diff --git a/internal/sanitize/path.go b/internal/sanitize/path.go
index 37eb30d..01b68c1 100644
--- a/internal/sanitize/path.go
+++ b/internal/sanitize/path.go
@@ -30,7 +30,7 @@ func SafeJoin(root, rel string) (string, error) {
 	return joined, nil
 }
 
-// ValidIdentifier returns true if s contains only [a-zA-Z0-9_-]. Useful for
+// ValidIdentifier returns true if s contains only [a-zA-Z0-9_.-]. Useful for
 // validating story IDs, agent IDs, and session names before using them in
 // filesystem paths or shell arguments.
 func ValidIdentifier(s string) bool {
@@ -50,3 +50,28 @@ func ValidIdentifier(s string) bool {
 	}
 	return true
 }
+
+// ValidTmuxTarget returns true if s is safe to pass to tmux as a session
+// target via -t. Stricter than ValidIdentifier: the `.` and `:` characters
+// have window/pane semantics in tmux target specifiers ("session.0",
+// "session:1") so we exclude both even though tmux session names CAN
+// technically contain them. A session name like "mysession.0" would
+// otherwise let an attacker (or a corrupted projection) target a specific
+// pane in a different session via the kill command.
+func ValidTmuxTarget(s string) bool {
+	if s == "" {
+		return false
+	}
+	for _, r := range s {
+		switch {
+		case r >= 'a' && r <= 'z',
+			r >= 'A' && r <= 'Z',
+			r >= '0' && r <= '9',
+			r == '_', r == '-':
+			continue
+		default:
+			return false
+		}
+	}
+	return true
+}
diff --git a/internal/sanitize/path_test.go b/internal/sanitize/path_test.go
index 7910ac9..5c96bb2 100644
--- a/internal/sanitize/path_test.go
+++ b/internal/sanitize/path_test.go
@@ -60,3 +60,30 @@ func TestValidIdentifier(t *testing.T) {
 		}
 	}
 }
+
+// TestValidTmuxTarget guards the stricter contract used by handleKill before
+// passing a session name to `tmux kill-session -t`. Unlike ValidIdentifier,
+// `.` and `:` are rejected — both are tmux target separators (`session.0`
+// targets pane 0, `session:1` targets window 1) and would let a corrupted
+// projection name kill the wrong tmux entity.
+func TestValidTmuxTarget(t *testing.T) {
+	for _, tc := range []struct {
+		in   string
+		want bool
+	}{
+		{"nxd-req-junior-1", true},
+		{"a_b", true},
+		{"abc123", true},
+		{"", false},
+		{"a/b", false},
+		{"a b", false},
+		{"a;b", false},
+		{"session.0", false}, // tmux pane separator
+		{"session:1", false}, // tmux window separator
+		{"../etc", false},
+	} {
+		if got := ValidTmuxTarget(tc.in); got != tc.want {
+			t.Errorf("ValidTmuxTarget(%q) = %v, want %v", tc.in, got, tc.want)
+		}
+	}
+}
diff --git a/internal/web/handler_success_test.go b/internal/web/handler_success_test.go
index da80bd3..698f5c6 100644
--- a/internal/web/handler_success_test.go
+++ b/internal/web/handler_success_test.go
@@ -81,6 +81,31 @@ func TestHandleKill_MalformedPayload(t *testing.T) {
 	}
 }
 
+// TestHandleKill_RejectsUnsafeStoredSessionName guards the
+// defense-in-depth check on the projection-read sessionName: even though
+// the spawn-time path validates names before insertion, a tampered or
+// migrated projection could carry a value like "nxd-session.0" — tmux
+// `.0` syntax then targets pane 0 of that session, killing the wrong
+// thing. handleKill now re-validates via sanitize.ValidTmuxTarget and
+// refuses the kill on failure.
+func TestHandleKill_RejectsUnsafeStoredSessionName(t *testing.T) {
+	s := newTestServer(t)
+	// Insert an agent whose SessionName contains a tmux pane separator —
+	// passes ValidIdentifier (which permits '.') but must fail the
+	// stricter ValidTmuxTarget.
+	if err := s.projStore.InsertAgent("agent-bad-session", "dev", "claude", "tmux", "nxd-session.0"); err != nil {
+		t.Fatalf("InsertAgent: %v", err)
+	}
+
+	resp := s.handleKill(mustMarshal(t, agentPayload{AgentID: "agent-bad-session"}))
+	if resp.Success {
+		t.Fatal("expected Success=false when session name has tmux pane separator")
+	}
+	if !strings.Contains(resp.Message, "failed validation") {
+		t.Errorf("expected 'failed validation' in message, got %q", resp.Message)
+	}
+}
+
 // TestHandleKill_AgentInListButNoSession covers the path where an
 // agent exists in the projection but its SessionName field is empty.
 // The handler refuses to issue a tmux kill in that case.
diff --git a/internal/web/handlers.go b/internal/web/handlers.go
index 313ba93..3bbc026 100644
--- a/internal/web/handlers.go
+++ b/internal/web/handlers.go
@@ -262,7 +262,20 @@ func (s *Server) handleKill(payload json.RawMessage) WSResponse {
 		return WSResponse{Type: "command_result", Action: action, Success: false, Message: "agent not found or no session"}
 	}
 
-	cmd := exec.Command("tmux", "kill-session", "-t", sessionName)
+	// Defense in depth: the sessionName was written into the projection at
+	// agent spawn time. A corrupted or tampered store could carry a string
+	// like "real-session.0" — tmux's `.0` syntax targets pane 0 of that
+	// session, so we'd kill the wrong window. ValidTmuxTarget excludes
+	// `.` and `:` for exactly this case.
+	if !sanitize.ValidTmuxTarget(sessionName) {
+		log.Printf("[ws] kill_agent rejecting unsafe session name %q for agent %s", sessionName, p.AgentID)
+		return WSResponse{Type: "command_result", Action: action, Success: false, Message: "stored session name failed validation"}
+	}
+
+	// The leading `=` is tmux's exact-match prefix: even if a future
+	// sessionName accidentally contained substring matches against other
+	// sessions, the kill targets only the literal name. Belt + braces.
+	cmd := exec.Command("tmux", "kill-session", "-t", "="+sessionName)
 	if err := cmd.Run(); err != nil {
 		log.Printf("[cmd] kill-session %s: %v", sessionName, err)
 		// Don't fail — session may already be dead.