diff --git a/.github/workflows/build-image.yml b/.github/workflows/build-image.yml
index eb709bd9..e221ba84 100644
--- a/.github/workflows/build-image.yml
+++ b/.github/workflows/build-image.yml
@@ -10,6 +10,9 @@ on:
     # still skipped.
     paths:
       - "Dockerfile"
+      - "ROLLBACK_UNSAFE_BEFORE"
+      - "scripts/check-rollback-boundary.sh"
+      - "scripts/test-check-rollback-boundary.sh"
       - "go.mod"
       - "go.sum"
       - "cmd/**"
@@ -77,6 +80,20 @@ jobs:
             dockerfile: tests/sdk-monitor/Dockerfile
     steps:
       - uses: actions/checkout@v7
+        with:
+          fetch-depth: 0
+
+      # Tags and workflow_dispatch can otherwise publish from an arbitrary
+      # branch. Release images must come from a commit already contained in
+      # main so the cumulative rollback-boundary history cannot be forked.
+      - name: Require publish source on main
+        if: github.event_name != 'pull_request'
+        shell: bash
+        run: |
+          set -euo pipefail
+          git fetch --force origin refs/heads/main:refs/remotes/origin/main
+          git merge-base --is-ancestor "$GITHUB_SHA" refs/remotes/origin/main \
+            || { echo "::error::Image publishing is allowed only for commits contained in main"; exit 1; }
 
       # QEMU registers binfmt_misc handlers so this amd64 runner can
       # cross-build arm64 layers under emulation. Without this step the
@@ -85,6 +102,21 @@ jobs:
       - uses: docker/setup-qemu-action@v4
       - uses: docker/setup-buildx-action@v4
 
+      # Hosted release automation refuses to infer rollback safety from a
+      # missing label. The tracked value is `none` while all migrations remain
+      # backward-compatible; once an irreversible migration ships, set its
+      # first affected release and carry that boundary forward cumulatively.
+      - name: Validate rollback compatibility boundary
+        id: rollback-boundary
+        shell: bash
+        env:
+          ROLLBACK_BASE_REF: ${{ github.event.pull_request.base.sha || github.event.before || '' }}
+          REQUIRE_ROLLBACK_BASE: true
+        run: |
+          set -euo pipefail
+          value=$(scripts/check-rollback-boundary.sh)
+          echo "value=$value" >> "$GITHUB_OUTPUT"
+
       # Skip login on PR builds: we don't push from a PR, and PRs from
       # forks only get a read-only GITHUB_TOKEN anyway.
       - name: Log in to GHCR
@@ -117,7 +149,9 @@ jobs:
           # push on real branch/tag/manual events.
           push: ${{ github.event_name != 'pull_request' }}
           tags: ${{ steps.meta.outputs.tags }}
-          labels: ${{ steps.meta.outputs.labels }}
+          labels: |
+            ${{ steps.meta.outputs.labels }}
+            org.e2a.rollback-unsafe-before=${{ steps.rollback-boundary.outputs.value }}
           platforms: linux/amd64,linux/arm64
           # Per-image cache scopes — without scoping, the two matrix
           # entries would clobber each other's cache and rebuild from
diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index ed03b1ee..bc0aa91e 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -40,10 +40,15 @@ jobs:
     timeout-minutes: 5
     steps:
       - uses: actions/checkout@v7
+        with:
+          fetch-depth: 0
       - uses: actions/setup-node@v7
         with:
           node-version: "22"
       - run: scripts/check-repository-text-integrity.sh
+        env:
+          ROLLBACK_BASE_REF: ${{ github.event.pull_request.base.sha || github.event.before || '' }}
+      - run: scripts/test-check-rollback-boundary.sh
       - run: node --test scripts/sync-agent-docs.test.mjs
 
   harness-tests:
diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index dab6928a..cf1d9856 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -286,6 +286,15 @@ pending migrations against a `schema_migrations` tracker table.
    renumber existing files.
