diff --git a/.github/workflows/ingest.yml b/.github/workflows/ingest.yml
index 749f5bb6786..4fc2f5776b6 100644
--- a/.github/workflows/ingest.yml
+++ b/.github/workflows/ingest.yml
@@ -107,7 +107,7 @@ jobs:
           fi
           git commit -m "Matched advisory candidates (snapshot)"
           {
-            echo "base=${GITHUB_SHA}"
+            echo "diff_base=${GITHUB_SHA}"
             echo "commit=$(git rev-parse HEAD)"
             echo "created=true"
           } >> "${GITHUB_OUTPUT}"
@@ -115,7 +115,7 @@ jobs:
       - name: Partition matched advisories
         if: steps.snapshot.outputs.created == 'true'
         env:
-          BASE_SHA: ${{ steps.snapshot.outputs.base }}
+          DIFF_BASE_SHA: ${{ steps.snapshot.outputs.diff_base }}
           SNAPSHOT_SHA: ${{ steps.snapshot.outputs.commit }}
           SHARD_OUTPUT: ${{ runner.temp }}/advisory-shards
           # Deltas at or below this many files publish as one standing PR;
@@ -126,7 +126,7 @@ jobs:
           mkdir -p "${SHARD_OUTPUT}"
           changed="${RUNNER_TEMP}/changed-advisories"
           git diff --name-only -z --no-renames --diff-filter=AM \
-            "${BASE_SHA}" "${SNAPSHOT_SHA}" -- advisories/ > "${changed}"
+            "${DIFF_BASE_SHA}" "${SNAPSHOT_SHA}" -- advisories/ > "${changed}"
           count="$(tr -dc '\0' < "${changed}" | wc -c)"
           if (( count <= SINGLE_PR_LIMIT )); then
             echo "Publishing ${count} paths as a single pull request."
@@ -149,7 +149,7 @@ jobs:
         if: steps.snapshot.outputs.created == 'true'
         env:
           GITHUB_TOKEN: ${{ secrets.HOMEBREW_GITHUB_PUBLIC_REPO_TOKEN || github.token }}
-          BASE_SHA: ${{ steps.snapshot.outputs.base }}
+          DIFF_BASE_SHA: ${{ steps.snapshot.outputs.diff_base }}
           SNAPSHOT_SHA: ${{ steps.snapshot.outputs.commit }}
           SHARD_OUTPUT: ${{ runner.temp }}/advisory-shards
         run: |
@@ -161,6 +161,25 @@ jobs:
             exit 1
           fi
 
+          # The matcher runs for over an hour, so main may advance after checkout.
+          # Base every published branch on main as it exists at publication time.
+          git fetch --no-tags origin +refs/heads/main:refs/remotes/origin/main
+          PUBLISH_BASE_SHA="$(git rev-parse origin/main)"
+          merge_status=0
+          PUBLISH_SNAPSHOT_TREE="$(
+            git merge-tree --write-tree --no-messages --merge-base="${DIFF_BASE_SHA}" \
+              "${PUBLISH_BASE_SHA}" "${SNAPSHOT_SHA}"
+          )" || merge_status=$?
+          if (( merge_status != 0 )); then
+            if (( merge_status == 1 )); then
+              echo "::error::Matched advisory changes conflict with main at publication time."
+            else
+              echo "::error::git merge-tree failed with exit status ${merge_status}."
+            fi
+            echo "${PUBLISH_SNAPSHOT_TREE}"
+            exit "${merge_status}"
+          fi
+
           # Commands must use `|| return` because this function runs in an `if`.
           publish_shard() {
             local manifest="$1"
@@ -177,14 +196,18 @@ jobs:
               body="Automated candidates via \`brew advisory-match --all\`, sharded by formula (SHA-256 bucket \`${shard}\`). See [CONTRIBUTING.md](https://github.com/${GITHUB_REPOSITORY}/blob/HEAD/CONTRIBUTING.md#reviewing-matched-candidates) for the review checklist."
             fi
 
-            git switch --discard-changes --force-create "${branch}" "${BASE_SHA}" || return
+            git switch --discard-changes --force-create "${branch}" "${PUBLISH_BASE_SHA}" || return
             git clean --force -d -- advisories/ || return
-            git restore --source="${SNAPSHOT_SHA}" --pathspec-from-file="${manifest}" \
+            git restore --source="${PUBLISH_SNAPSHOT_TREE}" --pathspec-from-file="${manifest}" \
               --pathspec-file-nul || return
             # Regenerate rebuilds data/advisories.json separately.
             git add advisories/ || return
+            if git diff --cached --quiet; then
+              echo "Shard ${shard} is already on main; nothing to publish."
+              return 0
+            fi
             git commit -m "${title}" \
-              -m "Base: ${BASE_SHA}" || return
+              -m "Base: ${PUBLISH_BASE_SHA}" || return
             # No lease: every branch is rebuilt from main on each run.
             git push --force origin "HEAD:refs/heads/${branch}" || return
 
@@ -219,6 +242,7 @@ jobs:
 
