diff --git a/.beads/issues.jsonl b/.beads/issues.jsonl
index ce619c2..fcd3b1e 100644
--- a/.beads/issues.jsonl
+++ b/.beads/issues.jsonl
@@ -32,7 +32,7 @@
 {"id":"ifcurl-cyw","title":"Phase 3c: Verify Forgejo merge-ability check works with ifcmerge","description":"Investigate whether Forgejo's merge-ability check (the 'branches can be merged automatically' indicator) correctly reflects ifcmerge's ability to handle .ifc files. Determine if Forgejo runs a trial git merge or uses a heuristic. If needed, patch Forgejo to run a trial git merge --no-commit to get an accurate result.","status":"closed","priority":3,"issue_type":"task","assignee":"Bruno Postle","owner":"bruno@postle.net","created_at":"2026-04-23T05:48:22Z","created_by":"Bruno Postle","updated_at":"2026-04-24T08:27:25Z","started_at":"2026-04-24T08:19:58Z","closed_at":"2026-04-24T08:27:25Z","close_reason":"Verified: git merge-tree --write-tree (Forgejo's code path for git ≥2.38) does invoke merge drivers. Bare repos require core.attributesFile in /etc/gitconfig — committed .gitattributes not read. Added gitattributes server-config file and updated deployment docs.","dependencies":[{"issue_id":"ifcurl-cyw","depends_on_id":"ifcurl-c69","type":"blocks","created_at":"2026-04-23T06:48:29Z","created_by":"Bruno Postle","metadata":"{}"}],"dependency_count":1,"dependent_count":0,"comment_count":0}
 {"id":"ifcurl-c69","title":"Phase 3c: Register ifcmerge as git merge driver on Forgejo server","description":"Configure ifcmerge as the git merge driver for .ifc files on the Forgejo server. Add to /etc/gitconfig or ~forgejo/.gitconfig: [merge \"ifcmerge\"] name = IFC merge driver / driver = ifcmerge %O %A %B %L. Add *.ifc merge=ifcmerge to server-side gitattributes. This allows Forgejo's 'merge automatically' path to invoke ifcmerge via git when merging .ifc files.","status":"closed","priority":3,"issue_type":"task","owner":"bruno@postle.net","created_at":"2026-04-23T05:48:21Z","created_by":"Bruno Postle","updated_at":"2026-04-24T08:15:20Z","closed_at":"2026-04-24T08:15:20Z","close_reason":"Documented ifcmerge driver registration (gitconfig-ifcmerge), gitattributes setup, merge direction constraint, and merge-commit strategy requirement","comments":[{"id":"019dbc54-bf0a-76e2-b64d-7d1681f0c730","issue_id":"ifcurl-c69","author":"Bruno Postle","text":"ifcmerge needs to be installed as a pair of merge tools because the merge process is asymmetrical and we need to be able to prefer one branch over another (because step-ids from one branch get rewritten in the merged file). In practice we prefer origin and/or main/master branches. there is code in the ifcopenshell bonsai ifcgit module for reference.","created_at":"2026-04-23T21:52:44Z"}],"dependency_count":0,"dependent_count":1,"comment_count":1}
 {"id":"ifcurl-8ns","title":"Future: Contribute ifc:// URL parsing to IfcOpenShell","description":"Once the Python core URL parsing and resolution logic is stable, contribute it to IfcOpenShell as a shared utility. Not a prerequisite for any phase — pursue after Phase 1 has been stable in production use for a while.","status":"open","priority":4,"issue_type":"task","owner":"bruno@postle.net","created_at":"2026-04-23T05:49:33Z","created_by":"Bruno Postle","updated_at":"2026-04-23T05:49:33Z","dependency_count":0,"dependent_count":0,"comment_count":0}
-{"id":"ifcurl-9yr","title":"Future: BCF export endpoint /bcf","description":"Add a /bcf endpoint to the preview service: POST /bcf with body {url: 'ifc://...'} returns a BCF zip file. Resolve the selector to a GUID set and construct a BCF viewpoint from the camera, clipping planes, and visibility state. Not planned before Phase 2 is complete.","status":"open","priority":4,"issue_type":"feature","owner":"bruno@postle.net","created_at":"2026-04-23T05:49:25Z","created_by":"Bruno Postle","updated_at":"2026-04-23T05:49:25Z","dependency_count":0,"dependent_count":0,"comment_count":0}
+{"id":"ifcurl-9yr","title":"Future: BCF export endpoint /bcf","description":"Add a /bcf endpoint to the preview service: POST /bcf with body {url: 'ifc://...'} returns a BCF zip file. Resolve the selector to a GUID set and construct a BCF viewpoint from the camera, clipping planes, and visibility state. Not planned before Phase 2 is complete.","status":"closed","priority":4,"issue_type":"feature","assignee":"Bruno Postle","owner":"bruno@postle.net","created_at":"2026-04-23T05:49:25Z","created_by":"Bruno Postle","updated_at":"2026-04-24T08:51:18Z","started_at":"2026-04-24T08:34:51Z","closed_at":"2026-04-24T08:51:18Z","close_reason":"bcf.py build_bcf() + /bcf POST endpoint + viewer BCF panel with JSZip client-side generation + 21 tests","dependency_count":0,"dependent_count":0,"comment_count":0}
