diff --git a/CHANGELOG.md b/CHANGELOG.md
index d1741992..485c48bb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,7 @@ All notable changes to OriginWeave are documented in this file. The format follo
 
 ### Added
 
+- Added a fail-closed Chrome-permission separation boundary so reviewed Manifest V3 compatibility permissions, including `downloads` and `nativeMessaging`, can never mint any OriginWeave Agent action authority.
 - Bound explicit extension-to-Agent grants to exclusive trusted-time expiry in addition to extension identity, session, browsing context, and canonical origin, so a same-origin grant cannot be reused at or after the deadline.
 - Bound explicit extension-to-Agent grants to the exact canonical origin in addition to extension identity, session, and browsing context, so a same-session navigation or port change cannot reuse the grant.
 - Rust workspace for independently reusable core, policy, destination, network, TLS, resource, and evidence modules.
diff --git a/crates/originweave-core/Cargo.toml b/crates/originweave-core/Cargo.toml
index 35c83b19..15bd2316 100644
--- a/crates/originweave-core/Cargo.toml
+++ b/crates/originweave-core/Cargo.toml
@@ -10,6 +10,9 @@ repository.workspace = true
 homepage.workspace = true
 publish = false
 
+[lib]
+path = "src/crate_root.rs"
+
 [dependencies]
 
 [lints]
diff --git a/crates/originweave-core/src/chrome_permission_authority.rs b/crates/originweave-core/src/chrome_permission_authority.rs
new file mode 100644
index 00000000..264c45f1
--- /dev/null
+++ b/crates/originweave-core/src/chrome_permission_authority.rs
@@ -0,0 +1,73 @@
+//! Separation between Chrome extension compatibility permissions and Agent authority.
+
+use crate::ActionKind;
+use std::fmt;
+
+/// Why a Chrome extension permission cannot authorize an OriginWeave Agent action.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum ChromePermissionAuthorityError {
+    /// The permission names a reviewed Chrome compatibility surface, not Agent authority.
+    CompatibilitySurfaceOnly,
+    /// The permission is not a reviewed Chrome surface and still grants no Agent capability.
+    UnrecognizedPermission,
+}
+
+impl fmt::Display for ChromePermissionAuthorityError {
+    fn fmt(&self, formatter: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let message = match self {
+            Self::CompatibilitySurfaceOnly => {
+                "Chrome compatibility permission cannot authorize an OriginWeave Agent action"
+            }
+            Self::UnrecognizedPermission => {
+                "Chrome permission is not a reviewed compatibility surface and cannot authorize an OriginWeave Agent action"
+            }
+        };
+        formatter.write_str(message)
+    }
+}
+
+impl std::error::Error for ChromePermissionAuthorityError {}
+
+const REVIEWED_CHROME_COMPATIBILITY_PERMISSIONS: &[&str] = &[
+    "bookmarks",
+    "commands",
+    "declarativeNetRequest",
+    "declarativeNetRequestWithHostAccess",
+    "downloads",
+    "history",
+    "nativeMessaging",
+    "scripting",
+    "sidePanel",
+    "storage",
+    "tabs",
+    "windows",
+];
+
+/// Refuse to treat a Chrome extension permission as OriginWeave Agent authority.
+///
+/// A successful Chrome compatibility proof never becomes an OriginWeave Agent
+/// capability. Adapters must keep browser compatibility evidence and explicit
+/// OriginWeave grants separate and call this boundary before exposing a typed
+/// action to policy. The action is accepted only to make that separation
+/// explicit at the adapter boundary; no action kind can make this function
+/// return success.
+pub fn chrome_permission_authorizes_agent_action(
+    permission: &str,
+    _action: ActionKind,
+) -> Result<(), ChromePermissionAuthorityError> {
+    if !is_exact_chrome_permission_token(permission) {
+        return Err(ChromePermissionAuthorityError::UnrecognizedPermission);
+    }
+    if REVIEWED_CHROME_COMPATIBILITY_PERMISSIONS.contains(&permission) {
+        return Err(ChromePermissionAuthorityError::CompatibilitySurfaceOnly);
+    }
+    Err(ChromePermissionAuthorityError::UnrecognizedPermission)
+}
+
+fn is_exact_chrome_permission_token(permission: &str) -> bool {
+    let mut characters = permission.chars();
+    let Some(first) = characters.next() else {
+        return false;
+    };
+    first.is_ascii_lowercase() && characters.all(|character| character.is_ascii_alphabetic())
+}
diff --git a/crates/originweave-core/src/crate_root.rs b/crates/originweave-core/src/crate_root.rs
new file mode 100644
index 00000000..399e21e5
--- /dev/null
+++ b/crates/originweave-core/src/crate_root.rs
@@ -0,0 +1,17 @@
+//! OriginWeave core contracts plus narrowly scoped adapter authority boundaries.
+//!
+//! The existing deterministic core remains implemented in `lib.rs`; this crate
+//! root re-exports that protected-main API and adds the independently reviewed
+//! Chrome-permission separation boundary without weakening existing authority.
+
+#![forbid(unsafe_code)]
+#![deny(missing_docs)]
+
+#[path = "lib.rs"]
+mod base;
+pub use base::*;
+
+mod chrome_permission_authority;
+pub use chrome_permission_authority::{
+    ChromePermissionAuthorityError, chrome_permission_authorizes_agent_action,
+};
diff --git a/crates/originweave-core/tests/chrome_permission_authority.rs b/crates/originweave-core/tests/chrome_permission_authority.rs
new file mode 100644
index 00000000..c78bf733
--- /dev/null
+++ b/crates/originweave-core/tests/chrome_permission_authority.rs
@@ -0,0 +1,61 @@
+use originweave_core::{
+    ActionKind, ChromePermissionAuthorityError, chrome_permission_authorizes_agent_action,
+};
+
+#[test]
+fn chrome_compatibility_permissions_never_mint_agent_authority() {
+    for permission in [
+        "downloads",
+        "bookmarks",
+        "history",
+        "storage",
+        "tabs",
+        "windows",
+        "scripting",
+        "commands",
+        "sidePanel",
+        "declarativeNetRequest",
+        "declarativeNetRequestWithHostAccess",
+        "nativeMessaging",
+    ] {
+        assert_eq!(
+            chrome_permission_authorizes_agent_action(permission, ActionKind::Download),
+            Err(ChromePermissionAuthorityError::CompatibilitySurfaceOnly)
+        );
+    }
+}
+
+#[test]
+fn malformed_or_unreviewed_chrome_permissions_remain_unrecognized() {
+    for permission in [
+        "",
+        "DOWNLOADS",
+        "downloads\nhttps://example.invalid",
+        "cookies",
+        "downloads ",
+    ] {
+        assert_eq!(
+            chrome_permission_authorizes_agent_action(permission, ActionKind::Download),
+            Err(ChromePermissionAuthorityError::UnrecognizedPermission)
+        );
+    }
+}
+
+#[test]
+fn chrome_permission_authority_errors_are_standard_credential_safe_errors() {
+    let cases = [
+        (
+            ChromePermissionAuthorityError::CompatibilitySurfaceOnly,
+            "Chrome compatibility permission cannot authorize an OriginWeave Agent action",
+        ),
+        (
+            ChromePermissionAuthorityError::UnrecognizedPermission,
+            "Chrome permission is not a reviewed compatibility surface and cannot authorize an OriginWeave Agent action",
+        ),
+    ];
+
+    for (error, expected_message) in cases {
+        assert_eq!(error.to_string(), expected_message);
+        assert!(std::error::Error::source(&error).is_none());
+    }
+}