 4. **Forward-only** — there are no down migrations. If you need to
    undo something, write a new migration that does it.
+5. **Declare binary rollback compatibility** — `ROLLBACK_UNSAFE_BEFORE` is
+   embedded as `org.e2a.rollback-unsafe-before` in release images. Leave it at
+   `none` only while every rollback-eligible retained production binary can
+   still run safely after every embedded migration applies. Hosted S1 enforces
+   that claim by booting those exact image digests against its migrated prod
+   clone. If a migration crosses that boundary, set the first affected release
+   semver and carry it forward unchanged; CI scans the reachable file history
+   and rejects any later reset or move. Tag and manual image publishing are
+   also restricted to commits already contained in `main`.
 
 Add or update tests in any Go package that writes raw SQL against the
 touched table. Higher-level e2e coverage doesn't catch query drift
diff --git a/ROLLBACK_UNSAFE_BEFORE b/ROLLBACK_UNSAFE_BEFORE
new file mode 100644
index 00000000..621e94f0
--- /dev/null
+++ b/ROLLBACK_UNSAFE_BEFORE
@@ -0,0 +1 @@
+none
diff --git a/scripts/check-repository-text-integrity.sh b/scripts/check-repository-text-integrity.sh
index 9f29f92e..d1c00d70 100755
--- a/scripts/check-repository-text-integrity.sh
+++ b/scripts/check-repository-text-integrity.sh
@@ -21,6 +21,13 @@ node -e '
 node scripts/sync-agent-docs.mjs --check
 node scripts/check-sdk-example-contracts.mjs
 
+scripts/check-rollback-boundary.sh >/dev/null
+if ! grep -Fq 'org.e2a.rollback-unsafe-before=${{ steps.rollback-boundary.outputs.value }}' \
+     .github/workflows/build-image.yml; then
+  echo "server image build must publish the rollback compatibility boundary" >&2
+  exit 1
+fi
+
 legacy_agent_calls="$(git grep -n -E 'webhooks\.(fetch_message|fetchMessage)|client\.messages\.reply|messages\.reply' -- \
   examples/agent-framework-webhooks/python/agent_webhooks \
   examples/agent-framework-webhooks/typescript/src \
diff --git a/scripts/check-rollback-boundary.sh b/scripts/check-rollback-boundary.sh
new file mode 100755
index 00000000..16c9cc6e
--- /dev/null
+++ b/scripts/check-rollback-boundary.sh
@@ -0,0 +1,72 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+repo_root="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+cd "$repo_root"
+
+boundary="$(tr -d '[:space:]' < ROLLBACK_UNSAFE_BEFORE)"
+if ! [[ "$boundary" =~ ^(none|[0-9]+\.[0-9]+\.[0-9]+(-[A-Za-z0-9.]+)?)$ ]]; then
+  echo "ROLLBACK_UNSAFE_BEFORE must be 'none' or an exact semver" >&2
+  exit 1
+fi
+
+# Once a release crosses a migration boundary, every descendant must carry the
+# same first-unsafe release. Comparing only one caller-supplied snapshot is not
+# enough: a tag/manual publish could omit that ref after an earlier commit had
+# already reset the file. Resolve a trusted ancestor, then scan the file's full
+# reachable HEAD history chronologically, including every commit in a PR.
