diff --git a/tools/workflow-engine/gitlab-world.test.ts b/tools/workflow-engine/gitlab-world.test.ts
new file mode 100644
index 0000000000..8a561db8c1
--- /dev/null
+++ b/tools/workflow-engine/gitlab-world.test.ts
@@ -0,0 +1,187 @@
+// Invariant tests for GitLabWorld per-host adapter (B-0867.15 PoC).
+
+import { describe, expect, test } from "bun:test";
+import { EMPTY_WORLD } from "./world.js";
+import { buildGitWorld } from "./git-world.js";
+import {
+  type GitLabResourceBudget,
+  type MrLifetime,
+  buildGitLabWorld,
+  canAffordGitLab,
+  gitLabRateLimitTier,
+  GITLAB_MR_UNIVERSE,
+  GITLAB_DISCUSSION_UNIVERSE,
+  GITLAB_PIPELINE_UNIVERSE,
+  GITLAB_REQUIRE_RESOLVED_VERDICT,
+  GITLAB_APPROVAL_NOT_MET_VERDICT,
+} from "./gitlab-world.js";
+
+describe("GitLabWorld constructor + inheritance", () => {
+  test("buildGitLabWorld extends GitWorld base", () => {
+    const gitWorld = buildGitWorld();
+    const gitLabWorld = buildGitLabWorld(gitWorld);
+    expect(gitLabWorld.forgeName).toBe("git");           // inherited
+    expect(gitLabWorld.forgeSpecialization).toBe("gitlab");
+    expect(gitLabWorld.branchUniverse.length).toBe(4);   // inherited from GitWorld
+    expect(gitLabWorld.commitUniverse.length).toBe(5);   // inherited
+  });
+
+  test("buildGitLabWorld populates MR + Discussion + Pipeline universes", () => {
+    const gitLabWorld = buildGitLabWorld(buildGitWorld());
+    expect(gitLabWorld.mrUniverse.length).toBe(6);
+    expect(gitLabWorld.discussionUniverse.length).toBe(2);
+    expect(gitLabWorld.pipelineUniverse.length).toBe(8);
+  });
+
+  test("buildGitLabWorld accepts optional resourceBudget", () => {
+    const budget: GitLabResourceBudget = {
+      restRemaining: 1500,
+      restLimit: 2000,
+      restResetAt: 1700000000,
+      graphqlRemaining: 800,
+      graphqlLimit: 1000,
+      graphqlResetAt: 1700000060,
+    };
+    const gitLabWorld = buildGitLabWorld(buildGitWorld(), budget);
+    expect(gitLabWorld.resourceBudget).toEqual(budget);
+  });
+
+  test("buildGitLabWorld omits resourceBudget when not provided", () => {
+    const gitLabWorld = buildGitLabWorld(buildGitWorld());
+    expect(gitLabWorld.resourceBudget).toBeUndefined();
+  });
+});
+
+describe("gitLabRateLimitTier tier boundaries", () => {
+  test("normal tier when remaining > 800", () => {
+    expect(gitLabRateLimitTier(2000)).toBe("normal");
+    expect(gitLabRateLimitTier(801)).toBe("normal");
+  });
+  test("cost-aware tier when 400 < remaining ≤ 800", () => {
+    expect(gitLabRateLimitTier(800)).toBe("cost-aware");
+    expect(gitLabRateLimitTier(401)).toBe("cost-aware");
+  });
+  test("extreme-cost-aware tier when 80 < remaining ≤ 400", () => {
+    expect(gitLabRateLimitTier(400)).toBe("extreme-cost-aware");
+    expect(gitLabRateLimitTier(81)).toBe("extreme-cost-aware");
+  });
+  test("pure-git tier when remaining ≤ 80", () => {
+    expect(gitLabRateLimitTier(80)).toBe("pure-git");
+    expect(gitLabRateLimitTier(0)).toBe("pure-git");
+  });
+});
+
+describe("canAffordGitLab budget enforcement", () => {
+  const baseWorld = buildGitLabWorld(buildGitWorld(), {
+    restRemaining: 100,
+    restLimit: 2000,
+    restResetAt: 1700000060,
+    graphqlRemaining: 50,
+    graphqlLimit: 1000,
+    graphqlResetAt: 1700000120,
+  });
+
+  test("within budget returns ok", () => {
+    const result = canAffordGitLab(baseWorld, { restCost: 5, graphqlCost: 5 });
+    expect(result.ok).toBe(true);
+  });
+
