diff --git a/docs/DESIGN.md b/docs/DESIGN.md
index 5703815..5a5fdc2 100644
--- a/docs/DESIGN.md
+++ b/docs/DESIGN.md
@@ -470,7 +470,8 @@ above, including:
   building needs a range limit at all, since the bot needs no arms to build. Agreed: keep a limit,
   but make it about what's on screen, so anything the player can see and aim at builds directly
   and anything beyond stays a blueprint built when the bot comes near. No limit at all would let
-  players build in places they've never been and link far claims instantly. Not built yet.
+  players build in places they've never been and link far claims instantly. Built: the range is
+  the visible view box, at least 64 grains from its centre and at most 200 when zoomed far out.
 - **Sticky materials slide freely on built surfaces (t61, 2026-10-27).** Stickiness (STICK) is the
   chance per tick a grain takes a diagonal step, which made clay lumps, snow, wood chunks, ash and
   dirt creep down sloped walls (a single clay grain never got down a 20-piece slope). The owner
diff --git a/docs/PROJECT_MAP.md b/docs/PROJECT_MAP.md
index 72ff5a9..ad7e1c5 100644
--- a/docs/PROJECT_MAP.md
+++ b/docs/PROJECT_MAP.md
@@ -315,9 +315,10 @@ Bars and other loose objects (ents), and the bag: adding, taking, costs and refu
 - functions: entSolidCell, entSupportCount, entFreeAt, entMark, spawnBar, removeEnt, entScrewAt, entScrewFree, entScrewMove, entScrewLane, entScrewEdge, entScrewEnter, entScrewTick, entDiagonalScrewTick, entBeltScrewEnter, entSlopeBeltTick, entTick, entsInCircle, bagAdd, bagTake, giveOrDrop, refundAt, canAfford, payCost, fmtMoney, costText, needText
 - variables: BAR_W, BAR_H, entCurM, droppedLast
 
-### src/sim/tools.js (52 lines)
+### src/sim/tools.js (65 lines)
 Tool helpers: reach, brushes, and plain digging.
-- functions: inReach, forBrush, drillReach, inDrillReach, toolDig
+- functions: setBuildRange, inBuildRange, inReach, forBrush, drillReach, inDrillReach, toolDig
+- variables: BUILD_MAX, buildBox
 
 ### src/sim/drill.js (161 lines)
 The resonance drill: tuned pulses that shatter one material.
@@ -342,7 +343,7 @@ Player actions on built things: take back, place a machine or mold, gates, F to
 Blueprints: copy, cut, flip, cost and paste (as ghosts) a region of built pieces.
 - functions: copyRegion, flipBlueprint, bpCost, pasteGhost, cutRegion, footprintBlocker, clearFootprint
 
-### src/sim/undo.js (87 lines)
+### src/sim/undo.js (93 lines)
 Undo (Ctrl+Z) for building, and the bot building ghosts.
 - functions: undoNote, undoPush, undoRecord, undoLast, clearAllGhosts, ghostTick, ghostTick0
 - variables: UNDO_MAX, undoStack, undoRec, ghostMissing, ghostIter
@@ -511,7 +512,7 @@ Keyboard and mouse listeners.
 - functions: isTyping
 - variables: canvas
 
-### src/ui/actions.js (242 lines)
+### src/ui/actions.js (243 lines)
 What input does: overlays, F, pick, copy / cut / paste, undo, building and tool use each frame. Which overlays are on, shown as a coloured frame round the screen and a named badge
 - functions: updateOverlayUI, toggleLens, doInteract, pickUnderCursor, aimBlock, stampOrigin, selModeHeld, startPaste, stopSelectAndPaste, plural, piecesIn, doUndo, endRmbRecord, dragDirFor, blockLabel, primaryPress, primaryRelease, secondaryPress, toolTick
 - variables: ovShown
@@ -544,7 +545,7 @@ Shared world colours, minimap and full map.
 - functions: resetMap, mapArea, updateMap, drawMap, mapFit, drawFullMap, openMap, zoomMap, zoomMinimap
 - variables: MAP_S, MAP
 
-### src/ui/loop.js (136 lines)
+### src/ui/loop.js (137 lines)
 The frame loop and boot.
