diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 3f4db6bf..0f2a013d 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -104,11 +104,29 @@ jobs:
       - if: always()
         run: python3 -m unittest discover -s tools/release/tests
 
+  # Whether this branch touches anything the build reads. tools/fork/docs-only.py
+  # says what counts, and main always builds. A skipped build still reports
+  # success below: Gitea's `github` job waits for that status, so this can't be
+  # a paths-ignore on the trigger, which would leave it waiting until it times
+  # out. If this job fails, the build is skipped and the report says failure.
+  changes:
+    if: ${{ vars.BUILD_ON == 'github' && github.ref_type == 'branch' }}
+    runs-on: ubuntu-latest
+    outputs:
+      docs_only: ${{ steps.check.outputs.docs_only }}
+    steps:
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          fetch-depth: 0
+      - id: check
+        run: python3 tools/fork/docs-only.py
+
   # The build, and that every test target compiles. The suites are not run:
   # they need a store, fixed ports and containers (docs/spec/
   # container-tests.md), and are run by hand.
   build:
-    if: ${{ vars.BUILD_ON == 'github' && github.ref_type == 'branch' }}
+    needs: [changes]
+    if: ${{ vars.BUILD_ON == 'github' && github.ref_type == 'branch' && needs.changes.outputs.docs_only == 'false' }}
     runs-on: ubuntu-latest
     env:
       CARGO_INCREMENTAL: "0"
@@ -583,7 +601,7 @@ jobs:
   # way round) count as passing; a failed or cancelled one does not.
   report:
     if: ${{ always() && vars.BUILD_ON == 'github' }}
-    needs: [start, fork-checks, build, version, publish, index, relay-publish, relay-index, release, binaries]
+    needs: [start, fork-checks, changes, build, version, publish, index, relay-publish, relay-index, release, binaries]
     runs-on: ubuntu-latest
     steps:
       - env:
diff --git a/tools/fork/README.md b/tools/fork/README.md
index eda05ab5..34966a35 100644
--- a/tools/fork/README.md
+++ b/tools/fork/README.md
@@ -127,3 +127,9 @@ tools/fork/expr-schema.py --check  # exit 1 if it's out of date (CI)
 
 Re-run it after anything that regenerates the registry, an upstream import
 included.
+
+## docs-only.py
+
+Says whether a branch changes only top-level `*.md` files and `docs/`. GitHub's
+`ci.yml` uses the answer to skip the build for those branches, and still reports
+success to Gitea. `main` always builds. `fork-checks` runs either way.
diff --git a/tools/fork/docs-only.py b/tools/fork/docs-only.py
new file mode 100755
index 00000000..5e2209d2
--- /dev/null
+++ b/tools/fork/docs-only.py
@@ -0,0 +1,71 @@
+#!/usr/bin/env python3
+# SPDX-FileCopyrightText: 2026 Coffey Labs LLC
+# SPDX-License-Identifier: AGPL-3.0-only
+"""
+Say whether a branch changes only documentation, so CI can skip its build.
+
+    tools/fork/docs-only.py        # prints docs_only=true or docs_only=false
+
+Under GitHub Actions the answer also goes to $GITHUB_OUTPUT.
+
+The whole branch is compared with where it left main, not just its latest
+push. A newer push cancels the run for the older one, so a documentation commit
+on top of a code commit must still build.
+
+Only top-level *.md files and docs/ count. Markdown elsewhere can be compiled
+in: vendor/calcard and vendor/sieve-rs include their READMEs as doc comments.
+
+main always builds. A push to main cancels the run before it, so a documentation
+merge skipping its build would leave the merge before it unchecked. When there's
+any doubt, the answer is false: a needless build only costs time.
+"""
+import os
+import re
+import subprocess
+import sys
+
+BASE = "origin/main"
+MAIN_REF = "refs/heads/main"
+DOCS = re.compile(r"(?:[^/]+\.md|docs/.+)", re.S)
+
+
+def is_docs_only(paths):
+    return bool(paths) and all(DOCS.fullmatch(p) for p in paths)
+
+
+def changed_paths(base=BASE, cwd=None):
+    """The branch's files, or None if git can't say. A rename lists both ends."""