           # A mode switch (standing <-> sharded) leaves the other mode's PRs
           # open with superseded content; close them and delete their branches.
+          # Every successful manifest counts as published, including those already on main.
           published=" "
           for manifest in "${manifests[@]}"; do
             shard="${manifest##*/}"
diff --git a/spec/workflows_spec.rb b/spec/workflows_spec.rb
index 2d6f8a84a24..bec65d2f580 100644
--- a/spec/workflows_spec.rb
+++ b/spec/workflows_spec.rb
@@ -100,6 +100,13 @@
     publish = steps.fetch(publish_index).fetch("run")
 
     expect(snapshot).to include('git switch --detach "${GITHUB_SHA}"')
+    expect(snapshot).to include('echo "diff_base=${GITHUB_SHA}"')
+    expect(snapshot).not_to include('echo "base=${GITHUB_SHA}"')
+    expect(steps.fetch(partition_index).dig("env", "DIFF_BASE_SHA")).to eq(
+      "${{ steps.snapshot.outputs.diff_base }}",
+    )
+    expect(steps.fetch(partition_index).fetch("env")).not_to have_key("BASE_SHA")
+    expect(partition).to include('"${DIFF_BASE_SHA}" "${SNAPSHOT_SHA}"')
     expect(partition).to include("bundle exec rake advisories:shard")
     expect(partition).to include("--no-renames")
     expect(partition).to include("--diff-filter=AM")
@@ -117,17 +124,44 @@
     )
     expect(publish).not_to include("gh auth setup-git")
     expect(publish).to include('manifests=("${SHARD_OUTPUT}"/*)')
-    expect(publish).to include('git switch --discard-changes --force-create "${branch}" "${BASE_SHA}"')
+    fetch_main = "git fetch --no-tags origin +refs/heads/main:refs/remotes/origin/main"
+    merge_snapshot =
+      'git merge-tree --write-tree --no-messages --merge-base="${DIFF_BASE_SHA}"'
+    switch_to_publish_base =
+      'git switch --discard-changes --force-create "${branch}" "${PUBLISH_BASE_SHA}"'
+    expect(publish).to include(fetch_main)
+    expect(publish).to include('PUBLISH_BASE_SHA="$(git rev-parse origin/main)"')
+    expect(publish).to include(merge_snapshot)
+    expect(publish).to include(')" || merge_status=$?')
+    expect(publish).to include("if (( merge_status == 1 ))")
+    expect(publish).to include("Matched advisory changes conflict with main")
+    expect(publish).to include("git merge-tree failed with exit status ${merge_status}")
+    expect(publish).to include('echo "${PUBLISH_SNAPSHOT_TREE}"')
+    expect(publish).to include('exit "${merge_status}"')
+    expect(publish).to include(switch_to_publish_base)
+    expect(publish.index(fetch_main)).to be < publish.index(merge_snapshot)
+    expect(publish.index(merge_snapshot)).to be < publish.index(switch_to_publish_base)
+    expect(steps.fetch(publish_index).dig("env", "DIFF_BASE_SHA")).to eq(
+      "${{ steps.snapshot.outputs.diff_base }}",
+    )
+    expect(steps.fetch(publish_index).fetch("env")).not_to have_key("BASE_SHA")
     expect(publish).to include("git clean --force -d -- advisories/")
-    expect(publish).to include('git restore --source="${SNAPSHOT_SHA}" --pathspec-from-file="${manifest}"')
+    expect(publish).to include(
+      'git restore --source="${PUBLISH_SNAPSHOT_TREE}" --pathspec-from-file="${manifest}"',
+    )
     expect(publish).to include("--pathspec-file-nul")
     expect(publish).not_to include("bundle exec rake advisories:concat")
     expect(publish).to include("git add advisories/")
     expect(publish).not_to include("git add advisories/ data/")
+    empty_guard = "if git diff --cached --quiet; then"
+    expect(publish).to include(empty_guard)
+    expect(publish).to include('echo "Shard ${shard} is already on main; nothing to publish."')
+    expect(publish.index("git add advisories/")).to be < publish.index(empty_guard)
+    expect(publish.index(empty_guard)).to be < publish.index('git commit -m "${title}"')
     expect(publish).to include('branch="matched-advisories"')
     expect(publish).to include('branch="matched-advisories-${shard}"')
     expect(publish).to include('git push --force origin "HEAD:refs/heads/${branch}"')
-    expect(publish).to include("Base: ${BASE_SHA}")
+    expect(publish).to include("Base: ${PUBLISH_BASE_SHA}")
     expect(publish).to include("No advisory shards were produced")
     expect(publish).to include("failed=0")
     expect(publish).to include('if publish_shard "${manifest}"')
@@ -135,6 +169,7 @@
     expect(publish).to include("--delete-branch")
     expect(publish).to include("matched-advisories(-[0-9a-f]{2})?$")
     expect(publish).to include('exit "${failed}"')
+    expect(publish).to include("including those already on main")
 
     # Reconciliation only runs after full publication, enumerates open PRs
     # with a checked paginated command, and propagates close failures.