 - functions: scanVisible, frame, boot
 - variables: lastT, acc, lastRender, frameMs, simMs, fpsSm, ambientT
diff --git a/docs/agent-log.md b/docs/agent-log.md
index 7c1ada7..3faa6f7 100644
--- a/docs/agent-log.md
+++ b/docs/agent-log.md
@@ -4,6 +4,19 @@ What agents have done, newest first. Add an entry for every piece of work: the d
 note or request it was for, what changed, the files touched, the tests run, and anything left
 open or not verified. Keep each entry short; the git history has the details.
 
+## 2026-10-27: t64 build range about the screen; out-of-range replace ghosts (Claude Code)
+
+- `setBuildRange`/`inBuildRange` in `src/sim/tools.js`: the UI sets the view box each frame
+  (`src/ui/loop.js`); half-sizes clamped to REACH..BUILD_MAX (64..200 grains); with no view (tests)
+  a REACH box around the bot. Used by `blockInReach`, machine/mold placement and previews,
+  right-click take-back, hover reach, and ghost building for blocks, machines and molds.
+- A replace (G) placed beyond range stores a replace flag on the ghost (`[code, v, extra, 1]`);
+  `ghostTick0` removes the old piece (refunded) and places the new one (charged), or re-sets a
+  filter's settings, when the bot comes in range.
+- Files: `src/sim/tools.js`, `src/sim/blocks.js`, `src/sim/undo.js`, `src/ui/actions.js`,
+  `src/ui/previews.js`, `src/ui/info.js`, `src/ui/loop.js`, `test/t_build.js`, `docs/DESIGN.md`,
+  `docs/PROJECT_MAP.md`. Tests: `python3 test/run_all.py --browser` all passed. Not played by hand.
+
 ## 2026-10-27: t49 throughput overlay (Claude Code)
 
 - The I overlay also shows grains per second at each line's open outlet, an orange glow on
diff --git a/src/sim/blocks.js b/src/sim/blocks.js
index 71431d7..380d497 100644
--- a/src/sim/blocks.js
+++ b/src/sim/blocks.js
@@ -209,7 +209,7 @@ function blockRect(bx0, by0, bx1, by1) {
   for (let y = Math.min(by0, by1); y <= Math.max(by0, by1); y++) for (let x = Math.min(bx0, bx1); x <= Math.max(bx0, bx1); x++) out.push([x, y]);
   return out;
 }
-function blockInReach(bx, by) { return inReach(bx * BS + BS / 2, by * BS + BS / 2); }
+function blockInReach(bx, by) { return inBuildRange(bx * BS + BS / 2, by * BS + BS / 2); }
 // Is block b already exactly this piece (same kind, variant and, for a filter, settings)?
 function blockIs(b, code, v, extra) {
   if (blockP[b] !== code) return false;
diff --git a/src/sim/tools.js b/src/sim/tools.js
index f3c51ff..b86bed1 100644
--- a/src/sim/tools.js
+++ b/src/sim/tools.js
@@ -1,5 +1,18 @@
 // Tool helpers: reach, brushes, and plain digging.
 // ---------- Tools ----------
+// Building reaches about as far as the screen shows (t64): the view box the UI sets each frame
+// (setBuildRange), at least REACH from its centre and at most BUILD_MAX; anything beyond becomes
+// a blueprint ghost. With no view (tests, headless), a REACH box around the bot.
