diff --git a/ACKNOWLEDGMENTS.md b/ACKNOWLEDGMENTS.md
index 38e184764..294289b7f 100644
--- a/ACKNOWLEDGMENTS.md
+++ b/ACKNOWLEDGMENTS.md
@@ -48,8 +48,8 @@ just composed.
 | Service | Image | Purpose | License |
 |---|---|---|---|
 | [SearXNG](https://github.com/searxng/searxng) | `searxng/searxng:2026.9.25-12f8b6515` (pinned tag; see compose) | Default metasearch backend | AGPL-3.0 |
-| [ChromaDB](https://github.com/chroma-core/chroma) | `chromadb/chroma:latest` | Vector store for memory / RAG | Apache-2.0 |
-| [ntfy](https://github.com/binwiederhier/ntfy) | `binwiederhier/ntfy` | Push notifications (self-hosted reminders) | Apache-2.0 / GPL-2.0 |
+| [ChromaDB](https://github.com/chroma-core/chroma) | `chromadb/chroma:1.5.9` (pinned digest; see compose) | Vector store for memory / RAG | Apache-2.0 |
+| [ntfy](https://github.com/binwiederhier/ntfy) | `binwiederhier/ntfy:v2.32.0` (pinned digest; see compose) | Push notifications (self-hosted reminders) | Apache-2.0 / GPL-2.0 |
 
 ## Bundled front-end libraries
 
diff --git a/docker-compose.gpu-amd.yml b/docker-compose.gpu-amd.yml
index 0e2cdbcb8..5c85014d6 100644
--- a/docker-compose.gpu-amd.yml
+++ b/docker-compose.gpu-amd.yml
@@ -127,7 +127,7 @@ services:
       - ${RENDER_GID:-render}
 
   chromadb:
-    image: docker.io/chromadb/chroma:latest
+    image: docker.io/chromadb/chroma:1.5.9@sha256:1e0b73a187a28757c572acba508c46f48c9e8b0acaf5c20e6d95cdedce1acdf6
     ports:
       - "${CHROMADB_BIND:-127.0.0.1}:8100:8000"
     volumes:
@@ -196,7 +196,7 @@ services:
     restart: unless-stopped
 
   ntfy:
-    image: docker.io/binwiederhier/ntfy
+    image: docker.io/binwiederhier/ntfy:v2.32.0@sha256:59250e1d8be2ee2186c4560322f625faef22ace380331f05b63f8842dbd62602
     command: serve
     ports:
       - "${NTFY_BIND:-127.0.0.1}:8091:80"
diff --git a/docker-compose.gpu-nvidia.yml b/docker-compose.gpu-nvidia.yml
index 5d997f28b..5c628fcfb 100644
--- a/docker-compose.gpu-nvidia.yml
+++ b/docker-compose.gpu-nvidia.yml
@@ -130,7 +130,7 @@ services:
               capabilities: [gpu]
 
   chromadb:
-    image: docker.io/chromadb/chroma:latest
+    image: docker.io/chromadb/chroma:1.5.9@sha256:1e0b73a187a28757c572acba508c46f48c9e8b0acaf5c20e6d95cdedce1acdf6
     ports:
       - "${CHROMADB_BIND:-127.0.0.1}:8100:8000"
     volumes:
@@ -199,7 +199,7 @@ services:
     restart: unless-stopped
 
   ntfy:
-    image: docker.io/binwiederhier/ntfy
+    image: docker.io/binwiederhier/ntfy:v2.32.0@sha256:59250e1d8be2ee2186c4560322f625faef22ace380331f05b63f8842dbd62602
     command: serve
     ports:
       - "${NTFY_BIND:-127.0.0.1}:8091:80"
diff --git a/docker-compose.yml b/docker-compose.yml
index 808471520..2b90227a3 100644
--- a/docker-compose.yml
+++ b/docker-compose.yml
@@ -108,7 +108,7 @@ services:
     restart: unless-stopped
 
   chromadb:
-    image: docker.io/chromadb/chroma:latest
+    image: docker.io/chromadb/chroma:1.5.9@sha256:1e0b73a187a28757c572acba508c46f48c9e8b0acaf5c20e6d95cdedce1acdf6
     ports:
       - "${CHROMADB_BIND:-127.0.0.1}:8100:8000"
     volumes:
@@ -177,7 +177,7 @@ services:
     restart: unless-stopped
 
   ntfy:
-    image: docker.io/binwiederhier/ntfy
+    image: docker.io/binwiederhier/ntfy:v2.32.0@sha256:59250e1d8be2ee2186c4560322f625faef22ace380331f05b63f8842dbd62602
     command: serve
     ports:
       - "${NTFY_BIND:-127.0.0.1}:8091:80"
diff --git a/tests/test_compose_sidecar_pins.py b/tests/test_compose_sidecar_pins.py
new file mode 100644
index 000000000..6ba9cb901
--- /dev/null
+++ b/tests/test_compose_sidecar_pins.py
@@ -0,0 +1,34 @@
+"""Sidecar images must be pinned to a versioned tag and an immutable digest.
+
+A bare or ``:latest`` reference changes underneath an existing install on the
+next ``docker compose pull``/``up``, so two hosts on the same commit can run
+different ChromaDB or ntfy builds. The ``odysseus`` service is excluded: it is
+the project's own image, selected through ``ODYSSEUS_IMAGE``. The standalone
+GPU files are held equal to these services by test_gpu_compose_standalone.py.
+"""
+
+import re
+from pathlib import Path
+
+import pytest
+import yaml
+
+COMPOSE = Path(__file__).resolve().parents[1] / "docker-compose.yml"
+PINNED = re.compile(r"^[\w.\-/]+:(?P<tag>[\w.\-]+)@sha256:[0-9a-f]{64}$")
+
+
+def _sidecars():
+    services = yaml.safe_load(COMPOSE.read_text(encoding="utf-8"))["services"]
+    return {name: svc["image"] for name, svc in services.items() if name != "odysseus"}
+
+
+def test_expected_sidecars_present():
+    assert {"chromadb", "searxng", "ntfy"} <= set(_sidecars())
+
+
+@pytest.mark.parametrize("name", sorted(_sidecars()))
+def test_sidecar_image_has_version_tag_and_digest(name):
+    image = _sidecars()[name]
+    match = PINNED.match(image)
+    assert match, f"{name}: expected <repo>:<tag>@sha256:<digest>, got {image!r}"
+    assert match.group("tag") != "latest", f"{name}: pin a release tag, not latest"