diff --git a/pyproject.toml b/pyproject.toml
index be84cf3..1bd64dc 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -10,7 +10,7 @@ readme = "README.md"
 requires-python = ">=3.11"
 dependencies = [
     "pyyaml",
-    "jfjoch-client",
+    "jfjoch-client>=1.0.0rc166",
     "pydantic",
     "numpy",
     "scipy",
diff --git a/src/aarecommon/math/jfjoch_gridscan_union.py b/src/aarecommon/math/jfjoch_gridscan_union.py
index 1dfadc4..070a672 100644
--- a/src/aarecommon/math/jfjoch_gridscan_union.py
+++ b/src/aarecommon/math/jfjoch_gridscan_union.py
@@ -40,7 +40,8 @@ the picture if the database wants one.
 from __future__ import annotations
 
 import io
-from typing import Any, Optional, Sequence
+from collections.abc import Sequence
+from typing import Any
 
 import numpy as np
 from pydantic import BaseModel, Field
@@ -68,7 +69,7 @@ except ImportError:  # pragma: no cover
         z: float = 0.0
 
 
-__all__ = ["analyse", "GridScanResult", "Centre", "Size", "Counts", "Thresholds"]
+__all__ = ["Centre", "Counts", "GridScanResult", "Size", "Thresholds", "analyse"]
 
 #: DAQStatusModel, or anything else carrying the beam: its ``.geom`` is a
 #: SampleGeometryModel, and a ScanResultPayloadModel has ``beam_size_mm`` itself.
@@ -98,10 +99,10 @@ class Centre(BaseModel):
     nx: float
     ny: float
     image_number: int = Field(description="Nearest collected image, for addressing")
-    x_um: Optional[float] = Field(
+    x_um: float | None = Field(
         None, description="Signed offset from the centre of cell (0,0), along the grid's own axes"
     )
-    y_um: Optional[float] = None
+    y_um: float | None = None
 
     def offset_mm(self) -> Coordinate:
         """Offset in mm from the centre of cell (0,0), as an aarecommon Coordinate.
@@ -123,17 +124,17 @@ class Size(BaseModel):
     narrower than the beam still reports a size instead of nothing.
     """
 
-    x_um: Optional[float] = None
-    y_um: Optional[float] = None
-    z_um: Optional[float] = Field(None, description="From an orthogonal scan, if passed")
-    x_um_deconv: Optional[float] = None
-    y_um_deconv: Optional[float] = None
-    z_um_deconv: Optional[float] = None
-    beam_x_um: Optional[float] = Field(None, description="Beam FWHM used to deconvolve x")
-    beam_y_um: Optional[float] = Field(None, description="Beam FWHM used to deconvolve y")
-    area_um2: Optional[float] = None
-    equiv_diameter_um: Optional[float] = None
-    volume_pl: Optional[float] = Field(None, description="Ellipsoid; needs all three axes")
+    x_um: float | None = None
+    y_um: float | None = None
+    z_um: float | None = Field(None, description="From an orthogonal scan, if passed")
+    x_um_deconv: float | None = None
+    y_um_deconv: float | None = None
+    z_um_deconv: float | None = None
+    beam_x_um: float | None = Field(None, description="Beam FWHM used to deconvolve x")
+    beam_y_um: float | None = Field(None, description="Beam FWHM used to deconvolve y")
+    area_um2: float | None = None
+    equiv_diameter_um: float | None = None
+    volume_pl: float | None = Field(None, description="Ellipsoid; needs all three axes")
     volume_deconvolved: bool = Field(
         False, description="True if volume_pl used the beam-removed axes"
     )
@@ -168,44 +169,50 @@ class Counts(BaseModel):
     n_noise: int = Field(description="Spots present but neither protein nor ice")
     n_blank: int
     peak_protein_spots: int
-    mean_protein_in_contour: Optional[float] = None
-    best_resolution_A: Optional[float] = Field(None, description="Best res inside the contour")
-    median_bkg: Optional[float] = None
-    ice_in_contour: Optional[float] = Field(
+    mean_protein_in_contour: float | None = None
+    best_resolution_A: float | None = Field(None, description="Best res inside the contour")
+    median_bkg: float | None = None
+    ice_in_contour: float | None = Field(
         None, description="Fraction of frames inside the contour showing ice"
     )
     n_fragments: int = Field(
         0, description="Separate blobs at the 50 % level; >1 means more than one region"
     )
     ice_fraction: float = Field(description="Fraction of frames showing ice")
-    unit_cell_agreement: Optional[float] = Field(
+    unit_cell_agreement: float | None = Field(
         None, description="Fraction of indexed frames in the contour sharing one cell"
     )
 
