diff --git a/deno.json b/deno.json
index 5cef24dc..3b08278b 100644
--- a/deno.json
+++ b/deno.json
@@ -5,7 +5,7 @@
   "author": "zumbrunn (https://zumbrunn.com)",
   "minimumDependencyAge": {
     "age": "P1D",
-    "exclude": ["jsr:@dune/core", "jsr:@dune/theme-api"]
+    "exclude": ["jsr:@dune/core", "jsr:@dune/plugin-api", "jsr:@dune/theme-api"]
   },
   "exports": {
     ".": "./src/mod.ts",
@@ -57,6 +57,7 @@
     "./watchdog": "./src/runtime/watchdog.ts"
   },
   "imports": {
+    "@dune/plugin-api": "jsr:@dune/plugin-api@^1",
     "@dune/theme-api": "jsr:@dune/theme-api@^1",
     "fresh": "jsr:@fresh/core@^2",
     "preact": "npm:preact@^10",
diff --git a/tests/contract/plugin_api_test.ts b/tests/contract/plugin_api_test.ts
new file mode 100644
index 00000000..a53d0cf6
--- /dev/null
+++ b/tests/contract/plugin_api_test.ts
@@ -0,0 +1,197 @@
+/**
+ * `@dune/plugin-api` is a leaf package that redeclares the plugin contract.
+ * These tests keep core and the package from drifting apart. They are mostly
+ * compile-time: `deno check` is the assertion.
+ *
+ * Two directions (plugin-api extraction plan, decision 10):
+ *  - what core hands a plugin is assignable to the package's types;
+ *  - what a plugin hands in (an engine, an editor manager, a record) is
+ *    accepted by core.
+ * Types that are identical on both sides are checked both ways.
+ */
+
+import { assertEquals } from "@std/assert";
+import type * as P from "@dune/plugin-api";
+import { isDuneError as pluginIsDuneError } from "@dune/plugin-api";
+import type { PageIndex } from "../../src/content/types.ts";
+import type {
+  ContentApi,
+  ContentSearchResult,
+  PagesOptions,
+  ResolvedPage,
+  TaxonomyTerm,
+} from "../../src/content/api.ts";
+import type {
+  FacetCounts,
+  InjectedSearchRecord,
+  SearchEngine,
+  SearchEngineCreateContext,
+  SearchFilter,
+  SearchOptions,
+  SearchRecordsCollectContext,
+  SearchResult,
+} from "../../src/search/engine.ts";
+import type { StorageAdapter, StorageEntry, StorageStat, WatchEvent } from "../../src/storage/types.ts";
+import type { Submission, SubmissionFile, SubmissionMeta, SubmissionStatus } from "../../src/forms/submissions.ts";
+import type { BlueprintField } from "../../src/blueprints/types.ts";
+import type { EditorIdentity, InlineEditManager, InlineEditRequestContext } from "../../src/inline-edit/types.ts";
+import type { ScheduledAction } from "../../src/workflow/types.ts";
+import type { PluginApp, PluginRouteContext } from "../../src/plugin-api/app.ts";
+import type { PluginServices } from "../../src/plugin-api/services.ts";
+import type { User, UserCreate } from "../../src/auth/types.ts";
+import type { UserStore } from "../../src/auth/user-store.ts";
+import type { Session } from "../../src/session/types.ts";
+import type { SessionManager } from "../../src/session/mod.ts";
+import type { HistoryEngine } from "../../src/history/engine.ts";
+import type { ContentDiff, ContentRevision } from "../../src/workflow/types.ts";
+import type { WorkflowEngine } from "../../src/workflow/engine.ts";
+import type { Scheduler } from "../../src/workflow/scheduler.ts";
+import type { StagingEngine } from "../../src/staging/engine.ts";
+import type { RateLimiter } from "../../src/security/rate-limit.ts";
