refactor: extract parseUnitId() to centralize unit ID parsing (#1282)

Replaces 30+ inline `unitId.split("/")` + destructuring patterns across
16 production files with a single `parseUnitId()` helper that returns
`{ milestone, slice?, task? }`. If the unit ID format ever changes,
only one function needs updating.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/resources/extensions/gsd/auto-dashboard.ts

@@ -20,6 +20,7 @@ import { parseRoadmap, parsePlan } from "./files.js";
 import { readFileSync, existsSync } from "node:fs";
 import { truncateToWidth, visibleWidth } from "@gsd/pi-tui";
 import { makeUI, GLYPH, INDENT } from "../shared/mod.js";
+import { parseUnitId } from "./unit-id.js";
 
 // ─── Dashboard Data ───────────────────────────────────────────────────────────
 
@@ -372,8 +373,9 @@ export function updateProgressWidget(
         lines.push("");
 
         const isHook = unitType.startsWith("hook/");
+        const hookParsed = isHook ? parseUnitId(unitId) : undefined;
         const target = isHook
-          ? (unitId.split("/").pop() ?? unitId)
+          ? (hookParsed!.task ?? hookParsed!.slice ?? unitId)
           : (task ? `${task.id}: ${task.title}` : unitId);
         const actionLeft = `${pad}${theme.fg("accent", "▸")} ${theme.fg("accent", verb)}  ${theme.fg("text", target)}`;
         const tierTag = tierBadge ? theme.fg("dim", `[${tierBadge}] `) : "";

src/resources/extensions/gsd/auto-idempotency.ts

@@ -18,6 +18,7 @@ import {
 import { resolveMilestoneFile } from "./paths.js";
 import { MAX_CONSECUTIVE_SKIPS, MAX_LIFETIME_DISPATCHES } from "./auto/session.js";
 import type { AutoSession } from "./auto/session.js";
+import { parseUnitId } from "./unit-id.js";
 
 export interface IdempotencyContext {
   s: AutoSession;
@@ -54,7 +55,7 @@ export function checkIdempotency(ictx: IdempotencyContext): IdempotencyResult {
       s.unitConsecutiveSkips.set(idempotencyKey, skipCount);
       if (skipCount > MAX_CONSECUTIVE_SKIPS) {
         // Cross-check: verify the unit's milestone is still active (#790)
-        const skippedMid = unitId.split("/")[0];
+        const skippedMid = parseUnitId(unitId).milestone;
         const skippedMilestoneComplete = skippedMid
           ? !!resolveMilestoneFile(basePath, skippedMid, "SUMMARY")
           : false;
@@ -110,7 +111,7 @@ export function checkIdempotency(ictx: IdempotencyContext): IdempotencyResult {
     const skipCount2 = (s.unitConsecutiveSkips.get(idempotencyKey) ?? 0) + 1;
     s.unitConsecutiveSkips.set(idempotencyKey, skipCount2);
     if (skipCount2 > MAX_CONSECUTIVE_SKIPS) {
-      const skippedMid2 = unitId.split("/")[0];
+      const skippedMid2 = parseUnitId(unitId).milestone;
       const skippedMilestoneComplete2 = skippedMid2
         ? !!resolveMilestoneFile(basePath, skippedMid2, "SUMMARY")
         : false;

src/resources/extensions/gsd/auto-observability.ts

@@ -12,6 +12,7 @@ import {
   formatValidationIssues,
 } from "./observability-validator.js";
 import type { ValidationIssue } from "./observability-validator.js";
+import { parseUnitId } from "./unit-id.js";
 
 export async function collectObservabilityWarnings(
   ctx: ExtensionContext,
@@ -22,10 +23,7 @@ export async function collectObservabilityWarnings(
   // Hook units have custom artifacts — skip standard observability checks
   if (unitType.startsWith("hook/")) return [];
 