 {"id":"ifcurl-vam","title":"Phase 6: Bonsai offer ifc:// URL when saving linked model reference","description":"When saving an IFCDOCUMENTREFERENCE in Bonsai, offer the option to write the location as an ifc:// URL rather than a relative path. When a relative path would traverse above the repository root, prompt the user to convert it to an ifc:// URL. Requires Phase 4 federation (ifcurl-bqt).","status":"deferred","priority":4,"issue_type":"feature","owner":"bruno@postle.net","created_at":"2026-04-23T05:49:16Z","created_by":"Bruno Postle","updated_at":"2026-04-23T06:15:13Z","defer_until":"2026-12-01T00:00:00Z","dependencies":[{"issue_id":"ifcurl-vam","depends_on_id":"ifcurl-bqt","type":"blocks","created_at":"2026-04-23T06:49:23Z","created_by":"Bruno Postle","metadata":"{}"}],"dependency_count":1,"dependent_count":0,"comment_count":0}
 {"id":"ifcurl-9k2","title":"Phase 5: IFC Viewer integration (equivalent of Phase 4)","description":"Add ifc:// OS protocol handler and 'Copy view URL' button to the IFC Viewer desktop application, equivalent to the Bonsai Phase 4 integration. Depends on Phase 4 being complete as a reference implementation.","status":"deferred","priority":4,"issue_type":"feature","owner":"bruno@postle.net","created_at":"2026-04-23T05:49:08Z","created_by":"Bruno Postle","updated_at":"2026-04-23T06:15:12Z","defer_until":"2026-12-01T00:00:00Z","dependencies":[{"issue_id":"ifcurl-9k2","depends_on_id":"ifcurl-0oj","type":"blocks","created_at":"2026-04-23T06:49:14Z","created_by":"Bruno Postle","metadata":"{}"}],"dependency_count":1,"dependent_count":0,"comment_count":0}
 {"id":"ifcurl-prc","title":"Phase 3c: 3D diff view for IFC PR merges","description":"After a PR merge completes, surface a 'View merge in 3D' button using the Phase 3b viewer. Use @thatopen/components highlighter to colour added, removed, and modified elements differently, driven by comparing the merged IFC against either parent. Requires Phase 3b viewer (ifcurl-i1s) to be working.","status":"open","priority":4,"issue_type":"feature","owner":"bruno@postle.net","created_at":"2026-04-23T05:48:30Z","created_by":"Bruno Postle","updated_at":"2026-04-23T05:48:30Z","dependencies":[{"issue_id":"ifcurl-prc","depends_on_id":"ifcurl-i1s","type":"blocks","created_at":"2026-04-23T06:48:36Z","created_by":"Bruno Postle","metadata":"{}"}],"dependency_count":1,"dependent_count":0,"comment_count":0}
diff --git a/forgejo/custom/public/assets/viewer.html b/forgejo/custom/public/assets/viewer.html
index bf95fd1..7614223 100644
--- a/forgejo/custom/public/assets/viewer.html
+++ b/forgejo/custom/public/assets/viewer.html
@@ -50,6 +50,9 @@
     .tb-btn:hover { background: rgba(255,255,255,0.14); }
     .tb-btn.active { color: #a8c8ff; background: rgba(80,120,255,0.2);
                      border-color: rgba(80,120,255,0.5); }
+    /* BCF panel */
+    #bcf-comment { flex: 3; }
+    #bcf-title   { flex: 2; }
     /* Status */
     #status {
       position: fixed; bottom: 12px; left: 50%;
@@ -76,7 +79,8 @@
       "three/addons/":        "https://esm.sh/three@0.177.0/examples/jsm/",
       "web-ifc":              "https://unpkg.com/web-ifc@0.0.75/web-ifc-api.js",
       "@thatopen/fragments":  "https://esm.sh/@thatopen/fragments@3.3.1?external=three,web-ifc",
-      "@thatopen/components": "https://esm.sh/@thatopen/components@3.3.1?external=three,web-ifc,@thatopen/fragments"
+      "@thatopen/components": "https://esm.sh/@thatopen/components@3.3.1?external=three,web-ifc,@thatopen/fragments",
+      "jszip":                "https://esm.sh/jszip@3.10.1"
     }
   }
   </script>
@@ -90,6 +94,7 @@
              spellcheck="false">
       <button id="clip-btn" class="tb-btn" title="Double-click model surface to add clipping plane">✂ clip</button>
       <button id="clip-clear-btn" class="tb-btn" title="Remove all clipping planes" style="display:none">⊗ clips</button>
+      <button id="bcf-btn" class="tb-btn" title="Export current view as a BCF issue">BCF</button>
       <div id="fov-wrap">
         FOV <input id="fov-input" class="ti" type="number" min="10" max="120" step="1"
                    title="Field of view (degrees)">°
@@ -118,6 +123,19 @@
                placeholder="IfcWall" spellcheck="false">
       </div>
     </div>
+    <!-- Row 3: BCF export form (hidden until BCF button clicked) -->
+    <div id="bcf-row" class="trow" style="display:none">
+      <div class="field" style="flex:2">
+        <label>title</label>
+        <input id="bcf-title" class="ti" type="text" placeholder="IFC View" spellcheck="false">
+      </div>
+      <div class="field" style="flex:3">
+        <label>comment</label>
+        <input id="bcf-comment" class="ti" type="text" placeholder="Optional note" spellcheck="false">
+      </div>
+      <button id="bcf-cancel-btn" class="tb-btn">✕</button>
+      <button id="bcf-export-btn" class="tb-btn">Export .bcf</button>
+    </div>
   </div>
   <div id="viewer"></div>
   <div id="status"></div>
@@ -125,6 +143,7 @@
   <script type="module">
     import * as THREE from "three";
     import * as OBC from "@thatopen/components";
+    import JSZip from "jszip";
     import { parseIfcUrl, toRawUrl, buildIfcUrl as _buildIfcUrl } from "./viewer-url.js";
 
     const params   = new URLSearchParams(window.location.search);
@@ -189,7 +208,27 @@
     // Disable camera controls while any input has focus.
