diff --git a/.github/scripts/check-wheel-python-matrix.sh b/.github/scripts/check-wheel-python-matrix.sh
new file mode 100755
index 00000000..112e85d5
--- /dev/null
+++ b/.github/scripts/check-wheel-python-matrix.sh
@@ -0,0 +1,144 @@
+#!/usr/bin/env bash
+#
+# Drift guard: fail when the release wheel-build interpreter matrix diverges
+# from the Python support pyproject.toml declares.
+#
+# AAASM-4446 shipped rc3 with cp312-only wheels while pyproject claimed
+# 3.12/3.13/3.14, so a 3.13 install silently fell back to a native-core-less
+# sdist and governance registration became a no-op. The root cause was a
+# hand-maintained build matrix drifting from `requires-python` + the Python
+# classifiers with nothing checking they agree. This script is that check.
+#
+# It compares three facts and fails (non-zero) on any mismatch:
+#   1. The "Programming Language :: Python :: 3.x" classifiers in pyproject.
+#   2. The PYTHON_INTERPRETERS list in release-python.yml (the versions every
+#      wheel job actually builds for — see that file's single-source comment).
+#   3. The `requires-python` lower bound in pyproject.
+# The built set must equal the classifier set, and the requires-python floor
+# must equal the lowest built version. It also fails if any wheel job hardcodes
+# a literal `--interpreter <version>` list, bypassing the single source.
+#
+# Run locally:
+#   bash .github/scripts/check-wheel-python-matrix.sh
+#
+# Run from CI: see .github/workflows/wheel-python-matrix.yml (runs on every PR
+# that touches pyproject.toml or the release workflow, not only at release).
+#
+# Usage (CLI, paths optional — default to the repo's files; overridable so the
+# fixture suite can point at temp copies):
+#   .github/scripts/check-wheel-python-matrix.sh [pyproject.toml] [release-python.yml]
+#
+# Owner: python-sdk release pipeline (AAASM-4453).
+
+# Print each "3.<minor>" declared by a Python classifier, numerically sorted,
+# deduped. The bare ":: 3" classifier (no minor) is intentionally excluded.
+extract_classifier_versions() {
+    local pyproject="$1"
+    grep -oE 'Programming Language :: Python :: 3\.[0-9]+' "$pyproject" \
+        | grep -oE '3\.[0-9]+' \
+        | sort -t. -k2,2n -u
+}
+
+# Print the lower-bound "3.<minor>" from `requires-python = ">=3.X,<4.0"`.
+extract_requires_python_floor() {
+    local pyproject="$1"
+    grep -E '^[[:space:]]*requires-python[[:space:]]*=' "$pyproject" \
+        | grep -oE '>=[[:space:]]*3\.[0-9]+' \
+        | grep -oE '3\.[0-9]+' \
+        | head -n1
+}
+
+# Print each version in the workflow's PYTHON_INTERPRETERS env value,
+# numerically sorted, deduped.
+extract_built_interpreters() {
+    local workflow="$1"
+    grep -E '^[[:space:]]*PYTHON_INTERPRETERS[[:space:]]*:' "$workflow" \
+        | head -n1 \
+        | grep -oE "'[^']*'" \
+        | tr -d "'" \
+        | tr ' ' '\n' \
+        | grep -E '^3\.[0-9]+$' \
+        | sort -t. -k2,2n -u
+}
+
+# check_wheel_python_matrix <pyproject_path> <workflow_path>
+# Returns 0 when the matrix matches declared support, non-zero otherwise.
+check_wheel_python_matrix() {
+    local pyproject="${1:-pyproject.toml}"
+    local workflow="${2:-.github/workflows/release-python.yml}"
+    local rc=0
+
+    if [[ ! -f "$pyproject" ]]; then
+        echo "::error::pyproject not found: $pyproject" >&2
+        return 1
+    fi
+    if [[ ! -f "$workflow" ]]; then
+        echo "::error::release workflow not found: $workflow" >&2
+        return 1
+    fi
+
+    local classifiers built floor
+    classifiers="$(extract_classifier_versions "$pyproject")"
+    built="$(extract_built_interpreters "$workflow")"
+    floor="$(extract_requires_python_floor "$pyproject")"
+
+    # A parse miss must fail loud, never pass vacuously — an empty extraction
+    # would otherwise make two empty strings compare equal.
