diff --git a/crates/op-orchestrator/src/concurrent.rs b/crates/op-orchestrator/src/concurrent.rs
new file mode 100644
index 000000000..986a4910d
--- /dev/null
+++ b/crates/op-orchestrator/src/concurrent.rs
@@ -0,0 +1,230 @@
+//! S3b-2 Task A1: concurrency decision + screen grouping.
+//!
+//! Port of `orchestrator.ts:780-810`.
+//! - `DesignRequest.concurrency` (added in `types.rs`)
+//! - `clamp_concurrency` — defensive [1, 6] clamp
+//! - `group_subtasks_by_screen` — group subtasks by screen; faithful to TS L785-801
+//! - `effective_concurrency` — concurrency decision; faithful to TS L803-810
+//!   (minus append-mode gate, which is S3b-4)
+//!
+//! Callers land in later S3b-2 tasks; scaffolding symbols are allowed to be unused.
+#![allow(dead_code)]
+
+use crate::plan::Subtask;
+
+/// A group of subtask indices that share the same screen.
+/// `screen` is the screen name; `indices` are into the plan's `subtasks` slice.
+#[derive(Debug, Clone, PartialEq)]
+pub(crate) struct ScreenGroup {
+    pub screen: String,
+    pub indices: Vec<usize>,
+}
+
+/// Clamps a raw concurrency value to the valid range `[1, 6]`.
+///
+/// This mirrors the store-side clamp in TS (store clamps to [1,6] before
+/// writing to `request.concurrency`). The Rust crate clamps defensively on
+/// the way in so callers need not worry about out-of-range values.
+pub(crate) fn clamp_concurrency(v: u32) -> u32 {
+    v.clamp(1, 6)
+}
+
+/// Groups subtasks by screen, faithfully porting `orchestrator.ts:785-801`.
+///
+/// Rules:
+/// - Only called when `concurrency > 1` (caller's responsibility).
+/// - `first_screen` = the `screen` of the first subtask that has one, else `"page"`.
+/// - A subtask with no `screen` falls back to `first_screen`.
+/// - Group order = first-seen order of distinct screen values.
+/// - If no subtask has a `screen`, returns an empty `Vec` (caller treats as
+///   single-screen; `effective_concurrency` will return 1 in that case).
+pub(crate) fn group_subtasks_by_screen(subtasks: &[Subtask]) -> Vec<ScreenGroup> {
+    let has_any_screen = subtasks.iter().any(|st| st.screen.is_some());
+    if !has_any_screen {
+        return vec![];
+    }
+
+    // first_screen = screen of first subtask that has one, else "page".
+    let first_screen: String = subtasks
+        .iter()
+        .find(|st| st.screen.is_some())
+        .and_then(|st| st.screen.clone())
+        .unwrap_or_else(|| "page".to_string());
+
+    let mut groups: Vec<ScreenGroup> = Vec::new();
+    // Map from screen name to index into `groups`.
+    let mut screen_map: std::collections::HashMap<String, usize> = std::collections::HashMap::new();
+
+    for (i, subtask) in subtasks.iter().enumerate() {
+        let screen = subtask
+            .screen
+            .clone()
+            .unwrap_or_else(|| first_screen.clone());
+
+        if let Some(&group_idx) = screen_map.get(&screen) {
+            groups[group_idx].indices.push(i);
+        } else {
+            screen_map.insert(screen.clone(), groups.len());
+            groups.push(ScreenGroup {
+                screen,
+                indices: vec![i],
+            });
+        }
+    }
+
+    groups
+}
+
+/// Computes the effective concurrency for a run.
+///
+/// Port of `orchestrator.ts:803-810`, minus the append-mode gate (S3b-4).
+///
+/// - `screen_group_count > 1` → `clamp_concurrency(concurrency)`.
+/// - else → `1`.
+pub(crate) fn effective_concurrency(concurrency: u32, screen_group_count: usize) -> u32 {
+    if screen_group_count > 1 {
+        clamp_concurrency(concurrency)
+    } else {
+        1
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::plan::{Region, Subtask};
+
+    fn subtask_with_screen(id: &str, screen: Option<&str>) -> Subtask {
+        Subtask {
+            id: id.into(),
+            label: id.into(),
+            id_prefix: id.into(),
+            region: Region {
+                width: 1200.0,
+                height: 400.0,
+            },
+            parent_frame_id: None,
+            elements: None,
+            screen: screen.map(|s| s.to_string()),
+        }
+    }
+
+    // ── effective_concurrency ──────────────────────────────────────────────────
+
+    /// (concurrency=1, 3 screens) → 1  (single-threaded even with many screens)
+    #[test]
+    fn effective_concurrency_one_concurrency_three_screens_gives_one() {
+        assert_eq!(effective_concurrency(1, 3), 1);
+    }
+
+    /// (concurrency=4, 1 screen) → 1  (only one group → sequential)
+    #[test]
+    fn effective_concurrency_four_concurrency_one_screen_gives_one() {
+        assert_eq!(effective_concurrency(4, 1), 1);
+    }
+
