fix(start-work): preserve existing works when starting an explicit new plan

src/hooks/start-work/context-info-builder.test.ts

@@ -168,4 +168,52 @@ describe("buildStartWorkContextInfo", () => {
     expect(existsSync(getBoulderFilePath(testDirectory))).toBe(true)
     expect(clearSpy).toHaveBeenCalledTimes(0)
   })
+
+  test("keeps existing works when explicit new plan is started", () => {
+    // given
+    writePlan("work-a", "## TODOs\n- [ ] 1. Work A")
+    const workBPath = writePlan("work-b", "## TODOs\n- [ ] 1. Work B")
+    writePlan("new-plan-c", "## TODOs\n- [ ] 1. Work C")
+
+    const initialState = createBoulderState(
+      join(testDirectory, ".sisyphus", "plans", "work-a.md"),
+      "session-a",
+      "atlas",
+      "/tmp/worktree-a",
+    )
+    writeBoulderState(testDirectory, initialState)
+
+    const workAId = initialState.active_work_id!
+    const withSecondWork = addBoulderWork(testDirectory, {
+      planPath: workBPath,
+      sessionId: "session-b",
+      agent: "atlas",
+      worktreePath: "/tmp/worktree-b",
+    })
+    expect(withSecondWork).not.toBeNull()
+    const workBId = Object.keys(withSecondWork!.works!).find((workId) => workId !== workAId)
+    expect(workBId).toBeDefined()
+
+    // when
+    buildStartWorkContextInfo({
+      ctx: createPluginInput(),
+      explicitPlanName: "new-plan-c",
+      existingState: readExistingState(),
+      sessionId: "session-c",
+      timestamp: "2026-05-11T00:00:00.000Z",
+      activeAgent: "atlas",
+      worktreePath: undefined,
+      worktreeBlock: "",
+    })
+
+    // then
+    const nextState = readBoulderState(testDirectory)
+    const workIds = Object.keys(nextState?.works ?? {})
+    expect(workIds.length).toBe(3)
+    expect(workIds).toContain(workAId)
+    expect(workIds).toContain(workBId!)
+    const workC = getWorkByPlanName(testDirectory, "new-plan-c")
+    expect(workC).not.toBeNull()
+    expect(workIds).toContain(workC!.work_id)
+  })
 })

src/hooks/start-work/context-info-builder.ts

@@ -48,19 +48,14 @@ function findPlanByName(plans: string[], requestedName: string): string | null {
   return normalizedPartialMatch || null
 }
 
-function buildAutoSelectedPlanContext(params: {
+function buildAutoSelectedPlanContextInfoOnly(params: {
   planPath: string
   sessionId: string
   timestamp: string
-  activeAgent: string
-  worktreePath: string | undefined
   worktreeBlock: string
-  directory: string
 }): string {
-  const { planPath, sessionId, timestamp, activeAgent, worktreePath, worktreeBlock, directory } = params
+  const { planPath, sessionId, timestamp, worktreeBlock } = params
   const progress = getPlanProgress(planPath)
-  const newState = createBoulderState(planPath, sessionId, activeAgent, worktreePath)
-  writeBoulderState(directory, newState)
 
   return `
 ## Auto-Selected Plan
@@ -75,6 +70,27 @@ ${worktreeBlock}
 boulder.json has been created. Read the plan and begin execution.`
 }
 
+function buildAutoSelectedPlanContextWithStateInit(params: {
+  planPath: string
+  sessionId: string
+  timestamp: string
+  activeAgent: string
+  worktreePath: string | undefined
+  worktreeBlock: string
+  directory: string
+}): string {
+  const { planPath, sessionId, timestamp, activeAgent, worktreePath, worktreeBlock, directory } = params
+  const newState = createBoulderState(planPath, sessionId, activeAgent, worktreePath)
+  writeBoulderState(directory, newState)
+
+  return buildAutoSelectedPlanContextInfoOnly({
+    planPath,
+    sessionId,
+    timestamp,
+    worktreeBlock,
+  })
+}
+
 function buildMissingPlanContext(explicitPlanName: string, allPlans: string[]): string {
   const incompletePlans = allPlans.filter((p) => !getPlanProgress(p).isComplete)
   if (incompletePlans.length > 0) {
@@ -218,14 +234,11 @@ function buildExplicitPlanContext(params: {
     worktreePath,
   })
 
-  return buildAutoSelectedPlanContext({
+  return buildAutoSelectedPlanContextInfoOnly({
     planPath: matchedPlan,
     sessionId,
     timestamp,
-    activeAgent,
-    worktreePath,
     worktreeBlock,
-    directory,
   })
 }
 
@@ -328,7 +341,7 @@ function buildPlanDiscoveryContext(params: {
   }
 
   if (incompletePlans.length === 1) {
-    return contextInfo + buildAutoSelectedPlanContext({
+    return contextInfo + buildAutoSelectedPlanContextWithStateInit({
       planPath: incompletePlans[0],
       sessionId,
       timestamp,