diff --git a/infrastructure/lib/problem-deploy/handlers/participant-handler/coordination-plugin-loader.ts b/infrastructure/lib/problem-deploy/handlers/participant-handler/coordination-plugin-loader.ts
new file mode 100644
index 000000000..28a77c9c9
--- /dev/null
+++ b/infrastructure/lib/problem-deploy/handlers/participant-handler/coordination-plugin-loader.ts
@@ -0,0 +1,100 @@
+import type { CoordinationContext, CoordinationPlugin } from "@tenkacloud/coordination-plugin-sdk";
+import {
+  type CoordinationDispatchInput,
+  type CoordinationDispatchOutcome,
+  dispatchCoordinationOp,
+  projectCoordinationForTeam,
+} from "./coordination-dispatch.js";
+import type { CoordinationStoreDeps } from "./coordination-store.js";
+
+/**
+ * ADR-028 D6 (#1420): 問題が同梱する coordination plugin の **動的 import loader**。
+ *
+ * 静的レジストリにすると platform が各問題に結合し、 community が coordination 付き問題を
+ * 追加するたびに platform の再デプロイが要る。 それは「問題は plugin、 platform は host」を壊し
+ * 問題カタログの community moat を殺す。 そこで plugin は問題 payload (ADR-008 の S3 配信経路) から
+ * 取得し runtime に `import()` で動的 load する — platform リリース不要で community が拡張できる。
+ *
+ * 実 import 関数は {@link PluginImporter} として注入する (= 本番は S3 から materialize した module を
+ * import、 test は fake)。 別 isolate sandbox は立てない (cost/複雑度、 ADR-028 D6): plugin の hook は
+ * SDK 契約上すべて純関数 (AWS SDK / fetch 非依存) で、 bug は当該 event の 1 row に閉じ
+ * (optimistic lock + DDB write fail で停止)、 platform 全体には波及しない。
+ */
+
+/** plugin module を解決する関数。 本番は S3 payload を materialize した module ref を import()。 */
+export type PluginImporter = (moduleRef: string) => Promise<unknown>;
+
+/**
+ * value が {@link CoordinationPlugin} 契約 (initialState + 3 必須 hook) を満たすか構造判定する。
+ * tick は optional なので問わない。 動的 load した未知 module を dispatcher に渡す前の門番。
+ */
+export function isCoordinationPlugin(
+  value: unknown,
+): value is CoordinationPlugin<unknown, unknown> {
+  if (typeof value !== "object" || value === null) return false;
+  const p = value as Record<string, unknown>;
+  return (
+    typeof p.initialState === "function" &&
+    typeof p.validateOp === "function" &&
+    typeof p.applyOp === "function" &&
+    typeof p.projectForTeam === "function"
+  );
+}
+
+/**
+ * plugin module を動的 import し、 default export (無ければ module 自体) が契約を満たせば返す。
+ * import 失敗 (= module 不在 / 構文エラー) や契約不一致は **null** を返す (= caller が safe fallback)。
+ * platform は問題依存の意味論を持たないので、 buggy / 未対応な問題でも participant API を壊さない。
+ */
+export async function loadCoordinationPlugin(
+  importer: PluginImporter,
+  moduleRef: string,
+): Promise<CoordinationPlugin<unknown, unknown> | null> {
+  let mod: unknown;
+  try {
+    mod = await importer(moduleRef);
+  } catch {
+    return null;
+  }
+  const candidate = (mod as { default?: unknown } | null)?.default ?? mod;
+  return isCoordinationPlugin(candidate) ? candidate : null;
+}
+
+/** plugin が load できなかった (= 問題が coordination 未対応 / 壊れている) ときの outcome。 */
+export type PluginUnavailable = { readonly kind: "plugin_unavailable" };
+
+/**
+ * 動的 load → dispatch を 1 経路にした orchestration (ADR-028 D6: 「plugin を load して dispatchOp」)。
+ * plugin が load できなければ `plugin_unavailable` を返し、 副作用 (DDB) には触れない。
+ */
+export async function loadAndDispatchCoordinationOp(
+  importer: PluginImporter,
+  moduleRef: string,
+  store: CoordinationStoreDeps,
+  input: CoordinationDispatchInput<unknown>,
+): Promise<CoordinationDispatchOutcome | PluginUnavailable> {
+  const plugin = await loadCoordinationPlugin(importer, moduleRef);
