diff --git a/ingestion/internal/api/handler.go b/ingestion/internal/api/handler.go
index 95621f8..6359f74 100644
--- a/ingestion/internal/api/handler.go
+++ b/ingestion/internal/api/handler.go
@@ -174,9 +174,6 @@ func writeHallNote(brainDir string, opts WriteNoteOptions) (string, error) {
 		return "", fmt.Errorf("claim envelope: %w", err)
 	}
 
-	if err := os.MkdirAll(filepath.Dir(dest), 0o755); err != nil {
-		return "", fmt.Errorf("create hall dir: %w", err)
-	}
 	emitted := make(map[string]bool, 6)
 
 	var fm strings.Builder
@@ -228,6 +225,12 @@ func writeHallNote(brainDir string, opts WriteNoteOptions) (string, error) {
 	if err != nil {
 		return "", err
 	}
+	// The directory is created only once the note is known to be writable:
+	// a refused write leaves nothing behind, not even an empty hall dir
+	// (brain#21; provenance refusal joins the envelope refusal here).
+	if err := os.MkdirAll(filepath.Dir(dest), 0o755); err != nil {
+		return "", fmt.Errorf("create hall dir: %w", err)
+	}
 	if err := os.WriteFile(dest, []byte(final), 0o644); err != nil {
 		return "", fmt.Errorf("write: %w", err)
 	}
diff --git a/ingestion/internal/api/provenance_test.go b/ingestion/internal/api/provenance_test.go
index e23cf68..ffb3a69 100644
--- a/ingestion/internal/api/provenance_test.go
+++ b/ingestion/internal/api/provenance_test.go
@@ -108,3 +108,15 @@ func TestRESTIngestStampsUnknown(t *testing.T) {
 		assert.Contains(t, strings.SplitN(string(b), "---\n", 3)[1], "origin: unknown\n", p)
 	}
 }
+
+// A refused write leaves nothing behind, not even an empty hall directory
+// (brain#21's invariant, round 3 non-blocking finding).
+func TestWriteNoteRefusedForProvenanceLeavesNoDirectory(t *testing.T) {
+	dir := t.TempDir()
+	_, err := api.WriteNote(dir, api.WriteNoteOptions{
+		Content: "# x\n\nbody", Wing: "freshwing", Hall: "facts", Filename: "x", Origin: "jwt", WrittenBy: "bad\xff",
+	})
+	require.Error(t, err)
+	_, statErr := os.Stat(filepath.Join(dir, "wiki", "freshwing"))
+	assert.True(t, os.IsNotExist(statErr), "no wing directory may be created by a refused write")
+}
diff --git a/ingestion/internal/mcp/provenance_escapes_test.go b/ingestion/internal/mcp/provenance_escapes_test.go
index bf2e48a..0783f1c 100644
--- a/ingestion/internal/mcp/provenance_escapes_test.go
+++ b/ingestion/internal/mcp/provenance_escapes_test.go
@@ -2,6 +2,7 @@ package mcp_test
 
 import (
 	"bytes"
+	"context"
 	"encoding/json"
 	"log/slog"
 	"os"
@@ -20,6 +21,7 @@ import (
 	"github.com/mathiasbq/hyperguild/ingestion/internal/capturehttp"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/classification"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/mcp"
+	"github.com/mathiasbq/hyperguild/ingestion/internal/pipeline"
 )
 
 // Escapes found by the PR #147 gatekeeper review. Each test drives the real
@@ -260,3 +262,64 @@ func TestToolsCallAuditErrorOutcomeCarriesNoArgumentValues(t *testing.T) {
 	}
 	assert.True(t, sawError, "the failing calls must still be recorded as errors")
 }
+
+// Round 3 (B1): brain_edit adding a block scalar containing an indented "---"
+// must not hide provenance from brain_query's reader.