+base_ref=${ROLLBACK_BASE_REF:-${1:-}}
+require_base=${REQUIRE_ROLLBACK_BASE:-false}
+
+case "$require_base" in
+  true|false) ;;
+  *) echo "REQUIRE_ROLLBACK_BASE must be true or false" >&2; exit 1 ;;
+esac
+
+if [ -z "$base_ref" ] || [[ "$base_ref" =~ ^0+$ ]]; then
+  base_ref=""
+  if git rev-parse --verify HEAD^ >/dev/null 2>&1; then
+    base_ref=HEAD^
+  fi
+fi
+
+if [ -z "$base_ref" ]; then
+  if [ "$require_base" = true ]; then
+    echo "a resolvable rollback-boundary base commit is required for publishing" >&2
+    exit 1
+  fi
+  printf '%s\n' "$boundary"
+  exit 0
+fi
+
+if ! git cat-file -e "${base_ref}^{commit}" 2>/dev/null; then
+  echo "rollback-boundary base is not a commit: ${base_ref}" >&2
+  exit 1
+fi
+if ! git merge-base --is-ancestor "$base_ref" HEAD; then
+  echo "rollback-boundary base ${base_ref} is not an ancestor of HEAD" >&2
+  exit 1
+fi
+
+historical_boundary=""
+while IFS= read -r commit; do
+  value="$(git show "${commit}:ROLLBACK_UNSAFE_BEFORE" 2>/dev/null | tr -d '[:space:]' || true)"
+  [ -n "$value" ] || continue
+  if ! [[ "$value" =~ ^(none|[0-9]+\.[0-9]+\.[0-9]+(-[A-Za-z0-9.]+)?)$ ]]; then
+    echo "historical ROLLBACK_UNSAFE_BEFORE at ${commit} is invalid: ${value}" >&2
+    exit 1
+  fi
+  if [ -z "$historical_boundary" ]; then
+    [ "$value" = none ] || historical_boundary="$value"
+  elif [ "$value" != "$historical_boundary" ]; then
+    echo "ROLLBACK_UNSAFE_BEFORE changed historically after ${historical_boundary}: ${value} at ${commit}" >&2
+    exit 1
+  fi
+done < <(git log --full-history --topo-order --reverse --format=%H HEAD -- ROLLBACK_UNSAFE_BEFORE)
+
+if [ -n "$historical_boundary" ] && [ "$boundary" != "$historical_boundary" ]; then
+  echo "ROLLBACK_UNSAFE_BEFORE is cumulative: ${historical_boundary} cannot change to ${boundary}" >&2
+  exit 1
+fi
+
+printf '%s\n' "$boundary"
diff --git a/scripts/test-check-rollback-boundary.sh b/scripts/test-check-rollback-boundary.sh
new file mode 100755
index 00000000..b1c3e85c
--- /dev/null
+++ b/scripts/test-check-rollback-boundary.sh
@@ -0,0 +1,91 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+source_root="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+tmp=$(mktemp -d "${TMPDIR:-/tmp}/rollback-boundary-test.XXXXXX")
+trap 'rm -rf "$tmp"' EXIT
+mkdir -p "$tmp/scripts"
+cp "$source_root/scripts/check-rollback-boundary.sh" "$tmp/scripts/"
+chmod +x "$tmp/scripts/check-rollback-boundary.sh"
+
+git -C "$tmp" init -q
+git -C "$tmp" config user.name Test
+git -C "$tmp" config user.email test@example.com
+printf '%s\n' none > "$tmp/ROLLBACK_UNSAFE_BEFORE"
+git -C "$tmp" add ROLLBACK_UNSAFE_BEFORE
+git -C "$tmp" commit -qm safe
+safe_sha=$(git -C "$tmp" rev-parse HEAD)
+
+printf '%s\n' 2.0.0 > "$tmp/ROLLBACK_UNSAFE_BEFORE"
+ROLLBACK_BASE_REF="$safe_sha" "$tmp/scripts/check-rollback-boundary.sh" >/dev/null
+git -C "$tmp" commit -qam boundary
+boundary_sha=$(git -C "$tmp" rev-parse HEAD)
+
+printf '%s\n' none > "$tmp/ROLLBACK_UNSAFE_BEFORE"
+if ROLLBACK_BASE_REF="$boundary_sha" "$tmp/scripts/check-rollback-boundary.sh" >/dev/null 2>&1; then
+  echo "resetting a cumulative rollback boundary unexpectedly passed" >&2
+  exit 1
+fi
+
+# Publishing entry points such as tag push and workflow_dispatch may not have a
+# useful event.before. Commit the reset and prove the checker derives HEAD^ and
+# still finds the earlier cumulative value in history.
