, "import-estx.cjs")).importEstx(fs.readFileSync(process.argv[3]));
-else if (command === "blob-tail") {
+else if (command === "baseline-size") {
+  context.input = require(path.join(base, "import-estx.cjs")).importEstx(
+    fs.readFileSync(process.argv[3]),
+  );
+  const baseline = vm.runInContext("EstBusiness.baseline(input)", context);
+  result = {
+    bytes: Buffer.byteLength(JSON.stringify(baseline), "utf8"),
+    archivePreserved: baseline.originalArchive === context.input.originalArchive,
+  };
+} else if (command === "blob-tail") {
   const { blob } = require(path.join(base, "import-estx.cjs")),
     bytes = Buffer.alloc(1536);
   bytes.writeUInt32LE(2, 516);
diff --git a/resources/tester/test_est_business.py b/resources/tester/test_est_business.py
index 92211aead..4b8b3fc7a 100644
--- a/resources/tester/test_est_business.py
+++ b/resources/tester/test_est_business.py
@@ -208,3 +208,40 @@ def test_original_numeric_fields_reject_invalid_saved_values():
     })()""",
     )
     assert "GN_" in result
+
+
+def test_baseline_keeps_current_inputs_once_and_preserves_working_binaries():
+    result = run(
+        "math",
+        """(()=>{
+      const p=EstNative.empty();
+      p.tables.CostMainDat=[{Index:1,Code:'C',LineData:'current',LineData_:'previous'}];
+      p.tables.CadMainDat=[{Code:'D',Data:{binary:'QUJD'}}];
+      p.tables.StuffcDat=[{Code:'M',InputTC:100}];p.originalArchive='original bytes';
+      const q=EstBusiness.baseline(p),again=EstBusiness.baseline(q);
+      return {p,q,again};
+    })()""",
+    )
+    for item in [result["q"], result["again"]]:
+        assert item["originalArchive"] == "original bytes"
+        assert item["tables"]["CadMainDat"][0]["Data"]["binary"] == "QUJD"
+        assert "CadMainDat" not in item["baseline"]["tables"]
+        assert "StuffcDat" not in item["baseline"]["tables"]
+        assert item["baseline"]["tables"]["CostMainDat"][0]["LineData"] == "current"
+        assert "LineData_" not in item["baseline"]["tables"]["CostMainDat"][0]
+    assert result["p"]["tables"]["CostMainDat"][0]["LineData_"] == "previous"
+
+
+def test_large_original_baseline_stays_within_the_save_request_limit():
+    import os
+    from pathlib import Path
+    import pytest
+
+    archive = Path(
+        os.environ.get("EST_REA_MAIN", "C:/Dasansoft/Est Plus 2/Data/ProjectData/main.estx")
+    )
+    if not archive.is_file():
+        pytest.skip("Original main.estx is not installed")
+    result = run("baseline-size", str(archive))
+    assert result["archivePreserved"]
+    assert result["bytes"] < 256 * 1024 * 1024
diff --git a/tools/est-test/README.md b/tools/est-test/README.md
index c47499ae0..df7b3fbbd 100644
--- a/tools/est-test/README.md
+++ b/tools/est-test/README.md
@@ -10,6 +10,8 @@
 
 Ctrl+S는 입력 중인 셀도 확정한 뒤 저장합니다. 연속 저장 요청은 순서대로 처리하며, 저장 대기 중의 추가 입력이나 새로 연 문서를 이전 저장 응답으로 덮어쓰지 않습니다. 실행 취소는 현재 작업 파일의 연결을 유지합니다.
 
+당초 확정 사본에는 재계산 입력을 한 번만 보관하며, 작업본의 도면·원본 압축 자료는 보존합니다. 설치된 `main.estx`의 당초 확정 후 실제 저장 API를 통과했고, 산근 77,395행과 원본 압축 해시 보존·계산 오류 0을 확인했습니다(요청 약 215MB, 저장 약 221MB).
+
 초기 자료는 기술정보 DB의 실제 REA 저장 샘플입니다. `REA 샘플 대조`에서 원본 저장값과 275개 계산 결과를 비교합니다. 테스트용 임의 요율을 원본 기본값으로 넣지 않습니다. 새 원가 서식의 요율은 사용자가 입력합니다.
 
 ## 검증과 남은 범위
diff --git a/tools/est-test/native-business.js b/tools/est-test/native-business.js
index cec72656b..aa743b86c 100644
--- a/tools/est-test/native-business.js
+++ b/tools/est-test/native-business.js
@@ -5,8 +5,29 @@ const EstBusiness = (() => {
   const rowKey = (row, index) => String(row.PkID ?? row.Code ?? index);
   const special = (row) => [57, 58, 62, 67].includes(num(row.LinkIndex));
   function baseline(project) {
-    const next = copy(project);
-    next.baseline = { tables: copy(project.tables), name: project.name };
+    const working = { ...project };
+    delete working.baseline;
+    const next = copy(working);
+    // Original recalculation replaces source catalogs from the working project.
+    // Keep its contract inputs once; drawings and pre-existing '_' copies stay in the working file.
+    const tables = {};
+    for (const table of [
+      "EstMainDat",
+      "HeadMainDat",
+      "HeadTitleDat",
+      "CostMainDat",
+      "CostTitleDat",
+      "MachTitleDat",
+      "Wonga",
+    ]) {
+      tables[table] = (project.tables[table] || []).map((row) => {
+        const values = Object.fromEntries(
+          Object.entries(row).filter(([key]) => !key.endsWith("_")),
+        );
+        return copy(values);
+      });
+    }
+    next.baseline = { tables, name: project.name };
     for (const rows of Object.values(next.baseline.tables))
       for (const row of rows) row.NewItem = false;
     for (const rows of Object.values(next.tables))