diff --git a/backend/internal/pkg/claude/constants.go b/backend/internal/pkg/claude/constants.go
index ad663bfec4..ee2efcd29f 100644
--- a/backend/internal/pkg/claude/constants.go
+++ b/backend/internal/pkg/claude/constants.go
@@ -20,12 +20,13 @@ const (
 	BetaFastMode                 = "fast-mode-2026-02-01"
 
 	// 新增（对齐官方 CLI 2.1.9x 以来的流量）
-	BetaPromptCachingScope      = "prompt-caching-scope-2026-01-05"
-	BetaEffort                  = "effort-2025-11-24"
-	BetaRedactThinking          = "redact-thinking-2026-02-12"
-	BetaContextManagement       = "context-management-2025-06-27"
-	BetaThinkingBindingControls = "thinking-binding-controls-2026-08-01"
-	BetaExtendedCacheTTL        = "extended-cache-ttl-2025-04-11"
+	BetaPromptCachingScope          = "prompt-caching-scope-2026-01-05"
+	BetaEffort                      = "effort-2025-11-24"
+	BetaRedactThinking              = "redact-thinking-2026-02-12"
+	BetaContextManagement           = "context-management-2025-06-27"
+	BetaThinkingBindingControls     = "thinking-binding-controls-2026-08-01"
+	BetaMidConversationOutputConfig = "mid-conversation-output-config-2026-07-01"
+	BetaExtendedCacheTTL            = "extended-cache-ttl-2025-04-11"
 
 	// server-side refusal fallback beta 字段族（beta Messages API 专有）。
 	// 客户端（Claude Code / SDK / OpenCode 等）会默认透传 body.fallbacks /
@@ -101,6 +102,7 @@ func FullClaudeCodeMimicryBetas() []string {
 		BetaEffort,
 		BetaContextManagement,
 		BetaThinkingBindingControls,
+		BetaMidConversationOutputConfig,
 		BetaExtendedCacheTTL,
 	}
 }
diff --git a/backend/internal/service/gateway_context_management_test.go b/backend/internal/service/gateway_context_management_test.go
index 0e48f2cb91..60dc89489c 100644
--- a/backend/internal/service/gateway_context_management_test.go
+++ b/backend/internal/service/gateway_context_management_test.go
@@ -186,6 +186,17 @@ func TestComputeFinalAnthropicBeta_OAuthMimic_IgnoresClientBeta(t *testing.T) {
 	require.True(t, ok)
 	require.False(t, strings.Contains(final, "custom-experimental-beta"),
 		"mimic 路径必须忽略客户端 anthropic-beta header")
+	require.True(t, anthropicBetaTokensContains(final, claude.BetaMidConversationOutputConfig),
+		"mimic 必须注入 mid-conversation output_config 控制所需的 beta")
+
+	// 显式 dropSet 仍能移除 mimic 注入的该 beta，且不会因此放行客户端未知 beta。
+	dropped, ok := s.computeFinalAnthropicBeta("oauth", true, "claude-sonnet-4-6", hdr, []byte(`{}`),
+		map[string]struct{}{claude.BetaMidConversationOutputConfig: {}})
+	require.True(t, ok)
+	require.False(t, anthropicBetaTokensContains(dropped, claude.BetaMidConversationOutputConfig),
+		"显式 dropSet 必须能移除新增的 mimic beta")
+	require.False(t, strings.Contains(dropped, "custom-experimental-beta"),
+		"dropSet 存在时 mimic 路径仍必须忽略客户端 anthropic-beta header")
 }
 
 func TestComputeFinalAnthropicBeta_OAuthTransparent_NonHaiku_PreservesClientContextManagement(t *testing.T) {
diff --git a/backend/internal/service/gateway_mid_conversation_output_config_test.go b/backend/internal/service/gateway_mid_conversation_output_config_test.go
new file mode 100644
index 0000000000..d551fae785
--- /dev/null
+++ b/backend/internal/service/gateway_mid_conversation_output_config_test.go
