diff --git a/commands.lua b/commands.lua
index 9a69e50..897ebf9 100644
--- a/commands.lua
+++ b/commands.lua
@@ -163,7 +163,7 @@ function draw_menu_bar()
           if pane.cursor_x == nil then
             add_panning_hotkeys_to_menu()
           else
-            assert(pane.cursor_y)
+            assert(pane.cursor_y, 'cursor fell off viewport')
             add_hotkey_to_menu('ctrl+e: stop editing')
             add_hotkey_to_menu('ctrl+h on drawing: help')
             add_hotkey_to_menu('ctrl+f: find')
@@ -671,7 +671,7 @@ function command.edit_note()
     add_error('no current note')
     return
   end
-  assert(not pane.editable)
+  assert(not pane.editable, 'pane already editable')
   stop_editing_all()
   pane.recent_updated = false
   pane.editable = true
@@ -681,10 +681,10 @@ function command.edit_note()
 end
 
 function command.exit_editing()
-  assert(Cursor_pane.col >= 1)
+  assert(Cursor_pane.col >= 1, 'no current pane')
   local pane = Surface[Cursor_pane.col][Cursor_pane.row]
-  assert(pane)
-  assert(pane.editable)
+  assert(pane, 'no current pane')
+  assert(pane.editable, 'current pane not editable')
   stop_editing(pane)
 end
 
@@ -853,7 +853,7 @@ end
 function finalize_search_all_pane()
   if Display_settings.search_all_query:sub(1,1) == '"' then
     -- support only a single quoted phrase by itself
-    assert(Display_settings.search_all_query:sub(#Display_settings.search_all_query) == '"')
+    assert(Display_settings.search_all_query:sub(#Display_settings.search_all_query) == '"', 'you can search for strings in quotes, but only one of them by itself')
     Display_settings.search_all_terms = {Display_settings.search_all_query:sub(2, #Display_settings.search_all_query-1)}
   else
     Display_settings.search_all_terms = split(Display_settings.search_all_query)
@@ -1168,17 +1168,17 @@ function remove_link(src, rel, target)
   if array.find(Non_unique_links, rel) then
 --?     print(('%s is non-unique'):format(rel))
     local arr = Links[src][rel]
-    assert(arr)
-    assert(type(arr) == 'table')
+    assert(arr, 'found no link')
+    assert(type(arr) == 'table', 'links not arranged in a table')
     local pos = array.find(arr, target)
 --?     print(('contains %s at index %s'):format(target, pos))
-    assert(pos)
+    assert(pos, "couldn't find link to remove in links table")
     table.remove(arr, pos)
     if #arr == 0 then
       Links[src][rel] = nil
     end
   else
-    assert(Links[src][rel] == target)
+    assert(Links[src][rel] == target, 'link at this rel is not the target; giving up')
     Links[src][rel] = nil
   end
 end
@@ -1345,7 +1345,7 @@ function command.unroll(rel)
     return
   end
   if #Surface[Cursor_pane.col] == 1 then
-    assert(Cursor_pane.row == 1)
+    assert(Cursor_pane.row == 1, "couldn't set current pane after unrolling")
     stop_editing(pane)  -- save any edits before we blow it away
     Surface[Cursor_pane.col] = column
   else
@@ -1471,7 +1471,7 @@ function command.neighbors()
   local column = {name=('neighbors of %s'):format(pane.id)}
   populate_neighbors_column(column, pane.id)
   if #Surface[Cursor_pane.col] == 1 then
-    assert(Cursor_pane.row == 1)
+    assert(Cursor_pane.row == 1, "couldn't repurpose column for neighbors")
     Surface[Cursor_pane.col] = column
   else
     table.insert(Surface, Cursor_pane.col+1, column)
@@ -1561,7 +1561,7 @@ function command.delete_note()
       Cursor_pane.row = 1
     end
   else
-    assert(not delete_cursor_column)
+    assert(not delete_cursor_column, "failed to delete note's column")
   end
   --
   while Cursor_pane.row > #Surface[Cursor_pane.col] do
@@ -1605,7 +1605,7 @@ function new_pane()
   local id = os.date('%Y/%m/%d/%H-%M-%S', t)
   print_and_log('new_pane: creating directory '..Directory..dirname(id))
   local status = App.mkdir(Directory..dirname(id))
-  assert(status)
+  assert(status, "failed to create directory for note")
   Links[id] = {}
   local pane = load_pane(id)
   if not file_exists(pane.filename) then
diff --git a/drawing.lua b/drawing.lua
index 65e4aa6..d7fdcad 100644
--- a/drawing.lua
+++ b/drawing.lua
@@ -34,7 +34,6 @@ function Drawing.draw(State, line_index, y)
 
 --?   print(State.left, State.right)
   for _,shape in ipairs(line.shapes) do
-    assert(shape)
     if State.editable and geom.on_shape(mx,my, line, shape) then
       App.color(Focus_stroke_color)
     else
@@ -114,8 +113,7 @@ function Drawing.draw_shape(drawing, shape, top, left,right)
   elseif shape.mode == 'deleted' then
     -- ignore
   else
-    print(shape.mode)
-    assert(false)
+    assert(false, ('unknown drawing mode %s'):format(shape.mode))
   end
 end
 
@@ -208,8 +206,7 @@ function Drawing.draw_pending_shape(drawing, top, left,right)
   elseif shape.mode == 'name' then
     -- nothing pending; changes are immediately committed
   else
-    print(shape.mode)
-    assert(false)
+    assert(false, ('unknown drawing mode %s'):format(shape.mode))
   end
 end
 
@@ -240,8 +237,7 @@ function Drawing.mouse_press(State, drawing_index, x,y, mouse_button)
   elseif State.current_drawing_mode == 'name' then
     -- nothing
   else
-    print(State.current_drawing_mode)
-    assert(false)
+    assert(false, ('unknown drawing mode %s'):format(State.current_drawing_mode))
   end
 end
 