-  const parts = unitId.split("/");
-  const mid = parts[0];
-  const sid = parts[1];
-  const tid = parts[2];
+  const { milestone: mid, slice: sid, task: tid } = parseUnitId(unitId);
 
   if (!mid || !sid) return [];
 

src/resources/extensions/gsd/auto-post-unit.ts

@@ -61,6 +61,7 @@ import {
 } from "./auto-dashboard.js";
 import { join } from "node:path";
 import { STATE_REBUILD_MIN_INTERVAL_MS } from "./auto-constants.js";
+import { parseUnitId } from "./unit-id.js";
 
 /**
  * Initialize a unit dispatch: stamp the current time, set `s.currentUnit`,
@@ -134,8 +135,7 @@ export async function postUnitPreVerification(pctx: PostUnitContext): Promise<"d
       let taskContext: TaskCommitContext | undefined;
 
       if (s.currentUnit.type === "execute-task") {
-        const parts = s.currentUnit.id.split("/");
-        const [mid, sid, tid] = parts;
+        const { milestone: mid, slice: sid, task: tid } = parseUnitId(s.currentUnit.id);
         if (mid && sid && tid) {
           const summaryPath = resolveTaskFile(s.basePath, mid, sid, tid, "SUMMARY");
           if (summaryPath) {
@@ -167,8 +167,8 @@ export async function postUnitPreVerification(pctx: PostUnitContext): Promise<"d
 
     // Doctor: fix mechanical bookkeeping
     try {
-      const scopeParts = s.currentUnit.id.split("/").slice(0, 2);
-      const doctorScope = scopeParts.join("/");
+      const { milestone, slice } = parseUnitId(s.currentUnit.id);
+      const doctorScope = slice ? `${milestone}/${slice}` : milestone;
       const sliceTerminalUnits = new Set(["complete-slice", "run-uat"]);
       const effectiveFixLevel = sliceTerminalUnits.has(s.currentUnit.type) ? "all" as const : "task" as const;
       const report = await runGSDDoctor(s.basePath, { fix: true, scope: doctorScope, fixLevel: effectiveFixLevel });
@@ -348,7 +348,7 @@ export async function postUnitPostVerification(pctx: PostUnitContext): Promise<"
   // instead of dispatching LLM sessions for complete-slice / validate-milestone.
   if (s.currentUnit?.type === "execute-task" && !s.stepMode) {
     try {
-      const [mid, sid] = s.currentUnit.id.split("/");
+      const { milestone: mid, slice: sid } = parseUnitId(s.currentUnit.id);
       if (mid && sid) {
         const state = await deriveState(s.basePath);
         if (state.phase === "summarizing" && state.activeSlice?.id === sid) {

src/resources/extensions/gsd/auto-recovery.ts

@@ -42,6 +42,7 @@ import { existsSync, mkdirSync, readFileSync, writeFileSync, unlinkSync } from "
 import { atomicWriteSync } from "./atomic-write.js";
 import { loadJsonFileOrNull } from "./json-persistence.js";
 import { dirname, join } from "node:path";
+import { parseUnitId } from "./unit-id.js";
 
 // ─── Artifact Resolution & Verification ───────────────────────────────────────
 
@@ -49,9 +50,7 @@ import { dirname, join } from "node:path";
  * Resolve the expected artifact for a unit to an absolute path.
  */
 export function resolveExpectedArtifactPath(unitType: string, unitId: string, base: string): string | null {
-  const parts = unitId.split("/");
-  const mid = parts[0]!;
-  const sid = parts[1];
+  const { milestone: mid, slice: sid, task: tid } = parseUnitId(unitId);
   switch (unitType) {
     case "research-milestone": {
       const dir = resolveMilestonePath(base, mid);
@@ -78,7 +77,6 @@ export function resolveExpectedArtifactPath(unitType: string, unitId: string, ba
       return dir ? join(dir, buildSliceFileName(sid!, "UAT-RESULT")) : null;
     }
     case "execute-task": {
-      const tid = parts[2];
       const dir = resolveSlicePath(base, mid, sid!);
       return dir && tid ? join(dir, "tasks", buildTaskFileName(tid, "SUMMARY")) : null;
     }
@@ -167,10 +165,7 @@ export function verifyExpectedArtifact(unitType: string, unitId: string, base: s
 
