diff --git a/.issueflows/03-solved-issues/issue458_original.md b/.issueflows/03-solved-issues/issue458_original.md
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+# Issue #458 — Stage 1.15: translate.py — dormant native⇄legacy frame translation in cellpy_file/
+
+GitHub: https://github.com/jepegit/cellpy/issues/458
+Labels: cellpy2-stage1
+
+## Goal
+
+Native-headers plan Phase 1: build `to_native(data)` / `to_legacy(data)` in
+`cellpy/readers/cellpy_file/translate.py` over `cellpycore.legacy.mapping`
+(dormant on v1.x — the Phase-3 flip wires it into `cellpy_file.load`), with the
+v8 → native → v8 round-trip and totality tests as the guards, and the #434
+value-parity comparator green against the translated frames.
+
+Depends on core#116 (mapping extensions; shipped in cellpycore 0.2.0).
+
+## Acceptance
+
+- v8 golden → to_native → to_legacy round-trip green; every column in the
+  golden file classified (mapped / legacy-only) — unknown fails; #434
+  comparator green.
diff --git a/.issueflows/03-solved-issues/issue458_plan.md b/.issueflows/03-solved-issues/issue458_plan.md
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+# Plan — issue #458 (Stage 1.15)
+
+1. **`cellpy/readers/cellpy_file/translate.py`** — all header knowledge comes
+   from `cellpycore.legacy.mapping` (core#116 / cellpycore 0.2.0); nothing is
+   declared locally:
+   - frame-level: `raw/steps/summary_to_native` and `_to_legacy` (summary
+     renames include the `{col}_{mode}` specific variants via
+     `mapping.expand_specific_columns`); renames touch only columns present.
+   - Data-level: `to_native(data)` injects `test_id = 0` where absent (native
+     composite group keys, D3); `to_legacy(data)` strips it again from
+     steps/summary so v8 → native → v8 is exact.
+   - `classify_legacy_columns(columns, family)` — the totality guard:
+     mapped / legacy-only / unknown.
+2. **Dormant scope:** lossless renames only. `date_time` → `epoch_time_utc`
+   derivation and the drop-and-recompute policy for summary cruft belong to
+   the Phase-3 importer, not this layer — round trips here are exact by
+   construction.
+3. **Quirks the totality guard surfaced (now documented classifications):**
+   the bridge's `index`-column sort quirk on steps (native-headers D4), and
+   `shifted_*_{gravimetric,areal}` summary variants in old files (legacy
+   `specific_columns` included the shifted capacities; the base is
+   legacy-only, so the variants are too).
+4. **Tests** (`tests/test_translate.py`): v8 golden round-trip
+   (`assert_frame_equal`, exact columns/order/dtypes), totality (unknown
+   column fails), #434 `assert_value_parity` green per family through the
+   translation, specific-variant two-way translation, family validation.
diff --git a/.issueflows/03-solved-issues/issue458_status.md b/.issueflows/03-solved-issues/issue458_status.md
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+# Status — issue #458 (Stage 1.15)
+
+- [x] Done
+
+## 2026-07-15
+
+- `cellpy_file/translate.py` landed (dormant; Phase 3 wires it into load).
+- v8 → native → v8 round-trip exact; totality guard classified two real
+  quirks (steps `index` column per D4; legacy-only `shifted_*` specific
+  variants); #434 comparator green through the translation for all three
+  frame families.
+- Full suite: **592 passed, 0 failed**. Stage-1 issue set complete.
diff --git a/cellpy/readers/cellpy_file/translate.py b/cellpy/readers/cellpy_file/translate.py
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+++ b/cellpy/readers/cellpy_file/translate.py
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+"""Dormant native ⇄ legacy frame translation for the cellpy-file boundary.
+
+Native-headers plan Phase 1 (issue #458, Stage 1.15): the translation layer
+that the cellpy 2 flip will run **once at I/O** instead of today's per-call
+rename sandwich. On the v1.x line this module is *dormant* — nothing in the
+runtime calls it yet; the round-trip and totality tests keep it honest until
+the flip (native-headers plan Phase 3) wires it into ``cellpy_file.load``.
