diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index d550baa5..1e4d3ede 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -1,6 +1,10 @@
 # CI for Tack.
 #
-# Three jobs, all independent so they report in parallel:
+# The jobs report in parallel. On a pull request, `changes` first says which
+# its files need (tools/ci_changes.py): a change to the docs alone runs lint
+# and docs; one to an example, lint and examples; anything in the packages,
+# the build or the workflows, everything. A job left out reports "skipping".
+# Pushes to main and runs by hand run everything.
 #
 #   docs       — builds the site strictly; publishes main's as `dev`
 #   tests      — the suite on Python 3.11/3.12/3.13 (Linux, CPU backend)
@@ -39,6 +43,37 @@ concurrency:
 
 jobs:
 
+  # ── What changed ───────────────────────────────────────────────────
+  # The groups a pull request's files need, as outputs the jobs below test:
+  # `code` (every test job, packaging and Metal), `docs`, `examples`. A path
+  # no rule of tools/ci_changes.py names is code. Lint needs no group.
+  changes:
+    name: what changed
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    outputs:
+      code: ${{ steps.classify.outputs.code }}
+      docs: ${{ steps.classify.outputs.docs }}
+      examples: ${{ steps.classify.outputs.examples }}
+
+    steps:
+      - uses: actions/checkout@v7
+        with:
+          # A pull request checks out its merge commit, whose first parent
+          # is the base it merges into.
+          fetch-depth: 2
+
+      - name: Classify the changed files
+        id: classify
+        shell: bash
+        run: |
+          if [ "$GITHUB_EVENT_NAME" = pull_request ]; then
+            git diff --name-only HEAD^1 HEAD | python3 tools/ci_changes.py >> "$GITHUB_OUTPUT"
+          else
+            printf 'code=true\ndocs=true\nexamples=true\n' >> "$GITHUB_OUTPUT"
+          fi
+          cat "$GITHUB_OUTPUT"
+
   # ── Lint ───────────────────────────────────────────────────────────
   # Cheapest job here and the one most likely to catch something before a
   # human looks. It found a real bug on its first run: an f-string with a
@@ -91,6 +126,8 @@ jobs:
 
   docs:
     name: docs
+    needs: changes
+    if: needs.changes.outputs.docs == 'true' || needs.changes.outputs.code == 'true'
     runs-on: ubuntu-latest
     timeout-minutes: 20
 
@@ -113,6 +150,14 @@ jobs:
       - name: Build the site
         run: uv run mkdocs build --strict
 
+      # Package tests that read the pages: each algorithm and export has its
+      # entry, and a VTK wrapper. A change to the docs alone runs no test job.
+      - name: The reference pages name everything
+        run: |
+          uv run --with pytest pytest -q -p no:cacheprovider \
+            packages/tack-vis/tests/test_reference_pages.py \
+            packages/tack-vis/tests/test_vtk_algorithms.py::test_every_algorithm_has_a_wrapper
+
   # ── Publish ────────────────────────────────────────────────────────
   #
   # Separate from the gate above because deploying and checking are
@@ -165,6 +210,8 @@ jobs:
   # ── The main gate ──────────────────────────────────────────────────
   tests:
     name: tests (py${{ matrix.python-version }}, ${{ matrix.shard }}/4)
+    needs: changes
+    if: needs.changes.outputs.code == 'true'
     runs-on: ubuntu-latest
     # The suite in four shares (conftest's --shard, by test-durations.json), each
     # serial: whole, it took 25-35+ min by 2026-10-29 (35 cancelled #212's py3.11
@@ -242,6 +289,8 @@ jobs:
   # repeats the whole dataset suite, which would crowd the main gate.
   ids-i64:
     name: dataset tests with 64-bit ids (${{ matrix.shard }}/4)
+    needs: changes
+    if: needs.changes.outputs.code == 'true'
     runs-on: ubuntu-latest
     # In four shares (conftest's --shard): whole, 17.5-45 min by 2026-10-29 (30
     # cancelled #212's run); in three shares, 17-25+ min a share by 2026-11-02
@@ -282,6 +331,8 @@ jobs:
   # (the vault's coefficient-layout proposal, D7).
   l2-cells:
     name: dataset tests with L2 cell-fastest (${{ matrix.shard }}/2)
+    needs: changes
+    if: needs.changes.outputs.code == 'true'
     runs-on: ubuntu-latest
     # In two shares (conftest's --shard): whole, on four workers, it ran 13 min by
     # 2026-10-29, and #213's run (more compiled kernels a worker) lost its runner
@@ -324,6 +375,8 @@ jobs:
   # warp-simdata's int32 ids rather than sharing them.
   warp:
     name: Warp backend and interop
+    needs: changes
+    if: needs.changes.outputs.code == 'true'
     runs-on: ubuntu-latest
     timeout-minutes: 20
 