   // execute-task must also have its checkbox marked [x] in the slice plan
   if (unitType === "execute-task") {
-    const parts = unitId.split("/");
-    const mid = parts[0];
-    const sid = parts[1];
-    const tid = parts[2];
+    const { milestone: mid, slice: sid, task: tid } = parseUnitId(unitId);
     if (mid && sid && tid) {
       const planAbs = resolveSliceFile(base, mid, sid, "PLAN");
       if (planAbs && existsSync(planAbs)) {
@@ -187,9 +182,7 @@ export function verifyExpectedArtifact(unitType: string, unitId: string, base: s
   // but omitted T{tid}-PLAN.md files would be marked complete, causing execute-task
   // to dispatch with a missing task plan (see issue #739).
   if (unitType === "plan-slice") {
-    const parts = unitId.split("/");
-    const mid = parts[0];
-    const sid = parts[1];
+    const { milestone: mid, slice: sid } = parseUnitId(unitId);
     if (mid && sid) {
       try {
         const planContent = readFileSync(absPath, "utf-8");
@@ -213,9 +206,8 @@ export function verifyExpectedArtifact(unitType: string, unitId: string, base: s
   // state machine keeps returning the same complete-slice unit (roadmap still shows
   // the slice incomplete), so dispatchNextUnit recurses forever.
   if (unitType === "complete-slice") {
-    const parts = unitId.split("/");
-    const mid = parts[0];
-    const sid = parts[1];
+    const { milestone: mid, slice: sid } = parseUnitId(unitId);
+
     if (mid && sid) {
       const dir = resolveSlicePath(base, mid, sid);
       if (dir) {
@@ -268,9 +260,7 @@ export function writeBlockerPlaceholder(unitType: string, unitId: string, base:
 }
 
 export function diagnoseExpectedArtifact(unitType: string, unitId: string, base: string): string | null {
-  const parts = unitId.split("/");
-  const mid = parts[0];
-  const sid = parts[1];
+  const { milestone: mid, slice: sid, task: tid } = parseUnitId(unitId);
   switch (unitType) {
     case "research-milestone":
       return `${relMilestoneFile(base, mid!, "RESEARCH")} (milestone research)`;
@@ -281,7 +271,6 @@ export function diagnoseExpectedArtifact(unitType: string, unitId: string, base:
     case "plan-slice":
       return `${relSliceFile(base, mid!, sid!, "PLAN")} (slice plan)`;
     case "execute-task": {
-      const tid = parts[2];
       return `Task ${tid} marked [x] in ${relSliceFile(base, mid!, sid!, "PLAN")} + summary written`;
     }
     case "complete-slice":
@@ -539,10 +528,7 @@ export async function selfHealRuntimeRecords(
  * These are shown when automatic reconciliation is not possible.
  */
 export function buildLoopRemediationSteps(unitType: string, unitId: string, base: string): string | null {
-  const parts = unitId.split("/");
-  const mid = parts[0];
-  const sid = parts[1];
-  const tid = parts[2];
+  const { milestone: mid, slice: sid, task: tid } = parseUnitId(unitId);
   switch (unitType) {
     case "execute-task": {
       if (!mid || !sid || !tid) break;

src/resources/extensions/gsd/auto-start.ts

@@ -64,6 +64,7 @@ import type { AutoSession } from "./auto/session.js";
 import { existsSync, mkdirSync, readdirSync, statSync, unlinkSync } from "node:fs";
 import { join } from "node:path";
 import { getErrorMessage } from "./error-utils.js";
+import { parseUnitId } from "./unit-id.js";
 
 export interface BootstrapDeps {
   shouldUseWorktreeIsolation: () => boolean;
@@ -139,7 +140,7 @@ export async function bootstrapAutoSession(
   if (crashLock && crashLock.pid !== process.pid) {
     // We already hold the session lock, so no concurrent session is running.
     // The crash lock is from a dead process — recover context from it.
-    const recoveredMid = crashLock.unitId.split("/")[0];
+    const recoveredMid = parseUnitId(crashLock.unitId).milestone;
     const milestoneAlreadyComplete = recoveredMid
       ? !!resolveMilestoneFile(base, recoveredMid, "SUMMARY")
       : false;

src/resources/extensions/gsd/auto-stuck-detection.ts

@@ -39,6 +39,7 @@ import {
 import type { AutoSession } from "./auto/session.js";
 import { existsSync, mkdirSync, writeFileSync } from "node:fs";
 import { join } from "node:path";
+import { parseUnitId } from "./unit-id.js";
 
 export interface StuckContext {
   s: AutoSession;
@@ -99,7 +100,7 @@ export async function checkStuckAndRecover(sctx: StuckContext): Promise<StuckRes
 