+
+All header knowledge comes from ``cellpycore.legacy.mapping`` (the authoritative
+lossless/total mapping, extended in core #116 with ``expand_specific_columns``);
+no column names are declared here.
+
+Import policies implemented (native-headers plan D3):
+
+- **Mapped columns** are renamed (both directions; summary includes the
+  ``{col}_{gravimetric|areal|absolute}`` specific variants).
+- **Legacy-only columns** pass through unchanged (the native schema tolerates
+  extras). Deriving ``epoch_time_utc`` from ``date_time`` and the
+  drop-and-recompute policy for summary cruft belong to the Phase-3 importer,
+  not this dormant layer — round trips here are lossless by construction.
+- **``test_id``** is injected (``= 0``) on the native side when absent
+  (native group keys are composite) and stripped again on the legacy side for
+  frames that never carried it (steps/summary), so v8 → native → v8 is exact.
+"""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+
+from cellpycore.config import default_schema
+from cellpycore.legacy import mapping
+
+if TYPE_CHECKING:
+    import pandas as pd
+
+    from cellpy.readers.data_structures import Data
+
+SPECIFIC_MODES = ("gravimetric", "areal", "absolute")
+
+_TEST_ID = "test_id"
+
+
+def _summary_native_to_legacy_rename() -> dict:
+    """Native → legacy summary rename incl. the specific-column variants."""
+    schema = default_schema()
+    return mapping.expand_specific_columns(
+        mapping.native_to_legacy_summary(),
+        schema.cycle.specific_columns,
+        SPECIFIC_MODES,
+    )
+
+
+def _rename_present(frame: "pd.DataFrame", rename: dict) -> "pd.DataFrame":
+    """Rename only the columns present; never touch anything else."""
+    present = {old: new for old, new in rename.items() if old in frame.columns}
+    return frame.rename(columns=present)
+
+
+# --- frame-level translation ---------------------------------------------------
+def raw_to_native(frame: "pd.DataFrame") -> "pd.DataFrame":
+    """Legacy raw frame → native ``RawCols`` names (extras pass through)."""
+    return _rename_present(frame, mapping.legacy_to_native_raw())
+
+
+def raw_to_legacy(frame: "pd.DataFrame") -> "pd.DataFrame":
+    """Native raw frame → legacy ``HeadersNormal`` names."""
+    rename = {native: legacy for native, legacy in mapping.RAW_PAIRS}
+    return _rename_present(frame, rename)
+
+
+def steps_to_native(frame: "pd.DataFrame") -> "pd.DataFrame":
+    """Legacy step table → native ``StepCols`` names (extras pass through)."""
+    return _rename_present(frame, mapping.legacy_to_native_step())
+
+
+def steps_to_legacy(frame: "pd.DataFrame") -> "pd.DataFrame":
+    """Native step table → legacy ``HeadersStepTable`` names."""
+    return _rename_present(frame, mapping.native_to_legacy_step())
+
+
+def summary_to_native(frame: "pd.DataFrame") -> "pd.DataFrame":
+    """Legacy summary → native ``CycleCols`` names incl. specific variants."""
+    rename = {v: k for k, v in _summary_native_to_legacy_rename().items()}
+    return _rename_present(frame, rename)
+
+
+def summary_to_legacy(frame: "pd.DataFrame") -> "pd.DataFrame":
+    """Native summary → legacy ``HeadersSummary`` names incl. specific variants."""
+    return _rename_present(frame, _summary_native_to_legacy_rename())
+
+
+# --- column classification (the totality guard) --------------------------------
+def classify_legacy_columns(columns, family: str) -> dict:
+    """Classify legacy columns as ``mapped`` / ``legacy-only`` / ``unknown``.
+
+    The importer's totality guard (native-headers plan Phase 1 tests): every
+    column in a v8 file must be classified — an ``unknown`` column fails the
+    round-trip test until it is deliberately categorized in the core mapping.
+
+    Args:
+        columns: Iterable of legacy column names.
+        family: ``"raw"``, ``"steps"``, or ``"summary"``.