@@ -256,7 +252,7 @@ function Drawing.update(State)
     -- just skip this frame
     return
   end
-  assert(drawing.mode == 'drawing')
+  assert(drawing.mode == 'drawing', 'Drawing.update: line is not a drawing')
   local pmx, pmy = App.mouse_x(), App.mouse_y()
   local mx = Drawing.coord(pmx-State.left, State.width)
   local my = Drawing.coord(pmy-line_cache.starty, State.width)
@@ -343,7 +339,7 @@ function Drawing.mouse_release(State, x,y, mouse_button)
           table.insert(drawing.shapes, drawing.pending)
         end
       elseif drawing.pending.mode == 'rectangle' then
-        assert(#drawing.pending.vertices <= 2)
+        assert(#drawing.pending.vertices <= 2, 'Drawing.mouse_release: rectangle has too many pending vertices')
         if #drawing.pending.vertices == 2 then
           local mx,my = Drawing.coord(x-State.left, State.width), Drawing.coord(y-line_cache.starty, State.width)
           if mx >= 0 and mx < 256 and my >= 0 and my < drawing.h then
@@ -358,7 +354,7 @@ function Drawing.mouse_release(State, x,y, mouse_button)
           -- too few points; draw nothing
         end
       elseif drawing.pending.mode == 'square' then
-        assert(#drawing.pending.vertices <= 2)
+        assert(#drawing.pending.vertices <= 2, 'Drawing.mouse_release: square has too many pending vertices')
         if #drawing.pending.vertices == 2 then
           local mx,my = Drawing.coord(x-State.left, State.width), Drawing.coord(y-line_cache.starty, State.width)
           if mx >= 0 and mx < 256 and my >= 0 and my < drawing.h then
@@ -387,8 +383,7 @@ function Drawing.mouse_release(State, x,y, mouse_button)
       elseif drawing.pending.mode == 'name' then
         -- drop it
       else
-        print(drawing.pending.mode)
-        assert(false)
+        assert(false, ('unknown drawing mode %s'):format(drawing.pending.mode))
       end
       State.lines.current_drawing.pending = {}
       State.lines.current_drawing = nil
@@ -546,7 +541,7 @@ function Drawing.keychord_press(State, chord)
         if Drawing.contains_point(shape, i) then
           if shape.mode == 'polygon' then
             local idx = table.find(shape.vertices, i)
-            assert(idx)
+            assert(idx, 'point to delete is not in vertices')
             table.remove(shape.vertices, idx)
             if #shape.vertices < 3 then
               shape.mode = 'deleted'
@@ -642,7 +637,6 @@ function Drawing.select_shape_at_mouse(State)
       if Drawing.in_drawing(drawing, line_cache, x,y, State.left,State.right) then
         local mx,my = Drawing.coord(x-State.left, State.width), Drawing.coord(y-line_cache.starty, State.width)
         for i,shape in ipairs(drawing.shapes) do
-          assert(shape)
           if geom.on_shape(mx,my, drawing, shape) then
             return drawing,line_cache,i,shape
           end
@@ -660,7 +654,6 @@ function Drawing.select_point_at_mouse(State)
       if Drawing.in_drawing(drawing, line_cache, x,y, State.left,State.right) then
         local mx,my = Drawing.coord(x-State.left, State.width), Drawing.coord(y-line_cache.starty, State.width)
         for i,point in ipairs(drawing.points) do
-          assert(point)
           if Drawing.near(point, mx,my, State.width) then
             return drawing_index,drawing,line_cache,i,point
           end
@@ -697,13 +690,12 @@ function Drawing.contains_point(shape, p)
   elseif shape.mode == 'deleted' then
     -- already done
   else
-    print(shape.mode)
-    assert(false)
+    assert(false, ('unknown drawing mode %s'):format(shape.mode))
   end
 end
 
 function Drawing.smoothen(shape)
-  assert(shape.mode == 'freehand')
+  assert(shape.mode == 'freehand', 'can only smoothen freehand shapes')
   for _=1,7 do
     for i=2,#shape.points-1 do
       local a = shape.points[i-1]
diff --git a/edit.lua b/edit.lua
index 858b81e..05363c1 100644
--- a/edit.lua
+++ b/edit.lua
@@ -82,7 +82,7 @@ function edit.initialize_state(top, left, right, font_height, line_height)  -- c
     cursor_x = nil,
     cursor_y = nil,
 
-    current_drawing_mode = 'line',
+    current_drawing_mode = 'line',  -- one of the available shape modes
     previous_drawing_mode = nil,  -- extra state for some ephemeral modes like moving/deleting/naming points
 
     font_height = font_height,
@@ -169,10 +169,7 @@ function edit.draw(State)
 --?   print_and_log(('edit.draw %s %d %d,%d'):format(State.id, State.top, State.left,State.right))
   State.button_handlers = {}
   App.color(Text_color)
-  if #State.lines ~= #State.line_cache then
-    log(2, os.date('%Y/%m/%d/%H-%M-%S')..(' line_cache is out of date; %d when it should be %d'):format(#State.line_cache, #State.lines))
-    assert(false)
-  end
+  assert(#State.lines == #State.line_cache, ('line_cache is out of date; %d elements when it should be %d'):format(#State.line_cache, #State.lines))
   State.cursor_x = nil
   State.cursor_y = nil
   local y = State.top
@@ -215,8 +212,7 @@ function edit.draw(State)
       Drawing.draw(State, line_index, y)
       y = y + Drawing.pixels(line.h, State.width) + Drawing_padding_bottom
     else
-      print(line.mode)
-      assert(false)
+      assert(false, ('unknown line mode %s'):format(line.mode))
     end
   end
   State.bottom = y
diff --git a/file.lua b/file.lua
index ebc00a7..cb289ba 100644
--- a/file.lua
+++ b/file.lua
@@ -66,7 +66,7 @@ function save_to_disk(State)
       if not f then
         error(err)
       end
-      assert(not err)
+      assert(not err, 'failed to save recent')
       f:write(State.id, '\n')
       f:close()
     end
@@ -79,7 +79,7 @@ function load_drawing(infile_next_line)
   local drawing = {mode='drawing', h=256/2, points={}, shapes={}, pending={}}
   while true do
     local line = infile_next_line()
-    assert(line)
+    assert(line, 'drawing in file is incomplete')
     if line == '```' then break end
     local shape = json.decode(line)
     if shape.mode == 'freehand' then
@@ -104,8 +104,7 @@ function load_drawing(infile_next_line)
     elseif shape.mode == 'deleted' then
       -- ignore
     else
-      print_and_log('load_drawing: '..shape.mode)
-      assert(false)
+      assert(false, ('unknown drawing mode %s'):format(shape.mode))
     end
     table.insert(drawing.shapes, shape)
   end
@@ -139,8 +138,7 @@ function store_drawing(outfile, drawing)
     elseif shape.mode == 'deleted' then
       -- ignore
     else
-      print_and_log('store_drawing: '..shape.mode)
-      assert(false)
+      assert(false, ('unknown drawing mode %s'):format(shape.mode))
     end
   end
   outfile:write('```\n')
@@ -158,7 +156,7 @@ end
 function save_links(id)
   local links_filename = Directory..id..'.json'
   local status = App.mkdir(dirname(links_filename))
-  assert(status)
+  assert(status, 'failed to create directory for links')
   log(2, 'save_links: '..id)
   if empty(Links[id]) then
     print_and_log('save_links: no links; getting rid of .json if it exists')
@@ -206,7 +204,7 @@ function load_drawing_from_array(iter, a, i)
   local line
   while true do
     i, line = iter(a, i)
-    assert(i)
+    assert(i, 'drawing in array is incomplete')
 --?     print(i)
     if line == '```' then break end
     local shape = json.decode(line)
@@ -232,8 +230,7 @@ function load_drawing_from_array(iter, a, i)
     elseif shape.mode == 'deleted' then
       -- ignore
     else
-      print_and_log('load_drawing_from_array: '..shape.mode)
-      assert(false)
+      assert(false, ('unknown drawing mode %s'):format(shape.mode))
     end
     table.insert(drawing.shapes, shape)
   end
diff --git a/geom.lua b/geom.lua
index 891e98d..ea007b7 100644
--- a/geom.lua
+++ b/geom.lua
@@ -38,8 +38,7 @@ function geom.on_shape(x,y, drawing, shape)
     return geom.angle_between(center.x,center.y, x,y, shape.start_angle,shape.end_angle)
   elseif shape.mode == 'deleted' then
   else
-    print(shape.mode)
-    assert(false)
+    assert(false, ('unknown drawing mode %s'):format(shape.mode))
   end
 end
 
