diff --git a/src/validation.py b/src/validation.py
new file mode 100644
index 0000000..c7fb169
--- /dev/null
+++ b/src/validation.py
@@ -0,0 +1,469 @@
+from __future__ import annotations
+
+from dataclasses import dataclass
+from datetime import datetime
+from pathlib import Path
+import shutil
+from typing import Dict, List, Optional, Set
+
+from .config import Config
+from .errors import ValidationError, ValidationIssue
+from .git_utils import ensure_repo, git_first_timestamp, git_timestamp
+from .manifest import load_manifest
+from .markdown_utils import convert_markdown, extract_title
+from .models import EvaluationResult, InheritList, PostPlan, Source, TaxonomyPlan
+from .state import State
+from .wp_cli import WordPressCLI
+
+
+@dataclass
+class _Context:
+    categories: InheritList
+    tags: InheritList
+    author: InheritList
+    renderer: Optional[str]
+    hard_line_breaks: bool
+    block_html: bool
+    subdirectories: InheritList
+    manifest_chain: List[Path]
+
+
+def validate(
+    config: Config,
+    state: State,
+    sync_repos: bool,
+    force_new: bool = False,
+    skip_wp_checks: bool = False,
+) -> EvaluationResult:
+    issues: List[ValidationIssue] = []
+
+    sources = _load_sources(config, sync_repos, issues)
+
+    posts: List[PostPlan] = []
+    for source, content_root in sources:
+        _validate_directory(
+            source=source,
+            directory=content_root,
+            context=_Context(
+                categories=InheritList(),
+                tags=InheritList(),
+                author=InheritList(),
+                renderer=config.renderer,
+                hard_line_breaks=config.hard_line_breaks,
+                block_html=config.block_html,
+                subdirectories=InheritList(),
+                manifest_chain=[],
+            ),
+            state=state,
+            issues=issues,
+            posts=posts,
+            force_new=force_new,
+        )
+
+    missing_categories: List[List[str]] = []
+    missing_tags: List[str] = []
+    if not skip_wp_checks:
+        if shutil.which("wp") is None:
+            issues.append(ValidationIssue("wp CLI not found in PATH", context=str(config.wordpress_root)))
+        categories = []
+        tag_names: Set[str] = set()
+        try:
+            wp = WordPressCLI(config.wordpress_root)
+            categories = wp.list_categories()
+            tags = wp.list_tags()
+            tag_names = {tag.name for tag in tags}
+        except Exception as exc:
+            issues.append(ValidationIssue(str(exc), context=str(config.wordpress_root)))
+
+        missing_categories, missing_tags = _plan_taxonomy(posts, categories, tag_names)
+
+    if issues:
+        raise ValidationError(issues)
+
+    return EvaluationResult(
+        posts=posts,
+        taxonomy_to_create=TaxonomyPlan(missing_categories=missing_categories, missing_tags=missing_tags),
+    )
+
+
+def _load_sources(
+    config: Config,
+    sync_repos: bool,
+    issues: List[ValidationIssue],
+) -> List[tuple[Source, Path]]:
+    sources: List[tuple[Source, Path]] = []
+
+    for repo in config.git_repositories:
+        repo_path = config.repo_storage_dir / repo.name
+        try:
+            ensure_repo(repo_path, repo.url, repo.branch, sync=sync_repos)
+        except Exception as exc:
+            issues.append(ValidationIssue(str(exc), context=str(repo_path)))
+            continue
+        content_root = repo_path / repo.root_subdir if repo.root_subdir else repo_path
+        if not content_root.exists():
+            issues.append(ValidationIssue("Repository content root missing", context=str(content_root)))
+            continue
+        sources.append(
+            (
+                Source(name=repo.name, root_path=content_root, identity_root=repo_path, kind="git"),
+                content_root,
+            )
+        )
+
+    for directory in config.directories:
+        root_path = directory.path
+        if not root_path.exists():
+            issues.append(ValidationIssue("Directory not found", context=str(root_path)))
+            continue
+        content_root = root_path / directory.root_subdir if directory.root_subdir else root_path
+        if not content_root.exists():