     // Final reconciliation pass for execute-task
     if (unitType === "execute-task") {
-      const [mid, sid, tid] = unitId.split("/");
+      const { milestone: mid, slice: sid, task: tid } = parseUnitId(unitId);
       if (mid && sid && tid) {
         const status = await inspectExecuteTaskDurability(basePath, unitId);
         if (status) {
@@ -168,7 +169,7 @@ export async function checkStuckAndRecover(sctx: StuckContext): Promise<StuckRes
     // Adaptive self-repair: each retry attempts a different remediation step.
     if (unitType === "execute-task") {
       const status = await inspectExecuteTaskDurability(basePath, unitId);
-      const [mid, sid, tid] = unitId.split("/");
+      const { milestone: mid, slice: sid, task: tid } = parseUnitId(unitId);
       if (status && mid && sid && tid) {
         if (status.summaryExists && !status.taskChecked) {
           const repaired = skipExecuteTask(basePath, mid, sid, tid, status, "self-repair", 0);

src/resources/extensions/gsd/auto-timeout-recovery.ts

@@ -18,6 +18,7 @@ import {
   writeBlockerPlaceholder,
 } from "./auto-recovery.js";
 import { existsSync } from "node:fs";
+import { parseUnitId } from "./unit-id.js";
 
 export interface RecoveryContext {
   basePath: string;
@@ -128,7 +129,7 @@ export async function recoverTimedOutUnit(
 
     // Retries exhausted — write missing durable artifacts and advance.
     const diagnostic = formatExecuteTaskRecoveryStatus(status);
-    const [mid, sid, tid] = unitId.split("/");
+    const { milestone: mid, slice: sid, task: tid } = parseUnitId(unitId);
     const skipped = mid && sid && tid
       ? skipExecuteTask(basePath, mid, sid, tid, status, reason, maxRecoveryAttempts)
       : false;

src/resources/extensions/gsd/auto-verification.ts

@@ -25,6 +25,7 @@ import { removePersistedKey } from "./auto-recovery.js";
 import type { AutoSession, PendingVerificationRetry } from "./auto/session.js";
 import { join } from "node:path";
 import { getErrorMessage } from "./error-utils.js";
+import { parseUnitId } from "./unit-id.js";
 
 export interface VerificationContext {
   s: AutoSession;
@@ -58,10 +59,9 @@ export async function runPostUnitVerification(
     const prefs = effectivePrefs?.preferences;
 
     // Read task plan verify field
-    const parts = s.currentUnit.id.split("/");
+    const { milestone: mid, slice: sid, task: tid } = parseUnitId(s.currentUnit.id);
     let taskPlanVerify: string | undefined;
-    if (parts.length >= 3) {
-      const [mid, sid, tid] = parts;
+    if (mid && sid && tid) {
       const planFile = resolveSliceFile(s.basePath, mid, sid, "PLAN");
       if (planFile) {
         const planContent = await loadFile(planFile);
@@ -153,9 +153,8 @@ export async function runPostUnitVerification(
 