+import type { RateLimitStore } from "../../src/security/rate-limit-store.ts";
+import type { Logger } from "../../src/core/logger.ts";
+import type { DuneConfig } from "../../src/config/types.ts";
+import type { DuneEngine } from "../../src/core/engine.ts";
+import type { Page } from "../../src/content/types.ts";
+import type { SearchManager } from "../../src/search/manager.ts";
+import type { FlexEngine } from "../../src/flex/engine.ts";
+import type { SubmissionManager } from "../../src/forms/submissions.ts";
+import type { SectionRegistry } from "../../src/sections/registry.ts";
+import type { MetricsCollector } from "../../src/metrics/mod.ts";
+import type { JobScheduler } from "../../src/jobs/mod.ts";
+import type { DuneAuthSystem } from "../../src/auth/authz.ts";
+import type { ThemeLoader } from "../../src/themes/loader.ts";
+import type { RouteResolver } from "../../src/routing/resolver.ts";
+import type { PluginRuntime } from "../../src/plugin-api/runtime.ts";
+import type {
+  DunePlugin,
+  FormSubmitPayload,
+  HookContext,
+  HookEvent,
+  HookRegistry,
+  MountApi,
+  PageMovePayload,
+  PluginApi,
+  PublicRouteRegistration,
+  ResponseTransformContext,
+  SystemActionPayload,
+} from "../../src/hooks/types.ts";
+import type {
+  AuthenticateResult,
+  Capabilities,
+  CapabilityContext,
+  CapabilityKey,
+  CapabilityReader,
+  CapabilityRegistrar,
+} from "../../src/plugin-api/capabilities.ts";
+import { isDuneError as coreIsDuneError } from "../../src/plugin-api/errors.ts";
+import { capabilityKey as coreCapabilityKey } from "../../src/plugin-api/capabilities.ts";
+
+// Both directions: identical on both sides.
+function same<A, B>(..._ok: [A] extends [B] ? ([B] extends [A] ? [] : [never]) : [never]): void {}
+// Core -> package only: core hands out something wider.
+function handsOut<Core extends Pkg, Pkg>(): void {}
+// Package -> core only: a plugin supplies it and core consumes it.
+function takesIn<Pkg extends Core, Core>(): void {}
+
+// --- what core hands a plugin ---------------------------------------------
+handsOut<PageIndex, P.PageIndex>();
+handsOut<ContentApi, P.ContentApi>();
+handsOut<ResolvedPage, P.ResolvedPage>();
+handsOut<ContentSearchResult, P.ContentSearchResult>();
+handsOut<TaxonomyTerm, P.TaxonomyTerm>();
+handsOut<PagesOptions, P.PagesOptions>();
+handsOut<SearchEngineCreateContext, P.SearchEngineCreateContext>();
+handsOut<SearchRecordsCollectContext, P.SearchRecordsCollectContext>();
+
+handsOut<PluginServices, P.PluginServices>();
+handsOut<HistoryEngine, P.HistoryEngine>();
+handsOut<WorkflowEngine, P.WorkflowEngine>();
+handsOut<Scheduler, P.Scheduler>();
+handsOut<StagingEngine, P.StagingEngine>();
+handsOut<RateLimiter, P.RateLimiter>();
+handsOut<Logger, P.Logger>();
+
+// --- what a plugin hands in -----------------------------------------------
+takesIn<P.SearchEngine, SearchEngine>();
+takesIn<P.InjectedSearchRecord, InjectedSearchRecord>();
+takesIn<P.InlineEditManager, InlineEditManager>();
+same<EditorIdentity, P.EditorIdentity>();
+same<InlineEditRequestContext, P.InlineEditRequestContext>();
+
+// --- identity: a provider supplies its own stores and cores consumes them -
+handsOut<User, P.User>();