+    if [[ -z "$classifiers" ]]; then
+        echo "::error::no 'Programming Language :: Python :: 3.x' classifiers found in $pyproject" >&2
+        return 1
+    fi
+    if [[ -z "$built" ]]; then
+        echo "::error::no PYTHON_INTERPRETERS list found in $workflow" >&2
+        return 1
+    fi
+    if [[ -z "$floor" ]]; then
+        echo "::error::could not parse requires-python lower bound in $pyproject" >&2
+        return 1
+    fi
+
+    # Core drift check: the versions wheels are built for must equal the
+    # versions pyproject advertises support for.
+    if [[ "$classifiers" != "$built" ]]; then
+        {
+            echo "::error::wheel build matrix diverges from pyproject Python classifiers"
+            printf 'pyproject classifiers : %s\n' "$(echo "$classifiers" | paste -sd' ' -)"
+            printf 'built (PYTHON_INTERPRETERS): %s\n' "$(echo "$built" | paste -sd' ' -)"
+        } >&2
+        rc=1
+    fi
+
+    # A requires-python floor bump without dropping the matching classifier +
+    # wheel (or vice versa) is itself a support-claim drift — catch it.
+    local min_built
+    min_built="$(echo "$built" | head -n1)"
+    if [[ "$floor" != "$min_built" ]]; then
+        echo "::error::requires-python floor (>=$floor) != lowest built interpreter ($min_built)" >&2
+        rc=1
+    fi
+
+    # No wheel job may hardcode a literal `--interpreter <version>` list: that
+    # would let one platform silently drift from the single source of truth,
+    # which is exactly the failure mode this guard exists to prevent.
+    local literal
+    literal="$(grep -nE -- '--interpreter[[:space:]]+[0-9]' "$workflow" || true)"
+    if [[ -n "$literal" ]]; then
+        {
+            echo "::error::a wheel job hardcodes --interpreter versions instead of \${{ env.PYTHON_INTERPRETERS }}:"
+            printf '%s\n' "$literal"
+        } >&2
+        rc=1
+    fi
+
+    if [[ "$rc" -eq 0 ]]; then
+        echo "OK: wheel build matrix matches pyproject Python support ($(echo "$built" | paste -sd' ' -)); requires-python floor >=$floor"
+    fi
+    return "$rc"
+}
+
+# When executed directly, run the check as a CLI. When sourced, define the
+# functions only and let the caller (the fixture suite) drive them.
+if [[ "${BASH_SOURCE[0]}" == "${0}" ]]; then
+    set -uo pipefail
+    check_wheel_python_matrix "${1:-}" "${2:-}"
+fi
diff --git a/.github/scripts/test_check-wheel-python-matrix.sh b/.github/scripts/test_check-wheel-python-matrix.sh
new file mode 100755
index 00000000..d3973f9a
--- /dev/null
+++ b/.github/scripts/test_check-wheel-python-matrix.sh
@@ -0,0 +1,141 @@
+#!/usr/bin/env bash
+#
+# Unit tests for `.github/scripts/check-wheel-python-matrix.sh`.
+#
+# Drives the drift guard that keeps the release wheel matrix in sync with the
+# Python support pyproject.toml declares (AAASM-4446 / AAASM-4453). Sources the
+# same file CI runs, then exercises it against synthetic pyproject/workflow
+# fixtures for both the passing and the drifting cases — plus one run against
+# the repo's real files to prove the shipped config actually passes.
+#
+# Run locally:
+#   bash .github/scripts/test_check-wheel-python-matrix.sh
+#
+# Run from CI: see .github/workflows/wheel-python-matrix.yml.
