diff --git a/.github/scripts/apidiff.sh b/.github/scripts/apidiff.sh
new file mode 100755
index 000000000..0d61a0e4b
--- /dev/null
+++ b/.github/scripts/apidiff.sh
@@ -0,0 +1,144 @@
+#!/usr/bin/env bash
+# Reports incompatible changes to the exported API of the repository's modules.
+#
+# Compares every module against a base revision and fails when apidiff calls a
+# change incompatible. Adding to the API is always allowed.
+#
+# Usage: apidiff.sh <target-ref>
+#
+# Why this exists: a trailing variadic parameter added to an exported function
+# changes that function's type. Every ordinary call site still compiles, so
+# neither the build nor the tests notice, while any caller holding the function
+# as a value stops compiling. That shipped once.
+set -euo pipefail
+
+TARGET_REF="${1:?usage: apidiff.sh <target-ref>}"
+
+# Compare against the merge base, not the target's tip. A branch cut before a
+# recent change to the target would otherwise be reported as having removed
+# whatever the target gained in the meantime, which is a false positive and the
+# quickest way to get a check like this ignored.
+BASE_REF="$(git merge-base HEAD "$TARGET_REF")"
+echo "Target $TARGET_REF, comparing against merge base ${BASE_REF:0:12}"
+
+WORK="$(mktemp -d)"
+BASE_TREE="$WORK/base"
+cleanup() {
+  git worktree remove --force "$BASE_TREE" >/dev/null 2>&1 || true
+  rm -rf "$WORK"
+}
+trap cleanup EXIT
+mkdir -p "$WORK/api"
+
+# Every module in the repository, as a path relative to the root.
+modules() {
+  find . -not -path '*/.git/*' -name go.mod -exec dirname {} \; | sort
+}
+
+# The module path declared by a directory. GOWORK=off because inside a
+# workspace `go list -m` reports every module in the workspace, not this one.
+module_path() { (cd "$1" && GOWORK=off go list -m); }
+
+snapshot_for() { echo "$WORK/api/$(echo "$1" | tr '/' '_').api"; }
+
+# apidiff narrates every internal package it skips, on stderr, which buries the
+# one line that says what actually went wrong under eighty that do not.
+report_error() { grep -v '^Ignoring internal package ' "$WORK/err" | sed 's/^/    /' || true; }
+
+# apidiff is asked for a whole module at a time. -m loads every package in the
+# module in one pass and skips internal/ by itself, where invoking it once per
+# package costs minutes on a module this size and reports the same thing.
+#
+# It exits non-zero only when it could not do its job; finding an incompatible
+# change is a successful run that prints to stdout. Keeping those two apart is
+# the difference between "nothing broke" and "nothing was compared", which look
+# identical from the outside.
+failed=0
+breaks=0
+
+echo "Collecting the API at the base revision"
+git worktree add --detach --quiet "$BASE_TREE" "$BASE_REF"
+(
+  cd "$BASE_TREE"
+  go work init >/dev/null 2>&1 || true
+  go work use -r . >/dev/null 2>&1 || true
+)
+
+while IFS= read -r dir; do
+  mod="$(module_path "$dir")"
+  snap="$(snapshot_for "$mod")"
+
+  # A module this change introduces has no baseline, which is not a problem.
+  if [ ! -d "$BASE_TREE/$dir" ]; then
+    echo "  $mod is new; nothing to compare against"
+    continue
+  fi
+
+  if (cd "$BASE_TREE/$dir" && apidiff -m -w "$snap" "$mod") 2>"$WORK/err"; then
+    echo "  $mod"
+  else
+    echo "  ERROR: could not read the API of $mod at the base revision"
+    report_error
+    failed=1
+  fi
+done < <(modules)
+
+echo "Comparing HEAD against it"
+while IFS= read -r dir; do
+  mod="$(module_path "$dir")"
+  snap="$(snapshot_for "$mod")"
+  [ -f "$snap" ] || continue
+
+  if ! out="$( (cd "$dir" && apidiff -m "$snap" "$mod") 2>"$WORK/err" )"; then
+    echo "  ERROR: could not read the API of $mod at HEAD"
+    report_error
+    failed=1
+    continue
+  fi
+
+  # apidiff prints an "Incompatible changes:" section only when there are some.