@@ -0,0 +1,318 @@
+//go:build unit
+
+package service
+
+import (
+	"bytes"
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/Wei-Shaw/sub2api/internal/config"
+	"github.com/Wei-Shaw/sub2api/internal/pkg/claude"
+	"github.com/gin-gonic/gin"
+	"github.com/stretchr/testify/require"
+	"github.com/tidwall/gjson"
+)
+
+// ============================================================================
+// message-level output_config ↔ mid-conversation-output-config beta
+// ============================================================================
+//
+// 背景：pi-ai（Harness 使用的 Anthropic provider）会为 opus5 生成形如
+//
+//	{"role":"system","content":[],"output_config":{"effort":"high"}}
+//
+// 的空控制消息，并在 anthropic-beta 中请求
+// mid-conversation-output-config-2026-07-01。OAuth mimic 会用
+// FullClaudeCodeMimicryBetas 覆盖客户端 beta；若该 beta 不在固定列表内，
+// body 消息级 output_config 与最终 header 不再对称 → 上游 400：
+//
+//	"output_config: Extra inputs are not permitted"
+//
+// 修复策略（剥字段，不注入 beta）：
+//   - header 缺该 token：仅为携带 message-level output_config 的消息剥此字段；
+//     role=system 且 content 无正文（缺失 / null / 空 string / 空 array / 仅空
+//     text 块）时整条删除；system 有正文保留正文与其余字段；user/assistant 只剥
+//     字段，绝不整条删除。
+//   - header 含该 token：完全保留。
+//   - 无任何 message-level output_config：字节 no-op。
+//   - 顶层 output_config / effort 不受该 beta 约束，本增量不触碰。
+//
+// 本文件只覆盖 Anthropic 直连路径的该增量；header 侧固定列表（mimic betas）
+// 的改动由并行增量负责。
+
+// ============================================================================
+// sanitizeAnthropicBodyForBetaTokens — message-level output_config
+// ============================================================================
+
+// ★ 主场景：缺 beta 时，多条空控制 system 消息整条删除，且用户顺序 / 顶层 effort
+// 不受影响；有正文 system 与 assistant/user 只剥字段。
+func TestSanitizeAnthropicBodyForBetaTokens_MidConversationOutputConfig_MultiMessage(t *testing.T) {
+	body := []byte(`{
+		"model":"claude-opus-5",
+		"output_config":{"effort":"high"},
+		"messages":[
+			{"role":"system","content":[],"output_config":{"effort":"high"}},
+			{"role":"user","content":"hello"},
+			{"role":"system","content":"You are helpful.","output_config":{"effort":"high"},"cache_control":{"type":"ephemeral"}},
+			{"role":"assistant","content":[{"type":"text","text":"hi"}],"output_config":{"effort":"high"}},
+			{"role":"system","content":[{"type":"text","text":""}],"output_config":{"effort":"high"}},
+			{"role":"system","content":null,"output_config":{"effort":"high"}},
+			{"role":"system","content":"","output_config":{"effort":"high"}},
+			{"role":"system","output_config":{"effort":"high"}}
+		]
+	}`)
+
+	// 模拟 OAuth mimic / 默认 API-key beta：无 mid-conversation-output-config beta
+	out, changed := sanitizeAnthropicBodyForBetaTokens(body, "claude-code-20250219,oauth-2025-04-20")
+	require.True(t, changed, "存在 message-level output_config 且缺 beta → 必须发生净化")
+
+	msgs := gjson.GetBytes(out, "messages").Array()
+	require.Len(t, msgs, 3, "空控制 system 整条删除；仅保留 user + 有正文 system + assistant")
+
+	require.Equal(t, "user", gjson.GetBytes(out, "messages.0.role").String())
+	require.Equal(t, "hello", gjson.GetBytes(out, "messages.0.content").String(), "用户消息顺序不得改变")
