diff --git a/extensions/shared/capability-intent.ts b/extensions/shared/capability-intent.ts
index f7da1d54..f473385f 100644
--- a/extensions/shared/capability-intent.ts
+++ b/extensions/shared/capability-intent.ts
@@ -7,8 +7,9 @@ const CAPABILITY_INTENT = {
   search:
     /\b(?:use|run)\s+(?:fd|rg)\b|\buse\s+(?:structured\s+)?(?:(?:file|code|content)\s+)?search\b|\b(?:structured|fast)\s+(?:file|code|content)\s+search\b|(?:使用|用|运行).{0,8}(?:fd|rg|git\s+(?:show|diff|log))|结构化(?:文件|代码|内容)搜索/iu,
   delegate:
-    /\bsubagents?\b|(?:^|[.!?]\s+)(?:please\s+)?(?:delegate|parallelize)\s+(?:this|the)\s+(?:task|work)\b|\b(?:can|could|would)\s+you\s+(?:please\s+)?(?:delegate|parallelize)\s+(?:this|the)\s+(?:task|work)\b|\bparallel\s+agents?\b|^\s*子代理\s*[,，:：]?\s*(?:(?:来|去|请|帮(?:我|忙)?|负责|给我)\s*)?(?:(?:先|全面|系统(?:性地)?|深入|快速|仔细)\s*)?(?:了解|查看|检查|审查|分析|研究|调研|梳理|探索|实现|修复|处理|执行|完成|阅读|总结)|(?:使用|用|启动|调用|来|开).{0,8}子代理|(?:多个?|多路)子代理|并行.{0,8}(?:代理|agent)|委派.{0,6}(?:任务|给|出去)/iu,
-  workflow: /\bworkflows?\b|(?:使用|用|运行|创建|构建).{0,8}工作流/iu,
+    /(?:^|[.!?]\s+)(?:please\s+)?(?:delegate|parallelize)\s+(?:this|the)\s+(?:task|work)\b|\b(?:can|could|would)\s+you\s+(?:please\s+)?(?:delegate|parallelize)\s+(?:this|the)\s+(?:task|work)\b|\b(?:use|spawn|start)\s+(?:a\s+)?subagents?\b|\bparallel\s+agents?\b|(?:使用|用|启动|调用).{0,8}subagents?\b|^\s*子代理\s*[,，:：]?\s*(?:(?:来|去|请|帮(?:我|忙)?|负责|给我)\s*)?(?:(?:先|全面|系统(?:性地)?|深入|快速|仔细)\s*)?(?:了解|查看|检查|审查|分析|研究|调研|梳理|探索|实现|修复|处理|执行|完成|阅读|总结)|(?:使用|用|启动|调用|来|开).{0,8}子代理|(?:多个?|多路)子代理|并行.{0,8}(?:代理|agent)|委派.{0,6}(?:任务|给|出去)/iu,
+  workflow:
+    /\b(?:use|run|start|create|build)\s+(?:a\s+)?workflows?\b|(?:使用|用|运行|创建|构建).{0,8}(?:工作流|workflows?)/iu,
   background:
     /\b(?:run|start|keep)\b.{0,40}\b(?:in the background|background\s+(?:process|terminal|job))\b|后台.{0,8}(?:运行|启动|进程|终端|任务)/iu,
   session:
@@ -31,8 +32,9 @@ function isExplicitClause(clause: string) {
 
 /**
  * One fail-closed interpretation of explicit user capability intent. The
- * English names `subagent` and `workflow` are reserved authorization words;
- * negated or conditional clauses remain inert. Runtime activation and
+ * English capability names require an imperative request context; references
+ * in identifiers, discussion, or comparisons stay inert. Negated or
+ * conditional clauses remain inert. Runtime activation and
  * pre-submit UI feedback both cross this seam, so they cannot drift.
  */
 export function capabilitiesRequestedByPrompt(prompt: string) {
diff --git a/tests/extensions/shared/capability-intent.test.ts b/tests/extensions/shared/capability-intent.test.ts
index 9b066702..967327e9 100644
--- a/tests/extensions/shared/capability-intent.test.ts
+++ b/tests/extensions/shared/capability-intent.test.ts
@@ -6,10 +6,7 @@ import {
 } from "../../../extensions/shared/capability-intent.ts";
 
 test("classifies explicit capability intent across English, Chinese, and mixed phrases", () => {
-  assert.deepEqual(capabilitiesRequestedByPrompt("subagent, workflow"), [
-    "delegate",
-    "workflow",
-  ]);
+  assert.deepEqual(capabilitiesRequestedByPrompt("subagent, workflow"), []);
   assert.deepEqual(
     capabilitiesRequestedByPrompt("用 Subagent 并行检查这三个模块"),
     ["delegate"],
@@ -31,10 +28,19 @@ test("classifies explicit capability intent across English, Chinese, and mixed p
   );
 });
 
