diff --git a/.github/workflows/mutation.yml b/.github/workflows/mutation.yml
index 818255f1..dea55947 100644
--- a/.github/workflows/mutation.yml
+++ b/.github/workflows/mutation.yml
@@ -68,7 +68,7 @@ jobs:
 
           echo "Running mutation testing for changed files:"
           echo "$changed_rs"
-          cargo mutants $file_args
+          make mutation-test ARGS="$file_args"
 
       - name: Upload mutation report
         uses: actions/upload-artifact@v4
@@ -195,7 +195,7 @@ jobs:
           tool: cargo-mutants
 
       - name: Run full mutation testing
-        run: cargo mutants
+        run: make mutation-test
         # Note: Do NOT fail on low score initially (report only mode)
         continue-on-error: true
 
diff --git a/Makefile b/Makefile
index 348bfa2f..08ef928c 100644
--- a/Makefile
+++ b/Makefile
@@ -66,3 +66,7 @@ docker-build-startos:
 	cd docker && \
 	docker compose build mostro-startos
 
+mutation-test:
+	@set -o pipefail; \
+	CARGO_MUTANTS_JOBS=2 MOSTRO_TEST_LN_PORT=18080 cargo mutants $(ARGS)
+
diff --git a/src/app.rs b/src/app.rs
index 938641a6..ddc304e5 100644
--- a/src/app.rs
+++ b/src/app.rs
@@ -292,6 +292,24 @@ async fn handle_message_action(
     }
 }
 
+/// True when a rumor's `created_at` falls outside the 10s replay window.
+/// Pulled out of `accept_event` so the boundary can be hit directly instead
+/// of only through a live wrapped event.
+fn is_stale(created_at: Timestamp, since_time: u64) -> bool {
+    created_at.as_secs() < since_time
+}
+
+/// True when the inner rumor is missing a required signature. Full-privacy
+/// clients reuse the trade key as identity and send unsigned rumors; any
+/// other identity/sender split must carry a valid inner signature.
+fn missing_inner_signature(
+    identity: PublicKey,
+    sender: PublicKey,
+    signature: Option<Signature>,
+) -> bool {
+    identity != sender && signature.is_none()
+}
+
 /// Decode and fully validate one relay event into a dispatchable
 /// `(action, message, unwrapped)` triple, or `None` if it must be skipped
 /// (failed PoW, wrong kind, spam-gate drop, decrypt failure, stale, missing
@@ -375,7 +393,7 @@ async fn accept_event(
         .checked_sub_signed(chrono::Duration::seconds(10))
         .unwrap()
         .timestamp() as u64;
-    if unwrapped.created_at.as_secs() < since_time {
+    if is_stale(unwrapped.created_at, since_time) {
         return None;
     }
     let message = unwrapped.message.clone();
@@ -384,7 +402,7 @@ async fn accept_event(
     // unsigned rumors. Any other shape must carry a valid inner
     // signature — unwrap_message already verified it, so if identity
     // and sender differ here without a signature we bail out.
-    if unwrapped.identity != unwrapped.sender && unwrapped.signature.is_none() {
+    if missing_inner_signature(unwrapped.identity, unwrapped.sender, unwrapped.signature) {
         tracing::warn!(
             "Missing inner signature: identity {} differs from trade key {}",
             unwrapped.identity,
@@ -610,6 +628,46 @@ mod tests {
         }
     }
 