+  test("REST exceeded returns ResourceBudgetExhausted with rest", () => {
+    const result = canAffordGitLab(baseWorld, { restCost: 200 });
+    expect(result.ok).toBe(false);
+    if (!result.ok) {
+      expect(result.feedback.kind).toBe("ResourceBudgetExhausted");
+      if (result.feedback.kind === "ResourceBudgetExhausted") {
+        expect(result.feedback.budget).toBe("rest");
+      }
+    }
+  });
+
+  test("GraphQL exceeded returns ResourceBudgetExhausted with graphql", () => {
+    const result = canAffordGitLab(baseWorld, { graphqlCost: 100 });
+    expect(result.ok).toBe(false);
+    if (!result.ok) {
+      expect(result.feedback.kind).toBe("ResourceBudgetExhausted");
+      if (result.feedback.kind === "ResourceBudgetExhausted") {
+        expect(result.feedback.budget).toBe("graphql");
+      }
+    }
+  });
+
+  test("no budget loaded → ok by default", () => {
+    const noBudgetWorld = buildGitLabWorld(buildGitWorld());
+    const result = canAffordGitLab(noBudgetWorld, { restCost: 1_000_000 });
+    expect(result.ok).toBe(true);
+  });
+});
+
+describe("reusable substrate exports", () => {
+  test("GITLAB_MR_UNIVERSE has 6 variants", () => {
+    expect(GITLAB_MR_UNIVERSE.length).toBe(6);
+    const kinds = GITLAB_MR_UNIVERSE.map((m) => m.kind);
+    expect(kinds).toContain("draft");
+    expect(kinds).toContain("opened");
+    expect(kinds).toContain("merged");
+  });
+  test("GITLAB_DISCUSSION_UNIVERSE has 2 variants", () => {
+    expect(GITLAB_DISCUSSION_UNIVERSE.length).toBe(2);
+    expect(GITLAB_DISCUSSION_UNIVERSE.map((d) => d.kind)).toEqual([
+      "unresolved",
+      "resolved",
+    ]);
+  });
+  test("GITLAB_PIPELINE_UNIVERSE has 8 variants (GitLab-native CI/CD)", () => {
+    expect(GITLAB_PIPELINE_UNIVERSE.length).toBe(8);
+    const kinds = GITLAB_PIPELINE_UNIVERSE.map((p) => p.kind);
+    expect(kinds).toContain("manual");  // GitLab-specific
+    expect(kinds).toContain("skipped"); // GitLab-specific (CI rules)
+  });
+  test("GITLAB_REQUIRE_RESOLVED_VERDICT is block with GitLab vocabulary", () => {
+    expect(GITLAB_REQUIRE_RESOLVED_VERDICT.kind).toBe("block");
+    if (GITLAB_REQUIRE_RESOLVED_VERDICT.kind === "block") {
+      expect(GITLAB_REQUIRE_RESOLVED_VERDICT.reason).toContain("GitLab");
+      expect(GITLAB_REQUIRE_RESOLVED_VERDICT.reason).toContain("discussions");
+    }
+  });
+  test("GITLAB_APPROVAL_NOT_MET_VERDICT is block with approval vocabulary", () => {
+    expect(GITLAB_APPROVAL_NOT_MET_VERDICT.kind).toBe("block");
+    if (GITLAB_APPROVAL_NOT_MET_VERDICT.kind === "block") {
+      expect(GITLAB_APPROVAL_NOT_MET_VERDICT.reason).toContain("approval");
+    }
+  });
+});
+
+describe("type-level MrLifetime exhaustive switch (compile check)", () => {
+  test("all 6 MR variants distinguishable", () => {
+    const variants: MrLifetime[] = [
+      { kind: "draft" },
+      { kind: "opened" },
+      { kind: "reviewer-assigned" },
+      { kind: "approved" },
+      { kind: "merged" },
+      { kind: "closed" },
+    ];
+    expect(variants.length).toBe(6);
+  });
+});
+
+describe("end-to-end composition (GitWorld → GitLabWorld → budget-check)", () => {
+  test("full chain: GitWorld base + GitLab specialization + budget enforcement", () => {
+    const gitWorld = buildGitWorld();
+    expect(EMPTY_WORLD.registry.size).toBe(0);
+    const gitLabWorld = buildGitLabWorld(gitWorld, {
+      restRemaining: 1500,
+      restLimit: 2000,
+      restResetAt: 1700000060,
+      graphqlRemaining: 800,