 
 class GridScanResult(BaseModel):
     found: bool
-    reason: Optional[str] = Field(None, description="Why not, when found is False")
-    file_prefix: Optional[str] = None
+    reason: str | None = Field(None, description="Why not, when found is False")
+    file_prefix: str | None = None
     n_fast: int = 0
     n_slow: int = 0
     channel: str = Field("", description="Which spot channel built the map")
-    centre: Optional[Centre] = None
-    size: Optional[Size] = None
-    counts: Optional[Counts] = None
-    unit_cell: Optional[list[float]] = Field(
+    centre: Centre | None = None
+    size: Size | None = None
+    counts: Counts | None = None
+    unit_cell: list[float] | None = Field(
         None, description="Median cell of the contour, when check_unit_cell is on"
     )
     warnings: list[str] = Field(
         default_factory=list, description="Reasons to distrust a found=True result"
     )
-    jpeg: Optional[bytes] = Field(
+    contour_cells: list[tuple[int, int]] | None = Field(
+        None,
+        description="(nx, ny) of every collected cell inside the chosen 50 % "
+        "blob -- the decision's own footprint, for drawing it over the sample "
+        "image without recomputing the map",
+    )
+    jpeg: bytes | None = Field(
         None,
         exclude=True,
         repr=False,
         description="The picture, in whichever single style was asked for",
     )
-    jpeg_cells: Optional[bytes] = Field(
+    jpeg_cells: bytes | None = Field(
         None,
         exclude=True,
         repr=False,
@@ -225,10 +232,10 @@ class Thresholds(BaseModel):
         "the bkg-only gate then rejects the true crystal",
     )
     ice_spots: int = Field(5, description="spots_ice at or above this counts as ice")
-    ice_ring: Optional[float] = Field(
+    ice_ring: float | None = Field(
         None, description="Also call ice if scan_result 'ice' exceeds this"
     )
-    beam_um: Optional[float] = Field(
+    beam_um: float | None = Field(
         None,
         description="Beam FWHM in um, used only when no DAQ status is passed. "
         "A single number is taken as a square beam",
@@ -284,7 +291,7 @@ def _upsample(a: np.ndarray, f: int) -> np.ndarray:
     return np.asarray(im.resize((nx * fx, ny * fy), Image.BICUBIC), dtype=float)
 
 
-def _norm(a: np.ndarray, floor: float = 0.0, ref: Optional[np.ndarray] = None) -> np.ndarray:
+def _norm(a: np.ndarray, floor: float = 0.0, ref: np.ndarray | None = None) -> np.ndarray:
     """Normalise for colour against the channel's own range, but never below floor.
 
     ``ref`` supplies the range: the bounds come from the frames actually measured,
@@ -372,7 +379,7 @@ _BEAM_PATHS = (
 )
 
 
-def _beam_um(obj: Optional[BeamSource]) -> tuple[Optional[float], Optional[float]]:
+def _beam_um(obj: BeamSource | None) -> tuple[float | None, float | None]:
     """Beam FWHM (x, y) in micrometres, dug out of a DAQ status or scan payload.
 
     The beam is a Coordinate, not one number: on a beamline with an 80 x 20 um
@@ -406,7 +413,7 @@ def _beam_um(obj: Optional[BeamSource]) -> tuple[Optional[float], Optional[float
     return None, None
 
 
-def _deconvolve(fwhm: Optional[float], beam: Optional[float]) -> Optional[float]:
+def _deconvolve(fwhm: float | None, beam: float | None) -> float | None:
     """Quadrature removal of the beam. None when the beam swallows the feature."""
     if fwhm is None or beam is None:
         return fwhm
@@ -489,11 +496,11 @@ def _classify(F: dict, th: Thresholds) -> Counts:
 
