diff --git a/crates/tracedecay-agent-hosts/src/agents/claude/tests.rs b/crates/tracedecay-agent-hosts/src/agents/claude/tests.rs
index ba7d200b50..0a6f9e0b76 100644
--- a/crates/tracedecay-agent-hosts/src/agents/claude/tests.rs
+++ b/crates/tracedecay-agent-hosts/src/agents/claude/tests.rs
@@ -19,15 +19,17 @@ fn deploy_rendered_bundle(home: &Path, tracedecay_bin: &str) -> PathBuf {
     deploy_dir
 }
 
-fn copy_rendered_bundle_to_native_cache(home: &Path, tracedecay_bin: &str) {
-    let source = plugin_deploy_dir(home);
-    let cache = claude_current_cached_plugin_root(home);
-    for (relative, _) in rendered_plugin_files(tracedecay_bin, &ServeArgs::default()).unwrap() {
-        let target = cache.join(relative);
-        std::fs::create_dir_all(target.parent().unwrap()).unwrap();
-        let contents = std::fs::read(source.join(relative)).unwrap();
-        std::fs::write(target, contents).unwrap();
-    }
+fn copy_rendered_bundle_to_native_cache(
+    home: &Path,
+    tracedecay_bin: &str,
+) -> Vec<(&'static str, String)> {
+    let rendered = rendered_plugin_files(tracedecay_bin, &ServeArgs::default()).unwrap();
+    super::super::native_cache_fixture::materialize_cached_bundle(
+        &plugin_deploy_dir(home),
+        &claude_current_cached_plugin_root(home),
+        rendered.iter().map(|(relative, _)| *relative),
+    );
+    rendered
 }
 
 fn write_native_activation(home: &Path, tracedecay_bin: &str) {
@@ -159,18 +161,7 @@ fn native_cache_content_drift_and_binary_relocation_require_refresh() {
         .iter()
         .map(|(relative, _)| (*relative).to_owned())
         .collect::<Vec<_>>();
-    for (relative, expected) in rendered {
-        assert_eq!(
-            std::fs::read_to_string(source.join(relative)).unwrap(),
-            expected,
-            "relocated marketplace bytes differ at {relative}"
-        );
-        assert_eq!(
-            std::fs::read_to_string(cache.join(relative)).unwrap(),
-            expected,
-            "refreshed native cache bytes differ at {relative}"
-        );
-    }
+    super::super::native_cache_fixture::assert_rendered_bundle_bytes(&source, &cache, &rendered);
     for root in [&source, &cache] {
         let unexpected = unexpected_bundle_entrypoints(
             root,
diff --git a/crates/tracedecay-agent-hosts/src/agents/codex/tests.rs b/crates/tracedecay-agent-hosts/src/agents/codex/tests.rs
index 57a8a22305..a8d4df9372 100644
--- a/crates/tracedecay-agent-hosts/src/agents/codex/tests.rs
+++ b/crates/tracedecay-agent-hosts/src/agents/codex/tests.rs
@@ -762,16 +762,17 @@ fn install_codex_personal_bootstrap(home: &Path, tracedecay_bin: &str) -> Result
     Ok(install_dir)
 }
 
-fn copy_rendered_bundle_to_native_cache(home: &Path, tracedecay_bin: &str) {
-    let source = codex_plugin_install_dir(home);
-    let cache = codex_plugin_current_cached_install_dir(home);
-    for (relative, _) in
-        rendered_global_plugin_files(tracedecay_bin, &ServeArgs::default()).unwrap()
-    {
-        let target = cache.join(relative);
-        std::fs::create_dir_all(target.parent().unwrap()).unwrap();
-        std::fs::copy(source.join(relative), target).unwrap();
-    }
+fn copy_rendered_bundle_to_native_cache(
+    home: &Path,
+    tracedecay_bin: &str,
+) -> Vec<(&'static str, String)> {
+    let rendered = rendered_global_plugin_files(tracedecay_bin, &ServeArgs::default()).unwrap();
+    super::super::native_cache_fixture::materialize_cached_bundle(
+        &codex_plugin_install_dir(home),
+        &codex_plugin_current_cached_install_dir(home),
+        rendered.iter().map(|(relative, _)| *relative),
+    );
+    rendered
 }
 
 fn write_exact_native_activation(home: &Path, tracedecay_bin: &str) {
@@ -1048,7 +1049,12 @@ fn native_cache_content_drift_and_binary_relocation_require_refresh() {
         "binary relocation must require refresh: {}",
         native_activation_diagnostics(profile, home.path(), new_bin),
     );
-    copy_rendered_bundle_to_native_cache(home.path(), new_bin);
+    let rendered = copy_rendered_bundle_to_native_cache(home.path(), new_bin);
+    super::super::native_cache_fixture::assert_rendered_bundle_bytes(
+        &codex_plugin_install_dir(home.path()),
+        &codex_plugin_current_cached_install_dir(home.path()),
+        &rendered,
+    );
     assert!(
         codex_plugin_is_natively_active(
             profile,
@@ -1061,6 +1067,69 @@ fn native_cache_content_drift_and_binary_relocation_require_refresh() {
     );
 }
 
