diff --git a/scripts/fixtures/stub-worker.mjs b/scripts/fixtures/stub-worker.mjs
index 1ad2ccc..ce777b6 100644
--- a/scripts/fixtures/stub-worker.mjs
+++ b/scripts/fixtures/stub-worker.mjs
@@ -1,7 +1,7 @@
 // Stands in for kimodo-resident-worker.py in the daemon's tests. It binds port 0, prints the ready line the
 // daemon waits for, and answers /health and /generate with what the daemon gave it. The --model value picks a
 // misbehaviour: crash-before-ready, hang-before-ready, sick, slow-health, stubborn, fail-generate,
-// refuse-generate, drop-generate, slow-generate, writer (appends to <output>.writes every 50 ms, never answers, ignores SIGTERM), slow-to-stop (no answer; prints a line and exits 1.5 s after SIGTERM), chatty (prints a line the moment it is healthy), encoder (a grandchild, then no answer), graceful (exits 0.5 s after SIGTERM), die-after-ready, bvh-outside (lists a BVH outside the job's directory), bvh-link (lists a symlink as its BVH), bvh-fifo (lists a FIFO as its BVH), plant-link (a normal answer with a file symlink and a directory symlink planted beside the BVH). A request with bvh writes <output>.bvh (motion/motion_NN.bvh for several samples) and lists it. Any other value is just the model the worker serves.
+// refuse-generate, drop-generate, slow-generate, writer (appends to <output>.writes every 50 ms, never answers, ignores SIGTERM), slow-to-stop (no answer; prints a line and exits 1.5 s after SIGTERM), chatty (prints a line the moment it is healthy), encoder (a grandchild, then no answer), graceful (exits 0.5 s after SIGTERM), die-after-ready, bvh-outside (lists a BVH outside the job's directory), bvh-link (lists a symlink as its BVH), bvh-fifo (lists a FIFO as its BVH), plant-link (a normal answer with a file symlink and a directory symlink planted beside the BVH), no-loops (a normal answer that leaves out the loops a loop request asked for), bad-loops (loops whose window is not a frame range), short-loops (one window fewer than samples). A loop request is answered with loops, sample k as {first 3 + k, last 223 + k}. A request with bvh writes <output>.bvh (motion/motion_NN.bvh for several samples) and lists it. Any other value is just the model the worker serves.
 import http from "node:http";
 import os from "node:os";
 import path from "node:path";
@@ -117,6 +117,11 @@ if (model === "hang-before-ready") {
           seed: payload.seed,
           metaSeed: payload.meta?.seed,
           bvh: payload.bvh,
+          ...(payload.loop && model !== "no-loops" ? {
+            loops: Array.from({ length: payload.numSamples - (model === "short-loops" ? 1 : 0) }, (_, k) => (model === "bad-loops"
+              ? { first: 5.5, last: 9, poseCost: 0, velocityCost: 0 }
+              : { first: 3 + k, last: 223 + k, poseCost: 0.03, velocityCost: 0.2 })),
+          } : {}),
           bvhStandardTpose: payload.bvhStandardTpose,
           model: model || "stub-model",
           pid: process.pid,
diff --git a/scripts/kimodo-resident-worker.py b/scripts/kimodo-resident-worker.py
index e855a32..27d45f6 100644
--- a/scripts/kimodo-resident-worker.py
+++ b/scripts/kimodo-resident-worker.py
@@ -1,6 +1,7 @@
 #!/usr/bin/env python3
 import argparse
 import json
+import math
 import os
 import shutil
 import sys
@@ -14,7 +15,7 @@ import torch
 from huggingface_hub.constants import HF_HOME
 
 from kimodo import DEFAULT_MODEL, load_model
-from kimodo.constraints import load_constraints_lst
+from kimodo.constraints import compute_global_heading, load_constraints_lst
 from kimodo.exports.motion_io import save_kimodo_npz
 from kimodo.meta import parse_prompts_from_meta
 from kimodo.model.load_model import TEXT_ENCODER_PRESETS
@@ -28,6 +29,7 @@ from kimodo.scripts.generate import (
 )
 from kimodo.tools import save_json, seed_everything
 
+import loop_window
 import text_encoding
 
 
@@ -184,6 +186,20 @@ def save_outputs(output, output_base, export_bvh, bvh_standard_tpose):
     return files
 
 
+def loop_windows(output):
+    """One loop window per sample, chosen from Kimodo's own posed joints (always the somaskel77 output skeleton) and the
+    heading it computes from them. Frame indices into the take, last inclusive."""