+func TestBrainEditBlockScalarFenceDoesNotHideProvenanceFromQuery(t *testing.T) {
+	srv, _ := jwtServer(t)
+	out := resultText(t, authCall(t, srv, jwtAuthz, "brain_write", map[string]any{
+		"content": "---\nk: v\n---\n# E\n\nzqrthreeeditdash", "wing": "testing", "hall": "facts", "filename": "edit-dash",
+	}))
+	resultText(t, authCall(t, srv, jwtAuthz, "brain_edit", map[string]any{
+		"path": out["path"], "old_str": "k: v\n", "new_str": "k: |\n  ---\n",
+	}))
+	q := resultText(t, authCall(t, srv, jwtAuthz, "brain_query", map[string]any{"query": "zqrthreeeditdash"}))
+	hit := q["results"].([]any)[0].(map[string]any)
+	assert.Equal(t, "jwt", hit["origin"])
+	assert.Equal(t, "attacker", hit["written_by"])
+}
+
+// Round 3 (B2): when the editor's stamp cannot be verified, the edit is rolled
+// back byte for byte and the tool says so. A JWT subject with an invalid UTF-8
+// byte cannot round-trip through YAML, which forces that path.
+func TestBrainEditRollsBackWhenTheStampCannotBeVerified(t *testing.T) {
+	dir := newBrainRepo(t)
+	cfg, err := classification.Load(dir)
+	require.NoError(t, err)
+	svc := capture.NewService(brainstore.New(dir), capFakeTracker{}, cfg, audit.NewSlogSink(nil))
+	srv := mcp.NewServer(dir, nil, nil, nil)
+	srv.WithCapture(svc, capFakeValidator{subject: "bad\xff"}, capStaticTok, "local-cli", capturehttp.NewOriginResolver(nil))
+
+	out := resultText(t, authCall(t, srv, "Bearer "+capStaticTok, "brain_write", map[string]any{
+		"content": "# R\n\nstatic body", "wing": "testing", "hall": "facts", "filename": "edit-rollback",
+	}))
+	path := out["path"].(string)
+	before, err := os.ReadFile(filepath.Join(dir, path))
+	require.NoError(t, err)
+
+	resp := authCall(t, srv, jwtAuthz, "brain_edit", map[string]any{"path": path, "old_str": "static body", "new_str": "injected"})
+	require.NotNil(t, resp["error"], "the edit must be reported as failed")
+	assert.Contains(t, resp["error"].(map[string]any)["message"], "rolled back")
+	after, err := os.ReadFile(filepath.Join(dir, path))
+	require.NoError(t, err)
+	assert.Equal(t, string(before), string(after), "the note must be byte-identical to before the edit")
+}
+
+// MCP brain_ingest (the LLM path) attributes its pages to the caller.
+func TestBrainIngestViaMCPStampsCaller(t *testing.T) {
+	dir := newBrainRepo(t)
+	cfg, err := classification.Load(dir)
+	require.NoError(t, err)
+	svc := capture.NewService(brainstore.New(dir), capFakeTracker{}, cfg, audit.NewSlogSink(nil))
+	stub := pipeline.Config{Complete: func(context.Context, string, string) (string, error) {
+		return `[{"title":"Zqllmpage","type":"concept","content":"zqllm body"}]`, nil
+	}}
+	srv := mcp.NewServer(dir, &stub, nil, nil)
+	srv.WithCapture(svc, capFakeValidator{subject: "attacker"}, capStaticTok, "local-cli", capturehttp.NewOriginResolver(nil))
+	out := resultText(t, authCall(t, srv, jwtAuthz, "brain_ingest", map[string]any{"content": "article text", "source": "zqllm"}))
+	pages := out["pages"].([]any)
+	require.NotEmpty(t, pages)
+	for _, p := range pages {
+		assertProvenance(t, srv, dir, p.(string), "jwt", "attacker")
+	}
+}
diff --git a/ingestion/internal/mcp/tools_notes.go b/ingestion/internal/mcp/tools_notes.go
index 17f0d5b..bcfa12a 100644
--- a/ingestion/internal/mcp/tools_notes.go
+++ b/ingestion/internal/mcp/tools_notes.go
@@ -110,10 +110,17 @@ func (s *Server) brainEdit(ctx context.Context, args json.RawMessage) (json.RawM
 	// verified, the edit is rolled back rather than left on disk unattributed
 	// (or carrying whatever provenance the edit wrote).