diff --git a/log_browser.lua b/log_browser.lua
index 6b71da6..b6195b9 100644
--- a/log_browser.lua
+++ b/log_browser.lua
@@ -75,7 +75,7 @@ function table.shallowcopy(x)
 end
 
 function log_browser.draw(State, hide_cursor)
-  assert(#State.lines == #State.line_cache)
+  assert(#State.lines == #State.line_cache, ('line_cache is out of date; %d elements when it should be %d'):format(#State.line_cache, #State.lines))
   local mouse_line_index = log_browser.line_index(State, App.mouse_x(), App.mouse_y())
   local y = State.top
   for line_index = State.screen_top1.line,#State.lines do
@@ -95,7 +95,7 @@ function log_browser.draw(State, hide_cursor)
           love.graphics.line(xleft,sectiony, xleft+50-2,sectiony)
           love.graphics.print(line.section_name, xleft+50,y)
           love.graphics.line(xleft+50+App.width(line.section_name)+2,sectiony, xright,sectiony)
-        else assert(line.section_end)
+        else assert(line.section_end, "log line has a section name, but it's neither the start nor end of a section")
           local sectiony = y+State.line_height-Section_border_padding_vertical
           love.graphics.line(xleft,y, xleft,sectiony)
           love.graphics.line(xright,y, xright,sectiony)
diff --git a/run.lua b/run.lua
index 188188c..52f1326 100644
--- a/run.lua
+++ b/run.lua
@@ -176,7 +176,7 @@ function run.initialize(arg)
     return
   end
 
-  assert(Directory)
+  assert(Directory, 'no directory to browse notes in')
   print('Directory initialized to '..Directory)
   Error_log.filename = Directory..'errors'
 
@@ -332,8 +332,7 @@ function refresh_pane_height(pane)
       -- nothing
       y = y + Drawing.pixels(line.h, Display_settings.column_width) + Drawing_padding_height
     else
-      print_and_log('refresh_pane_height: '..line.mode)
-      assert(false)
+      assert(false, ('unknown line mode %s'):format(line.mode))
     end
   end
   if Links[pane.id] and not empty(Links[pane.id]) then
@@ -436,8 +435,7 @@ function run.draw()
       end
     end
   else
-    print_and_log('run.draw: '..Display_settings.mode)
-    assert(false)
+    assert(false, ('pensieve is in an unknown mode %s'):format(Display_settings.mode))
   end
   if Grab_pane then
     local old_top, old_left, old_right = Grab_pane.top, Grab_pane.left, Grab_pane.right
@@ -467,11 +465,11 @@ function run.draw()
 end
 
 function draw_normal_mode()
-  assert(Cursor_pane.col)
-  assert(Cursor_pane.row)
+  assert(Cursor_pane.col, 'no current column')
+  assert(Cursor_pane.row, 'no current row')
 --?   print('draw', Display_settings.x, Display_settings.y)
   for _,pane in ipairs(Panes_to_draw) do
-    assert(pane.top)
+    assert(pane.top, "pane has no top coordinate; there's likely a problem in plan_draw")
 --?     if pane.column_index == 1 and pane.pane_index == 1 then
 --?       print('draw', pane.id, 'from y', pane.top, 'down to screen height', App.screen.height)
 --?       print('screen top', pane.screen_top1.line, pane.screen_top1.pos)
@@ -517,7 +515,7 @@ function should_show_pane(pane, sy)
 end
 
 function draw_title(pane)
-  assert(pane.title)
+  assert(pane.title, 'pane has no title')
   App.color(Pane_title_color)
   App.screen.print(pane.title, pane.left, pane.top-Margin_above -5-Line_height)
   App.color(Pane_title_background_color)
@@ -675,7 +673,9 @@ function run.quit()
 end
 
 function run.settings()
-  assert(Directory_error == nil)  -- this will go into an infinite loop if handle_error ever tries to save settings
+  -- avoid an infinite loop if handle_error ever tries to save settings
+  assert(Directory_error == nil,
+        "tried to save settings when we couldn't determine the directory to browse notes in")
   -- side effect: save notes-related settings inside Directory
   local column_names = {}
   for _,column in ipairs(Surface) do
@@ -690,7 +690,7 @@ function run.settings()
         surface_x=Display_settings.x,
         surface_y=Display_settings.y,
       }))
-  assert(status)
+  assert(status, 'failed to write settings')
 
   if Settings == nil then Settings = {} end
   Settings.x, Settings.y, Settings.displayindex = App.screen.position()
@@ -717,8 +717,7 @@ function run.mouse_press(x,y, mouse_button)
       end
     end
   else
-    print_and_log('run.mouse_press: '..Display_settings.mode)
-    assert(false)
+    assert(false, ('pensieve is in an unknown mode %s'):format(Display_settings.mode))
   end
 end
 
@@ -874,8 +873,7 @@ function run.text_input(t)
       end
     end
   else
-    print_and_log('run.text_input: '..Display_settings.mode)
-    assert(false)
+    assert(false, ('pensieve is in an unknown mode %s'):format(Display_settings.mode))
   end
 end
 
@@ -911,7 +909,7 @@ function run.keychord_press(chord, key)
             command.exit_editing()
           elseif pane.cursor_x == nil then
             -- ignore if cursor is not visible on screen
-            assert(pane.cursor_y == nil)
+            assert(pane.cursor_y == nil, 'cursor x is not set but y is set')
             panning_keychord_press(chord, key)
             plan_draw()
           else
@@ -949,8 +947,7 @@ function run.keychord_press(chord, key)
       keychord_press_in_maximize_mode(chord, key)
     end
   else
-    print_and_log('run.keychord_press: '..Display_settings.mode)
-    assert(false)
+    assert(false, ('pensieve is in an unknown mode %s'):format(Display_settings.mode))
   end
 end
 