+#
+# Exits 0 when every case passes, 1 otherwise.
+#
+# Refs AAASM-4453.
+
+set -uo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+REPO_ROOT="$(cd "${SCRIPT_DIR}/../.." && pwd)"
+# shellcheck source=.github/scripts/check-wheel-python-matrix.sh
+source "${SCRIPT_DIR}/check-wheel-python-matrix.sh"
+
+# Literal GitHub Actions env reference — kept in a single-quoted variable so
+# bash never tries to expand `${{ ... }}`. The non-expansion is the point here.
+# shellcheck disable=SC2016
+ENVREF='${{ env.PYTHON_INTERPRETERS }}'
+
+WORKDIR="$(mktemp -d)"
+trap 'rm -rf "$WORKDIR"' EXIT
+
+pass=0
+fail=0
+total=0
+
+# assert_exit <label> <expected_rc> <actual_rc>
+assert_exit() {
+    local label="$1" expected="$2" actual="$3"
+    total=$((total + 1))
+    if [[ "$expected" == "$actual" ]]; then
+        printf 'OK    %-52s rc=%s\n' "$label" "$actual"
+        pass=$((pass + 1))
+    else
+        printf 'FAIL  %-52s expected rc=%s actual rc=%s\n' "$label" "$expected" "$actual" >&2
+        fail=$((fail + 1))
+    fi
+}
+
+# write_pyproject <path> <requires-python> <classifier-minor>...
+write_pyproject() {
+    local path="$1" requires="$2"; shift 2
+    {
+        echo '[project]'
+        echo "requires-python = \"$requires\""
+        echo 'classifiers = ['
+        echo '    "Programming Language :: Python :: 3",'
+        local v
+        for v in "$@"; do
+            echo "    \"Programming Language :: Python :: $v\","
+        done
+        echo ']'
+    } > "$path"
+}
+
+# write_workflow <path> <PYTHON_INTERPRETERS value> [interpreter-arg]
+# interpreter-arg defaults to the env reference (the correct, non-drifting form).
+write_workflow() {
+    local path="$1" interpreters="$2" arg="${3:-$ENVREF}"
+    {
+        echo 'env:'
+        echo "  PYTHON_INTERPRETERS: '$interpreters'"
+        echo 'jobs:'
+        echo '  build-macos:'
+        echo '    steps:'
+        echo "      - run: maturin build --release --interpreter $arg"
+    } > "$path"
+}
+
+# run_guard <pyproject> <workflow> -> echoes the guard's exit code
+run_guard() {
+    check_wheel_python_matrix "$1" "$2" >/dev/null 2>&1
+    echo "$?"
+}
+
+echo "== real repo files (shipped config must pass) =="
+assert_exit "real pyproject.toml + release-python.yml" 0 \
+    "$(run_guard "${REPO_ROOT}/pyproject.toml" "${REPO_ROOT}/.github/workflows/release-python.yml")"
+
+echo "== synthetic: matching matrix passes =="
+write_pyproject "${WORKDIR}/ok.toml" ">=3.12,<4.0" 3.12 3.13 3.14
+write_workflow  "${WORKDIR}/ok.yml"  "3.12 3.13 3.14"
+assert_exit "classifiers == built, floor == min" 0 \
+    "$(run_guard "${WORKDIR}/ok.toml" "${WORKDIR}/ok.yml")"
+
+echo "== synthetic: drift is caught =="
+# Classifier claims a version the matrix does not build (the AAASM-4446 shape).
+write_pyproject "${WORKDIR}/extra_classifier.toml" ">=3.12,<4.0" 3.12 3.13 3.14
+write_workflow  "${WORKDIR}/short_matrix.yml" "3.12"
+assert_exit "classifiers superset of built (cp313/cp314 unbuilt)" 1 \
+    "$(run_guard "${WORKDIR}/extra_classifier.toml" "${WORKDIR}/short_matrix.yml")"
+
+# Matrix builds a version pyproject does not advertise.
