diff --git a/pscp.c b/pscp.c
index e53f1dee..bcf18dcf 100644
--- a/pscp.c
+++ b/pscp.c
@@ -127,14 +127,10 @@ static void tell_user(FILE *stream, const char *fmt, ...)
     sfree(str2);
 }
 
-/*
- * In pscp, all agent requests should be synchronous, so this is a
- * never-called stub.
- */
 void agent_schedule_callback(void (*callback)(void *, void *, int),
                              void *callback_ctx, void *data, int len)
 {
-    assert(!"We shouldn't be here");
+    unreachable("all PSCP agent requests should be synchronous");
 }
 
 /*
diff --git a/psftp.c b/psftp.c
index 73f58f3e..c0d2f9df 100644
--- a/psftp.c
+++ b/psftp.c
@@ -2428,14 +2428,10 @@ static bool verbose = false;
 
 void ldisc_echoedit_update(Ldisc *ldisc) { }
 
-/*
- * In psftp, all agent requests should be synchronous, so this is a
- * never-called stub.
- */
 void agent_schedule_callback(void (*callback)(void *, void *, int),
                              void *callback_ctx, void *data, int len)
 {
-    assert(!"We shouldn't be here");
+    unreachable("all PSFTP agent requests should be synchronous");
 }
 
 /*
diff --git a/ssh2transport.c b/ssh2transport.c
index 1ed0edd7..e577c5ac 100644
--- a/ssh2transport.c
+++ b/ssh2transport.c
@@ -314,8 +314,8 @@ static struct kexinit_algorithm *ssh2_kexinit_addalg(struct kexinit_algorithm
             list[i].name = name;
             return &list[i];
         }
-    assert(!"No space in KEXINIT list");
-    return NULL;
+
+    unreachable("Should never run out of space in KEXINIT list");
 }
 
 bool ssh2_common_filter_queue(PacketProtocolLayer *ppl)
diff --git a/sshshare.c b/sshshare.c
index c70d5da1..4d124187 100644
--- a/sshshare.c
+++ b/sshshare.c
@@ -1292,8 +1292,7 @@ void share_got_pkt_from_server(ssh_sharing_connstate *cs, int type,
         break;
 
       default:
-        assert(!"This packet type should never have come from ssh.c");
-        break;
+        unreachable("This packet type should never have come from ssh.c");
     }
 }
 
diff --git a/unix/gtkdlg.c b/unix/gtkdlg.c
index b2f44242..cd3f8092 100644
--- a/unix/gtkdlg.c
+++ b/unix/gtkdlg.c
@@ -357,7 +357,7 @@ char *dlg_editbox_get(union control *ctrl, dlgparam *dp)
         return dupstr(gtk_entry_get_text(GTK_ENTRY(uc->entry)));
     }
 
-    assert(!"We shouldn't get here");
+    unreachable("bad control type in editbox_get");
 }
 
 #if !GTK_CHECK_VERSION(2,4,0)
@@ -395,7 +395,7 @@ void dlg_listbox_clear(union control *ctrl, dlgparam *dp)
         return;
     }
 #endif
-    assert(!"We shouldn't get here");
+    unreachable("bad control type in listbox_clear");
 }
 
 void dlg_listbox_del(union control *ctrl, dlgparam *dp, int index)
@@ -429,7 +429,7 @@ void dlg_listbox_del(union control *ctrl, dlgparam *dp, int index)
         return;
     }
 #endif
-    assert(!"We shouldn't get here");
+    unreachable("bad control type in listbox_del");
 }
 
 void dlg_listbox_add(union control *ctrl, dlgparam *dp, char const *text)
@@ -587,7 +587,7 @@ void dlg_listbox_addwithid(union control *ctrl, dlgparam *dp,
         goto done;
     }
 #endif
-    assert(!"We shouldn't get here");
+    unreachable("bad control type in listbox_addwithid");
     done:
     dp->flags &= ~FLAG_UPDATING_COMBO_LIST;
 }
@@ -626,7 +626,7 @@ int dlg_listbox_getid(union control *ctrl, dlgparam *dp, int index)
         return ret;
     }
 #endif
-    assert(!"We shouldn't get here");
+    unreachable("bad control type in listbox_getid");
     return -1;                         /* placate dataflow analysis */
 }
 