+    #[test]
+    fn is_stale_rejects_events_older_than_the_cutoff() {
+        assert!(is_stale(Timestamp::from(99), 100));
+    }
+
+    #[test]
+    fn is_stale_accepts_events_exactly_at_the_cutoff() {
+        assert!(!is_stale(Timestamp::from(100), 100));
+    }
+
+    #[test]
+    fn is_stale_accepts_events_newer_than_the_cutoff() {
+        assert!(!is_stale(Timestamp::from(101), 100));
+    }
+
+    #[test]
+    fn missing_inner_signature_true_when_identity_and_sender_differ_unsigned() {
+        let identity = create_test_keys().public_key();
+        let sender = create_test_keys().public_key();
+        assert!(missing_inner_signature(identity, sender, None));
+    }
+
+    #[test]
+    fn missing_inner_signature_false_when_identity_equals_sender_unsigned() {
+        let same = create_test_keys().public_key();
+        assert!(!missing_inner_signature(same, same, None));
+    }
+
+    #[test]
+    fn missing_inner_signature_false_when_signed_even_if_identity_differs() {
+        let identity = create_test_keys().public_key();
+        let sender_keys = create_test_keys();
+        let sig = sender_keys.sign_schnorr(&nostr_sdk::secp256k1::Message::from_digest([7u8; 32]));
+        assert!(!missing_inner_signature(
+            identity,
+            sender_keys.public_key(),
+            Some(sig)
+        ));
+    }
+
     #[test]
     fn test_warning_msg_all_error_types() {
         let action = Action::NewOrder;
@@ -866,6 +924,184 @@ mod tests {
         }
     }
 
+    mod accept_event_tests {
+        use super::*;
+        use crate::app::context::test_utils::{test_settings, TestContextBuilder};
+        use mostro_core::prelude::*;
+        use sqlx::SqlitePool;
+        use std::sync::Arc;
+
+        async fn create_test_ctx() -> AppContext {
+            let pool = Arc::new(SqlitePool::connect(":memory:").await.unwrap());
+            TestContextBuilder::new()
+                .with_pool(pool)
+                .with_settings(test_settings())
+                .build()
+        }
+
+        #[allow(deprecated)] // Transport::GiftWrap: still the tested default; see #786.
+        async fn wrap_test_order(
+            identity_keys: &Keys,
+            trade_keys: &Keys,
+            receiver: PublicKey,
+        ) -> Event {
+            // RestoreSession is the one action whose payload is required to
+            // be None (mostro-core's `verify()`), which keeps this helper
+            // free of a hand-built `Payload::Order`.
+            wrap_message_with(
+                Transport::GiftWrap,
+                &create_test_message(Action::RestoreSession, None),
+                identity_keys,
+                trade_keys,
+                receiver,
+                WrapOptions::default(),
+            )
+            .await
+            .expect("gift wrap succeeds")
+        }
+
+        #[tokio::test]
+        async fn accepts_a_validly_wrapped_event_and_returns_the_action() {
+            let ctx = create_test_ctx().await;
+            let identity_keys = create_test_keys();
+            let trade_keys = create_test_keys();
+            let receiver_keys = create_test_keys();
+            let event =
+                wrap_test_order(&identity_keys, &trade_keys, receiver_keys.public_key()).await;
+
+            let result = accept_event(
+                &ctx,
+                &event,
+                &receiver_keys,
+                0,
+                0,
+                NostrKind::GiftWrap,
+                false,
+            )
+            .await;
+
+            let (action, _message, unwrapped) = result.expect("valid event should be accepted");
+            assert_eq!(action, Action::RestoreSession);
+            assert_eq!(unwrapped.sender, trade_keys.public_key());
+            assert_eq!(unwrapped.identity, identity_keys.public_key());
+        }
+
+        #[tokio::test]
+        async fn rejects_an_event_of_the_wrong_kind() {
+            let ctx = create_test_ctx().await;
+            let identity_keys = create_test_keys();
+            let trade_keys = create_test_keys();
+            let receiver_keys = create_test_keys();
+            let event =
+                wrap_test_order(&identity_keys, &trade_keys, receiver_keys.public_key()).await;
+
+            // The event is a real, validly-signed GiftWrap; accepted_kind is
+            // deliberately mismatched to exercise the kind-gate on its own.
+            let result = accept_event(
+                &ctx,
+                &event,
+                &receiver_keys,
+                0,
+                0,
+                NostrKind::TextNote,
+                false,
+            )
+            .await;
+
+            assert!(result.is_none());
+        }
+
+        #[tokio::test]
+        async fn rejects_an_event_not_addressed_to_the_receiver() {
+            let ctx = create_test_ctx().await;
+            let identity_keys = create_test_keys();
+            let trade_keys = create_test_keys();
+            let intended_receiver = create_test_keys();
+            let eavesdropper = create_test_keys();
+            let event =
+                wrap_test_order(&identity_keys, &trade_keys, intended_receiver.public_key()).await;
+
+            // unwrap_incoming can't decrypt a rumor sealed for someone else.
+            let result = accept_event(
+                &ctx,
+                &event,
+                &eavesdropper,
+                0,
+                0,
+                NostrKind::GiftWrap,
+                false,
+            )
+            .await;
+
+            assert!(result.is_none());
+        }
+
+        /// Ensures a spam gate is installed for this binary run without
+        /// clobbering one another test already installed — `SPAM_GATE` is a
+        /// process-wide `OnceLock`, so at most one instance ever wins, and
+        /// every test only relies on `is_known` being false for its own
+        /// freshly-generated (never-registered) pubkey.
+        fn ensure_spam_gate_installed() {
+            let _ = crate::spam_gate::SpamGate::new(crate::spam_gate::REPLAY_WINDOW_SECS)
+                .install_global();
+        }
+
+        #[tokio::test]
+        async fn unknown_first_contact_sender_clearing_the_pow_bar_is_accepted() {
+            ensure_spam_gate_installed();
+            let ctx = create_test_ctx().await;
+            let identity_keys = create_test_keys();
+            let trade_keys = create_test_keys();
+            let receiver_keys = create_test_keys();
+            let event =
+                wrap_test_order(&identity_keys, &trade_keys, receiver_keys.public_key()).await;
+
+            // is_v2 = true routes through the spam gate; a never-seen sender
+            // pubkey is always unknown, and pow_first_contact = 0 is
+            // trivially cleared, so the first-contact lane must let it
+            // through.
+            let result = accept_event(
+                &ctx,
+                &event,
+                &receiver_keys,
+                0,
+                0,
+                NostrKind::GiftWrap,
+                true,
+            )
+            .await;
+
+            assert!(result.is_some());
+        }
+
+        #[tokio::test]
+        async fn unknown_first_contact_sender_below_the_pow_bar_is_dropped() {
+            ensure_spam_gate_installed();
+            let ctx = create_test_ctx().await;
+            let identity_keys = create_test_keys();
+            let trade_keys = create_test_keys();
+            let receiver_keys = create_test_keys();
+            let event =
+                wrap_test_order(&identity_keys, &trade_keys, receiver_keys.public_key()).await;
+
+            // pow_first_contact = u8::MAX demands more leading-zero bits than
+            // any real event id will ever have, so this deterministically
+            // fails the first-contact PoW toll.
+            let result = accept_event(
+                &ctx,
+                &event,
+                &receiver_keys,
+                0,
+                u8::MAX,
+                NostrKind::GiftWrap,
+                true,
+            )
+            .await;
+
+            assert!(result.is_none());
+        }
+    }
+
     mod handle_message_action_tests {
         use super::*;
         use crate::app::context::test_utils::{test_settings, TestContextBuilder};
diff --git a/src/lightning/invoice.rs b/src/lightning/invoice.rs
index abd4664e..f66ec1ae 100644
--- a/src/lightning/invoice.rs
+++ b/src/lightning/invoice.rs
@@ -243,7 +243,12 @@ mod tests {
             )
             .layer(tower_http::cors::CorsLayer::permissive());
 