+const BUILD_MAX = 200;
+let buildBox = null;
+function setBuildRange(cx, cy, w, h) {
+  const hw = Math.max(REACH, Math.min(BUILD_MAX, w / 2)), hh = Math.max(REACH, Math.min(BUILD_MAX, h / 2));
+  if (!buildBox) buildBox = { cx, cy, hw, hh }; else { buildBox.cx = cx; buildBox.cy = cy; buildBox.hw = hw; buildBox.hh = hh; }
+}
+function inBuildRange(x, y) {
+  if (!buildBox) { const dx = x - (bot.x + bot.w / 2), dy = y - (bot.y + bot.h / 2); return Math.abs(dx) <= REACH && Math.abs(dy) <= REACH; }
+  return Math.abs(x - buildBox.cx) <= buildBox.hw && Math.abs(y - buildBox.cy) <= buildBox.hh;
+}
 function inReach(x, y) { const dx = x - (bot.x + bot.w / 2), dy = y - (bot.y + bot.h / 2); return dx * dx + dy * dy <= REACH * REACH; }
 function forBrush(cx, cy, r, fn) {
   const r2 = (r + 0.5) * (r + 0.5);
diff --git a/src/sim/undo.js b/src/sim/undo.js
index b62f818..e62deac 100644
--- a/src/sim/undo.js
+++ b/src/sim/undo.js
@@ -54,13 +54,19 @@ function ghostTick0() {
   while (budget > 0 && checks-- > 0 && ghostBlocks.size) {
     let nx = ghostIter.next();
     if (nx.done) { ghostIter = ghostBlocks.entries(); nx = ghostIter.next(); if (nx.done) break; }
-    const [b, [code, v, e]] = nx.value;
+    const [b, [code, v, e, swap]] = nx.value;
     const bx = b % BW, by = (b / BW) | 0;
     if (blockP[b] === code && blockV[b] === (isFilter(code) ? v & 128 : v)) {
       if (isFilter(code) && e && !sameFilter(filterInfo(b), cleanFilterCfg(e))) setFilter(b, e); // same piece, new settings
       ghostBlocks.delete(b); ghostBlocked.delete(b); continue;
     }
     if (!blockInReach(bx, by)) continue;
+    // a replacing ghost takes down what is there (refunded) and builds itself, like a replace in reach
+    if (swap && blockP[b]) {
+      if (blockP[b] === code && isFilter(code)) { blockV[b] = filterVariant(code, v); setFilter(b, e); filtersDirty = true; ghostBlocks.delete(b); ghostBlocked.delete(b); budget--; continue; }
+      if (!canAfford(pieceCost(code, v), 1)) { ghostMissing++; continue; }
+      removeBlock(bx, by);
+    }
     const err = canPlaceBlock(bx, by, code);
     if (err) { if (err.startsWith('Needs')) ghostBlocked.add(b); continue; }
     if (!canAfford(pieceCost(code, v), 1)) { ghostMissing++; continue; }
@@ -70,7 +76,7 @@ function ghostTick0() {
   }
   for (let k = ghostMachines.length - 1; k >= 0 && budget > 0; k--) {
     const g = ghostMachines[k], P = PIECE_BY_ID[g.type];
-    if (!inReach(g.bx * BS + P.w / 2, g.by * BS + P.h / 2)) continue;
+    if (!inBuildRange(g.bx * BS + P.w / 2, g.by * BS + P.h / 2)) continue;
     if (canPlaceMachine(g.type, g.bx, g.by)) continue;
     if (!canAfford(P.cost, 1)) { ghostMissing++; continue; }
     placeMachineAt(g.type, g.dir, g.bx, g.by); ghostMachines.splice(k, 1); budget--;
@@ -78,7 +84,7 @@ function ghostTick0() {
   }
   for (let k = ghostMolds.length - 1; k >= 0 && budget > 0; k--) {
     const g = ghostMolds[k], md = MOLDS[g.kind];
-    if (!inReach(g.bx * BS + md.w / 2, g.by * BS + md.h / 2)) continue;
+    if (!inBuildRange(g.bx * BS + md.w / 2, g.by * BS + md.h / 2)) continue;
     if (canPlaceMold(g.kind, g.bx, g.by)) continue;