@@ -362,6 +415,8 @@ jobs:
   # warns (an annotation on the run) rather than failing the job.
   vtk-master:
     name: VTK master
+    needs: changes
+    if: needs.changes.outputs.code == 'true'
     runs-on: ubuntu-latest
     timeout-minutes: 25
 
@@ -409,6 +464,8 @@ jobs:
   # job rather than skip, and oversubscription lets 3 ranks run on any runner.
   mpi:
     name: mpi
+    needs: changes
+    if: needs.changes.outputs.code == 'true'
     runs-on: ubuntu-latest
     timeout-minutes: 30
     env:
@@ -467,6 +524,8 @@ jobs:
   # — so it needs its own job with a deliberately minimal install.
   packaging:
     name: GPU install needs no llvmlite
+    needs: changes
+    if: needs.changes.outputs.code == 'true'
     runs-on: ubuntu-latest
     timeout-minutes: 10
 
@@ -512,6 +571,8 @@ jobs:
   # of tack-core's when it comes first) fails here.
   wheels:
     name: Built wheels install and import
+    needs: changes
+    if: needs.changes.outputs.code == 'true'
     runs-on: ubuntu-latest
     timeout-minutes: 10
 
@@ -564,6 +625,8 @@ jobs:
   # the log says what was really checked rather than just "green".
   examples:
     name: examples
+    needs: changes
+    if: needs.changes.outputs.examples == 'true' || needs.changes.outputs.code == 'true'
     runs-on: ubuntu-latest
     timeout-minutes: 20
 
@@ -594,6 +657,8 @@ jobs:
   # quietly omit the hardware cases.
   metal:
     name: metal (${{ matrix.shard }}/4)
+    needs: changes
+    if: needs.changes.outputs.code == 'true'
     runs-on: macos-latest
     # The suite in four shares (conftest's --shard), one process each: whole, it
     # took 42 min of a 45 min limit when macOS runners were short (2026-10-29),
diff --git a/CLAUDE.md b/CLAUDE.md
index 97ea3226..0f001326 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -92,6 +92,8 @@ validation round or a merge gate, once per machine, not for iteration.
 
 CI's long jobs run in shares across runners (`--shard K/N`, the root conftest's: test files dealt by their seconds in `test-durations.json`, longest first, to the least-loaded share, so every share collects the same suite and they are disjoint; by test count they came out 27 against 14 minutes; refresh the table with `uv run pytest -n auto -q --durations=0 | python tools/test_durations.py`), each share one process as before: the Metal job in four (Metal tests never run under xdist; whole, it reached 42 of its 45 minutes when macOS runners were short, and a third 25 of 30), the Linux `tests` per Python version and `ids-i64` in four (whole, 25-45 minutes by 2026-10-29; in three shares, 17-28 minutes a share once #259 and #260 merged, and `ids-i64`'s 25-minute limit cancelled a third).
 
