diff --git a/openspec/changes/support-index-md-as-section-file/.openspec.yaml b/openspec/changes/support-index-md-as-section-file/.openspec.yaml
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+schema: spec-driven
+created: 2026-05-20
diff --git a/openspec/changes/support-index-md-as-section-file/design.md b/openspec/changes/support-index-md-as-section-file/design.md
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+## Context
+
+Docforge hardcodes `const sectionFile = "_index.md"` to identify section index files — nodes that define section metadata without fetching upstream content. Hugo uses `_index.md` as its section index convention, but VitePress and other modern SSGs use `index.md`. When repos migrate from Hugo to VitePress and rename their section files, docforge silently breaks: it treats `index.md` as a normal content file, attempts source resolution, and fails.
+
+The docsy plugin already checks for both filenames (`pkg/manifestplugins/docsy/plugin.go:22`), proving this gap was recognized but never propagated to the core resolution logic.
+
+## Goals / Non-Goals
+
+**Goals:**
+
+- Recognize `index.md` (without underscore) as a valid section index file alongside `_index.md`
+- Maintain identical behavior for existing `_index.md` manifests
+- Keep the change minimal and contained — no new configuration surfaces
+
+**Non-Goals:**
+
+- Making the set of section file names user-configurable (the set is well-known and stable)
+- Changing `--hugo-section-files` CLI behavior or FSWriter output normalization
+- Refactoring the entire index file handling system
+- Supporting arbitrary filenames as section indices
+
+## Decisions
+
+### Decision 1: Package-level helper function over configuration
+
+**Choice:** Add `isSectionFile(name string) bool` helper that checks both names.
+
+**Alternatives considered:**
+
+| Option | Description | Why rejected |
+|--------|-------------|--------------|
+| Wire `IndexFileNames` config into resolution | Thread existing config through manifest resolver | Unnecessary plumbing for a two-element set that won't grow |
+| Unify under `--hugo-section-files` flag | Single config point for all section file handling | High-risk refactor, mixes output normalization with input detection |
+
+**Rationale:** The set `{"_index.md", "index.md"}` is a well-known convention pair unlikely to expand. A simple predicate function is easier to audit, test, and maintain than configuration threading.
+
+### Decision 2: Inline checks for cross-package call sites
+
+**Choice:** Use inline `name == "_index.md" || name == "index.md"` in packages outside `pkg/manifest/` (fswriter, frontmatter, alias plugin) rather than exporting the helper.
+
+**Rationale:** Exporting a helper from `pkg/manifest` would create import dependencies from packages that currently don't depend on it. The two-name check is trivial enough that duplication is preferable to coupling.
+
+### Decision 3: Preserve TrimSuffix ordering in HugoPrettyPath
+
+**Choice:** Trim `_index` before `index` in `HugoPrettyPath`.
+
+**Rationale:** If `index` were trimmed first from `_index.md`, it would leave a trailing `_` in the path. The longer prefix must be stripped first.
+
+## Risks / Trade-offs
+
+| Risk | Likelihood | Impact | Mitigation |
+|------|-----------|--------|------------|
+| `index.md` with a `source:` field treated as section file | None | N/A | Check requires BOTH `isSectionFile(name)` AND `source == ""` |
+| Hugo output produces `index.md` instead of `_index.md` | None | N/A | FSWriter line 27 normalization is unchanged — output always uses `_index.md` in Hugo mode |
+| Alias plugin generates wrong suffix for `index.md` | Low | Low | Covered by test — same empty-suffix behavior as `_index.md` |
+| Future SSG introduces third convention name | Very low | Low | Add one more case to `isSectionFile` — trivial change |
diff --git a/openspec/changes/support-index-md-as-section-file/proposal.md b/openspec/changes/support-index-md-as-section-file/proposal.md
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+## Why
+
+The constant `sectionFile = "_index.md"` is hardcoded throughout docforge. When a node has `file: _index.md` and no `source:`, docforge skips source resolution and auto-generates a frontmatter-only section index. This is the mechanism that allows downstream manifests to define section metadata without fetching content from upstream sources.
+
+The problem: `index.md` (without the underscore) does NOT trigger this special case. VitePress and other modern static site generators use `index.md` as their section index convention instead of Hugo's `_index.md`. When downstream repos rename `_index.md` to `index.md`, docforge treats the file as a normal content file, resolves its source, and fetches real content — breaking the overlay mechanism silently.
