diff --git a/crates/op-orchestrator/src/concurrent_tests.rs b/crates/op-orchestrator/src/concurrent_tests.rs
index 89aa3284b..d7ffb2a0a 100644
--- a/crates/op-orchestrator/src/concurrent_tests.rs
+++ b/crates/op-orchestrator/src/concurrent_tests.rs
@@ -34,6 +34,7 @@ fn subtask_with_screen(id: &str, screen: Option<&str>) -> Subtask {
         elements: None,
         screen: screen.map(|s| s.to_string()),
         generated_root_id: None,
+        existing_section_labels: None,
     }
 }
 
@@ -64,6 +65,7 @@ fn make_plan_with_subtasks(subtask_ids: &[&str]) -> crate::plan::OrchestratorPla
                 elements: None,
                 screen: None,
                 generated_root_id: None,
+                existing_section_labels: None,
             })
             .collect(),
         style_guide_name: None,
@@ -77,6 +79,7 @@ fn make_req() -> crate::types::DesignRequest {
         provider: None,
         design_md: None,
         concurrency: 2,
+        append_context: None,
     }
 }
 
diff --git a/crates/op-orchestrator/src/concurrent_tests_b2.rs b/crates/op-orchestrator/src/concurrent_tests_b2.rs
index 6c81478e1..34f25dd47 100644
--- a/crates/op-orchestrator/src/concurrent_tests_b2.rs
+++ b/crates/op-orchestrator/src/concurrent_tests_b2.rs
@@ -21,6 +21,7 @@ fn make_req() -> crate::types::DesignRequest {
         provider: None,
         design_md: None,
         concurrency: 2,
+        append_context: None,
     }
 }
 
@@ -51,6 +52,7 @@ fn make_two_screen_plan() -> crate::plan::OrchestratorPlan {
                 elements: None,
                 screen: Some("login".into()),
                 generated_root_id: None,
+                existing_section_labels: None,
             },
             crate::plan::Subtask {
                 id: "s1".into(),
@@ -64,6 +66,7 @@ fn make_two_screen_plan() -> crate::plan::OrchestratorPlan {
                 elements: None,
                 screen: Some("home".into()),
                 generated_root_id: None,
+                existing_section_labels: None,
             },
         ],
         style_guide_name: None,
@@ -86,6 +89,7 @@ fn make_three_screen_plan() -> crate::plan::OrchestratorPlan {
         elements: None,
         screen: Some(screen.into()),
         generated_root_id: None,
+        existing_section_labels: None,
     };
     OrchestratorPlan {
         root_frame: RootFrameSpec {
diff --git a/crates/op-orchestrator/src/dashboard_columns_tests.rs b/crates/op-orchestrator/src/dashboard_columns_tests.rs
index 0c97bba76..5348f11af 100644
--- a/crates/op-orchestrator/src/dashboard_columns_tests.rs
+++ b/crates/op-orchestrator/src/dashboard_columns_tests.rs
@@ -32,6 +32,7 @@ fn subtask(id: &str, label: &str, elements: Option<&str>) -> Subtask {
         elements: elements.map(String::from),
         screen: None,
         generated_root_id: None,
+        existing_section_labels: None,
     }
 }
 
diff --git a/crates/op-orchestrator/src/dashboard_columns_tests_b.rs b/crates/op-orchestrator/src/dashboard_columns_tests_b.rs
index 1c4ceef92..e0fbf43f5 100644
--- a/crates/op-orchestrator/src/dashboard_columns_tests_b.rs
+++ b/crates/op-orchestrator/src/dashboard_columns_tests_b.rs
@@ -33,6 +33,7 @@ fn subtask(id: &str, label: &str, elements: Option<&str>) -> Subtask {
         elements: elements.map(String::from),
         screen: None,
         generated_root_id: None,
+        existing_section_labels: None,
     }
 }
 