     function onInputFocus() { if (cameraControls) cameraControls.enabled = false; }
     function onInputBlur()  { if (cameraControls) cameraControls.enabled = true;  }
-    const allInputs = [urlInput, fovInput, repoInput, refInput, pathInput, selectorInput];
+    const bcfBtn       = document.getElementById("bcf-btn");
+    const bcfRow       = document.getElementById("bcf-row");
+    const bcfTitleIn   = document.getElementById("bcf-title");
+    const bcfCommentIn = document.getElementById("bcf-comment");
+    const bcfCancelBtn = document.getElementById("bcf-cancel-btn");
+    const bcfExportBtn = document.getElementById("bcf-export-btn");
+
+    function bcfPanelOpen(open) {
+      bcfRow.style.display = open ? "" : "none";
+      bcfBtn.classList.toggle("active", open);
+    }
+
+    bcfBtn.addEventListener("click", () => bcfPanelOpen(bcfRow.style.display === "none"));
+    bcfCancelBtn.addEventListener("click", () => bcfPanelOpen(false));
+    bcfExportBtn.addEventListener("click", async () => {
+      await generateBcf(bcfTitleIn.value.trim(), bcfCommentIn.value.trim());
+      bcfPanelOpen(false);
+    });
+
+    const allInputs = [urlInput, fovInput, repoInput, refInput, pathInput, selectorInput,
+                       bcfTitleIn, bcfCommentIn];
     for (const el of allInputs) {
       el.addEventListener("focus",       onInputFocus);
       el.addEventListener("blur",        onInputBlur);
@@ -406,6 +445,111 @@
       return true;
     }
 
+    // -----------------------------------------------------------------------
+    // BCF 2.1 export — build zip client-side and trigger download.
+    // -----------------------------------------------------------------------
+    async function generateBcf(title, comment) {
+      if (threeCamera) syncCameraUrl(threeCamera);
+      const src = urlInput.value.trim();
+      if (!src) return;
+
+      const qIdx = src.indexOf("?");
+      const qs   = new URLSearchParams(qIdx < 0 ? "" : src.slice(qIdx + 1));
+
+      const camV  = (qs.get("camera") ?? "").split(",").map(Number);
+      const cam   = camV.length === 9 && !camV.some(isNaN) ? camV : null;
+      const fov   = cam ? (parseFloat(qs.get("fov")) || null) : null;
+      const scale = cam ? (parseFloat(qs.get("scale")) || null) : null;
+      const clips = qs.getAll("clip")
+        .map(s => s.split(",").map(Number))
+        .filter(v => v.length === 6 && !v.some(isNaN));
+      const selector = qs.get("selector") || "";
+
+      const topicGuid = crypto.randomUUID();
+      const vpGuid    = cam ? crypto.randomUUID() : null;
+      const now       = new Date().toISOString().replace(/\.\d{3}Z$/, "Z");
+
+      function esc(s) {
+        return String(s).replace(/&/g,"&amp;").replace(/</g,"&lt;")
+                        .replace(/>/g,"&gt;").replace(/"/g,"&quot;");
+      }
+      function xyz(tag, a, b, c) {
+        return `<${tag}><X>${a}</X><Y>${b}</Y><Z>${c}</Z></${tag}>`;
+      }
+
+      const versionXml = `<?xml version="1.0" encoding="utf-8"?>
+<Version VersionId="2.1" xsi:noNamespaceSchemaLocation="https://raw.githubusercontent.com/buildingSMART/BCF-XML/release_2_1/Schemas/version.xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
+  <DetailedVersion>2.1</DetailedVersion>
+</Version>`;