+
+	require.Equal(t, "system", gjson.GetBytes(out, "messages.1.role").String())
+	require.Equal(t, "You are helpful.", gjson.GetBytes(out, "messages.1.content").String())
+	require.False(t, gjson.GetBytes(out, "messages.1.output_config").Exists(),
+		"有正文 system 只剥 output_config，不整条删除")
+	require.True(t, gjson.GetBytes(out, "messages.1.cache_control").Exists(),
+		"system 其余字段必须保留")
+
+	require.Equal(t, "assistant", gjson.GetBytes(out, "messages.2.role").String())
+	require.Equal(t, "hi", gjson.GetBytes(out, "messages.2.content.0.text").String())
+	require.False(t, gjson.GetBytes(out, "messages.2.output_config").Exists(),
+		"user/assistant 只剥字段，绝不整条删除")
+
+	// 顶层 output_config / effort 不属于该 beta 保护范围，必须原样保留
+	require.Equal(t, "high", gjson.GetBytes(out, "output_config.effort").String())
+	require.Equal(t, "claude-opus-5", gjson.GetBytes(out, "model").String())
+}
+
+// 表驱动：system 消息 content「无正文」的各种形态都必须整条删除。
+func TestSanitizeAnthropicBodyForBetaTokens_MidConversationOutputConfig_EmptySystemVariants(t *testing.T) {
+	cases := []struct {
+		name string
+		msg  string
+	}{
+		{"missing_content", `{"role":"system","output_config":{"effort":"high"}}`},
+		{"null_content", `{"role":"system","content":null,"output_config":{"effort":"high"}}`},
+		{"empty_string_content", `{"role":"system","content":"","output_config":{"effort":"high"}}`},
+		{"empty_array_content", `{"role":"system","content":[],"output_config":{"effort":"high"}}`},
+		{"only_empty_text_block", `{"role":"system","content":[{"type":"text","text":""}],"output_config":{"effort":"high"}}`},
+		{"only_empty_text_blocks", `{"role":"system","content":[{"type":"text","text":""},{"type":"text","text":""}],"output_config":{"effort":"high"}}`},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			body := []byte(`{"messages":[` + tc.msg + `,{"role":"user","content":"hi"}],"output_config":{"effort":"high"}}`)
+			out, changed := sanitizeAnthropicBodyForBetaTokens(body, "oauth-2025-04-20")
+			require.True(t, changed)
+
+			msgs := gjson.GetBytes(out, "messages").Array()
+			require.Len(t, msgs, 1, "无正文的 system 控制消息必须整条删除")
+			require.Equal(t, "user", msgs[0].Get("role").String())
+			require.Equal(t, "hi", msgs[0].Get("content").String())
+			require.Equal(t, "high", gjson.GetBytes(out, "output_config.effort").String(),
+				"顶层 effort 不得被连带修改")
+		})
+	}
+}
+
+// 表驱动：system 有正文（含未来非 text 内容块 / 未知块）时不得整条删除，
+// 只剥 message-level output_config，正文原样保留。
+func TestSanitizeAnthropicBodyForBetaTokens_MidConversationOutputConfig_SystemWithBodyKept(t *testing.T) {
+	cases := []struct {
+		name    string
+		content string
+	}{
+		{"non_empty_string", `"You are helpful."`},
+		{"text_block", `[{"type":"text","text":"hi"}]`},
+		{"text_with_empty_sibling", `[{"type":"text","text":""},{"type":"text","text":"hi"}]`},
+		{"future_non_text_block", `[{"type":"image","source":{"type":"base64","media_type":"image/png","data":"AA=="}}]`},
+		{"unknown_block", `[{"foo":"bar"}]`},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			body := []byte(`{"messages":[{"role":"system","content":` + tc.content + `,"output_config":{"effort":"high"}}]}`)