@@ -711,7 +711,7 @@ int dlg_listbox_index(union control *ctrl, dlgparam *dp)
         return ret;
     }
 #endif
-    assert(!"We shouldn't get here");
+    unreachable("bad control type in listbox_index");
     return -1;                         /* placate dataflow analysis */
 }
 
@@ -768,7 +768,7 @@ bool dlg_listbox_issel(union control *ctrl, dlgparam *dp, int index)
         return ret;
     }
 #endif
-    assert(!"We shouldn't get here");
+    unreachable("bad control type in listbox_issel");
     return false;                      /* placate dataflow analysis */
 }
 
@@ -837,7 +837,7 @@ void dlg_listbox_select(union control *ctrl, dlgparam *dp, int index)
         return;
     }
 #endif
-    assert(!"We shouldn't get here");
+    unreachable("bad control type in listbox_select");
 }
 
 void dlg_text_set(union control *ctrl, dlgparam *dp, char const *text)
@@ -884,8 +884,7 @@ void dlg_label_change(union control *ctrl, dlgparam *dp, char const *text)
         shortcut_highlight(uc->label, ctrl->listbox.shortcut);
         break;
       default:
-        assert(!"This shouldn't happen");
-        break;
+        unreachable("bad control type in label_change");
     }
 }
 
@@ -1016,8 +1015,7 @@ void dlg_set_focus(union control *ctrl, dlgparam *dp)
             break;
         }
 #endif
-        assert(!"We shouldn't get here");
-        break;
+        unreachable("bad control type in set_focus");
     }
 }
 
@@ -2730,8 +2728,7 @@ gint win_key_press(GtkWidget *widget, GdkEventKey *event, gpointer data)
                     break;
                 }
 #endif
-                assert(!"We shouldn't get here");
-                break;
+                unreachable("bad listbox type in win_key_press");
             }
             break;
         }
diff --git a/windows/winctrls.c b/windows/winctrls.c
index f30ccf39..3389ce00 100644
--- a/windows/winctrls.c
+++ b/windows/winctrls.c
@@ -1658,9 +1658,7 @@ void winctrl_layout(struct dlgparam *dp, struct winctrls *wc,
             sfree(escaped);
             break;
           default:
-            assert(!"Can't happen");
-            num_ids = 0;               /* placate gcc */
-            break;
+            unreachable("bad control type in winctrl_layout");
         }
 
         /*
@@ -2096,8 +2094,7 @@ int dlg_radiobutton_get(union control *ctrl, dlgparam *dp)
     for (i = 0; i < c->ctrl->radio.nbuttons; i++)
         if (IsDlgButtonChecked(dp->hwnd, c->base_id + 1 + i))
             return i;
-    assert(!"No radio button was checked?!");
-    return 0;
+    unreachable("no radio button was checked");
 }
 
 void dlg_checkbox_set(union control *ctrl, dlgparam *dp, bool checked)
@@ -2286,8 +2283,7 @@ void dlg_label_change(union control *ctrl, dlgparam *dp, char const *text)
         id = c->base_id;
         break;
       default:
-        assert(!"Can't happen");
-        break;
+        unreachable("bad control type in label_change");
     }
     if (escaped) {
         SetDlgItemText(dp->hwnd, id, escaped);
diff --git a/windows/winpgnt.c b/windows/winpgnt.c
index 83c516c2..718feb9f 100644
--- a/windows/winpgnt.c
+++ b/windows/winpgnt.c
@@ -1119,14 +1119,10 @@ void spawn_cmd(const char *cmdline, const char *args, int show)
     }
 }
 
-/*
- * This is a can't-happen stub, since Pageant never makes
- * asynchronous agent requests.
- */
 void agent_schedule_callback(void (*callback)(void *, void *, int),
                              void *callback_ctx, void *data, int len)
 {
-    assert(!"We shouldn't get here");
+    unreachable("all Pageant's own agent requests should be synchronous");
 }
 
 void cleanup_exit(int code)