diff --git a/crates/originweave-policy/tests/extension_mutation_isolation.rs b/crates/originweave-policy/tests/extension_mutation_isolation.rs
new file mode 100644
index 0000000..97d9b18
--- /dev/null
+++ b/crates/originweave-policy/tests/extension_mutation_isolation.rs
@@ -0,0 +1,335 @@
+#![allow(clippy::expect_used)]
+
+//! Keep extension proposal-grant evaluation separate from ordinary action policy.
+//!
+//! OriginWeave does not yet implement an adapter that converts an extension proposal into an
+//! [`ActionRequest`]. These regressions therefore prove two independent fail-closed boundaries:
+//! the exact extension/session/context grant permits only `ProposeTypedAction`, while an ordinary
+//! user-sourced action request remains subject to the core policy decision shown in each test.
+
+use std::collections::BTreeSet;
+
+use originweave_core::{
+    ActionIntentDigest, ActionKind, ActionRequest, ApprovalEvidence, BrowserSessionId,
+    BrowsingContextId, Capability, ExecutionPurpose, ExtensionAccessDecision,
+    ExtensionAccessRequest, ExtensionAgentCapability, ExtensionAgentGrant, ExtensionId,
+    InstructionSource, Origin, PolicyContext, RobotsDecision, SecretDelivery, SessionMode,
+    evaluate_extension_access,
+};
+use originweave_policy::{Decision, DenialReason, evaluate};
+
+const VALID_INTENT: &str =
+    "sha256:0123456789abcdef0123456789abcdef0123456789abcdef0123456789abcdef";
+
+fn extension_id() -> ExtensionId {
+    ExtensionId::parse("abcdefghijklmnopabcdefghijklmnop").expect("valid extension id")
+}
+
+fn browser_session() -> BrowserSessionId {
+    BrowserSessionId::new(17).expect("nonzero browser session")
+}
+
+fn browsing_context() -> BrowsingContextId {
+    BrowsingContextId::new(23).expect("nonzero browsing context")
+}
+
+fn origin(value: &str) -> Origin {
+    Origin::parse(value).expect("valid test origin")
+}
+
+fn intent() -> ActionIntentDigest {
+    ActionIntentDigest::parse(VALID_INTENT).expect("valid intent digest")
+}
+
+fn action_proposal_grant() -> ExtensionAgentGrant {
+    ExtensionAgentGrant::new(
+        extension_id(),
+        browser_session(),
+        browsing_context(),
+        [ExtensionAgentCapability::ProposeTypedAction],
+    )
+}
+
+fn assert_proposal_grant_is_independently_allowed(grant: &ExtensionAgentGrant) {
+    let request = ExtensionAccessRequest::new(
+        extension_id(),
+        browser_session(),
+        browsing_context(),
+        ExtensionAgentCapability::ProposeTypedAction,
+    );
+    assert_eq!(
+        evaluate_extension_access(&request, Some(grant)),
+        ExtensionAccessDecision::Allow
+    );
+}
+
+#[test]
+fn extension_proposal_grant_is_independent_of_cross_origin_mutation_policy() {
+    let grant = action_proposal_grant();
+    assert_proposal_grant_is_independently_allowed(&grant);
+
+    let source = origin("https://source.example");
+    let target = origin("https://target.example");
+    let context = PolicyContext::new(
+        SessionMode::AgentTask,
+        ExecutionPurpose::UserDelegatedTask,
+        BTreeSet::from([Capability::Submit]),
+        BTreeSet::from([source.clone(), target.clone()]),
+        BTreeSet::from([target.clone()]),
+        RobotsDecision::Allowed,
+        ApprovalEvidence::None,
+    );
+    let proposed = ActionRequest::new(
+        ActionKind::Submit,
+        source,
+        target,
+        InstructionSource::User,
+        SecretDelivery::None,
+        intent(),
+    );
+
+    assert_eq!(
+        evaluate(&proposed, &context),
