diff --git a/src/resources/extensions/gsd/auto-observability.ts b/src/resources/extensions/gsd/auto-observability.ts
index ddcc0bf3d..0715a9ac4 100644
--- a/src/resources/extensions/gsd/auto-observability.ts
+++ b/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 [];
 
diff --git a/src/resources/extensions/gsd/auto-recovery.ts b/src/resources/extensions/gsd/auto-recovery.ts
index 42c7cac97..9181d7fe8 100644
--- a/src/resources/extensions/gsd/auto-recovery.ts
+++ b/src/resources/extensions/gsd/auto-recovery.ts
@@ -195,13 +195,8 @@ export function verifyExpectedArtifact(
   // Reactive-execute: verify that each dispatched task's summary exists.
   // The unitId encodes the batch: "{mid}/{sid}/reactive+T02,T03"
   if (unitType === "reactive-execute") {
-    const parts = unitId.split("/");
-    const mid = parts[0];
-    const sidAndBatch = parts[1];
-    const batchPart = parts[2]; // "reactive+T02,T03"
-    if (!mid || !sidAndBatch || !batchPart) return false;
-
-    const sid = sidAndBatch;
+    const { milestone: mid, slice: sid, task: batchPart } = parseUnitId(unitId);
+    if (!mid || !sid || !batchPart) return false;
     const plusIdx = batchPart.indexOf("+");
     if (plusIdx === -1) {
       // Legacy format "reactive" without batch IDs — fall back to "any summary"
@@ -233,10 +228,7 @@ export function verifyExpectedArtifact(
   // Gate-evaluate: verify that each dispatched gate has been resolved in the DB.
   // The unitId encodes the batch: "{mid}/{sid}/gates+Q3,Q4"
   if (unitType === "gate-evaluate") {
-    const parts = unitId.split("/");
-    const mid = parts[0];
-    const sid = parts[1];
-    const batchPart = parts[2]; // "gates+Q3,Q4"
+    const { milestone: mid, slice: sid, task: batchPart } = parseUnitId(unitId);
     if (!mid || !sid || !batchPart) return false;
 
     const plusIdx = batchPart.indexOf("+");
@@ -286,10 +278,7 @@ export function verifyExpectedArtifact(
   // execute-task: DB status is authoritative. Fall back to heading-style plan
   // detection when the DB is unavailable (unmigrated projects).
   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 dbTask = getTask(mid, sid, tid);
       if (dbTask) {
@@ -319,9 +308,7 @@ export function verifyExpectedArtifact(
   // 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 {
         // DB primary path — get task IDs to verify task plan files exist
@@ -356,9 +343,7 @@ export function verifyExpectedArtifact(
   // complete-slice: DB status is authoritative for whether the slice is done.
   // Fall back to file-based check (roadmap [x]) when DB is unavailable.
   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) {
@@ -430,10 +415,7 @@ export function writeBlockerPlaceholder(
   // Without this, the DB status stays "pending" and the dispatch loop
   // re-derives the same task indefinitely (#2531).
   if (unitType === "execute-task" && isDbAvailable()) {
-    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) {
       try { updateTaskStatus(mid, sid, tid, "complete", new Date().toISOString()); } catch { /* non-fatal */ }
     }
@@ -558,10 +540,7 @@ export function buildLoopRemediationSteps(
   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;
diff --git a/src/resources/extensions/gsd/auto.ts b/src/resources/extensions/gsd/auto.ts
index c807a878d..9d6fd4019 100644
--- a/src/resources/extensions/gsd/auto.ts
+++ b/src/resources/extensions/gsd/auto.ts
@@ -17,6 +17,7 @@ import type {
 } from "@gsd/pi-coding-agent";
 
 import { deriveState } from "./state.js";
+import { parseUnitId } from "./unit-id.js";
 import type { GSDState } from "./types.js";
 import { getManifestStatus } from "./files.js";
 export { inlinePriorMilestoneSummary } from "./files.js";
@@ -1280,8 +1281,7 @@ function ensurePreconditions(
   base: string,
   state: GSDState,
 ): void {
-  const parts = unitId.split("/");
-  const mid = parts[0]!;
+  const { milestone: mid, slice: sid } = parseUnitId(unitId);
 
   const mDir = resolveMilestonePath(base, mid);
   if (!mDir) {
@@ -1289,8 +1289,7 @@ function ensurePreconditions(
     mkdirSync(join(newDir, "slices"), { recursive: true });
   }
 
