diff --git a/surfsense_backend/app/services/okf/serializer.py b/surfsense_backend/app/services/okf/serializer.py
new file mode 100644
index 000000000..c866badf9
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+++ b/surfsense_backend/app/services/okf/serializer.py
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+"""Serialize SurfSense knowledge into Open Knowledge Format (OKF v0.1).
+
+Pure functions with no HTTP / MCP / framework dependencies: given a
+:class:`~app.db.Document` (and, for listings, its neighbours) they return
+OKF-conformant markdown. Every consumer (ZIP export, REST, MCP, agents) calls
+these rather than re-implementing frontmatter.
+
+Spec: https://github.com/GoogleCloudPlatform/knowledge-catalog/blob/main/okf/SPEC.md
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import Any
+
+import yaml
+
+from app.db import Document
+from app.services.okf.type_mapping import okf_resource, okf_type
+
+# Reserved OKF filenames; never used for concept documents.
+INDEX_FILENAME = "index.md"
+LOG_FILENAME = "log.md"
+
+_FRONTMATTER_DELIMITER = "---"
+
+
+def _tags_from_metadata(metadata: dict[str, Any] | None) -> list[str] | None:
+    if not isinstance(metadata, dict):
+        return None
+    raw = metadata.get("tags")
+    if isinstance(raw, list):
+        tags = [str(t).strip() for t in raw if str(t).strip()]
+        return tags or None
+    return None
+
+
+def _timestamp(document: Document) -> str | None:
+    when = document.updated_at or document.created_at
+    if when is None:
+        return None
+    # ISO 8601, matching Google's sample bundles (e.g. 2026-05-28T22:49:59+00:00).
+    return when.isoformat()
+
+
+def build_frontmatter(document: Document) -> dict[str, Any]:
+    """Build the ordered OKF frontmatter mapping for a document.
+
+    Only ``type`` is required; recommended keys are included only when we have a
+    value. Insertion order is preserved in the emitted YAML.
+    """
+    metadata = document.document_metadata if isinstance(document.document_metadata, dict) else {}
+
+    frontmatter: dict[str, Any] = {"type": okf_type(document.document_type)}
+
+    resource = okf_resource(document.document_type, metadata)
+    if resource:
+        frontmatter["resource"] = resource
+
+    title = (document.title or "").strip()
+    if title:
+        frontmatter["title"] = title
+
+    description = metadata.get("description")
+    if isinstance(description, str) and description.strip():
+        frontmatter["description"] = description.strip()
+
+    tags = _tags_from_metadata(metadata)
+    if tags:
+        frontmatter["tags"] = tags
+
+    timestamp = _timestamp(document)
+    if timestamp:
+        frontmatter["timestamp"] = timestamp
+
+    return frontmatter
+
+
+def render_frontmatter(frontmatter: dict[str, Any]) -> str:
+    """Render a frontmatter mapping as a YAML block delimited by ``---``."""
+    body = yaml.safe_dump(
+        frontmatter,
+        sort_keys=False,
+        allow_unicode=True,
+        default_flow_style=False,
+    )
+    return f"{_FRONTMATTER_DELIMITER}\n{body}{_FRONTMATTER_DELIMITER}\n"
+
+
+def document_to_concept(document: Document, *, body: str) -> str:
+    """Serialize a document as an OKF concept: frontmatter + ``body``.
+
+    ``body`` is caller-resolved markdown, keeping this a pure formatting step.
+    """
+    frontmatter = render_frontmatter(build_frontmatter(document))
+    body_text = (body or "").strip("\n")
+    return f"{frontmatter}\n{body_text}\n"
+
+
+@dataclass(frozen=True)
+class ConceptRef:
+    """One concept entry for an ``index.md`` listing."""
+
+    title: str
+    filename: str  # relative to the directory, e.g. "orders.md"
+    type: str  # OKF type, used as the grouping heading
+    description: str | None = None
+
+
+@dataclass(frozen=True)
+class SubdirRef:
+    """One subdirectory entry for an ``index.md`` listing."""
+
+    name: str  # directory name, e.g. "tables"
+    description: str | None = None
+
+
+@dataclass(frozen=True)
+class LogEntry:
+    """One line of an OKF ``log.md``: a concept and when it last changed."""
+
+    title: str
+    timestamp: str | None = None  # ISO-8601, or None when unknown
+
+
+def folder_to_log(entries: list[LogEntry]) -> str:
+    """Build a minimal OKF ``log.md`` body for one directory.
+
+    Lists each concept newest-first with the time it last changed (from a
+    document's ``updated_at``/``created_at``); undated entries sort last. Returns
+    an empty string when there is nothing to log.
+
+    ponytail: this is a last-touched summary, not a per-field change history. A
+    fuller changelog would read ``DocumentVersion`` rows; upgrade there if
+    consumers ever need diffs rather than "what changed and when".
+    """
+    if not entries:
+        return ""
+    ordered = sorted(
+        entries,
+        key=lambda e: (e.timestamp is not None, e.timestamp or "", e.title),
+        reverse=True,
+    )
+    lines = ["# Change Log", ""]
+    for entry in ordered:
+        when = f" - {entry.timestamp}" if entry.timestamp else ""
+        lines.append(f"* {entry.title}{when}")
+    return "\n".join(lines) + "\n"
+
+
+def _index_bullet(title: str, link: str, description: str | None) -> str:
+    bullet = f"* [{title}]({link})"
+    if description:
+        # Keep index descriptions to a single line.
+        bullet += f" - {' '.join(description.split())}"
+    return bullet
+
+
+def folder_to_index(
+    *,
+    concepts: list[ConceptRef] | None = None,
+    subdirectories: list[SubdirRef] | None = None,
+) -> str:
+    """Build an OKF ``index.md`` body (no frontmatter) for one directory.
+
+    Subdirectories are listed under a ``# Subdirectories`` heading and concepts
+    are grouped under their ``type`` heading, mirroring Google's sample bundles.
+    Returns an empty string when there is nothing to list.
+    """
+    concepts = concepts or []
+    subdirectories = subdirectories or []
+    sections: list[str] = []
+
+    if subdirectories:
+        lines = ["# Subdirectories", ""]
+        for sub in sorted(subdirectories, key=lambda s: s.name.lower()):
+            lines.append(
+                _index_bullet(sub.name, f"{sub.name}/{INDEX_FILENAME}", sub.description)
+            )
+        sections.append("\n".join(lines))
+
+    by_type: dict[str, list[ConceptRef]] = {}
+    for concept in concepts:
+        by_type.setdefault(concept.type, []).append(concept)
+    for type_heading in sorted(by_type):
+        lines = [f"# {type_heading}", ""]
+        for concept in sorted(by_type[type_heading], key=lambda c: c.title.lower()):
+            lines.append(
+                _index_bullet(concept.title, concept.filename, concept.description)
+            )
+        sections.append("\n".join(lines))
+
+    return ("\n\n".join(sections) + "\n") if sections else ""