+
+      let viewpointXml = null;
+      if (vpGuid && cam) {
+        const [px,py,pz, dx,dy,dz, ux,uy,uz] = cam;
+        const camXml = fov != null
+          ? `<PerspectiveCamera>
+    ${xyz("CameraViewPoint",px,py,pz)}
+    ${xyz("CameraDirection",dx,dy,dz)}
+    ${xyz("CameraUpVector",ux,uy,uz)}
+    <FieldOfView>${fov}</FieldOfView>
+  </PerspectiveCamera>`
+          : scale != null
+          ? `<OrthogonalCamera>
+    ${xyz("CameraViewPoint",px,py,pz)}
+    ${xyz("CameraDirection",dx,dy,dz)}
+    ${xyz("CameraUpVector",ux,uy,uz)}
+    <ViewToWorldScale>${scale}</ViewToWorldScale>
+  </OrthogonalCamera>`
+          : "";
+        const clipsXml = clips.length
+          ? `<ClippingPlanes>
+    ${clips.map(([cx,cy,cz,nx,ny,nz]) =>
+        `<ClippingPlane>${xyz("Location",cx,cy,cz)}${xyz("Direction",nx,ny,nz)}</ClippingPlane>`
+      ).join("\n    ")}
+  </ClippingPlanes>` : "";
+        viewpointXml = `<?xml version="1.0" encoding="utf-8"?>
+<VisualizationInfo Guid="${vpGuid}">
+  <Components><Visibility DefaultVisibility="true"/></Components>
+  ${camXml}
+  ${clipsXml}
+</VisualizationInfo>`;
+      }
+
+      const desc = selector
+        ? `${esc(src)}\nselector: ${esc(selector)}`
+        : esc(src);
+      const vpEntry  = vpGuid ? `<Viewpoints Guid="${vpGuid}"><Viewpoint>viewpoint.bcfv</Viewpoint></Viewpoints>` : "";
+      const cmtEntry = comment
+        ? `<Comment Guid="${crypto.randomUUID()}">
+    <Date>${now}</Date><Author>anonymous</Author>
+    <Comment>${esc(comment)}</Comment>
+    ${vpGuid ? `<Viewpoint Guid="${vpGuid}"/>` : ""}
+  </Comment>` : "";
+
+      const markupXml = `<?xml version="1.0" encoding="utf-8"?>
+<Markup>
+  <Topic Guid="${topicGuid}" TopicType="Coordination" TopicStatus="Open">
+    <Title>${esc(title || "IFC View")}</Title>
+    <Description>${desc}</Description>
+    <CreationDate>${now}</CreationDate>
+    <CreationAuthor>anonymous</CreationAuthor>
+    ${vpEntry}
+  </Topic>
+  ${cmtEntry}
+</Markup>`;
+
+      const zip = new JSZip();
+      zip.file("bcf.version", versionXml);
+      zip.folder(topicGuid).file("markup.bcf", markupXml);
+      if (vpGuid && viewpointXml) zip.folder(topicGuid).file("viewpoint.bcfv", viewpointXml);
+
+      const blob = await zip.generateAsync({ type: "blob" });
+      const a = document.createElement("a");
+      a.href = URL.createObjectURL(blob);
+      a.download = "view.bcf";
+      a.click();
+      URL.revokeObjectURL(a.href);
+    }
+
     // -----------------------------------------------------------------------
     // Main
     // -----------------------------------------------------------------------
diff --git a/ifcurl/bcf.py b/ifcurl/bcf.py
new file mode 100644
index 0000000..be45773
--- /dev/null
+++ b/ifcurl/bcf.py
@@ -0,0 +1,171 @@
+# IFC URL — resolve and render ifc:// URLs
+# Copyright (C) 2026 Bruno Postle <bruno@postle.net>
+#
+# This file is part of IFC URL.
+#
+# IFC URL is free software: you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# IFC URL is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU Lesser General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with IFC URL.  If not, see <http://www.gnu.org/licenses/>.
+
+"""Build BCF 2.1 zip archives from ifc:// view state.