@@ -1203,8 +1200,7 @@ function bring_cursor_of_cursor_pane_in_view(dir)
     if not horizontally_ok then
       Display_settings.y = cursor_sy - 3*Line_height
     end
-  else
-    assert(dir == 'down')
+  elseif dir == 'down' then
     if not vertically_ok then
       Display_settings.x = left_edge_sx + Display_settings.column_width + Margin_right + Padding_horizontal - App.screen.width
     end
@@ -1213,7 +1209,9 @@ function bring_cursor_of_cursor_pane_in_view(dir)
       Display_settings.y = cursor_sy + Text.search_bar_height(pane) - (App.screen.height - Header_height)
       -- Bah, temporarily giving up on debugging.
       Display_settings.y = Display_settings.y + Line_height
-      assert(App.screen.height - (cursor_sy-Display_settings.y) > 1.5*Line_height)
+      assert(App.screen.height - (cursor_sy-Display_settings.y) > 1.5*Line_height, 'ugh, ancient bug is back: panning the viewport when cursor falls off')
+    else
+      assert(false, ('unknown dir %s'):format(dir))
     end
   end
   Display_settings.x = math.max(Display_settings.x, 0)
@@ -1414,7 +1412,7 @@ end
 --            - let loc, y_offset = schema1_of_y(pane, y)
 --              y - y_offset == y_of_schema1(pane, loc)
 function schema1_of_y(pane, y)
-  assert(y >= 0)
+  assert(y >= 0, 'something is at negative y on the surface')
   local y_offset = y
   for i=1,#pane.lines do
 --?     print('--', y_offset)
@@ -1427,7 +1425,7 @@ function schema1_of_y(pane, y)
       else
         local nlines = math.floor(y_offset/pane.line_height)
 --?         print(y_offset, pane.line_height, nlines)
-        assert(nlines >= 0 and nlines < #pane.line_cache[i].screen_line_starting_pos)
+        assert(nlines >= 0 and nlines < #pane.line_cache[i].screen_line_starting_pos, 'error in mapping y coordinate to schema-1')
         local pos = pane.line_cache[i].screen_line_starting_pos[nlines+1]  -- switch to 1-indexing
         y_offset = y_offset - nlines*pane.line_height
         return {line=i, pos=pos}, y_offset
@@ -1459,7 +1457,7 @@ function stop_editing_all()
       end
     end
   end
-  assert(edit_count <= 1)
+  assert(edit_count <= 1, 'multiple panes were editable')
 end
 
 function stop_editing(pane)
diff --git a/search.lua b/search.lua
index b870c96..a16c3f1 100644
--- a/search.lua
+++ b/search.lua
@@ -151,7 +151,7 @@ function rfind(s, pat, i, plain)
   local rendpos = rs:find(rpat, ri, plain)
   if rendpos == nil then return nil end
   local endpos = #s - rendpos + 1
-  assert (endpos >= #pat)
+  assert (endpos >= #pat, ('rfind: endpos %d should be >= #pat %d at this point'):format(endpos, #pat))
   return endpos-#pat+1
 end
 
diff --git a/select.lua b/select.lua
index 9b2a278..9ede1da 100644
--- a/select.lua
+++ b/select.lua
@@ -33,13 +33,13 @@ function Text.clip_selection(State, line_index, apos, bpos)
     -- fully contained
     return apos,bpos
   elseif a_ge then
-    assert(maxl == line_index)
+    assert(maxl == line_index, ('maxl %d not equal to line_index %d'):format(maxl, line_index))
     return apos,maxp
   elseif b_lt then
-    assert(minl == line_index)
+    assert(minl == line_index, ('minl %d not equal to line_index %d'):format(minl, line_index))
     return minp,bpos
   else
-    assert(minl == maxl and minl == line_index)
+    assert(minl == maxl and minl == line_index, ('minl %d, maxl %d and line_index %d are not all equal'):format(minl, maxl, line_index))
     return minp,maxp
   end
 end
@@ -127,7 +127,7 @@ function Text.delete_selection_without_undo(State)
     State.lines[minl].data = State.lines[minl].data:sub(1, min_offset-1)..State.lines[minl].data:sub(max_offset)
     return
   end
-  assert(minl < maxl)
+  assert(minl < maxl, ('minl %d not < maxl %d'):format(minl, maxl))
   local rhs = State.lines[maxl].data:sub(max_offset)
   for i=maxl,minl+1,-1 do
     table.remove(State.lines, i)
@@ -154,7 +154,7 @@ function Text.selection(State)
   if minl == maxl then
     return State.lines[minl].data:sub(min_offset, max_offset-1)
   end
-  assert(minl < maxl)
+  assert(minl < maxl, ('minl %d not < maxl %d'):format(minl, maxl))
   local result = {State.lines[minl].data:sub(min_offset)}
   for i=minl+1,maxl-1 do
     if State.lines[i].mode == 'text' then
diff --git a/source_commands.lua b/source_commands.lua
index 62e7733..c911044 100644
--- a/source_commands.lua
+++ b/source_commands.lua
@@ -136,7 +136,7 @@ end
 