-        let listener = TcpListener::bind("127.0.0.1:8080").await.unwrap();
+        // Must match lnurl.rs's MOSTRO_TEST_LN_PORT override (default 8080).
+        let port = std::env::var("MOSTRO_TEST_LN_PORT")
+            .ok()
+            .and_then(|v| v.parse::<u16>().ok())
+            .unwrap_or(8080);
+        let listener = TcpListener::bind(("127.0.0.1", port)).await.unwrap();
         let addr = listener.local_addr().unwrap();
         let port = addr.port();
 
diff --git a/src/lnurl.rs b/src/lnurl.rs
index c1c2175e..c93d9f84 100644
--- a/src/lnurl.rs
+++ b/src/lnurl.rs
@@ -39,7 +39,14 @@ async fn extract_lnurl(address: &str) -> Result<String, MostroError> {
             None => return Err(MostroInternalErr(ServiceError::LnAddressParseError)),
         };
         let base_url = if cfg!(test) {
-            format!("http://{domain}:8080")
+            // Overridable so test runs can dodge a port already taken on the
+            // host (e.g. cargo-mutants) without disturbing the 8080 default
+            // other tests assert on.
+            let port = std::env::var("MOSTRO_TEST_LN_PORT")
+                .ok()
+                .and_then(|v| v.parse::<u16>().ok())
+                .unwrap_or(8080);
+            format!("http://{domain}:{port}")
         } else {
             format!("https://{domain}")
         };
@@ -260,11 +267,20 @@ mod tests {
 
