diff --git a/bootc-build/sign-and-publish/action.yml b/bootc-build/sign-and-publish/action.yml
index 009b354b..660fe693 100644
--- a/bootc-build/sign-and-publish/action.yml
+++ b/bootc-build/sign-and-publish/action.yml
@@ -30,6 +30,22 @@ inputs:
   image-name:
     description: "Short image name for SBOM file path (e.g. bluefin)"
     default: ""
+  new-bundle-format:
+    description: >
+      Whether cosign writes signatures in the Sigstore "new bundle format" (OCI 1.1
+      referrers) instead of the legacy `sha256-<digest>.sig` tag.
+
+      Also controls `--use-signing-config`, which cosign requires to agree with it:
+      `--new-bundle-format=false` with the default `--use-signing-config=true` is
+      rejected outright ("must provide --new-bundle-format or --bundle where
+      applicable with --signing-config or --use-signing-config").
+
+      MUST stay "false" for bootc/podman consumers. containers/image — which backs
+      podman, skopeo and `bootc switch` — only discovers signatures via the legacy
+      `.sig` tag, so a policy.json `sigstoreSigned` entry cannot see new-format
+      signatures. cosign 3.x flipped this default to true, which silently produced
+      images that `cosign verify` accepts but podman rejects. See projectbluefin/common#977.
+    default: "false"
   certificate-identity-regexp:
     description: >
       Regexp for cosign verify --certificate-identity-regexp. Defaults to known signing
@@ -80,18 +96,20 @@ runs:
       if: inputs.signing-mode == 'keyless'
       uses: nick-fields/retry@ad984534de44a9489a53aefd81eb77f87c70dc60 # v4
       env:
+        NEW_BUNDLE_FORMAT: ${{ inputs.new-bundle-format }}
         IMAGE: ${{ inputs.image }}
         DIGEST: ${{ inputs.digest }}
       with:
         timeout_minutes: 5
         max_attempts: 3
         retry_wait_seconds: 30
-        command: cosign sign -y "${IMAGE}@${DIGEST}"
+        command: cosign sign -y --new-bundle-format="${NEW_BUNDLE_FORMAT}" --use-signing-config="${NEW_BUNDLE_FORMAT}" "${IMAGE}@${DIGEST}"
 
     - name: Sign container image (key-based)
       if: inputs.signing-mode == 'key'
       uses: nick-fields/retry@ad984534de44a9489a53aefd81eb77f87c70dc60 # v4
       env:
+        NEW_BUNDLE_FORMAT: ${{ inputs.new-bundle-format }}
         IMAGE: ${{ inputs.image }}
         DIGEST: ${{ inputs.digest }}
         COSIGN_PRIVATE_KEY: ${{ inputs.signing-key }}
@@ -99,7 +117,7 @@ runs:
         timeout_minutes: 5
         max_attempts: 3
         retry_wait_seconds: 30
-        command: cosign sign -y --key env://COSIGN_PRIVATE_KEY "${IMAGE}@${DIGEST}"
+        command: cosign sign -y --new-bundle-format="${NEW_BUNDLE_FORMAT}" --use-signing-config="${NEW_BUNDLE_FORMAT}" --key env://COSIGN_PRIVATE_KEY "${IMAGE}@${DIGEST}"
 
     - name: Verify signature
       if: inputs.signing-mode == 'keyless'
@@ -115,6 +133,31 @@ runs:
           --certificate-oidc-issuer="https://token.actions.githubusercontent.com" \
           "${IMAGE}@${DIGEST}"
 