 	var a brainEditArgs
-	_ = json.Unmarshal(args, &a)
-	var prior []byte
-	if t, rerr := notes.Resolve(s.brainDir, a.Path, notes.ScopeNotes); rerr == nil {
-		prior, _ = os.ReadFile(t.Abs)
+	if err := json.Unmarshal(args, &a); err != nil {
+		return nil, fmt.Errorf("parse args: %w", err)
+	}
+	target, err := notes.Resolve(s.brainDir, a.Path, notes.ScopeNotes)
+	if err != nil {
+		return nil, err
+	}
+	// No snapshot, no edit: without one a failed stamp could not be undone.
+	prior, err := os.ReadFile(target.Abs)
+	if err != nil {
+		return nil, fmt.Errorf("snapshot before edit: %w", err)
 	}
 	res, out, err := s.editWithScope(args, notes.ScopeNotes)
 	if err != nil {
@@ -124,8 +131,10 @@ func (s *Server) brainEdit(ctx context.Context, args json.RawMessage) (json.RawM
 	// to the editor after every edit (PR #147 review, escape E1).
 	origin, writtenBy := provenanceFrom(ctx)
 	if serr := api.StampProvenanceFile(s.brainDir, res.Path, origin, writtenBy); serr != nil {
-		if t, rerr := notes.Resolve(s.brainDir, res.Path, notes.ScopeNotes); rerr == nil && prior != nil {
-			_ = os.WriteFile(t.Abs, prior, 0o644)
+		if werr := os.WriteFile(target.Abs, prior, 0o644); werr != nil {
+			// Say exactly what state the note is in: edited, unverified.
+			return nil, fmt.Errorf("stamp provenance failed (%v) AND the edit could NOT be rolled back: %w; "+
+				"%s carries the edit under its previous attribution", serr, werr, res.Path)
 		}
 		return nil, fmt.Errorf("edit rolled back, stamp provenance: %w", serr)
 	}
diff --git a/ingestion/internal/provenance/provenance.go b/ingestion/internal/provenance/provenance.go
index 62d4b8b..1343e0d 100644
--- a/ingestion/internal/provenance/provenance.go
+++ b/ingestion/internal/provenance/provenance.go
@@ -85,9 +85,15 @@ func Apply(content string, s Stamp) (string, error) {
 	content = foldFrontmatterCR(content)
 	block, rest, ok := sharedfm.Split([]byte(content))
 	var out string
-	if !ok {
+	switch {
+	case !ok && strings.HasPrefix(strings.ReplaceAll(content, "\r\n", "\n"), "---\n"):
+		// A fence was opened but cannot be closed (e.g. a "---x" line).
+		// Prepending a second block would demote the original frontmatter
+		// into the body, so refuse instead (PR #147 review round 3).
+		return "", fmt.Errorf("%w: unclosed frontmatter fence", ErrUnverifiable)
+	case !ok:
 		out = "---\n" + stamp + "---\n" + content
-	} else {
+	default:
 		out = "---\n" + plainGroups(string(block)) + stamp + "---\n" + strings.TrimPrefix(string(rest), "\n")
 	}
 	if err := verify(out, s); err != nil {
@@ -203,11 +209,18 @@ func foldFrontmatterCR(content string) string {
 	return strings.ReplaceAll(s[:4+end], "\r", "\n") + s[4+end:]
 }
 
-// scalar renders a bare word as-is and anything else as a JSON string (valid
-// YAML double-quoted), so a principal name can never break the line.
