diff --git a/packages/ludic.nav/README.md b/packages/ludic.nav/README.md
index 70d99108..59c4541d 100644
--- a/packages/ludic.nav/README.md
+++ b/packages/ludic.nav/README.md
@@ -71,3 +71,13 @@ across the seams, a tile under water with no polygons, and the set saved and loa
 `native/build.sh` fetches Recast & Detour v1.6.0, checks its SHA-256, and builds Recast, Detour
 and the shim (`native/shim/nav_shim.cpp`) into `lib/<target>/` - the same script on the Mac and on
 the PC (Git Bash, the LLVM installer's clang). `native/LICENSE-recastnavigation` ships with it.
+
+## Crowds (phase 18)
+
+`nav_crowd_start(kind, max, max_radius)` puts a DetourCrowd on a kind's mesh, with the mesh's tastes as
+its filters. `nav_crowd_add(kind, x, y, z, radius, height, speed, filter)` adds a walker (snapped to
+the mesh, -1 off it), `nav_crowd_target` and `nav_crowd_speed` say where and how fast, and
+`nav_crowd_step(dt)` steps every crowd. `nav_crowd_read` puts a walker into `nav_agent_x/y/z/vx/vz`,
+and `nav_crowd_us()` is what the stepping has cost. The mechanic decides and the crowd moves, and
+dropping a mesh drops its crowd first. On the test meadow, twenty walkers crossing head-on never come
+closer than their two radii, and a step costs about 12 us.
diff --git a/packages/ludic.nav/crowd.ludic b/packages/ludic.nav/crowd.ludic
new file mode 100644
index 00000000..f0930676
--- /dev/null
+++ b/packages/ludic.nav/crowd.ludic
@@ -0,0 +1,61 @@
+# crowd.ludic - a crowd per kind of walker (phase 18): many walkers steered along their paths at
+# once, each kept clear of the others. The mechanic decides where and how fast; the crowd moves.
+
+# a crowd on kind's mesh for at most max walkers, none wider than max_radius; false with no mesh
+export function nav_crowd_start(nav_st: mut NavState, kind: int, max: int, max_radius: float) -> bool {
+  let h = nv_mesh(nav_st, kind)
+  if h == null { return false }
+  nv_crowd_drop(nav_st, kind)
+  nav_st.nv_crowds[kind] = nvc_crowd_new(h, max, max_radius)
+  return nav_st.nv_crowds[kind] != null
+}
+function nv_crowd(nav_st: NavState, kind: int) -> pointer {
+  if kind < 0 or kind >= NAV_KINDS { return null }
+  return nav_st.nv_crowds[kind]
+}
+# let kind's crowd go (before its mesh, which it walks on)
+function nv_crowd_drop(nav_st: mut NavState, kind: int) -> void {
+  if kind < 0 or kind >= NAV_KINDS or nav_st.nv_crowds[kind] == null { return }
+  nvc_crowd_free(nav_st.nv_crowds[kind])
+  nav_st.nv_crowds[kind] = null
+}
+
+# a walker at (x, y, z) with its width, height, top speed and taste: its index, or -1
+export function nav_crowd_add(nav_st: NavState, kind: int, x: float, y: float, z: float, radius: float, height: float, speed: float, filter: int) -> int {
+  let c = nv_crowd(nav_st, kind)
+  if c == null { return -1 }
+  return nvc_crowd_add(c, x, y, z, radius, height, speed, filter)
+}
+export function nav_crowd_remove(nav_st: NavState, kind: int, i: int) -> void {
+  let c = nv_crowd(nav_st, kind)
+  if c != null and i >= 0 { nvc_crowd_remove(c, i) }
+}
+# where walker i is going, snapped to the mesh; false off it
+export function nav_crowd_target(nav_st: NavState, kind: int, i: int, x: float, y: float, z: float) -> bool {
+  let c = nv_crowd(nav_st, kind)
+  return c != null and i >= 0 and nvc_crowd_target(c, i, x, y, z) == 1
+}
+export function nav_crowd_speed(nav_st: NavState, kind: int, i: int, speed: float) -> void {