+        Decision::Deny(DenialReason::CrossOriginMutation)
+    );
+}
+
+#[test]
+fn extension_proposal_grant_is_independent_of_write_origin_policy() {
+    let grant = action_proposal_grant();
+    assert_proposal_grant_is_independently_allowed(&grant);
+
+    let site = origin("https://app.example");
+    let context = PolicyContext::new(
+        SessionMode::AgentTask,
+        ExecutionPurpose::UserDelegatedTask,
+        BTreeSet::from([Capability::Submit]),
+        BTreeSet::from([site.clone()]),
+        BTreeSet::new(),
+        RobotsDecision::Allowed,
+        ApprovalEvidence::None,
+    );
+    let proposed = ActionRequest::new(
+        ActionKind::Submit,
+        site.clone(),
+        site,
+        InstructionSource::User,
+        SecretDelivery::None,
+        intent(),
+    );
+
+    assert_eq!(
+        evaluate(&proposed, &context),
+        Decision::Deny(DenialReason::OriginNotWritable)
+    );
+}
+
+#[test]
+fn extension_proposal_grant_is_independent_of_crawler_mutation_policy() {
+    let grant = action_proposal_grant();
+    assert_proposal_grant_is_independently_allowed(&grant);
+
+    let site = origin("https://public.example");
+    let context = PolicyContext::new(
+        SessionMode::Crawler,
+        ExecutionPurpose::PublicCrawl,
+        BTreeSet::from([Capability::Submit]),
+        BTreeSet::from([site.clone()]),
+        BTreeSet::from([site.clone()]),
+        RobotsDecision::Allowed,
+        ApprovalEvidence::None,
+    );
+    let proposed = ActionRequest::new(
+        ActionKind::Submit,
+        site.clone(),
+        site,
+        InstructionSource::User,
+        SecretDelivery::None,
+        intent(),
+    );
+
+    assert_eq!(
+        evaluate(&proposed, &context),
+        Decision::Deny(DenialReason::CrawlerMutation)
+    );
+}
+
+#[test]
+fn extension_proposal_grant_is_independent_of_mode_purpose_policy() {
+    let grant = action_proposal_grant();
+    assert_proposal_grant_is_independently_allowed(&grant);
+
+    let site = origin("https://public.example");
+    let context = PolicyContext::new(
+        SessionMode::AgentTask,
+        ExecutionPurpose::PublicCrawl,
+        BTreeSet::from([Capability::Observe]),
+        BTreeSet::from([site.clone()]),
+        BTreeSet::new(),
+        RobotsDecision::Allowed,
+        ApprovalEvidence::None,
+    );
+    let proposed = ActionRequest::new(
+        ActionKind::Observe,
+        site.clone(),
+        site,
+        InstructionSource::User,
+        SecretDelivery::None,
+        intent(),
+    );
+
+    assert_eq!(
+        evaluate(&proposed, &context),
+        Decision::Deny(DenialReason::ModePurposeMismatch)
+    );
+}
+
+#[test]
+fn extension_proposal_grant_is_independent_of_disallowed_robots_policy() {
+    let grant = action_proposal_grant();
+    assert_proposal_grant_is_independently_allowed(&grant);
+
+    let site = origin("https://public.example");
+    let context = PolicyContext::new(
+        SessionMode::Crawler,
+        ExecutionPurpose::PublicCrawl,
+        BTreeSet::from([Capability::Observe]),
+        BTreeSet::from([site.clone()]),
+        BTreeSet::new(),
+        RobotsDecision::Disallowed,
+        ApprovalEvidence::None,
+    );
+    let proposed = ActionRequest::new(
+        ActionKind::Observe,
+        site.clone(),
+        site,
+        InstructionSource::User,
+        SecretDelivery::None,
+        intent(),
+    );
+
+    assert_eq!(
+        evaluate(&proposed, &context),
+        Decision::Deny(DenialReason::RobotsDisallowed)