+
+The docsy plugin (`pkg/manifestplugins/docsy/plugin.go:22`) already checks for both filenames, proving this was a known gap that was never propagated to the other checks.
+
+## What Changes
+
+- Replace `const sectionFile = "_index.md"` with a package-level helper function `isSectionFile(name string) bool` that matches both `_index.md` and `index.md`
+- Update all hardcoded `_index.md` checks in production code to use the new helper or check both names
+- Update `HugoPrettyPath` in `node.go` to also strip the `index` prefix (resolving an existing TODO)
+
+## Design
+
+### Approach chosen: Minimal helper function
+
+Three approaches were considered:
+
+| Approach | Description | Trade-off |
+|----------|-------------|-----------|
+| **A (chosen)** | Add `isSectionFile` helper, check both names | Minimal change, low risk, solves the problem |
+| B | Wire `IndexFileNames` config into manifest resolution | More plumbing, configurable but unnecessary complexity |
+| C | Unify all index file handling under `--hugo-section-files` | Full refactor, high risk for minimal gain |
+
+**Rationale:** The set of valid section file names (`_index.md`, `index.md`) is well-known and unlikely to grow. A simple helper function is easier to understand, test, and maintain than threading configuration through the manifest resolution layer.
+
+### Implementation
+
+```go
+// pkg/manifest/manifest.go
+
+// isSectionFile returns true if the given filename is a section index file.
+// Both Hugo (_index.md) and VitePress (index.md) conventions are supported.
+func isSectionFile(name string) bool {
+    return name == "_index.md" || name == "index.md"
+}
+```
+
+**Call sites to update:**
+
+1. `pkg/manifest/manifest.go:256` — `resolveManifestLinks` early return
+   ```go
+   // Before:
+   if node.File == sectionFile && node.Source == "" {
+   // After:
+   if isSectionFile(node.File) && node.Source == "" {
+   ```
+
+2. `pkg/writers/fswriter.go:30` — frontmatter generation guard
+   ```go
+   // Before:
+   if f.Hugo && name == "_index.md" && node != nil && node.Frontmatter != nil && docBlob == nil {
+   // After:
+   if f.Hugo && (name == "_index.md" || name == "index.md") && node != nil && node.Frontmatter != nil && docBlob == nil {
+   ```
+   Note: Line 27 (`name = "_index.md"`) stays unchanged — that's the output normalization for Hugo mode via `IndexFileNames`, which is a separate concern.
+
+3. `pkg/nodeplugins/markdown/document/frontmatter/frontmatter.go:108` — fallback index detection
+   ```go
+   // Before:
+   return name == "_index.md"
+   // After:
+   return name == "_index.md" || name == "index.md"
+   ```
+
+4. `pkg/manifestplugins/alias/plugin.go:40` — alias suffix for section files
+   ```go
+   // Before:
+   if child.Name() == "_index.md" {
+   // After:
+   if child.Name() == "_index.md" || child.Name() == "index.md" {
+   ```
+
+5. `pkg/manifest/node.go:67` — `HugoPrettyPath` prefix stripping
+   ```go
+   // Before:
+   name = strings.TrimSuffix(name, "_index")
+   // After:
+   name = strings.TrimSuffix(name, "_index")
+   name = strings.TrimSuffix(name, "index")
+   ```
+   Note: order matters — `_index` must be trimmed first, otherwise `index` would leave a trailing `_`.
+
+6. `pkg/manifestplugins/docsy/plugin.go:22` — already handles both, no change needed.