+    skeleton = model.output_skeleton
+    hips = skeleton.bone_index["Hips"]
+    windows = []
+    for joints in output["posed_joints"]:
+        heading = compute_global_heading(torch.from_numpy(joints[None]), skeleton)[0]
+        angle = torch.atan2(heading[:, 1], heading[:, 0]).numpy()
+        first, last, pose_cost, velocity_cost = loop_window.loop_window(joints, angle, model.fps, math.ceil(len(joints) / 2), hips)
+        windows.append({"first": first, "last": last, "poseCost": pose_cost, "velocityCost": velocity_cost})
+    return windows
+
+
 def save_example_dirs(output_base, texts, num_frames, num_samples, constraints_path):
     output_stem = os.path.splitext(output_base)[0].rstrip(os.sep)
     base_name = os.path.basename(output_stem)
@@ -319,6 +335,7 @@ def generate(payload):
             request_bool(payload, "bvh"),
             request_bool(payload, "bvhStandardTpose") or request_bool(payload, "bvh_standard_tpose"),
         )
+        loops = loop_windows(output) if request_bool(payload, "loop") else None
         if request_bool(payload, "saveExampleDir") or request_bool(payload, "save_example_dir"):
             files.extend(save_example_dirs(output_base, texts, num_frames, num_samples, constraints_path))
         generation_count += 1
@@ -347,6 +364,7 @@ def generate(payload):
         "constraints": constraints_path,
         "seed": seed,
         "files": files,
+        **({"loops": loops} if loops is not None else {}),
         "encoder": encoder_record,
         "worker": worker_identity,
         "timing": {
diff --git a/scripts/test-vili-daemon.mjs b/scripts/test-vili-daemon.mjs
index 66f9819..0e827c9 100644
--- a/scripts/test-vili-daemon.mjs
+++ b/scripts/test-vili-daemon.mjs
@@ -1441,6 +1441,81 @@ test("fbx refusals happen before anything is written or spawned", async () => {
   }
 });
 
+test("loop asks for the seamless window, and the window is the FBX's frame range", async () => {
+  const daemon = await startDaemon();
+  try {
+    const constraints = [{ type: "root2d", frame_indices: [0, 10], smooth_root_2d: [[0, 0], [0, 0.5]] }];
+    const answer = await generate(daemon, exportRequest({ clipName: "walk", seed: 7, loop: true, constraints }));
+    assert.equal(answer.status, 200);
+    const job = answer.body;
+    assert.equal(job.request.loop, true);
+    assert.deepEqual(job.worker.loops, [{ first: 3, last: 223, poseCost: 0.03, velocityCost: 0.2 }]);
+    assert.deepEqual(job.request.constraints, constraints, "the record keeps inline constraints as sent");
+    assert.deepEqual(JSON.parse(readFileSync(path.join(job.artifactDir, "constraints.json"), "utf8")), constraints);
+    assert.equal(JSON.parse(readFileSync(path.join(job.artifactDir, "request.json"), "utf8")).loop, true, "the worker is asked for the loop");
+    assert.equal(job.worker.constraints, path.join(job.artifactDir, "constraints.json"), "the worker still gets the path");
+    const [run] = exportStarts(daemon);
+    assert.deepEqual(run.argv.slice(run.argv.indexOf("--") + 3), ["walk", "--frames", "4", "224"], "Blender's frames count from 1");
+    assert.deepEqual(job.export.clips.map((clip) => clip.frames), [[1, 60]], "the clip's frames are what Blender reports");
+
+    const several = await generate(daemon, exportRequest({ clipName: "pair", numSamples: 2, loop: true }));
+    assert.equal(several.status, 200);
+    assert.deepEqual(several.body.worker.loops.map((loop) => [loop.first, loop.last]), [[3, 223], [4, 224]]);
+    const frames = Object.fromEntries(exportStarts(daemon).slice(1).map((start) => {
+      const [, , clip, ...rest] = start.argv.slice(start.argv.indexOf("--") + 1);
+      return [clip, rest];
+    }));
+    assert.deepEqual(frames, { pair_00: ["--frames", "4", "224"], pair_01: ["--frames", "5", "225"] }, "each sample is exported over its own window");
+
+    const plain = await generate(daemon, exportRequest({ clipName: "plain" }));
+    assert.equal(plain.status, 200);
+    assert.equal(plain.body.worker.loops, undefined);
+    assert.equal(plain.body.request.loop, undefined);
+    assert.equal(JSON.parse(readFileSync(path.join(plain.body.artifactDir, "request.json"), "utf8")).loop, undefined);
+    assert.equal(exportStarts(daemon).at(-1).argv.at(-1), "plain", "no window, no --frames");