 def analyse(
     scan_result: ScanResult,
-    grid_scan: Optional[GridScan] = None,
+    grid_scan: GridScan | None = None,
     *,
-    daq: Optional[BeamSource] = None,
-    thresholds: Optional[Thresholds] = None,
-    z_um: Optional[float] = None,
+    daq: BeamSource | None = None,
+    thresholds: Thresholds | None = None,
+    z_um: float | None = None,
     jpeg: bool = True,
 ) -> GridScanResult:
     """Analyse one grid scan.
@@ -610,6 +617,9 @@ def analyse(
         if keep.shape == inside.shape:
             inside = inside & keep
     counts.n_fragments = n_frag
+    if cell.any():
+        yy_c, xx_c = np.nonzero(cell)
+        out.contour_cells = [(int(x), int(y)) for y, x in zip(yy_c, xx_c)]
 
     if line:
         # One position wide: the 50 % crossings either side of the peak. The outer
@@ -640,7 +650,7 @@ def analyse(
         w_cells = ((xx.max() - xx.min() + 1) / mx * nx_n, (yy.max() - yy.min() + 1) / my * ny_n)
 
     # the nearest image actually collected, for the DAQ to address
-    iy, ix = int(round(np.clip(gy, 0, ny_n - 1))), int(round(np.clip(gx, 0, nx_n - 1)))
+    iy, ix = round(np.clip(gy, 0, ny_n - 1)), round(np.clip(gx, 0, nx_n - 1))
     num = int(F["number"][iy, ix])
     if num < 0:  # that cell was never collected
         yy2, xx2 = np.nonzero(F["number"] >= 0)
@@ -742,7 +752,7 @@ def _edge(mask: np.ndarray, width: int = 1) -> np.ndarray:
     return mask & ~core
 
 
-def _pictures(F, inside, centre, res: "GridScanResult", th: Thresholds, obj_level: float) -> None:
+def _pictures(F, inside, centre, res: GridScanResult, th: Thresholds, obj_level: float) -> None:
     """Fill res.jpeg, and res.jpeg_cells too when both styles were asked for."""
     want = ("smooth", "cells") if th.jpeg_style == "both" else (th.jpeg_style,)
     res.jpeg = _render(F, inside, centre, res, th, obj_level, want[0])
@@ -751,7 +761,7 @@ def _pictures(F, inside, centre, res: "GridScanResult", th: Thresholds, obj_leve
 
 
 def _render(
-    F, inside, centre, res: "GridScanResult", th: Thresholds, obj_level: float, style: str
+    F, inside, centre, res: GridScanResult, th: Thresholds, obj_level: float, style: str
 ) -> bytes:
     """Map, contours, crosshair and a one-line caption, as JPEG.
 
diff --git a/src/aarecommon/models/gridscan_decision.py b/src/aarecommon/models/gridscan_decision.py
new file mode 100644
index 0000000..b8cfeda
--- /dev/null
+++ b/src/aarecommon/models/gridscan_decision.py
@@ -0,0 +1,36 @@
+"""The grid-scan centring decision, as taken at the beamline.
+
+The DAQ runs the analysis (``aarecommon.math.jfjoch_gridscan_union``) to pick
+the collection position; this wraps its result verbatim with provenance and
+rides on ``RasterPayloadModel.decision`` into AareDB, whose Results viewer
+displays THE decision instead of recomputing one -- two implementations of the
+same analysis drifting apart is how a UI ends up contradicting the beamline.
+
+The pictures are excluded from the result's serialization by the model itself;
+the DAQ ships them through the ordinary image pipeline.
+"""
+
+from typing import Any
+
+from pydantic import BaseModel, Field
+
+from aarecommon.math.jfjoch_gridscan_union import GridScanResult
+
+GRIDSCAN_DECISION_CONTRACT = 1
+
+
+class GridScanDecision(BaseModel):
+    contract: int = GRIDSCAN_DECISION_CONTRACT
+    algorithm: str = Field(
+        description="Which analysis took the decision, e.g. a "
+        "GridscanAnalysisMode value such as 'jfjoch_gridscan_union'"
+    )
+    algorithm_version: str | None = Field(
+        None, description="aarecommon version (or commit) the DAQ ran"
+    )
+    thresholds: dict[str, Any] | None = Field(
+        None,
+        description="Thresholds.model_dump() used at decision time, so a "
+        "stored decision states its own tuning",
+    )
+    result: GridScanResult
diff --git a/src/aarecommon/models/raster_grid.py b/src/aarecommon/models/raster_grid.py
index ed9a1b0..db37e4a 100644
--- a/src/aarecommon/models/raster_grid.py
+++ b/src/aarecommon/models/raster_grid.py
@@ -6,6 +6,7 @@ from pydantic import AfterValidator, BaseModel, Field
 
