diff --git a/packages/ludic.physics/bodies.ludic b/packages/ludic.physics/bodies.ludic
index a3d4fa94..519803ab 100644
--- a/packages/ludic.physics/bodies.ludic
+++ b/packages/ludic.physics/bodies.ludic
@@ -16,7 +16,10 @@ export property PhysPose {
 function ph_add(physics_st: mut PhysicsState, shape: int, x: float, y: float, z: float, yaw: float, motion: int, layer: int, mass: float) -> int {
   let s = ph_shape(physics_st, shape)
   if physics_st.ph_world == null or s == null { return -1 }
-  return jph_body_add(physics_st.ph_world, s, x, y, z, yaw, motion, layer, mass, 0.6, 0.1)
+  let id = jph_body_add(physics_st.ph_world, s, x, y, z, yaw, motion, layer, mass, 0.6, 0.1)
+  # 25.5a: a world full to the bodies it was opened for is a failure, never a thing quietly not there
+  if id < 0 and jph_world_bodies(physics_st.ph_world) >= physics_st.ph_max { Mem.over("a physics body past phys_open's max_bodies") }
+  return id
 }
 
 # the ground: a heightfield or a mesh at the world's origin
diff --git a/packages/ludic.physics/index.ludic b/packages/ludic.physics/index.ludic
index 4de40a96..7e39e3ba 100644
--- a/packages/ludic.physics/index.ludic
+++ b/packages/ludic.physics/index.ludic
@@ -8,6 +8,7 @@ import "state.ludic"
 import "fact_pool.ludic"
 import "shape_ids.ludic"
 import "owned.ludic"
+import "world.ludic"
 import "shapes.ludic"
 import "bodies.ludic"
 import "queries.ludic"
diff --git a/packages/ludic.physics/lib/macos-arm64/libludicjolt.dylib b/packages/ludic.physics/lib/macos-arm64/libludicjolt.dylib
index 10610d51..18dd4b42 100755
--- a/packages/ludic.physics/lib/macos-arm64/libludicjolt.dylib
+++ b/packages/ludic.physics/lib/macos-arm64/libludicjolt.dylib
@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:5b79fd1c0c2c8d7d5225ef4251d4e64fb454283a972cf2b16e68a83b953daf65
+oid sha256:8237112881ad63698c7c1b32c45597b50a1e8f8f0aa4e99298ed2f8979f63c0a
 size 1161504
diff --git a/packages/ludic.physics/native.ludic b/packages/ludic.physics/native.ludic
