diff --git a/changelog.json b/changelog.json
index df0b80a..7d71afd 100644
--- a/changelog.json
+++ b/changelog.json
@@ -5,12 +5,14 @@
     "en": [
       "An entity that is a single island group now fills the IOTA reference on its own — no callbook subscription needed.",
       "CAT sharing can now speak TCI instead of Hamlib, so a TCI-only program reaches whatever radio OpsLog is on.",
-      "Lookup cache: a TTL of 0 switches it off, so a callbook record you are correcting is re-read every time."
+      "Lookup cache: a TTL of 0 switches it off, so a callbook record you are correcting is re-read every time.",
+      "TCI: when the radio forbids transmitting, PTT now says so instead of doing nothing silently."
     ],
     "fr": [
       "Une entité qui est un seul groupe d’îles remplit désormais la référence IOTA toute seule, sans abonnement callbook.",
       "Le partage CAT peut désormais parler TCI au lieu de Hamlib : un logiciel TCI atteint la radio, quelle qu’elle soit.",
-      "Cache des recherches : un TTL à 0 le désactive, pour relire à chaque fois une fiche callbook en cours de correction."
+      "Cache des recherches : un TTL à 0 le désactive, pour relire à chaque fois une fiche callbook en cours de correction.",
+      "TCI : quand la radio interdit l’émission, le PTT le dit désormais au lieu de ne rien faire en silence."
     ]
   },
   {
diff --git a/internal/cat/tci.go b/internal/cat/tci.go
index 8bb132f..1468422 100644
--- a/internal/cat/tci.go
+++ b/internal/cat/tci.go
@@ -46,6 +46,18 @@ type TCI struct {
 	mode   string
 	split  bool
 	tx     bool
+	// txAllowed is what the radio last said about TRANSMIT PERMISSION.
+	//
+	// TX_ENABLE is sent by ExpertSDR when a client connects and again whenever
+	// the band changes, "in case transmitter permission was changed" (§4.3). When
+	// it is false the radio silently ignores TRX — which is exactly what an
+	// operator sees as "PTT does nothing", with no error anywhere to explain it.
+	//
+	// txAllowedKnown keeps an OLDER ExpertSDR, or a TCI-compatible program that
+	// never sends TX_ENABLE at all, from being treated as refusing: without a
+	// word from the radio we key and let it decide.
+	txAllowed      bool
+	txAllowedKnown bool
 
 	lastSig string // last logged state signature (log only on change)
 
@@ -109,6 +121,10 @@ func (t *TCI) Connect() error {
 	t.mu.Lock()
 	t.conn = conn
 	t.ready = false
+	// Forget the previous session's transmit permission: the radio announces it
+	// again on connect, and a refusal remembered from a band we have since left
+	// would block PTT until it did.
+	t.txAllowed, t.txAllowedKnown = false, false
 	t.mu.Unlock()
 	debugLog.Printf("TCI: connected to %s", url)
 	go t.reader(conn)
@@ -298,7 +314,22 @@ func (t *TCI) SetMode(mode string) error {
 }
 
 // SetPTT keys or unkeys the transmitter (VFO 0).
+//
+// A refusal by the radio is reported rather than swallowed. ExpertSDR announces
+// transmit permission with TX_ENABLE and then simply IGNORES trx when it is
+// false — out-of-band frequency, TX disabled in the program, no PA. The command
+// went out, nothing happened, and nothing anywhere said why. Now the operator
+// is told, and the message names the place to look.
 func (t *TCI) SetPTT(on bool) error {
+	if on {
+		t.mu.Lock()
+		known, allowed := t.txAllowedKnown, t.txAllowed
+		t.mu.Unlock()
+		if known && !allowed {
+			return fmt.Errorf("the radio is refusing to transmit (TCI reports TX disabled) — " +
+				"check the frequency is inside a transmit band and that TX is enabled in ExpertSDR")
+		}
+	}
 	return t.send(fmt.Sprintf("trx:0,%t;", on))
 }
 
