diff --git a/ingestion/internal/mcp/tools_notes.go b/ingestion/internal/mcp/tools_notes.go
index df99825..b2c5269 100644
--- a/ingestion/internal/mcp/tools_notes.go
+++ b/ingestion/internal/mcp/tools_notes.go
@@ -187,10 +187,9 @@ func (s *Server) editWithScope(args json.RawMessage, scope notes.Scope) (notes.R
 	// replace has no frontmatter awareness at all, so it could rewrite an
 	// existing half_life:/applies:/claim_provenance: line straight past the
 	// validator entirely. Scoped to notes only (governance files have no
-	// claim-envelope concept) and gated on a cheap substring check so an
-	// edit that never touches these field names pays nothing extra.
-	if scope == notes.ScopeNotes && touchesClaimEnvelope(a.OldStr, a.NewStr) {
-		if verr := validateEnvelopeAfterEdit(s.brainDir, a.Path, a.OldStr, a.NewStr); verr != nil {
+	// claim-envelope concept).
+	if scope == notes.ScopeNotes {
+		if verr := checkClaimEnvelope(s.brainDir, a.Path, a.OldStr, a.NewStr); verr != nil {
 			return notes.Result{}, nil, fmt.Errorf("claim envelope: %w", verr)
 		}
 	}
@@ -245,42 +244,59 @@ func touchesFrontmatter(oldStr, newStr string) bool {
 	return false
 }
 
-// touchesClaimEnvelope reports whether oldStr/newStr mention any of the
-// three claim-envelope field names — the cheap gate that lets an ordinary
-// body edit skip the read-and-validate round trip entirely.
-func touchesClaimEnvelope(oldStr, newStr string) bool {
-	for _, s := range []string{oldStr, newStr} {
-		if strings.Contains(s, claim.FieldHalfLife) || strings.Contains(s, claim.FieldApplies) || strings.Contains(s, claim.FieldProvenance) {
-			return true
-		}
-	}
-	return false
-}
-
-// validateEnvelopeAfterEdit previews what oldStr->newStr would produce —
-// the exact strings.Replace(before, oldStr, newStr, 1) formula notes.Edit
+// checkClaimEnvelope previews what old_str->new_str would produce — the
+// exact strings.Replace(before, oldStr, newStr, 1) formula notes.Edit
 // itself uses internally — and validates the resulting frontmatter's claim
 // envelope before the real edit is allowed to run. Fail-closed: an invalid
 // result blocks the edit entirely, the same posture writeHallNote/UpdateNote
 // already apply on create/supersede.
 //
+// Gated on old_str appearing within the note's frontmatter BLOCK, not on
+// old_str/new_str mentioning a claim-envelope field NAME — found in review:
+// a field-name-substring gate misses a value-only edit like
+// "asserted"->"guessed" on a claim_provenance line, which never mentions
+// the field name at all. That is exactly the shape of edit a minimal,
+// natural diff produces, so a field-name gate was closing the issue in
+// name only. Checking block containment instead means any edit that
+// couldn't possibly touch the frontmatter (its target text only exists in
+// the body) is still skipped cheaply, while nothing inside the fence can
+// slip through unvalidated regardless of which token it targets.
+//
 // This does not replicate notes.Edit's uniqueness/CRLF/conflict-marker
 // checks — if old_str is absent or ambiguous, this preview may not match
 // what Edit ultimately does, but Edit independently and correctly rejects
 // those cases at write time regardless of what this function decides, so
 // the only thing riding on this preview's accuracy is the envelope check
 // itself, not write safety.
-func validateEnvelopeAfterEdit(brainDir, path, oldStr, newStr string) error {
+func checkClaimEnvelope(brainDir, path, oldStr, newStr string) error {
 	before, err := notes.Read(brainDir, path)
 	if err != nil {
 		// Let notes.Edit itself produce the real error (missing file, etc.)
 		// — this function's job is the envelope, not read failures.
 		return nil
 	}
+	block, ok := frontmatterBlock(string(before))
+	if !ok || !strings.Contains(block, oldStr) {
+		return nil // old_str isn't in the frontmatter block at all
+	}
 	after := strings.Replace(string(before), oldStr, newStr, 1)
 	return claim.ValidateEnvelope(frontmatterScalars(after))
 }
 