+  let c = nv_crowd(nav_st, kind)
+  if c != null and i >= 0 { nvc_crowd_speed(c, i, speed) }
+}
+# every crowd one step on
+export function nav_crowd_step(nav_st: NavState, dt: float) -> void {
+  for k in 0 .. NAV_KINDS { if nav_st.nv_crowds[k] != null { nvc_crowd_update(nav_st.nv_crowds[k], dt) } }
+}
+# walker i read into nav_agent_*; false when there is none
+export function nav_crowd_read(nav_st: NavState, kind: int, i: int) -> bool {
+  let c = nv_crowd(nav_st, kind)
+  return c != null and i >= 0 and nvc_crowd_read(c, i, nav_st.nv_agent) == 1
+}
+export function nav_agent_x(nav_st: NavState) -> float { return nav_st.nv_agent[0] }
+export function nav_agent_y(nav_st: NavState) -> float { return nav_st.nv_agent[1] }
+export function nav_agent_z(nav_st: NavState) -> float { return nav_st.nv_agent[2] }
+export function nav_agent_vx(nav_st: NavState) -> float { return nav_st.nv_agent[3] }
+export function nav_agent_vz(nav_st: NavState) -> float { return nav_st.nv_agent[5] }
+# microseconds the crowds have spent stepping
+export function nav_crowd_us(nav_st: NavState) -> int {
+  var us = 0
+  for k in 0 .. NAV_KINDS { if nav_st.nv_crowds[k] != null { us += nvc_crowd_us(nav_st.nv_crowds[k]) } }
+  return us
+}
diff --git a/packages/ludic.nav/index.ludic b/packages/ludic.nav/index.ludic
index 3f380a8d..59dfbfc5 100644
--- a/packages/ludic.nav/index.ludic
+++ b/packages/ludic.nav/index.ludic
@@ -8,3 +8,4 @@ import "native.ludic"
 import "state.ludic"
 import "build.ludic"
 import "query.ludic"
+import "crowd.ludic"
diff --git a/packages/ludic.nav/lib/macos-arm64/libludicnav.dylib b/packages/ludic.nav/lib/macos-arm64/libludicnav.dylib
index 7d84075a..7314e9b3 100755
--- a/packages/ludic.nav/lib/macos-arm64/libludicnav.dylib
+++ b/packages/ludic.nav/lib/macos-arm64/libludicnav.dylib
@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:969c1ee0010c0fe88abcdfceb1a166031a8f6519b9e77a27c27331cbe2036e0b
-size 176912
+oid sha256:f373d9a1d4d37d907d7df0869a2ba4e8b2a70de55cb33c6f635cea49486efe2f
+size 198240
diff --git a/packages/ludic.nav/lib/windows-x64/ludicnav.dll b/packages/ludic.nav/lib/windows-x64/ludicnav.dll
index 0bdc9341..b0c6761f 100644
--- a/packages/ludic.nav/lib/windows-x64/ludicnav.dll
+++ b/packages/ludic.nav/lib/windows-x64/ludicnav.dll
@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:0ef49349d93dbc3b94c3743f80fa56f2acf4f5e02f16faba6ec13823b5705aaa
-size 337920
+oid sha256:a7626d464f2588054610dc27a655d67acf66649ea120120705cdf5f1fca5fe34
+size 374784
diff --git a/packages/ludic.nav/lib/windows-x64/ludicnav.lib b/packages/ludic.nav/lib/windows-x64/ludicnav.lib
index 6bcfe136..32d4c3af 100644
--- a/packages/ludic.nav/lib/windows-x64/ludicnav.lib
+++ b/packages/ludic.nav/lib/windows-x64/ludicnav.lib
@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:0ea766bfe59613b41a1566f41de772d49602191ad7e188bef3bcf453c55d6114
-size 4022
+oid sha256:754f4ea0ce90f7b512d9e6e12f59c5babaf1d071dfd8a30faff93bba8af52cee
+size 5804
diff --git a/packages/ludic.nav/native.ludic b/packages/ludic.nav/native.ludic
index 190af731..7414cfb5 100644
--- a/packages/ludic.nav/native.ludic
+++ b/packages/ludic.nav/native.ludic
@@ -15,4 +15,13 @@ extern function nvc_us(h: pointer) -> int = "nav_us"
 extern function nvc_raycast(h: pointer, f: int, sx: float, sy: float, sz: float, ex: float, ez: float) -> int = "nav_raycast"
