diff --git a/scripts/artifacts.sh b/scripts/artifacts.sh
new file mode 100755
index 0000000..948706a
--- /dev/null
+++ b/scripts/artifacts.sh
@@ -0,0 +1,150 @@
+#!/bin/sh
+# Single definition of WHAT EACH PUBLISHED ARTIFACT IS BUILT FROM, and the
+# version derived from it. Milestone 313; generalises the shape
+# extension/scripts/packaging.sh established for the extension alone.
+#
+# Four artifacts, four independent versions. An artifact whose shipped files
+# did not change keeps its version and does not rebuild — that is the whole
+# point, and it is why each path set must match its Dockerfile rather than
+# being a plausible guess. Getting a set wrong is quiet in BOTH directions:
+#
+#   too narrow -> a pin serves stale bytes, because the version did not move
+#                 when the content did. This is the dangerous one.
+#   too wide   -> the artifact re-versions and rebuilds for a change it does
+#                 not ship. Merely wasteful.
+#
+# tests/test_artifact_paths.py asserts every COPY source in each Dockerfile is
+# covered here, so adding a COPY without updating this file fails CI.
+#
+# POSIX sh only — CI's run shell is busybox on some paths.
+#
+# -f (no pathname expansion) is load-bearing for the whole script: the lists
+# below are iterated with deliberate word-splitting, and without it the shell
+# would glob `frontend/test/**` against the working tree and silently narrow
+# the pattern. Callers substituting the output need their own `set -f` too;
+# the two guards protect different expansions.
+set -euf
+
+ROOT=$(git rev-parse --show-toplevel)
+
+# --- what each artifact ships ------------------------------------------------
+#
+# Each set includes its own Dockerfile and requirements: changing a base image
+# or a pin changes the artifact just as surely as changing a source file.
+#
+# web (Dockerfile, context `.`) — the runtime stage copies backend/, alembic/,
+# alembic.ini, entrypoint.sh and requirements.txt; the frontend-builder stage
+# copies frontend/ and the runtime takes its `dist` output.
+#
+# frontend/test is excluded: `npm run build` is vite, which builds from src/,
+# index.html and public/ and never reads test/. It lands in the builder layer
+# but not in `dist`, so it cannot reach the shipped image.
+#
+# The web image ALSO bundles the signed XPI (build.yml downloads it into
+# frontend/public/extension/ before the docker build), so an extension change
+# changes the web image. The extension's packaged set is appended in cmd_paths
+# rather than restated — one definition, per #2397.
+WEB_PATHS='Dockerfile requirements.txt backend alembic alembic.ini entrypoint.sh frontend :(exclude)frontend/test :(exclude)frontend/test/**'
+
+# ml (Dockerfile.ml, context `.`) — no frontend, no extension. Note it copies
+# BOTH requirements-ml.txt and requirements.txt.
+ML_PATHS='Dockerfile.ml requirements-ml.txt requirements.txt backend alembic alembic.ini entrypoint.sh'
+
+# agent (agent/Dockerfile, context `agent`) — copies requirements.txt and
+# fc_agent only. agent/README.md, agent/docker-compose.yml and agent/ruff.toml
+# live in the directory but never reach the image, so they must not re-version
+# it: this is deliberately NOT `agent/`.
+AGENT_PATHS='agent/Dockerfile agent/requirements.txt agent/fc_agent'
+
+usage() {
+  echo "usage: artifacts.sh {paths|revision|version|tag} {web|ml|agent|extension}" >&2
+  exit 2
+}
+
+# The extension's packaged set, read from its own definition rather than
+# copied. packaging.sh emits `:(exclude)extension/...` entries, so the bare
+# `extension` include has to come with them.
+ext_paths() {
+  echo "extension $(sh "$ROOT/extension/scripts/packaging.sh" pathspec)"
+}
+
+cmd_paths() {
+  case "$1" in
+    web)       echo "$WEB_PATHS $(ext_paths)" ;;
+    ml)        echo "$ML_PATHS" ;;
+    agent)     echo "$AGENT_PATHS" ;;
+    extension) ext_paths ;;
+    *)         usage ;;
+  esac
+}
+
+# "<unix ts> <sha>" of the newest commit touching this artifact's shipped set.
+# Unquoted on purpose: the pathspec must word-split into separate args.
+# Globbing is already off script-wide.
