diff --git a/sandbox/SPEC.md b/sandbox/SPEC.md
index 4f9e4025d..a0cd42afc 100644
--- a/sandbox/SPEC.md
+++ b/sandbox/SPEC.md
@@ -386,7 +386,7 @@ Long-horizon work outlives the sandbox that started it. A host that suspends and
 
 ## 14. The environment manifest
 
-A durable run is only the same run if it resumes into the same toolchain — checkable by hash, not asserted by trust. The **environment manifest** is one canonical, content-addressed object standing for the declared environment: base image (pinned by digest where honest claims require it), tool features, environment variables and mounts. Declarations are ingested from the open `devcontainer.json` format — its **declarative subset only**: fields that execute commands or widen the sandbox (`build`, the lifecycle `*Command` hooks, `privileged`, …) are refused, and unknown fields are refused rather than silently dropped. Networking fields are excluded from identity: the network surface belongs to the grant (§5). The manifest is JCS-canonicalized, BLAKE3-addressed, Ed25519-signed (§7.3 convention); `SnapshotRef.manifest` (§13) records it at suspension and resumption **MUST** be refused on a drifted address. Conformance class **C7**. See [`spec/environment-manifest.md`](spec/environment-manifest.md); the reference `sandbox-attest::manifest` implements it.
+A durable run is only the same run if it resumes into the same toolchain — checkable by hash, not asserted by trust. The **environment manifest** is one canonical, content-addressed object standing for the declared environment: base image and tool features (each pinned by digest where honest claims require it: a manifest is pinned only when both are), environment variables and mounts. Declarations are ingested from the open `devcontainer.json` format — its **declarative subset only**: fields that execute commands or widen the sandbox (`build`, the lifecycle `*Command` hooks, `privileged`, …) are refused, and unknown fields are refused rather than silently dropped. Networking fields are excluded from identity: the network surface belongs to the grant (§5). The manifest is JCS-canonicalized, BLAKE3-addressed, Ed25519-signed (§7.3 convention); `SnapshotRef.manifest` (§13) records it at suspension and resumption **MUST** be refused on a drifted address. Conformance class **C7**. See [`spec/environment-manifest.md`](spec/environment-manifest.md); the reference `sandbox-attest::manifest` implements it.
 
 ## 15. Warm pools
 
diff --git a/sandbox/sandbox-rs/crates/sandbox-attest/src/manifest.rs b/sandbox/sandbox-rs/crates/sandbox-attest/src/manifest.rs
index ae2f4898b..e18a08d5e 100644
--- a/sandbox/sandbox-rs/crates/sandbox-attest/src/manifest.rs
+++ b/sandbox/sandbox-rs/crates/sandbox-attest/src/manifest.rs
@@ -29,12 +29,22 @@ use crate::jcs;
 /// One tool feature (the devcontainer `features` table, normalized).
 #[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
 pub struct Feature {
-    /// The feature reference as declared (e.g. `ghcr.io/devcontainers/features/rust:1`).
+    /// The feature reference as declared. A reference with a digest
+    /// (`ghcr.io/devcontainers/features/rust@sha256:…`) names fixed bytes; a
+    /// tag (`ghcr.io/devcontainers/features/rust:1`) floats, and resolves to
+    /// whatever its publisher last pushed under it.
     pub id: String,
     /// The feature's options, JCS-canonicalized JSON text ("" when empty).
     pub options: String,
 }
 
+impl Feature {
+    /// Whether the reference is addressed by digest, and so names fixed bytes.
+    pub fn is_pinned(&self) -> bool {
+        self.id.contains("@sha256:")
+    }
+}
+
 /// One mount, normalized.
 #[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
 pub struct Mount {
@@ -190,9 +200,22 @@ impl EnvironmentManifest {
         Ok(EnvironmentManifest { image, image_digest, features, env, mounts, remote_user })
     }
 
-    /// Whether the environment is fully pinned (digest-addressed base image).
-    /// Only a pinned manifest can honestly claim "same address ⇒ same bytes".
+    /// Whether the toolchain is fully pinned: the base image AND every
+    /// feature are addressed by digest. Only then does "same address ⇒ same
+    /// bytes" hold. A pinned image with a feature declared by tag (`rust:1`)
+    /// is not pinned: the tag floats, so two hosts can resolve one address to
+    /// different toolchains.
+    ///
+    /// Mounted content is data, not toolchain, and is never part of the
+    /// address: a pinned manifest fixes what the unit runs with, not what its
+    /// mounts hold.
     pub fn is_pinned(&self) -> bool {
+        self.image_pinned() && self.features.iter().all(Feature::is_pinned)
+    }
+
+    /// Whether the base image alone is addressed by digest. Necessary for
+    /// [`EnvironmentManifest::is_pinned`], not sufficient.
+    pub fn image_pinned(&self) -> bool {
         self.image_digest.is_some()
     }
 
@@ -311,7 +334,9 @@ mod tests {
     #[test]
     fn ingest_canonicalize_address() {
         let m = EnvironmentManifest::from_devcontainer(SAMPLE).unwrap();
-        assert!(m.is_pinned());
+        // The image is digest-addressed, but `rust:1` and `node:1` float.
+        assert!(m.image_pinned());
+        assert!(!m.is_pinned());
         assert_eq!(m.env, vec![("A".into(), "1".into()), ("B".into(), "2".into())]);
         assert_eq!(m.features.len(), 2);
         assert_eq!(m.features[0].id, "ghcr.io/devcontainers/features/node:1");
@@ -351,9 +376,33 @@ mod tests {
     #[test]
     fn unpinned_image_is_marked() {
         let m = EnvironmentManifest::from_devcontainer(r#"{"image": "ubuntu:24.04"}"#).unwrap();
+        assert!(!m.image_pinned());
         assert!(!m.is_pinned());
     }
 