+takesIn<P.UserCreate, UserCreate>();
+handsOut<UserStore, P.UserStore>();
+handsOut<Session, P.Session>();
+handsOut<SessionManager, P.SessionManager>();
+same<ContentRevision, P.ContentRevision>();
+same<ContentDiff, P.ContentDiff>();
+same<RateLimitStore, P.RateLimitStore>();
+
+// --- identical both ways ---------------------------------------------------
+same<StorageAdapter, P.StorageAdapter>();
+same<StorageEntry, P.StorageEntry>();
+same<StorageStat, P.StorageStat>();
+same<WatchEvent, P.WatchEvent>();
+same<Submission, P.Submission>();
+same<SubmissionFile, P.SubmissionFile>();
+same<SubmissionMeta, P.SubmissionMeta>();
+same<SubmissionStatus, P.SubmissionStatus>();
+same<BlueprintField, P.BlueprintField>();
+same<ScheduledAction, P.ScheduledAction>();
+same<SearchResult, P.SearchResult>();
+same<SearchFilter, P.SearchFilter>();
+same<SearchOptions, P.SearchOptions>();
+same<FacetCounts, P.FacetCounts>();
+same<PluginApp, P.PluginApp>();
+same<PluginRouteContext, P.PluginRouteContext>();
+
+// --- slice 2b: config, runtime, hooks, plugin object, capabilities ---------
+handsOut<DuneConfig, P.DuneConfig>();
+handsOut<DuneEngine, P.PluginEngine>();
+handsOut<Page, P.Page>();
+handsOut<RouteResolver, P.RouteResolver>();
+handsOut<ThemeLoader, P.ThemeLoader>();
+handsOut<SearchManager, P.PluginSearch>();
+handsOut<FlexEngine, P.FlexEngine>();
+handsOut<SubmissionManager, P.SubmissionManager>();
+handsOut<SectionRegistry, P.SectionRegistry>();
+handsOut<MetricsCollector, P.MetricsCollector>();
+handsOut<JobScheduler, P.JobScheduler>();
+handsOut<DuneAuthSystem, P.PluginAuthz>();
+handsOut<PluginRuntime, P.PluginRuntime>();
+handsOut<MountApi, P.MountApi>();
+handsOut<PluginApi, P.PluginApi>();
+handsOut<HookContext, P.HookContext>();
+handsOut<HookRegistry, P.HookRegistry>();
+handsOut<ResponseTransformContext, P.ResponseTransformContext>();
+handsOut<CapabilityContext, P.CapabilityContext>();
+handsOut<CapabilityReader, P.CapabilityReader>();
+handsOut<CapabilityRegistrar, P.CapabilityRegistrar>();
+
+// What a plugin supplies and core consumes.
+takesIn<P.DunePlugin, DunePlugin>();
+takesIn<P.PublicRouteRegistration, PublicRouteRegistration>();
+takesIn<P.Capabilities, Capabilities>();
+// Extension points a plugin owns are typed keys, not entries in a shared interface.
+same<CapabilityKey<unknown>, P.CapabilityKey<unknown>>();
+
+same<HookEvent, P.HookEvent>();
+same<PageMovePayload, P.PageMovePayload>();
+same<FormSubmitPayload, P.FormSubmitPayload>();
+same<SystemActionPayload, P.SystemActionPayload>();
+same<AuthenticateResult, P.AuthenticateResult>();
+
+Deno.test("isDuneError behaves the same in the package and in core", () => {
+  const err = { code: "ssrf_blocked" };
+  for (const code of ["ssrf_blocked", "body_too_large", "weak_password", "duplicate_email"] as const) {
+    assertEquals(pluginIsDuneError(err, code), coreIsDuneError(err, code));
+  }
+  assertEquals(pluginIsDuneError(new Error("x"), "ssrf_blocked"), false);
+});
+
+Deno.test("a capability key made by the package is the key core reads", async () => {
+  const { capabilityKey: pluginCapabilityKey } = await import("@dune/plugin-api");
+  assertEquals(pluginCapabilityKey("bar.actions"), coreCapabilityKey("bar.actions"));
+});