+      graphqlLimit: 1000,
+      graphqlResetAt: 1700000120,
+    });
+    expect(gitLabWorld.forgeSpecialization).toBe("gitlab");
+    expect(gitLabWorld.mrUniverse.length).toBe(6);
+    expect(gitLabWorld.pipelineUniverse.length).toBe(8);
+    const budgetCheck = canAffordGitLab(gitLabWorld, { restCost: 10, graphqlCost: 5 });
+    expect(budgetCheck.ok).toBe(true);
+    expect(gitLabRateLimitTier(gitLabWorld.resourceBudget!.restRemaining)).toBe("normal");
+  });
+});
diff --git a/tools/workflow-engine/gitlab-world.ts b/tools/workflow-engine/gitlab-world.ts
new file mode 100644
index 0000000000..e3083c0405
--- /dev/null
+++ b/tools/workflow-engine/gitlab-world.ts
@@ -0,0 +1,305 @@
+// tools/workflow-engine/gitlab-world.ts
+//
+// B-0867.15 — GitLabWorld per-host adapter PoC.
+//
+// Aaron 2026-05-28 lane-status framing: workflow-engine substrate Lane 2
+// (GitHub accelerator/workflow). B-0867.15 names per-host adapters
+// extending PR #5775's GitWorld → GitHubWorld specialization hierarchy
+// to GitLab + Gitea + Bitbucket + Codeberg + Sourcehut.
+//
+// This file ships the GitLabWorld instantiation — first per-host adapter
+// extending the base. Pattern: same as GitHubWorld but with forge-
+// specific lifetime variants + GitLab-native substrate (merge requests
+// instead of pull requests; discussions instead of review threads;
+// pipelines as first-class CI/CD substrate).
+//
+// Composes with:
+// - PR #5775 git-world.ts (GitWorld base + GitHubWorld first specialization)
+// - PR #5776 world-hierarchy.ts (Clifford → DBSP → Git → forge-specific)
+// - B-0867.15 backlog row (per-host adapters target)
+// - .claude/rules/asymmetric-authorship (per-forge feedback variants substrate-entity-authored)
+// - .claude/rules/monad-propagation (Result<T, GitLabFeedback> shape)
+
+import {
+  registerLifetimePair,
+  type ComposedKey,
+  type LifetimeState,
+  type StandardVerdict,
+} from "./world.js";
+import { type GitWorld } from "./git-world.js";
+
+// ─────────────────────────────────────────────────────────────────────
+// GitLab forge-specific lifetime types
+// ─────────────────────────────────────────────────────────────────────
+
+/**
+ * Merge Request lifetime (GitLab's analog of GitHub's PR).
+ *
+ * GitLab MR state machine variants per GitLab REST API v4 spec:
+ * https://docs.gitlab.com/ee/api/merge_requests.html
+ */
+export interface MrLifetime extends LifetimeState {
+  readonly kind: "draft" | "opened" | "reviewer-assigned" | "approved" | "merged" | "closed";
+}
+
+/**
+ * Discussion lifetime (GitLab's analog of GitHub's review thread).
+ *
+ * GitLab discussions can be resolvable or non-resolvable; this DU covers
+ * the resolvable shape used by code review.
+ */
+export interface DiscussionLifetime extends LifetimeState {
+  readonly kind: "unresolved" | "resolved";
+}
+
+/**
+ * Pipeline lifetime (GitLab-native; first-class CI/CD substrate).
+ *
+ * GitLab makes CI/CD pipelines first-class in the API model. Pipeline
+ * state-machine variants per GitLab REST API v4 spec.
+ */
+export interface PipelineLifetime extends LifetimeState {
+  readonly kind:
+    | "created"
+    | "pending"
+    | "running"
+    | "success"
+    | "failed"
+    | "canceled"
+    | "skipped"
+    | "manual";
+}
+
+// ─────────────────────────────────────────────────────────────────────
+// GitLab resource-allocation substrate
+// ─────────────────────────────────────────────────────────────────────
+
+/**
+ * GitLab resource budget (forge-specific).