+newest() {
+  # shellcheck disable=SC2046
+  set -- "$(cd "$ROOT" && git log --format='%ct %H' HEAD -- $(cmd_paths "$1") \
+            | sort -n | tail -1)"
+  if [ -z "$1" ]; then
+    echo "artifacts.sh: no commit touches this artifact's shipped files" >&2
+    exit 1
+  fi
+  echo "$1"
+}
+
+# Formatted through git rather than date(1): busybox date does not reliably
+# accept `-d @<epoch>`, and git's own --date=format-local is available wherever
+# git is. TZ=UTC so the value does not depend on the runner's timezone.
+fmt() {
+  (cd "$ROOT" && TZ=UTC git show -s --format=%cd --date="format-local:$2" "$1")
+}
+
+# Leading zeros stripped so every segment is a plain integer — some version
+# validators reject `08`, and a leading zero buys nothing. `0000` (midnight)
+# must survive as `0`, not as the empty string.
+strip0() {
+  printf '%s' "$1" | sed -e 's/^0*//' -e 's/^$/0/'
+}
+
+# The IDENTITY of an artifact's content: the commit its shipped files last
+# changed in. This — not the tag — is what decides whether a build can be
+# skipped, because the published tag is only day-precise and two different
+# builds can share it.
+cmd_revision() {
+  echo "$(newest "$1")" | cut -d' ' -f2 | cut -c1-12
+}
+
+# The ORDERING KEY: full precision, YYYY.M.D.HHMM. Used by the extension,
+# where the value is what Firefox compares to decide whether an update exists
+# — two same-day builds MUST be distinguishable or the second never reaches
+# anyone.
+cmd_version() {
+  sha=$(echo "$(newest "$1")" | cut -d' ' -f2)
+  printf '%s.%s.%s.%s\n' \
+    "$(fmt "$sha" %Y)" \
+    "$(strip0 "$(fmt "$sha" %m)")" \
+    "$(strip0 "$(fmt "$sha" %d)")" \
+    "$(strip0 "$(fmt "$sha" %H%M)")"
+}
+
+# The PUBLISHED IMAGE TAG: day precision, YYYY.M.D. Deliberately coarser than
+# the ordering key, per the operator 2026-08-28 — same-day work is not
+# something worth pinning, so a second build the same day replaces the first
+# rather than accumulating a tag nobody would roll back to. Safe only because
+# skip decisions key on cmd_revision, never on this.
+cmd_tag() {
+  sha=$(echo "$(newest "$1")" | cut -d' ' -f2)
+  printf '%s.%s.%s\n' \
+    "$(fmt "$sha" %Y)" \
+    "$(strip0 "$(fmt "$sha" %m)")" \
+    "$(strip0 "$(fmt "$sha" %d)")"
+}
+
+[ $# -ge 2 ] || usage
+case "$1" in
+  paths)    cmd_paths "$2" ;;
+  revision) cmd_revision "$2" ;;
+  version)  cmd_version "$2" ;;
+  tag)      cmd_tag "$2" ;;
+  *)        usage ;;
+esac
diff --git a/tests/test_artifact_paths.py b/tests/test_artifact_paths.py
new file mode 100644
index 0000000..c226f5b
--- /dev/null
+++ b/tests/test_artifact_paths.py
@@ -0,0 +1,138 @@
+"""`scripts/artifacts.sh` path sets must match what the Dockerfiles copy.
+
+Each published artifact's version derives from the newest commit touching its
+own shipped file set (milestone 313). The whole scheme rests on those sets
+being right, and both ways of being wrong are silent:
+
+* **too narrow** — a file ships but is not in the set, so the version does not
+  move when the content does, and a pin serves stale bytes. This is the
+  dangerous direction and the one this module exists for.
+* **too wide** — a file is in the set but never reaches the image, so the
+  artifact re-versions and rebuilds for a change it does not ship.
+
+Nothing else notices either. The version still derives, CI still goes green,
+and the mismatch only surfaces as "I pinned that build and got the wrong
+bytes". So the Dockerfiles are read here and compared against the declaration.
+"""
+from __future__ import annotations
+
+import re
+import subprocess
+from pathlib import Path
+
+import pytest
+
+ROOT = Path(__file__).resolve().parent.parent
+
+# artifact -> (dockerfile, build context relative to the repo root)
+ARTIFACTS = {
+    "web": ("Dockerfile", ""),
+    "ml": ("Dockerfile.ml", ""),
+    "agent": ("agent/Dockerfile", "agent"),
+}
+
+# COPY --from=<stage> copies from an earlier build stage, not from the build
+# context, so its source is not a repo path and cannot be in a path set.