+// frontmatterBlock returns the raw text between doc's opening and closing
+// `---` fences, or ok=false if doc has no (properly closed) fence.
+func frontmatterBlock(doc string) (block string, ok bool) {
+	if !strings.HasPrefix(doc, "---\n") {
+		return "", false
+	}
+	rest := doc[len("---\n"):]
+	end := strings.Index(rest, "\n---\n")
+	if end < 0 {
+		return "", false
+	}
+	return rest[:end], true
+}
+
 // frontmatterScalars extracts top-level "key: value" scalar pairs from
 // doc's leading fence block, for the claim-envelope check only — not a
 // general frontmatter parser. Indented lines (YAML block-list items,
@@ -288,15 +304,11 @@ func validateEnvelopeAfterEdit(brainDir, path, oldStr, newStr string) error {
 // hyperguild#126/#128 in the sibling packages this mirrors.
 func frontmatterScalars(doc string) map[string]string {
 	out := map[string]string{}
-	if !strings.HasPrefix(doc, "---\n") {
+	block, ok := frontmatterBlock(doc)
+	if !ok {
 		return out
 	}
-	rest := doc[len("---\n"):]
-	end := strings.Index(rest, "\n---\n")
-	if end < 0 {
-		return out
-	}
-	for _, line := range strings.Split(rest[:end], "\n") {
+	for _, line := range strings.Split(block, "\n") {
 		if line != "" && (line[0] == ' ' || line[0] == '\t') {
 			continue
 		}
diff --git a/ingestion/internal/mcp/tools_notes_test.go b/ingestion/internal/mcp/tools_notes_test.go
index dbe6fcc..5989cde 100644
--- a/ingestion/internal/mcp/tools_notes_test.go
+++ b/ingestion/internal/mcp/tools_notes_test.go
@@ -116,6 +116,31 @@ func TestBrainEdit_RejectsAnInvalidClaimEnvelopeWrittenViaStringReplace(t *testi
 	assert.Contains(t, string(got), "claim_provenance: asserted", "rejected edit must leave the note byte-identical")
 }
 
+func TestBrainEdit_RejectsAValueOnlyEditThatBreaksTheEnvelope(t *testing.T) {
+	// Found in review: the original gate (touchesClaimEnvelope) checked
+	// whether old_str/new_str mention the FIELD NAME — but a minimal diff
+	// (the natural shape of any real edit, model-generated or human) can
+	// target just the VALUE token and never mention the field name at all.
+	// "asserted" -> "guessed" on a claim_provenance line walked straight
+	// past the field-name substring check and wrote an invalid enum value
+	// with no error — the exact bypass #123 was supposed to close, closed
+	// in name only.
+	dir := newBrainRepo(t)
+	note := filepath.Join(dir, "wiki/homelab/facts/n.md")
+	require.NoError(t, os.WriteFile(note,
+		[]byte("---\ntitle: 'n'\nhalf_life: durable\napplies: universal\nclaim_provenance: asserted\n---\n\nbody\n"), 0o644))
+	srv := mcp.NewServer(dir, nil, nil, nil)
+
+	resp := call(t, srv, "brain_edit", map[string]any{
+		"path": "wiki/homelab/facts/n.md", "old_str": "asserted", "new_str": "guessed",
+	})
+	require.Contains(t, resp, "error", "%v", resp)
+
+	got, err := os.ReadFile(note)
+	require.NoError(t, err)
+	assert.Contains(t, string(got), "claim_provenance: asserted", "rejected edit must leave the note byte-identical")
+}
+
 func TestBrainEdit_AllowsAValidClaimEnvelopeEdit(t *testing.T) {
 	dir := newBrainRepo(t)
 	note := filepath.Join(dir, "wiki/homelab/facts/n.md")