-test("reserved English capability words authorize the matching groups", () => {
+test("English references without an imperative stay inert", () => {
   assert.deepEqual(
     capabilitiesRequestedByPrompt("Compare Subagent and Workflow"),
-    ["delegate", "workflow"],
+    [],
+  );
+  for (const prompt of [
+    "git checkout -b subagent-matching",
+    "Please review the subagent.ts implementation",
+    "讨论 workflow 和 subagent 的区别",
+  ]) assert.deepEqual(capabilitiesRequestedByPrompt(prompt), [], prompt);
+  assert.deepEqual(
+    capabilitiesRequestedByPrompt("Use a subagent to review this change"),
+    ["delegate"],
   );
 });
 
diff --git a/web/host/web-host.ts b/web/host/web-host.ts
index 6203e647..9cde64dd 100644
--- a/web/host/web-host.ts
+++ b/web/host/web-host.ts
@@ -541,6 +541,28 @@ export class WebHost {
         cursor: this.sequence,
       });
     }
+    if (url.pathname === "/api/prompt/cancel" && request.method === "POST") {
+      const body = await this.readJson(request);
+      if (typeof body.sessionId !== "string" || body.sessionId !== this.runtime.sessionManager.getSessionId()) {
+        return this.json(response, 409, {
+          code: "SESSION_CONFLICT",
+          error: "Only the active Web session accepts cancellation",
+        });
+      }
+      try {
+        if (!this.runtime.interruptTurn) {
+          return this.json(response, 501, {
+            code: "PROMPT_CANCEL_FAILED",
+            error: "Turn cancellation is unavailable",
+          });
+        }
+        const result = await this.runtime.interruptTurn({ expectedSessionId: body.sessionId });
+        return this.json(response, 200, result);
+      } catch (error) {
+        const failure = this.runtimeRequestFailure(error, "PROMPT_CANCEL_FAILED", "prompt cancellation failed");
+        return this.json(response, failure.status, { code: failure.code, error: failure.error });
+      }
+    }
     if (request.method !== "GET") {
       return this.json(response, 405, { error: "method not allowed" });
     }
diff --git a/web/runtime/pi-runtime.ts b/web/runtime/pi-runtime.ts
index cf792d39..cac27ef4 100644
--- a/web/runtime/pi-runtime.ts
+++ b/web/runtime/pi-runtime.ts
@@ -454,6 +454,24 @@ export class PiWebRuntime implements WebRuntimeController {
     await requestAdmission;
   }
 
+  async interruptTurn(options?: { expectedSessionId?: string }) {
+    this.assertActive();
+    this.assertWorkspaceSelected();
+    const session = this.runtime.session;
+    const sessionId = session.sessionManager.getSessionId();
+    if (options?.expectedSessionId !== undefined && options.expectedSessionId !== sessionId) {
+      throw new WebRuntimeRequestError(
+        "Only the active Web session accepts cancellation",
+        "SESSION_CONFLICT",
+        409,
+      );
+    }
+    if (!session.isStreaming) return { state: "already_settled" as const };
+    await session.abort();
+    this.emit("prompt_cancelled", { sessionId });
+    return { state: "cancelled" as const };
+  }
+
   newSession(workspacePath: string, options?: WebSessionCreationOptions) {
     return this.serializeControllerMutation(() =>
       this.createNewSession(workspacePath, options),
diff --git a/web/runtime/types.ts b/web/runtime/types.ts
index 2b66c3f0..c8d4649f 100644
--- a/web/runtime/types.ts
+++ b/web/runtime/types.ts
@@ -10,6 +10,7 @@ export type WebRuntimeRequestErrorCode =
   | "MODEL_NOT_AVAILABLE"
   | "SESSION_CONFLICT"
   | "PROMPT_REJECTED"
+  | "PROMPT_CANCEL_FAILED"
   | "WORKSPACE_REQUIRED";
 
 export class WebRuntimeRequestError extends Error {
@@ -55,6 +56,9 @@ export interface WebRuntimeController {
   readonly sessionManager: SessionManager;
   isIdle(): boolean;
   sendPrompt(content: string, options?: WebPromptOptions): Promise<void>;
+  interruptTurn?(options?: { expectedSessionId?: string }): Promise<{
+    state: "cancelled" | "already_settled";
+  }>;
   newSession(
     workspacePath: string,
     options?: WebSessionCreationOptions,