     // Write verification evidence JSON
     const attempt = s.verificationRetryCount.get(s.currentUnit.id) ?? 0;
-    if (parts.length >= 3) {
+    if (mid && sid && tid) {
       try {
-        const [mid, sid, tid] = parts;
         const sDir = resolveSlicePath(s.basePath, mid, sid);
         if (sDir) {
           const tasksDir = join(sDir, "tasks");

src/resources/extensions/gsd/auto.ts

@@ -105,6 +105,7 @@ import { computeBudgets, resolveExecutorContextWindow } from "./context-budget.j
 import { GSDError, GSD_ARTIFACT_MISSING } from "./errors.js";
 import { join } from "node:path";
 import { sep as pathSep } from "node:path";
+import { parseUnitId } from "./unit-id.js";
 import { readdirSync, readFileSync, existsSync, mkdirSync, writeFileSync, unlinkSync, statSync } from "node:fs";
 import { atomicWriteSync } from "./atomic-write.js";
 import { nativeIsRepo, nativeInit, nativeAddAll, nativeCommit } from "./native-git-bridge.js";
@@ -1748,8 +1749,7 @@ async function dispatchNextUnit(
 function ensurePreconditions(
   unitType: string, unitId: string, base: string, state: GSDState,
 ): void {
-  const parts = unitId.split("/");
-  const mid = parts[0]!;
+  const { milestone: mid } = parseUnitId(unitId);
 
   const mDir = resolveMilestonePath(base, mid);
   if (!mDir) {
@@ -1757,8 +1757,8 @@ function ensurePreconditions(
     mkdirSync(join(newDir, "slices"), { recursive: true });
   }
 
-  if (parts.length >= 2) {
-    const sid = parts[1]!;
+  const sid = parseUnitId(unitId).slice;
+  if (sid) {
 
     const mDirResolved = resolveMilestonePath(base, mid);
     if (mDirResolved) {

src/resources/extensions/gsd/complexity-classifier.ts

@@ -6,6 +6,7 @@ import { existsSync, readFileSync } from "node:fs";
 import { join } from "node:path";
 import { gsdRoot } from "./paths.js";
 import { getAdaptiveTierAdjustment } from "./routing-history.js";
+import { parseUnitId } from "./unit-id.js";
 
 // ─── Types ───────────────────────────────────────────────────────────────────
 
@@ -180,15 +181,14 @@ function analyzePlanComplexity(
   basePath: string,
 ): TaskAnalysis | null {
   // Check if this is a milestone-level plan (more complex) vs single slice
-  const parts = unitId.split("/");
-  if (parts.length === 1) {
+  const { milestone: mid, slice: sid } = parseUnitId(unitId);
+  if (!sid) {
     // Milestone-level planning is always at least standard
     return { tier: "standard", reason: "milestone-level planning" };
   }
 
   // For slice planning, try to read the context/research to gauge complexity
   // If research exists and is large, bump to heavy
-  const [mid, sid] = parts;
   const researchPath = join(gsdRoot(basePath), mid, "slices", sid, "RESEARCH.md");
   try {
     if (existsSync(researchPath)) {
@@ -210,10 +210,8 @@ function analyzePlanComplexity(
  */
 function extractTaskMetadata(unitId: string, basePath: string): TaskMetadata {
   const meta: TaskMetadata = {};
-  const parts = unitId.split("/");
-  if (parts.length !== 3) return meta;
-
-  const [mid, sid, tid] = parts;
+  const { milestone: mid, slice: sid, task: tid } = parseUnitId(unitId);
+  if (!mid || !sid || !tid) return meta;
   const taskPlanPath = join(gsdRoot(basePath), mid, "slices", sid, "tasks", `${tid}-PLAN.md`);
 
   try {

src/resources/extensions/gsd/dispatch-guard.ts

@@ -5,6 +5,7 @@ import { readdirSync } from "node:fs";
 import { resolveMilestoneFile, milestonesDir } from "./paths.js";
 import { parseRoadmapSlices } from "./roadmap-slices.js";
 import { findMilestoneIds } from "./guided-flow.js";
+import { parseUnitId } from "./unit-id.js";
 
 const SLICE_DISPATCH_TYPES = new Set([
   "research-slice",
@@ -39,7 +40,7 @@ function readRoadmapFromDisk(base: string, milestoneId: string): string | null {
 export function getPriorSliceCompletionBlocker(base: string, _mainBranch: string, unitType: string, unitId: string): string | null {
   if (!SLICE_DISPATCH_TYPES.has(unitType)) return null;
 