+write_pyproject "${WORKDIR}/narrow_classifier.toml" ">=3.12,<4.0" 3.12 3.13
+write_workflow  "${WORKDIR}/wide_matrix.yml" "3.12 3.13 3.14"
+assert_exit "built superset of classifiers" 1 \
+    "$(run_guard "${WORKDIR}/narrow_classifier.toml" "${WORKDIR}/wide_matrix.yml")"
+
+echo "== synthetic: requires-python floor drift is caught =="
+# Floor says >=3.13 but 3.12 is still built + classified.
+write_pyproject "${WORKDIR}/floor.toml" ">=3.13,<4.0" 3.12 3.13
+write_workflow  "${WORKDIR}/floor.yml"  "3.12 3.13"
+assert_exit "requires-python floor > lowest built version" 1 \
+    "$(run_guard "${WORKDIR}/floor.toml" "${WORKDIR}/floor.yml")"
+
+echo "== synthetic: hardcoded --interpreter literal is caught =="
+write_pyproject "${WORKDIR}/lit.toml" ">=3.12,<4.0" 3.12 3.13 3.14
+write_workflow  "${WORKDIR}/lit.yml"  "3.12 3.13 3.14" "3.12 3.13 3.14"
+assert_exit "job hardcodes --interpreter versions" 1 \
+    "$(run_guard "${WORKDIR}/lit.toml" "${WORKDIR}/lit.yml")"
+
+echo "== synthetic: unparseable inputs fail loud, not vacuously pass =="
+# pyproject with only the bare ":: 3" classifier (no minor versions).
+write_pyproject "${WORKDIR}/noclass.toml" ">=3.12,<4.0"
+write_workflow  "${WORKDIR}/noclass.yml"  "3.12"
+assert_exit "no Python minor classifiers" 1 \
+    "$(run_guard "${WORKDIR}/noclass.toml" "${WORKDIR}/noclass.yml")"
+
+# workflow with no PYTHON_INTERPRETERS env at all.
+write_pyproject "${WORKDIR}/nolist.toml" ">=3.12,<4.0" 3.12
+printf 'env:\n  OTHER: x\n' > "${WORKDIR}/nolist.yml"
+assert_exit "no PYTHON_INTERPRETERS in workflow" 1 \
+    "$(run_guard "${WORKDIR}/nolist.toml" "${WORKDIR}/nolist.yml")"
+
+echo
+echo "Summary: ${pass} passed, ${fail} failed, ${total} total"
+if (( fail > 0 )); then
+    exit 1
+fi
diff --git a/.github/workflows/release-python.yml b/.github/workflows/release-python.yml
index ec4d5502..a6df4159 100644
--- a/.github/workflows/release-python.yml
+++ b/.github/workflows/release-python.yml
@@ -42,7 +42,18 @@ env:
   # before each maturin build so it lands at agent_assembly/bin/aasm
   # inside the wheel (matches runtime.py's WHEEL_BUNDLED_BIN search path).
   AASM_BINARY_RELEASE_REPO: ai-agent-assembly/agent-assembly
+  # Host/tooling interpreter for jobs that only need *a* Python (sbom, sdist,
+  # the sidecar-staging steps) — not the set wheels are built for.
   PYTHON_VERSION: '3.12'
+  # SINGLE SOURCE OF TRUTH for the CPython versions the release builds wheels
+  # for. Every platform wheel job feeds this to maturin's --interpreter; the
+  # macOS jobs also install exactly these via setup-python. Adding/removing a
+  # supported version is a one-line edit here — and the wheel-python-matrix
+  # drift guard (`.github/workflows/wheel-python-matrix.yml`) fails any PR where
+  # this list diverges from pyproject.toml's requires-python + Python
+  # classifiers, so this can never silently lag declared support again
+  # (AAASM-4446 / AAASM-4453). Space-separated: maturin accepts each X.Y tag.
