diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index cdaf544..d826336 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -20,7 +20,7 @@ jobs:
       - uses: astral-sh/setup-uv@c18668ad3cf93ea998bef934396af7bb5c839dc7  # v10.2.0
         with:
           enable-cache: true
-          # Distinguishes this job's cache key from pytest (py3.13)'s. Without
+          # Distinguishes this job's cache key from the pytest job on the same Python. Without
           # it both race on the same key and one fails to save.
           cache-suffix: lint
       # The ruff and ty hooks are `uv run --no-sync`, so the venv must exist
@@ -49,7 +49,7 @@ jobs:
       matrix:
         # The supported range, matching requires-python, not a pin: widen it by hand when
         # a Python is added to or dropped from support.
-        python-version: ["3.11", "3.12", "3.13", "3.14"]
+        python-version: ["3.11", "3.12", "3.13", "3.14", "3.15"]
     steps:
       - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1  # v7.0.1
         with:
diff --git a/README.md b/README.md
index e21e962..0320ad8 100644
--- a/README.md
+++ b/README.md
@@ -140,7 +140,7 @@ tightened.
 
 ## CI
 
-Every pull request runs lint, the pytest matrix (Python 3.11 through 3.14),
+Every pull request runs lint, the pytest matrix (Python 3.11 through 3.15),
 and a bootstrap smoke test that generates two projects from the template
 (one flat, one with DDD layering) and runs the full gate on each, catching
 any file that uses a placeholder the bootstrap script doesn't know about.
diff --git a/decisions/2026-10-13-python-version-matrix.md b/decisions/2026-10-13-python-version-matrix.md
new file mode 100644
index 0000000..28fedbf
--- /dev/null
+++ b/decisions/2026-10-13-python-version-matrix.md
@@ -0,0 +1,55 @@
+# Python version matrix follows the development pin
+
+## Decision: widen the pytest matrix in the same PR that bumps `.python-version`
+
+When Renovate raises `.python-version` to a new minor Python, that PR also adds
+the version to the `pytest` matrix in `.github/workflows/ci.yaml` and updates
+the CI section of `README.md`. If the new version fails the matrix, close the
+bump PR and keep `.python-version` on the previous minor until the dependencies
+catch up. Do not merge the pin without the matrix entry.
+
+## Context: why it came up
+
+Renovate bumped `.python-version` from 3.14 to 3.15 (#58) and it merged on
+green CI. The pin selects the interpreter for local development and the `lint`
+job only; the matrix is listed by hand and stayed at 3.11–3.14. The result was
+that the interpreter everyone develops on was outside the range CI tests.
+
+The matrix comment already says to widen it by hand. Nothing tied that to the
+event that makes it due.
+
+## Alternatives considered
+
+- **Have Renovate update the matrix too.** A custom regex manager could
+  rewrite the list. Rejected: adding a supported Python is a support decision,
+  and a newly released Python is the point where a dependency without wheels
+  for it breaks `uv sync`. That needs a person to look at the result, which a
+  bump that also widens the matrix would hide inside one green or red run.
+- **Leave the matrix until a separate support review.** Leaves a gap of
+  unknown length where local `task dev:check` is the only test on the pinned
+  version, and CI cannot report a failure local runs miss.
+- **Keep `.python-version` on the oldest supported Python.** Testing the floor
+  locally is defensible, but it turns down every Renovate bump forever and
+  means development never meets the new interpreter until users do.
+
+## Reasoning
+
+The bump PR is the one moment the gap opens, and it already arrives as a PR
+that needs merging by hand. Making the matrix entry part of accepting it costs
+one line and turns the new version's CI run into the check for whether the
+pin should move at all. A project whose dependencies are not ready finds out
+before the pin lands rather than after.
+
+## Trade-offs accepted
+
+- Merging a Python bump is no longer a single click; each project's bump PR
+  needs the matrix line added by hand.
+- One more matrix job per pull request for as long as the old floor is kept.
+- uv can lag a CPython release: at 3.15.0 its managed downloads offered only
+  3.15.0rc3, so CI tested the release candidate while a Mac with Homebrew's
+  3.15.0 tested the final. Accepted as temporary; it clears when uv ships the
+  final build.
+
+## Supersedes
+
+Nothing.