diff --git a/AGENTS.md b/AGENTS.md
index de3b4b0..83ff63d 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -146,8 +146,9 @@ into `templates/factory/code/pyproject.toml.tmpl` yourself.
   (`RECIPE_INVENTORY_SCOPE_TREE`), the Dockerfile-only one claims just what it
   emitted. Never rewrite `pyproject.toml` or `uv.lock` to make a path resolve.
 - **The runtime image is pinned.** `docker-image` rejects a bare name and
-  `:latest`. The default `codeflydev/python` companion is built by core and
-  repinned on release — prefer it.
+  `:latest`. The default `ghcr.io/codefly-dev/python` companion is built from
+  core and pinned by the digest of its multi-platform index — prefer it. Its tag
+  must equal the linked core's companion version (test-enforced).
 - **A user's `flake.nix` is never overwritten.** The embedded flake is written
   only when the source dir has none.
 - **Release needs cgo.** The binary links core's tree-sitter grammars, so
diff --git a/main.go b/main.go
index bcf76e0..97ec008 100644
--- a/main.go
+++ b/main.go
@@ -79,20 +79,39 @@ type Settings struct {
 
 	// RuntimeImage overrides the default codefly-built runtime image.
 	// Format: "name:tag". Plain "name" and ":latest" are rejected —
-	// pinning is enforced. Leave empty to use codeflydev/python:<ver>
+	// pinning is enforced. Leave empty to use ghcr.io/codefly-dev/python:<ver>
 	// (recommended; companion is rebuilt + pinned on every codefly
 	// release). Field named RuntimeImage (not DockerImage) to avoid
 	// colliding with services.Base.DockerImage(req).
 	RuntimeImage string `yaml:"docker-image"`
 }
 
-// runtimeImage is the codefly-built Python runtime companion —
-// python:3.13.1-alpine3.21 + codefly CLI + uv 0.5.29. Built from
-// core/companions/python/. Users can override via the Python settings
-// (DockerImage field) but NOT recommended — the companion image is
-// the mode-consistent default and gets the same tool set as every
-// other codefly-built image.
-var runtimeImage = &resources.DockerImage{Name: "codeflydev/python", Tag: "0.0.1"}
+// runtimeImage is the codefly-built Python runtime companion, built from
+// core/companions/python/ and published by the CLI to ghcr.io/codefly-dev as
+// a linux/amd64 + linux/arm64 image index. Users can override it through the
+// Python settings (docker-image) but should not: the companion is the
+// mode-consistent default and carries the same tool set as every other
+// codefly-built image.
+//
+// It is pinned by the digest of the multi-platform INDEX, never of one
+// platform's manifest: the build recipe is rendered for every target platform
+// and each resolves its own entry from the index. The legacy
+// docker.io/codeflydev/python:0.0.1 this replaces was a single amd64 manifest,
+// so every linux/arm64 image build ran amd64 binaries under emulation and died
+// on its first RUN ("Invalid ELF image for this architecture").
+//
+// Tag is the companion version of the core this agent links
+// (core/companions/python/info.codefly.yaml); a test holds the two together,
+// so bumping core without re-pinning here fails the build. Re-pin the digest
+// from the registry when the tag moves:
+//
+//	docker buildx imagetools inspect ghcr.io/codefly-dev/python:<tag>
+var runtimeImage = &resources.DockerImage{
+	Repository: resources.ImageRegistry,
+	Name:       "python",
+	Tag:        "0.0.5",
+	Digest:     "sha256:67d6fe825765fc30f94131ee551e3800f5b6fa4e07094b47d036904387cb75a7",
+}
 
 // uvVersion is the uv the emitted build recipe resolves its lockfile with.
 //
diff --git a/runtime_image_test.go b/runtime_image_test.go
new file mode 100644
index 0000000..abae237
--- /dev/null
+++ b/runtime_image_test.go
@@ -0,0 +1,33 @@
+package main
+
+import (
+	"context"
+	"regexp"
+	"testing"
+
+	"github.com/codefly-dev/core/companions/python"
+	"github.com/codefly-dev/core/resources"
+	"github.com/stretchr/testify/require"
+)
+
+// The default runtime image is the Python companion of the core this agent
+// links, from the registry codefly publishes to. A core bump that moves the
+// companion version fails here until the pin, and its digest, move with it.
+func TestRuntimeImageIsTheLinkedCoreCompanion(t *testing.T) {
+	companion, err := python.CompanionImage(context.Background())
+	require.NoError(t, err)
+	require.Equal(t, companion.Repository, runtimeImage.Repository)
+	require.Equal(t, companion.Name, runtimeImage.Name)
+	require.Equal(t, companion.Tag, runtimeImage.Tag,
+		"re-pin runtimeImage (tag and index digest) to core's python companion")
+}
+
+// A tag alone lets the registry move the image under a built recipe, and a
+// platform manifest digest would pin every target platform to one
+// architecture. The pin is a digest; that it names the multi-platform index is
+// what the comment on runtimeImage records and a reviewer checks.
+func TestRuntimeImageIsDigestPinned(t *testing.T) {
+	require.Regexp(t, regexp.MustCompile(`^sha256:[0-9a-f]{64}$`), runtimeImage.Digest)
+	require.Equal(t, resources.ImageRegistry+"/python@"+runtimeImage.Digest, runtimeImage.FullName(),
+		"the recipe's FROM must resolve by digest")
+}