+			out, changed := sanitizeAnthropicBodyForBetaTokens(body, "oauth-2025-04-20")
+			require.True(t, changed)
+
+			msgs := gjson.GetBytes(out, "messages").Array()
+			require.Len(t, msgs, 1, "有正文 system 不得整条删除")
+			require.Equal(t, "system", msgs[0].Get("role").String())
+			require.False(t, msgs[0].Get("output_config").Exists(),
+				"有正文 system 必须剥掉 message-level output_config")
+			require.Equal(t, tc.content, msgs[0].Get("content").Raw,
+				"正文必须原样保留；非 text / 未知内容块不得被误当空而删除")
+		})
+	}
+}
+
+// header 含该 beta → 完全保留，字节 no-op（即使是空控制 system 消息）。
+func TestSanitizeAnthropicBodyForBetaTokens_MidConversationOutputConfig_ByteNoopWhenBetaPresent(t *testing.T) {
+	body := []byte(`{"model":"claude-opus-5","output_config":{"effort":"high"},"messages":[{"role":"system","content":[],"output_config":{"effort":"high"}},{"role":"user","content":"hi"}]}`)
+	out, changed := sanitizeAnthropicBodyForBetaTokens(body,
+		"claude-code-20250219,oauth-2025-04-20,"+claude.BetaMidConversationOutputConfig)
+	require.False(t, changed, "header 含 mid-conversation-output-config beta → 完全保留")
+	require.True(t, bytes.Equal(body, out), "含 beta 时必须字节 no-op")
+}
+
+// 无 message-level output_config（仅有顶层 output_config，或完全没有）→ 字节 no-op。
+func TestSanitizeAnthropicBodyForBetaTokens_MidConversationOutputConfig_NoFieldByteNoop(t *testing.T) {
+	bodies := [][]byte{
+		// 只有顶层 output_config（effort），消息无该字段
+		[]byte(`{"model":"claude-opus-5","output_config":{"effort":"high"},"messages":[{"role":"system","content":[]},{"role":"user","content":"hi"}]}`),
+		// 完全没有 output_config
+		[]byte(`{"model":"claude-opus-5","messages":[{"role":"user","content":"hi"}]}`),
+	}
+	for i, body := range bodies {
+		out, changed := sanitizeAnthropicBodyForBetaTokens(body, "oauth-2025-04-20")
+		require.Falsef(t, changed, "case %d: 无 message-level output_config → 不得改动", i)
+		require.Truef(t, bytes.Equal(body, out), "case %d: 必须字节 no-op", i)
+	}
+}
+
+// 幂等：净化后的 body 再跑一次必须是 no-op。
+func TestSanitizeAnthropicBodyForBetaTokens_MidConversationOutputConfig_Idempotent(t *testing.T) {
+	body := []byte(`{"model":"claude-opus-5","output_config":{"effort":"high"},"messages":[` +
+		`{"role":"system","content":[],"output_config":{"effort":"high"}},` +
+		`{"role":"system","content":"sys","output_config":{"effort":"high"}},` +
+		`{"role":"user","content":"hi"}]}`)
+
+	first, changed := sanitizeAnthropicBodyForBetaTokens(body, "oauth-2025-04-20")
+	require.True(t, changed)
+
+	second, changedAgain := sanitizeAnthropicBodyForBetaTokens(first, "oauth-2025-04-20")
+	require.False(t, changedAgain, "二次净化必须为 no-op（幂等）")
+	require.True(t, bytes.Equal(first, second))
+}
+
+// ============================================================================
+// 真实 request 联动
+// ============================================================================
+
+// OAuth mimic 路径真实 request 联动，两 case 明确期望：
+//   - 默认：mimic 固定列表带该 beta → outgoing header 含 token，空控制 system 消息保留；
+//   - policy filter 命中（经 gin context 的 betaPolicyFilterSetKey 缓存注入该 token，
+//     走真实 policy filter/dropSet 路径）→ outgoing header 无 token，空控制 system
+//     消息整条删除。