+// scalar renders a bare word as-is only when YAML reads it back as that same
+// string; anything else (true, 123, null, ~, punctuation, newlines) becomes
+// a double-quoted string, so a principal name can never break the line or
+// change type.
 func scalar(v string) string {
 	if bareScalar.MatchString(v) {
-		return v
+		var back any
+		if yaml.Unmarshal([]byte(v), &back) == nil {
+			if s, ok := back.(string); ok && s == v {
+				return v
+			}
+		}
 	}
 	return fmt.Sprintf("%q", v)
 }
diff --git a/ingestion/internal/provenance/provenance_test.go b/ingestion/internal/provenance/provenance_test.go
new file mode 100644
index 0000000..c666c8a
--- /dev/null
+++ b/ingestion/internal/provenance/provenance_test.go
@@ -0,0 +1,86 @@
+package provenance
+
+import (
+	"errors"
+	"strings"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"gopkg.in/yaml.v3"
+)
+
+func fmOf(t *testing.T, doc string) map[string]any {
+	t.Helper()
+	parts := strings.SplitN(doc, "---\n", 3)
+	require.Len(t, parts, 3, doc)
+	var m map[string]any
+	require.NoError(t, yaml.Unmarshal([]byte(parts[1]), &m), parts[1])
+	return m
+}
+
+// The whole-document backstop: a stamp that would not read back as written
+// is refused, never written. A principal with an invalid UTF-8 byte quotes
+// to "\xff", which YAML decodes as U+00FF, so it cannot round-trip.
+func TestApplyRefusesAStampThatDoesNotReadBack(t *testing.T) {
+	_, err := Apply("---\ntitle: t\n---\nbody\n", Stamp{Origin: "jwt", WrittenBy: "bad\xff"})
+	assert.True(t, errors.Is(err, ErrUnverifiable), "got %v", err)
+}
+
+// A principal that YAML would decode as a non-string (true, 123, null) is
+// quoted, so the stamp reads back as the same string.
+func TestApplyQuotesPrincipalsYAMLWouldNotReadAsStrings(t *testing.T) {
+	for _, p := range []string{"true", "123", "null", "1e3", "0x1F", "yes", "~"} {
+		out, err := Apply("---\ntitle: t\n---\nbody\n", Stamp{Origin: "jwt", WrittenBy: p})
+		require.NoError(t, err, p)
+		assert.Equal(t, p, fmOf(t, out)["written_by"], p)
+	}
+}
+
+// A group that does not decode on its own is dropped; its neighbours stay.
+func TestApplyDropsGroupsThatDoNotDecodeAlone(t *testing.T) {
+	out, err := Apply("---\nkeep: 1\ntitle: a: b\nalso: 2\n---\nbody\n", Stamp{Origin: "static", WrittenBy: "local-cli"})
+	require.NoError(t, err)
+	m := fmOf(t, out)
+	assert.Equal(t, 1, m["keep"])
+	assert.Equal(t, 2, m["also"])
+	assert.NotContains(t, m, "title")
+}
+
+func TestApplyKeepsFirstOfDuplicateKeys(t *testing.T) {
+	out, err := Apply("---\ntitle: first\ntitle: second\nTitle: third\n---\nbody\n", Stamp{})
+	require.NoError(t, err)
+	m := fmOf(t, out)
+	assert.Equal(t, "first", m["title"])
+	assert.NotContains(t, m, "Title")
+}
+
+func TestApplyFoldsLoneCRSoItCannotHideALine(t *testing.T) {
+	out, err := Apply("---\ntitle: x\rorigin: static\n---\nbody\n", Stamp{Origin: "jwt", WrittenBy: "a"})
+	require.NoError(t, err)
+	assert.NotContains(t, out, "\r")
+	assert.Equal(t, "jwt", fmOf(t, out)["origin"])
+	assert.Equal(t, "x", fmOf(t, out)["title"])
+}
+
+func TestApplyDropsTabIndentedContinuations(t *testing.T) {
+	out, err := Apply("---\ntitle: x\n\torigin: static\nk: v\n---\nbody\n", Stamp{Origin: "jwt", WrittenBy: "a"})
+	require.NoError(t, err)
+	m := fmOf(t, out)
+	assert.Equal(t, "jwt", m["origin"])
+	assert.Equal(t, "v", m["k"])
+}
+
+// Content that opens a fence the shared splitter cannot close (e.g. a
+// "---x" line) is refused rather than given a second, prepended block that
+// demotes the original frontmatter into the body.