     if (!canAfford(wallCost(g.wall, md.walls), 1)) { ghostMissing++; continue; }
     addMold(g.kind, g.bx, g.by, g.wall, false); ghostMolds.splice(k, 1); budget--;
diff --git a/src/ui/actions.js b/src/ui/actions.js
index f6ff395..c4c7ace 100644
--- a/src/ui/actions.js
+++ b/src/ui/actions.js
@@ -131,7 +131,7 @@ function primaryPress(shift) {
     else {
       const { bw, bh } = pieceSize(P);
       const { bx, by } = stampOrigin(bw, bh);
-      if (!inReach(bx * BS + bw * BS / 2, by * BS + bh * BS / 2)) { setStatus('Out of reach.'); playSfx('error'); return; }
+      if (!inBuildRange(bx * BS + bw * BS / 2, by * BS + bh * BS / 2)) { setStatus('Out of reach.'); playSfx('error'); return; }
       const place = () => P.kind === 'mold' ? placeMoldAt(P.mold, bx, by, GAME.wallMat) : placeMachineAt(P.id, GAME.pieceDir[P.id] || 0, bx, by);
       const check = () => P.kind === 'mold' ? canPlaceMold(P.mold, bx, by) : canPlaceMachine(P.id, bx, by);
       let err = '';
@@ -178,7 +178,8 @@ function primaryRelease(shift) {
       for (const [bx, by] of far) {
         const b = by * BW + bx;
         if (bx < 0 || by < 0 || bx >= BW || by >= BH || blockIs(b, code, v, ex) || ghostBlocks.has(b)) continue;
-        ghostBlocks.set(b, ex ? [code, v, ex] : [code, v]); g.blocks.push([bx, by, code]); ghosts++;
+        // held G over a different piece: the ghost replaces it when the bot comes in range
+        ghostBlocks.set(b, GAME.replace && blockP[b] ? [code, v, ex, 1] : ex ? [code, v, ex] : [code, v]); g.blocks.push([bx, by, code]); ghosts++;
       }
       if (ghosts) undoNote(g);
       return res;
@@ -230,7 +231,7 @@ function toolTick() {
       if (r.placed) { playSfx('blow'); hudDirty = true; } else if (r.status) setStatus(r.status);
     } else if (t === 'build') {
       const key = `${(ax / BS) | 0},${(ay / BS) | 0}`;
-      if (!GAME.rmbSeen.has(key) && inReach(ax, ay)) {
+      if (!GAME.rmbSeen.has(key) && inBuildRange(ax, ay)) {
         GAME.rmbSeen.add(key);
         const keep = undoRec; undoRec = GAME.rmbRec;
         const msg = removePieceAt(ax, ay);
diff --git a/src/ui/info.js b/src/ui/info.js
index 6a4e9ff..ea798f6 100644
--- a/src/ui/info.js
+++ b/src/ui/info.js
@@ -158,7 +158,7 @@ function hoverInfo(ax, ay) {
     const hint = furnaceJunkHint(mo).trim(); if (hint) t2.push(hint);
     if (player.lens) t2.push(fmtT(temp[i]));
   }
-  const reachOk = toolId() === 'drill' || inReach(ax, ay);
+  const reachOk = toolId() === 'drill' || (toolId() === 'build' ? inBuildRange(ax, ay) : inReach(ax, ay));
   if (!reachOk) { t2.push('out of reach'); warn = 'out of reach'; }
   // what it does, unless the cursor is on a machine or mold, which have their own details
   const what = bar ? bar.m : !mo && !id && !(p && p !== P_GRATE && p !== P_PIPE) && m !== BELTL && m !== BELTR && m !== PIPE ? (m === EMPTY && bgm[i] ? bgm[i] : m) : 0;
diff --git a/src/ui/loop.js b/src/ui/loop.js
index 70c1797..f4c8c69 100644
--- a/src/ui/loop.js
+++ b/src/ui/loop.js
@@ -54,6 +54,7 @@ function frame(now) {
   // grid under the cursor does not slide around mid-line.
