diff --git a/src/hooks/start-work/index.test.ts b/src/hooks/start-work/index.test.ts
index 3a12ca2c8..31f73fdfb 100644
--- a/src/hooks/start-work/index.test.ts
+++ b/src/hooks/start-work/index.test.ts
@@ -151,5 +151,90 @@ describe("start-work hook", () => {
       expect(output.parts[0].text).not.toContain("$TIMESTAMP")
       expect(output.parts[0].text).toMatch(/\d{4}-\d{2}-\d{2}T/)
     })
+
+    test("should auto-select when only one incomplete plan among multiple plans", async () => {
+      // #given - multiple plans but only one incomplete
+      const plansDir = join(TEST_DIR, ".sisyphus", "plans")
+      mkdirSync(plansDir, { recursive: true })
+
+      // Plan 1: complete (all checked)
+      const plan1Path = join(plansDir, "plan-complete.md")
+      writeFileSync(plan1Path, "# Plan Complete\n- [x] Task 1\n- [x] Task 2")
+
+      // Plan 2: incomplete (has unchecked)
+      const plan2Path = join(plansDir, "plan-incomplete.md")
+      writeFileSync(plan2Path, "# Plan Incomplete\n- [ ] Task 1\n- [x] Task 2")
+
+      const hook = createStartWorkHook(createMockPluginInput())
+      const output = {
+        parts: [{ type: "text", text: "Start Sisyphus work session" }],
+      }
+
+      // #when
+      await hook["chat.message"](
+        { sessionID: "session-123" },
+        output
+      )
+
+      // #then - should auto-select the incomplete plan, not ask user
+      expect(output.parts[0].text).toContain("Auto-Selected Plan")
+      expect(output.parts[0].text).toContain("plan-incomplete")
+      expect(output.parts[0].text).not.toContain("Multiple Plans Found")
+    })
+
+    test("should wrap multiple plans message in system-reminder tag", async () => {
+      // #given - multiple incomplete plans
+      const plansDir = join(TEST_DIR, ".sisyphus", "plans")
+      mkdirSync(plansDir, { recursive: true })
+
+      const plan1Path = join(plansDir, "plan-a.md")
+      writeFileSync(plan1Path, "# Plan A\n- [ ] Task 1")
+
+      const plan2Path = join(plansDir, "plan-b.md")
+      writeFileSync(plan2Path, "# Plan B\n- [ ] Task 2")
+
+      const hook = createStartWorkHook(createMockPluginInput())
+      const output = {
+        parts: [{ type: "text", text: "Start Sisyphus work session" }],
+      }
+
+      // #when
+      await hook["chat.message"](
+        { sessionID: "session-123" },
+        output
+      )
+
+      // #then - should use system-reminder tag format
+      expect(output.parts[0].text).toContain("<system-reminder>")
+      expect(output.parts[0].text).toContain("</system-reminder>")
+      expect(output.parts[0].text).toContain("Multiple Plans Found")
+    })
+
+    test("should use 'ask user' prompt style for multiple plans", async () => {
+      // #given - multiple incomplete plans
+      const plansDir = join(TEST_DIR, ".sisyphus", "plans")
+      mkdirSync(plansDir, { recursive: true })
+
+      const plan1Path = join(plansDir, "plan-x.md")
+      writeFileSync(plan1Path, "# Plan X\n- [ ] Task 1")
+
+      const plan2Path = join(plansDir, "plan-y.md")
+      writeFileSync(plan2Path, "# Plan Y\n- [ ] Task 2")
+
+      const hook = createStartWorkHook(createMockPluginInput())
+      const output = {
+        parts: [{ type: "text", text: "Start Sisyphus work session" }],
+      }
+
+      // #when
+      await hook["chat.message"](
+        { sessionID: "session-123" },
+        output
+      )
+
+      // #then - should prompt agent to ask user, not ask directly
+      expect(output.parts[0].text).toContain("Ask the user")
+      expect(output.parts[0].text).not.toContain("Which plan would you like to work on?")
+    })
   })
 })
diff --git a/src/hooks/start-work/index.ts b/src/hooks/start-work/index.ts
index d58c2455d..d7a8c6922 100644
--- a/src/hooks/start-work/index.ts
+++ b/src/hooks/start-work/index.ts
@@ -80,6 +80,7 @@ Looking for new plans...`
 
       if (!existingState || getPlanProgress(existingState.active_plan).isComplete) {
         const plans = findPrometheusPlans(ctx.directory)
+        const incompletePlans = plans.filter(p => !getPlanProgress(p).isComplete)
         
         if (plans.length === 0) {
           contextInfo += `
@@ -88,8 +89,14 @@ Looking for new plans...`
 
 No Prometheus plan files found at .sisyphus/plans/
 Use Prometheus to create a work plan first: /plan "your task"`
-        } else if (plans.length === 1) {
-          const planPath = plans[0]
+        } else if (incompletePlans.length === 0) {
+          contextInfo += `
+
+## All Plans Complete
+
+All ${plans.length} plan(s) are complete. Create a new plan with: /plan "your task"`
+        } else if (incompletePlans.length === 1) {
+          const planPath = incompletePlans[0]
           const progress = getPlanProgress(planPath)
           const newState = createBoulderState(planPath, sessionId)
           writeBoulderState(ctx.directory, newState)
@@ -106,7 +113,7 @@ Use Prometheus to create a work plan first: /plan "your task"`
 
 boulder.json has been created. Read the plan and begin execution.`
         } else {
-          const planList = plans.map((p, i) => {
+          const planList = incompletePlans.map((p, i) => {
             const progress = getPlanProgress(p)
             const stat = require("node:fs").statSync(p)
             const modified = new Date(stat.mtimeMs).toISOString()
@@ -115,6 +122,7 @@ boulder.json has been created. Read the plan and begin execution.`
 
           contextInfo += `
 
+<system-reminder>
 ## Multiple Plans Found
 
 Current Time: ${timestamp}
@@ -122,7 +130,8 @@ Session ID: ${sessionId}
 
 ${planList}
 
-Which plan would you like to work on? Reply with the number or plan name.`
+Ask the user which plan to work on. Present the options above and wait for their response.
+</system-reminder>`
         }
       }