-  if (parts.length >= 2) {
-    const sid = parts[1]!;
+  if (sid !== undefined) {
 
     const mDirResolved = resolveMilestonePath(base, mid);
     if (mDirResolved) {
diff --git a/src/resources/extensions/gsd/custom-execution-policy.ts b/src/resources/extensions/gsd/custom-execution-policy.ts
index 6912c83f4..656873682 100644
--- a/src/resources/extensions/gsd/custom-execution-policy.ts
+++ b/src/resources/extensions/gsd/custom-execution-policy.ts
@@ -14,6 +14,7 @@
 import type { ExecutionPolicy } from "./execution-policy.js";
 import type { RecoveryAction, CloseoutResult } from "./engine-types.js";
 import { runCustomVerification } from "./custom-verification.js";
+import { parseUnitId } from "./unit-id.js";
 
 export class CustomExecutionPolicy implements ExecutionPolicy {
   private readonly runDir: string;
@@ -48,8 +49,8 @@ export class CustomExecutionPolicy implements ExecutionPolicy {
     unitId: string,
     _context: { basePath: string },
   ): Promise<"continue" | "retry" | "pause"> {
-    const parts = unitId.split("/");
-    const stepId = parts[parts.length - 1];
+    const { milestone, slice, task } = parseUnitId(unitId);
+    const stepId = task ?? slice ?? milestone;
     return runCustomVerification(this.runDir, stepId);
   }
 
diff --git a/src/resources/extensions/gsd/custom-workflow-engine.ts b/src/resources/extensions/gsd/custom-workflow-engine.ts
index 49e71a4bd..8c2a168ea 100644
--- a/src/resources/extensions/gsd/custom-workflow-engine.ts
+++ b/src/resources/extensions/gsd/custom-workflow-engine.ts
@@ -33,6 +33,7 @@ import {
 } from "./graph.js";
 import { injectContext } from "./context-injector.js";
 import type { WorkflowDefinition, StepDefinition } from "./definition-loader.js";
+import { parseUnitId } from "./unit-id.js";
 
 /** Read and parse the frozen DEFINITION.yaml from a run directory. */
 export function readFrozenDefinition(runDir: string): WorkflowDefinition {
@@ -181,8 +182,8 @@ export class CustomWorkflowEngine implements WorkflowEngine {
     const graph = state.raw as WorkflowGraph;
 
     // Extract stepId from "<workflowName>/<stepId>"
-    const parts = completedStep.unitId.split("/");
-    const stepId = parts[parts.length - 1];
+    const { milestone, slice, task } = parseUnitId(completedStep.unitId);
+    const stepId = task ?? slice ?? milestone;
 
     const updatedGraph = markStepComplete(graph, stepId);
     writeGraph(this.runDir, updatedGraph);
diff --git a/src/resources/extensions/gsd/dispatch-guard.ts b/src/resources/extensions/gsd/dispatch-guard.ts
index 33d0687e4..811ff4cf7 100644
--- a/src/resources/extensions/gsd/dispatch-guard.ts
+++ b/src/resources/extensions/gsd/dispatch-guard.ts
@@ -2,6 +2,7 @@
 
 import { resolveMilestoneFile } from "./paths.js";
 import { findMilestoneIds } from "./guided-flow.js";
+import { parseUnitId } from "./unit-id.js";
 import { isDbAvailable, getMilestoneSlices } from "./gsd-db.js";
 import { parseRoadmap } from "./parsers-legacy.js";
 import { readFileSync } from "node:fs";
@@ -22,7 +23,7 @@ export function getPriorSliceCompletionBlocker(
 ): 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.
diff --git a/src/resources/extensions/gsd/rule-registry.ts b/src/resources/extensions/gsd/rule-registry.ts
index 6f818080f..e8ac7c13e 100644
--- a/src/resources/extensions/gsd/rule-registry.ts
+++ b/src/resources/extensions/gsd/rule-registry.ts
@@ -20,20 +20,19 @@ import type {
 import { resolvePostUnitHooks, resolvePreDispatchHooks } from "./preferences.js";
 import { existsSync, readFileSync, writeFileSync, mkdirSync } from "node:fs";
 import { join } from "node:path";
+import { parseUnitId } from "./unit-id.js";
 