 extern function nvc_random_near(h: pointer, f: int, x: float, y: float, z: float, r: float, seed: int, out: pointer) -> int = "nav_random_near"
 extern function nvc_stage(size: int) -> pointer = "nav_stage"
+extern function nvc_crowd_new(h: pointer, max: int, max_radius: float) -> pointer = "nav_crowd_new"
+extern function nvc_crowd_free(c: pointer) -> void = "nav_crowd_free"
+extern function nvc_crowd_add(c: pointer, x: float, y: float, z: float, radius: float, height: float, speed: float, filter: int) -> int = "nav_crowd_add"
+extern function nvc_crowd_remove(c: pointer, i: int) -> void = "nav_crowd_remove"
+extern function nvc_crowd_target(c: pointer, i: int, x: float, y: float, z: float) -> int = "nav_crowd_target"
+extern function nvc_crowd_speed(c: pointer, i: int, speed: float) -> void = "nav_crowd_speed"
+extern function nvc_crowd_update(c: pointer, dt: float) -> void = "nav_crowd_update"
+extern function nvc_crowd_read(c: pointer, i: int, out: pointer) -> int = "nav_crowd_read"
+extern function nvc_crowd_us(c: pointer) -> int = "nav_crowd_us"
 extern function nvc_load_staged(buf: pointer, size: int) -> pointer = "nav_load_staged"
diff --git a/packages/ludic.nav/native/build.sh b/packages/ludic.nav/native/build.sh
index 6bc4323a..bf963d02 100755
--- a/packages/ludic.nav/native/build.sh
+++ b/packages/ludic.nav/native/build.sh
@@ -1,7 +1,7 @@
 #!/bin/sh
 # builds lib/<target>/ for ludic.nav: Recast & Detour at a pinned tag, and the shim over them.
-# Recast builds the mesh, Detour answers it; DetourTileCache and DetourCrowd come with phases 17.8
-# and 18. Polygon refs are 64-bit (DT_POLYREF64): an 8 km map in 64 m tiles is 16384 tiles, which
+# Recast builds the mesh, Detour answers it, DetourCrowd steers many walkers at once (phase 18);
+# DetourTileCache would come with 17.8. Polygon refs are 64-bit (DT_POLYREF64): an 8 km map in 64 m tiles is 16384 tiles, which
 # 32-bit refs would leave 256 polygons each. Objects go to build/native; a few seconds on eight cores.
 set -eu
 PKG="$(cd "$(dirname "$0")/.." && pwd)"
@@ -14,12 +14,12 @@ native_fetch "$SRC" "https://github.com/recastnavigation/recastnavigation/archiv
 OBJ="$PKG/build/native/$(native_target)"
 mkdir -p "$OBJ"
 CXX="$(native_cxx)"
-INC="-I$SRC/Recast/Include -I$SRC/Detour/Include"
+INC="-I$SRC/Recast/Include -I$SRC/Detour/Include -I$SRC/DetourCrowd/Include"
 FLAGS="$(native_cflags) -std=c++17 -ffp-contract=off -fno-exceptions -fno-rtti -DNDEBUG -DDT_POLYREF64 $INC"
 case "$(native_target)" in windows-*) FLAGS="$FLAGS -D_CRT_SECURE_NO_WARNINGS" ;; esac
 JOBS="$(getconf _NPROCESSORS_ONLN 2>/dev/null || echo 4)"
 
-( cd "$SRC" && ls Recast/Source/*.cpp Detour/Source/*.cpp ) | while read -r f; do
+( cd "$SRC" && ls Recast/Source/*.cpp Detour/Source/*.cpp DetourCrowd/Source/*.cpp ) | while read -r f; do
   o="$OBJ/$(basename "$f" .cpp).o"
   [ "$o" -nt "$SRC/$f" ] || echo "$f $o"
 done | xargs -P "$JOBS" -n 2 sh -c '"$0" '"$FLAGS"' -c "'"$SRC"'/$1" -o "$2"' "$CXX"
diff --git a/packages/ludic.nav/native/shim/nav_crowd.inl b/packages/ludic.nav/native/shim/nav_crowd.inl
new file mode 100644
index 00000000..2a81400c
--- /dev/null
+++ b/packages/ludic.nav/native/shim/nav_crowd.inl
@@ -0,0 +1,87 @@
+// nav_crowd.inl - DetourCrowd over a navmesh: many walkers steered along their paths at once, each
+// kept clear of the others. The mechanic decides where and how fast; the crowd only moves them.
