diff --git a/ingestion/internal/api/frontmatter.go b/ingestion/internal/api/frontmatter.go
new file mode 100644
index 0000000..6569f15
--- /dev/null
+++ b/ingestion/internal/api/frontmatter.go
@@ -0,0 +1,97 @@
+package api
+
+import "strings"
+
+// frontmatter is an ordered, line-preserving view of a note's YAML
+// frontmatter block. It deliberately avoids a full YAML round-trip: the
+// brain writes flat `key: value` frontmatter by hand, and a yaml.v3
+// re-marshal would reorder keys and strip comments. Preserving the
+// original lines verbatim keeps brain_update a surgical edit — only the
+// keys it manages (updated_at, supersedes, supersede_reason) change.
+type frontmatter struct {
+	lines []fmLine
+}
+
+// fmLine is one frontmatter line. For `key: value` lines, key and value
+// are populated; for blank lines, comments, or anything that isn't a
+// simple scalar pair, key is empty and raw holds the line verbatim.
+type fmLine struct {
+	key   string
+	value string
+	raw   string
+}
+
+// parseFrontmatter splits src into its frontmatter block and body. A
+// frontmatter block is recognised only when the file opens with a `---`
+// fence and a closing `---` fence follows. Otherwise the whole input is
+// the body and the returned frontmatter is empty.
+func parseFrontmatter(src string) (frontmatter, string) {
+	var fm frontmatter
+	if !strings.HasPrefix(src, "---\n") {
+		return fm, src
+	}
+	rest := src[len("---\n"):]
+	end := strings.Index(rest, "\n---\n")
+	if end < 0 {
+		// Opening fence with no closing fence — treat as bodyless content.
+		return fm, src
+	}
+	block := rest[:end]
+	body := rest[end+len("\n---\n"):]
+
+	for _, line := range strings.Split(block, "\n") {
+		key, val, ok := strings.Cut(line, ":")
+		key = strings.TrimSpace(key)
+		if !ok || key == "" || strings.HasPrefix(strings.TrimSpace(line), "#") {
+			fm.lines = append(fm.lines, fmLine{raw: line})
+			continue
+		}
+		fm.lines = append(fm.lines, fmLine{key: key, value: strings.TrimSpace(val)})
+	}
+	return fm, body
+}
+
+// get returns the value for key, or "" if absent.
+func (f *frontmatter) get(key string) string {
+	for _, l := range f.lines {
+		if l.key == key {
+			return l.value
+		}
+	}
+	return ""
+}
+
+// set overrides the value for an existing key in place, or appends a new
+// `key: value` line when the key is absent.
+func (f *frontmatter) set(key, value string) {
+	for i := range f.lines {
+		if f.lines[i].key == key {
+			f.lines[i].value = value
+			return
+		}
+	}
+	f.lines = append(f.lines, fmLine{key: key, value: value})
+}
+
+// render serialises the frontmatter back into a `---`-fenced block. An
+// empty frontmatter renders to the empty string so bodies without a
+// header stay header-less.
+func (f *frontmatter) render() string {
+	if len(f.lines) == 0 {
+		return ""
+	}
+	var b strings.Builder
+	b.WriteString("---\n")
+	for _, l := range f.lines {
+		if l.key == "" {
+			b.WriteString(l.raw)
+		} else {
+			b.WriteString(l.key)
+			b.WriteString(": ")
+			b.WriteString(l.value)
+		}
+		b.WriteByte('\n')
+	}
+	b.WriteString("---\n")
+	return b.String()
+}
diff --git a/ingestion/internal/api/frontmatter_test.go b/ingestion/internal/api/frontmatter_test.go
new file mode 100644
index 0000000..6bcb2e3
--- /dev/null
+++ b/ingestion/internal/api/frontmatter_test.go
@@ -0,0 +1,61 @@
+package api
+
+import (
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestParseFrontmatterSplitsHeaderAndBody(t *testing.T) {
+	src := "---\nwing: jepa-fx\nhall: facts\ncreated_at: 2026-01-01T00:00:00Z\n---\n# Title\n\nbody text\n"
+	fm, body := parseFrontmatter(src)
+
+	assert.Equal(t, "jepa-fx", fm.get("wing"))
+	assert.Equal(t, "facts", fm.get("hall"))
+	assert.Equal(t, "2026-01-01T00:00:00Z", fm.get("created_at"))
+	assert.Equal(t, "# Title\n\nbody text\n", body)
+}
+
+func TestParseFrontmatterNoHeader(t *testing.T) {
+	src := "# Just a body\n\nno frontmatter here\n"
+	fm, body := parseFrontmatter(src)
+
+	assert.Empty(t, fm.lines)
+	assert.Equal(t, src, body)
+}
+
+func TestFrontmatterSetOverridesExistingKey(t *testing.T) {
+	fm, _ := parseFrontmatter("---\nwing: a\nupdated_at: old\n---\nbody\n")
+	fm.set("updated_at", "new")
+
+	assert.Equal(t, "new", fm.get("updated_at"))
+	// No duplicate key.