+    /// Pinned means the whole toolchain: a digest image with digest features
+    /// is pinned; one feature by tag beside them is not; and a mount, whose
+    /// content is never addressed, does not change either answer.
+    #[test]
+    fn pinned_means_every_feature_is_digest_addressed() {
+        let doc = |rust: &str| {
+            format!(
+                r#"{{"image": "ghcr.io/acme/builder@sha256:0011aabb",
+                    "features": {{"{rust}": {{}},
+                                  "ghcr.io/devcontainers/features/node@sha256:ccdd": {{}}}},
+                    "mounts": ["source=cache,target=/cache,type=volume"]}}"#
+            )
+        };
+        let pinned = EnvironmentManifest::from_devcontainer(&doc("ghcr.io/devcontainers/features/rust@sha256:eeff")).unwrap();
+        assert!(pinned.features.iter().all(Feature::is_pinned));
+        assert!(pinned.is_pinned());
+        let floating = EnvironmentManifest::from_devcontainer(&doc("ghcr.io/devcontainers/features/rust:1")).unwrap();
+        assert!(floating.image_pinned());
+        assert!(!floating.is_pinned(), "a feature by tag leaves the toolchain unpinned");
+        let no_features = EnvironmentManifest::from_devcontainer(r#"{"image": "ghcr.io/acme/builder@sha256:0011aabb"}"#).unwrap();
+        assert!(no_features.is_pinned());
+    }
+
     #[test]
     fn sign_verify_and_tamper() {
         let m = EnvironmentManifest::from_devcontainer(SAMPLE).unwrap();
diff --git a/sandbox/sandbox-rs/crates/sandbox-conformance/src/lib.rs b/sandbox/sandbox-rs/crates/sandbox-conformance/src/lib.rs
index e32d84af4..a44ef5b26 100644
--- a/sandbox/sandbox-rs/crates/sandbox-conformance/src/lib.rs
+++ b/sandbox/sandbox-rs/crates/sandbox-conformance/src/lib.rs
@@ -286,7 +286,15 @@ fn c7() -> Result<(), String> {
         "name": "cosmetic", "forwardPorts": [3000]
     }"#;
     let m = EnvironmentManifest::from_devcontainer(doc).map_err(|e| format!("{e:?}"))?;
-    want(m.is_pinned(), "digest-addressed image is pinned")?;
+    want(m.image_pinned(), "digest-addressed image is pinned")?;
+    // The image is pinned, but `rust:1` floats: the toolchain is not.
+    want(!m.is_pinned(), "a feature declared by tag leaves the manifest unpinned")?;
+    let pinned_doc = doc.replace(
+        "ghcr.io/devcontainers/features/rust:1",
+        "ghcr.io/devcontainers/features/rust@sha256:eeff",
+    );
+    let pinned = EnvironmentManifest::from_devcontainer(&pinned_doc).map_err(|e| format!("{e:?}"))?;
+    want(pinned.is_pinned(), "digest image and digest features are pinned")?;
     let id = m.content_id().map_err(|e| format!("{e:?}"))?;
     // Determinism: key order / whitespace must not change the address.
     let shuffled = doc.replace("\"B\": \"2\", \"A\": \"1\"", "\"A\":\"1\",\"B\":\"2\"");
diff --git a/sandbox/spec/environment-manifest.md b/sandbox/spec/environment-manifest.md
index 6fd2ac8e4..754a7c750 100644
--- a/sandbox/spec/environment-manifest.md
+++ b/sandbox/spec/environment-manifest.md
@@ -18,8 +18,8 @@ industry. This standard ingests its **declarative subset**:
 
 | accepted | meaning |
 |---|---|
-| `image` | base image reference; a `@sha256:` digest marks the manifest **pinned** |
-| `features` | tool features, normalized to (id, JCS-canonical options), sorted |
+| `image` | base image reference; a `@sha256:` digest pins the image |
+| `features` | tool features, normalized to (id, JCS-canonical options), sorted; a reference with a `@sha256:` digest is pinned, one by tag (`:1`) floats |
 | `containerEnv` | environment variables, sorted |
 | `mounts` | string or object form, normalized to (source, target, kind), sorted by target |
 | `remoteUser` | the user the unit runs as |
@@ -45,10 +45,17 @@ its BLAKE3 address is the environment's identity; it is signed Ed25519 over
 the canonical bytes, mirroring the attestation convention (§7.3). Key order
 and whitespace in the source document do not affect the address.
 
-A manifest without an image digest is **unpinned**: its address identifies a
-declaration, not bytes. Hosts SHOULD treat unpinned manifests the way the
-energy receipt treats estimates — usable, but labelled, and never the basis
-for a "same environment" claim across hosts.
+A manifest is **pinned** only when its image and every feature are addressed
+by digest. Otherwise it is **unpinned**: its address identifies a
+declaration, not bytes — a tag such as `rust:1` resolves to whatever its
+publisher last pushed, so a pinned image beside a feature by tag does not fix
+the toolchain. Hosts SHOULD treat unpinned manifests the way the energy
+receipt treats estimates — usable, but labelled, and never the basis for a
+"same environment" claim across hosts.
+
+Mounted content is data, not toolchain, and is never part of the address. A
+pinned manifest fixes what a unit runs with; it says nothing about what its
+mounts hold.
 
 ## Use in durable runs