index 775751e2..f1e13c12 100644
--- a/packages/ludic.physics/native.ludic
+++ b/packages/ludic.physics/native.ludic
@@ -1,6 +1,6 @@
 # native.ludic - the shim's symbols (native/shim/jph_shim.cpp). A world and a shape are handles
 # the package keeps; a body is an int id. None of this is exported.
-extern function jph_world_new(max_bodies: int, threads: int) -> pointer = "jph_world_new"
+extern function jph_world_new(max_bodies: int, threads: int, max_pairs: int, max_contacts: int, temp_kb: int) -> pointer = "jph_world_new"
 extern function jph_world_free(w: pointer) -> void = "jph_world_free"
 extern function jph_world_gravity(w: pointer, y: float) -> void = "jph_world_gravity"
 extern function jph_world_step(w: pointer, dt: float, sub: int) -> int = "jph_world_step"
diff --git a/packages/ludic.physics/native/shim/jph_shim.cpp b/packages/ludic.physics/native/shim/jph_shim.cpp
index b12f54f4..92f09a7e 100644
--- a/packages/ludic.physics/native/shim/jph_shim.cpp
+++ b/packages/ludic.physics/native/shim/jph_shim.cpp
@@ -93,11 +93,11 @@ public:
 
 struct World {
   // pairs and contacts are for what MOVES: a valley of sixty thousand still trunks makes none
-  // between them, so these are Jolt's own sizes, not the body count's; the temporary memory falls
-  // back to the heap rather than aborting a step
-  World(int max_bodies, int threads)
-      : temp(32 * 1024 * 1024), jobs(cMaxPhysicsJobs, cMaxPhysicsBarriers, threads) {
-    sys.Init(max_bodies, 0, 65536, 20480, bpl, ovb, olp);
+  // between them, so they are sized for the moving things, not the body count; the temporary memory
+  // falls back to the heap rather than aborting a step. All four are the package's (phys_open)
+  World(int max_bodies, int threads, int max_pairs, int max_contacts, int temp_kb)
+      : temp(size_t(temp_kb) * 1024), jobs(cMaxPhysicsJobs, cMaxPhysicsBarriers, threads) {
+    sys.Init(max_bodies, 0, max_pairs, max_contacts, bpl, ovb, olp);
     sys.SetContactListener(&contacts);
   }
   TempAllocatorImplWithMallocFallback temp;
diff --git a/packages/ludic.physics/native/shim/jph_world.inl b/packages/ludic.physics/native/shim/jph_world.inl
index 41e2253b..348f523f 100644
--- a/packages/ludic.physics/native/shim/jph_world.inl
+++ b/packages/ludic.physics/native/shim/jph_world.inl
@@ -1,13 +1,14 @@
 // jph_world.inl - a physics world: made, stepped, emptied of its contacts, freed
 
-// a world for up to max_bodies; threads 0 means the machine's cores less one, at most four
-JPH_SHIM void *jph_world_new(int max_bodies, int threads) {
+// a world for up to max_bodies, max_pairs touching pairs and max_contacts contact constraints, with a
+// temp heap of temp_kb; threads 0 means the machine's cores less one, at most four
+JPH_SHIM void *jph_world_new(int max_bodies, int threads, int max_pairs, int max_contacts, int temp_kb) {
   if (jph_init() != 0) return nullptr;
   if (threads <= 0) {
     int hw = int(std::thread::hardware_concurrency()) - 1;
     threads = hw < 1 ? 1 : (hw > 4 ? 4 : hw);
   }
-  World *w = new World(max_bodies, threads);
+  World *w = new World(max_bodies, threads, max_pairs, max_contacts, temp_kb);
   w->sys.SetGravity(Vec3(0.0f, -9.81f, 0.0f));
   return w;
 }
diff --git a/packages/ludic.physics/shapes.ludic b/packages/ludic.physics/shapes.ludic
index 0820c2b2..c599ac18 100644
--- a/packages/ludic.physics/shapes.ludic
+++ b/packages/ludic.physics/shapes.ludic
@@ -1,43 +1,5 @@
-# shapes.ludic - the world opened and closed, and shapes: made in C, kept here, named by an int.
-# A shape may be used by any number of bodies; phys_close frees them all.
-
-# a world for up to max_bodies (threads 0: the cores less one, at most four); false if C refused
-export function phys_open(physics_st: mut PhysicsState, max_bodies: int, threads: int) -> bool {
-  phys_close(physics_st)
-  ph_buffers()
-  physics_st.ph_world = jph_world_new(max_bodies, threads)
-  ph_own_reset(physics_st, max_bodies)
-  physics_st.ph_gen += 1
-  physics_st.ph_acc = 0.0
-  physics_st.ph_steps = 0
-  return physics_st.ph_world != null
-}
-
-export function phys_is_open(physics_st: PhysicsState) -> bool { return physics_st.ph_world != null }
-# which world this is: a body, shape or walker id kept from an older one means nothing now
-export function phys_generation(physics_st: PhysicsState) -> int { return physics_st.ph_gen }
-
-# everything let go: the world, every body in it and every shape (a world swap, a quit); the lists
-# that held them are emptied and kept, since nothing is given back
-export function phys_close(physics_st: mut PhysicsState) -> void {
-  if physics_st.ph_walkers != null {
-    for i in 0 .. len(physics_st.ph_walkers) {
-      if physics_st.ph_walkers[i] != null { jph_char_free(physics_st.ph_walkers[i]) }
-    }
-  }
-  if physics_st.ph_walkers != null { List.clear(physics_st.ph_walkers) }
-  if physics_st.ph_world != null { jph_world_free(physics_st.ph_world) }
-  physics_st.ph_world = null
-  if physics_st.ph_shapes != null {
-    for i in 0 .. len(physics_st.ph_shapes) {
-      if physics_st.ph_shapes[i] != null { jph_shape_free(physics_st.ph_shapes[i]) }
-    }
-  }
-  if physics_st.ph_shapes != null { List.clear(physics_st.ph_shapes) }
-  List.clear(physics_st.ph_spare)
-  physics_st.ph_nf = 0
-  if physics_st.ph_facts != null { q_clear(physics_st.ph_facts) }
-}
+# shapes.ludic - shapes: made in C, kept here, named by an int. A shape may be used by any number of
+# bodies; phys_close (world.ludic) frees them all.
