diff --git a/routes/sources.js b/routes/sources.js
index 4347774..5b2352e 100644
--- a/routes/sources.js
+++ b/routes/sources.js
@@ -224,6 +224,41 @@ module.exports = function(pool) {
         }
     });
 
+    // given a key column value, look up sibling dim_group column values from source
+    // returns { sibling_col: value, ... } if exactly one match, null if none or ambiguous
+    router.get('/sources/:id/lookup', async (req, res) => {
+        const { col, value } = req.query;
+        if (!col || value == null || value === '') return res.json(null);
+        try {
+            const [srcResult, metaResult] = await Promise.all([
+                pool.query(`SELECT schema, tname FROM pf.source WHERE id = $1`, [req.params.id]),
+                pool.query(`SELECT * FROM pf.col_meta WHERE source_id = $1 ORDER BY opos`, [req.params.id])
+            ]);
+            if (srcResult.rows.length === 0) return res.status(404).json({ error: 'Source not found' });
+
+            const keyCol = metaResult.rows.find(c => c.cname === col && c.is_key && c.dim_group);
+            if (!keyCol) return res.json(null);
+
+            const siblings = metaResult.rows.filter(c =>
+                c.dim_group === keyCol.dim_group && c.cname !== col
+            );
+            if (!siblings.length) return res.json(null);
+
+            const { schema, tname } = srcResult.rows[0];
+            const sibCols = siblings.map(c => `"${c.cname}"`).join(', ');
+            const result = await pool.query(
+                `SELECT DISTINCT ${sibCols} FROM "${schema}"."${tname}" WHERE "${col}" = $1 LIMIT 2`,
+                [value]
+            );
+
+            if (result.rows.length !== 1) return res.json(null);
+            res.json(result.rows[0]);
+        } catch (err) {
+            console.error(err);
+            res.status(500).json({ error: err.message });
+        }
+    });
+
     // set or clear the default Perspective layout for a source.
     // Body: a Perspective view config (group_by, split_by, columns, plugin_config, …).
     // Pass null or {} to clear.
diff --git a/ui/src/views/Forecast.jsx b/ui/src/views/Forecast.jsx
index 93192ca..94e85fd 100644
--- a/ui/src/views/Forecast.jsx
+++ b/ui/src/views/Forecast.jsx
@@ -436,6 +436,21 @@ export default function Forecast({ sources = [], sourceId, versionId, refreshSou
     } catch (err) { flash(err.message, 'error') }
   }
 
+  async function lookupDerivedCols(col, value, setter) {
+    if (!sourceId || !value.trim()) return
+    const res = await fetch(`/api/sources/${sourceId}/lookup?col=${encodeURIComponent(col)}&value=${encodeURIComponent(value)}`)
+    if (!res.ok) return
+    const derived = await res.json()
+    if (!derived) return
+    setter(prev => {
+      const next = { ...prev }
+      for (const [k, v] of Object.entries(derived)) {
+        if (!prev[k] || prev[k] === '') next[k] = String(v ?? '')
+      }
+      return next
+    })
+  }
+
   function buildEffectiveSlice(raw) {
     const dimCols  = new Set(colMetaRef.current.filter(c => c.role === 'dimension').map(c => c.cname))
     const dateCols = new Set(colMetaRef.current.filter(c => c.role === 'date').map(c => c.cname))
@@ -842,9 +857,15 @@ export default function Forecast({ sources = [], sourceId, versionId, refreshSou
                 {activeOp === 'recode' && <>
                   <p className="text-gray-400">New values for dimensions to replace. Leave blank to keep.</p>
                   {dimCols.map(c => (
-                    <Row key={c.cname} label={c.cname}>
-                      <input value={recodeSet[c.cname] || ''} onChange={e => setRecodeSet(s => ({ ...s, [c.cname]: e.target.value }))}
-                        placeholder={slice[c.cname] || '—'} className={`${inp} font-mono`} />
+                    <Row key={c.cname} label={c.label || c.cname}>
+                      <input
+                        value={recodeSet[c.cname] || ''}
+                        onChange={e => setRecodeSet(s => ({ ...s, [c.cname]: e.target.value }))}
+                        onBlur={c.is_key && c.dim_group
+                          ? e => lookupDerivedCols(c.cname, e.target.value, setRecodeSet)
+                          : undefined}
+                        placeholder={slice[c.cname] || '—'}
+                        className={`${inp} font-mono`} />
                     </Row>
                   ))}
                   <Row label="Note"><input value={recodeNote} onChange={e => setRecodeNote(e.target.value)} placeholder="optional" className={inp} /></Row>
@@ -855,9 +876,15 @@ export default function Forecast({ sources = [], sourceId, versionId, refreshSou
                 {activeOp === 'clone' && <>
                   <p className="text-gray-400">Override dimensions on cloned rows. Leave blank to keep.</p>
                   {dimCols.map(c => (
-                    <Row key={c.cname} label={c.cname}>
-                      <input value={cloneSet[c.cname] || ''} onChange={e => setCloneSet(s => ({ ...s, [c.cname]: e.target.value }))}
-                        placeholder={slice[c.cname] || '—'} className={`${inp} font-mono`} />
+                    <Row key={c.cname} label={c.label || c.cname}>
+                      <input
+                        value={cloneSet[c.cname] || ''}
+                        onChange={e => setCloneSet(s => ({ ...s, [c.cname]: e.target.value }))}
+                        onBlur={c.is_key && c.dim_group
+                          ? e => lookupDerivedCols(c.cname, e.target.value, setCloneSet)
+                          : undefined}
+                        placeholder={slice[c.cname] || '—'}
+                        className={`${inp} font-mono`} />
                     </Row>
                   ))}
                   <Row label="Scale"><input type="number" step="any" value={cloneScale} onChange={e => setCloneScale(e.target.value)} className={inp} /></Row>