+            issues.append(ValidationIssue("Directory content root missing", context=str(content_root)))
+            continue
+        sources.append(
+            (
+                Source(name=directory.name, root_path=content_root, identity_root=root_path, kind="dir"),
+                content_root,
+            )
+        )
+
+    return sources
+
+
+def _validate_directory(
+    source: Source,
+    directory: Path,
+    context: _Context,
+    state: State,
+    issues: List[ValidationIssue],
+    posts: List[PostPlan],
+    force_new: bool,
+) -> None:
+    manifest_path = directory / ".wp-materialize.json"
+    manifest = load_manifest(manifest_path, issues)
+    if manifest is None:
+        return
+
+    effective_categories = _merge_inherit(context.categories, manifest.categories)
+    effective_tags = _merge_inherit(context.tags, manifest.tags)
+    effective_author = _merge_inherit(context.author, manifest.author)
+    effective_renderer = manifest.renderer if manifest.renderer is not None else context.renderer
+    effective_hard_line_breaks = (
+        manifest.hard_line_breaks
+        if manifest.hard_line_breaks is not None
+        else context.hard_line_breaks
+    )
+    effective_block_html = (
+        manifest.block_html
+        if manifest.block_html is not None
+        else context.block_html
+    )
+    effective_subdirs = _merge_inherit(context.subdirectories, manifest.subdirectories)
+
+    manifest_chain = context.manifest_chain + [manifest.path]
+
+    for file_name, spec in manifest.files.items():
+        file_path = directory / file_name
+        if not file_path.exists():
+            issues.append(ValidationIssue("File not found", context=str(file_path)))
+            continue
+
+        try:
+            content = file_path.read_text(encoding="utf-8")
+        except Exception as exc:
+            issues.append(ValidationIssue(f"Failed to read file: {exc}", context=str(file_path)))
+            continue
+
+        title = spec.title
+        markdown_body = content
+        if spec.use_heading_level is not None:
+            extracted = extract_title(
+                content,
+                level=spec.use_heading_level,
+                strict=spec.use_heading_strict,
+                context=str(file_path),
+                issues=issues,
+            )
+            if extracted is None:
+                continue
+            title, markdown_body = extracted
+        elif not title:
+            issues.append(ValidationIssue("Missing title (title or use_heading_as_title required)", context=str(file_path)))
+            continue
+
+        resolved_categories = _resolve_overrides(effective_categories, spec.categories)
+        resolved_tags = _resolve_overrides(effective_tags, spec.tags)
+
+        resolved_categories = _normalize_list(resolved_categories, "category", str(file_path), issues)
+        resolved_tags = _normalize_list(resolved_tags, "tag", str(file_path), issues)
+        resolved_author = _resolve_author(effective_author.content, str(file_path), issues)
+
+        resolved_renderer = spec.renderer if spec.renderer is not None else effective_renderer
+        resolved_hard_line_breaks = (
+            spec.hard_line_breaks
+            if spec.hard_line_breaks is not None
+            else effective_hard_line_breaks
+        )
+        resolved_block_html = (
+            spec.block_html
+            if spec.block_html is not None
+            else effective_block_html
+        )
+        html = convert_markdown(
+            markdown_body,
+            context=str(file_path),
+            issues=issues,
+            renderer=resolved_renderer or "default",
+            hard_line_breaks=resolved_hard_line_breaks,
+            block_html=resolved_block_html,
+        )
+        if html is None:
+            continue
+
+        relative_path = _relative_path(file_path, source.identity_root, issues)
+        if relative_path is None:
+            continue
+
+        timestamps = []
+        ts = _timestamp_for_path(source, source.identity_root, relative_path, issues)
+        if ts is None:
+            continue
+        timestamps.append(ts)
+
+        for manifest_file in manifest_chain:
+            manifest_rel = _relative_path(manifest_file, source.identity_root, issues)