+
+    Returns:
+        dict: ``{column_name: classification}``.
+    """
+    if family == "raw":
+        mapped = set(mapping.legacy_to_native_raw())
+        legacy_only = set(mapping.LEGACY_ONLY_RAW)
+    elif family == "steps":
+        mapped = set(mapping.legacy_to_native_step())
+        legacy_only = set(mapping.LEGACY_ONLY_STEP)
+    elif family == "summary":
+        mapped = set(_summary_native_to_legacy_rename().values())
+        legacy_only = set(mapping.LEGACY_ONLY_CYCLE)
+    else:
+        raise ValueError(
+            f"unknown family {family!r}; expected 'raw', 'steps', or 'summary'"
+        )
+
+    out = {}
+    for col in columns:
+        if col in mapped:
+            out[col] = "mapped"
+        elif col in legacy_only:
+            out[col] = "legacy-only"
+        elif family == "summary" and col.startswith("aux_"):
+            # the legacy aux_ prefix marker covers a family of columns
+            out[col] = "legacy-only"
+        elif family == "summary" and _is_legacy_only_specific_variant(col):
+            # legacy specific_columns included the shifted_* capacities, so
+            # old files carry e.g. shifted_charge_capacity_gravimetric —
+            # legacy-only base, legacy-only variant
+            out[col] = "legacy-only"
+        elif family == "steps" and col == "index":
+            # the bridge's index-column sort quirk (native-headers plan D4)
+            out[col] = "legacy-only"
+        elif col == _TEST_ID:
+            out[col] = "legacy-only"
+        else:
+            out[col] = "unknown"
+    return out
+
+
+def _is_legacy_only_specific_variant(col: str) -> bool:
+    """True for ``{legacy-only col}_{mode}`` summary variants in old files."""
+    for mode in SPECIFIC_MODES:
+        suffix = f"_{mode}"
+        if col.endswith(suffix):
+            return col.removesuffix(suffix) in mapping.LEGACY_ONLY_CYCLE
+    return False
+
+
+# --- Data-level translation -----------------------------------------------------
+def to_native(data: "Data") -> "Data":
+    """Translate a legacy-named ``Data`` object's frames to native names.
+
+    Renames in place on the passed object (frames are replaced, not mutated),
+    and injects ``test_id = 0`` where absent — native group keys are composite
+    ``(test_id, cycle_num, …)`` (native-headers plan D3).
+    """
+    if getattr(data, "raw", None) is not None and len(data.raw.columns):
+        raw = raw_to_native(data.raw)
+        if _TEST_ID not in raw.columns:
+            raw = raw.assign(**{_TEST_ID: 0})
+        data.raw = raw
+    if getattr(data, "steps", None) is not None and len(data.steps.columns):
+        steps = steps_to_native(data.steps)
+        if _TEST_ID not in steps.columns:
+            steps = steps.assign(**{_TEST_ID: 0})
+        data.steps = steps
+    if getattr(data, "summary", None) is not None and len(data.summary.columns):
+        summary = summary_to_native(data.summary)
+        if _TEST_ID not in summary.columns:
+            summary = summary.assign(**{_TEST_ID: 0})
+        data.summary = summary
+    return data
+
+
+def to_legacy(data: "Data", *, injected_test_id: bool = True) -> "Data":
+    """Translate a native-named ``Data`` object's frames back to legacy names.
+
+    Args:
+        data: The object whose frames get renamed (replaced, not mutated).
+        injected_test_id: Strip the ``test_id`` column from steps/summary
+            (legacy never carried it there; raw keeps its own legacy
+            ``test_id``). Set False to keep it.