+
+BCF (BIM Collaboration Format) 2.1 structure produced here::
+
+    bcf.version
+    <topic-guid>/
+        markup.bcf      — title, comment, viewpoint reference
+        viewpoint.bcfv  — camera, clipping planes, component selection
+"""
+
+from __future__ import annotations
+
+import io
+import uuid
+import zipfile
+import xml.etree.ElementTree as ET
+from datetime import datetime, timezone
+
+
+_VERSION_XML = b"""\
+<?xml version="1.0" encoding="utf-8"?>
+<Version VersionId="2.1" xsi:noNamespaceSchemaLocation="https://raw.githubusercontent.com/buildingSMART/BCF-XML/release_2_1/Schemas/version.xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
+  <DetailedVersion>2.1</DetailedVersion>
+</Version>"""
+
+
+def _xyz(parent: ET.Element, tag: str, x: float, y: float, z: float) -> None:
+    el = ET.SubElement(parent, tag)
+    ET.SubElement(el, "X").text = str(x)
+    ET.SubElement(el, "Y").text = str(y)
+    ET.SubElement(el, "Z").text = str(z)
+
+
+def _viewpoint_xml(
+    guid: str,
+    camera: tuple[float, ...],
+    fov: float | None,
+    scale: float | None,
+    clips: list[tuple[float, ...]],
+    guids: list[str] | None,
+    visibility: str,
+) -> bytes:
+    root = ET.Element("VisualizationInfo", Guid=guid)
+
+    comps = ET.SubElement(root, "Components")
+    if guids:
+        if visibility == "isolate":
+            vis_el = ET.SubElement(comps, "Visibility", DefaultVisibility="false")
+            exc = ET.SubElement(vis_el, "Exceptions")
+            for g in guids:
+                ET.SubElement(exc, "Component", IfcGuid=g)
+        else:
+            sel = ET.SubElement(comps, "Selection")
+            for g in guids:
+                ET.SubElement(sel, "Component", IfcGuid=g)
+            ET.SubElement(comps, "Visibility", DefaultVisibility="true")
+    else:
+        ET.SubElement(comps, "Visibility", DefaultVisibility="true")
+
+    px, py, pz, dx, dy, dz, ux, uy, uz = camera
+    if fov is not None:
+        cam = ET.SubElement(root, "PerspectiveCamera")
+        _xyz(cam, "CameraViewPoint", px, py, pz)
+        _xyz(cam, "CameraDirection", dx, dy, dz)
+        _xyz(cam, "CameraUpVector", ux, uy, uz)
+        ET.SubElement(cam, "FieldOfView").text = f"{fov:.4f}"
+    elif scale is not None:
+        cam = ET.SubElement(root, "OrthogonalCamera")
+        _xyz(cam, "CameraViewPoint", px, py, pz)
+        _xyz(cam, "CameraDirection", dx, dy, dz)
+        _xyz(cam, "CameraUpVector", ux, uy, uz)
+        ET.SubElement(cam, "ViewToWorldScale").text = f"{scale:.4f}"
+
+    if clips:
+        cps = ET.SubElement(root, "ClippingPlanes")
+        for clip in clips:
+            cp = ET.SubElement(cps, "ClippingPlane")
+            _xyz(cp, "Location",  clip[0], clip[1], clip[2])
+            _xyz(cp, "Direction", clip[3], clip[4], clip[5])
+
+    return b'<?xml version="1.0" encoding="utf-8"?>\n' + ET.tostring(root, encoding="unicode").encode()
+
+
+def _markup_xml(
+    topic_guid: str,
+    vp_guid: str | None,
+    title: str,
+    comment: str,
+    author: str,
+    now: str,
+) -> bytes:
+    root = ET.Element("Markup")
+    topic = ET.SubElement(root, "Topic",
+                          Guid=topic_guid, TopicType="Coordination", TopicStatus="Open")
+    ET.SubElement(topic, "Title").text = title or "IFC View"
+    ET.SubElement(topic, "CreationDate").text = now
+    ET.SubElement(topic, "CreationAuthor").text = author
+    if vp_guid:
+        vps = ET.SubElement(topic, "Viewpoints", Guid=vp_guid)
+        ET.SubElement(vps, "Viewpoint").text = "viewpoint.bcfv"
+
+    if comment and comment.strip():
+        c = ET.SubElement(root, "Comment", Guid=str(uuid.uuid4()))
+        ET.SubElement(c, "Date").text = now
+        ET.SubElement(c, "Author").text = author
+        ET.SubElement(c, "Comment").text = comment
+        if vp_guid:
+            ET.SubElement(c, "Viewpoint", Guid=vp_guid)
+
+    return b'<?xml version="1.0" encoding="utf-8"?>\n' + ET.tostring(root, encoding="unicode").encode()
+
+
+def build_bcf(
+    camera: tuple[float, ...] | None = None,
+    fov: float | None = None,
+    scale: float | None = None,
+    clips: list[tuple[float, ...]] | None = None,
+    guids: list[str] | None = None,
+    visibility: str = "highlight",
+    title: str = "IFC View",
+    comment: str = "",
+    author: str = "anonymous",
+) -> bytes:
+    """Build a BCF 2.1 zip archive and return the raw bytes.
+
+    :param camera: 9-tuple (px, py, pz, dx, dy, dz, ux, uy, uz) in IFC coords.
+    :param fov: Perspective field of view in degrees.  Mutually exclusive with *scale*.
+    :param scale: Orthographic view-to-world scale.  Mutually exclusive with *fov*.
+    :param clips: List of 6-tuples (px, py, pz, nx, ny, nz) clipping planes.
+    :param guids: IfcGloballyUniqueId strings for selected/visible elements.
+    :param visibility: ``'highlight'``, ``'ghost'``, or ``'isolate'``.
+    :param title: BCF topic title.
+    :param comment: Optional comment text added to the topic.
+    :param author: Author string recorded in the BCF markup.
+    :returns: Bytes of a valid BCF 2.1 zip archive.