+//
+// 两 case 都断言 user 文本、消息数、顶层 effort 原值；期望为显式常量，不引用
+// FullClaudeCodeMimicryBetas（避免把实现列表当 expected）。
+func TestBuildUpstreamRequestOAuthMimic_MidConversationOutputConfig(t *testing.T) {
+	cases := []struct {
+		name              string
+		policyFilterDrops bool
+		wantHeaderHasBeta bool
+		wantMsgLen        int
+		wantFieldOnFirst  bool
+	}{
+		{"default_mimic_keeps_beta", false, true, 2, true},
+		{"policy_filter_drops_beta", true, false, 1, false},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			gin.SetMode(gin.TestMode)
+			rec := httptest.NewRecorder()
+			c, _ := gin.CreateTestContext(rec)
+			c.Request = httptest.NewRequest(http.MethodPost, "/v1/messages", nil)
+			if tc.policyFilterDrops {
+				c.Set(betaPolicyFilterSetKey, map[string]struct{}{claude.BetaMidConversationOutputConfig: {}})
+			}
+
+			account := &Account{ID: 701, Platform: PlatformAnthropic, Type: AccountTypeOAuth,
+				Credentials: map[string]any{"access_token": "oauth-tok"},
+				Status:      StatusActive,
+				Schedulable: true,
+			}
+			body := []byte(`{"model":"claude-opus-5","output_config":{"effort":"high"},"messages":[` +
+				`{"role":"system","content":[],"output_config":{"effort":"high"}},` +
+				`{"role":"user","content":"hello"}]}`)
+
+			svc := &GatewayService{cfg: &config.Config{}}
+			req, _, err := svc.buildUpstreamRequest(
+				context.Background(), c, account, body,
+				"oauth-tok", "oauth", "claude-opus-5", false, true, // mimicClaudeCode=true
+			)
+			require.NoError(t, err)
+
+			outBody := readUpstreamBodyForTest(t, req)
+			outBeta := getHeaderRaw(req.Header, "anthropic-beta")
+			require.Equalf(t, tc.wantHeaderHasBeta,
+				anthropicBetaTokensContains(outBeta, claude.BetaMidConversationOutputConfig),
+				"outgoing anthropic-beta 必须与 filter 结果一致（outgoing beta=%q）", outBeta)
+
+			msgs := gjson.GetBytes(outBody, "messages").Array()
+			require.Len(t, msgs, tc.wantMsgLen,
+				"空控制 system 消息：header 带 beta 保留 / 缺 beta 整条删除")
+			require.Equal(t, tc.wantFieldOnFirst, msgs[0].Get("output_config").Exists())
+
+			var userContent string
+			for _, m := range msgs {
+				if m.Get("role").String() == "user" {
+					userContent = m.Get("content").String()
+				}
+			}
+			require.Equal(t, "hello", userContent, "用户消息文本必须始终保留")
+
+			require.Equal(t, "high", gjson.GetBytes(outBody, "output_config.effort").String(),
+				"顶层 effort 不受该 beta 约束")
+		})
+	}
+}
+
+// API-key passthrough 透传路径：客户端 header 带/不带该 beta 时，
+// 出站 header 与 body 的 message-level output_config 必须同进同退。
+func TestBuildUpstreamRequestAnthropicAPIKeyPassthrough_MidConversationOutputConfigConsistentWithClientHeader(t *testing.T) {
+	cases := []struct {
+		name       string
+		clientBeta string
+		wantField  bool
+		wantMsgLen int
+	}{
+		{
+			name:       "client_header_has_beta",
+			clientBeta: "oauth-2025-04-20," + claude.BetaMidConversationOutputConfig,
+			wantField:  true,
+			wantMsgLen: 2,
+		},
+		{
+			name:       "client_header_missing_beta",
+			clientBeta: "oauth-2025-04-20",
+			wantField:  false,
+			wantMsgLen: 1,
+		},
+	}
+	for _, tc := range cases {
+		t.Run(tc.name, func(t *testing.T) {
