diff --git a/internal/gateway/anthropic.go b/internal/gateway/anthropic.go
index d7b9750f..fc086b38 100644
--- a/internal/gateway/anthropic.go
+++ b/internal/gateway/anthropic.go
@@ -44,6 +44,22 @@ type aBlock struct {
 	CacheControl map[string]string `json:"cache_control,omitempty"`
 }
 
+// MarshalJSON writes a thinking block's text even when it is empty.
+// Messages requires the field: a signed block with no text (Claude Code's
+// thinking when its display is omitted) sent without it is refused,
+// "messages.N.content.0.thinking.thinking: Field required" (#1447). Every
+// other block keeps its omitempty fields.
+func (b aBlock) MarshalJSON() ([]byte, error) {
+	type plain aBlock
+	if b.Type != "thinking" {
+		return json.Marshal(plain(b))
+	}
+	return json.Marshal(struct {
+		plain
+		Thinking string `json:"thinking"`
+	}{plain(b), b.Thinking})
+}
+
 // ephemeral marks a prompt-cache breakpoint.
 var ephemeral = map[string]string{"type": "ephemeral"}
 
diff --git a/internal/gateway/thinking_block_test.go b/internal/gateway/thinking_block_test.go
new file mode 100644
index 00000000..05529a9d
--- /dev/null
+++ b/internal/gateway/thinking_block_test.go
@@ -0,0 +1,38 @@
+package gateway
+
+import (
+	"encoding/json"
+	"strings"
+	"testing"
+)
+
+// A signed thinking block with no text goes with "thinking": "", the
+// field Messages requires. Without it OpenCode Go's Claude models answer
+// "messages.1.content.0.thinking.thinking: Field required" (#1447).
+func TestSignedThinkingWithNoTextKeepsTheField(t *testing.T) {
+	r := &Request{Messages: []Message{
+		{Role: "user", Parts: []Part{{Kind: Text, Text: "hi"}}},
+		{Role: "assistant", Parts: []Part{{Kind: Thinking, Signature: "abc123"}, {Kind: Text, Text: "hello"}}},
+		{Role: "user", Parts: []Part{{Kind: Text, Text: "go on"}}},
+	}}
+	var out struct {
+		Messages []struct {
+			Content []map[string]any `json:"content"`
+		} `json:"messages"`
+	}
+	body := buildAnthropic(r, "claude-haiku-5-5")
+	if err := json.Unmarshal(body, &out); err != nil {
+		t.Fatal(err)
+	}
+	think := out.Messages[1].Content[0]
+	if v, ok := think["thinking"]; !ok || v != "" || think["signature"] != "abc123" {
+		t.Fatalf("thinking block = %v: a signed block with no text must carry \"thinking\": \"\"", think)
+	}
+	// Other blocks keep leaving their empty fields out.
+	if text := out.Messages[1].Content[1]; len(text) != 2 || text["text"] != "hello" {
+		t.Fatalf("text block = %v", text)
+	}
+	if strings.Count(string(body), `"thinking":`) != 1 {
+		t.Fatalf("only the thinking block names thinking: %s", body)
+	}
+}