 // ─── Artifact Path Resolution ──────────────────────────────────────────────
 
 export function resolveHookArtifactPath(basePath: string, unitId: string, artifactName: string): string {
-  const parts = unitId.split("/");
-  if (parts.length === 3) {
-    const [mid, sid, tid] = parts;
-    return join(basePath, ".gsd", "milestones", mid, "slices", sid, "tasks", `${tid}-${artifactName}`);
+  const { milestone, slice, task } = parseUnitId(unitId);
+  if (task !== undefined && slice !== undefined) {
+    return join(basePath, ".gsd", "milestones", milestone, "slices", slice, "tasks", `${task}-${artifactName}`);
   }
-  if (parts.length === 2) {
-    const [mid, sid] = parts;
-    return join(basePath, ".gsd", "milestones", mid, "slices", sid, artifactName);
+  if (slice !== undefined) {
+    return join(basePath, ".gsd", "milestones", milestone, "slices", slice, artifactName);
   }
-  return join(basePath, ".gsd", "milestones", parts[0], artifactName);
+  return join(basePath, ".gsd", "milestones", milestone, artifactName);
 }
 
 // ─── Dispatch Rule Conversion ──────────────────────────────────────────────
@@ -212,7 +211,7 @@ export class RuleRegistry {
       };
 
       // Build prompt with variable substitution
-      const [mid, sid, tid] = triggerUnitId.split("/");
+      const { milestone: mid, slice: sid, task: tid } = parseUnitId(triggerUnitId);
       let prompt = config.prompt
         .replace(/\{milestoneId\}/g, mid ?? "")
         .replace(/\{sliceId\}/g, sid ?? "")
@@ -291,7 +290,7 @@ export class RuleRegistry {
       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 ?? "")
@@ -506,7 +505,7 @@ export class RuleRegistry {
     this.cycleCounts.set(cycleKey, currentCycle);
     this.activeHook.cycle = currentCycle;
 
-    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 ?? "")
diff --git a/src/resources/extensions/gsd/tests/complete-milestone.test.ts b/src/resources/extensions/gsd/tests/complete-milestone.test.ts
index 0173dffd3..78211dc0e 100644
--- a/src/resources/extensions/gsd/tests/complete-milestone.test.ts
+++ b/src/resources/extensions/gsd/tests/complete-milestone.test.ts
@@ -5,6 +5,7 @@ import { join, dirname } from "node:path";
 import { tmpdir } from "node:os";
 import { fileURLToPath } from "node:url";
 import { invalidateAllCaches } from '../cache.ts';
+import { parseUnitId } from "../unit-id.ts";
 
 // loadPrompt reads from ~/.gsd/agent/extensions/gsd/prompts/ (main checkout).
 // In a worktree the file may not exist there yet, so we resolve prompts
@@ -222,8 +223,7 @@ describe("complete-milestone", () => {
 
       const unitType = "complete-milestone";
       const unitId = "M001";
-      const parts = unitId.split("/");
-      const mid = parts[0]!;
+      const { milestone: mid } = parseUnitId(unitId);
 
       // This is the exact logic from diagnoseExpectedArtifact for "complete-milestone"
       const result = `${relMilestoneFile(base, mid, "SUMMARY")} (milestone summary)`;
diff --git a/src/resources/extensions/gsd/tests/reactive-executor.test.ts b/src/resources/extensions/gsd/tests/reactive-executor.test.ts
index 458cc1bc5..589fe39e7 100644
--- a/src/resources/extensions/gsd/tests/reactive-executor.test.ts
+++ b/src/resources/extensions/gsd/tests/reactive-executor.test.ts
@@ -15,6 +15,7 @@ import {
 } from "../reactive-graph.ts";
 import { validatePreferences } from "../preferences-validation.ts";
 import type { ReactiveExecutionState } from "../types.ts";
+import { parseUnitId } from "../unit-id.ts";
 
 // ─── Preference Validation ────────────────────────────────────────────────
 
@@ -441,11 +442,10 @@ test("unitId batch encoding round-trips correctly", () => {
   const unitId = `${mid}/${sid}/reactive+${selected.join(",")}`;
 
   // Parse it back
-  const parts = unitId.split("/");
-  assert.equal(parts[0], "M001");
-  assert.equal(parts[1], "S01");
-  const batchPart = parts[2];
-  const plusIdx = batchPart.indexOf("+");
+  const { milestone, slice, task: batchPart } = parseUnitId(unitId);
+  assert.equal(milestone, "M001");
+  assert.equal(slice, "S01");
+  const plusIdx = batchPart!.indexOf("+");
   assert.ok(plusIdx > 0, "Should have + separator");
   const batchIds = batchPart.slice(plusIdx + 1).split(",");
   assert.deepEqual(batchIds, ["T02", "T03", "T05"]);
diff --git a/src/resources/extensions/gsd/tests/retry-state-reset.test.ts b/src/resources/extensions/gsd/tests/retry-state-reset.test.ts
index dabbc4d2c..1eb8b259a 100644
--- a/src/resources/extensions/gsd/tests/retry-state-reset.test.ts
+++ b/src/resources/extensions/gsd/tests/retry-state-reset.test.ts
@@ -16,6 +16,7 @@ import {
   resolveHookArtifactPath,
 } from "../post-unit-hooks.ts";
 import { uncheckTaskInPlan } from "../undo.ts";
+import { parseUnitId } from "../unit-id.ts";
 