-  const [targetMid, targetSid] = unitId.split("/");
+  const { milestone: targetMid, slice: targetSid } = parseUnitId(unitId);
   if (!targetMid || !targetSid) return null;
 
   // Use findMilestoneIds to respect custom queue order.

src/resources/extensions/gsd/metrics.ts

@@ -18,6 +18,7 @@ import type { ExtensionContext } from "@gsd/pi-coding-agent";
 import { gsdRoot } from "./paths.js";
 import { getAndClearSkills } from "./skill-telemetry.js";
 import { loadJsonFile, loadJsonFileOrNull, saveJsonFile } from "./json-persistence.js";
+import { parseUnitId } from "./unit-id.js";
 
 // Re-export from shared — canonical implementation lives in format-utils.
 export { formatTokenCount } from "../shared/mod.js";
@@ -290,9 +291,8 @@ export function aggregateByPhase(units: UnitMetrics[]): PhaseAggregate[] {
 export function aggregateBySlice(units: UnitMetrics[]): SliceAggregate[] {
   const map = new Map<string, SliceAggregate>();
   for (const u of units) {
-    const parts = u.id.split("/");
-    // Slice ID is parts[0]/parts[1] if it exists, else parts[0]
-    const sliceId = parts.length >= 2 ? `${parts[0]}/${parts[1]}` : parts[0];
+    const { milestone, slice } = parseUnitId(u.id);
+    const sliceId = slice ? `${milestone}/${slice}` : milestone;
     let agg = map.get(sliceId);
     if (!agg) {
       agg = { sliceId, units: 0, tokens: emptyTokens(), cost: 0, duration: 0 };

src/resources/extensions/gsd/post-unit-hooks.ts

@@ -15,6 +15,7 @@ import { resolvePostUnitHooks, resolvePreDispatchHooks } from "./preferences.js"
 import { existsSync, readFileSync, writeFileSync, mkdirSync } from "node:fs";
 import { join } from "node:path";
 import { gsdRoot } from "./paths.js";
+import { parseUnitId } from "./unit-id.js";
 
 // ─── Hook Queue State ──────────────────────────────────────────────────────
 
@@ -149,7 +150,7 @@ function dequeueNextHook(basePath: string): HookDispatchResult | null {
     };
 
     // Build the prompt with variable substitution
-    const [mid, sid, tid] = triggerUnitId.split("/");
+    const { milestone: mid, slice: sid, task: tid } = parseUnitId(triggerUnitId);
     const prompt = config.prompt
       .replace(/\{milestoneId\}/g, mid ?? "")
       .replace(/\{sliceId\}/g, sid ?? "")
@@ -208,16 +209,14 @@ function handleHookCompletion(basePath: string): HookDispatchResult | null {
  *   - Milestone-level (M001):    .gsd/M001/{artifact}
  */
 export function resolveHookArtifactPath(basePath: string, unitId: string, artifactName: string): string {
-  const parts = unitId.split("/");
-  if (parts.length === 3) {
-    const [mid, sid, tid] = parts;
+  const { milestone: mid, slice: sid, task: tid } = parseUnitId(unitId);
+  if (mid && sid && tid) {
     return join(gsdRoot(basePath), mid, "slices", sid, "tasks", `${tid}-${artifactName}`);
   }
-  if (parts.length === 2) {
-    const [mid, sid] = parts;
+  if (mid && sid) {
     return join(gsdRoot(basePath), mid, "slices", sid, artifactName);
   }
-  return join(gsdRoot(basePath), parts[0], artifactName);
+  return join(gsdRoot(basePath), mid, artifactName);
 }
 
 // ═══════════════════════════════════════════════════════════════════════════
@@ -253,7 +252,7 @@ export function runPreDispatchHooks(
     return { action: "proceed", prompt, firedHooks: [] };
   }
 
-  const [mid, sid, tid] = unitId.split("/");
+  const { milestone: mid, slice: sid, task: tid } = parseUnitId(unitId);
   const substitute = (text: string): string =>
     text
       .replace(/\{milestoneId\}/g, mid ?? "")
@@ -466,7 +465,7 @@ export function triggerHookManually(
   activeHook.cycle = currentCycle;
 