     #[tokio::test]
     async fn extract_lnurl_builds_wellknown_url_for_lightning_address() {
-        // cfg!(test) pins lightning addresses to the local test host form.
+        // cfg!(test) pins lightning addresses to the local test host form,
+        // on MOSTRO_TEST_LN_PORT (default 8080) so this stays correct under
+        // a port override (e.g. cargo-mutants dodging a taken 8080).
+        let port = std::env::var("MOSTRO_TEST_LN_PORT")
+            .ok()
+            .and_then(|v| v.parse::<u16>().ok())
+            .unwrap_or(8080);
         let extracted = extract_lnurl("alice@127.0.0.1")
             .await
             .expect("lightning address parses");
-        assert_eq!(extracted, "http://127.0.0.1:8080/.well-known/lnurlp/alice");
+        assert_eq!(
+            extracted,
+            format!("http://127.0.0.1:{port}/.well-known/lnurlp/alice")
+        );
     }
 
     #[tokio::test]
diff --git a/src/nip33.rs b/src/nip33.rs
index 2d0a26ad..cf55fa20 100644
--- a/src/nip33.rs
+++ b/src/nip33.rs
@@ -1117,6 +1117,31 @@ mod tests {
         );
     }
 
+    #[test]
+    fn create_event_does_not_duplicate_a_caller_supplied_expiration_tag() {
+        // Order events (kind 38383) always get an auto expiration tag from
+        // config when one isn't already present. Pre-supplying a real NIP-40
+        // `TagKind::Expiration` tag must suppress the auto-add, exercising
+        // the `||` in create_event's has_expiration_tag check on its
+        // `TagKind::Expiration` arm rather than only the `Custom("expiration")` arm.
+        init_test_settings();
+        let keys = Keys::generate();
+        let extra_tags = Tags::from_list(vec![Tag::expiration(Timestamp::from(123_456_u64))]);
+
+        let order = super::new_order_event(&keys, "", "order-id".to_string(), extra_tags)
+            .expect("order event");
+
+        let expiration_tags = order
+            .tags
+            .iter()
+            .filter(|t| matches!(t.kind(), TagKind::Expiration))
+            .count();
+        assert_eq!(
+            expiration_tags, 1,
+            "caller-supplied expiration tag must not be duplicated"
+        );
+    }
+
     // ── create_rating_tag ────────────────────────────────────────────────
 
     #[test]
diff --git a/src/spam_gate.rs b/src/spam_gate.rs
index 0de5bf46..936d00c4 100644
--- a/src/spam_gate.rs
+++ b/src/spam_gate.rs
@@ -216,20 +216,23 @@ mod tests {
 
     #[test]
     fn install_global_then_second_install_is_rejected() {
-        // The OnceLock is process-wide, so this single test owns both the
-        // first (successful) install and the AlreadyInstalled rejection.
-        let first = SpamGate::new(REPLAY_WINDOW_SECS).install_global();
-        assert!(first.is_ok(), "first install must succeed");
+        // The OnceLock is process-wide and shared with every other test in
+        // this binary (e.g. app.rs's accept_event tests also install a
+        // gate), so another test may have already won the race to install
+        // before this one runs. Only "some install eventually succeeds, and
+        // every attempt after that is rejected" is deterministic here — not
+        // which attempt is "first".
+        let _ = SpamGate::new(REPLAY_WINDOW_SECS).install_global();
         assert!(
             SpamGate::global().is_some(),
-            "global() must expose the installed gate"
+            "global() must expose an installed gate"
         );
 
         let second = SpamGate::new(REPLAY_WINDOW_SECS).install_global();
         assert_eq!(
             second,
             Err(InstallError::AlreadyInstalled),
-            "a second install must be refused, not panic"
+            "an install attempted after one already succeeded must be refused, not panic"
         );
     }