 function move_candidate_to_front(s)
   local index = array.find(File_navigation.all_candidates, s)
-  assert(index)
+  assert(index, 'file missing from manifest')
   table.remove(File_navigation.all_candidates, index)
   table.insert(File_navigation.all_candidates, 1, s)
 end
diff --git a/source_edit.lua b/source_edit.lua
index 12c2bae..6259ebe 100644
--- a/source_edit.lua
+++ b/source_edit.lua
@@ -158,14 +158,8 @@ end
 function edit.draw(State, hide_cursor, show_line_numbers)
   State.button_handlers = {}
   App.color(Text_color)
-  if #State.lines ~= #State.line_cache then
-    print(('line_cache is out of date; %d when it should be %d'):format(#State.line_cache, #State.lines))
-    assert(false)
-  end
-  if not Text.le1(State.screen_top1, State.cursor1) then
-    print(State.screen_top1.line, State.screen_top1.pos, State.cursor1.line, State.cursor1.pos)
-    assert(false)
-  end
+  assert(#State.lines == #State.line_cache, ('line_cache is out of date; %d elements when it should be %d'):format(#State.line_cache, #State.lines))
+  assert(Text.le1(State.screen_top1, State.cursor1), ('screen_top (line=%d,pos=%d) is below cursor (line=%d,pos=%d)'):format(State.screen_top1.line, State.screen_top1.pos, State.cursor1.line, State.cursor1.pos))
   State.cursor_x = nil
   State.cursor_y = nil
   local y = State.top
@@ -209,8 +203,7 @@ function edit.draw(State, hide_cursor, show_line_numbers)
       Drawing.draw(State, line_index, y)
       y = y + Drawing.pixels(line.h, State.width) + Drawing_padding_bottom
     else
-      print(line.mode)
-      assert(false)
+      assert(false, ('unknown line mode %s'):format(line.mode))
     end
   end
   State.screen_bottom1 = screen_bottom1
diff --git a/source_file.lua b/source_file.lua
index dbb6f95..6d04f6b 100644
--- a/source_file.lua
+++ b/source_file.lua
@@ -63,7 +63,7 @@ function load_drawing(infile_next_line)
   local drawing = {mode='drawing', h=256/2, points={}, shapes={}, pending={}}
   while true do
     local line = infile_next_line()
-    assert(line)
+    assert(line, 'drawing in file is incomplete')
     if line == '```' then break end
     local shape = json.decode(line)
     if shape.mode == 'freehand' then
@@ -88,8 +88,7 @@ function load_drawing(infile_next_line)
     elseif shape.mode == 'deleted' then
       -- ignore
     else
-      print(shape.mode)
-      assert(false)
+      assert(false, ('unknown drawing mode %s'):format(shape.mode))
     end
     table.insert(drawing.shapes, shape)
   end
@@ -123,8 +122,7 @@ function store_drawing(outfile, drawing)
     elseif shape.mode == 'deleted' then
       -- ignore
     else
-      print(shape.mode)
-      assert(false)
+      assert(false, ('unknown drawing mode %s'):format(shape.mode))
     end
   end
   outfile:write('```\n')
@@ -162,7 +160,7 @@ function load_drawing_from_array(iter, a, i)
   local line
   while true do
     i, line = iter(a, i)
-    assert(i)
+    assert(i, 'drawing in array is incomplete')
 --?     print(i)
     if line == '```' then break end
     local shape = json.decode(line)
@@ -188,8 +186,7 @@ function load_drawing_from_array(iter, a, i)
     elseif shape.mode == 'deleted' then
       -- ignore
     else
-      print(shape.mode)
-      assert(false)
+      assert(false, ('unknown drawing mode %s'):format(shape.mode))
     end
     table.insert(drawing.shapes, shape)
   end
diff --git a/source_select.lua b/source_select.lua
index 9b2a278..9ede1da 100644
--- a/source_select.lua
+++ b/source_select.lua
@@ -33,13 +33,13 @@ function Text.clip_selection(State, line_index, apos, bpos)
     -- fully contained
     return apos,bpos
   elseif a_ge then
-    assert(maxl == line_index)
+    assert(maxl == line_index, ('maxl %d not equal to line_index %d'):format(maxl, line_index))
     return apos,maxp
   elseif b_lt then
-    assert(minl == line_index)
+    assert(minl == line_index, ('minl %d not equal to line_index %d'):format(minl, line_index))
     return minp,bpos
   else
-    assert(minl == maxl and minl == line_index)
+    assert(minl == maxl and minl == line_index, ('minl %d, maxl %d and line_index %d are not all equal'):format(minl, maxl, line_index))
     return minp,maxp
   end
 end
@@ -127,7 +127,7 @@ function Text.delete_selection_without_undo(State)
     State.lines[minl].data = State.lines[minl].data:sub(1, min_offset-1)..State.lines[minl].data:sub(max_offset)
     return
   end
-  assert(minl < maxl)
+  assert(minl < maxl, ('minl %d not < maxl %d'):format(minl, maxl))
   local rhs = State.lines[maxl].data:sub(max_offset)
   for i=maxl,minl+1,-1 do
     table.remove(State.lines, i)
@@ -154,7 +154,7 @@ function Text.selection(State)
   if minl == maxl then
     return State.lines[minl].data:sub(min_offset, max_offset-1)
   end
-  assert(minl < maxl)
+  assert(minl < maxl, ('minl %d not < maxl %d'):format(minl, maxl))
   local result = {State.lines[minl].data:sub(min_offset)}
   for i=minl+1,maxl-1 do
     if State.lines[i].mode == 'text' then
diff --git a/source_text.lua b/source_text.lua
index fb5123a..8b34d52 100644
--- a/source_text.lua
+++ b/source_text.lua
@@ -17,7 +17,7 @@ function Text.draw(State, line_index, y, startpos, hide_cursor, show_line_number
     love.graphics.print(line_index, State.left-Line_number_width*App.width('m')+10,y)
   end
   initialize_color()
-  assert(#line_cache.screen_line_starting_pos >= 1)
+  assert(#line_cache.screen_line_starting_pos >= 1, 'line cache missing screen line info')
   for i=1,#line_cache.screen_line_starting_pos do
     local pos = line_cache.screen_line_starting_pos[i]
     if pos < startpos then
@@ -209,7 +209,7 @@ function Text.text_input(State, t)
 end
 
 function Text.insert_at_cursor(State, t)
-  assert(State.lines[State.cursor1.line].mode == 'text')
+  assert(State.lines[State.cursor1.line].mode == 'text', 'line is not text')
   local byte_offset = Text.offset(State.lines[State.cursor1.line].data, State.cursor1.pos)
   State.lines[State.cursor1.line].data = string.sub(State.lines[State.cursor1.line].data, 1, byte_offset-1)..t..string.sub(State.lines[State.cursor1.line].data, byte_offset)
   Text.clear_screen_line_cache(State, State.cursor1.line)
@@ -286,7 +286,7 @@ function Text.keychord_press(State, chord)
       Text.redraw_all(State)  -- if we're scrolling, reclaim all fragments to avoid memory leaks
     end
     Text.clear_screen_line_cache(State, State.cursor1.line)
-    assert(Text.le1(State.screen_top1, State.cursor1))
+    assert(Text.le1(State.screen_top1, State.cursor1), ('screen_top (line=%d,pos=%d) is below cursor (line=%d,pos=%d)'):format(State.screen_top1.line, State.screen_top1.pos, State.cursor1.line, State.cursor1.pos))
     schedule_save(State)
     record_undo_event(State, {before=before, after=snapshot(State, State.cursor1.line)})
   elseif chord == 'delete' then
@@ -452,7 +452,7 @@ function Text.pagedown(State)
 end
 