+
+struct Crowd {
+  dtCrowd *crowd = nullptr;
+  Nav *nav = nullptr;
+  long long ns = 0;    // time spent stepping it, for the measure (18.8)
+};
+
+// a crowd of at most max walkers, none wider than max_radius, with the mesh's tastes as its filters
+NAV_SHIM void *nav_crowd_new(void *h, int max, float max_radius) {
+  Nav *n = static_cast<Nav *>(h);
+  Crowd *c = new Crowd();
+  c->nav = n;
+  c->crowd = dtAllocCrowd();
+  if (!c->crowd || !c->crowd->init(max, max_radius, n->mesh)) { dtFreeCrowd(c->crowd); delete c; return nullptr; }
+  for (int f = 0; f < NAV_FILTERS && f < DT_CROWD_MAX_QUERY_FILTER_TYPE; ++f) *c->crowd->getEditableFilter(f) = n->filters[f];
+  return c;
+}
+NAV_SHIM void nav_crowd_free(void *p) {
+  Crowd *c = static_cast<Crowd *>(p);
+  if (!c) return;
+  dtFreeCrowd(c->crowd);
+  delete c;
+}
+
+// a walker at (x, y, z): its index, or -1 when the crowd is full or the point is off the mesh
+NAV_SHIM int nav_crowd_add(void *p, float x, float y, float z, float radius, float height, float speed, int filter) {
+  Crowd *c = static_cast<Crowd *>(p);
+  dtCrowdAgentParams ap;
+  memset(&ap, 0, sizeof(ap));
+  ap.radius = radius;
+  ap.height = height;
+  ap.maxSpeed = speed;
+  ap.maxAcceleration = speed * 4.0f;
+  ap.collisionQueryRange = radius * 12.0f;
+  ap.pathOptimizationRange = radius * 30.0f;
+  ap.separationWeight = 2.0f;
+  ap.updateFlags = DT_CROWD_ANTICIPATE_TURNS | DT_CROWD_OBSTACLE_AVOIDANCE | DT_CROWD_SEPARATION | DT_CROWD_OPTIMIZE_TOPO | DT_CROWD_OPTIMIZE_VIS;
+  ap.obstacleAvoidanceType = 3;
+  ap.queryFilterType = (unsigned char)(filter >= 0 && filter < NAV_FILTERS ? filter : 0);
+  float pos[3] = {x, y, z}, near[3];
+  dtPolyRef ref = 0;
+  c->nav->query->findNearestPoly(pos, c->nav->ext, c->crowd->getFilter(ap.queryFilterType), &ref, near);
+  if (!ref) return -1;
+  return c->crowd->addAgent(near, &ap);
+}
+NAV_SHIM void nav_crowd_remove(void *p, int i) { static_cast<Crowd *>(p)->crowd->removeAgent(i); }
+
+// send walker i toward (x, y, z), snapped to the mesh; 0 when the point is off it
+NAV_SHIM int nav_crowd_target(void *p, int i, float x, float y, float z) {
+  Crowd *c = static_cast<Crowd *>(p);
+  const dtCrowdAgent *a = c->crowd->getAgent(i);
+  if (!a || !a->active) return 0;
+  float e[3] = {x, y, z}, near[3];
+  dtPolyRef ref = 0;
+  c->nav->query->findNearestPoly(e, c->nav->ext, c->crowd->getFilter(a->params.queryFilterType), &ref, near);
+  if (!ref) return 0;
+  return c->crowd->requestMoveTarget(i, ref, near) ? 1 : 0;
+}
+// a new top speed, as a walker goes from a walk to a run
+NAV_SHIM void nav_crowd_speed(void *p, int i, float speed) {
+  Crowd *c = static_cast<Crowd *>(p);
+  const dtCrowdAgent *a = c->crowd->getAgent(i);
+  if (!a || !a->active) return;
+  dtCrowdAgentParams ap = a->params;
+  ap.maxSpeed = speed;
+  ap.maxAcceleration = speed * 4.0f;