+    # `cosign verify` succeeds against BOTH the legacy `.sig` tag and the new
+    # OCI 1.1 bundle format, so it cannot detect a format regression on its own —
+    # that is exactly how cosign 3.x's default flip shipped months of images that
+    # verified cleanly but that podman/bootc could not validate.
+    # containers/image (podman, skopeo, `bootc switch`) resolves signatures ONLY
+    # via `sha256-<digest>.sig`, so assert that tag exists in the registry.
+    - name: Assert legacy .sig tag exists (podman/bootc compatibility)
+      if: inputs.new-bundle-format == 'false'
+      shell: bash
+      env:
+        IMAGE: ${{ inputs.image }}
+        DIGEST: ${{ inputs.digest }}
+      run: |
+        set -euo pipefail
+        sig_tag="${DIGEST/:/-}.sig"
+        if ! cosign download signature "${IMAGE}@${DIGEST}" >/dev/null 2>&1; then
+          echo "::error::No legacy signature tag ${IMAGE}:${sig_tag} in the registry."
+          echo "::error::cosign verify passed, but containers/image (podman, skopeo, bootc)"
+          echo "::error::only discovers signatures via the .sig tag, so this image would be"
+          echo "::error::rejected by a policy.json sigstoreSigned entry. Check that"
+          echo "::error::--new-bundle-format=false reached the cosign sign invocation."
+          exit 1
+        fi
+        echo "Legacy signature tag present: ${IMAGE}:${sig_tag}"
+
     - name: Install Syft
       if: inputs.generate-sbom == 'true'
       id: setup-syft
@@ -193,18 +236,20 @@ runs:
       if: inputs.generate-sbom == 'true' && inputs.signing-mode == 'keyless'
       uses: nick-fields/retry@ad984534de44a9489a53aefd81eb77f87c70dc60 # v4
       env:
+        NEW_BUNDLE_FORMAT: ${{ inputs.new-bundle-format }}
         IMAGE: ${{ inputs.image }}
         SBOM_DIGEST: ${{ steps.upload-sbom.outputs.sbom-digest }}
       with:
         timeout_minutes: 5
         max_attempts: 3
         retry_wait_seconds: 30
-        command: cosign sign -y "${IMAGE}@${SBOM_DIGEST}"
+        command: cosign sign -y --new-bundle-format="${NEW_BUNDLE_FORMAT}" --use-signing-config="${NEW_BUNDLE_FORMAT}" "${IMAGE}@${SBOM_DIGEST}"
 
     - name: Sign SBOM artifact (key-based)
       if: inputs.generate-sbom == 'true' && inputs.signing-mode == 'key'
       uses: nick-fields/retry@ad984534de44a9489a53aefd81eb77f87c70dc60 # v4
       env:
+        NEW_BUNDLE_FORMAT: ${{ inputs.new-bundle-format }}
         IMAGE: ${{ inputs.image }}
         SBOM_DIGEST: ${{ steps.upload-sbom.outputs.sbom-digest }}
         COSIGN_PRIVATE_KEY: ${{ inputs.signing-key }}
@@ -212,7 +257,7 @@ runs:
         timeout_minutes: 5
         max_attempts: 3
         retry_wait_seconds: 30
-        command: cosign sign -y --key env://COSIGN_PRIVATE_KEY "${IMAGE}@${SBOM_DIGEST}"
+        command: cosign sign -y --new-bundle-format="${NEW_BUNDLE_FORMAT}" --use-signing-config="${NEW_BUNDLE_FORMAT}" --key env://COSIGN_PRIVATE_KEY "${IMAGE}@${SBOM_DIGEST}"
 
     - name: GitHub Build Provenance Attestation
       if: inputs.push-attestation == 'true'
diff --git a/docs/skills/composite-actions/action-reference.md b/docs/skills/composite-actions/action-reference.md
index da7e2c04..99a885be 100644
--- a/docs/skills/composite-actions/action-reference.md
+++ b/docs/skills/composite-actions/action-reference.md
@@ -166,6 +166,22 @@ Two signing modes:
 - `keyless` (default): OIDC/Fulcio via `cosign sign -y`. **Requires** `id-token: write` in the calling job. Validated early — fails immediately if `ACTIONS_ID_TOKEN_REQUEST_URL` is unset.
 - `key`: `cosign sign -y --key env://COSIGN_PRIVATE_KEY`. Requires `inputs.signing-key` to be set.
 
+**Signature format — `new-bundle-format` (default `"false"`). Do not change this.**
+
+All four `cosign sign` invocations pass `--new-bundle-format=false`. cosign 3.x flipped
+this default to `true`, which writes the signature as an OCI 1.1 referrer under a
+`sha256-<digest>` tag instead of the legacy `sha256-<digest>.sig` tag.
+
+`containers/image` — the library behind podman, skopeo and `bootc switch` — discovers
+signatures **only** via the `.sig` tag. A new-format signature is therefore invisible to
+a `policy.json` `sigstoreSigned` entry: the image is signed, `cosign verify` passes, and
+podman still rejects it. Because `cosign verify` accepts both formats, nothing in CI
+catches the regression, which is why the `Assert legacy .sig tag exists` step exists —
+it queries the registry directly for the `.sig` tag and fails the build if it is absent.
+
+Only set `new-bundle-format: "true"` if every consumer of the image verifies with cosign
+rather than with podman/bootc policy.
+
 **Step order (important):** gen-sbom → GitHub SBOM attestation → ORAS attach → sign SBOM artifact → SLSA provenance attestation.
 
 **SBOM flow** (when `generate-sbom: true`): Syft generates SPDX JSON → `actions/attest` with `sbom-path` creates a GitHub-native SBOM attestation in the attestation store → ORAS attaches the same SPDX JSON as an OCI referrer artifact → cosign signs the ORAS artifact digest. Both are needed: ORAS serves OCI-native consumers; the GitHub attestation store serves `gh attestation verify` and GitHub-native consumers.