+			gin.SetMode(gin.TestMode)
+			rec := httptest.NewRecorder()
+			c, _ := gin.CreateTestContext(rec)
+			c.Request = httptest.NewRequest(http.MethodPost, "/v1/messages", nil)
+			c.Request.Header.Set("Anthropic-Beta", tc.clientBeta)
+
+			body := []byte(`{"model":"claude-opus-5","output_config":{"effort":"high"},"messages":[` +
+				`{"role":"system","content":[],"output_config":{"effort":"high"}},` +
+				`{"role":"user","content":"hello"}]}`)
+
+			svc := &GatewayService{cfg: &config.Config{}}
+			req, _, err := svc.buildUpstreamRequestAnthropicAPIKeyPassthrough(
+				context.Background(), c, newAnthropicAPIKeyPassthroughAccountForBetaTest(), body, "token",
+			)
+			require.NoError(t, err)
+
+			outBody := readUpstreamBodyForTest(t, req)
+			outBeta := getHeaderRaw(req.Header, "anthropic-beta")
+
+			require.Equalf(t, tc.wantField, anthropicBetaTokensContains(outBeta, claude.BetaMidConversationOutputConfig),
+				"出站 header 必须与客户端传入的 beta 一致（outgoing beta=%q）", outBeta)
+			require.Equal(t, tc.wantField, gjson.GetBytes(outBody, "messages.0.output_config").Exists(),
+				"header/body 必须一致")
+			require.Len(t, gjson.GetBytes(outBody, "messages").Array(), tc.wantMsgLen,
+				"缺 beta 时空控制 system 消息整条删除；带 beta 时保留")
+		})
+	}
+}
diff --git a/backend/internal/service/gateway_request.go b/backend/internal/service/gateway_request.go
index ab12f4ad22..a099f99463 100644
--- a/backend/internal/service/gateway_request.go
+++ b/backend/internal/service/gateway_request.go
@@ -918,6 +918,16 @@ const anthropicBetaContextManagementToken = "context-management-2025-06-27"
 //   - 缺 token 时上游拒收：
 //     "thinking.adaptive.block_binding: Extra inputs are not permitted"
 //
+// message-level output_config 场景：
+//   - pi-ai（Harness 使用的 Anthropic provider）会为 opus5 生成形如
+//     `{"role":"system","content":[],"output_config":{"effort":"high"}}` 的控制消息，
+//     并请求 `mid-conversation-output-config-2026-07-01` beta
+//   - 该 output_config 是 **message 级**字段，只有该 beta 保护；顶层 output_config/effort
+//     不受它约束
+//   - OAuth mimic 用 FullClaudeCodeMimicryBetas 覆盖客户端 beta；固定列表漏该 beta 时
+//     body 字段与 header 不对称 → 上游报 "output_config: Extra inputs are not permitted"
+//   - 缺 token 时净化消息级 output_config（详见 stripAnthropicMessageOutputConfigUnlessBeta）
+//
 // 本函数按最终发送的 anthropic-beta header 决定是否保留 body 中的上述字段：
 // 缺对应 beta token → strip；客户端 header 已带对应 beta → 保留（不过度删除）。
 // 这将限制完全建立在 "能力维度" 上，与 model 名 / token type / mimicry 子路径无关。
@@ -964,6 +974,12 @@ func sanitizeAnthropicBodyForBetaTokens(body []byte, anthropicBetaHeader string)
 		body, changed = b, true
 	}
 
+	// messages[].output_config：mid-conversation-output-config beta 专属字段。
+	// 顶层 output_config / effort 不受该 beta 约束，本分支只净化消息内字段。
+	if b, deleted := stripAnthropicMessageOutputConfigUnlessBeta(body, anthropicBetaHeader); deleted {
+		body, changed = b, true
+	}
+
 	return body, changed
 }
 
@@ -1006,6 +1022,106 @@ func anthropicBetaTokensContains(header, token string) bool {
 	return false
 }
 
+// stripAnthropicMessageOutputConfigUnlessBeta 在 anthropic-beta header 缺
+// mid-conversation-output-config beta 时，净化 **messages[].output_config**：