+    /// (concurrency=4, 3 screens) → 4
+    #[test]
+    fn effective_concurrency_four_concurrency_three_screens_gives_four() {
+        assert_eq!(effective_concurrency(4, 3), 4);
+    }
+
+    /// Clamp: (concurrency=99, 3 screens) → 6
+    #[test]
+    fn effective_concurrency_clamps_to_six() {
+        assert_eq!(effective_concurrency(99, 3), 6);
+    }
+
+    // ── group_subtasks_by_screen ───────────────────────────────────────────────
+
+    /// Basic grouping: [login, home, login] → 2 groups {login:[0,2], home:[1]}
+    #[test]
+    fn group_subtasks_three_entries_two_screens() {
+        let subtasks = vec![
+            subtask_with_screen("a", Some("login")),
+            subtask_with_screen("b", Some("home")),
+            subtask_with_screen("c", Some("login")),
+        ];
+        let groups = group_subtasks_by_screen(&subtasks);
+        assert_eq!(groups.len(), 2);
+        assert_eq!(groups[0].screen, "login");
+        assert_eq!(groups[0].indices, vec![0, 2]);
+        assert_eq!(groups[1].screen, "home");
+        assert_eq!(groups[1].indices, vec![1]);
+    }
+
+    /// A subtask with no screen falls back to first_screen.
+    #[test]
+    fn group_subtasks_no_screen_falls_back_to_first_screen() {
+        let subtasks = vec![
+            subtask_with_screen("a", Some("login")),
+            subtask_with_screen("b", None), // no screen → "login"
+            subtask_with_screen("c", Some("home")),
+        ];
+        let groups = group_subtasks_by_screen(&subtasks);
+        // "login" gets index 0 and 1 (b bucketed under first_screen="login")
+        assert_eq!(groups.len(), 2);
+        assert_eq!(groups[0].screen, "login");
+        assert_eq!(groups[0].indices, vec![0, 1]);
+        assert_eq!(groups[1].screen, "home");
+        assert_eq!(groups[1].indices, vec![2]);
+    }
+
+    /// All subtasks have no screen → empty result (no groups).
+    #[test]
+    fn group_subtasks_all_no_screen_returns_empty() {
+        let subtasks = vec![
+            subtask_with_screen("a", None),
+            subtask_with_screen("b", None),
+        ];
+        let groups = group_subtasks_by_screen(&subtasks);
+        assert!(groups.is_empty());
+    }
+
+    /// Empty subtask slice → empty result.
+    #[test]
+    fn group_subtasks_empty_slice_returns_empty() {
+        let groups = group_subtasks_by_screen(&[]);
+        assert!(groups.is_empty());
+    }
+
+    /// Group order is first-seen order of distinct screen values.
+    #[test]
+    fn group_subtasks_preserves_first_seen_order() {
+        let subtasks = vec![
+            subtask_with_screen("a", Some("profile")),
+            subtask_with_screen("b", Some("settings")),
+            subtask_with_screen("c", Some("profile")),
+            subtask_with_screen("d", Some("dashboard")),
+        ];
+        let groups = group_subtasks_by_screen(&subtasks);
+        assert_eq!(groups.len(), 3);
+        assert_eq!(groups[0].screen, "profile");
+        assert_eq!(groups[1].screen, "settings");
+        assert_eq!(groups[2].screen, "dashboard");
+    }
+
+    /// first_screen fallback is "page" when no subtask has a screen
+    /// (but `has_any_screen` is false, so returns empty — tested separately).
+    /// Here we test that a None screen at the START falls back to first_screen
+    /// from a LATER subtask — i.e., first_screen is derived from the first
+    /// subtask that HAS a screen.
+    #[test]
+    fn group_subtasks_no_screen_at_start_uses_later_first_screen() {
+        let subtasks = vec![
+            subtask_with_screen("a", None),         // no screen
+            subtask_with_screen("b", Some("home")), // first with screen → first_screen="home"
+            subtask_with_screen("c", Some("settings")),
+        ];
+        let groups = group_subtasks_by_screen(&subtasks);
+        // "home" first (b has screen), but "a" has no screen → bucketed under first_screen="home"
+        assert_eq!(groups.len(), 2);
+        assert_eq!(groups[0].screen, "home");
+        // "a" (index 0) bucketed under "home" — it's first because it's processed first
+        // and maps to first_screen="home" which is first seen when processing "a".
+        assert!(groups[0].indices.contains(&0));
+        assert!(groups[0].indices.contains(&1));
+        assert_eq!(groups[1].screen, "settings");
+        assert_eq!(groups[1].indices, vec![2]);
+    }
+}
diff --git a/crates/op-orchestrator/src/lib.rs b/crates/op-orchestrator/src/lib.rs
index 098db5393..f326b006d 100644
--- a/crates/op-orchestrator/src/lib.rs
+++ b/crates/op-orchestrator/src/lib.rs
@@ -24,6 +24,7 @@ pub mod types;
 pub mod variables;
 
