diff --git a/src/hooks/start-work/index.test.ts b/src/hooks/start-work/index.test.ts
index 26b87eba4..565b2decb 100644
--- a/src/hooks/start-work/index.test.ts
+++ b/src/hooks/start-work/index.test.ts
@@ -417,7 +417,7 @@ describe("start-work hook", () => {
       detectSpy.mockRestore()
     })
 
-    test("should inject model-decides instructions when no --worktree flag", async () => {
+    test("should NOT inject worktree instructions when no --worktree flag", async () => {
       // given - single plan, no worktree flag
       const plansDir = join(testDir, ".sisyphus", "plans")
       mkdirSync(plansDir, { recursive: true })
@@ -431,10 +431,10 @@ describe("start-work hook", () => {
       // when
       await hook["chat.message"]({ sessionID: "session-123" }, output)
 
-      // then - model-decides instructions should appear
-      expect(output.parts[0].text).toContain("Worktree Setup Required")
-      expect(output.parts[0].text).toContain("git worktree list --porcelain")
-      expect(output.parts[0].text).toContain("git worktree add")
+      // then - no worktree instructions should appear
+      expect(output.parts[0].text).not.toContain("Worktree Setup Required")
+      expect(output.parts[0].text).not.toContain("Worktree Active")
+      expect(output.parts[0].text).not.toContain("git worktree list --porcelain")
     })
 
     test("should inject worktree path when --worktree flag is valid", async () => {
@@ -452,8 +452,10 @@ describe("start-work hook", () => {
       // when
       await hook["chat.message"]({ sessionID: "session-123" }, output)
 
-      // then - validated path shown, no model-decides instructions
-      expect(output.parts[0].text).toContain("**Worktree**: /validated/worktree")
+      // then - strong worktree active instructions shown
+      expect(output.parts[0].text).toContain("Worktree Active")
+      expect(output.parts[0].text).toContain("/validated/worktree")
+      expect(output.parts[0].text).toContain("subagent")
       expect(output.parts[0].text).not.toContain("Worktree Setup Required")
     })
 
@@ -548,8 +550,10 @@ describe("start-work hook", () => {
       // when
       await hook["chat.message"]({ sessionID: "session-789" }, output)
 
-      // then - shows existing worktree, no model-decides instructions
+      // then - shows strong worktree active instructions
+      expect(output.parts[0].text).toContain("Worktree Active")
       expect(output.parts[0].text).toContain("/existing/wt")
+      expect(output.parts[0].text).toContain("subagent")
       expect(output.parts[0].text).not.toContain("Worktree Setup Required")
     })
   })
diff --git a/src/hooks/start-work/start-work-hook.ts b/src/hooks/start-work/start-work-hook.ts
index 03cdb540e..81ec85cc3 100644
--- a/src/hooks/start-work/start-work-hook.ts
+++ b/src/hooks/start-work/start-work-hook.ts
@@ -34,26 +34,28 @@ function findPlanByName(plans: string[], requestedName: string): string | null {
   return partialMatch || null
 }
 
-const MODEL_DECIDES_WORKTREE_BLOCK = `
-## Worktree Setup Required
+function createWorktreeActiveBlock(worktreePath: string): string {
+  return `
+## Worktree Active
 
-No worktree specified. Before starting work, you MUST choose or create one:
+**Worktree**: \`${worktreePath}\`
 
-1. \`git worktree list --porcelain\` — list existing worktrees
-2. Create if needed: \`git worktree add <absolute-path> <branch-or-HEAD>\`
-3. Update \`.sisyphus/boulder.json\` — add \`"worktree_path": "<absolute-path>"\`
-4. Work exclusively inside that worktree directory`
+**CRITICAL — DO NOT FORGET**: You are working inside a git worktree. ALL operations MUST be performed exclusively within this worktree directory.
+- Every file read, write, edit, and git operation MUST target paths under: \`${worktreePath}\`
+- When delegating tasks to subagents, you MUST include the worktree path in your delegation prompt so they also operate exclusively within the worktree
+- NEVER operate on the main repository directory — always use the worktree path above`
+}
 
 function resolveWorktreeContext(
   explicitWorktreePath: string | null,
 ): { worktreePath: string | undefined; block: string } {
   if (explicitWorktreePath === null) {
-    return { worktreePath: undefined, block: MODEL_DECIDES_WORKTREE_BLOCK }
+    return { worktreePath: undefined, block: "" }
   }
 
   const validatedPath = detectWorktreePath(explicitWorktreePath)
   if (validatedPath) {
-    return { worktreePath: validatedPath, block: `\n**Worktree**: ${validatedPath}` }
+    return { worktreePath: validatedPath, block: createWorktreeActiveBlock(validatedPath) }
   }
 
   return {
@@ -165,7 +167,7 @@ No incomplete plans available. Create a new plan with: /plan "your task"`
             appendSessionId(ctx.directory, sessionId)
           }
 
-          const worktreeDisplay = effectiveWorktree ? `\n**Worktree**: ${effectiveWorktree}` : worktreeBlock
+          const worktreeDisplay = effectiveWorktree ? createWorktreeActiveBlock(effectiveWorktree) : worktreeBlock
 
           contextInfo = `
 ## Active Work Session Found