+
+### What does NOT change
+
+- The `const sectionFile` is removed, but its semantics are preserved via `isSectionFile`
+- `--hugo-section-files` CLI flag behavior is unchanged (still controls output rename in FSWriter)
+- The FSWriter's line 27 (`name = "_index.md"`) stays — this is Hugo output normalization, separate from section file detection
+- All existing `_index.md` manifests continue working identically
+
+## Capabilities
+
+### New Capabilities
+
+- Manifests can use `file: index.md` (without source) as a section index, equivalent to `file: _index.md`
+
+### Modified Capabilities
+
+- `section-file-detection`: Both `_index.md` and `index.md` without a `source:` field are treated as auto-generated section indices
+- `alias-generation`: Alias suffix calculation treats `index.md` the same as `_index.md` (empty suffix)
+- `frontmatter-generation`: The FSWriter generates frontmatter-only content for both `_index.md` and `index.md` when `docBlob` is nil
+- `index-file-detection`: The frontmatter processor's `nodeIsIndexFile` fallback recognizes both names
+- `hugo-pretty-path`: URL path generation correctly strips `index` prefix (not just `_index`)
+
+## Impact
+
+- **CLI**: No flag changes
+- **Manifest schema**: `file: index.md` without `source:` now behaves identically to `file: _index.md` without `source:` — previously it would attempt (and likely fail) source resolution
+- **Code**: ~5 files modified, no packages added or removed
+- **Downstream consumers**: Repos that renamed `_index.md` to `index.md` (e.g., VitePress migrations) will work correctly without needing to revert the rename
+- **Backwards compatibility**: Existing `_index.md` usage is entirely unaffected
+- **Risk**: Low — the change is additive and each call site is a simple boolean expansion
+
+## Testing
+
+- Update existing `manifest_test.go` `_index.md` test entry to also cover `index.md` variant
+- Add a test case in `resolveManifestLinks` for `file: index.md` with no source (should return nil)
+- Update alias plugin tests to verify `index.md` gets empty suffix
+- Update frontmatter tests to verify `index.md` is detected as index file
diff --git a/openspec/changes/support-index-md-as-section-file/specs/alias-generation/spec.md b/openspec/changes/support-index-md-as-section-file/specs/alias-generation/spec.md
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+++ b/openspec/changes/support-index-md-as-section-file/specs/alias-generation/spec.md
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+## MODIFIED Requirements
+
+### Requirement: Alias suffix for section index files
+The alias plugin SHALL assign an empty suffix to nodes whose name equals `_index.md` OR `index.md`. All other nodes SHALL receive a suffix derived from their name.
+
+#### Scenario: Node named _index.md gets empty alias suffix
+- **WHEN** a child node has name `_index.md`
+- **THEN** the alias plugin SHALL assign an empty string as the alias suffix
+
+#### Scenario: Node named index.md gets empty alias suffix
+- **WHEN** a child node has name `index.md`
+- **THEN** the alias plugin SHALL assign an empty string as the alias suffix
+
+#### Scenario: Node with regular name gets normal alias suffix
+- **WHEN** a child node has name `getting-started.md`
+- **THEN** the alias plugin SHALL assign a suffix derived from the node name
diff --git a/openspec/changes/support-index-md-as-section-file/specs/frontmatter-generation/spec.md b/openspec/changes/support-index-md-as-section-file/specs/frontmatter-generation/spec.md
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+++ b/openspec/changes/support-index-md-as-section-file/specs/frontmatter-generation/spec.md
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+## MODIFIED Requirements
+
+### Requirement: Frontmatter-only generation for section index files
+The FSWriter in Hugo mode SHALL generate frontmatter-only content for nodes named `_index.md` OR `index.md` when the document blob is nil and frontmatter is present.
+
+#### Scenario: Hugo mode generates frontmatter for _index.md with nil blob
+- **WHEN** Hugo mode is enabled AND name is `_index.md` AND node has frontmatter AND docBlob is nil
+- **THEN** the FSWriter SHALL write a file containing only the frontmatter
+
+#### Scenario: Hugo mode generates frontmatter for index.md with nil blob
+- **WHEN** Hugo mode is enabled AND name is `index.md` AND node has frontmatter AND docBlob is nil
+- **THEN** the FSWriter SHALL write a file containing only the frontmatter
+
+#### Scenario: Non-index file with nil blob does not get frontmatter generation
+- **WHEN** Hugo mode is enabled AND name is `readme.md` AND docBlob is nil
+- **THEN** the FSWriter SHALL NOT generate frontmatter-only content
diff --git a/openspec/changes/support-index-md-as-section-file/specs/hugo-pretty-path/spec.md b/openspec/changes/support-index-md-as-section-file/specs/hugo-pretty-path/spec.md
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+++ b/openspec/changes/support-index-md-as-section-file/specs/hugo-pretty-path/spec.md
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+## MODIFIED Requirements
+
+### Requirement: HugoPrettyPath strips both index prefixes
+The `HugoPrettyPath` function SHALL strip both `_index` and `index` prefixes from filenames when generating URL paths. The `_index` prefix MUST be stripped before `index` to avoid leaving a trailing underscore.