 // ─── Fixture Helpers ───────────────────────────────────────────────────────
 
@@ -183,8 +184,7 @@ test('Full retry reset: all steps combined', () => {
       retryArtifact: "NEEDS-REWORK.md",
     };
 
-    const parts = trigger.unitId.split("/");
-    const [mid, sid, tid] = parts;
+    const { milestone: mid, slice: sid, task: tid } = parseUnitId(trigger.unitId);
 
     // Simulate completedUnits
     let completedUnits = [
@@ -264,8 +264,7 @@ test('Retry reset: idempotent when artifacts already missing', () => {
     };
 
     // These should not throw even with missing files
-    const parts = trigger.unitId.split("/");
-    const [mid, sid, tid] = parts;
+    const { milestone: mid, slice: sid, task: tid } = parseUnitId(trigger.unitId);
 
     // Uncheck — returns false because no PLAN file
     const uncheckResult = uncheckTaskInPlan(base, mid, sid, tid);
diff --git a/src/resources/extensions/gsd/undo.ts b/src/resources/extensions/gsd/undo.ts
index 1db75a845..3d0c589b2 100644
--- a/src/resources/extensions/gsd/undo.ts
+++ b/src/resources/extensions/gsd/undo.ts
@@ -7,6 +7,7 @@ import type { ExtensionCommandContext, ExtensionAPI } from "@gsd/pi-coding-agent
 import { existsSync, readFileSync, writeFileSync, unlinkSync, readdirSync } from "node:fs";
 import { join } from "node:path";
 import { nativeRevertCommit, nativeRevertAbort } from "./native-git-bridge.js";
+import { parseUnitId } from "./unit-id.js";
 import { deriveState } from "./state.js";
 import { invalidateAllCaches } from "./cache.js";
 import { gsdRoot, resolveTasksDir, resolveSlicePath, resolveTaskFile, buildTaskFileName, buildSliceFileName } from "./paths.js";
@@ -65,11 +66,11 @@ export async function handleUndo(args: string, ctx: ExtensionCommandContext, _pi
   }
 
   // 1. Delete summary artifact
-  const parts = unitId.split("/");
+  const { milestone, slice, task } = parseUnitId(unitId);
   let summaryRemoved = false;
-  if (parts.length === 3) {
+  if (task !== undefined && slice !== undefined) {
     // Task-level: M001/S01/T01
-    const [mid, sid, tid] = parts;
+    const [mid, sid, tid] = [milestone, slice, task];
     const tasksDir = resolveTasksDir(basePath, mid, sid);
     if (tasksDir) {
       const summaryFile = join(tasksDir, buildTaskFileName(tid, "SUMMARY"));
@@ -78,9 +79,9 @@ export async function handleUndo(args: string, ctx: ExtensionCommandContext, _pi
         summaryRemoved = true;
       }
     }
-  } else if (parts.length === 2) {
+  } else if (slice !== undefined) {
     // Slice-level: M001/S01
-    const [mid, sid] = parts;
+    const [mid, sid] = [milestone, slice];
     const slicePath = resolveSlicePath(basePath, mid, sid);
     if (slicePath) {
       for (const suffix of ["SUMMARY", "COMPLETE"]) {
@@ -95,8 +96,8 @@ export async function handleUndo(args: string, ctx: ExtensionCommandContext, _pi
 
   // 2. 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" && task !== undefined && slice !== undefined) {
+    const [mid, sid, tid] = [milestone, slice, task];
     planUpdated = uncheckTaskInPlan(basePath, mid, sid, tid);
   }
 
diff --git a/src/resources/extensions/gsd/unit-runtime.ts b/src/resources/extensions/gsd/unit-runtime.ts
index 8384ea401..a433ac9da 100644
--- a/src/resources/extensions/gsd/unit-runtime.ts
+++ b/src/resources/extensions/gsd/unit-runtime.ts
@@ -8,6 +8,7 @@ import {
   resolveTaskFile,
 } from "./paths.js";
 import { loadFile, parseTaskPlanMustHaves, countMustHavesMentionedInSummary } from "./files.js";
+import { parseUnitId } from "./unit-id.js";
 
 export type UnitRuntimePhase =
   | "dispatched"
@@ -128,7 +129,7 @@ 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");