diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 8b34bc9..8b54fbf 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -23,7 +23,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest, windows-latest, macos-latest]
-        python-version: ["3.10", "3.13"]
+        python-version: ["3.11", "3.13"]
     steps:
       - uses: actions/checkout@v4
       - uses: actions/setup-python@v5
diff --git a/AGENTS.md b/AGENTS.md
index 3025523..d8c3198 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -78,7 +78,7 @@ unchanged (contract §1).
    behaviour starts in Milestone 2.
 4. **Tests** use simulated data generated in the tests; real data only as an optional
    check. Each formula is tested against a direct implementation (plan §11).
-5. **Style** as in morphobricks-preprocessing: Python 3.10+, `ruff` (line length 100,
+5. **Style** as in morphobricks-preprocessing: Python 3.11+ (MorphoBricks decision D01), `ruff` (line length 100,
    NumPy docstrings), small modules with one job each, comments that explain why.
 6. **Done means** `ruff check .`, `ruff format --check .` and `pytest` pass and the docs
    are updated. Report honestly what was verified and what was not.
diff --git a/docs/contract.md b/docs/contract.md
index ea3d4f7..6368115 100644
--- a/docs/contract.md
+++ b/docs/contract.md
@@ -624,7 +624,7 @@ UTF-8, comma-separated, one header row, one row per plane, rows in a determinist
 
 | Column | Type | Meaning |
 |---|---|---|
-| `position_id` | text or blank | From reliable metadata only; never inferred from file names. |
+| `position_id` | text or blank | The XY position within its technical repeat (MorphoBricks decision D02): from reliable metadata, or the image file name without extension when one file holds exactly one position, as morphobricks-preprocessing writes it. Blank only in tables written before it was filled. |
 | `image_file` | text | File name. |
 | `z`, `c`, `t` | integer | Zero-based plane coordinates, always written; 0 for an axis the image does not have (preprocessing contract §2). |
 | `condition`, `biological_repeat`, `technical_repeat` | text | Folder labels; `single` when absent. Always read as text. |
diff --git a/pyproject.toml b/pyproject.toml
index c6ecb9a..203fc72 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -9,7 +9,7 @@ description = "MorphoBricks normalization brick: per-plane statistics, their fil
 readme = "README.md"
 license = "MIT"
 license-files = ["LICENSE"]
-requires-python = ">=3.10"
+requires-python = ">=3.11"
 authors = [{ name = "CellMigrationLab" }]
 keywords = ["microscopy", "image analysis", "histogram", "normalization", "MorphoBricks"]
 classifiers = [
@@ -41,7 +41,7 @@ addopts = "-ra"
 
 [tool.ruff]
 line-length = 100
-target-version = "py310"
+target-version = "py311"
 src = ["src", "tests"]
 
 [tool.ruff.lint]
diff --git a/src/morphobricks_normalization/records.py b/src/morphobricks_normalization/records.py
index cb23f78..63d836e 100644
--- a/src/morphobricks_normalization/records.py
+++ b/src/morphobricks_normalization/records.py
@@ -32,7 +32,9 @@ class PlaneRecord:
         Zero-based plane coordinates. An axis the image does not have, or has
         with length 1, has coordinate 0; None is stored as 0.
     position_id
-        Position identifier from reliable metadata, or None.
+        The XY position within its technical repeat (MorphoBricks decision D02):
+        from reliable metadata, or the file stem when one file is one position; None
+        only for records written before the field was filled.
     """
 
     source_relative_path: str