+    const kept = await generate(daemon, exportRequest({ constraints_path: "/some/where.json", constraints: undefined }));
+    assert.equal(kept.body.request.constraints, "/some/where.json", "a path stays a path");
+  } finally {
+    await daemon.stop();
+  }
+});
+
+test("loop is a boolean and needs fbx, refused before anything is written or spawned", async () => {
+  const daemon = await startDaemon();
+  try {
+    for (const bad of [{ fbx: false, loop: true }, { fbx: undefined, loop: true }, { loop: "yes canary" }, { loop: 1 }, { loop: null }, { loop: "true" }]) {
+      const answer = await generate(daemon, exportRequest(bad));
+      assert.equal(answer.status, 400, JSON.stringify(bad));
+      assert.deepEqual(answer.body, { ok: false, error: "Invalid fbx request." });
+      assert.ok(!answer.raw.includes("canary"), "the refusal repeated the input");
+    }
+    assert.equal(existsSync(path.join(daemon.stateRoot, "artifacts")), false, "no artifact directory was made");
+    assert.deepEqual(jobFiles(daemon), []);
+    assert.deepEqual(daemon.spawns(), [], "no worker was started");
+    assert.equal(exportStarts(daemon).length, 0);
+    assert.equal((await generate(daemon, exportRequest({ loop: false }))).status, 200, "loop false is no loop");
+    assert.equal((await generate(daemon, { prompt: "a person waves", duration: 2, loop: false })).status, 200);
+  } finally {
+    await daemon.stop();
+  }
+});
+
+test("a loop request whose answer has no usable window for a sample fails without exporting that sample", async () => {
+  for (const [model, exported] of [["no-loops", 0], ["bad-loops", 0], ["short-loops", 1]]) {
+    const daemon = await startDaemon();
+    try {
+      const answer = await generate(daemon, exportRequest({ model, loop: true, numSamples: 2 }));
+      assert.equal(answer.status, 502, model);
+      assert.equal(answer.body.ok, false);
+      assert.equal(answer.body.error, "fbx-export-failed (no loop window)");
+      assert.equal(exportStarts(daemon).length, exported, `${model}: only a sample with a window is exported`);
+      assert.equal(answer.body.export.clips.length, exported);
+    } finally {
+      await daemon.stop();
+    }
+  }
+});
+
 test("the BVH is never given to Blender unless it is a regular file inside the job's directory", async () => {
   const daemon = await startDaemon();
   try {
diff --git a/scripts/vili-daemon.mjs b/scripts/vili-daemon.mjs
index 908ca98..77e4966 100644
--- a/scripts/vili-daemon.mjs
+++ b/scripts/vili-daemon.mjs
@@ -592,8 +592,9 @@ async function readInside(file, directory) {
 // the BVH and writes a private FBX in a directory made for this export alone, and the FBX is then created beside the
 // BVH exclusively, so nothing planted at either path is read, written through or replaced. The FBX counts only if
 // Blender exits 0, prints its result line and its file exists. A failure names the exit or the signal and never
-// Blender's output.
-async function exportClip(bvh, fbx, clipName, timeoutMs, artifactDir) {
+// Blender's output. A window {first, last} (BVH frame indices, last inclusive) is handed to Blender as the frame
+// range to export, in Blender's frames, which count from 1; the daemon never chooses one.
+async function exportClip(bvh, fbx, clipName, timeoutMs, artifactDir, window = null) {
   const input = await readInside(bvh, artifactDir);
   if (input === null) return { error: "fbx-export-refused" };
   guard();
@@ -606,6 +607,7 @@ async function exportClip(bvh, fbx, clipName, timeoutMs, artifactDir) {
     const run = await runBlender([
       "-b", "--factory-startup", "--python-use-system-env", "--python-exit-code", "1",
       "--python", exportScript, "--", privateBvh, privateFbx, clipName,
+      ...(window ? ["--frames", String(window.first + 1), String(window.last + 1)] : []),
     ], timeoutMs);
     let info = null;
     if (run.code === 0 && run.result !== null) {
@@ -648,8 +650,9 @@ async function sweepWork() {
 
 // Every BVH the worker listed becomes an FBX, or the job fails. A listed BVH must be a regular file inside the job's
 // artifact directory, and that is decided when its export begins (exportClip), never earlier. Sample N of several is
-// <clipName>_NN.