+#[test]
+fn native_cache_same_size_unchanged_mtime_content_drift_deactivates() {
+    let home = tempfile::tempdir().unwrap();
+    let profile = &tracedecay_runtime_core::config::ProfileRoot::under_home(home.path());
+    write_exact_native_activation(home.path(), TEST_BIN);
+    assert!(
+        codex_plugin_is_natively_active(
+            profile,
+            home.path(),
+            Some((TEST_BIN, &ServeArgs::default()))
+        )
+        .unwrap(),
+        "initial native activation: {}",
+        native_activation_diagnostics(profile, home.path(), TEST_BIN),
+    );
+
+    let rendered = rendered_global_plugin_files(TEST_BIN, &ServeArgs::default()).unwrap();
+    let relative = rendered
+        .iter()
+        .find(|(relative, _)| relative.starts_with("skills/"))
+        .map(|(relative, _)| *relative)
+        .expect("the Codex bundle ships at least one skill file");
+    let cached = codex_plugin_current_cached_install_dir(home.path()).join(relative);
+    let original = std::fs::read(&cached).unwrap();
+    let modified = std::fs::metadata(&cached).unwrap().modified().unwrap();
+    let mut drifted = original.clone();
+    let letter = drifted
+        .iter()
+        .rposition(u8::is_ascii_alphabetic)
+        .expect("a rendered skill file contains letters");
+    drifted[letter] = if drifted[letter] == b'a' { b'b' } else { b'a' };
+    assert_eq!(
+        drifted.len(),
+        original.len(),
+        "the drift must keep {relative} the same size"
+    );
+    assert_ne!(drifted, original);
+
+    std::fs::write(&cached, &drifted).unwrap();
+    std::fs::OpenOptions::new()
+        .write(true)
+        .open(&cached)
+        .unwrap()
+        .set_modified(modified)
+        .unwrap();
+    assert_eq!(
+        std::fs::metadata(&cached).unwrap().modified().unwrap(),
+        modified,
+        "the drift must keep {relative}'s mtime"
+    );
+
+    assert!(
+        !codex_plugin_is_natively_active(
+            profile,
+            home.path(),
+            Some((TEST_BIN, &ServeArgs::default()))
+        )
+        .unwrap(),
+        "a same-size byte change with an unchanged mtime must deactivate: {}",
+        native_activation_diagnostics(profile, home.path(), TEST_BIN),
+    );
+}
+
 #[test]
 fn redeploy_preserves_foreign_discovery_and_support_bytes() {
     let home = tempfile::tempdir().unwrap();
diff --git a/crates/tracedecay-agent-hosts/src/agents/mod.rs b/crates/tracedecay-agent-hosts/src/agents/mod.rs
index 28e807c12e..a831884907 100644
--- a/crates/tracedecay-agent-hosts/src/agents/mod.rs
+++ b/crates/tracedecay-agent-hosts/src/agents/mod.rs
@@ -28,6 +28,9 @@ pub mod kilo;
 pub mod kimi;
 pub mod kiro;
 mod mcp_registration;
+#[cfg(test)]
+#[allow(clippy::unwrap_used, clippy::expect_used)]
+mod native_cache_fixture;
 pub mod opencode;
 pub mod pi;
 pub mod plugin_bundle;
diff --git a/crates/tracedecay-agent-hosts/src/agents/native_cache_fixture.rs b/crates/tracedecay-agent-hosts/src/agents/native_cache_fixture.rs
new file mode 100644
index 0000000000..b2de81ea9b
--- /dev/null
+++ b/crates/tracedecay-agent-hosts/src/agents/native_cache_fixture.rs
@@ -0,0 +1,59 @@
+//! Byte-level fixture for a host's native plugin cache.
+//!
+//! Claude Code and Codex each copy the rendered plugin source into their own
+//! versioned cache, and TraceDecay's activation probes decide whether the host
+//! loads the plugin by comparing those cached bytes against the rendered
+//! bundle. Fixtures have to reproduce the host's copy as a plain byte copy.
+
+use std::path::Path;
+
+use tracedecay_domain::canonical_text::sha256_hex;
+
+/// Materialize the rendered `relatives` from `source` into the host's native
+/// `cache` root.
+///
+/// Deliberately reads and writes bytes instead of [`std::fs::copy`]. On Linux
+/// `fs::copy` takes the kernel copy-offload path (`copy_file_range` /
+/// `sendfile`), and the Linux CI filesystem has served a same-length cache file
+/// whose bytes were not the source's for a bundle the installer had just
+/// republished; macOS copies through `fcopyfile`, so the same fixture passed
+/// locally. The activation probes compare content digests, so only a byte copy
+/// is a faithful stand-in for the host's own copy.
+pub(crate) fn materialize_cached_bundle<'a>(
+    source: &Path,
+    cache: &Path,
+    relatives: impl IntoIterator<Item = &'a str>,
+) {
+    for relative in relatives {
+        let target = cache.join(relative);
+        std::fs::create_dir_all(target.parent().unwrap()).unwrap();
+        let contents = std::fs::read(source.join(relative)).unwrap();
+        std::fs::write(&target, &contents).unwrap();
+    }
+}
+
+/// Assert `source` and `cache` both hold exactly the bytes `rendered` declares.
+///
+/// A host-cache refresh is a content contract: the fixture must not leave a
+/// same-length file with different bytes behind, and a divergence has to name
+/// the file and both digests instead of only failing the activation probe.
+pub(crate) fn assert_rendered_bundle_bytes(
+    source: &Path,
+    cache: &Path,
+    rendered: &[(&str, String)],
+) {
+    for (relative, expected) in rendered {
+        for (label, root) in [("source", source), ("cache", cache)] {
+            let observed = std::fs::read(root.join(relative)).unwrap();
+            assert!(
+                observed == expected.as_bytes(),
+                "{label}/{relative}: refreshed bundle is not byte-exact; \
+                 actual(len={} sha256={}), expected(len={} sha256={})",
+                observed.len(),
+                sha256_hex(&observed),
+                expected.len(),
+                sha256_hex(expected.as_bytes()),
+            );
+        }
+    }
+}