 function Text.up(State)
-  assert(State.lines[State.cursor1.line].mode == 'text')
+  assert(State.lines[State.cursor1.line].mode == 'text', 'line is not text')
 --?   print('up', State.cursor1.line, State.cursor1.pos, State.screen_top1.line, State.screen_top1.pos)
   local screen_line_starting_pos, screen_line_index = Text.pos_at_start_of_screen_line(State, State.cursor1)
   if screen_line_starting_pos == 1 then
@@ -478,7 +478,7 @@ function Text.up(State)
     end
   else
     -- move up one screen line in current line
-    assert(screen_line_index > 1)
+    assert(screen_line_index > 1, 'bumped up against top screen line in line')
     local new_screen_line_starting_pos = State.line_cache[State.cursor1.line].screen_line_starting_pos[screen_line_index-1]
     local new_screen_line_starting_byte_offset = Text.offset(State.lines[State.cursor1.line].data, new_screen_line_starting_pos)
     local s = string.sub(State.lines[State.cursor1.line].data, new_screen_line_starting_byte_offset)
@@ -495,9 +495,9 @@ function Text.up(State)
 end
 
 function Text.down(State)
-  assert(State.lines[State.cursor1.line].mode == 'text')
+  assert(State.lines[State.cursor1.line].mode == 'text', 'line is not text')
 --?   print('down', State.cursor1.line, State.cursor1.pos, State.screen_top1.line, State.screen_top1.pos, State.screen_bottom1.line, State.screen_bottom1.pos)
-  assert(State.cursor1.pos)
+  assert(State.cursor1.pos, 'cursor has no pos')
   if Text.cursor_at_final_screen_line(State) then
     -- line is done, skip to next text line
 --?     print('cursor at final screen line of its line')
@@ -571,7 +571,7 @@ function Text.word_left(State)
     if State.cursor1.pos == 1 then
       break
     end
-    assert(State.cursor1.pos > 1)
+    assert(State.cursor1.pos > 1, 'bumped up against start of line')
     if Text.match(State.lines[State.cursor1.line].data, State.cursor1.pos-1, '%s') then
       break
     end
@@ -605,15 +605,14 @@ end
 
 function Text.match(s, pos, pat)
   local start_offset = Text.offset(s, pos)
-  assert(start_offset)
   local end_offset = Text.offset(s, pos+1)
-  assert(end_offset > start_offset)
+  assert(end_offset > start_offset, ('end_offset %d not > start_offset %d'):format(end_offset, start_offset))
   local curr = s:sub(start_offset, end_offset-1)
   return curr:match(pat)
 end
 
 function Text.left(State)
-  assert(State.lines[State.cursor1.line].mode == 'text')
+  assert(State.lines[State.cursor1.line].mode == 'text', 'line is not text')
   if State.cursor1.pos > 1 then
     State.cursor1.pos = State.cursor1.pos-1
   else
@@ -646,7 +645,7 @@ function Text.right(State)
 end
 
 function Text.right_without_scroll(State)
-  assert(State.lines[State.cursor1.line].mode == 'text')
+  assert(State.lines[State.cursor1.line].mode == 'text', 'line is not text')
   if State.cursor1.pos <= utf8.len(State.lines[State.cursor1.line].data) then
     State.cursor1.pos = State.cursor1.pos+1
   else
@@ -671,7 +670,7 @@ function Text.pos_at_start_of_screen_line(State, loc1)
       return spos,i
     end
   end
-  assert(false)
+  assert(false, ('invalid pos %d'):format(loc1.pos))
 end
 
 function Text.pos_at_end_of_screen_line(State, loc1)
@@ -685,7 +684,7 @@ function Text.pos_at_end_of_screen_line(State, loc1)
     end
     most_recent_final_pos = spos-1
   end
-  assert(false)
+  assert(false, ('invalid pos %d'):format(loc1.pos))
 end
 
 function Text.cursor_at_final_screen_line(State)
@@ -710,7 +709,7 @@ function Text.move_cursor_down_to_next_text_line_while_scrolling_again_if_necess
   end
   -- hack: insert a text line at bottom of file if necessary
   if State.cursor1.line > #State.lines then
-    assert(State.cursor1.line == #State.lines+1)
+    assert(State.cursor1.line == #State.lines+1, 'tried to ensure bottom line of file is text, but failed')
     table.insert(State.lines, {mode='text', data=''})
     table.insert(State.line_cache, {})
   end
@@ -742,8 +741,8 @@ function Text.snap_cursor_to_bottom_of_screen(State)
       end
       y = y - h
     else
-      assert(top2.line > 1)
-      assert(State.lines[top2.line-1].mode == 'drawing')
+      assert(top2.line > 1, 'tried to snap cursor to buttom of screen but failed')
+      assert(State.lines[top2.line-1].mode == 'drawing', "expected a drawing but it's not")
       -- We currently can't draw partial drawings, so either skip it entirely
       -- or not at all.
       local h = Drawing_padding_height + Drawing.pixels(State.lines[top2.line-1].h, State.width)
@@ -775,7 +774,7 @@ end
 function Text.to_pos_on_line(State, line_index, mx, my)
   local line = State.lines[line_index]
   local line_cache = State.line_cache[line_index]
-  assert(my >= line_cache.starty)
+  assert(my >= line_cache.starty, 'failed to map y pixel to line')
   -- duplicate some logic from Text.draw
   local y = line_cache.starty
   local start_screen_line_index = Text.screen_line_index(line_cache.screen_line_starting_pos, line_cache.startpos)
@@ -798,7 +797,7 @@ function Text.to_pos_on_line(State, line_index, mx, my)
     end
     y = nexty
   end
-  assert(false)
+  assert(false, 'failed to map y pixel to line')
 end
 
 function Text.screen_line_width(State, line_index, i)
@@ -864,7 +863,7 @@ function Text.nearest_cursor_pos(line, x, left)
       leftpos = curr
     end
   end
-  assert(false)
+  assert(false, 'failed to map x pixel to pos')
 end
 
 -- return the nearest index of line (in utf8 code points) which lies entirely
@@ -895,7 +894,7 @@ function Text.nearest_pos_less_than(line, x)
       left = curr
     end
   end
-  assert(false)
+  assert(false, 'failed to map x pixel to pos')
 end
 
 function Text.x_after(s, pos)
@@ -926,7 +925,7 @@ function Text.to2(State, loc1)
       break
     end
   end
-  assert(result.screen_pos)
+  assert(result.screen_pos, 'failed to convert schema-1 coordinate to schema-2')
   return result
 end
 