diff --git a/crates/op-orchestrator/src/dashboard_columns_tests_b2.rs b/crates/op-orchestrator/src/dashboard_columns_tests_b2.rs
index c287b5b1d..c9ac231e6 100644
--- a/crates/op-orchestrator/src/dashboard_columns_tests_b2.rs
+++ b/crates/op-orchestrator/src/dashboard_columns_tests_b2.rs
@@ -34,6 +34,7 @@ fn subtask(id: &str, label: &str, elements: Option<&str>) -> Subtask {
         elements: elements.map(String::from),
         screen: None,
         generated_root_id: None,
+        existing_section_labels: None,
     }
 }
 
diff --git a/crates/op-orchestrator/src/dashboard_columns_tests_b3.rs b/crates/op-orchestrator/src/dashboard_columns_tests_b3.rs
index b5c2bd7ab..f9ba15ae7 100644
--- a/crates/op-orchestrator/src/dashboard_columns_tests_b3.rs
+++ b/crates/op-orchestrator/src/dashboard_columns_tests_b3.rs
@@ -33,6 +33,7 @@ fn st(id: &str, label: &str, width: f64, height: f64) -> Subtask {
         elements: None,
         screen: None,
         generated_root_id: None,
+        existing_section_labels: None,
     }
 }
 
@@ -49,6 +50,7 @@ fn sidebar_st(height: f64) -> Subtask {
         elements: Some("nav links".into()),
         screen: None,
         generated_root_id: None,
+        existing_section_labels: None,
     }
 }
 
diff --git a/crates/op-orchestrator/src/plan.rs b/crates/op-orchestrator/src/plan.rs
index 6646df674..5ebc4d9c4 100644
--- a/crates/op-orchestrator/src/plan.rs
+++ b/crates/op-orchestrator/src/plan.rs
@@ -5,11 +5,11 @@
 //! `buildFallbackPlanFromPrompt`)。
 
 use crate::types::DesignRequest;
-use serde::Deserialize;
+use serde::{Deserialize, Serialize};
 
 /// 根 frame 的一个 fill —— 对齐 TS canonical `[{type,color}]`。
 /// 规划只需 solid 色;非 solid 项保留 `color` 字段(可能为空)。
-#[derive(Debug, Clone, PartialEq, Deserialize)]
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub struct PlanFill {
     #[serde(rename = "type", default)]
     pub kind: String,
@@ -18,14 +18,14 @@ pub struct PlanFill {
 }
 
 /// 一个 subtask 区域的尺寸。
-#[derive(Debug, Clone, PartialEq, Deserialize)]
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub struct Region {
     pub width: f64,
     pub height: f64,
 }
 
 /// 根 frame 规格。字段对齐规划语料 `decomposition.md` 的 `rootFrame`。
-#[derive(Debug, Clone, PartialEq, Deserialize)]
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub struct RootFrameSpec {
     pub id: String,
     pub name: String,
@@ -54,7 +54,8 @@ impl RootFrameSpec {
 }
 
 /// 一个生成子任务 —— 对应设计里的一个区块。
-#[derive(Debug, Clone, PartialEq, Deserialize)]
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
 pub struct Subtask {
     pub id: String,
     pub label: String,
@@ -75,10 +76,14 @@ pub struct Subtask {
     /// `SubTask.generatedRootId`。仪器由 run.rs 填写;规划阶段为 None。
     #[serde(skip)]
     pub generated_root_id: Option<String>,
+    /// 从 AppendContext 传播而来 —— 告知 sub-agent 哪些已有区块不要重复。
+    /// port of TS `SubTask.existingSectionLabels` (`ai-types.ts:134`)。
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub existing_section_labels: Option<Vec<String>>,
 }
 
 /// 规划阶段的完整产物。字段对齐规划语料 `decomposition.md`。
-#[derive(Debug, Clone, PartialEq, Deserialize)]
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
 pub struct OrchestratorPlan {
     #[serde(rename = "rootFrame")]
     pub root_frame: RootFrameSpec,
@@ -177,6 +182,7 @@ pub fn build_fallback_plan(req: &DesignRequest) -> OrchestratorPlan {
                 elements: None,
                 screen: None,
                 generated_root_id: None,
+                existing_section_labels: None,
             }
         })
         .collect();
@@ -211,6 +217,7 @@ mod tests {
             provider: None,
             design_md: None,
             concurrency: 1,
+            append_context: None,
         }
     }
 