+    """
+    topic_guid = str(uuid.uuid4())
+    vp_guid = str(uuid.uuid4()) if camera is not None else None
+    now = datetime.now(timezone.utc).strftime("%Y-%m-%dT%H:%M:%SZ")
+
+    buf = io.BytesIO()
+    with zipfile.ZipFile(buf, "w", zipfile.ZIP_DEFLATED) as zf:
+        zf.writestr("bcf.version", _VERSION_XML)
+        zf.writestr(
+            f"{topic_guid}/markup.bcf",
+            _markup_xml(topic_guid, vp_guid, title, comment, author, now),
+        )
+        if vp_guid is not None:
+            zf.writestr(
+                f"{topic_guid}/viewpoint.bcfv",
+                _viewpoint_xml(vp_guid, camera, fov, scale, clips or [], guids, visibility),
+            )
+    return buf.getvalue()
diff --git a/ifcurl/service.py b/ifcurl/service.py
index 9f6a3d9..b3d11cf 100644
--- a/ifcurl/service.py
+++ b/ifcurl/service.py
@@ -54,8 +54,8 @@ from pydantic import BaseModel
 
 from ifcurl import render as render_mod
 from ifcurl.auth import get_token_for_host
+from ifcurl.bcf import build_bcf
 from ifcurl.git import fetch_ifc
-
 from ifcurl.url import IfcUrl
 
 app = FastAPI(
@@ -91,6 +91,17 @@ def _is_private_ip(host: str) -> bool:
         return False
 
 
+def _ssrf_check(ifc_url: IfcUrl) -> None:
+    """Raise HTTPException if the URL fails SSRF protection checks."""
+    if ifc_url.transport == "local":
+        raise HTTPException(status_code=403, detail="Local file transport is not permitted in service mode")
+    if _allowed_hosts is not None:
+        if ifc_url.host not in _allowed_hosts:
+            raise HTTPException(status_code=403, detail=f"Host {ifc_url.host!r} is not in the allowed-hosts list")
+    elif _is_private_ip(ifc_url.host):
+        raise HTTPException(status_code=403, detail="Requests to private/loopback addresses are not permitted")
+
+
 # ---------------------------------------------------------------------------
 # Tier 2: (commit_hexsha, path) → IFC bytes
 # ---------------------------------------------------------------------------
@@ -243,6 +254,13 @@ class PreviewRequest(BaseModel):
     """
 
 
+class BcfRequest(BaseModel):
+    url: str
+    title: str = "IFC View"
+    comment: str = ""
+    token: str | None = None
+
+
 # ---------------------------------------------------------------------------
 # Endpoints
 # ---------------------------------------------------------------------------
@@ -275,13 +293,7 @@ def preview(request: PreviewRequest) -> Response:
         raise HTTPException(status_code=400, detail="URL has no 'path' parameter")
 
     # --- SSRF protection ---
-    if ifc_url.transport == "local":
-        raise HTTPException(status_code=403, detail="Local file transport is not permitted in service mode")
-    if _allowed_hosts is not None:
-        if ifc_url.host not in _allowed_hosts:
-            raise HTTPException(status_code=403, detail=f"Host {ifc_url.host!r} is not in the allowed-hosts list")
-    elif _is_private_ip(ifc_url.host):
-        raise HTTPException(status_code=403, detail="Requests to private/loopback addresses are not permitted")
+    _ssrf_check(ifc_url)
 
     # --- Tier 4 / 4m: cached PNG ---
     if ifc_url.is_mutable_ref():
@@ -382,3 +394,61 @@ def preview(request: PreviewRequest) -> Response:
             media_type="image/png",
             headers={"Cache-Control": "public, max-age=31536000, immutable"},
         )
+
+
+@app.post("/bcf")
+def bcf_export(request: BcfRequest) -> Response:
+    """Generate a BCF 2.1 zip from an ifc:// URL viewpoint.
+
+    Returns ``application/octet-stream`` with a ``.bcf`` zip file containing
+    the camera, clipping planes, and (when a selector is present) the resolved
+    component GUID selection.
+    """
+    try:
+        ifc_url = IfcUrl.parse(request.url)
+    except ValueError as exc:
+        raise HTTPException(status_code=400, detail=str(exc)) from exc
+
+    if ifc_url.path is None:
+        raise HTTPException(status_code=400, detail="URL has no 'path' parameter")
+
+    _ssrf_check(ifc_url)
+
+    # Resolve selector → component GUIDs when present.
