diff --git a/test/fixtures/tracker_id_baseline.txt b/test/fixtures/tracker_id_baseline.txt
new file mode 100644
index 0000000..3bf4dd5
--- /dev/null
+++ b/test/fixtures/tracker_id_baseline.txt
@@ -0,0 +1,52 @@
+# Tracker ids already present in lib/.
+# Read by the "no lib/ file gains a tracker id" test in
+# test/statifier_examples/tracker_id_test.exs, which defines the id shape.
+# One line per file and id: the file, the id, and how many times the file
+# carries it, tab-separated. A new id is refused, not added here; a line is
+# lowered or deleted when an id leaves its file.
+lib/statifier_examples/application.ex	se-1cl	1
+lib/statifier_examples/card_auth.ex	se-0u1	1
+lib/statifier_examples/card_auth.ex	se-2ox	1
+lib/statifier_examples/card_auth.ex	se-bv9	1
+lib/statifier_examples/card_auth.ex	se-ihm	1
+lib/statifier_examples/card_auth/legacy_check.ex	se-bv9	1
+lib/statifier_examples/charts.ex	se-06z	2
+lib/statifier_examples/charts.ex	se-4dt	3
+lib/statifier_examples/charts.ex	se-6ag	1
+lib/statifier_examples/charts/async_calls.ex	se-d74	1
+lib/statifier_examples/charts/durable.ex	se-29d	1
+lib/statifier_examples/charts/durable.ex	se-4dt	3
+lib/statifier_examples/charts/durable.ex	se-6ag	2
+lib/statifier_examples/charts/durable.ex	se-cqr	1
+lib/statifier_examples/charts/durable.ex	se-j87	1
+lib/statifier_examples/charts/durable.ex	se-w4i	2
+lib/statifier_examples/charts/durable.ex	sp-2yx	1
+lib/statifier_examples/charts/execution.ex	se-0ay	1
+lib/statifier_examples/charts/execution.ex	se-29d	1
+lib/statifier_examples/charts/execution.ex	se-b2f	1
+lib/statifier_examples/charts/fan_out.ex	sb-kha0	1
+lib/statifier_examples/charts/fan_out.ex	se-j87	1
+lib/statifier_examples/charts/fixture.ex	se-06z	1
+lib/statifier_examples/charts/step.ex	se-06z	1
+lib/statifier_examples/charts/subchart.ex	se-4dt	1
+lib/statifier_examples/charts/subchart.ex	se-6ag	1
+lib/statifier_examples/charts/subchart.ex	st-pvpz	1
+lib/statifier_examples/charts/sync_adapter.ex	se-4dt	2
+lib/statifier_examples/documents.ex	se-1cl	1
+lib/statifier_examples/persistence.ex	se-1tro	1
+lib/statifier_examples/persistence.ex	se-6ag	1
+lib/statifier_examples/persistence.ex	se-j87	1
+lib/statifier_examples/persistence.ex	sp-11w	1
+lib/statifier_examples/signup.ex	st-pvpz	1
+lib/statifier_examples/signup/handlers.ex	se-dyo	1
+lib/statifier_examples/signup/invites.ex	se-j87	1
+lib/statifier_examples/signup/journey.ex	se-19h	1
+lib/statifier_examples/signup/journey.ex	se-e68	2
+lib/statifier_examples/signup/journey.ex	se-w4i	1
+lib/statifier_examples/signup/promotion.ex	se-j87	1
+lib/statifier_examples/signup/screens.ex	se-19h	1
+lib/statifier_examples/signup/screens.ex	se-e68	1
+lib/statifier_examples/signup/signup_step.ex	se-1mc	1
+lib/statifier_examples_web/components/signup_elements.ex	se-19h	1
+lib/statifier_examples_web/live/editor_live.ex	se-1cl	1
+lib/statifier_examples_web/live/signup_screens_live.ex	se-19h	2
diff --git a/test/statifier_examples/tracker_id_test.exs b/test/statifier_examples/tracker_id_test.exs
new file mode 100644
index 0000000..c9d7182
--- /dev/null
+++ b/test/statifier_examples/tracker_id_test.exs
@@ -0,0 +1,150 @@
+defmodule StatifierExamples.TrackerIdTest do
+  @moduledoc """
+  No file under `lib/` gains a tracker id.
+
+  A tracker id names an issue in one of the family's trackers: a repository
+  prefix, a dash, and three or four base36 characters (`se-` then `06z`,
+  say). In a moduledoc or a comment it points a reader at an issue instead
+  of saying what the code does and why, and the issue text is not shipped
+  with the code. Write the substance instead.
+
+  ## Mechanism: a committed baseline
+
+  `lib/` already carries such ids. Each one is counted in the committed
+  baseline, `test/fixtures/tracker_id_baseline.txt`: per file, the id and
+  how many times the file carries it. A count above the baseline fails,
+  naming the file and the id; a count below it fails too, naming the
+  baseline line to lower or delete, so an id that leaves a file cannot come
+  back unseen. An existing occurrence that moves within its file changes no
+  count. Like `StatifierExamples.PrivateIdTest`, it needs no git history and
+  runs the same locally and in CI.
+
+  ## What it does not catch
+
+    * An id of a prefix missing from `@prefixes`: a tracker added to the
+      family later is enforced only once its prefix is listed there.