 
 # a crate: half extents in metres
 @creates(PhysShape)
diff --git a/packages/ludic.physics/state.ludic b/packages/ludic.physics/state.ludic
index 44cf4664..8800767b 100644
--- a/packages/ludic.physics/state.ludic
+++ b/packages/ludic.physics/state.ludic
@@ -42,6 +42,7 @@ export state PhysicsState {
   ph_step: float = 0.016666668    # seconds a step
   ph_sub: int = 1                 # collision steps in each
   ph_max_steps: int = 4           # a slow frame catches up this far and no further
+  ph_max: int = 0                 # the bodies the world was opened for: one more is a failure, not a -1
   ph_acc: float = 0.0
   ph_steps: int = 0
   ph_gen: int = 0                 # how many worlds have been opened: an id from an older one is stale
@@ -75,3 +76,9 @@ export function phys_config(physics_st: mut PhysicsState, step: float, sub: int,
   physics_st.ph_hard = hard
 }
 function ph_facts__new() -> Queue<PhysFact> { return queue_new("physics.facts") }
+
+# what moves, sized: touching pairs, contact constraints and the step's temp heap (KB). A valley's still
+# things make no pairs between them; drops, boats and the walkers do. Past a size a step fails loudly
+export const PH_PAIRS: int = 4096
+export const PH_CONTACTS: int = 2048
+export const PH_TEMP_KB: int = 4096
diff --git a/packages/ludic.physics/system.ludic b/packages/ludic.physics/system.ludic
index c2a06a95..facf9d67 100644
--- a/packages/ludic.physics/system.ludic
+++ b/packages/ludic.physics/system.ludic
@@ -23,7 +23,8 @@ export function phys_step(physics_st: mut PhysicsState) -> void {
   if physics_st.ph_world == null { return }
   ph_buffers()
   ph_buoy_all(physics_st)
-  jph_world_step(physics_st.ph_world, physics_st.ph_step, physics_st.ph_sub)
+  # 25.5a: Jolt's pairs or contacts full (PH_PAIRS, PH_CONTACTS) is a failure: past it things pass through
+  if jph_world_step(physics_st.ph_world, physics_st.ph_step, physics_st.ph_sub) != 0 { Mem.over("a physics step past phys_open's pairs or contacts") }
   physics_st.ph_steps += 1
   ph_contacts(physics_st)
 }
diff --git a/packages/ludic.physics/tests/jolt_floor_test.ludic b/packages/ludic.physics/tests/jolt_floor_test.ludic
new file mode 100644
index 00000000..71541f16
--- /dev/null
+++ b/packages/ludic.physics/tests/jolt_floor_test.ludic
@@ -0,0 +1,25 @@
+# jolt_floor_test.ludic - what an empty world costs in Jolt's own heap at the game's size (plan 25):
+# the floor under every body, from the body table, the broad phase, the contact caches and the temp heap
+import "ludic.physics"
+import "ludic.base"
+program JoltFloorTest {
+  numbers float
+  function floor_at(physics_st: mut PhysicsState, bodies: int) -> long {
+    expect(phys_open(physics_st, 64, 1))
+    phys_close(physics_st)
+    let before = phys_heap_bytes()
+    expect(phys_open(physics_st, bodies, 0))
+    let f = phys_heap_bytes() - before
+    phys_close(physics_st)
+    return f
+  }
+