+            if manifest_rel is None:
+                continue
+            ts_manifest = _timestamp_for_path(source, source.identity_root, manifest_rel, issues)
+            if ts_manifest is None:
+                continue
+            timestamps.append(ts_manifest)
+
+        source_timestamp = max(timestamps)
+        identity = f"{source.name}:{relative_path}"
+        cached_entry = state.posts.get(identity)
+        cached_ts = cached_entry.source_timestamp if cached_entry else None
+        should_update = True if force_new else (cached_ts is None or source_timestamp > cached_ts)
+        created_on, last_modified = _resolve_post_datetimes(
+            source=source,
+            identity_root=source.identity_root,
+            relative_path=relative_path,
+            spec=spec,
+            issues=issues,
+        )
+
+        posts.append(
+            PostPlan(
+                source=source,
+                identity=identity,
+                relative_path=relative_path,
+                absolute_path=file_path,
+                title=title,
+                html=html,
+                categories=resolved_categories,
+                tags=resolved_tags,
+                author=resolved_author,
+                source_timestamp=source_timestamp,
+                cached_timestamp=cached_ts,
+                should_update=should_update,
+                created_on=created_on,
+                last_modified=last_modified,
+            )
+        )
+
+    for subdir in effective_subdirs.content:
+        subdir_path = directory / subdir
+        if not subdir_path.exists():
+            issues.append(ValidationIssue("Missing subdirectory", context=str(subdir_path)))
+            continue
+        _validate_directory(
+            source=source,
+            directory=subdir_path,
+            context=_Context(
+                categories=effective_categories,
+                tags=effective_tags,
+                author=effective_author,
+                renderer=effective_renderer,
+                hard_line_breaks=effective_hard_line_breaks,
+                block_html=effective_block_html,
+                subdirectories=effective_subdirs,
+                manifest_chain=manifest_chain,
+            ),
+            state=state,
+            issues=issues,
+            posts=posts,
+            force_new=force_new,
+        )
+
+
+def _merge_inherit(parent: InheritList, child: InheritList) -> InheritList:
+    if child.inherit:
+        content = parent.content + child.content
+    else:
+        content = child.content
+    return InheritList(content=content, inherit=True)
+
+
+def _resolve_overrides(parent: InheritList, override: Optional[InheritList]) -> List[str]:
+    if override is None:
+        return list(parent.content)
+    if override.inherit:
+        return parent.content + override.content
+    return list(override.content)
+
+
+def _normalize_list(values: List[str], label: str, context: str, issues: List[ValidationIssue]) -> List[str]:
+    normalized: List[str] = []
+    seen: Set[str] = set()
+    for value in values:
+        cleaned = value.strip()
+        if not cleaned:
+            issues.append(ValidationIssue(f"Empty {label} entry", context=context))
+            continue
+        if label == "category":
+            parts = [part.strip() for part in cleaned.split("/")]
+            if any(not part for part in parts):
+                issues.append(ValidationIssue(f"Invalid category path: {cleaned}", context=context))
+                continue
+            cleaned = "/".join(parts)
+        if cleaned not in seen:
+            seen.add(cleaned)
+            normalized.append(cleaned)
+    return normalized
+
+
+def _resolve_author(values: List[str], context: str, issues: List[ValidationIssue]) -> Optional[str]:
+    normalized = _normalize_list(values, "author", context, issues)
+    if not normalized:
+        return None
+    if len(normalized) > 1:
+        issues.append(ValidationIssue("Multiple authors specified; only one is allowed", context=context))
+        return None
+    return normalized[0]
+
+
+def _relative_path(path: Path, root: Path, issues: List[ValidationIssue]) -> Optional[str]:
+    try:
+        return str(path.relative_to(root))
+    except ValueError:
+        issues.append(ValidationIssue("Path is outside identity root", context=str(path)))
+        return None
+
+
+def _timestamp_for_path(
+    source: Source,
+    identity_root: Path,
+    relative_path: str,
+    issues: List[ValidationIssue],
+) -> Optional[int]:
+    if source.kind == "git":
+        try:
+            return git_timestamp(identity_root, relative_path)
+        except Exception as exc:
+            issues.append(ValidationIssue(str(exc), context=relative_path))
+            return None
+    try:
+        return int((identity_root / relative_path).stat().st_mtime)
+    except Exception as exc:
+        issues.append(ValidationIssue(f"Timestamp lookup failed: {exc}", context=relative_path))
+        return None
+
+
+def _resolve_post_datetimes(
+    source: Source,
+    identity_root: Path,
+    relative_path: str,
+    spec,
+    issues: List[ValidationIssue],
+) -> tuple[Optional[str], Optional[str]]:
+    created_dt = spec.created_on
+    modified_dt = spec.last_modified
+
+    if created_dt is None or modified_dt is None:
+        inferred = _infer_file_timestamps(source, identity_root, relative_path, issues)
+        if inferred is None:
+            return None, None
+        inferred_created, inferred_modified = inferred
+        if created_dt is None:
+            created_dt = datetime.fromtimestamp(inferred_created)
+        if modified_dt is None:
+            modified_dt = datetime.fromtimestamp(inferred_modified)
+
+    if created_dt and modified_dt and modified_dt < created_dt:
+        issues.append(
+            ValidationIssue("last_modified cannot be earlier than created_on", context=relative_path)
+        )
+        return None, None
+
+    created_on = _format_wp_datetime(created_dt) if created_dt else None
+    last_modified = _format_wp_datetime(modified_dt) if modified_dt else None
+    return created_on, last_modified
+
+
+def _infer_file_timestamps(
+    source: Source,
+    identity_root: Path,
+    relative_path: str,
+    issues: List[ValidationIssue],
+) -> Optional[tuple[int, int]]:
+    if source.kind == "git":
+        try:
+            created_ts = git_first_timestamp(identity_root, relative_path)
+            modified_ts = git_timestamp(identity_root, relative_path)
+            return created_ts, modified_ts
+        except Exception:
+            pass
+    try:
+        stat = (identity_root / relative_path).stat()
+        return int(stat.st_ctime), int(stat.st_mtime)
+    except Exception as exc:
+        issues.append(ValidationIssue(f"Timestamp lookup failed: {exc}", context=relative_path))
+        return None
+
+
+def _format_wp_datetime(value: datetime) -> str:
+    return value.strftime("%Y-%m-%d %H:%M:%S")
+
+
+def _plan_taxonomy(
+    posts: List[PostPlan],
+    categories,  # list of CategoryTerm
+    existing_tags: Set[str],
+) -> tuple[List[List[str]], List[str]]:
+    category_map: Dict[tuple[int, str], int] = {}
+    for category in categories:
+        category_map[(category.parent, category.name)] = category.term_id
+
+    missing_paths: List[List[str]] = []
+    seen_missing: Set[tuple[str, ...]] = set()
+    missing_tags: List[str] = []
+    seen_tags: Set[str] = set()
+
+    for post in posts:
+        if not post.should_update:
+            continue
+        for tag in post.tags:
+            if tag not in existing_tags:
+                if tag not in seen_tags:
+                    seen_tags.add(tag)
+                    missing_tags.append(tag)
+        for path in post.categories:
+            segments = [segment for segment in path.split("/") if segment]
+            if not segments:
+                continue
+            parent = 0
+            missing = False
+            for segment in segments:
+                key = (parent, segment)
+                if key in category_map:
+                    parent = category_map[key]
+                    continue
+                missing = True
+                break
+            if missing:
+                key = tuple(segments)
+                if key not in seen_missing:
+                    seen_missing.add(key)
+                    missing_paths.append(list(segments))
+
+    return missing_paths, missing_tags