+_COPY = re.compile(r"^\s*COPY\s+(?!--from=)(?P<args>.+)$", re.MULTILINE)
+
+
+def declared_paths(artifact: str) -> list[str]:
+    out = subprocess.run(
+        ["sh", str(ROOT / "scripts" / "artifacts.sh"), "paths", artifact],
+        capture_output=True, text=True, check=True, cwd=ROOT,
+    ).stdout
+    return out.split()
+
+
+def includes(artifact: str) -> list[str]:
+    """The set minus its `:(exclude)…` entries."""
+    return [p for p in declared_paths(artifact) if not p.startswith(":(exclude)")]
+
+
+def copy_sources(dockerfile: str, context: str) -> list[str]:
+    """Repo-relative sources of every context COPY in a Dockerfile."""
+    text = (ROOT / dockerfile).read_text()
+    sources: list[str] = []
+    for m in _COPY.finditer(text):
+        args = m.group("args").split()
+        # Last arg is the destination; everything before it is a source.
+        for src in args[:-1]:
+            # `frontend/package-lock.json*` — the glob is an optional-file
+            # idiom; the directory it sits in is what matters for coverage.
+            src = src.rstrip("*")
+            sources.append(f"{context}/{src}" if context else src)
+    return sources
+
+
+def covered_by(path: str, include: str) -> bool:
+    """`path` ships if an include names it or one of its ancestors."""
+    path = path.rstrip("/").lstrip("./")
+    include = include.rstrip("/")
+    return path == include or path.startswith(include + "/")
+
+
+@pytest.mark.parametrize("artifact", sorted(ARTIFACTS))
+def test_every_copied_path_is_in_the_artifacts_path_set(artifact):
+    """The too-narrow direction — the one that serves stale bytes on a pin."""
+    dockerfile, context = ARTIFACTS[artifact]
+    inc = includes(artifact)
+    for src in copy_sources(dockerfile, context):
+        assert any(covered_by(src, i) for i in inc), (
+            f"{dockerfile} copies {src!r} into the {artifact} image, but no "
+            f"include in scripts/artifacts.sh covers it. The {artifact} "
+            f"version will not move when that file changes, so a pinned build "
+            f"will serve stale bytes. Add it to the path set.\n"
+            f"  declared includes: {inc}"
+        )
+
+
+@pytest.mark.parametrize("artifact", sorted(ARTIFACTS))
+def test_the_dockerfile_itself_is_in_the_path_set(artifact):
+    """Changing a base image or a RUN changes the artifact as surely as
+    changing a source file, so each set must include its own Dockerfile."""
+    dockerfile, _ = ARTIFACTS[artifact]
+    assert any(covered_by(dockerfile, i) for i in includes(artifact)), (
+        f"{dockerfile} is not in the {artifact} path set — a base-image bump "
+        f"would not move the version."
+    )
+
+
+def test_the_web_image_versions_on_an_extension_change():
+    """The web image bundles the signed XPI, so the extension's packaged files
+    are part of what it ships. Miss this and `:latest` serves a NEW extension
+    under an unchanged web version — a pin that quietly disagrees with itself.
+    """
+    inc = includes("web")
+    assert any(covered_by("extension/background/background.js", i) for i in inc), (
+        "the web path set does not cover the extension's packaged files, but "
+        "build.yml downloads the signed XPI into frontend/public/extension/ "
+        "before the docker build"
+    )
+
+
+@pytest.mark.parametrize(
+    "artifact, path",
+    [
+        # Deliberate exclusions — the too-wide direction. Each of these lives
+        # beside shipped code but never reaches an image, and including it
+        # would re-version the artifact for a change it does not carry.
+        ("agent", "agent/README.md"),
+        ("agent", "agent/ruff.toml"),
+        ("agent", "agent/docker-compose.yml"),
+        # vite builds from src/, index.html and public/; it never reads test/,
+        # so a frontend test change cannot reach `dist`.
+        ("web", "frontend/test/gallery.spec.js"),
+    ],
+)
+def test_files_that_never_reach_an_image_do_not_version_it(artifact, path):
+    paths = declared_paths(artifact)
+    excluded = [p[len(":(exclude)"):] for p in paths if p.startswith(":(exclude)")]
+    inc = [p for p in paths if not p.startswith(":(exclude)")]
+
+    included = any(covered_by(path, i) for i in inc)
+    exempted = any(covered_by(path, e.rstrip("*").rstrip("/")) for e in excluded)
+    assert not included or exempted, (
+        f"{path} is in the {artifact} path set but is not copied into the "
+        f"image — it would re-version and rebuild {artifact} for a change it "
+        f"does not ship."
+    )