+  c->crowd->updateAgentParameters(i, &ap);
+}
+
+NAV_SHIM void nav_crowd_update(void *p, float dt) {
+  Crowd *c = static_cast<Crowd *>(p);
+  long long t0 = nav_now_ns();
+  c->crowd->update(dt, nullptr);
+  c->ns += nav_now_ns() - t0;
+}
+
+// walker i's position and velocity into out[0..5]; 0 when there is no such walker
+NAV_SHIM int nav_crowd_read(void *p, int i, float *out) {
+  const dtCrowdAgent *a = static_cast<Crowd *>(p)->crowd->getAgent(i);
+  if (!a || !a->active) return 0;
+  dtVcopy(out, a->npos);
+  dtVcopy(out + 3, a->vel);
+  return 1;
+}
+NAV_SHIM int nav_crowd_us(void *p) { return static_cast<int>(static_cast<Crowd *>(p)->ns / 1000); }
diff --git a/packages/ludic.nav/native/shim/nav_shim.cpp b/packages/ludic.nav/native/shim/nav_shim.cpp
index 7710e6cb..79dec5c9 100644
--- a/packages/ludic.nav/native/shim/nav_shim.cpp
+++ b/packages/ludic.nav/native/shim/nav_shim.cpp
@@ -6,6 +6,7 @@
 #include <DetourNavMeshBuilder.h>
 #include <DetourNavMeshQuery.h>
 #include <DetourCommon.h>
+#include <DetourCrowd.h>
 #include <cstring>
 #include <cstdlib>
 #include <cmath>
@@ -77,3 +78,4 @@ const dtQueryFilter *nav_filter(Nav *n, int f) { return &n->filters[f >= 0 && f
 #include "nav_build.inl"
 #include "nav_tiles.inl"
 #include "nav_query.inl"
+#include "nav_crowd.inl"
diff --git a/packages/ludic.nav/state.ludic b/packages/ludic.nav/state.ludic
index cf0f240c..ca4edf58 100644
--- a/packages/ludic.nav/state.ludic
+++ b/packages/ludic.nav/state.ludic
@@ -45,6 +45,8 @@ export state NavState {
   nv_asked: int = 0                # paths asked for, found (two corners or more), and cut short
   nv_found: int = 0
   nv_short: int = 0
+  nv_crowds: []pointer = nv_none() # a crowd per kind, on that kind's mesh (crowd.ludic)
+  nv_agent: []float = floats(6)    # the last walker read: position, velocity
 }
 function nv_none() -> []pointer {
   let m = new []pointer
@@ -57,6 +59,7 @@ function nv_mesh(nav_st: NavState, kind: int) -> pointer {
 }
 function nv_set(nav_st: mut NavState, kind: int, h: pointer) -> bool {
   if kind < 0 or kind >= NAV_KINDS { return false }
+  nv_crowd_drop(nav_st, kind)
   if nav_st.nv_meshes[kind] != null { nvc_free(nav_st.nv_meshes[kind]) }
   nav_st.nv_meshes[kind] = h
   return h != null
diff --git a/packages/ludic.nav/tests/crowd_test.ludic b/packages/ludic.nav/tests/crowd_test.ludic
new file mode 100644
index 00000000..4e0fc3d6
--- /dev/null
+++ b/packages/ludic.nav/tests/crowd_test.ludic
@@ -0,0 +1,84 @@
+# crowd_test.ludic - a crowd on the test meadow: ten walkers crossing ten head-on, and none walks
+# through another; a crowd goes with its mesh, and nothing off the mesh joins it
+import "ludic.nav"
+import "ludic.base"
+import "fakes/meadow.ludic"
+program CrowdTest {
+  numbers float
+
+  state CrowdTestState {
+    ids: []int = new []int
+    gx: []float = new []float