 pub mod cleanup;
+pub mod concurrent;
 pub mod prompt;
 pub mod run;
 pub mod scaffold;
diff --git a/crates/op-orchestrator/src/plan.rs b/crates/op-orchestrator/src/plan.rs
index 66cee337a..7ee507607 100644
--- a/crates/op-orchestrator/src/plan.rs
+++ b/crates/op-orchestrator/src/plan.rs
@@ -205,6 +205,7 @@ mod tests {
             model: None,
             provider: None,
             design_md: None,
+            concurrency: 1,
         }
     }
 
diff --git a/crates/op-orchestrator/src/plan_normalize.rs b/crates/op-orchestrator/src/plan_normalize.rs
index 22673c921..a79e2731d 100644
--- a/crates/op-orchestrator/src/plan_normalize.rs
+++ b/crates/op-orchestrator/src/plan_normalize.rs
@@ -56,6 +56,7 @@ mod tests {
             model: None,
             provider: None,
             design_md: None,
+            concurrency: 1,
         }
     }
 
diff --git a/crates/op-orchestrator/src/plan_repair_tests.rs b/crates/op-orchestrator/src/plan_repair_tests.rs
index 769672e31..73a38a2d9 100644
--- a/crates/op-orchestrator/src/plan_repair_tests.rs
+++ b/crates/op-orchestrator/src/plan_repair_tests.rs
@@ -242,6 +242,7 @@ fn req(prompt: &str) -> DesignRequest {
         model: None,
         provider: None,
         design_md: None,
+        concurrency: 1,
     }
 }
 
@@ -264,6 +265,7 @@ fn req_with_design_md(prompt: &str) -> DesignRequest {
             layout_principles: None,
             generation_notes: None,
         }),
+        concurrency: 1,
     }
 }
 
diff --git a/crates/op-orchestrator/src/prompt.rs b/crates/op-orchestrator/src/prompt.rs
index eb28e8ed0..da71548c9 100644
--- a/crates/op-orchestrator/src/prompt.rs
+++ b/crates/op-orchestrator/src/prompt.rs
@@ -229,6 +229,7 @@ mod tests {
             model: Some("claude".into()),
             provider: None,
             design_md: None,
+            concurrency: 1,
         }
     }
 
