diff --git a/main.lua b/main.lua
index d776fd0..5f3dfa3 100644
--- a/main.lua
+++ b/main.lua
@@ -421,6 +421,46 @@ local function tag_labels(tag_set)
     return labels
 end
 
+-- Build gaps: what the current board is shopping for. A tag is a gap when
+-- some owned joker wants it and no OTHER owned joker gives it (same rule as
+-- scoring and theme discovery: a joker cannot feed itself). The reverse -- a
+-- give no other owned joker wants -- is an untapped hook a future purchase
+-- could pay off. Engine roles (mult/chips/xmult) are skipped for the same
+-- reason they have no TAG_LABEL: they hint nothing specific. Returns two
+-- tag arrays in TAG_ORDER, so the display is stable across hovers.
+function JCA.build_gaps()
+    local looking, untapped = {}, {}
+    if not (G.jokers and G.jokers.cards) then return looking, untapped end
+    local entries = {}
+    for _, c in ipairs(G.jokers.cards) do
+        if is_joker(c) then
+            local e = JCA.db[c.config.center.key]
+            if e then entries[#entries + 1] = e end
+        end
+    end
+    -- Compare by index, not table identity: two copies of the same joker
+    -- share one db entry, and they DO feed each other's cluster tags.
+    local function others_have(side, tag, self_idx)
+        for i, e in ipairs(entries) do
+            if i ~= self_idx and e[side][tag] then return true end
+        end
+    end
+    local want_gap, give_hook = {}, {}
+    for i, e in ipairs(entries) do
+        for t in pairs(e.wants) do
+            if TAG_LABEL[t] and not others_have('gives', t, i) then want_gap[t] = true end
+        end
+        for t in pairs(e.gives) do
+            if TAG_LABEL[t] and not others_have('wants', t, i) then give_hook[t] = true end
+        end
+    end
+    for _, t in ipairs(data.tag_order) do
+        if want_gap[t] then looking[#looking + 1] = t end
+        if give_hook[t] then untapped[#untapped + 1] = t end
+    end
+    return looking, untapped
+end
+
 local function tooltip_rows(card)
     -- Touch mode enlarges every advice row for phone screens (see config.lua).
     local S = JCA.config.touch_mode and 1.4 or 1
@@ -1250,18 +1290,43 @@ end
 -- in an overlay. The overlay reuses catalog_tabs(); the page cyclers work
 -- unchanged because vanilla create_tabs also names its body 'tab_contents'.
 
+-- Shopping list shown at the top of the in-run overlay: the tags the board
+-- wants but nothing else on it gives, and the hooks it gives that nothing
+-- pays off yet. Teaching content, so learning mode keeps it.
+local function build_gaps_rows()
+    local looking, untapped = JCA.build_gaps()
+    local nodes = {}
+    local function line(prefix, tags, colour)
+        if #tags == 0 then return end
+        local labels = {}
+        for _, t in ipairs(tags) do labels[#labels + 1] = {name = TAG_LABEL[t]} end
+        for i, l in ipairs(wrap_names(labels, 58, 2)) do
+            nodes[#nodes + 1] = {n = G.UIT.R, config = {align = 'cm', padding = 0.02}, nodes = {
+                {n = G.UIT.T, config = {text = (i == 1 and prefix or '  ') .. l,
+                    colour = colour, scale = 0.3, shadow = true}},
+            }}
+        end
+    end
+    line('Your build is looking for: ', looking, G.C.GOLD)
+    line('Untapped on your board: ', untapped, G.C.UI.TEXT_LIGHT)
+    if #nodes == 0 then return nil end
+    return {n = G.UIT.R, config = {align = 'cm', padding = 0.04}, nodes = nodes}
+end
+
 G.FUNCS.jca_open_catalog = function()
+    local contents = {}
+    local ok, gaps = pcall(build_gaps_rows)
+    if ok and gaps then contents[#contents + 1] = gaps end
+    contents[#contents + 1] = {n = G.UIT.R, config = {align = 'tm', padding = 0}, nodes = {
+        create_tabs({
+            tabs = catalog_tabs(),
+            snap_to_nav = true,
+            colour = G.C.BOOSTER,
+        }),
+    }}
     G.FUNCS.overlay_menu{definition = create_UIBox_generic_options({
         back_func = 'exit_overlay_menu',
-        contents = {
-            {n = G.UIT.R, config = {align = 'tm', padding = 0}, nodes = {
-                create_tabs({
-                    tabs = catalog_tabs(),
-                    snap_to_nav = true,
-                    colour = G.C.BOOSTER,
-                }),
-            }},
-        },
+        contents = contents,
     })}
 end
 
diff --git a/test.lua b/test.lua
index 1bc0464..a0bddee 100644
--- a/test.lua
+++ b/test.lua
@@ -400,6 +400,35 @@ ok(cut == board[1] or cut == board[2],
     'the sell advisor flags one of the two clashing jokers',
     cut and cut.config.center.key or 'nil')
 
+--------------------------------------------------------------------------------
+section('Engine: build gaps name what the board is shopping for')
+--------------------------------------------------------------------------------
+
+-- Baron alone: wants held-card retriggers and hand size, and nothing on the
+-- board provides either; its copy_target give has no copier to pay it off.
+field('j_baron')
+local looking, untapped = JCA.build_gaps()
+eq(table.concat(looking, ','), 'retrig_held,hand_size',
+    'a lone Baron is shopping for its enablers, in TAG_ORDER')
+eq(table.concat(untapped, ','), 'copy_target',
+    'its copy_target hook is untapped until a copier arrives')
+
+-- Mime fills the retrig_held gap (and only that gap).
+field('j_baron', 'j_mime')
+looking = JCA.build_gaps()
+eq(table.concat(looking, ','), 'hand_size', 'Mime closes the retrig_held gap')
+
+-- Two copies of the same cluster joker feed each other: the entries share one
+-- db table, so the check must compare board slots, not table identity.
+field('j_jolly', 'j_jolly')
+looking = JCA.build_gaps()
+eq(table.concat(looking, ','), '', 'two Jolly Jokers are not shopping for Pairs')
+
+-- One alone is: a cluster member cannot feed itself.
+field('j_jolly')
+looking = JCA.build_gaps()
+eq(table.concat(looking, ','), 'pair', 'a lone cluster member still wants its cluster')
+
 --------------------------------------------------------------------------------
 section('Data: themes are reachable')
 --------------------------------------------------------------------------------