   // Build the prompt with variable substitution
-  const [mid, sid, tid] = unitId.split("/");
+  const { milestone: mid, slice: sid, task: tid } = parseUnitId(unitId);
   const prompt = hook.prompt
     .replace(/\{milestoneId\}/g, mid ?? "")
     .replace(/\{sliceId\}/g, sid ?? "")

src/resources/extensions/gsd/undo.ts

@@ -9,6 +9,7 @@ import { deriveState } from "./state.js";
 import { invalidateAllCaches } from "./cache.js";
 import { gsdRoot, resolveTasksDir, resolveSlicePath, buildTaskFileName } from "./paths.js";
 import { sendDesktopNotification } from "./notifications.js";
+import { parseUnitId } from "./unit-id.js";
 
 /**
  * Undo the last completed unit: revert git commits, remove from completed-units,
@@ -62,11 +63,10 @@ export async function handleUndo(args: string, ctx: ExtensionCommandContext, _pi
   writeFileSync(completedKeysFile, JSON.stringify(keys), "utf-8");
 
   // 3. Delete summary artifact
-  const parts = unitId.split("/");
+  const { milestone: mid, slice: sid, task: tid } = parseUnitId(unitId);
   let summaryRemoved = false;
-  if (parts.length === 3) {
+  if (mid && sid && tid) {
     // Task-level: M001/S01/T01
-    const [mid, sid, tid] = parts;
     const tasksDir = resolveTasksDir(basePath, mid, sid);
     if (tasksDir) {
       const summaryFile = join(tasksDir, buildTaskFileName(tid, "SUMMARY"));
@@ -75,9 +75,8 @@ export async function handleUndo(args: string, ctx: ExtensionCommandContext, _pi
         summaryRemoved = true;
       }
     }
-  } else if (parts.length === 2) {
+  } else if (mid && sid) {
     // Slice-level: M001/S01
-    const [mid, sid] = parts;
     const slicePath = resolveSlicePath(basePath, mid, sid);
     if (slicePath) {
       // Try common summary filenames
@@ -93,8 +92,7 @@ export async function handleUndo(args: string, ctx: ExtensionCommandContext, _pi
 
   // 4. Uncheck task in PLAN if execute-task
   let planUpdated = false;
-  if (unitType === "execute-task" && parts.length === 3) {
-    const [mid, sid, tid] = parts;
+  if (unitType === "execute-task" && mid && sid && tid) {
     planUpdated = uncheckTaskInPlan(basePath, mid, sid, tid);
   }
 

src/resources/extensions/gsd/unit-id.ts

@@ -0,0 +1,14 @@
+// GSD Extension — Unit ID Parsing
+// Centralizes the milestone/slice/task decomposition of unit ID strings.
+
+export interface ParsedUnitId {
+  milestone: string;
+  slice?: string;
+  task?: string;
+}
+
+/** Parse a unit ID string (e.g. "M1/S1/T1") into its milestone, slice, and task components. */
+export function parseUnitId(unitId: string): ParsedUnitId {
+  const [milestone, slice, task] = unitId.split("/");
+  return { milestone: milestone!, slice, task };
+}

src/resources/extensions/gsd/unit-runtime.ts

@@ -9,6 +9,7 @@ import {
 } from "./paths.js";
 import { loadFile, parseTaskPlanMustHaves, countMustHavesMentionedInSummary } from "./files.js";
 import { loadJsonFileOrNull, saveJsonFile } from "./json-persistence.js";
+import { parseUnitId } from "./unit-id.js";
 
 export type UnitRuntimePhase =
   | "dispatched"
@@ -131,7 +132,7 @@ export async function inspectExecuteTaskDurability(
   basePath: string,
   unitId: string,
 ): Promise<ExecuteTaskRecoveryStatus | null> {
-  const [mid, sid, tid] = unitId.split("/");
+  const { milestone: mid, slice: sid, task: tid } = parseUnitId(unitId);
   if (!mid || !sid || !tid) return null;
 
   const planAbs = resolveSliceFile(basePath, mid, sid, "PLAN");