@@ -968,7 +967,7 @@ function Text.offset(s, pos1)
   if result == nil then
     print(pos1, #s, s)
   end
-  assert(result)
+  assert(result, "Text.offset returned nil; this is likely a failure to handle utf8")
   return result
 end
 
diff --git a/source_undo.lua b/source_undo.lua
index d3d5f0f..e5dea93 100644
--- a/source_undo.lua
+++ b/source_undo.lua
@@ -36,11 +36,11 @@ end
 -- Make copies of objects; the rest of the app may mutate them in place, but undo requires immutable histories.
 function snapshot(State, s,e)
   -- Snapshot everything by default, but subset if requested.
-  assert(s)
+  assert(s, 'failed to snapshot operation for undo history')
   if e == nil then
     e = s
   end
-  assert(#State.lines > 0)
+  assert(#State.lines > 0, 'failed to snapshot operation for undo history')
   if s < 1 then s = 1 end
   if s > #State.lines then s = #State.lines end
   if e < 1 then e = 1 end
@@ -65,8 +65,7 @@ function snapshot(State, s,e)
     elseif line.mode == 'drawing' then
       table.insert(event.lines, {mode='drawing', h=line.h, points=deepcopy(line.points), shapes=deepcopy(line.shapes), pending={}})
     else
-      print(line.mode)
-      assert(false)
+      assert(false, ('unknown line mode %s'):format(line.mode))
     end
   end
   return event
@@ -80,22 +79,22 @@ function patch(lines, from, to)
 --?     lines[from.start_line] = to.lines[1]
 --?     return
 --?   end
-  assert(from.start_line == to.start_line)
+  assert(from.start_line == to.start_line, 'failed to patch undo operation')
   for i=from.end_line,from.start_line,-1 do
     table.remove(lines, i)
   end
-  assert(#to.lines == to.end_line-to.start_line+1)
+  assert(#to.lines == to.end_line-to.start_line+1, 'failed to patch undo operation')
   for i=1,#to.lines do
     table.insert(lines, to.start_line+i-1, to.lines[i])
   end
 end
 
 function patch_placeholders(line_cache, from, to)
-  assert(from.start_line == to.start_line)
+  assert(from.start_line == to.start_line, 'failed to patch undo operation')
   for i=from.end_line,from.start_line,-1 do
     table.remove(line_cache, i)
   end
-  assert(#to.lines == to.end_line-to.start_line+1)
+  assert(#to.lines == to.end_line-to.start_line+1, 'failed to patch undo operation')
   for i=1,#to.lines do
     table.insert(line_cache, to.start_line+i-1, {})
   end
diff --git a/text.lua b/text.lua
index 4e0735d..2f80d91 100644
--- a/text.lua
+++ b/text.lua
@@ -13,7 +13,7 @@ function Text.draw(State, line_index, y, startpos)
   local final_screen_line_starting_pos = startpos  -- track value to return
   Text.populate_screen_line_starting_pos(State, line_index)
   Text.populate_link_offsets(State, line_index)
-  assert(#line_cache.screen_line_starting_pos >= 1)
+  assert(#line_cache.screen_line_starting_pos >= 1, 'line cache missing screen line info')
   for i=1,#line_cache.screen_line_starting_pos do
     local pos = line_cache.screen_line_starting_pos[i]
     if pos < startpos then
@@ -205,7 +205,7 @@ function Text.text_input(State, t)
 end
 
 function Text.insert_at_cursor(State, t)
-  assert(State.lines[State.cursor1.line].mode == 'text')
+  assert(State.lines[State.cursor1.line].mode == 'text', 'line is not text')
   local byte_offset = Text.offset(State.lines[State.cursor1.line].data, State.cursor1.pos)
   State.lines[State.cursor1.line].data = string.sub(State.lines[State.cursor1.line].data, 1, byte_offset-1)..t..string.sub(State.lines[State.cursor1.line].data, byte_offset)
   Text.clear_screen_line_cache(State, State.cursor1.line)
@@ -282,7 +282,7 @@ function Text.keychord_press(State, chord)
       Text.redraw_all(State)  -- if we're scrolling, reclaim all fragments to avoid memory leaks
     end
     Text.clear_screen_line_cache(State, State.cursor1.line)
-    assert(Text.le1(State.screen_top1, State.cursor1))
+    assert(Text.le1(State.screen_top1, State.cursor1), ('screen_top (line=%d,pos=%d) is below cursor (line=%d,pos=%d)'):format(State.screen_top1.line, State.screen_top1.pos, State.cursor1.line, State.cursor1.pos))
     schedule_save(State)
     record_undo_event(State, {before=before, after=snapshot(State, State.cursor1.line)})
   elseif chord == 'delete' then
@@ -459,7 +459,7 @@ function Text.pagedown(State)
 end
 
 function Text.up(State)
-  assert(State.lines[State.cursor1.line].mode == 'text')
+  assert(State.lines[State.cursor1.line].mode == 'text', 'line is not text')
 --?   print('up', State.cursor1.line, State.cursor1.pos, State.screen_top1.line, State.screen_top1.pos)
   local screen_line_starting_pos, screen_line_index = Text.pos_at_start_of_screen_line(State, State.cursor1)
   if screen_line_starting_pos == 1 then
@@ -485,7 +485,7 @@ function Text.up(State)
     end
   else
     -- move up one screen line in current line
-    assert(screen_line_index > 1)
+    assert(screen_line_index > 1, 'bumped up against top screen line in line')
     local new_screen_line_starting_pos = State.line_cache[State.cursor1.line].screen_line_starting_pos[screen_line_index-1]
     local new_screen_line_starting_byte_offset = Text.offset(State.lines[State.cursor1.line].data, new_screen_line_starting_pos)
     local s = string.sub(State.lines[State.cursor1.line].data, new_screen_line_starting_byte_offset)
@@ -502,9 +502,9 @@ function Text.up(State)
 end
 
 function Text.down(State)
-  assert(State.lines[State.cursor1.line].mode == 'text')
---?   print('down', State.screen_top1.line, State.screen_top1.pos, State.cursor1.line, State.cursor1.pos, State.screen_bottom1.line, State.screen_bottom1.pos)
-  assert(State.cursor1.pos)
+  assert(State.lines[State.cursor1.line].mode == 'text', 'line is not text')
+--?   print('down', State.cursor1.line, State.cursor1.pos, State.screen_top1.line, State.screen_top1.pos, State.screen_bottom1.line, State.screen_bottom1.pos)
+  assert(State.cursor1.pos, 'cursor has no pos')
   if Text.cursor_at_final_screen_line(State) then
     -- line is done, skip to next text line
 --?     print('cursor at final screen line of its line')
@@ -579,7 +579,7 @@ function Text.word_left(State)
     if State.cursor1.pos == 1 then
       break
     end
-    assert(State.cursor1.pos > 1)
+    assert(State.cursor1.pos > 1, 'bumped up against start of line')
     if Text.match(State.lines[State.cursor1.line].data, State.cursor1.pos-1, '%s') then
       break
     end
@@ -613,15 +613,14 @@ end
 
 function Text.match(s, pos, pat)
   local start_offset = Text.offset(s, pos)
-  assert(start_offset)
   local end_offset = Text.offset(s, pos+1)
-  assert(end_offset > start_offset)
+  assert(end_offset > start_offset, ('end_offset %d not > start_offset %d'):format(end_offset, start_offset))
   local curr = s:sub(start_offset, end_offset-1)
   return curr:match(pat)
 end
 
