diff --git a/ingestion/internal/mcp/tools_context.go b/ingestion/internal/mcp/tools_context.go
index 9ccfcec..d906f20 100644
--- a/ingestion/internal/mcp/tools_context.go
+++ b/ingestion/internal/mcp/tools_context.go
@@ -11,6 +11,7 @@ import (
 	"time"
 
 	"github.com/mathiasbq/hyperguild/ingestion/internal/search"
+	"github.com/mathiasbq/hyperguild/ingestion/internal/textutil"
 )
 
 // brainContextArgs is the input shape of brain_context. project_root is
@@ -170,10 +171,7 @@ func (s *Server) brainContext(ctx context.Context, args json.RawMessage) (json.R
 // without re-running the frontmatter parser. The ellipsis suffix matches
 // the convention used in search.excerpt.
 func truncateExcerpt(s string, maxLen int) string {
-	if len(s) <= maxLen {
-		return s
-	}
-	return s[:maxLen] + "…"
+	return textutil.Truncate(s, maxLen)
 }
 
 // readExcerpt loads a doc relative to brainDir, strips its frontmatter,
@@ -193,10 +191,7 @@ func readExcerpt(brainDir, relPath string, maxLen int) string {
 	if len(parts) == 3 {
 		body = strings.TrimSpace(parts[2])
 	}
-	if len(body) > maxLen {
-		return body[:maxLen] + "…"
-	}
-	return body
+	return textutil.Truncate(body, maxLen)
 }
 
 // max1 returns the maximum of n and 1, used to guard against divide-by-zero
diff --git a/ingestion/internal/search/search.go b/ingestion/internal/search/search.go
index ec75d5b..b0c75fe 100644
--- a/ingestion/internal/search/search.go
+++ b/ingestion/internal/search/search.go
@@ -12,6 +12,7 @@ import (
 	"strings"
 
 	"github.com/mathiasbq/hyperguild/ingestion/internal/brain"
+	"github.com/mathiasbq/hyperguild/ingestion/internal/textutil"
 	"github.com/mathiasbq/hyperguild/ingestion/internal/vectorstore"
 )
 