+    result = subprocess.run(
+        ["git", "diff", "--name-only", "--no-renames", "-z", f"{base}...HEAD"],
+        cwd=cwd,
+        capture_output=True,
+        text=True,
+    )
+    if result.returncode != 0:
+        print(result.stderr, file=sys.stderr)
+        return None
+    return [p for p in result.stdout.split("\0") if p]
+
+
+def answer(ref, base=BASE, cwd=None):
+    if ref == MAIN_REF:
+        return False
+    paths = changed_paths(base, cwd)
+    if paths is None:
+        return False
+    for path in paths:
+        print(f"changed: {path}")
+    return is_docs_only(paths)
+
+
+def main():
+    line = f"docs_only={str(answer(os.environ.get('GITHUB_REF', ''))).lower()}"
+    print(line)
+    output = os.environ.get("GITHUB_OUTPUT")
+    if output:
+        with open(output, "a", encoding="utf-8") as f:
+            f.write(line + "\n")
+
+
+if __name__ == "__main__":
+    main()
diff --git a/tools/fork/tests/test_docs_only.py b/tools/fork/tests/test_docs_only.py
new file mode 100644
index 00000000..d98d3ed7
--- /dev/null
+++ b/tools/fork/tests/test_docs_only.py
@@ -0,0 +1,115 @@
+# SPDX-FileCopyrightText: 2026 Coffey Labs LLC
+# SPDX-License-Identifier: AGPL-3.0-only
+"""Tests for tools/fork/docs-only.py: python3 -m unittest discover -s tools/fork/tests"""
+import contextlib
+import importlib.util
+import io
+import os
+import subprocess
+import tempfile
+import unittest
+from pathlib import Path
+
+HERE = os.path.dirname(os.path.abspath(__file__))
+spec = importlib.util.spec_from_file_location("docs_only", os.path.join(HERE, "..", "docs-only.py"))
+docs_only = importlib.util.module_from_spec(spec)
+spec.loader.exec_module(docs_only)
+
+
+class Paths(unittest.TestCase):
+    def test_documentation(self):
+        for paths in (["README.md"], ["CHANGELOG.md", "docs/spec/SPEC.md"], ["docs/img/a.png"]):
+            self.assertTrue(docs_only.is_docs_only(paths), paths)
+
+    def test_not_documentation(self):
+        for paths in (
+            [],
+            ["Cargo.toml"],
+            ["README.md", "crates/main/src/main.rs"],
+            ["vendor/calcard/README.md"],
+            ["crates/main/README.md"],
+            [".github/workflows/ci.yml"],
+            ["docs"],
+            ["README.md\nsrc/main.rs"],
+        ):
+            self.assertFalse(docs_only.is_docs_only(paths), paths)
+
+
+class Branches(unittest.TestCase):
+    def setUp(self):
+        tmp = tempfile.TemporaryDirectory()
+        self.addCleanup(tmp.cleanup)
+        self.repo = Path(tmp.name)
+        self.git("init", "-q", "-b", "main")
+        self.commit({"README.md": "one\n", "src.rs": "fn main() {}\n"})
+        self.git("branch", "base")
+        self.git("switch", "-q", "-c", "work")
+
+    def git(self, *args):
+        subprocess.run(
+            [
+                "git",
+                "-c", "user.name=t",
+                "-c", "user.email=t@example.org",
+                "-c", "commit.gpgsign=false",
+                *args,
+            ],
+            cwd=self.repo,
+            check=True,
+            capture_output=True,
+        )
+
+    def commit(self, files):
+        for name, text in files.items():
+            path = self.repo / name
+            path.parent.mkdir(parents=True, exist_ok=True)
+            path.write_text(text)
+        self.git("add", "-A")
+        self.git("commit", "-q", "-m", "change")
+
+    def answer(self, ref="refs/heads/work", base="base"):
+        with contextlib.redirect_stdout(io.StringIO()):
+            return docs_only.answer(ref, base, self.repo)
+
+    def test_docs_commit_is_docs_only(self):
+        self.commit({"README.md": "two\n", "docs/a.md": "a\n"})
+        self.assertTrue(self.answer())
+
+    def test_code_commit_builds(self):
+        self.commit({"src.rs": "fn main() { }\n"})
+        self.assertFalse(self.answer())
+
+    def test_docs_commit_on_top_of_code_still_builds(self):
+        self.commit({"src.rs": "fn main() { }\n"})
+        self.commit({"README.md": "two\n"})
+        self.assertFalse(self.answer())
+
+    def test_nothing_changed_builds(self):
+        self.assertFalse(self.answer())
+
+    def test_moving_code_into_docs_builds(self):
+        self.git("mv", "src.rs", "docs-src.rs")
+        (self.repo / "docs").mkdir()
+        self.git("mv", "docs-src.rs", "docs/src.rs")
+        self.git("commit", "-q", "-m", "move")
+        self.assertFalse(self.answer())
+
+    def test_code_landing_on_main_since_the_branch_left_it_does_not_count(self):
+        self.commit({"README.md": "two\n"})
+        self.git("switch", "-q", "base")
+        self.commit({"src.rs": "fn main() { }\n"})
+        self.git("switch", "-q", "work")
+        self.assertTrue(self.answer())
+
+    def test_main_always_builds(self):
+        self.commit({"README.md": "two\n"})
+        self.assertFalse(self.answer("refs/heads/main"))
+
+    def test_git_failing_builds(self):
+        self.commit({"README.md": "two\n"})
+        with contextlib.redirect_stderr(io.StringIO()):
+            self.assertFalse(self.answer(base="no-such-ref"))
+
+
+if __name__ == "__main__":
+    unittest.main()