+  PYTHON_INTERPRETERS: '3.12 3.13 3.14'
   # protoc binary version + per-arch SHA256 sums (cross-verified against
   # the GitHub release API's `digest:` field on the v32.1 release assets).
   # Bump in one place when upgrading protoc.
@@ -278,7 +289,7 @@ jobs:
           # silently falls back to the native-core-less hatchling sdist
           # (AAASM-4446). The manylinux image ships all CPythons; maturin resolves
           # each X.Y tag to the matching /opt/python interpreter in-image.
-          args: --release --out dist --interpreter 3.12 3.13 3.14
+          args: --release --out dist --interpreter ${{ env.PYTHON_INTERPRETERS }}
           manylinux: auto
           # The manylinux2014 image (CentOS 7-based) lacks protoc; aa-proto's
           # build.rs needs it via prost-build for proto3 syntax. The yum/dnf
@@ -373,7 +384,7 @@ jobs:
           # See linux-x86_64 above: build one wheel per supported CPython so no
           # supported interpreter falls back to the native-core-less sdist
           # (AAASM-4446). manylinux ships all CPythons; maturin resolves each tag.
-          args: --release --out dist --interpreter 3.12 3.13 3.14
+          args: --release --out dist --interpreter ${{ env.PYTHON_INTERPRETERS }}
           manylinux: auto
           # See linux-x86_64 above for rationale + security model. Same
           # SHA-verified protoc binary download.
@@ -405,17 +416,25 @@ jobs:
     runs-on: macos-14  # Apple silicon runner
     steps:
       - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0  # v7
+      - name: Expand interpreter list for setup-python
+        id: pythons
+        # macOS runners have no manylinux image bundling every CPython, so each
+        # interpreter the Build wheel step targets must be installed here first.
+        # setup-python wants a newline-separated list while PYTHON_INTERPRETERS
+        # (the single source of truth) is space-separated for maturin — translate
+        # here so adding a version stays a one-line env edit (AAASM-4453).
+        env:
+          PYTHON_INTERPRETERS: ${{ env.PYTHON_INTERPRETERS }}
+        run: |
+          set -euo pipefail
+          {
+            echo 'versions<<__EOF__'
+            printf '%s' "$PYTHON_INTERPRETERS" | tr ' ' '\n'
+            printf '\n__EOF__\n'
+          } >> "$GITHUB_OUTPUT"
       - uses: actions/setup-python@ece7cb06caefa5fff74198d8649806c4678c61a1  # v6
         with:
-          # macOS runners have no manylinux image bundling every CPython, so
-          # each interpreter the Build wheel step targets below must be installed
-          # here first — otherwise maturin finds only the runner's default python
-          # and the wheel ships for that single version, the AAASM-4446 gap.
-          # Keep this list in lockstep with the --interpreter list below.
-          python-version: |
-            3.12
-            3.13
-            3.14
+          python-version: ${{ steps.pythons.outputs.versions }}
       - name: Stage aasm sidecar binary
         env:
           GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
@@ -471,7 +490,7 @@ jobs:
           # supported interpreter falls back to the native-core-less sdist
           # (AAASM-4446). Each X.Y here is provided by the multi-version
           # setup-python step above (macOS has no manylinux image).
-          args: --release --out dist --interpreter 3.12 3.13 3.14
+          args: --release --out dist --interpreter ${{ env.PYTHON_INTERPRETERS }}
       - name: Upload wheel artifact
         uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a  # v7
         with:
@@ -484,17 +503,25 @@ jobs:
     runs-on: macos-15-intel  # Intel runner (macos-13 sunset 2025-09-19)
     steps:
       - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0  # v7
+      - name: Expand interpreter list for setup-python
+        id: pythons
+        # macOS runners have no manylinux image bundling every CPython, so each
+        # interpreter the Build wheel step targets must be installed here first.
+        # setup-python wants a newline-separated list while PYTHON_INTERPRETERS
+        # (the single source of truth) is space-separated for maturin — translate
+        # here so adding a version stays a one-line env edit (AAASM-4453).