@@ -427,8 +428,5 @@ func excerpt(content string, maxLen int) string {
 	if len(parts) == 3 {
 		body = strings.TrimSpace(parts[2])
 	}
-	if len(body) > maxLen {
-		return body[:maxLen] + "…"
-	}
-	return body
+	return textutil.Truncate(body, maxLen)
 }
diff --git a/ingestion/internal/textutil/truncate.go b/ingestion/internal/textutil/truncate.go
new file mode 100644
index 0000000..e525f50
--- /dev/null
+++ b/ingestion/internal/textutil/truncate.go
@@ -0,0 +1,45 @@
+// Package textutil holds small text helpers shared across the ingestion
+// service.
+package textutil
+
+import "unicode/utf8"
+
+// Ellipsis marks a value that was cut short.
+const Ellipsis = "…"
+
+// Truncate shortens s to at most limit CHARACTERS, appending an ellipsis when
+// anything was dropped.
+//
+// It exists because four call sites did `s[:limit] + "…"` (#93). Both halves
+// of that are byte operations: len() counts bytes and a slice cuts bytes, so
+// on Swedish text — where å ä ö are two bytes and an em dash is three — the
+// cut lands mid-character and leaves an orphaned lead byte. The result is
+// invalid UTF-8.
+//
+// That failed loudly in exactly one place, wiki/index.md, which is written
+// raw to disk and then rejected by every strict decoder. The other three fed
+// MCP responses, where encoding/json silently substitutes U+FFFD — so those
+// had been quietly eating the last character of truncated excerpts with no
+// error anywhere.
+//
+// The marker is appended only when content was actually dropped: an ellipsis
+// on complete text is a lie about the content.
+func Truncate(s string, limit int) string {
+	if limit <= 0 {
+		return ""
+	}
+	if utf8.RuneCountInString(s) <= limit {
+		return s
+	}
+
+	// Walk runes rather than converting to []rune: the input can be a whole
+	// note body, and this runs on the read path.
+	count := 0
+	for i := range s {
+		if count == limit {
+			return s[:i] + Ellipsis
+		}
+		count++
+	}
+	return s + Ellipsis
+}
diff --git a/ingestion/internal/textutil/truncate_test.go b/ingestion/internal/textutil/truncate_test.go
new file mode 100644
index 0000000..2dc0f03
--- /dev/null
+++ b/ingestion/internal/textutil/truncate_test.go
@@ -0,0 +1,66 @@
+// ingestion/internal/textutil/truncate_test.go
+package textutil_test
+
+import (
+	"strings"
+	"testing"
+	"unicode/utf8"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/textutil"
+)
+
+func TestTruncate_ShortStringsAreUntouched(t *testing.T) {
+	for _, s := range []string{"", "kort", "räv", "a — b"} {
+		if got := textutil.Truncate(s, 100); got != s {
+			t.Errorf("Truncate(%q, 100) = %q, want it unchanged", s, got)
+		}
+	}
+}
+
+func TestTruncate_NeverCutsInsideACharacter(t *testing.T) {
+	// The property, not an example: whatever the limit, the result must be
+	// valid UTF-8. Byte-slicing Swedish text (å ä ö) or an em dash produces
+	// an orphaned lead byte, which is what corrupted wiki/index.md (#93).
+	s := strings.Repeat("räv — ål — ökäd ", 20)
+
+	for limit := 0; limit <= len(s)+5; limit++ {
+		got := textutil.Truncate(s, limit)
+		if !utf8.ValidString(got) {
+			t.Fatalf("Truncate(s, %d) produced invalid UTF-8: %q", limit, got)
+		}
+	}
+}
+
+func TestTruncate_CountsCharactersNotBytes(t *testing.T) {
+	// "räv" is 4 bytes, 3 runes. A byte-based limit of 3 would cut inside the
+	// "ä"; a rune-based one returns it whole.
+	if got := textutil.Truncate("räv", 3); got != "räv" {
+		t.Errorf("Truncate(\"räv\", 3) = %q, want the whole word", got)
+	}
+
+	got := textutil.Truncate("räven springer", 5)
+	if utf8.RuneCountInString(got) != 6 { // 5 runes + the ellipsis
+		t.Errorf("Truncate(...) = %q, want 5 runes plus the marker", got)
+	}
+	if !strings.HasSuffix(got, "…") {
+		t.Errorf("Truncate(...) = %q, want it marked as truncated", got)
+	}
+}
+
+func TestTruncate_MarksOnlyWhenSomethingWasDropped(t *testing.T) {
+	// An ellipsis on text that was not cut is a lie about the content.
+	if got := textutil.Truncate("räv", 10); strings.HasSuffix(got, "…") {
+		t.Errorf("Truncate(%q, 10) = %q, want no truncation marker", "räv", got)
+	}
+	if got := textutil.Truncate("räv", 3); strings.HasSuffix(got, "…") {
+		t.Errorf("exact-length input must not be marked: %q", got)
+	}
+}
+
+func TestTruncate_NonPositiveLimitYieldsNothingRatherThanPanicking(t *testing.T) {
+	for _, limit := range []int{0, -1, -100} {
+		if got := textutil.Truncate("räven", limit); got != "" {
+			t.Errorf("Truncate(_, %d) = %q, want empty", limit, got)
+		}
+	}
+}
diff --git a/ingestion/internal/wiki/index.go b/ingestion/internal/wiki/index.go
index b80841a..3a0ea2f 100644
--- a/ingestion/internal/wiki/index.go
+++ b/ingestion/internal/wiki/index.go
@@ -6,6 +6,8 @@ import (
 	"os"
 	"path/filepath"
 	"strings"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/textutil"
 )
 
 // RebuildIndex writes brain/wiki/index.md from the current wiki contents.
@@ -62,10 +64,7 @@ func pageFirstSentence(brainDir string, e Entry) string {
 		if line == "" || strings.HasPrefix(line, "#") {
 			continue
 		}
-		if len(line) > 100 {
-			return line[:100] + "…"
-		}
-		return line
+		return textutil.Truncate(line, 100)
 	}
 	return ""
 }