+    guids: list[str] | None = None
+    if ifc_url.selector:
+        token = request.token
+        if token is None and ifc_url.host:
+            token = get_token_for_host(ifc_url.host)
+        try:
+            hexsha, ifc_bytes = fetch_ifc(ifc_url, token=token)
+        except (ImportError, ValueError) as exc:
+            raise HTTPException(status_code=404, detail=str(exc)) from exc
+
+        cached = _t2_get(hexsha, ifc_url.path)
+        ifc_bytes = cached if cached is not None else ifc_bytes
+        if cached is None:
+            _t2_put(hexsha, ifc_url.path, ifc_bytes)
+
+        model = _load_model(ifc_bytes)
+        try:
+            matched = list(ifcopenshell.util.selector.filter_elements(model, ifc_url.selector))
+        except Exception as exc:
+            raise HTTPException(status_code=422, detail=f"Invalid selector: {exc}") from exc
+        guids = [e.GlobalId for e in matched if hasattr(e, "GlobalId") and e.GlobalId]
+
+    bcf_bytes = build_bcf(
+        camera=ifc_url.camera,
+        fov=ifc_url.fov,
+        scale=ifc_url.scale,
+        clips=ifc_url.clips or None,
+        guids=guids,
+        visibility=ifc_url.visibility,
+        title=request.title,
+        comment=request.comment,
+    )
+    return Response(
+        content=bcf_bytes,
+        media_type="application/octet-stream",
+        headers={"Content-Disposition": 'attachment; filename="view.bcf"'},
+    )
diff --git a/tests/test_bcf.py b/tests/test_bcf.py
new file mode 100644
index 0000000..46f0ea3
--- /dev/null
+++ b/tests/test_bcf.py
@@ -0,0 +1,196 @@
+"""Tests for ifcurl.bcf and the /bcf service endpoint."""
+
+from __future__ import annotations
+
+import io
+import zipfile
+from unittest.mock import patch
+
+import pytest
+from fastapi.testclient import TestClient
+
+from ifcurl.bcf import build_bcf
+from ifcurl.service import _t2_cache, _t3_cache, app, configure_allowed_hosts
+
+client = TestClient(app)
+
+FAKE_HEXSHA = "b" * 40
+MUTABLE_URL = "ifc://example.com/org/repo@heads/main?path=model.ifc"
+CAMERA_URL  = (
+    "ifc://example.com/org/repo@heads/main"
+    "?path=model.ifc&camera=1,2,3,0,0,-1,0,1,0&fov=60"
+)
+CLIP_URL = (
+    "ifc://example.com/org/repo@heads/main"
+    "?path=model.ifc&camera=0,0,5,0,0,-1,0,1,0&fov=60"
+    "&clip=0,0,2,0,0,-1"
+)
+
+
+@pytest.fixture(autouse=True)
+def reset_service(tmp_path, monkeypatch):
+    monkeypatch.setattr("ifcurl.service.user_cache_dir", lambda *a, **kw: str(tmp_path))
+    _t2_cache.clear()
+    _t3_cache.clear()
+    configure_allowed_hosts(None)
+    yield
+    _t2_cache.clear()
+    _t3_cache.clear()
+    configure_allowed_hosts(None)
+
+
+def _zip_names(data: bytes) -> set[str]:
+    with zipfile.ZipFile(io.BytesIO(data)) as zf:
+        return set(zf.namelist())
+
+
+def _zip_read(data: bytes, name: str) -> str:
+    with zipfile.ZipFile(io.BytesIO(data)) as zf:
+        return zf.read(name).decode()
+
+
+# ---------------------------------------------------------------------------
+# build_bcf unit tests
+# ---------------------------------------------------------------------------
+
+
+class TestBuildBcf:
+    def test_returns_valid_zip(self):
+        data = build_bcf()
+        assert zipfile.is_zipfile(io.BytesIO(data))
+
+    def test_contains_version_file(self):
+        data = build_bcf()
+        assert "bcf.version" in _zip_names(data)
+
+    def test_contains_markup(self):
+        data = build_bcf(title="Test topic")
+        names = _zip_names(data)
+        markup_files = [n for n in names if n.endswith("markup.bcf")]
+        assert len(markup_files) == 1
+        content = _zip_read(data, markup_files[0])
+        assert "Test topic" in content
+
+    def test_no_viewpoint_without_camera(self):
+        data = build_bcf()
+        names = _zip_names(data)
+        assert not any(n.endswith("viewpoint.bcfv") for n in names)
+
+    def test_viewpoint_present_with_camera(self):
+        cam = (1.0, 2.0, 3.0, 0.0, 0.0, -1.0, 0.0, 1.0, 0.0)
+        data = build_bcf(camera=cam, fov=60.0)
+        names = _zip_names(data)
+        assert any(n.endswith("viewpoint.bcfv") for n in names)
+
+    def test_perspective_camera_in_viewpoint(self):
+        cam = (1.0, 2.0, 3.0, 0.0, 0.0, -1.0, 0.0, 1.0, 0.0)
+        data = build_bcf(camera=cam, fov=60.0)
+        vp_file = next(n for n in _zip_names(data) if n.endswith("viewpoint.bcfv"))
+        content = _zip_read(data, vp_file)
+        assert "PerspectiveCamera" in content
+        assert "<FieldOfView>60.0000</FieldOfView>" in content
+
+    def test_orthographic_camera_in_viewpoint(self):
+        cam = (0.0, 0.0, 10.0, 0.0, 0.0, -1.0, 0.0, 1.0, 0.0)
+        data = build_bcf(camera=cam, scale=50.0)