+ *
+ * GitLab uses per-minute rate limits (RateLimit-Remaining + RateLimit-
+ * Reset headers). Tier varies by plan; free tier defaults are typically
+ * 2000/min authenticated for REST. Self-hosted instances can configure
+ * limits per project / per user.
+ *
+ * Distinct from GitHub's per-hour 5000 budget; this models the per-minute
+ * rolling window GitLab uses.
+ */
+export interface GitLabResourceBudget {
+  readonly restRemaining: number;
+  readonly restLimit: number;        // per-minute window; tier-dependent
+  readonly restResetAt: number;      // unix timestamp
+  readonly graphqlRemaining: number;
+  readonly graphqlLimit: number;     // GitLab GraphQL has its own budget
+  readonly graphqlResetAt: number;
+}
+
+/**
+ * GitLab-specific rate-limit tier per framework's rate-limit substrate.
+ *
+ * Tiers shifted from GitHub's 5000/hour to GitLab's typical 2000/minute
+ * authenticated. Thresholds preserve the same operational substrate
+ * (normal / cost-aware / extreme-cost-aware / pure-git) at scaled
+ * boundaries.
+ */
+export type GitLabRateLimitTier =
+  | "normal"             // > 800 remaining (40% of typical 2000/min)
+  | "cost-aware"         // 400-800
+  | "extreme-cost-aware" // 80-400
+  | "pure-git";          // 0-80
+
+export function gitLabRateLimitTier(remaining: number): GitLabRateLimitTier {
+  if (remaining > 800) return "normal";
+  if (remaining > 400) return "cost-aware";
+  if (remaining > 80) return "extreme-cost-aware";
+  return "pure-git";
+}
+
+// ─────────────────────────────────────────────────────────────────────
+// GitLabWorld specialization
+// ─────────────────────────────────────────────────────────────────────
+
+/**
+ * GitLabWorld — specialization of GitWorld for GitLab forge.
+ *
+ * Per Aaron 2026-05-28 lane-status framing (Lane 2 GitHub accelerator):
+ * B-0867.15 names per-host adapters as extension target. GitLabWorld is
+ * the first concrete adapter beyond GitHubWorld; demonstrates the
+ * pattern that GiteaWorld + BitbucketWorld + CodebergWorld + SourcehutWorld
+ * follow.
+ *
+ * Inherits all GitWorld substrate + adds:
+ * - MR substrate (MrLifetime; analog of GitHub PR)
+ * - Discussion substrate (DiscussionLifetime; analog of GitHub review thread)
+ * - Pipeline substrate (PipelineLifetime; GitLab-native first-class CI/CD)
+ * - Resource-allocation substrate (REST + GraphQL per-minute budgets)
+ * - GitLab-specific optimizations (merge-train, approval rules, suggestion patches)
+ */
+export interface GitLabWorld extends GitWorld {
+  readonly forgeName: "git";  // inherits GitWorld base
+  readonly forgeSpecialization: "gitlab";
+  readonly mrUniverse: ReadonlyArray<MrLifetime>;
+  readonly discussionUniverse: ReadonlyArray<DiscussionLifetime>;
+  readonly pipelineUniverse: ReadonlyArray<PipelineLifetime>;
+  readonly resourceBudget?: GitLabResourceBudget;
+}
+
+/**
+ * Build the GitLabWorld substrate from a base GitWorld.
+ */
+export function buildGitLabWorld(
+  gitWorld: GitWorld,
+  resourceBudget?: GitLabResourceBudget,
+): GitLabWorld {
+  return {
+    ...gitWorld,
+    forgeSpecialization: "gitlab",
+    mrUniverse: [
+      { kind: "draft" },
+      { kind: "opened" },
+      { kind: "reviewer-assigned" },
+      { kind: "approved" },
+      { kind: "merged" },
+      { kind: "closed" },
+    ],
+    discussionUniverse: [
+      { kind: "unresolved" },
+      { kind: "resolved" },
+    ],
+    pipelineUniverse: [
+      { kind: "created" },
+      { kind: "pending" },
+      { kind: "running" },
+      { kind: "success" },
+      { kind: "failed" },
+      { kind: "canceled" },
+      { kind: "skipped" },
+      { kind: "manual" },
+    ],
+    ...(resourceBudget !== undefined && { resourceBudget }),
+  };
+}
+
+// ─────────────────────────────────────────────────────────────────────
+// GitLab-specific feedback substrate
+// ─────────────────────────────────────────────────────────────────────
+
+export type GitLabFeedback =
+  | { kind: "UnsupportedGitLabFeature"; feature: string }
+  | { kind: "ResourceBudgetExhausted"; budget: "rest" | "graphql"; resetAt: number }
+  | { kind: "ApprovalRulesNotMet"; required: number; actual: number }
+  | { kind: "MergeBlocked"; reason: string };
+
+export type GitLabResult<T> =
+  | { ok: true; world: T }
+  | { ok: false; feedback: GitLabFeedback };
+
+/**
+ * Check if a GitLab operation is within budget.
