diff --git a/internal/modelgateway/anthropic.go b/internal/modelgateway/anthropic.go
index 06e9f0c51..dfa634380 100644
--- a/internal/modelgateway/anthropic.go
+++ b/internal/modelgateway/anthropic.go
@@ -10,8 +10,9 @@ import (
 
 const anthropicVersion = "2023-06-01"
 
-// These protocol capabilities were observed with the pinned Claude 2.1.303 CLI and, for
-// Claude models over Anthropic Messages, the pinned OpenCode 1.19.3 CLI. A CLI upgrade must
+// These protocol capabilities were observed with the pinned Claude 2.1.303 CLI, including when
+// it is started through Agent SDK 0.3.303 / ACP adapter 0.94.0, and, for Claude models over
+// Anthropic Messages, the pinned OpenCode 1.19.3 CLI. A CLI, SDK or adapter upgrade must
 // exercise its requests before extending this set.
 var claudeBetaHeaders = map[string]bool{
 	"claude-code-20250219":                     true,
diff --git a/internal/modelgateway/anthropic_test.go b/internal/modelgateway/anthropic_test.go
index a0367fd5e..a1252487e 100644
--- a/internal/modelgateway/anthropic_test.go
+++ b/internal/modelgateway/anthropic_test.go
@@ -16,12 +16,16 @@ import (
 
 const pinnedClaudeBetas = "claude-code-20250219,interleaved-thinking-2025-05-14,thinking-token-count-2026-05-13,context-management-2025-06-27,prompt-caching-scope-2026-01-05,mid-conversation-tool-changes-2026-07-01,effort-2025-11-24"
 
+// The first request the bundled Claude Code 2.1.303 sent for claude-haiku-5-5 when run directly
+// with the adapter's alias and model environment, so it carries no Agent SDK additions.
+const pinnedClaudeHaikuBetas = "interleaved-thinking-2025-05-14,thinking-token-count-2026-05-13,context-management-2025-06-27,prompt-caching-scope-2026-01-05,claude-code-20250219,per-turn-control-2026-07-01,mid-conversation-tool-changes-2026-07-01,effort-2025-11-24"
+
 // Each pinned CLI's exact first-request header for a Claude model is forwarded unchanged.
 func TestPinnedCLIClaudeModelBetas(t *testing.T) {
 	for name, betas := range map[string]string{
 		// Claude Code 2.1.303 run directly with the adapter's alias and model environment, with
 		// claude-haiku-5-5; the Agent SDK's own additions are covered separately below.
-		"claude": "interleaved-thinking-2025-05-14,thinking-token-count-2026-05-13,context-management-2025-06-27,prompt-caching-scope-2026-01-05,claude-code-20250219,per-turn-control-2026-07-01,mid-conversation-tool-changes-2026-07-01,effort-2025-11-24",
+		"claude": pinnedClaudeHaikuBetas,
 		// OpenCode 1.19.3 through @ai-sdk/anthropic, with claude-haiku-5-5.
 		"opencode": "thinking-binding-controls-2026-08-01,structured-outputs-2025-11-13",
 	} {
@@ -73,6 +77,33 @@ func TestPinnedClaudeAgentSDKCapabilities(t *testing.T) {
 	}
 }
 
+// The documented production combination: the same CLI started through Agent SDK 0.3.303 / ACP
+// adapter 0.94.0 for claude-haiku-5-5 declares the direct run's betas plus the SDK's additions.
+// The header is composed from those two captured sets rather than captured end to end, so this
+// pins that the gateway accepts and forwards the union unchanged, not the client's header order.
+func TestPinnedClaudeACPHaikuBetaCombination(t *testing.T) {
+	betas := pinnedClaudeHaikuBetas + ",structured-outputs-2025-12-15,thinking-display-updates-2026-08-18"
+	var calls atomic.Int64
+	service, _ := anthropicFixture(t, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		calls.Add(1)
+		if r.Header.Get("Anthropic-Beta") != betas || r.URL.RawQuery != "beta=true" {
+			t.Error("ACP and haiku betas changed while forwarding")
+		}
+		w.Header().Set("Content-Type", "application/json")
+		_, _ = io.WriteString(w, "{\"id\":\"m\",\"type\":\"message\",\"role\":\"assistant\",\"model\":\"fixture-model\",\"content\":[{\"type\":\"text\",\"text\":\"{}\"}],\"stop_reason\":\"end_turn\",\"usage\":{\"input_tokens\":3,\"output_tokens\":1}}")
+	}))
+	request := httptest.NewRequest(http.MethodPost, "/runtime/anthropic/v1/messages?beta=true", strings.NewReader("{\"model\":\"fixture-model\",\"messages\":[],\"max_tokens\":16}"))
+	request.Header.Set("Authorization", "Bearer "+strings.Repeat("r", 40))
+	request.Header.Set("Content-Type", "application/json")
+	request.Header.Set("Anthropic-Version", anthropicVersion)
+	request.Header.Set("Anthropic-Beta", betas)
+	response := httptest.NewRecorder()
+	service.ModelHandler(response, request)
+	if response.Code != 200 || calls.Load() != 1 {
+		t.Fatalf("ACP haiku betas rejected: status=%d calls=%d", response.Code, calls.Load())
+	}
+}
+
 // anthropicFixture keeps the upstream key separate from the workload's short-lived token.
 func anthropicFixture(t *testing.T, handler http.Handler) (*Service, *fakePolicy) {
 	t.Helper()