@@ -366,6 +367,7 @@ mod tests {
             model: Some("claude-haiku".into()),
             provider: None,
             design_md: None,
+            concurrency: 1,
         };
         let full_cr =
             build_subagent_prompt(&st, &plan(), &basic_req, AbortFlag::new(), false, false);
@@ -451,6 +453,7 @@ mod tests {
             model: Some("claude-sonnet".into()),
             provider: None,
             design_md: None,
+            concurrency: 1,
         };
         let long_prompt = "x".repeat(5000); // >= 4200 chars
         let long_req = DesignRequest {
@@ -458,6 +461,7 @@ mod tests {
             model: Some("claude-sonnet".into()),
             provider: None,
             design_md: None,
+            concurrency: 1,
         };
         let short_pp = build_orchestrator_prompt(&short_req, PlanningMode::Rich, AbortFlag::new());
         let long_pp = build_orchestrator_prompt(&long_req, PlanningMode::Rich, AbortFlag::new());
@@ -476,6 +480,7 @@ mod tests {
             model: Some("deepseek-v4-pro".into()),
             provider: None,
             design_md: None,
+            concurrency: 1,
         };
         let pp = build_orchestrator_prompt(&ds_req, PlanningMode::Rich, AbortFlag::new());
         // Short bucket base: 300_000ms × 2.0 = 600_000ms
@@ -529,12 +534,14 @@ mod tests {
             model: Some("claude-sonnet".into()),
             provider: None,
             design_md: None,
+            concurrency: 1,
         };
         let long_req = DesignRequest {
             prompt: "x".repeat(5000),
             model: Some("claude-sonnet".into()),
             provider: None,
             design_md: None,
+            concurrency: 1,
         };
         let short_cr = build_subagent_prompt(
             &subtask(),
@@ -566,6 +573,7 @@ mod tests {
             model: Some("claude-haiku".into()),
             provider: None,
             design_md: None,
+            concurrency: 1,
         };
         let cr = build_subagent_prompt(
             &subtask(),
diff --git a/crates/op-orchestrator/src/run.rs b/crates/op-orchestrator/src/run.rs
index 6d084a922..c2799508b 100644
--- a/crates/op-orchestrator/src/run.rs
+++ b/crates/op-orchestrator/src/run.rs
@@ -402,6 +402,7 @@ mod tests {
             model: None,
             provider: None,
             design_md: None,
+            concurrency: 1,
         }
     }
 
@@ -413,6 +414,7 @@ mod tests {
             model: Some("gpt-4o".into()),
             provider: None,
             design_md: None,
+            concurrency: 1,
         }
     }
 
@@ -424,6 +426,7 @@ mod tests {
             model: Some("glm-4".into()),
             provider: None,
             design_md: None,
+            concurrency: 1,
         }
     }
 
diff --git a/crates/op-orchestrator/src/subagent.rs b/crates/op-orchestrator/src/subagent.rs
index 1346f001e..664c455b5 100644
--- a/crates/op-orchestrator/src/subagent.rs
+++ b/crates/op-orchestrator/src/subagent.rs
@@ -104,6 +104,7 @@ mod tests {
             model: None,
             provider: None,
             design_md: None,
+            concurrency: 1,
         }
     }
 
diff --git a/crates/op-orchestrator/src/types.rs b/crates/op-orchestrator/src/types.rs
index 0216c373e..8e62ce434 100644
--- a/crates/op-orchestrator/src/types.rs
+++ b/crates/op-orchestrator/src/types.rs
@@ -167,6 +167,10 @@ pub struct DesignRequest {
     pub provider: Option<String>,
     /// 当前文档的 design.md(若有)—— 规划 prompt 据此走 design.md 分支。
     pub design_md: Option<jian_ops_schema::DesignMdSpec>,
+    /// 并发度:允许同时运行的 screen-group worker 数。
+    /// 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,
 }
 
 #[cfg(test)]
diff --git a/crates/op-orchestrator/src/variables.rs b/crates/op-orchestrator/src/variables.rs
index 8434eff9e..1e9fcaeef 100644
--- a/crates/op-orchestrator/src/variables.rs
+++ b/crates/op-orchestrator/src/variables.rs
@@ -46,6 +46,7 @@ mod tests {
             model: None,
             provider: None,
             design_md: None,
+            concurrency: 1,
         });
         assert!(seed_commands(&plan).is_empty());
         let sink = VecDocSink::new();