+//   - 仅为携带 message-level output_config 的消息剥该字段；
+//   - 若该消息 role=system 且 content 无正文（缺失 / null / 空 string / 空 array /
+//     仅空 text 块），整条删除（pi-ai 为 opus5 生成的空 system 控制消息即此形态）；
+//   - system 有正文则保留正文与其余字段；user/assistant 只剥字段，绝不整条删除；
+//   - 无任何消息携带该字段时返回原 body（字节 no-op）。
+//
+// header 含该 beta 时完全保留。顶层 output_config / effort 不属于该 beta 保护范围，
+// 本函数不做任何处理。多条删除用「稳健重建」实现，保留其余字段与消息先后顺序。
+func stripAnthropicMessageOutputConfigUnlessBeta(body []byte, anthropicBetaHeader string) ([]byte, bool) {
+	if anthropicBetaTokensContains(anthropicBetaHeader, claude.BetaMidConversationOutputConfig) {
+		return body, false
+	}
+	// 快速路径：body 中不含 output_config 字面量时无需解析。
+	if !bytes.Contains(body, []byte("output_config")) {
+		return body, false
+	}
+	msgsRes := gjson.GetBytes(body, "messages")
+	if !msgsRes.Exists() || !msgsRes.IsArray() {
+		return body, false
+	}
+
+	hasMessageOutputConfig := false
+	for _, msg := range msgsRes.Array() {
+		if msg.Get("output_config").Exists() {
+			hasMessageOutputConfig = true
+			break
+		}
+	}
+	if !hasMessageOutputConfig {
+		return body, false
+	}
+
+	var messages []json.RawMessage
+	if err := json.Unmarshal([]byte(msgsRes.Raw), &messages); err != nil {
+		// gjson 的 IsArray 只做形态判断、不保证 JSON 完整合法；此分支保守返回原 body。
+		return body, false
+	}
+
+	changed := false
+	rebuilt := make([]json.RawMessage, 0, len(messages))
+	for _, msg := range messages {
+		if !gjson.GetBytes(msg, "output_config").Exists() {
+			rebuilt = append(rebuilt, msg)
+			continue
+		}
+		changed = true
+
+		// 空正文的 system 控制消息整条删除；其余消息只剥字段。
+		if gjson.GetBytes(msg, "role").String() == "system" &&
+			!anthropicMessageContentHasBody(gjson.GetBytes(msg, "content")) {
+			continue
+		}
+		stripped, err := sjson.DeleteBytes(msg, "output_config")
+		if err != nil {
+			// 不应发生：字段存在且 msg 是合法 JSON。保守整条保留，不产出半成品。
+			rebuilt = append(rebuilt, msg)
+			continue
+		}
+		rebuilt = append(rebuilt, json.RawMessage(stripped))
+	}
+	if !changed {
+		return body, false
+	}
+
+	rebuiltBytes, err := json.Marshal(rebuilt)
+	if err != nil {
+		return body, false
+	}
+	out, err := sjson.SetRawBytes(body, "messages", rebuiltBytes)
+	if err != nil {
+		return body, false
+	}
+	return out, true
+}
+
+// anthropicMessageContentHasBody 判断单条消息的 content 是否携带正文。
+// 返回 false 表示「无正文」：content 缺失 / null / 空 string / 空 array /
+// 仅由空 text 块构成。未知或非 text 内容块（image / tool_use / tool_result 等）
+// 一律视为有正文，避免把未来新增内容块误判为空而连带删除整条 system 消息。
+func anthropicMessageContentHasBody(content gjson.Result) bool {
+	switch {
+	case !content.Exists():
+		return false
+	case content.Type == gjson.String:
+		return content.String() != ""
+	case content.IsArray():
+		var blocks []any
+		if err := json.Unmarshal([]byte(content.Raw), &blocks); err != nil {
+			return true // 无法解析时保守视为有正文
+		}
+		cleaned, _ := stripEmptyTextBlocksFromSlice(blocks)
+		return len(cleaned) > 0
+	default:
+		// null 视为无正文；object / number / bool 等非标准形态保守保留。
+		return content.Type != gjson.Null
+	}
+}
+
 // FilterSignatureSensitiveBlocksForRetry is a stronger retry filter for cases where upstream errors indicate
 // signature/thought_signature validation issues involving tool blocks.
 //