 function Text.left(State)
-  assert(State.lines[State.cursor1.line].mode == 'text')
+  assert(State.lines[State.cursor1.line].mode == 'text', 'line is not text')
   if State.cursor1.pos > 1 then
     State.cursor1.pos = State.cursor1.pos-1
   else
@@ -654,7 +653,7 @@ function Text.right(State)
 end
 
 function Text.right_without_scroll(State)
-  assert(State.lines[State.cursor1.line].mode == 'text')
+  assert(State.lines[State.cursor1.line].mode == 'text', 'line is not text')
   if State.cursor1.pos <= utf8.len(State.lines[State.cursor1.line].data) then
     State.cursor1.pos = State.cursor1.pos+1
   else
@@ -679,7 +678,7 @@ function Text.pos_at_start_of_screen_line(State, loc1)
       return spos,i
     end
   end
-  assert(false)
+  assert(false, ('invalid pos %d'):format(loc1.pos))
 end
 
 function Text.pos_at_end_of_screen_line(State, loc1)
@@ -693,7 +692,7 @@ function Text.pos_at_end_of_screen_line(State, loc1)
     end
     most_recent_final_pos = spos-1
   end
-  assert(false)
+  assert(false, ('invalid pos %d'):format(loc1.pos))
 end
 
 function Text.cursor_at_final_screen_line(State)
@@ -718,7 +717,7 @@ function Text.move_cursor_down_to_next_text_line_while_scrolling_again_if_necess
   end
   -- hack: insert a text line at bottom of file if necessary
   if State.cursor1.line > #State.lines then
-    assert(State.cursor1.line == #State.lines+1)
+    assert(State.cursor1.line == #State.lines+1, 'tried to ensure bottom line of file is text, but failed')
     table.insert(State.lines, {mode='text', data=''})
     table.insert(State.line_cache, {})
   end
@@ -759,8 +758,8 @@ function Text.snap_cursor_to_bottom_of_screen(State)
       end
       y = y - h
     else
-      assert(top2.line > 1)
-      assert(State.lines[top2.line-1].mode == 'drawing')
+      assert(top2.line > 1, 'tried to snap cursor to buttom of screen but failed')
+      assert(State.lines[top2.line-1].mode == 'drawing', "expected a drawing but it's not")
       -- We currently can't draw partial drawings, so either skip it entirely
       -- or not at all.
       local h = Drawing_padding_height + Drawing.pixels(State.lines[top2.line-1].h, State.width)
@@ -793,7 +792,7 @@ function Text.to_pos_on_line(State, line_index, mx, my)
 --?   print('Text.to_pos_on_line', State.left, State.right, App.screen.width, mx)
   local line = State.lines[line_index]
   local line_cache = State.line_cache[line_index]
-  assert(my >= line_cache.starty)
+  assert(my >= line_cache.starty, 'failed to map y pixel to line')
   -- duplicate some logic from Text.draw
   local y = line_cache.starty
   local start_screen_line_index = Text.screen_line_index(line_cache.screen_line_starting_pos, line_cache.startpos)
@@ -816,7 +815,7 @@ function Text.to_pos_on_line(State, line_index, mx, my)
     end
     y = nexty
   end
-  assert(false)
+  assert(false, 'failed to map y pixel to line')
 end
 
 function Text.screen_line_width(State, line_index, i)
@@ -894,7 +893,7 @@ function Text.nearest_cursor_pos(line, x, left)
       leftpos = curr
     end
   end
-  assert(false)
+  assert(false, 'failed to map x pixel to pos')
 end
 
 -- return the nearest index of line (in utf8 code points) which lies entirely
@@ -925,7 +924,7 @@ function Text.nearest_pos_less_than(line, x)
       left = curr
     end
   end
-  assert(false)
+  assert(false, 'failed to map x pixel to pos')
 end
 
 function Text.x_after(s, pos)
@@ -956,7 +955,7 @@ function Text.to2(State, loc1)
       break
     end
   end
-  assert(result.screen_pos)
+  assert(result.screen_pos, 'failed to convert schema-1 coordinate to schema-2')
   return result
 end
 
@@ -1004,7 +1003,7 @@ function Text.offset(s, pos1)
   if result == nil then
     print(pos1, #s, s)
   end
-  assert(result)
+  assert(result, "Text.offset returned nil; this is likely a failure to handle utf8")
   return result
 end
 
diff --git a/undo.lua b/undo.lua
index d3d5f0f..1ed3ce7 100644
--- a/undo.lua
+++ b/undo.lua
@@ -36,11 +36,11 @@ end
 -- Make copies of objects; the rest of the app may mutate them in place, but undo requires immutable histories.
 function snapshot(State, s,e)
   -- Snapshot everything by default, but subset if requested.
-  assert(s)
+  assert(s, 'failed to snapshot operation for undo history')
   if e == nil then
     e = s
   end
-  assert(#State.lines > 0)
+  assert(#State.lines > 0, 'failed to snapshot operation for undo history')
   if s < 1 then s = 1 end
   if s > #State.lines then s = #State.lines end
   if e < 1 then e = 1 end
@@ -66,7 +66,7 @@ function snapshot(State, s,e)
       table.insert(event.lines, {mode='drawing', h=line.h, points=deepcopy(line.points), shapes=deepcopy(line.shapes), pending={}})
     else
       print(line.mode)
-      assert(false)
+      assert(false, ('unknown line mode %s'):format(line.mode))
     end
   end
   return event
@@ -80,22 +80,22 @@ function patch(lines, from, to)
 --?     lines[from.start_line] = to.lines[1]
 --?     return
 --?   end
-  assert(from.start_line == to.start_line)
+  assert(from.start_line == to.start_line, 'failed to patch undo operation')
   for i=from.end_line,from.start_line,-1 do
     table.remove(lines, i)
   end
-  assert(#to.lines == to.end_line-to.start_line+1)
+  assert(#to.lines == to.end_line-to.start_line+1, 'failed to patch undo operation')
   for i=1,#to.lines do
     table.insert(lines, to.start_line+i-1, to.lines[i])
   end
 end
 
 function patch_placeholders(line_cache, from, to)
-  assert(from.start_line == to.start_line)
+  assert(from.start_line == to.start_line, 'failed to patch undo operation')
   for i=from.end_line,from.start_line,-1 do
     table.remove(line_cache, i)
   end
-  assert(#to.lines == to.end_line-to.start_line+1)
+  assert(#to.lines == to.end_line-to.start_line+1, 'failed to patch undo operation')
   for i=1,#to.lines do
     table.insert(line_cache, to.start_line+i-1, {})
   end