@@ -265,4 +272,72 @@ mod tests {
         assert_eq!(long.subtasks.len(), 3);
         assert!(!long.subtasks.is_empty());
     }
+
+    // ── Task A1: Subtask.existing_section_labels ──────────────────────────────
+
+    /// Subtask accepts existing_section_labels: None without breaking compilation.
+    #[test]
+    fn subtask_existing_section_labels_none_compiles() {
+        let st = Subtask {
+            id: "hero".into(),
+            label: "Hero".into(),
+            region: Region {
+                width: 1200.0,
+                height: 400.0,
+            },
+            id_prefix: "hero".into(),
+            parent_frame_id: None,
+            elements: None,
+            screen: None,
+            generated_root_id: None,
+            existing_section_labels: None,
+        };
+        assert!(st.existing_section_labels.is_none());
+    }
+
+    /// Subtask accepts a populated existing_section_labels vec.
+    #[test]
+    fn subtask_existing_section_labels_some_compiles() {
+        let st = Subtask {
+            id: "features".into(),
+            label: "Features".into(),
+            region: Region {
+                width: 1200.0,
+                height: 400.0,
+            },
+            id_prefix: "features".into(),
+            parent_frame_id: None,
+            elements: None,
+            screen: None,
+            generated_root_id: None,
+            existing_section_labels: Some(vec!["Hero".into(), "About".into()]),
+        };
+        let labels = st.existing_section_labels.as_ref().unwrap();
+        assert_eq!(labels[0], "Hero");
+        assert_eq!(labels[1], "About");
+    }
+
+    /// Subtask with existing_section_labels = Some([]) serializes without the field.
+    #[test]
+    fn subtask_existing_section_labels_omitted_when_none() {
+        let st = Subtask {
+            id: "hero".into(),
+            label: "Hero".into(),
+            region: Region {
+                width: 1200.0,
+                height: 400.0,
+            },
+            id_prefix: "hero".into(),
+            parent_frame_id: None,
+            elements: None,
+            screen: None,
+            generated_root_id: None,
+            existing_section_labels: None,
+        };
+        let json = serde_json::to_string(&st).expect("serialize");
+        assert!(
+            !json.contains("existingSectionLabels"),
+            "existingSectionLabels should be omitted when None, got: {json}"
+        );
+    }
 }
diff --git a/crates/op-orchestrator/src/plan_normalize.rs b/crates/op-orchestrator/src/plan_normalize.rs
index a447c7bfc..8a990fd63 100644
--- a/crates/op-orchestrator/src/plan_normalize.rs
+++ b/crates/op-orchestrator/src/plan_normalize.rs
@@ -82,6 +82,7 @@ mod tests {
             provider: None,
             design_md: None,
             concurrency: 1,
+            append_context: None,
         }
     }
 
@@ -98,6 +99,7 @@ mod tests {
             elements: None,
             screen: None,
             generated_root_id: None,
+            existing_section_labels: None,
         }
     }
 
@@ -172,6 +174,7 @@ mod tests {
             provider: None,
             design_md: None,
             concurrency: 1,
+            append_context: None,
         }
     }
 