+  if printf '%s' "$out" | grep -q '^Incompatible changes:'; then
+    echo
+    echo "=== $mod ==="
+    printf '%s\n' "$out" | sed -n '/^Incompatible changes:/,/^$/p'
+    breaks=1
+  fi
+done < <(modules)
+
+# A module that could not be read was not checked at all. Reporting that as a
+# clean run is the one outcome worse than not having the check, so it fails
+# here, and the breaking-change label below does not cover it.
+if [ "$failed" -ne 0 ]; then
+  cat <<'MSG'
+
+The comparison did not complete: the errors above mean some module's API was
+never read, so nothing about it was verified. Fix those before reading this
+check as "no breaking changes".
+MSG
+  exit 1
+fi
+
+if [ "$breaks" -eq 0 ]; then
+  echo "No incompatible changes."
+  exit 0
+fi
+
+cat <<'MSG'
+
+Incompatible API changes found.
+
+Adding to the API is always allowed; the changes above remove something or
+change its type. Note that adding a variadic parameter to an existing exported
+function counts, even though every call site still compiles.
+
+If the break is deliberate, label the pull request "breaking-change". The
+comparison still runs and still prints what changed, so the break stays on the
+record, but it stops failing the check.
+MSG
+
+if [ "${ALLOW_BREAKING:-false}" = "true" ]; then
+  echo
+  echo 'Labelled "breaking-change", so this is reported rather than enforced.'
+  exit 0
+fi
+exit 1
diff --git a/.github/workflows/apidiff.yml b/.github/workflows/apidiff.yml
new file mode 100644
index 000000000..cbdf21683
--- /dev/null
+++ b/.github/workflows/apidiff.yml
@@ -0,0 +1,55 @@
+# Guards the exported API of the module against accidental breaking changes.
+#
+# The build and the tests do not catch every break. Adding a trailing variadic
+# parameter to an exported function changes that function's type: every ordinary
+# call site still compiles, so nothing here went red, while any caller holding
+# the function as a value stops compiling. That reached a release once, which is
+# why this check exists.
+name: API diff
+
+on:
+  pull_request:
+    branches: [ "main", "v1" ]
+
+  workflow_dispatch:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  apidiff:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd  # v6.0.2
+        with:
+          # apidiff compares against the merge base, which needs the history
+          # rather than the single commit a default checkout fetches.
+          fetch-depth: 0
+
+      - name: Setup
+        uses: ./.github/actions/setup
+
+      - name: Initialize workspace
+        run: go work init && go work use -r .
+
+      - name: Install apidiff
+        run: go install golang.org/x/exp/cmd/apidiff@v0.0.0-20250911091902-df9299821621
+
+      - name: Compare the exported API against the base branch
+        # The ref goes through the environment rather than being interpolated
+        # into the command. A ${{ }} expansion inside run: is substituted into
+        # the shell text before the shell sees it, so anything controllable in
+        # the value becomes code; passing it as a quoted variable keeps it data.
+        env:
+          BASE_REF: ${{ github.base_ref }}
+          # A pull request labelled breaking-change still gets compared, and
+          # still prints what changed, so the break is recorded rather than
+          # waved through invisibly. It just does not fail the check. Without
+          # an escape hatch the only way to land a deliberate break is to
+          # disable the check, which is how checks get removed entirely.
+          ALLOW_BREAKING: ${{ contains(github.event.pull_request.labels.*.name, 'breaking-change') }}
+        run: .github/scripts/apidiff.sh "origin/$BASE_REF"