   if (GAME.drag) { lookX = GAME.lookHold.x; lookY = GAME.lookHold.y; } else { GAME.lookHold.x = lookX; GAME.lookHold.y = lookY; }
   if (!GAME.drag || Math.hypot(bot.vx, bot.vy) > 0.08) updateCamera(dt, lookX, lookY);
+  { const v = viewCells(); setBuildRange(R.camX + v.w / 2, R.camY + v.h / 2, v.w, v.h); }
   R.frame++;
   drawWorld();
   drawCracks();
diff --git a/src/ui/previews.js b/src/ui/previews.js
index 53a0c5f..c44ea69 100644
--- a/src/ui/previews.js
+++ b/src/ui/previews.js
@@ -67,7 +67,7 @@ function drawPreviews() {
   ctx.save();
   const reachOk = inReach(ax, ay);
   const sel = GAME.selDrag ? GAME.selDrag.mode : selModeHeld(), pasting = !sel && GAME.paste && GAME.clip;
-  if (!reachOk && t !== 'drill' && !sel) {
+  if (!reachOk && t !== 'drill' && t !== 'build' && !sel) {
     drawReachCircle(REACH, 0.35);
   }
   drawDrillPulses();
@@ -143,7 +143,7 @@ function drawPreviews() {
       const { bx, by } = stampOrigin(bw, bh);
       const err = P.kind === 'mold' ? canPlaceMold(P.mold, bx, by) : canPlaceMachine(P.id, bx, by);
       const swap = !!err && GAME.replace && /already/.test(err) && !footprintBlocker(bx, by, bw, bh);
-      const ok = (!err || swap) && inReach(bx * BS + bw * BS / 2, by * BS + bh * BS / 2) && canAfford(pieceCostNow(P), 1);
+      const ok = (!err || swap) && inBuildRange(bx * BS + bw * BS / 2, by * BS + bh * BS / 2) && canAfford(pieceCostNow(P), 1);
       const [x, y, w, h] = blockRectPx(bx, by, bw, bh);
       if (swap && ok) { ctx.fillStyle = 'rgba(255,190,90,0.35)'; ctx.fillRect(x, y, w, h); }
       if (P.kind === 'mold') {
diff --git a/test/t_build.js b/test/t_build.js
index b033d54..8bfb3b7 100644
--- a/test/t_build.js
+++ b/test/t_build.js
@@ -79,4 +79,29 @@ run('for (let y = 120; y < 122; y++) for (let x = 60; x < 62; x++) setCell(y * W
 const n = run('clearHardSpecks(true)');
 check('a 4-grain speck of iron ore in stone becomes stone; a 100-grain vein stays', n === 1 && count(run, run('FEV')) === 100 && run('mat[120 * W + 60]') === run('STONE'), `${n} specks, ${count(run, run('FEV'))} vein`);
 
+// t64: building reaches about as far as the screen shows, and a replace placed out of range
+// becomes a ghost that replaces the old piece when the bot comes in range.
+fresh();
+check('with no view the build range is a REACH box around the bot', run('buildBox = null; inBuildRange(bot.x + bot.w / 2 + REACH, bot.y) && !inBuildRange(bot.x + bot.w / 2 + REACH + 2, bot.y)'));
+check('the build range follows the view box', run('(() => { setBuildRange(200, 100, 300, 160); const r = inBuildRange(340, 100) && !inBuildRange(352, 100) && inBuildRange(200, 180) && !inBuildRange(200, 190); buildBox = null; return r; })()'));
+check('a tiny view still reaches REACH and a huge one stops at BUILD_MAX', run('(() => { setBuildRange(500, 500, 20, 20000); const r = buildBox.hw === REACH && buildBox.hh === BUILD_MAX; buildBox = null; return r; })()'));
+fresh();
+const swap = run(`(() => {
+  placeBlock(BP_BELT, 1, 50, 20, true); const b = 20 * BW + 50, m0 = money;
+  ghostBlocks.set(b, [BP_SCREW, 1, undefined, 1]);
+  bot.x = 20; bot.y = 20; for (let k = 0; k < 30; k++) ghostTick();
+  const before = blockP[b];
+  bot.x = 50 * BS - 4; bot.y = 20 * BS - 10; for (let k = 0; k < 30; k++) ghostTick();
+  return { before, after: blockP[b], v: blockV[b], spent: m0 - money, ghosts: ghostBlocks.size };
+})()`);
+check('an out-of-range replace ghost leaves the old piece until the bot comes near', swap.before === run('BP_BELT'), JSON.stringify(swap));
+check('then replaces it: the new piece is built and the ghost is gone', swap.after === run('BP_SCREW') && swap.v === 1 && swap.ghosts === 0, JSON.stringify(swap));
+check('the replace charges the new piece and refunds the old one', swap.spent === 25 - 12, JSON.stringify(swap));
+fresh();
+check('a plain ghost over a different piece still leaves it alone', run(`(() => {
+  placeBlock(BP_BELT, 1, 30, 20, true); const b = 20 * BW + 30;
+  ghostBlocks.set(b, [BP_SCREW, 1]); bot.x = 30 * BS - 4; bot.y = 20 * BS - 10;
+  for (let k = 0; k < 30; k++) ghostTick();
+  return blockP[b] === BP_BELT && ghostBlocks.has(b);
+})()`));
 done();