+On a pull request, ci.yml's `changes` job says which jobs its files need (`tools/ci_changes.py`, first match wins; a path no rule names is `code`): `code` gates every test job, packaging, wheels and Metal; `docs` (with `code`) the docs job, which also runs the package tests that read its pages (`test_reference_pages.py`, `test_every_algorithm_has_a_wrapper`); `examples` (with `code`) the examples job; lint always runs. Pushes to `main` and `workflow_dispatch` run everything. A new test that reads a file outside the packages (as `test_threading_benchmark.py` reads `benchmarks/`) needs that path classified as `code`.
+
 A test module that compares in f64 (against VTK, say) takes conftest's `f64_backend` as an autouse fixture, so it runs on every backend with f64 (the CPU, CUDA, HIP, Level Zero) whatever the run's default device, never on Metal, which has none; pinning it to the CPU would lose the GPUs. Metal is covered by `test_dataset_devices`' device without f64 (`test_cell_quality.py`, `test_feature_edges.py`).
 
 Do not edit a `.py` file while a run that imported it is still going:
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index fec989a0..97a17b47 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -157,6 +157,12 @@ its own backends, so GPU memory grows with the worker count. The `timing` tests
 skip themselves on a busy host, so a parallel run skips them; run `-m timing`
 alone when they matter. CI runs the suite serially (`uv run pytest`).
 
+On a pull request CI runs the jobs its files need (`tools/ci_changes.py`): a
+change to the docs alone runs lint and the docs build, one to an example lint
+and the examples; a change to the packages, the build or the workflows runs
+everything. The jobs left out report "skipping". Pushes to `main` run
+everything.
+
 ## The thing to know about the tests
 
