diff --git a/module-ci-action/README.md b/module-ci-action/README.md
index d71ce3f..ea5f446 100644
--- a/module-ci-action/README.md
+++ b/module-ci-action/README.md
@@ -10,7 +10,7 @@ triggering event:
 | Event | Mode | What it does |
 |-------|------|--------------|
 | `pull_request` opened / synchronize / reopened | **preview** | Creates each changed output type the Control Plane does not have yet, then validates each changed module and registers a **feature-branch preview** pinned to the PR head commit. Optionally upserts a PR comment. |
-| `push` (to your default branch) | **publish** | Applies each changed output type, then uploads each changed module and **publishes** it (PREVIEW → PUBLISHED). |
+| `push` (to your default branch) | **publish** | Applies each changed output type, then uploads each changed module and **publishes** it (PREVIEW → PUBLISHED), declaring compatibility from the registry state — see **Compatibility declarations** below. |
 | `pull_request` **closed without merge** | **cleanup** | Deletes the preview each changed module owns — **only if** the preview belongs to a commit from this PR. |
 | `pull_request` **closed by merge** | **no-op** | Skipped: the concurrent `push` (publish) run re-uploads and publishes the module at the merge commit, so it owns the slot. Running cleanup here would be redundant and could race the publish. |
 
@@ -50,6 +50,30 @@ committed.
 If your trees are not at the repo root, set `path-prefix` (e.g. `infra/` when they
 live at `infra/modules/...` and `infra/outputs/...`).
 
+## Compatibility declarations
+
+`raptor publish` refuses to publish over an already-published version without an
+explicit `--backward-compatible yes`, and refuses a new contract version without
+`--versionupgrade` (raptor ≥ v0.1.106). A bare publish therefore fails for every
+module a repository has already shipped.
+
+In a repo-first flow the author has already declared, in git:
+
+| What the author did | What CI passes |
+|---|---|
+| Edited `modules/{intent}/{flavor}/0.2` in place | `--backward-compatible yes` |
+| Added a new `modules/{intent}/{flavor}/0.3` directory | `--versionupgrade` |
+| Added the first module of a type/flavor | nothing |
+
+The action reads the published versions of that `intent/flavor` back off the
+Control Plane and passes the matching flag. Version **ordering** stays raptor's
+job: a "bump" that is not actually greater than every published version is
+rejected there, and should be.
+
+If the listing cannot be read, the module fails rather than publishing with a
+guessed declaration — reading a failed count as "nothing is published" would send
+a first-publication for a module that already has one.
+
 ## Output types
 
 A module names its output type by reference (`@namespace/name`). The Control Plane
diff --git a/module-ci-action/action.yml b/module-ci-action/action.yml
index 3769d19..6baf181 100644
--- a/module-ci-action/action.yml
+++ b/module-ci-action/action.yml
@@ -584,8 +584,52 @@ runs:
             echo "::endgroup::"; continue
           fi
 
+          # raptor refuses to publish over an already-published version without an
+          # explicit compatibility declaration, and refuses a NEW version without
+          # --versionupgrade. A bare publish therefore fails for every module a
+          # repository has already shipped.
+          #
+          # In a repo-first flow the author HAS declared, in git: editing
+          # modules/{intent}/{flavor}/0.2 in place says "compatible"; adding a 0.3
+          # directory says "breaking". Read that choice back off the registry and
+          # pass the matching flag. Version ORDERING stays raptor's job — a bump
+          # that is not actually greater is rejected there, and must be.
+          if ! MODULES_JSON="$(raptor get iac-module -o json 2>/dev/null)"; then
+            echo "::error::Could not list modules to determine the compatibility declaration for ${REF}"
+            echo "${REF} (publish)" >> "$FAILURES"
+            echo "::endgroup::"; continue
+          fi
+          family() {
+            printf '%s' "$MODULES_JSON" | jq -r --arg t "$TYPE" --arg f "$FLAVOR" --arg v "$1" \
+              '(if type=="array" then . else [] end)
+               | map(select(.intent==$t and .flavor==$f and .stage=="PUBLISHED"
+                            and ($v=="" or .version==$v)))
+               | length' 2>/dev/null || echo ""
+          }
+          PUBLISHED_ANY="$(family "")"
+          PUBLISHED_TARGET="$(family "$VERSION")"
+          if [ -z "$PUBLISHED_ANY" ] || [ -z "$PUBLISHED_TARGET" ]; then
+            # A failed count is not "nothing is published" — that reading would send
+            # a first-publication (no flag) for a module that already has one.
+            echo "::error::Could not read published versions of ${TYPE}/${FLAVOR} for ${REF}"
+            echo "${REF} (publish)" >> "$FAILURES"
+            echo "::endgroup::"; continue
+          fi
+
+          DECLARATION=""
+          if [ "$PUBLISHED_TARGET" != "0" ]; then
+            DECLARATION="--backward-compatible yes"
+            echo "${VERSION} is already published: declaring a compatible update."
+          elif [ "$PUBLISHED_ANY" != "0" ]; then
+            DECLARATION="--versionupgrade"
+            echo "${VERSION} is a new contract version for ${TYPE}/${FLAVOR}: declaring a version upgrade."
+          else
+            echo "First publication of ${TYPE}/${FLAVOR}: no declaration needed."
+          fi
+
           echo "Publishing ${REF}..."
-          if ! raptor publish iac-module "$REF"; then
+          # shellcheck disable=SC2086  # $DECLARATION is a deliberate empty-or-flag word list
+          if ! raptor publish iac-module "$REF" $DECLARATION; then
             echo "::error::Publish failed for ${REF}"
             echo "${REF} (publish)" >> "$FAILURES"
             echo "::endgroup::"; continue