+	assert.Equal(t, 1, strings.Count(fm.render(), "updated_at:"))
+}
+
+func TestFrontmatterSetAppendsNewKey(t *testing.T) {
+	fm, _ := parseFrontmatter("---\nwing: a\n---\nbody\n")
+	fm.set("supersedes", "abc123")
+
+	out := fm.render()
+	assert.Contains(t, out, "wing: a")
+	assert.Contains(t, out, "supersedes: abc123")
+}
+
+func TestFrontmatterRenderPreservesCustomFields(t *testing.T) {
+	src := "---\nwing: a\nhall: facts\ncustom_field: keep-me\ntags: [x, y]\n---\nbody\n"
+	fm, _ := parseFrontmatter(src)
+	fm.set("updated_at", "2026-06-22T00:00:00Z")
+
+	out := fm.render()
+	assert.Contains(t, out, "custom_field: keep-me")
+	assert.Contains(t, out, "tags: [x, y]")
+	assert.Contains(t, out, "updated_at: 2026-06-22T00:00:00Z")
+}
+
+func TestFrontmatterRenderRoundTrips(t *testing.T) {
+	src := "---\nwing: a\nhall: facts\n---\n"
+	fm, _ := parseFrontmatter(src)
+	assert.Equal(t, src, fm.render())
+}
diff --git a/ingestion/internal/api/update.go b/ingestion/internal/api/update.go
new file mode 100644
index 0000000..9fd88f8
--- /dev/null
+++ b/ingestion/internal/api/update.go
@@ -0,0 +1,131 @@
+package api
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"time"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/brain"
+)
+
+// ContentHash returns the lowercase hex sha256 of b. It is the note's
+// content_hash handle: brain_write / brain_update return it, brain_get
+// recomputes it from the file on disk, and brain_update stamps the prior
+// note's hash into the new note's `supersedes` frontmatter.
+func ContentHash(b []byte) string {
+	sum := sha256.Sum256(b)
+	return hex.EncodeToString(sum[:])
+}
+
+// resolveWithin maps a brainDir-relative path to an absolute path and
+// guarantees it does not escape brainDir. Returns the cleaned relPath
+// (forward-slashed) and the absolute path.
+func resolveWithin(brainDir, relPath string) (rel, abs string, err error) {
+	clean := filepath.Clean("/" + filepath.ToSlash(relPath))
+	rel = strings.TrimPrefix(clean, "/")
+	abs = filepath.Join(brainDir, filepath.FromSlash(rel))
+	check, err := filepath.Rel(brainDir, abs)
+	if err != nil || check == ".." || strings.HasPrefix(check, ".."+string(filepath.Separator)) {
+		return "", "", fmt.Errorf("path %q escapes brain dir", relPath)
+	}
+	return rel, abs, nil
+}
+
+// UpdateNoteOptions identifies the note to supersede and supplies its new
+// body. Path takes precedence; otherwise the target is resolved from
+// Wing/Hall/Slug via brain.NotePath.
+type UpdateNoteOptions struct {
+	Path    string // brainDir-relative path; takes precedence over wing/hall/slug
+	Wing    string
+	Hall    string
+	Slug    string
+	Content string // new full body (whole-note replace)
+	Reason  string // optional; stamped as supersede_reason
+}
+
+// UpdateNote supersedes an existing note in place. It replaces the body
+// with opts.Content, preserves the existing frontmatter (created_at,
+// wing, hall, and any custom fields), and stamps updated_at, supersedes
+// (the prior content hash), and supersede_reason (when given).
+//
+// It never creates: if the target does not exist, it returns an error so
+// the caller can fall back to brain_write. Returns the note's relPath,
+// the new content hash, and the prior content hash.