+    * An id that spells one of `@css_words`: those are the CSS class names
+      in `lib/` that share the shape (`sr-only`), excluded by name.
+    * An author who adds a baseline line beside a new id. The baseline's
+      own header says a new id is refused, not added there, and a reviewer
+      reads the diff.
+  """
+  use ExUnit.Case, async: true
+
+  # Every tracker prefix in the family, one per repository.
+  @prefixes ~w(ece enc eq ots pts px rd rs sb sd se sob sp sr st sui)
+
+  # CSS class names in lib/ with a tracker id's shape. A class name is an
+  # English word where an id has minted characters, so it is excluded by
+  # name; everything else of the shape is an id.
+  @css_words ~w(only)
+
+  @tracker_id Regex.compile!(
+                "\\b(?:" <>
+                  Enum.join(@prefixes, "|") <>
+                  ")-(?!(?:" <> Enum.join(@css_words, "|") <> ")\\b)[a-z0-9]{3,4}\\b"
+              )
+
+  @baseline "test/fixtures/tracker_id_baseline.txt"
+
+  defp repo_path(relative), do: __DIR__ |> Path.join("../../#{relative}") |> Path.expand()
+
+  # Every file under lib/, globbed at run time rather than listed, so a file
+  # added later is enforced the moment it exists.
+  defp lib_files do
+    root = repo_path("")
+
+    root
+    |> Path.join("lib/**/*")
+    |> Path.wildcard()
+    |> Enum.filter(&File.regular?/1)
+    |> Enum.map(&Path.relative_to(&1, root))
+    |> Enum.sort()
+  end
+
+  defp tracker_ids(text), do: @tracker_id |> Regex.scan(text) |> Enum.map(&hd/1)
+
+  defp tracker_id_counts(files) do
+    for file <- files,
+        {id, count} <-
+          file |> repo_path() |> File.read!() |> tracker_ids() |> Enum.frequencies(),
+        into: %{},
+        do: {{file, id}, count}
+  end
+
+  defp read_baseline do
+    @baseline
+    |> repo_path()
+    |> File.read!()
+    |> String.split("\n", trim: true)
+    |> Enum.reject(&String.starts_with?(&1, "#"))
+    |> Map.new(fn line ->
+      [file, id, count] = String.split(line, "\t")
+      {{file, id}, String.to_integer(count)}
+    end)
+  end
+
+  # Every invented id below is assembled from two halves at run time, so
+  # this file's own source carries none of them.
+  defp invented(prefix, rest), do: prefix <> rest
+
+  # sabotage: drop "only" from @css_words -> red on the `sr-only` negative;
+  # shorten the id shape to three characters -> red on a four-character
+  # positive (both run 2026-09-30).
+  test "the tracker-id pattern matches each prefix's id shape and no class name" do
+    positives =
+      for prefix <- @prefixes, rest <- ["zz9", "zz9q"] do
+        id = invented(prefix <> "-", rest)
+        {"filed as #{id} last week", id}
+      end ++
+        [
+          {"(#{invented("se-", "zz9")}.2)", invented("se-", "zz9")},
+          {"Bead `#{invented("sb-", "zz9q")}` builds it", invented("sb-", "zz9q")},
+          {"# is a second one (#{invented("sp-", "zz9")}).", invented("sp-", "zz9")}
+        ]
+
+    for {text, id} <- positives, do: assert(tracker_ids(text) == [id], text)
+
+    negatives = [
+      ~s(<span class="sr-only">Actions</span>),
+      ~s(<div class="sb-editor myapp-plan">),
+      "`--sb-block-accent` and `--sb-block-accent-tint`",
+      ~s(@accent_token "--sb-accent-myapp"),
+      "the first-run path",
+      "a post-1998 engine",
+      "released as v0.32.0",
+      "ADR-0005 part (iii)"
+    ]
+
+    for text <- negatives, do: assert(tracker_ids(text) == [], text)
+  end
+
+  # sabotage: plant an invented tracker id in charts/fixture.ex's moduledoc
+  # -> red, naming the file and the id; add a second copy of an id
+  # documents.ex already carries -> red ("2 now, 1 before"); delete the id
+  # from charts/fan_out.ex's moduledoc -> red on the second assertion
+  # (all three run 2026-09-30).
+  test "no lib/ file gains a tracker id" do
+    baseline = read_baseline()
+    current = tracker_id_counts(lib_files())
+
+    added =
+      for {{file, id} = key, count} <- current,
+          count > Map.get(baseline, key, 0),
+          do: "#{file}: #{id} (#{count} now, #{Map.get(baseline, key, 0)} before)"
+
+    gone =
+      for {{file, id} = key, before} <- baseline,
+          Map.get(current, key, 0) < before,
+          do: "#{file}\t#{id}\t#{before}"
+
+    assert added == [],
+           "new tracker ids in lib/ - write the substance instead:\n" <>
+             Enum.join(Enum.sort(added), "\n")
+
+    assert gone == [],
+           "ids removed since #{@baseline} was written - lower or delete " <>
+             "these lines there so the id cannot come back unseen:\n" <>
+             Enum.join(Enum.sort(gone), "\n")
+  end
+end