+    """
+    if getattr(data, "raw", None) is not None and len(data.raw.columns):
+        data.raw = raw_to_legacy(data.raw)
+    if getattr(data, "steps", None) is not None and len(data.steps.columns):
+        steps = steps_to_legacy(data.steps)
+        if injected_test_id and _TEST_ID in steps.columns:
+            steps = steps.drop(columns=[_TEST_ID])
+        data.steps = steps
+    if getattr(data, "summary", None) is not None and len(data.summary.columns):
+        summary = summary_to_legacy(data.summary)
+        if injected_test_id and _TEST_ID in summary.columns:
+            summary = summary.drop(columns=[_TEST_ID])
+        data.summary = summary
+    return data
diff --git a/tests/test_translate.py b/tests/test_translate.py
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--- /dev/null
+++ b/tests/test_translate.py
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+"""Round-trip / totality tests for the dormant ``cellpy_file.translate`` layer.
+
+Native-headers plan Phase 1 (issue #458, Stage 1.15): the translation is
+dormant on v1.x — these tests are what keep it correct until the Phase-3 flip
+wires it into ``cellpy_file.load``:
+
+1. v8 golden file → ``to_native`` → ``to_legacy`` reproduces every frame
+   exactly (columns, order, values — lossless by construction).
+2. Totality: every column in the golden v8 frames is classified (mapped or
+   documented legacy-only) — an unclassified column fails until deliberately
+   categorized in the core mapping.
+3. The Stage-0 value-parity comparator (#434) is green between the legacy
+   frames and their translated native twins.
+"""
+
+import pandas as pd
+import pytest
+from pandas.testing import assert_frame_equal
+
+from cellpy import cellreader, log
+from cellpy.readers.cellpy_file import translate
+from tests import fdv
+from tests.parity import assert_value_parity
+
+log.setup_logging(default_level="DEBUG", testing=True)
+
+
+@pytest.fixture(scope="module")
+def golden_frames():
+    cell = cellreader.CellpyCell()
+    cell.load(fdv.cellpy_file_path)
+    return {
+        "raw": cell.data.raw.copy(),
+        "steps": cell.data.steps.copy(),
+        "summary": cell.data.summary.copy(),
+    }
+
+
+class _FrameCarrier:
+    """Minimal Data stand-in: the translate layer only touches the frames."""
+
+    def __init__(self, frames):
+        self.raw = frames["raw"].copy()
+        self.steps = frames["steps"].copy()
+        self.summary = frames["summary"].copy()
+
+
+def test_v8_round_trip_is_lossless(golden_frames):
+    carrier = _FrameCarrier(golden_frames)
+    translate.to_native(carrier)
+
+    # native side really is native (and carries the injected test_id)
+    assert "datapoint_num" in carrier.raw.columns
+    assert "cycle_num" in carrier.steps.columns
+    assert "cycle_num" in carrier.summary.columns
+    assert (carrier.steps["test_id"] == 0).all()
+    assert (carrier.summary["test_id"] == 0).all()
+
+    translate.to_legacy(carrier)
+    for family in ("raw", "steps", "summary"):
+        assert_frame_equal(
+            getattr(carrier, family), golden_frames[family], check_dtype=True
+        )
+
+
+def test_totality_every_golden_column_is_classified(golden_frames):
+    unknown = {}
+    for family in ("raw", "steps", "summary"):
+        classified = translate.classify_legacy_columns(
+            golden_frames[family].columns, family
+        )
+        bad = [col for col, kind in classified.items() if kind == "unknown"]
+        if bad:
+            unknown[family] = bad
+    assert not unknown, f"unclassified legacy columns: {unknown}"
+
+
+@pytest.mark.parametrize("family", ["raw", "steps", "summary"])
+def test_value_parity_comparator_green_through_translation(golden_frames, family):
+    carrier = _FrameCarrier(golden_frames)
+    translate.to_native(carrier)
+    assert_value_parity(golden_frames[family], getattr(carrier, family), family)
+
+
+def test_summary_specific_columns_translate_both_ways(golden_frames):
+    summary = golden_frames["summary"]
+    native = translate.summary_to_native(summary)
+    # a specific variant translated to its native name
+    assert "test_cumulated_charge_capacity_gravimetric" in native.columns
+    back = translate.summary_to_legacy(native)
+    assert list(back.columns) == list(summary.columns)
+
+
+def test_classify_rejects_unknown_family():
+    with pytest.raises(ValueError, match="family"):
+        translate.classify_legacy_columns(["a"], "bogus")