+func TestApplyRefusesAnUnclosableFrontmatterFence(t *testing.T) {
+	_, err := Apply("---\ntitle: t\n---x\norigin: static\n---\nbody\n", Stamp{Origin: "jwt", WrittenBy: "a"})
+	assert.True(t, errors.Is(err, ErrUnverifiable), "got %v", err)
+}
+
+func TestApplyAddsFrontmatterToContentWithoutAny(t *testing.T) {
+	out, err := Apply("# plain\n\norigin: static is prose\n", Stamp{Origin: "static", WrittenBy: "local-cli"})
+	require.NoError(t, err)
+	assert.True(t, strings.HasPrefix(out, "---\norigin: static\nwritten_by: local-cli\n---\n# plain"))
+}
diff --git a/ingestion/internal/search/provenance_test.go b/ingestion/internal/search/provenance_test.go
index ecc15fa..4c61573 100644
--- a/ingestion/internal/search/provenance_test.go
+++ b/ingestion/internal/search/provenance_test.go
@@ -19,3 +19,11 @@ func TestExtractProvenanceIgnoresBodyLines(t *testing.T) {
 	assert.Equal(t, "jwt", o)
 	assert.Equal(t, "attacker", w)
 }
+
+// Round 3 (B1): an indented "---" inside a block scalar is content, not the
+// closing fence — the same rule as frontmatter.Split.
+func TestExtractProvenanceUsesTheSharedFenceRule(t *testing.T) {
+	o, w := extractProvenance("---\nk: |\n  ---\norigin: jwt\nwritten_by: attacker\n---\nbody\n")
+	assert.Equal(t, "jwt", o)
+	assert.Equal(t, "attacker", w)
+}
diff --git a/ingestion/internal/search/search.go b/ingestion/internal/search/search.go
index 0fdab19..c272a61 100644
--- a/ingestion/internal/search/search.go
+++ b/ingestion/internal/search/search.go
@@ -5,6 +5,7 @@ import (
 	"bufio"
 	"context"
 	"fmt"
+	sharedfm "github.com/mathiasbq/hyperguild/ingestion/internal/frontmatter"
 	"log/slog"
 	"os"
 	"path/filepath"
@@ -296,33 +297,32 @@ func hydrate(brainDir, relPath string) (Result, error) {
 }
 
 // extractProvenance reads origin/written_by from the leading frontmatter
-// block only — a body line that happens to start with "origin:" is not
-// provenance.
+// block only, located with the shared fence rule (frontmatter.Split: a "---"
+// at column 0) so an indented "---" inside a block scalar is content, not a
+// closing fence (PR #147 review round 3). A body line is never provenance.
 func extractProvenance(content string) (origin, writtenBy string) {
-	scanner := bufio.NewScanner(strings.NewReader(content))
-	inFrontmatter := false
-	for scanner.Scan() {
-		line := scanner.Text()
-		if strings.TrimSpace(line) == "---" {
-			if !inFrontmatter {
-				inFrontmatter = true
-				continue
-			}
-			break
+	block, _, ok := sharedfm.Split([]byte(content))
+	if !ok {
+		return "", ""
+	}
+	for _, line := range strings.Split(string(block), "\n") {
+		if line == "" || line[0] == ' ' || line[0] == '\t' {
+			continue
 		}
-		if !inFrontmatter {
-			return "", ""
-		}
-		key, val, ok := strings.Cut(line, ":")
-		if !ok || strings.HasPrefix(line, " ") || strings.HasPrefix(line, "\t") {
+		key, val, found := strings.Cut(line, ":")
+		if !found {
 			continue
 		}
 		v := strings.Trim(strings.TrimSpace(val), `"'`)
 		switch strings.TrimSpace(key) {
 		case "origin":
-			origin = v
+			if origin == "" {
+				origin = v
+			}
 		case "written_by":
-			writtenBy = v
+			if writtenBy == "" {
+				writtenBy = v
+			}
 		}
 	}
 	return origin, writtenBy