diff --git a/.changeset/auto-inject-experimental-cache.md b/.changeset/auto-inject-experimental-cache.md
new file mode 100644
index 0000000000..244a1ade17
--- /dev/null
+++ b/.changeset/auto-inject-experimental-cache.md
@@ -0,0 +1,5 @@
+---
+"emdash": patch
+---
+
+Fixes a runtime crash where API routes (publish, unpublish, schedule) and templates (`Astro.cache.set`) hit `Cannot read properties of undefined` when the host project hadn't opted into Astro's `experimental.cache`. The integration now auto-injects `memoryCache()` as the default provider; hosts that already configured a provider keep theirs.
diff --git a/packages/core/src/astro/integration/index.ts b/packages/core/src/astro/integration/index.ts
index 98a69f5d10..c43a36e8b5 100644
--- a/packages/core/src/astro/integration/index.ts
+++ b/packages/core/src/astro/integration/index.ts
@@ -11,6 +11,7 @@
  */
 
 import type { AstroIntegration, AstroIntegrationLogger } from "astro";
+import { memoryCache } from "astro/config";
 
 import { validateAllowedOrigins, validateOriginShape } from "../../auth/allowed-origins.js";
 import type { ResolvedPlugin } from "../../plugins/types.js";
@@ -271,11 +272,34 @@ export function emdash(config: EmDashConfig = {}): AstroIntegration {
 								},
 							];
 
+				// EmDash relies on Astro's experimental response cache:
+				// - API routes (publish, unpublish, schedule, etc.) call `cache.invalidate()`
+				//   to bust tagged entries when content changes.
+				// - Templates and user pages call `Astro.cache.set(cacheHint)` so list/detail
+				//   routes are cacheable per-tag.
+				// When the host project does not opt into `experimental.cache`, both call sites
+				// hit `undefined` at runtime and crash with `Cannot read properties of undefined`.
+				// Auto-inject `memoryCache()` as the default provider so the contract holds for
+				// every host (graft path, starter templates, Deploy-to-Cloudflare button, etc.).
+				// Hosts that already configured a provider keep theirs.
+				const existingCacheProvider = (
+					astroConfig.experimental as { cache?: { provider?: unknown } } | undefined
+				)?.cache?.provider;
+				const cacheConfig = {
+					cache: {
+						provider: existingCacheProvider ?? memoryCache(),
+					},
+				};
+
 				updateConfig({
 					security: securityConfig,
 					// fonts is a valid AstroConfig key but may not be in the
 					// type definition for the minimum supported Astro version
 					...({ fonts: emdashFonts } as Record<string, unknown>),
+					// experimental.cache is opt-in in Astro 6 and undefined by default;
+					// cast through unknown because the experimental typing varies across
+					// Astro minor versions.
+					...({ experimental: cacheConfig } as Record<string, unknown>),
 					vite: createViteConfig(
 						{
 							serializableConfig,
diff --git a/packages/core/tests/unit/astro/integration/cache-config.test.ts b/packages/core/tests/unit/astro/integration/cache-config.test.ts
new file mode 100644
index 0000000000..5c74f453af
--- /dev/null
+++ b/packages/core/tests/unit/astro/integration/cache-config.test.ts
@@ -0,0 +1,79 @@
+import { describe, expect, it, vi } from "vitest";
+
+// `createViteConfig` resolves `@emdash-cms/admin/dist` via require.resolve, which
+// fails in a fresh checkout where the admin package hasn't been built yet. Stub
+// it: this test only cares about the experimental.cache injection in
+// `updateConfig({...})`, not the vite config payload.
+vi.mock("../../../../src/astro/integration/vite-config.js", () => ({
+	createViteConfig: () => ({}),
+}));
+
+const { emdash } = await import("../../../../src/astro/integration/index.js");
+
+/**
+ * Regression tests for the experimental.cache provider auto-injection.
+ *
+ * Without this, hosts that haven't opted into `experimental.cache` get an
+ * undefined `cache` parameter in the API route context, and any call to
+ * `cache.enabled` / `cache.invalidate()` (publish, unpublish, schedule, etc.)
+ * or `Astro.cache.set(cacheHint)` (templates) crashes with
+ * `TypeError: Cannot read properties of undefined`.
+ *
+ * Tracked in:
+ *   - https://github.com/emdash-cms/emdash/issues/962 (DX umbrella)
+ *   - https://github.com/emdash-cms/emdash/issues/959 (Deploy-to-Cloudflare 500)
+ *   - https://github.com/emdash-cms/emdash/issues/945 (Astro.cache.set undefined)
+ */
+describe("emdash integration: experimental.cache injection", () => {
+	function runConfigSetup(hostConfig: Record<string, unknown>) {
+		const integration = emdash({});
+		const setup = integration.hooks["astro:config:setup"];
+		if (!setup) throw new Error("astro:config:setup hook missing");
+
+		const updates: Array<Record<string, unknown>> = [];
+		void setup({
+			injectRoute: vi.fn(),
+			injectScript: vi.fn(),
+			addMiddleware: vi.fn(),
+			addClientDirective: vi.fn(),
+			addDevToolbarApp: vi.fn(),
+			addRenderer: vi.fn(),
+			addWatchFile: vi.fn(),
+			updateConfig: (cfg: Record<string, unknown>) => {
+				updates.push(cfg);
+				return cfg;
+			},
+			isRestart: false,
+			logger: {
+				info: vi.fn(),
+				warn: vi.fn(),
+				error: vi.fn(),
+				debug: vi.fn(),
+				fork: vi.fn(),
+			} as never,
+			config: hostConfig as never,
+			command: "build",
+		} as never);
+
+		return updates;
+	}
+
+	it("auto-injects a default cache provider when the host has not opted in", () => {
+		const updates = runConfigSetup({});
+		const cacheUpdate = updates.find((u) => "experimental" in u);
+		expect(cacheUpdate, "expected an updateConfig call with experimental.cache").toBeDefined();
+		const experimental = (cacheUpdate as { experimental: { cache: { provider: unknown } } })
+			.experimental;
+		expect(experimental.cache.provider).toBeDefined();
+		expect(experimental.cache.provider).not.toBeNull();
+	});
+
+	it("preserves the host's existing cache provider when one is configured", () => {
+		const hostProvider = { kind: "host-supplied" };
+		const updates = runConfigSetup({ experimental: { cache: { provider: hostProvider } } });
+		const cacheUpdate = updates.find((u) => "experimental" in u);
+		const experimental = (cacheUpdate as { experimental: { cache: { provider: unknown } } })
+			.experimental;
+		expect(experimental.cache.provider).toBe(hostProvider);
+	});
+});