+//
+// Embeddings are NOT refreshed here. The rewritten file's mtime advances,
+// which the mtime-driven vectorstore.Sync ticker uses to re-embed it on
+// its next pass — the same out-of-band mechanism brain_write relies on.
+func UpdateNote(brainDir string, opts UpdateNoteOptions) (relPath, contentHash, priorHash string, err error) {
+	if opts.Content == "" {
+		return "", "", "", fmt.Errorf("content is required")
+	}
+
+	var rel string
+	if opts.Path != "" {
+		rel = opts.Path
+	} else {
+		full, perr := brain.NotePath(brainDir, opts.Wing, opts.Hall, opts.Slug)
+		if perr != nil {
+			return "", "", "", perr
+		}
+		rel, _ = filepath.Rel(brainDir, full)
+		rel = filepath.ToSlash(rel)
+	}
+
+	rel, abs, err := resolveWithin(brainDir, rel)
+	if err != nil {
+		return "", "", "", err
+	}
+
+	prior, err := os.ReadFile(abs)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return "", "", "", fmt.Errorf("note %q does not exist: use brain_write to create", rel)
+		}
+		return "", "", "", fmt.Errorf("read target: %w", err)
+	}
+	priorHash = ContentHash(prior)
+
+	fm, _ := parseFrontmatter(string(prior))
+	fm.set("updated_at", time.Now().UTC().Format(time.RFC3339))
+	fm.set("supersedes", priorHash)
+	if opts.Reason != "" {
+		fm.set("supersede_reason", opts.Reason)
+	}
+
+	out := []byte(fm.render() + opts.Content)
+	if err := os.WriteFile(abs, out, 0o644); err != nil {
+		return "", "", "", fmt.Errorf("write: %w", err)
+	}
+	return rel, ContentHash(out), priorHash, nil
+}
+
+// ReadNote reads the note at the brainDir-relative relPath and returns
+// its parsed frontmatter, body, and content hash. It is the read-after-
+// write primitive behind brain_get: the hash it returns equals the hash
+// brain_write / brain_update returned for the same bytes.
+func ReadNote(brainDir, relPath string) (fm map[string]string, body, contentHash string, err error) {
+	_, abs, err := resolveWithin(brainDir, relPath)
+	if err != nil {
+		return nil, "", "", err
+	}
+	raw, err := os.ReadFile(abs)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, "", "", fmt.Errorf("note %q does not exist", relPath)
+		}
+		return nil, "", "", fmt.Errorf("read note: %w", err)
+	}
+	parsed, body := parseFrontmatter(string(raw))
+	fm = make(map[string]string, len(parsed.lines))
+	for _, l := range parsed.lines {
+		if l.key != "" {
+			fm[l.key] = l.value
+		}
+	}
+	return fm, body, ContentHash(raw), nil
+}
diff --git a/ingestion/internal/api/update_test.go b/ingestion/internal/api/update_test.go
new file mode 100644
index 0000000..d677a29
--- /dev/null
+++ b/ingestion/internal/api/update_test.go
@@ -0,0 +1,130 @@
+package api
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// seedNote writes a note directly to disk and returns its relPath.
+func seedNote(t *testing.T, brainDir, rel, content string) string {
+	t.Helper()
+	full := filepath.Join(brainDir, filepath.FromSlash(rel))
+	require.NoError(t, os.MkdirAll(filepath.Dir(full), 0o755))
+	require.NoError(t, os.WriteFile(full, []byte(content), 0o644))
+	return rel
+}
+
+func TestUpdateNoteSupersedesAndStamps(t *testing.T) {
+	brainDir := t.TempDir()
+	rel := seedNote(t, brainDir, "wiki/jepa-fx/facts/val-vol.md",
+		"---\nwing: jepa-fx\nhall: facts\ncreated_at: 2026-01-01T00:00:00Z\ncustom: keep-me\n---\n# Old\n\nold body\n")
+
+	relPath, hash, priorHash, err := UpdateNote(brainDir, UpdateNoteOptions{
+		Path:    rel,
+		Content: "# New\n\nnew body\n",
+		Reason:  "facts changed",
+	})
+	require.NoError(t, err)
+	assert.Equal(t, rel, relPath)
+	assert.NotEmpty(t, hash)
+	assert.NotEmpty(t, priorHash)
+	assert.NotEqual(t, hash, priorHash)
+
+	got, err := os.ReadFile(filepath.Join(brainDir, filepath.FromSlash(rel)))
+	require.NoError(t, err)
+	s := string(got)
+	// Body replaced.