@@ -189,6 +192,7 @@ mod tests {
             elements: None,
             screen: None,
             generated_root_id: None,
+            existing_section_labels: None,
         };
         // chart subtask — LLM-provided height 300, within [inferred*0.6, inferred*1.6]
         // inferred for "chart" = 320  →  min=192, max=512  →  300 in range → keep 300
@@ -204,6 +208,7 @@ mod tests {
             elements: None,
             screen: None,
             generated_root_id: None,
+            existing_section_labels: None,
         };
         // metric subtask — LLM-provided height 0 (invalid) → use inferred = 160
         let st_metric = Subtask {
@@ -218,6 +223,7 @@ mod tests {
             elements: None,
             screen: None,
             generated_root_id: None,
+            existing_section_labels: None,
         };
         OrchestratorPlan {
             root_frame: RootFrameSpec {
@@ -317,6 +323,7 @@ mod tests {
                 elements: None,
                 screen: None,
                 generated_root_id: None,
+                existing_section_labels: None,
             }],
             style_guide_name: None,
         };
@@ -356,6 +363,7 @@ mod tests {
                 elements: None,
                 screen: None,
                 generated_root_id: None,
+                existing_section_labels: None,
             }],
             style_guide_name: None,
         };
diff --git a/crates/op-orchestrator/src/plan_repair.rs b/crates/op-orchestrator/src/plan_repair.rs
index f60d854ab..d823d6c46 100644
--- a/crates/op-orchestrator/src/plan_repair.rs
+++ b/crates/op-orchestrator/src/plan_repair.rs
@@ -359,6 +359,7 @@ fn coerce_subtask(
             elements: None,
             screen: None,
             generated_root_id: None,
+            existing_section_labels: None,
         });
     }
 
@@ -410,6 +411,7 @@ fn coerce_subtask(
         elements,
         screen,
         generated_root_id: None,
+        existing_section_labels: None,
     })
 }
 
diff --git a/crates/op-orchestrator/src/plan_repair_tests.rs b/crates/op-orchestrator/src/plan_repair_tests.rs
index 73a38a2d9..7c2829ed3 100644
--- a/crates/op-orchestrator/src/plan_repair_tests.rs
+++ b/crates/op-orchestrator/src/plan_repair_tests.rs
@@ -243,6 +243,7 @@ fn req(prompt: &str) -> DesignRequest {
         provider: None,
         design_md: None,
         concurrency: 1,
+        append_context: None,
     }
 }
 
@@ -266,6 +267,7 @@ fn req_with_design_md(prompt: &str) -> DesignRequest {
             generation_notes: None,
         }),
         concurrency: 1,
+        append_context: None,
     }
 }
 
diff --git a/crates/op-orchestrator/src/prompt.rs b/crates/op-orchestrator/src/prompt.rs
index e3efbee0d..055180068 100644
--- a/crates/op-orchestrator/src/prompt.rs
+++ b/crates/op-orchestrator/src/prompt.rs
@@ -230,6 +230,7 @@ mod tests {
             provider: None,
             design_md: None,
             concurrency: 1,
+            append_context: None,
         }
     }
 
@@ -310,6 +311,7 @@ mod tests {
             elements: None,
             screen: None,
             generated_root_id: None,
+            existing_section_labels: None,
         };
         let cr = build_subagent_prompt(&st, &plan(), &req(), AbortFlag::new(), false, false);
         assert!(cr.user_prompt.contains("Hero"));
@@ -331,6 +333,7 @@ mod tests {
             elements: None,
             screen: None,
             generated_root_id: None,
+            existing_section_labels: None,
         };
         // minimal_skills=true: the system prompt should contain "schema" skill
         // content and "jsonl-format" skill content, but NOT layout/text-rules etc.
@@ -362,6 +365,7 @@ mod tests {
             elements: None,
             screen: None,
             generated_root_id: None,
+            existing_section_labels: None,
         };
         // req() uses model "claude" which is Full tier — no narrowing.
         // Use a basic-tier model to test narrowing.
@@ -371,6 +375,7 @@ mod tests {
             provider: None,
             design_md: None,
             concurrency: 1,
+            append_context: None,
         };
         let full_cr =
             build_subagent_prompt(&st, &plan(), &basic_req, AbortFlag::new(), false, false);
@@ -396,6 +401,7 @@ mod tests {
             elements: None,
             screen: None,
             generated_root_id: None,
+            existing_section_labels: None,
         };
         // req() uses "claude" which maps to Full tier → reduced_complexity is no-op
         let full_cr = build_subagent_prompt(&st, &plan(), &req(), AbortFlag::new(), false, false);
@@ -458,6 +464,7 @@ mod tests {
             provider: None,
             design_md: None,
             concurrency: 1,
+            append_context: None,
         };
         let long_prompt = "x".repeat(5000); // >= 4200 chars
         let long_req = DesignRequest {
@@ -466,6 +473,7 @@ mod tests {
             provider: None,
             design_md: None,
             concurrency: 1,
+            append_context: None,
         };
         let short_pp = build_orchestrator_prompt(&short_req, PlanningMode::Rich, AbortFlag::new());
         let long_pp = build_orchestrator_prompt(&long_req, PlanningMode::Rich, AbortFlag::new());
@@ -485,6 +493,7 @@ mod tests {
             provider: None,
             design_md: None,
             concurrency: 1,
+            append_context: None,
         };
         let pp = build_orchestrator_prompt(&ds_req, PlanningMode::Rich, AbortFlag::new());
         // Short bucket base: 300_000ms × 2.0 = 600_000ms
@@ -508,6 +517,7 @@ mod tests {
             elements: None,
             screen: None,
             generated_root_id: None,
+            existing_section_labels: None,
         }
     }
 