-async function exportListed(listed, artifactDir, clipName, timeoutMs) {
+// <clipName>_NN. With loops (the worker's answer, an array when the request asked for a loop, else undefined), sample k
+// is exported over loops[k], and a job whose answer has no usable window for a sample fails.
+async function exportListed(listed, artifactDir, clipName, timeoutMs, loops = undefined) {
   const bvhs = [];
   for (const file of Array.isArray(listed) ? listed : []) {
     if (typeof file !== "string" || !file.endsWith(".bvh")) continue;
@@ -660,7 +663,11 @@ async function exportListed(listed, artifactDir, clipName, timeoutMs) {
   const clips = [];
   for (const [index, bvh] of bvhs.entries()) {
     const name = bvhs.length === 1 ? clipName : `${clipName}_${String(index).padStart(2, "0")}`;
-    const done = await exportClip(bvh, `${bvh.slice(0, -".bvh".length)}.fbx`, name, timeoutMs, artifactDir);
+    const window = loops === undefined ? null : loops[index];
+    if (window !== null && !(Number.isInteger(window?.first) && Number.isInteger(window?.last) && window.first >= 0 && window.last > window.first)) {
+      return { clips, error: "fbx-export-failed (no loop window)" };
+    }
+    const done = await exportClip(bvh, `${bvh.slice(0, -".bvh".length)}.fbx`, name, timeoutMs, artifactDir, window);
     if (done.error) return { clips, error: done.error };
     clips.push({ ...done.clip, file: path.relative(artifactDir, done.clip.file).split(path.sep).join("/") });
   }
@@ -864,8 +871,10 @@ async function generate(request, response) {
   // One export plan: fbx implies the BVH and its standard T-pose, which the FBX is made from.
   const fbx = boolValue(body.fbx);
   const askedTpose = firstDefined(body.bvhStandardTpose, body.bvh_standard_tpose);
-  if (fbx && ((askedTpose !== undefined && !boolValue(askedTpose))
-    || (body.clipName !== undefined && !(typeof body.clipName === "string" && CLIP_NAME.test(body.clipName))))) {
+  const loop = body.loop === true;
+  if ((body.loop !== undefined && typeof body.loop !== "boolean") || (loop && !fbx)
+    || (fbx && ((askedTpose !== undefined && !boolValue(askedTpose))
+      || (body.clipName !== undefined && !(typeof body.clipName === "string" && CLIP_NAME.test(body.clipName)))))) {
     json(response, 400, { ok: false, error: "Invalid fbx request." });
     return;
   }
@@ -934,9 +943,10 @@ async function generate(request, response) {
     numSamples,
     diffusion_steps: diffusionSteps,
     numTransitionFrames: firstDefined(body.numTransitionFrames, body.num_transition_frames),
-    constraints: constraintsPath,
+    constraints: body.constraints !== undefined && typeof body.constraints !== "string" ? body.constraints : constraintsPath,
     seed,
     fbx: fbx || undefined,
+    loop: loop || undefined,
     clipName,
     cfg: body.cfg,
     cfgType: firstDefined(body.cfgType, body.cfg_type),
@@ -967,6 +977,7 @@ async function generate(request, response) {
       numTransitionFrames: firstDefined(body.numTransitionFrames, body.num_transition_frames),
       constraints: constraintsPath,
       seed,
+      loop: loop || undefined,
       cfg: body.cfg,
       cfgType: firstDefined(body.cfgType, body.cfg_type),
       cfgWeight: firstDefined(body.cfgWeight, body.cfg_weight),
@@ -993,7 +1004,7 @@ async function generate(request, response) {
   let exported = null;
   if (fbx && result.ok) {
     const timeoutMs = Math.min(EXPORT_TIMEOUT_MS, Math.max(1, Number(firstDefined(body.exportTimeoutMs, EXPORT_TIMEOUT_MS)) || EXPORT_TIMEOUT_MS));
-    exported = await exportListed(result.body?.files, artifactDir, clipName, timeoutMs);
+    exported = await exportListed(result.body?.files, artifactDir, clipName, timeoutMs, loop ? (Array.isArray(result.body?.loops) ? result.body.loops : []) : undefined);
     if (exported.error) result = { ok: false, error: exported.error, body: result.body };
   }