+  if (!plugin) return { kind: "plugin_unavailable" };
+  return dispatchCoordinationOp(store, plugin, input);
+}
+
+/**
+ * 動的 load → projection の read 経路 (書き込みなし、 portal polling 用)。 plugin が load できなければ
+ * `fallbackProjection` を返す (= 他 team の機密を出さない安全な既定)。
+ */
+export async function loadAndProjectCoordinationForTeam(
+  importer: PluginImporter,
+  moduleRef: string,
+  store: CoordinationStoreDeps,
+  input: {
+    readonly tenantId: string;
+    readonly eventId: string;
+    readonly teamId: string;
+    readonly ctx: CoordinationContext;
+    readonly fallbackProjection: unknown;
+  },
+): Promise<unknown> {
+  const plugin = await loadCoordinationPlugin(importer, moduleRef);
+  if (!plugin) return input.fallbackProjection;
+  return projectCoordinationForTeam(store, plugin, input);
+}
diff --git a/infrastructure/test/problem-deploy/coordination-plugin-loader.test.ts b/infrastructure/test/problem-deploy/coordination-plugin-loader.test.ts
new file mode 100644
index 000000000..702d7afaa
--- /dev/null
+++ b/infrastructure/test/problem-deploy/coordination-plugin-loader.test.ts
@@ -0,0 +1,164 @@
+import { GetCommand, PutCommand } from "@aws-sdk/lib-dynamodb";
+import type { CoordinationPlugin } from "@tenkacloud/coordination-plugin-sdk";
+import { describe, expect, it, vi } from "vitest";
+import {
+  isCoordinationPlugin,
+  loadAndDispatchCoordinationOp,
+  loadAndProjectCoordinationForTeam,
+  loadCoordinationPlugin,
+  type PluginImporter,
+} from "../../lib/problem-deploy/handlers/participant-handler/coordination-plugin-loader.js";
+import type { CoordinationStoreDeps } from "../../lib/problem-deploy/handlers/participant-handler/coordination-store.js";
+
+/**
+ * ADR-028 D6 (#1420): 問題同梱 coordination plugin の動的 import loader を pin する。
+ * import 失敗 / 契約不一致 → safe fallback、 load 成功 → 既存 dispatcher へ委譲、 を観測する。
+ */
+
+interface CounterState {
+  readonly count: number;
+}
+type CounterOp = { kind: "inc" } | { kind: "bad" };
+
+const counter: CoordinationPlugin<CounterState, CounterOp, { count: number }> = {
+  initialState: () => ({ count: 0 }),
+  validateOp: (_s, _t, op) => (op.kind === "bad" ? { ok: false, error: "bad_op" } : { ok: true }),
+  applyOp: (s) => ({ count: s.count + 1 }),
+  projectForTeam: (s) => ({ count: s.count }),
+};
+
+const ctx = { eventId: "e1", teamIds: ["t1", "t2"] };
+const dispatchInput = {
+  tenantId: "tn1",
+  eventId: "e1",
+  teamId: "t1",
+  op: { kind: "inc" } as CounterOp,
+  ctx,
+  fallbackProjection: { count: -1 },
+  nowIso: "2026-06-01T00:00:00Z",
+};
+
+/** GetCommand → getItem を返し、 PutCommand → ok。 conflict 経路は dispatcher 側 test で網羅済み。 */
+function fakeStore(getItem?: Record<string, unknown>): CoordinationStoreDeps {
+  const send = vi.fn(async (cmd: unknown) => {
+    if (cmd instanceof GetCommand) return { Item: getItem };
+    if (cmd instanceof PutCommand) return {};
+    throw new Error("unexpected command");
+  });
+  return { ddb: { send } as never, tableName: "Deployments" };
+}
+
+/** moduleRef を無視して固定 module を返す importer。 */
+const importerOf =
+  (mod: unknown): PluginImporter =>
+  async () =>
+    mod;
+const throwingImporter: PluginImporter = async () => {
+  throw new Error("module not found");
+};
+
+describe("isCoordinationPlugin", () => {
+  it("should accept an object with all required hooks", () => {
+    expect(isCoordinationPlugin(counter)).toBe(true);
+  });
+