 **A green run does not mean every backend was exercised.** Each backend's tests
diff --git a/tools/ci_changes.py b/tools/ci_changes.py
new file mode 100644
index 00000000..308cb8e2
--- /dev/null
+++ b/tools/ci_changes.py
@@ -0,0 +1,85 @@
+# SPDX-FileCopyrightText: Copyright (c) Kitware Inc.
+# SPDX-License-Identifier: BSD-3-Clause
+
+"""Which of CI's jobs a pull request's changed files need.
+
+Reads the changed paths (one a line, ``git diff --name-only``) on standard
+input and prints ``group=true`` or ``group=false`` for each group, which
+ci.yml's ``changes`` job appends to ``$GITHUB_OUTPUT``:
+
+- ``code``: the packages, their tests, the build and the workflows. Every
+  test job, the packaging jobs and Metal.
+- ``docs``: the site's pages and the prose around the code. The docs job.
+- ``examples``: the shipped examples. The examples job.
+
+Lint runs on every change and is no group. A path no rule names is code, so
+a new kind of file runs everything until a rule says otherwise.
+
+    python3 tools/ci_changes.py < changed.txt
+"""
+
+import sys
+
+GROUPS = ("code", "docs", "examples")
+
+# (test, groups), first match wins. A test is a prefix ("docs/"), a whole path
+# ("mkdocs.yml") or, for the packages' examples, a function.
+
+
+def _example(path):
+    parts = path.split("/")
+    return len(parts) >= 4 and parts[0] == "packages" and parts[2] == "examples"
+
+
+def _prose(path):
+    """Markdown outside the packages, which the site or a reader reads, nothing runs."""
+    return (path.endswith(".md") and not path.startswith("packages/")) or path.endswith("/CLAUDE.md")
+
+
+RULES = (
+    # Outside the workspace, ruff and the SPDX check; nothing runs it.
+    ("archive/", ()),
+    # Tested by lint alone (tools/ tests itself there).
+    ("tools/", ()),
+    (_example, ("examples",)),
+    ("docs/", ("docs",)),
+    ("overrides/", ("docs",)),
+    ("mkdocs.yml", ("docs",)),
+    ("changelog.d/", ()),           # lint checks the entries
+    ("LICENSE", ()),
+    (_prose, ("docs",)),
+)
+
+
+def _matches(test, path):
+    if callable(test):
+        return test(path)
+    return path.startswith(test) if test.endswith("/") else path == test
+
+
+def groups_of(path):
+    """The groups one changed path needs."""
+    for test, groups in RULES:
+        if _matches(test, path):
+            return set(groups)
+    return {"code"}
+
+
+def classify(paths):
+    """``{group: needed}`` for a change's paths."""
+    needed = set()
+    for path in paths:
+        needed |= groups_of(path)
+    return {group: group in needed for group in GROUPS}
+
+
+def main():
+    paths = [line.strip() for line in sys.stdin if line.strip()]
+    for path in paths:
+        print(f"{path}: {', '.join(sorted(groups_of(path))) or 'lint'}", file=sys.stderr)
+    for group, needed in classify(paths).items():
+        print(f"{group}={'true' if needed else 'false'}")
+
+
+if __name__ == "__main__":
+    main()
diff --git a/tools/test_ci_changes.py b/tools/test_ci_changes.py
new file mode 100644
index 00000000..7af4c9a5
--- /dev/null
+++ b/tools/test_ci_changes.py
@@ -0,0 +1,57 @@
+# SPDX-FileCopyrightText: Copyright (c) Kitware Inc.
+# SPDX-License-Identifier: BSD-3-Clause
+
+"""``tools/ci_changes.py``: a pull request's paths choose CI's jobs."""
+
+import subprocess
+import sys
+
+import ci_changes
+import pytest
+
+
+@pytest.mark.parametrize("path, groups", [
+    ("packages/tack-core/src/tack/runtime/cuda_backend.py", {"code"}),
+    ("packages/tack-vis/tests/test_streamlines.py", {"code"}),
+    ("packages/tack-vis/tests/data/vtk/README.md", {"code"}),
+    ("packages/tack-core/README.md", {"code"}),       # the package's description
+    ("packages/tack-vis/tools/make_tables.py", {"code"}),
+    ("pyproject.toml", {"code"}),
+    ("uv.lock", {"code"}),
+    ("conftest.py", {"code"}),
+    ("test-durations.json", {"code"}),
+    (".github/workflows/ci.yml", {"code"}),
+    ("benchmarks/threading_decisions.py", {"code"}),  # test_threading_benchmark runs it
+    ("something/new.txt", {"code"}),
+    ("packages/tack-core/examples/01_hello_tack.py", {"examples"}),
+    ("packages/tack-vis/examples/data/README.md", {"examples"}),
+    ("docs/vis-guide/reference/algorithms.md", {"docs"}),
+    ("docs/examples/validate.py", {"docs"}),
+    ("docs/_ext/tack_namespace.py", {"docs"}),
+    ("mkdocs.yml", {"docs"}),
+    ("overrides/main.html", {"docs"}),
+    ("README.md", {"docs"}),
+    ("CONTRIBUTING.md", {"docs"}),
+    ("CLAUDE.md", {"docs"}),
+    ("packages/tack-vis/CLAUDE.md", {"docs"}),
+    ("changelog.d/ci-path-filter.md", set()),
+    ("tools/changelog.py", set()),
+    ("archive/tack-rendering/NOTES.md", set()),
+    ("LICENSE", set()),
+])
+def test_each_path_has_its_groups(path, groups):
+    assert ci_changes.groups_of(path) == groups
+
+
+def test_a_change_needs_the_union_of_its_paths():
+    assert ci_changes.classify(["docs/install.md", "packages/tack-core/examples/02_saxpy.py"]) == {
+        "code": False, "docs": True, "examples": True}
+    assert ci_changes.classify(["changelog.d/x.md", "tools/spdx_headers.py"]) == {
+        "code": False, "docs": False, "examples": False}
+    assert ci_changes.classify(["docs/index.md", "uv.lock"])["code"]
+
+
+def test_prints_one_output_a_group():
+    out = subprocess.run([sys.executable, "ci_changes.py"], input="docs/index.md\n\n",
+                         capture_output=True, text=True, check=True).stdout
+    assert out.splitlines() == ["code=false", "docs=true", "examples=false"]