diff --git a/emitterc.go b/emitterc.go
index 0f47c9ca..48b9ba40 100644
--- a/emitterc.go
+++ b/emitterc.go
@@ -25,6 +25,7 @@ package yaml
 import (
 	"bytes"
 	"fmt"
+	"unicode/utf8"
 )
 
 // Flush the buffer if needed.
@@ -162,10 +163,9 @@ func yaml_emitter_emit(emitter *yaml_emitter_t, event *yaml_event_t) bool {
 // Check if we need to accumulate more events before emitting.
 //
 // We accumulate extra
-//  - 1 event for DOCUMENT-START
-//  - 2 events for SEQUENCE-START
-//  - 3 events for MAPPING-START
-//
+//   - 1 event for DOCUMENT-START
+//   - 2 events for SEQUENCE-START
+//   - 3 events for MAPPING-START
 func yaml_emitter_need_more_events(emitter *yaml_emitter_t) bool {
 	if emitter.events_head == len(emitter.events) {
 		return true
@@ -241,7 +241,7 @@ func yaml_emitter_increase_indent(emitter *yaml_emitter_t, flow, indentless bool
 			emitter.indent += 2
 		} else {
 			// Everything else aligns to the chosen indentation.
-			emitter.indent = emitter.best_indent*((emitter.indent+emitter.best_indent)/emitter.best_indent)
+			emitter.indent = emitter.best_indent * ((emitter.indent + emitter.best_indent) / emitter.best_indent)
 		}
 	}
 	return true
@@ -968,15 +968,18 @@ func yaml_emitter_check_empty_mapping(emitter *yaml_emitter_t) bool {
 
 // Check if the next node can be expressed as a simple key.
 func yaml_emitter_check_simple_key(emitter *yaml_emitter_t) bool {
-	length := 0
+	// first check length in bytes, since the fast majority of strings are
+	// within 1024 bytes, so take the faster route first. Only if we must,
+	// we check the length in runes and take the expensive route.
+	bLength := 0
 	switch emitter.events[emitter.events_head].typ {
 	case yaml_ALIAS_EVENT:
-		length += len(emitter.anchor_data.anchor)
+		bLength += len(emitter.anchor_data.anchor)
 	case yaml_SCALAR_EVENT:
 		if emitter.scalar_data.multiline {
 			return false
 		}
-		length += len(emitter.anchor_data.anchor) +
+		bLength += len(emitter.anchor_data.anchor) +
 			len(emitter.tag_data.handle) +
 			len(emitter.tag_data.suffix) +
 			len(emitter.scalar_data.value)
@@ -984,20 +987,51 @@ func yaml_emitter_check_simple_key(emitter *yaml_emitter_t) bool {
 		if !yaml_emitter_check_empty_sequence(emitter) {
 			return false
 		}
-		length += len(emitter.anchor_data.anchor) +
+		bLength += len(emitter.anchor_data.anchor) +
 			len(emitter.tag_data.handle) +
 			len(emitter.tag_data.suffix)
 	case yaml_MAPPING_START_EVENT:
 		if !yaml_emitter_check_empty_mapping(emitter) {
 			return false
 		}
-		length += len(emitter.anchor_data.anchor) +
+		bLength += len(emitter.anchor_data.anchor) +
 			len(emitter.tag_data.handle) +
 			len(emitter.tag_data.suffix)
 	default:
 		return false
 	}
-	return length <= 128
+
+	// length represents bytes, not runes.
+	if bLength <= 1024 {
+		// 1024 or less bytes are also 1024 or less runes.
+		// fast majority of cases are handled here.
+		return true
+	}
+	if bLength > 4096 {
+		// 4096 or more bytes are always 1024 or more runes.
+		return false
+	}
+
+	// we must convert and count runes, which is more expensive but less common.
+	rLength := 0
+	switch emitter.events[emitter.events_head].typ {
+	case yaml_ALIAS_EVENT:
+		rLength += utf8.RuneCount(emitter.anchor_data.anchor)
+	case yaml_SCALAR_EVENT:
+		rLength += utf8.RuneCount(emitter.anchor_data.anchor) +
+			utf8.RuneCount(emitter.tag_data.handle) +
+			utf8.RuneCount(emitter.tag_data.suffix) +
+			utf8.RuneCount(emitter.scalar_data.value)
+	case yaml_SEQUENCE_START_EVENT:
+		rLength += utf8.RuneCount(emitter.anchor_data.anchor) +
+			utf8.RuneCount(emitter.tag_data.handle) +
+			utf8.RuneCount(emitter.tag_data.suffix)
+	case yaml_MAPPING_START_EVENT:
+		rLength += utf8.RuneCount(emitter.anchor_data.anchor) +
+			utf8.RuneCount(emitter.tag_data.handle) +
+			utf8.RuneCount(emitter.tag_data.suffix)
+	}
+	return rLength <= 1024
 }
 
 // Determine an acceptable scalar style.
diff --git a/encode_test.go b/encode_test.go
index 4a8bf2e2..cd0cc305 100644
--- a/encode_test.go
+++ b/encode_test.go
@@ -349,6 +349,16 @@ var marshalTests = []struct {
 		"a: \"1:1\"\n",
 	},
 
+	// Issue 849: simple key is <= 1024 runes, not <= 1024 bytes
+	{
+		map[string]string{strings.Repeat("a", 1024): "a"},
+		strings.Repeat("a", 1024) + ": a" + "\n",
+	},
+	{
+		map[string]string{strings.Repeat("你", 1024): "a"},
+		strings.Repeat("你", 1024) + ": a" + "\n",
+	},
+
 	// Binary data.
 	{
 		map[string]string{"a": "\x00"},
diff --git a/go.mod b/go.mod
index f407ea32..67aca269 100644
--- a/go.mod
+++ b/go.mod
@@ -1,5 +1,5 @@
-module "gopkg.in/yaml.v3"
+module gopkg.in/yaml.v3
 
-require (
-	"gopkg.in/check.v1" v0.0.0-20161208181325-20d25e280405
-)
+go 1.23.0
+
+require gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405
diff --git a/go.sum b/go.sum
new file mode 100644
index 00000000..d277f45c
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,2 @@
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=