+ *
+ * Same shape as GitHubWorld's canAfford; per-minute window vs per-hour.
+ */
+export interface GitLabOperationCost {
+  readonly restCost?: number;
+  readonly graphqlCost?: number;
+}
+
+export function canAffordGitLab(
+  world: GitLabWorld,
+  cost: GitLabOperationCost,
+): GitLabResult<GitLabWorld> {
+  const budget = world.resourceBudget;
+  if (!budget) {
+    return { ok: true, world };
+  }
+  const restCost = cost.restCost ?? 0;
+  const graphqlCost = cost.graphqlCost ?? 0;
+  if (restCost > budget.restRemaining) {
+    return {
+      ok: false,
+      feedback: {
+        kind: "ResourceBudgetExhausted",
+        budget: "rest",
+        resetAt: budget.restResetAt,
+      },
+    };
+  }
+  if (graphqlCost > budget.graphqlRemaining) {
+    return {
+      ok: false,
+      feedback: {
+        kind: "ResourceBudgetExhausted",
+        budget: "graphql",
+        resetAt: budget.graphqlResetAt,
+      },
+    };
+  }
+  return { ok: true, world };
+}
+
+/**
+ * Convenience: register a lifetime pair in the GitLabWorld.
+ */
+export function registerInGitLab<
+  A extends LifetimeState,
+  B extends LifetimeState,
+  T,
+>(
+  world: GitLabWorld,
+  pairName: string,
+  matrix: ReadonlyMap<ComposedKey<A, B>, T>,
+): GitLabWorld {
+  return registerLifetimePair(world, pairName, matrix);
+}
+
+// ─────────────────────────────────────────────────────────────────────
+// Reusable substrate exports
+// ─────────────────────────────────────────────────────────────────────
+
+export const GITLAB_MR_UNIVERSE: ReadonlyArray<MrLifetime> = [
+  { kind: "draft" },
+  { kind: "opened" },
+  { kind: "reviewer-assigned" },
+  { kind: "approved" },
+  { kind: "merged" },
+  { kind: "closed" },
+];
+
+export const GITLAB_DISCUSSION_UNIVERSE: ReadonlyArray<DiscussionLifetime> = [
+  { kind: "unresolved" },
+  { kind: "resolved" },
+];
+
+export const GITLAB_PIPELINE_UNIVERSE: ReadonlyArray<PipelineLifetime> = [
+  { kind: "created" },
+  { kind: "pending" },
+  { kind: "running" },
+  { kind: "success" },
+  { kind: "failed" },
+  { kind: "canceled" },
+  { kind: "skipped" },
+  { kind: "manual" },
+];
+
+/**
+ * Reusable verdict for GitLab's "must resolve all discussions before
+ * merge" branch protection pattern (settings/general/merge_request).
+ *
+ * Same shape as GITHUB's REQUIRE_RESOLVED_VERDICT but named with
+ * GitLab vocabulary.
+ */
+export const GITLAB_REQUIRE_RESOLVED_VERDICT: StandardVerdict = {
+  kind: "block",
+  reason: "GitLab require_all_discussions_resolved: unresolved discussions block merge",
+};
+
+/**
+ * Reusable verdict for GitLab's "approval rules not met" pattern.
+ *
+ * GitLab MRs can have approval rules requiring N approvals from specific
+ * groups; this verdict captures the blocked state.
+ */
+export const GITLAB_APPROVAL_NOT_MET_VERDICT: StandardVerdict = {
+  kind: "block",
+  reason: "GitLab approval rules not met: required approvals missing",
+};