+git -C "$tmp" commit -qam reset
+if REQUIRE_ROLLBACK_BASE=true "$tmp/scripts/check-rollback-boundary.sh" >/dev/null 2>&1; then
+  echo "history-aware reset with an omitted base unexpectedly passed" >&2
+  exit 1
+fi
+
+printf '%s\n' 2.0.0 > "$tmp/ROLLBACK_UNSAFE_BEFORE"
+if REQUIRE_ROLLBACK_BASE=true "$tmp/scripts/check-rollback-boundary.sh" >/dev/null 2>&1; then
+  echo "a transient committed reset hidden by a later restore unexpectedly passed" >&2
+  exit 1
+fi
+
+if ROLLBACK_BASE_REF=not-a-commit REQUIRE_ROLLBACK_BASE=true \
+   "$tmp/scripts/check-rollback-boundary.sh" >/dev/null 2>&1; then
+  echo "an unresolvable required base unexpectedly passed" >&2
+  exit 1
+fi
+
+# Return to the clean boundary commit. The committed reset remains proven above
+# but must not contaminate the unchanged-boundary success case below.
+git -C "$tmp" restore ROLLBACK_UNSAFE_BEFORE
+git -C "$tmp" checkout -q "$boundary_sha"
+
+printf '%s\n' 2.1.0 > "$tmp/ROLLBACK_UNSAFE_BEFORE"
+if ROLLBACK_BASE_REF="$boundary_sha" "$tmp/scripts/check-rollback-boundary.sh" >/dev/null 2>&1; then
+  echo "moving a cumulative rollback boundary unexpectedly passed" >&2
+  exit 1
+fi
+
+printf '%s\n' 2.0.0 > "$tmp/ROLLBACK_UNSAFE_BEFORE"
+ROLLBACK_BASE_REF="$boundary_sha" "$tmp/scripts/check-rollback-boundary.sh" >/dev/null
+
+# Default path-history simplification can hide a boundary commit on a merged
+# side branch when the merge keeps main's `none`. Full history must still see
+# that reachable boundary and reject publishing the merge result as safe.
+merge_repo="$tmp/merge-history"
+mkdir -p "$merge_repo/scripts"
+cp "$source_root/scripts/check-rollback-boundary.sh" "$merge_repo/scripts/"
+chmod +x "$merge_repo/scripts/check-rollback-boundary.sh"
+git -C "$merge_repo" init -q -b main
+git -C "$merge_repo" config user.name Test
+git -C "$merge_repo" config user.email test@example.com
+printf '%s\n' none > "$merge_repo/ROLLBACK_UNSAFE_BEFORE"
+git -C "$merge_repo" add ROLLBACK_UNSAFE_BEFORE
+git -C "$merge_repo" commit -qm safe-root
+git -C "$merge_repo" checkout -qb boundary-side
+printf '%s\n' 2.0.0 > "$merge_repo/ROLLBACK_UNSAFE_BEFORE"
+git -C "$merge_repo" commit -qam side-boundary
+git -C "$merge_repo" checkout -q main
+printf '%s\n' unrelated > "$merge_repo/README.md"
+git -C "$merge_repo" add README.md
+git -C "$merge_repo" commit -qm main-work
+git -C "$merge_repo" merge -q --no-ff -s ours boundary-side -m merged-side-history
+if REQUIRE_ROLLBACK_BASE=true "$merge_repo/scripts/check-rollback-boundary.sh" >/dev/null 2>&1; then
+  echo "a rollback boundary hidden on a merged side branch unexpectedly passed" >&2
+  exit 1
+fi
+
+echo "rollback boundary checks: PASS"