+    gz: []float = new []float
+  }
+
+  # ten from the west side to the east and ten back, in rows 2.4 m apart that meet in the middle
+  function cross(nav_st: mut NavState, ct: mut CrowdTestState) -> void {
+    expect(nav_build(nav_st, NAV_PERSON, ground(40, 0), new NavConfig))
+    expect(nav_crowd_start(nav_st, NAV_PERSON, 32, 0.6))
+    for i in 0 .. 20 {
+      let east = i < 10
+      let z = 8.0 + float(i % 10) * 2.4
+      var x0 = 4.0
+      var x1 = 36.0
+      if not east {
+        x0 = 36.0
+        x1 = 4.0
+      }
+      let id = nav_crowd_add(nav_st, NAV_PERSON, x0, 0.0, z, 0.35, 1.8, 1.4, 0)
+      expect(id >= 0)
+      expect(nav_crowd_target(nav_st, NAV_PERSON, id, x1, 0.0, z))
+      push(ct.ids, id)
+      push(ct.gx, x1)
+      push(ct.gz, z)
+    }
+  }
+  # the closest any two walkers are now
+  function closest(nav_st: NavState, ct: CrowdTestState, xs: []float, zs: []float) -> float {
+    for i in 0 .. 20 {
+      nav_crowd_read(nav_st, NAV_PERSON, ct.ids[i])
+      xs[i] = nav_agent_x(nav_st)
+      zs[i] = nav_agent_z(nav_st)
+    }
+    var best = 1000.0
+    for i in 0 .. 20 {
+      for j in i + 1 .. 20 { best = Math.min(best, Math.sqrt((xs[i] - xs[j]) * (xs[i] - xs[j]) + (zs[i] - zs[j]) * (zs[i] - zs[j]))) }
+    }
+    return best
+  }
+
+  test "twenty cross head-on and none walks through another; all arrive" (nav_st: mut NavState, ct: mut CrowdTestState) {
+    cross(nav_st, ct)
+    let xs = floats(20)
+    let zs = floats(20)
+    var worst = 1000.0
+    for k in 0 .. 1800 {
+      nav_crowd_step(nav_st, 1.0 / 60.0)
+      worst = Math.min(worst, closest(nav_st, ct, xs, zs))
+    }
+    var arrived = 0
+    for i in 0 .. 20 {
+      nav_crowd_read(nav_st, NAV_PERSON, ct.ids[i])
+      let dx = nav_agent_x(nav_st) - ct.gx[i]
+      let dz = nav_agent_z(nav_st) - ct.gz[i]
+      if dx * dx + dz * dz < 1.5 * 1.5 { arrived += 1 }
+    }
+    print(`crowd: closest two ever {worst} m (two radii 0.7), {arrived} of 20 arrived, {nav_crowd_us(nav_st)} us over 1800 steps`)
+    expect(worst > 0.55)
+    expect(arrived >= 18)
+    nav_reset(nav_st)
+  }
+
+  test "a crowd goes with its mesh, and a walker off the mesh is refused" (nav_st: mut NavState) {
+    expect(not nav_crowd_start(nav_st, NAV_PERSON, 8, 0.6))
+    expect(nav_build(nav_st, NAV_PERSON, ground(40, 0), new NavConfig))
+    expect(nav_crowd_start(nav_st, NAV_PERSON, 8, 0.6))
+    expect_eq(nav_crowd_add(nav_st, NAV_PERSON, -50.0, 0.0, -50.0, 0.35, 1.8, 1.4, 0), -1)
+    let a = nav_crowd_add(nav_st, NAV_PERSON, 5.0, 0.0, 5.0, 0.35, 1.8, 1.4, 0)
+    expect(a >= 0)
+    expect(not nav_crowd_target(nav_st, NAV_PERSON, a, -50.0, 0.0, -50.0))
+    nav_drop(nav_st, NAV_PERSON)
+    expect(not nav_crowd_read(nav_st, NAV_PERSON, a))
+    expect_eq(nav_crowd_add(nav_st, NAV_PERSON, 5.0, 0.0, 5.0, 0.35, 1.8, 1.4, 0), -1)
+  }
+}