@@ -540,6 +550,7 @@ mod tests {
             provider: None,
             design_md: None,
             concurrency: 1,
+            append_context: None,
         };
         let long_req = DesignRequest {
             prompt: "x".repeat(5000),
@@ -547,6 +558,7 @@ mod tests {
             provider: None,
             design_md: None,
             concurrency: 1,
+            append_context: None,
         };
         let short_cr = build_subagent_prompt(
             &subtask(),
@@ -579,6 +591,7 @@ mod tests {
             provider: None,
             design_md: None,
             concurrency: 1,
+            append_context: None,
         };
         let cr = build_subagent_prompt(
             &subtask(),
diff --git a/crates/op-orchestrator/src/run_tests.rs b/crates/op-orchestrator/src/run_tests.rs
index e259a5b45..942d446d2 100644
--- a/crates/op-orchestrator/src/run_tests.rs
+++ b/crates/op-orchestrator/src/run_tests.rs
@@ -13,6 +13,7 @@ fn req() -> DesignRequest {
         provider: None,
         design_md: None,
         concurrency: 1,
+        append_context: None,
     }
 }
 
@@ -25,6 +26,7 @@ fn req_standard() -> DesignRequest {
         provider: None,
         design_md: None,
         concurrency: 1,
+        append_context: None,
     }
 }
 
@@ -37,6 +39,7 @@ fn req_basic() -> DesignRequest {
         provider: None,
         design_md: None,
         concurrency: 1,
+        append_context: None,
     }
 }
 
diff --git a/crates/op-orchestrator/src/run_tests_c2.rs b/crates/op-orchestrator/src/run_tests_c2.rs
index 1b25dff9b..b739e2a52 100644
--- a/crates/op-orchestrator/src/run_tests_c2.rs
+++ b/crates/op-orchestrator/src/run_tests_c2.rs
@@ -57,6 +57,7 @@ fn req_concurrent() -> DesignRequest {
         provider: None,
         design_md: None,
         concurrency: 2,
+        append_context: None,
     }
 }
 
@@ -69,6 +70,7 @@ fn req_sequential() -> DesignRequest {
         provider: None,
         design_md: None,
         concurrency: 1,
+        append_context: None,
     }
 }
 
diff --git a/crates/op-orchestrator/src/run_tests_c3.rs b/crates/op-orchestrator/src/run_tests_c3.rs
index 0c8776e0e..464fce950 100644
--- a/crates/op-orchestrator/src/run_tests_c3.rs
+++ b/crates/op-orchestrator/src/run_tests_c3.rs
@@ -56,6 +56,7 @@ fn req_dashboard() -> DesignRequest {
         provider: None,
         design_md: None,
         concurrency: 1,
+        append_context: None,
     }
 }
 