@@ -392,6 +423,14 @@ func (t *TCI) handle(msg string) {
 		if get(0) == "0" {
 			t.tx = get(1) == "true"
 		}
+	case "tx_enable":
+		if get(0) == "0" {
+			allowed := get(1) == "true"
+			if !t.txAllowedKnown || t.txAllowed != allowed {
+				debugLog.Printf("TCI: the radio %s transmitting", map[bool]string{true: "allows", false: "REFUSES"}[allowed])
+			}
+			t.txAllowed, t.txAllowedKnown = allowed, true
+		}
 	default:
 		lname := strings.ToLower(name)
 		// A click on one of our panorama spots comes back as
diff --git a/internal/cat/tci_ptt_test.go b/internal/cat/tci_ptt_test.go
new file mode 100644
index 0000000..b338d30
--- /dev/null
+++ b/internal/cat/tci_ptt_test.go
@@ -0,0 +1,80 @@
+//go:build windows
+
+package cat
+
+import "testing"
+
+// feed pushes messages at the backend the way the radio would.
+func feed(t *TCI, msgs ...string) {
+	for _, m := range msgs {
+		t.handle(m)
+	}
+}
+
+// "PTT via CAT does nothing on TCI."
+//
+// ExpertSDR announces transmit permission with TX_ENABLE — on connect, and
+// again whenever the band changes "in case transmitter permission was changed"
+// (§4.3 of the protocol document). When it is false the radio simply IGNORES
+// trx. OpsLog sent the documented command, the radio discarded it, and nothing
+// anywhere said why: the operator pressed a dead key.
+func TestPTTIsRefusedOutLoudWhenTheRadioForbidsTransmitting(t *testing.T) {
+	tci := NewTCI("localhost", 40001, "FT8", false)
+	feed(tci, "tx_enable:0,false")
+
+	err := tci.SetPTT(true)
+	if err == nil {
+		t.Fatal("keying was accepted while the radio forbids transmitting — the operator gets no reason at all")
+	}
+	// The message has to name where to look; "PTT failed" sends nobody anywhere.
+	for _, want := range []string{"transmit", "ExpertSDR"} {
+		if !contains(err.Error(), want) {
+			t.Errorf("the refusal reads %q, which does not mention %q", err.Error(), want)
+		}
+	}
+
+	// Unkeying is never blocked. Whatever the radio thinks about permission, a
+	// request to STOP transmitting must always reach it.
+	if err := tci.SetPTT(false); err != nil && contains(err.Error(), "refusing") {
+		t.Errorf("unkeying was refused: %v", err)
+	}
+}
+
+// Permission comes back when the operator returns to a band they may use, and
+// PTT has to come back with it — not stay blocked until OpsLog is restarted.
+func TestPermissionGrantedAgainRestoresPTT(t *testing.T) {
+	tci := NewTCI("localhost", 40001, "FT8", false)
+	feed(tci, "tx_enable:0,false")
+	if err := tci.SetPTT(true); err == nil {
+		t.Fatal("keying was accepted while forbidden")
+	}
+	feed(tci, "tx_enable:0,true")
+	// No connection here, so the send fails — but it must fail as a TRANSPORT
+	// error, never as a refusal.
+	if err := tci.SetPTT(true); err != nil && contains(err.Error(), "refusing") {
+		t.Errorf("still refusing after permission was granted: %v", err)
+	}
+}
+
+// A radio that never mentions TX_ENABLE — an older ExpertSDR, or one of the
+// other programs that speak TCI — must not be treated as refusing. Silence is
+// not a "no": we key, and let the radio decide.
+func TestSilenceAboutPermissionIsNotARefusal(t *testing.T) {
+	tci := NewTCI("localhost", 40001, "FT8", false)
+	if err := tci.SetPTT(true); err != nil && contains(err.Error(), "refusing") {
+		t.Errorf("a radio that never sent TX_ENABLE was treated as forbidding transmit: %v", err)
+	}
+}
+
+func contains(s, sub string) bool {
+	return len(sub) == 0 || (len(s) >= len(sub) && indexOf(s, sub) >= 0)
+}
+
+func indexOf(s, sub string) int {
+	for i := 0; i+len(sub) <= len(s); i++ {
+		if s[i:i+len(sub)] == sub {
+			return i
+		}
+	}
+	return -1
+}