diff --git a/docs/context-compact.md b/docs/context-compact.md
index af991d91..86c20a63 100644
--- a/docs/context-compact.md
+++ b/docs/context-compact.md
@@ -47,7 +47,7 @@ context:
 - `auto_compact.enabled`
   控制是否启用基于 token 阈值的自动压缩；默认关闭。
 - `auto_compact.input_token_threshold`
-  当会话累计输入 token 数达到此阈值时触发自动压缩；默认 100000。
+  当会话累计输入 token 数达到此阈值时触发自动压缩；默认 `0`（自动推导），推导失败时回退到 `fallback_input_token_threshold`（默认 `100000`）。
 
 ## 自动压缩
 
diff --git a/docs/guides/configuration.md b/docs/guides/configuration.md
index 4a0191e1..c03f3209 100644
--- a/docs/guides/configuration.md
+++ b/docs/guides/configuration.md
@@ -83,6 +83,8 @@ context:
 | `context.compact.micro_compact_disabled` | 是否关闭默认启用的 micro compact |
 | `context.auto_compact.enabled` | 是否启用自动压缩 |
 | `context.auto_compact.input_token_threshold` | 自动压缩输入 token 阈值 |
+| `context.auto_compact.reserve_tokens` | 自动阈值推导时预留 token 缓冲（`resolved_threshold = context_window - reserve_tokens`） |
+| `context.auto_compact.fallback_input_token_threshold` | 自动推导失败时使用的保底阈值 |
 
 ### `runtime` 字段
 
diff --git a/internal/runtime/run.go b/internal/runtime/run.go
index 93415b36..031eb092 100644
--- a/internal/runtime/run.go
+++ b/internal/runtime/run.go
@@ -359,7 +359,7 @@ func (s *Service) autoCompactThreshold(ctx context.Context, cfg config.Config) i
 	}
 	if s != nil && s.autoCompactThresholdResolver != nil {
 		threshold, err := s.autoCompactThresholdResolver.ResolveAutoCompactThreshold(ctx, cfg)
-		if err == nil {
+		if err == nil && threshold > 0 {
 			return threshold
 		}
 	}
diff --git a/internal/runtime/runtime_test.go b/internal/runtime/runtime_test.go
index 8ad229d7..a67f70de 100644
--- a/internal/runtime/runtime_test.go
+++ b/internal/runtime/runtime_test.go
@@ -4297,6 +4297,58 @@ func TestAutoCompactThresholdFallsBackWhenResolverErrors(t *testing.T) {
 	}
 }
 
+func TestAutoCompactThresholdFallsBackWhenResolverReturnsZeroWithoutError(t *testing.T) {
+	t.Parallel()
+
+	service := &Service{}
+	service.SetAutoCompactThresholdResolver(autoCompactThresholdResolverFunc(
+		func(ctx context.Context, cfg config.Config) (int, error) {
+			return 0, nil
+		},
+	))
+
+	cfg := config.Config{
+		Context: config.ContextConfig{
+			AutoCompact: config.AutoCompactConfig{
+				Enabled:                     true,
+				InputTokenThreshold:         0,
+				FallbackInputTokenThreshold: 88000,
+			},
+		},
+	}
+
+	threshold := service.autoCompactThreshold(context.Background(), cfg)
+	if threshold != 88000 {
+		t.Fatalf("expected fallback threshold == 88000, got %d", threshold)
+	}
+}
+
+func TestAutoCompactThresholdFallsBackWhenResolverReturnsNegativeWithoutError(t *testing.T) {
+	t.Parallel()
+
+	service := &Service{}
+	service.SetAutoCompactThresholdResolver(autoCompactThresholdResolverFunc(
+		func(ctx context.Context, cfg config.Config) (int, error) {
+			return -1, nil
+		},
+	))
+
+	cfg := config.Config{
+		Context: config.ContextConfig{
+			AutoCompact: config.AutoCompactConfig{
+				Enabled:                     true,
+				InputTokenThreshold:         0,
+				FallbackInputTokenThreshold: 88000,
+			},
+		},
+	}
+
+	threshold := service.autoCompactThreshold(context.Background(), cfg)
+	if threshold != 88000 {
+		t.Fatalf("expected fallback threshold == 88000, got %d", threshold)
+	}
+}
+
 func TestAutoCompactThresholdImplicitModeWithoutResolverUsesFallback(t *testing.T) {
 	t.Parallel()