+        env:
+          PYTHON_INTERPRETERS: ${{ env.PYTHON_INTERPRETERS }}
+        run: |
+          set -euo pipefail
+          {
+            echo 'versions<<__EOF__'
+            printf '%s' "$PYTHON_INTERPRETERS" | tr ' ' '\n'
+            printf '\n__EOF__\n'
+          } >> "$GITHUB_OUTPUT"
       - uses: actions/setup-python@ece7cb06caefa5fff74198d8649806c4678c61a1  # v6
         with:
-          # macOS runners have no manylinux image bundling every CPython, so
-          # each interpreter the Build wheel step targets below must be installed
-          # here first — otherwise maturin finds only the runner's default python
-          # and the wheel ships for that single version, the AAASM-4446 gap.
-          # Keep this list in lockstep with the --interpreter list below.
-          python-version: |
-            3.12
-            3.13
-            3.14
+          python-version: ${{ steps.pythons.outputs.versions }}
       - name: Stage aasm sidecar binary
         env:
           GH_TOKEN: ${{ secrets.GITHUB_TOKEN }}
@@ -550,7 +577,7 @@ jobs:
           # supported interpreter falls back to the native-core-less sdist
           # (AAASM-4446). Each X.Y here is provided by the multi-version
           # setup-python step above (macOS has no manylinux image).
-          args: --release --out dist --interpreter 3.12 3.13 3.14
+          args: --release --out dist --interpreter ${{ env.PYTHON_INTERPRETERS }}
       - name: Upload wheel artifact
         uses: actions/upload-artifact@043fb46d1a93c77aae656e7c1c64a875d1fc6a0a  # v7
         with:
diff --git a/.github/workflows/wheel-python-matrix.yml b/.github/workflows/wheel-python-matrix.yml
new file mode 100644
index 00000000..6b69edea
--- /dev/null
+++ b/.github/workflows/wheel-python-matrix.yml
@@ -0,0 +1,55 @@
+name: Wheel Python matrix drift guard
+
+# Fail any PR where the release wheel-build interpreter matrix diverges from the
+# Python support pyproject.toml declares. AAASM-4446 shipped cp312-only wheels
+# while pyproject claimed 3.13/3.14, so a 3.13 install silently fell back to a
+# native-core-less sdist. This runs the guard on EVERY PR that touches either
+# side of that contract (not only at release), so the drift is caught in review
+# rather than after publish.
+#
+# Triggers on the inputs the guard reads plus its own script/test/workflow, so a
+# change to any of them re-runs the check.
+#
+# Refs AAASM-4446 / AAASM-4453.
+
+on:
+  push:
+    branches:
+      - master
+    paths:
+      - "pyproject.toml"
+      - ".github/workflows/release-python.yml"
+      - ".github/scripts/check-wheel-python-matrix.sh"
+      - ".github/scripts/test_check-wheel-python-matrix.sh"
+      - ".github/workflows/wheel-python-matrix.yml"
+  pull_request:
+    branches:
+      - master
+    paths:
+      - "pyproject.toml"
+      - ".github/workflows/release-python.yml"
+      - ".github/scripts/check-wheel-python-matrix.sh"
+      - ".github/scripts/test_check-wheel-python-matrix.sh"
+      - ".github/workflows/wheel-python-matrix.yml"
+
+permissions:
+  contents: read
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  wheel-python-matrix:
+    name: Check wheel matrix matches declared Python support
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0  # v7
+      - name: Lint guard + test scripts
+        run: |
+          shellcheck .github/scripts/check-wheel-python-matrix.sh
+          shellcheck -x .github/scripts/test_check-wheel-python-matrix.sh
+      - name: Assert wheel matrix matches pyproject Python support
+        run: bash .github/scripts/check-wheel-python-matrix.sh
+      - name: Run drift-guard fixture suite
+        run: bash .github/scripts/test_check-wheel-python-matrix.sh