+    );
+}
+
+#[test]
+fn extension_proposal_grant_is_independent_of_unknown_robots_policy() {
+    let grant = action_proposal_grant();
+    assert_proposal_grant_is_independently_allowed(&grant);
+
+    let site = origin("https://public.example");
+    let context = PolicyContext::new(
+        SessionMode::Crawler,
+        ExecutionPurpose::PublicCrawl,
+        BTreeSet::from([Capability::Observe]),
+        BTreeSet::from([site.clone()]),
+        BTreeSet::new(),
+        RobotsDecision::Unknown,
+        ApprovalEvidence::None,
+    );
+    let proposed = ActionRequest::new(
+        ActionKind::Observe,
+        site.clone(),
+        site,
+        InstructionSource::User,
+        SecretDelivery::None,
+        intent(),
+    );
+
+    assert_eq!(
+        evaluate(&proposed, &context),
+        Decision::Deny(DenialReason::RobotsUnknown)
+    );
+}
+
+#[test]
+fn extension_proposal_grant_is_independent_of_missing_robots_policy() {
+    let grant = action_proposal_grant();
+    assert_proposal_grant_is_independently_allowed(&grant);
+
+    let site = origin("https://public.example");
+    let context = PolicyContext::new(
+        SessionMode::Crawler,
+        ExecutionPurpose::PublicCrawl,
+        BTreeSet::from([Capability::Observe]),
+        BTreeSet::from([site.clone()]),
+        BTreeSet::new(),
+        RobotsDecision::NotApplicable,
+        ApprovalEvidence::None,
+    );
+    let proposed = ActionRequest::new(
+        ActionKind::Observe,
+        site.clone(),
+        site,
+        InstructionSource::User,
+        SecretDelivery::None,
+        intent(),
+    );
+
+    assert_eq!(
+        evaluate(&proposed, &context),
+        Decision::Deny(DenialReason::RobotsNotApplicable)
+    );
+}
+
+#[test]
+fn extension_proposal_grant_is_independent_of_non_delegable_r5_policy() {
+    let grant = action_proposal_grant();
+    assert_proposal_grant_is_independently_allowed(&grant);
+
+    let site = origin("https://consent.example");
+    let context = PolicyContext::new(
+        SessionMode::AgentTask,
+        ExecutionPurpose::UserDelegatedTask,
+        BTreeSet::from([Capability::LegalConsent]),
+        BTreeSet::from([site.clone()]),
+        BTreeSet::from([site.clone()]),
+        RobotsDecision::Allowed,
+        ApprovalEvidence::None,
+    );
+    let proposed = ActionRequest::new(
+        ActionKind::LegalConsent,
+        site.clone(),
+        site,
+        InstructionSource::User,
+        SecretDelivery::None,
+        intent(),
+    );
+
+    assert_eq!(
+        evaluate(&proposed, &context),
+        Decision::Deny(DenialReason::ForbiddenRisk)
+    );
+}
+
+#[test]
+fn extension_proposal_grant_is_independent_of_human_mode_policy() {
+    let grant = action_proposal_grant();
+    assert_proposal_grant_is_independently_allowed(&grant);
+
+    let site = origin("https://human.example");
+    let context = PolicyContext::new(
+        SessionMode::Human,
+        ExecutionPurpose::UserDelegatedTask,
+        BTreeSet::from([Capability::Navigate]),
+        BTreeSet::from([site.clone()]),
+        BTreeSet::new(),
+        RobotsDecision::NotApplicable,
+        ApprovalEvidence::None,
+    );
+    let proposed = ActionRequest::new(
+        ActionKind::Navigate,
+        site.clone(),
+        site,
+        InstructionSource::User,
+        SecretDelivery::None,
+        intent(),
+    );
+
+    assert_eq!(
+        evaluate(&proposed, &context),
+        Decision::Deny(DenialReason::HumanModeNotAgentControlled)
+    );
+}