+  it("should reject null and non-objects", () => {
+    expect(isCoordinationPlugin(null)).toBe(false);
+    expect(isCoordinationPlugin("plugin")).toBe(false);
+    expect(isCoordinationPlugin(42)).toBe(false);
+  });
+
+  it("should reject an object missing any required hook", () => {
+    expect(isCoordinationPlugin({ validateOp() {}, applyOp() {}, projectForTeam() {} })).toBe(
+      false,
+    );
+    expect(isCoordinationPlugin({ initialState() {}, applyOp() {}, projectForTeam() {} })).toBe(
+      false,
+    );
+    expect(isCoordinationPlugin({ initialState() {}, validateOp() {}, projectForTeam() {} })).toBe(
+      false,
+    );
+    expect(isCoordinationPlugin({ initialState() {}, validateOp() {}, applyOp() {} })).toBe(false);
+  });
+});
+
+describe("loadCoordinationPlugin", () => {
+  it("should return the plugin from a default export", async () => {
+    expect(await loadCoordinationPlugin(importerOf({ default: counter }), "ref")).toBe(counter);
+  });
+
+  it("should return the plugin when the module itself is the plugin", async () => {
+    expect(await loadCoordinationPlugin(importerOf(counter), "ref")).toBe(counter);
+  });
+
+  it("should return null when the importer throws", async () => {
+    expect(await loadCoordinationPlugin(throwingImporter, "ref")).toBeNull();
+  });
+
+  it("should return null when the module is null or fails the contract", async () => {
+    expect(await loadCoordinationPlugin(importerOf(null), "ref")).toBeNull();
+    expect(await loadCoordinationPlugin(importerOf({ default: {} }), "ref")).toBeNull();
+  });
+});
+
+describe("loadAndDispatchCoordinationOp", () => {
+  it("should load the plugin and dispatch a valid op", async () => {
+    const outcome = await loadAndDispatchCoordinationOp(
+      importerOf(counter),
+      "ref",
+      fakeStore(undefined),
+      dispatchInput,
+    );
+    expect(outcome).toEqual({ kind: "ok", projection: { count: 1 } });
+  });
+
+  it("should surface the plugin's rejection for an invalid op", async () => {
+    const outcome = await loadAndDispatchCoordinationOp(
+      importerOf(counter),
+      "ref",
+      fakeStore(undefined),
+      { ...dispatchInput, op: { kind: "bad" } },
+    );
+    expect(outcome).toEqual({ kind: "rejected", error: "bad_op" });
+  });
+
+  it("should return plugin_unavailable when the plugin cannot be loaded", async () => {
+    const outcome = await loadAndDispatchCoordinationOp(
+      throwingImporter,
+      "ref",
+      fakeStore(undefined),
+      dispatchInput,
+    );
+    expect(outcome).toEqual({ kind: "plugin_unavailable" });
+  });
+});
+
+describe("loadAndProjectCoordinationForTeam", () => {
+  const projectInput = {
+    tenantId: "tn1",
+    eventId: "e1",
+    teamId: "t1",
+    ctx,
+    fallbackProjection: { count: -1 },
+  };
+
+  it("should project the loaded plugin's per-team view", async () => {
+    const out = await loadAndProjectCoordinationForTeam(
+      importerOf(counter),
+      "ref",
+      fakeStore({ state: { count: 5 }, version: 1 }),
+      projectInput,
+    );
+    expect(out).toEqual({ count: 5 });
+  });
+
+  it("should return the fallback projection when the plugin is unavailable", async () => {
+    const out = await loadAndProjectCoordinationForTeam(
+      throwingImporter,
+      "ref",
+      fakeStore(undefined),
+      projectInput,
+    );
+    expect(out).toEqual({ count: -1 });
+  });
+});