+
+#### Scenario: _index.md is stripped to produce clean path
+- **WHEN** `HugoPrettyPath` processes a node with file `_index.md`
+- **THEN** the resulting path SHALL have the `_index` portion removed
+
+#### Scenario: index.md is stripped to produce clean path
+- **WHEN** `HugoPrettyPath` processes a node with file `index.md`
+- **THEN** the resulting path SHALL have the `index` portion removed
+
+#### Scenario: Regular filename is not affected by index stripping
+- **WHEN** `HugoPrettyPath` processes a node with file `getting-started.md`
+- **THEN** the resulting path SHALL retain the filename (minus `.md` extension)
diff --git a/openspec/changes/support-index-md-as-section-file/specs/index-file-detection/spec.md b/openspec/changes/support-index-md-as-section-file/specs/index-file-detection/spec.md
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+++ b/openspec/changes/support-index-md-as-section-file/specs/index-file-detection/spec.md
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+## MODIFIED Requirements
+
+### Requirement: Fallback index file detection recognizes both conventions
+The frontmatter processor's `nodeIsIndexFile` fallback SHALL return true for files named `_index.md` OR `index.md`.
+
+#### Scenario: _index.md is detected as index file
+- **WHEN** the frontmatter processor checks if `_index.md` is an index file
+- **THEN** it SHALL return true
+
+#### Scenario: index.md is detected as index file
+- **WHEN** the frontmatter processor checks if `index.md` is an index file
+- **THEN** it SHALL return true
+
+#### Scenario: Regular file is not detected as index file
+- **WHEN** the frontmatter processor checks if `overview.md` is an index file
+- **THEN** it SHALL return false
diff --git a/openspec/changes/support-index-md-as-section-file/specs/section-file-detection/spec.md b/openspec/changes/support-index-md-as-section-file/specs/section-file-detection/spec.md
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+++ b/openspec/changes/support-index-md-as-section-file/specs/section-file-detection/spec.md
@@ -0,0 +1,20 @@
+## MODIFIED Requirements
+
+### Requirement: Section file detection skips source resolution
+The manifest resolver SHALL treat a node as a section index file when its `file` field equals `_index.md` OR `index.md` AND its `source` field is empty. Section index files MUST NOT undergo source resolution — they are auto-generated with frontmatter only.
+
+#### Scenario: Node with file _index.md and no source is a section file
+- **WHEN** a manifest node has `file: _index.md` and `source: ""`
+- **THEN** the resolver SHALL skip source resolution and return nil
+
+#### Scenario: Node with file index.md and no source is a section file
+- **WHEN** a manifest node has `file: index.md` and `source: ""`
+- **THEN** the resolver SHALL skip source resolution and return nil
+
+#### Scenario: Node with file index.md and a source is NOT a section file
+- **WHEN** a manifest node has `file: index.md` and `source: "https://example.com/content.md"`
+- **THEN** the resolver SHALL proceed with normal source resolution
+
+#### Scenario: Node with a non-index filename is NOT a section file
+- **WHEN** a manifest node has `file: readme.md` and `source: ""`
+- **THEN** the resolver SHALL proceed with normal resolution (not treated as section file)
diff --git a/openspec/changes/support-index-md-as-section-file/tasks.md b/openspec/changes/support-index-md-as-section-file/tasks.md
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+++ b/openspec/changes/support-index-md-as-section-file/tasks.md
@@ -0,0 +1,21 @@
+## 1. Core Helper
+
+- [ ] 1.1 Add `isSectionFile(name string) bool` helper in `pkg/manifest/manifest.go` and remove `const sectionFile`
+- [ ] 1.2 Update `resolveManifestLinks` to use `isSectionFile(node.File)` instead of `node.File == sectionFile`
+
+## 2. Cross-Package Call Sites
+
+- [ ] 2.1 Update `pkg/writers/fswriter.go` frontmatter guard to check both `_index.md` and `index.md`
+- [ ] 2.2 Update `pkg/nodeplugins/markdown/document/frontmatter/frontmatter.go` `nodeIsIndexFile` to return true for `index.md`
+- [ ] 2.3 Update `pkg/manifestplugins/alias/plugin.go` to treat `index.md` as section file for alias suffix
+
+## 3. HugoPrettyPath
+
+- [ ] 3.1 Update `pkg/manifest/node.go` `HugoPrettyPath` to strip `index` prefix after `_index` (order matters)
+
+## 4. Tests
+
+- [ ] 4.1 Add test case in `manifest_test.go` for `file: index.md` with no source (should skip resolution)
+- [ ] 4.2 Update alias plugin tests to verify `index.md` gets empty suffix
+- [ ] 4.3 Update frontmatter tests to verify `index.md` is detected as index file
+- [ ] 4.4 Add/update `HugoPrettyPath` test for `index.md` stripping