@@ -66,6 +67,7 @@ fn req_non_dashboard() -> DesignRequest {
         provider: None,
         design_md: None,
         concurrency: 1,
+        append_context: None,
     }
 }
 
diff --git a/crates/op-orchestrator/src/scaffold.rs b/crates/op-orchestrator/src/scaffold.rs
index 75e0c14fc..bdf88ed4a 100644
--- a/crates/op-orchestrator/src/scaffold.rs
+++ b/crates/op-orchestrator/src/scaffold.rs
@@ -304,6 +304,7 @@ mod tests {
             elements: None,
             screen: screen.map(|s| s.to_string()),
             generated_root_id: None,
+            existing_section_labels: None,
         }
     }
 
diff --git a/crates/op-orchestrator/src/scaffold_tests_c2.rs b/crates/op-orchestrator/src/scaffold_tests_c2.rs
index 60d6f7315..19d9898a1 100644
--- a/crates/op-orchestrator/src/scaffold_tests_c2.rs
+++ b/crates/op-orchestrator/src/scaffold_tests_c2.rs
@@ -60,6 +60,7 @@ fn dashboard_plan(root_id: &str) -> OrchestratorPlan {
                 elements: None,
                 screen: None,
                 generated_root_id: None,
+                existing_section_labels: None,
             },
             Subtask {
                 id: "revenue-chart".into(),
@@ -73,6 +74,7 @@ fn dashboard_plan(root_id: &str) -> OrchestratorPlan {
                 elements: None,
                 screen: None,
                 generated_root_id: None,
+                existing_section_labels: None,
             },
             Subtask {
                 id: "kpi-metrics".into(),
@@ -86,6 +88,7 @@ fn dashboard_plan(root_id: &str) -> OrchestratorPlan {
                 elements: None,
                 screen: None,
                 generated_root_id: None,
+                existing_section_labels: None,
             },
         ],
         style_guide_name: None,
diff --git a/crates/op-orchestrator/src/subagent.rs b/crates/op-orchestrator/src/subagent.rs
index d4c9ba4ca..aaac891c5 100644
--- a/crates/op-orchestrator/src/subagent.rs
+++ b/crates/op-orchestrator/src/subagent.rs
@@ -105,6 +105,7 @@ mod tests {
             provider: None,
             design_md: None,
             concurrency: 1,
+            append_context: None,
         }
     }
 
@@ -138,6 +139,7 @@ mod tests {
             elements: None,
             screen: None,
             generated_root_id: None,
+            existing_section_labels: None,
         }
     }
 
diff --git a/crates/op-orchestrator/src/types.rs b/crates/op-orchestrator/src/types.rs
index 7eb5f4d59..9e84eeff0 100644
--- a/crates/op-orchestrator/src/types.rs
+++ b/crates/op-orchestrator/src/types.rs
@@ -5,6 +5,7 @@
 
 use futures::stream::BoxStream;
 use op_editor_core::{EditorCommand, EditorState};
+use serde::{Deserialize, Serialize};
 use std::sync::atomic::{AtomicBool, Ordering};
 use std::sync::Arc;
 use std::time::Duration;
@@ -164,8 +165,26 @@ impl std::fmt::Display for OrchestratorError {
 
 impl std::error::Error for OrchestratorError {}
 
+/// 追加上下文 —— 当用户要求扩展已有页面时由 host 填入。
+///
+/// Port of `AppendContext` in `apps/web/src/services/ai/ai-types.ts:28-37`.
+/// 当存在时,编排器跳过创建新根 frame,将生成的区块插入现有目标 frame。
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
+pub struct AppendContext {
+    /// 新 sub-agent 区块应插入的 frame id。
+    pub target_parent_id: String,
+    /// 目标 frame 的宽度(用于给 sub-agent 确定区域尺寸)。
+    pub target_width: f64,
+    /// 现有顶层区块的标签列表 —— sub-agent 被告知不要重复这些。
+    pub existing_section_labels: Vec<String>,
+    /// 目标 frame 属于移动页面(宽度 ≤ 480)时为 true。
+    pub is_mobile: bool,
+}
+
 /// 编排器输入 —— 一次设计请求。
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Serialize, Deserialize)]
+#[serde(rename_all = "camelCase")]
 pub struct DesignRequest {
     pub prompt: String,
     pub model: Option<String>,
@@ -176,6 +195,10 @@ pub struct DesignRequest {
     /// 调用方应传 store-clamped 值 [1,6];crate 内部防御性 clamp。
     /// 默认为 1(顺序执行)。Port of TS `request.concurrency ?? 1`.
     pub concurrency: u32,
+    /// 追加模式上下文 —— 仅当 host 检测到 append intent 时填入。
+    /// Port of `AIDesignRequest.context.appendContext` in `ai-types.ts:51`.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub append_context: Option<AppendContext>,
 }
 