+	assert.Contains(t, s, "# New")
+	assert.NotContains(t, s, "old body")
+	// Prior fields preserved.
+	assert.Contains(t, s, "wing: jepa-fx")
+	assert.Contains(t, s, "hall: facts")
+	assert.Contains(t, s, "created_at: 2026-01-01T00:00:00Z")
+	assert.Contains(t, s, "custom: keep-me")
+	// Supersession stamped.
+	assert.Contains(t, s, "updated_at:")
+	assert.Contains(t, s, "supersedes: "+priorHash)
+	assert.Contains(t, s, "supersede_reason: facts changed")
+}
+
+func TestUpdateNoteResolvesByWingHallSlug(t *testing.T) {
+	brainDir := t.TempDir()
+	seedNote(t, brainDir, "wiki/jepa-fx/facts/val-vol.md",
+		"---\nwing: jepa-fx\nhall: facts\n---\nold\n")
+
+	relPath, _, _, err := UpdateNote(brainDir, UpdateNoteOptions{
+		Wing: "jepa-fx", Hall: "facts", Slug: "val-vol",
+		Content: "new\n",
+	})
+	require.NoError(t, err)
+	assert.Equal(t, "wiki/jepa-fx/facts/val-vol.md", relPath)
+}
+
+func TestUpdateNoteErrorsOnMissingAndDoesNotCreate(t *testing.T) {
+	brainDir := t.TempDir()
+
+	_, _, _, err := UpdateNote(brainDir, UpdateNoteOptions{
+		Wing: "jepa-fx", Hall: "facts", Slug: "ghost",
+		Content: "x\n",
+	})
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "does not exist")
+
+	// No file created.
+	_, statErr := os.Stat(filepath.Join(brainDir, "wiki/jepa-fx/facts/ghost.md"))
+	assert.True(t, os.IsNotExist(statErr), "missing-target update must not create a note")
+}
+
+func TestUpdateNoteRejectsTraversal(t *testing.T) {
+	brainDir := t.TempDir()
+	_, _, _, err := UpdateNote(brainDir, UpdateNoteOptions{
+		Path:    "../escape.md",
+		Content: "x\n",
+	})
+	require.Error(t, err)
+}
+
+func TestReadNoteReturnsFrontmatterBodyHash(t *testing.T) {
+	brainDir := t.TempDir()
+	rel := seedNote(t, brainDir, "wiki/jepa-fx/facts/n.md",
+		"---\nwing: jepa-fx\nhall: facts\n---\n# Body\n\ntext\n")
+
+	fm, body, hash, err := ReadNote(brainDir, rel)
+	require.NoError(t, err)
+	assert.Equal(t, "jepa-fx", fm["wing"])
+	assert.Equal(t, "facts", fm["hall"])
+	assert.Equal(t, "# Body\n\ntext\n", body)
+
+	// Hash matches ContentHash of the raw bytes on disk (round-trip).
+	raw, _ := os.ReadFile(filepath.Join(brainDir, filepath.FromSlash(rel)))
+	assert.Equal(t, ContentHash(raw), hash)
+}
+
+func TestReadNoteRejectsTraversal(t *testing.T) {
+	brainDir := t.TempDir()
+	_, _, _, err := ReadNote(brainDir, "../../etc/passwd")
+	require.Error(t, err)
+}
+
+func TestUpdateThenReadRoundTripsHash(t *testing.T) {
+	brainDir := t.TempDir()
+	rel := seedNote(t, brainDir, "wiki/a/facts/n.md", "---\nwing: a\nhall: facts\n---\nold\n")
+
+	_, hash, _, err := UpdateNote(brainDir, UpdateNoteOptions{Path: rel, Content: "new\n"})
+	require.NoError(t, err)
+
+	_, _, readHash, err := ReadNote(brainDir, rel)
+	require.NoError(t, err)
+	assert.Equal(t, hash, readHash, "update content_hash must round-trip through ReadNote")
+}
+
+func TestContentHashStable(t *testing.T) {
+	assert.Equal(t, ContentHash([]byte("abc")), ContentHash([]byte("abc")))
+	assert.NotEqual(t, ContentHash([]byte("abc")), ContentHash([]byte("abd")))
+	assert.True(t, strings.HasPrefix(ContentHash([]byte("")), "")) // hex, non-panicking
+}