 from aarecommon.math.coordinate import Coordinate, SmargonCoordinate, positive_coords
 from aarecommon.math.sample_geometry import SampleGeometryModel
+from aarecommon.models.gridscan_decision import GridScanDecision
 
 
 class RasterGridRequest(BaseModel):
@@ -92,3 +93,6 @@ class RasterPayloadModel(BaseModel):
     cell_size_pxl: Annotated[Coordinate, AfterValidator(positive_coords)]
     beam_mark_pxl: tuple[float, float]
     beam_size_mm: Annotated[Coordinate, AfterValidator(positive_coords)]
+    # The centring decision as taken at the beamline (None from DAQs that
+    # have not adopted it yet); AareDB stores and displays it verbatim.
+    decision: GridScanDecision | None = None
diff --git a/tests/test_gridscan_decision.py b/tests/test_gridscan_decision.py
new file mode 100644
index 0000000..cbd1cfa
--- /dev/null
+++ b/tests/test_gridscan_decision.py
@@ -0,0 +1,68 @@
+"""GridScanDecision wire contract: wraps the analysis result verbatim."""
+
+import json
+
+import numpy as np
+
+from aarecommon.math.jfjoch_gridscan_union import Thresholds, analyse
+from aarecommon.models.gridscan_decision import GridScanDecision
+from aarecommon.models.raster_grid import RasterPayloadModel
+
+
+def _synthetic_scan(ny_n=20, nx_n=20):
+    """A gaussian protein blob on a flat loop, as plain dicts (jfjoch-free)."""
+    rng = np.random.default_rng(0)
+    yy, xx = np.mgrid[0:ny_n, 0:nx_n]
+    blob = 80 * np.exp(-(((xx - 7) ** 2 + (yy - 12) ** 2) / 8.0))
+    images = [
+        {
+            "number": int(y * nx_n + x),
+            "nx": int(x),
+            "ny": int(y),
+            "spots": int(blob[y, x] + 10 + rng.integers(0, 3)),
+            "spots_low_res": int(4 + rng.integers(0, 2)),
+            "spots_ice": 0,
+            "bkg": float(30 + 40 * np.exp(-(((x - 9) ** 2 + (y - 10) ** 2) / 90.0))),
+            "res": 2.0,
+        }
+        for y in range(ny_n)
+        for x in range(nx_n)
+    ]
+    return {"file_prefix": "decision-test", "images": images}
+
+
+def test_decision_round_trips_without_pictures(request):
+    th = Thresholds.model_validate({})
+    r = analyse(_synthetic_scan(), {"step_x_um": 10.0, "step_y_um": 10.0}, thresholds=th)
+    assert r.found
+    assert r.contour_cells, "the chosen 50 % blob must ship its cells"
+    assert all(0 <= x < r.n_fast and 0 <= y < r.n_slow for x, y in r.contour_cells)
+    # the centre lies within the contour's bounding box
+    assert r.centre is not None
+    xs = [x for x, _ in r.contour_cells]
+    ys = [y for _, y in r.contour_cells]
+    assert min(xs) - 1 <= r.centre.nx <= max(xs) + 1
+    assert min(ys) - 1 <= r.centre.ny <= max(ys) + 1
+
+    d = GridScanDecision(
+        algorithm="jfjoch_gridscan_union",
+        algorithm_version="test",
+        thresholds=th.model_dump(),
+        result=r,
+    )
+    wire = json.loads(d.model_dump_json())
+    assert wire["contract"] == 1
+    assert "jpeg" not in wire["result"], "pictures never ride the JSON"
+    assert wire["result"]["centre"]["image_number"] == r.centre.image_number
+    assert wire["thresholds"]["object_union_spots"] is True
+    back = GridScanDecision.model_validate(wire)
+    assert back.result.centre is not None
+    assert back.result.contour_cells is not None
+    assert back.result.centre.nx == r.centre.nx
+    assert [tuple(c) for c in back.result.contour_cells] == r.contour_cells
+
+
+def test_payload_decision_is_optional():
+    fields = RasterPayloadModel.model_fields
+    assert "decision" in fields
+    assert fields["decision"].default is None