+  # it was 51839 KB at 200000 bodies with 65536 pairs, 20480 contacts and a 32 MB temp heap
+  test "an empty world at the game's size costs under 16 MB" (physics_st: mut PhysicsState) {
+    let a = floor_at(physics_st, 200000)
+    let b = floor_at(physics_st, 98304)
+    print(`jolt floor: {a / long(1024)} KB at 200000 bodies, {b / long(1024)} KB at 98304`)
+    expect(b < long(16 * 1024 * 1024))
+    expect(a > b)
+  }
+}
diff --git a/packages/ludic.physics/world.ludic b/packages/ludic.physics/world.ludic
new file mode 100644
index 00000000..d7d83ee6
--- /dev/null
+++ b/packages/ludic.physics/world.ludic
@@ -0,0 +1,43 @@
+# world.ludic - the world opened and closed: its sizes, and everything in it let go at once
+
+# a world for up to max_bodies (threads 0: the cores less one, at most four); false if C refused.
+# Pairs, contacts and the temp heap are sized for the things that MOVE (PH_PAIRS, ...)
+export function phys_open(physics_st: mut PhysicsState, max_bodies: int, threads: int) -> bool { return phys_open_sized(physics_st, max_bodies, threads, PH_PAIRS, PH_CONTACTS, PH_TEMP_KB) }
+# the same with Jolt's other three sizes given: touching pairs, contact constraints, the temp heap in KB
+export function phys_open_sized(physics_st: mut PhysicsState, max_bodies: int, threads: int, max_pairs: int, max_contacts: int, temp_kb: int) -> bool {
+  phys_close(physics_st)
+  ph_buffers()
+  physics_st.ph_world = jph_world_new(max_bodies, threads, max_pairs, max_contacts, temp_kb)
+  physics_st.ph_max = max_bodies
+  ph_own_reset(physics_st, max_bodies)
+  physics_st.ph_gen += 1
+  physics_st.ph_acc = 0.0
+  physics_st.ph_steps = 0
+  return physics_st.ph_world != null
+}
+
+export function phys_is_open(physics_st: PhysicsState) -> bool { return physics_st.ph_world != null }
+# which world this is: a body, shape or walker id kept from an older one means nothing now
+export function phys_generation(physics_st: PhysicsState) -> int { return physics_st.ph_gen }
+
+# everything let go: the world, every body in it and every shape (a world swap, a quit); the lists
+# that held them are emptied and kept, since nothing is given back
+export function phys_close(physics_st: mut PhysicsState) -> void {
+  if physics_st.ph_walkers != null {
+    for i in 0 .. len(physics_st.ph_walkers) {
+      if physics_st.ph_walkers[i] != null { jph_char_free(physics_st.ph_walkers[i]) }
+    }
+  }
+  if physics_st.ph_walkers != null { List.clear(physics_st.ph_walkers) }
+  if physics_st.ph_world != null { jph_world_free(physics_st.ph_world) }
+  physics_st.ph_world = null
+  if physics_st.ph_shapes != null {
+    for i in 0 .. len(physics_st.ph_shapes) {
+      if physics_st.ph_shapes[i] != null { jph_shape_free(physics_st.ph_shapes[i]) }
+    }
+  }
+  if physics_st.ph_shapes != null { List.clear(physics_st.ph_shapes) }
+  List.clear(physics_st.ph_spare)
+  physics_st.ph_nf = 0
+  if physics_st.ph_facts != null { q_clear(physics_st.ph_facts) }
+}