+        vp_file = next(n for n in _zip_names(data) if n.endswith("viewpoint.bcfv"))
+        content = _zip_read(data, vp_file)
+        assert "OrthogonalCamera" in content
+        assert "ViewToWorldScale" in content
+
+    def test_clipping_planes_in_viewpoint(self):
+        cam = (0.0, 0.0, 5.0, 0.0, 0.0, -1.0, 0.0, 1.0, 0.0)
+        data = build_bcf(camera=cam, fov=60.0, clips=[(0.0, 0.0, 2.0, 0.0, 0.0, -1.0)])
+        vp_file = next(n for n in _zip_names(data) if n.endswith("viewpoint.bcfv"))
+        content = _zip_read(data, vp_file)
+        assert "ClippingPlane" in content
+
+    def test_comment_in_markup(self):
+        cam = (0.0, 0.0, 5.0, 0.0, 0.0, -1.0, 0.0, 1.0, 0.0)
+        data = build_bcf(camera=cam, fov=60.0, comment="Look at this crack")
+        markup = next(n for n in _zip_names(data) if n.endswith("markup.bcf"))
+        content = _zip_read(data, markup)
+        assert "Look at this crack" in content
+
+    def test_guids_in_viewpoint_selection(self):
+        cam = (0.0, 0.0, 5.0, 0.0, 0.0, -1.0, 0.0, 1.0, 0.0)
+        data = build_bcf(camera=cam, fov=60.0, guids=["abc123", "def456"])
+        vp_file = next(n for n in _zip_names(data) if n.endswith("viewpoint.bcfv"))
+        content = _zip_read(data, vp_file)
+        assert 'IfcGuid="abc123"' in content
+        assert 'IfcGuid="def456"' in content
+
+    def test_isolate_visibility_uses_exceptions(self):
+        cam = (0.0, 0.0, 5.0, 0.0, 0.0, -1.0, 0.0, 1.0, 0.0)
+        data = build_bcf(camera=cam, fov=60.0, guids=["abc123"], visibility="isolate")
+        vp_file = next(n for n in _zip_names(data) if n.endswith("viewpoint.bcfv"))
+        content = _zip_read(data, vp_file)
+        assert 'DefaultVisibility="false"' in content
+        assert "Exceptions" in content
+
+
+# ---------------------------------------------------------------------------
+# /bcf service endpoint tests
+# ---------------------------------------------------------------------------
+
+
+class TestBcfEndpoint:
+    def test_returns_zip_bytes(self):
+        r = client.post("/bcf", json={"url": CAMERA_URL})
+        assert r.status_code == 200
+        assert zipfile.is_zipfile(io.BytesIO(r.content))
+
+    def test_content_disposition_header(self):
+        r = client.post("/bcf", json={"url": CAMERA_URL})
+        assert "view.bcf" in r.headers["content-disposition"]
+
+    def test_contains_viewpoint_when_camera_in_url(self):
+        r = client.post("/bcf", json={"url": CAMERA_URL})
+        names = _zip_names(r.content)
+        assert any(n.endswith("viewpoint.bcfv") for n in names)
+
+    def test_no_viewpoint_when_no_camera(self):
+        r = client.post("/bcf", json={"url": MUTABLE_URL})
+        assert r.status_code == 200
+        names = _zip_names(r.content)
+        assert not any(n.endswith("viewpoint.bcfv") for n in names)
+
+    def test_title_and_comment_in_markup(self):
+        r = client.post("/bcf", json={
+            "url": CAMERA_URL, "title": "My issue", "comment": "Fix this"
+        })
+        markup = next(n for n in _zip_names(r.content) if n.endswith("markup.bcf"))
+        content = _zip_read(r.content, markup)
+        assert "My issue" in content
+        assert "Fix this" in content
+
+    def test_clip_plane_in_viewpoint(self):
+        r = client.post("/bcf", json={"url": CLIP_URL})
+        vp = next(n for n in _zip_names(r.content) if n.endswith("viewpoint.bcfv"))
+        assert "ClippingPlane" in _zip_read(r.content, vp)
+
+    def test_ssrf_local_transport_rejected(self):
+        r = client.post("/bcf", json={"url": "ifc:///some/path@HEAD?path=m.ifc"})
+        assert r.status_code == 403
+
+    def test_ssrf_private_ip_rejected(self):
+        r = client.post("/bcf", json={"url": "ifc://192.168.1.1/org/repo@HEAD?path=m.ifc"})
+        assert r.status_code == 403
+
+    def test_invalid_url_returns_400(self):
+        r = client.post("/bcf", json={"url": "https://example.com/model.ifc"})
+        assert r.status_code == 400
+
+    def test_selector_resolves_guids(self, model_with_geometry):
+        ifc_bytes = model_with_geometry.to_string().encode()
+
+        def mock_fetch(ifc_url, token=None):
+            return FAKE_HEXSHA, ifc_bytes
+
+        selector_url = MUTABLE_URL.replace("?path=model.ifc", "?path=model.ifc&selector=IfcWall")
+        with patch("ifcurl.service.fetch_ifc", mock_fetch):
+            r = client.post("/bcf", json={"url": selector_url})
+        assert r.status_code == 200
+        vp_files = [n for n in _zip_names(r.content) if n.endswith("viewpoint.bcfv")]
+        # No camera → no viewpoint even with selector
+        assert len(vp_files) == 0