 #[cfg(test)]
@@ -201,4 +224,89 @@ mod tests {
         flag.set();
         assert!(clone.is_set());
     }
+
+    // ── Task A1: AppendContext + DesignRequest.append_context ─────────────────
+
+    /// AppendContext serde round-trips with all 4 fields (camelCase wire names).
+    #[test]
+    fn append_context_serde_round_trip() {
+        let ctx = AppendContext {
+            target_parent_id: "frame-abc".into(),
+            target_width: 390.0,
+            existing_section_labels: vec!["Hero".into(), "Pricing".into()],
+            is_mobile: true,
+        };
+        let json = serde_json::to_string(&ctx).expect("serialize");
+        // Wire names are camelCase
+        assert!(
+            json.contains("targetParentId"),
+            "expected targetParentId in {json}"
+        );
+        assert!(
+            json.contains("targetWidth"),
+            "expected targetWidth in {json}"
+        );
+        assert!(
+            json.contains("existingSectionLabels"),
+            "expected existingSectionLabels in {json}"
+        );
+        assert!(json.contains("isMobile"), "expected isMobile in {json}");
+        let back: AppendContext = serde_json::from_str(&json).expect("deserialize");
+        assert_eq!(back.target_parent_id, "frame-abc");
+        assert_eq!(back.target_width, 390.0);
+        assert_eq!(back.existing_section_labels, vec!["Hero", "Pricing"]);
+        assert!(back.is_mobile);
+    }
+
+    /// DesignRequest accepts append_context: None without breaking compilation.
+    #[test]
+    fn design_request_append_context_none_compiles() {
+        let req = DesignRequest {
+            prompt: "test".into(),
+            model: None,
+            provider: None,
+            design_md: None,
+            concurrency: 1,
+            append_context: None,
+        };
+        assert!(req.append_context.is_none());
+    }
+
+    /// DesignRequest accepts a populated AppendContext.
+    #[test]
+    fn design_request_append_context_some_compiles() {
+        let ctx = AppendContext {
+            target_parent_id: "p1".into(),
+            target_width: 1200.0,
+            existing_section_labels: vec![],
+            is_mobile: false,
+        };
+        let req = DesignRequest {
+            prompt: "extend page".into(),
+            model: None,
+            provider: None,
+            design_md: None,
+            concurrency: 1,
+            append_context: Some(ctx),
+        };
+        assert!(req.append_context.is_some());
+    }
+
+    /// DesignRequest without append_context serializes without the field.
+    #[test]
+    fn design_request_append_context_omitted_from_json_when_none() {
+        let req = DesignRequest {
+            prompt: "test".into(),
+            model: None,
+            provider: None,
+            design_md: None,
+            concurrency: 1,
+            append_context: None,
+        };
+        let json = serde_json::to_string(&req).expect("serialize");
+        assert!(
+            !json.contains("appendContext"),
+            "appendContext should be omitted when None, got: {json}"
+        );
+    }
 }
diff --git a/crates/op-orchestrator/src/variables.rs b/crates/op-orchestrator/src/variables.rs
index 1e9fcaeef..05ffd2bfe 100644
--- a/crates/op-orchestrator/src/variables.rs
+++ b/crates/op-orchestrator/src/variables.rs
@@ -47,6 +47,7 @@ mod tests {
             provider: None,
             design_md: None,
             concurrency: 1,
+            append_context: None,
         });
         assert!(seed_commands(&plan).is_empty());
         let sink = VecDocSink::new();