diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index e366b2fc6..496941d18 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -58,6 +58,15 @@ setting only to the vnext build, not the live build. If vnext builds a different
 source branch, set the intended label explicitly instead. Changing the badge on
 a deployed static site requires rebuilding its frontend assets.
 
+## Incremental API builds
+
+API reference cache keys include the deployment commit and repository URL emitted
+by the shared page head. Changing either value must regenerate cached API pages:
+mixing old and new `git-commit-id` metadata makes the deployment guard correctly
+fall back to full-page navigation. Builds with unchanged data and deployment
+metadata can still reuse their API pages. Keep the head metadata and API cache
+keys in sync when changing build identity handling.
+
 ## 🆘 Getting help
 
 - **Issues**: Report bugs or request features via [GitHub Issues](https://github.com/microsoft/aspire.dev/issues)
diff --git a/src/frontend/src/components/starlight/Head.astro b/src/frontend/src/components/starlight/Head.astro
index ef3ae838a..7e220ceab 100644
--- a/src/frontend/src/components/starlight/Head.astro
+++ b/src/frontend/src/components/starlight/Head.astro
@@ -9,17 +9,13 @@ import AccessibleCodeButtons from '../AccessibleCodeButtons.astro';
 import SiteTour from '../SiteTour.astro';
 import { getStructuredData } from '@utils/structured-data';
 import { getOgMetadata } from '@utils/page-metadata';
+import { deploymentMetadata } from '@utils/deployment-metadata';
 
 // The Poppins weights that text in the first viewport renders with. The same files back the
 // @font-face rules in site.css, so these resolve to the already-emitted assets.
 const preloadedFonts = [poppinsRegular, poppinsSemiBold, poppinsBold];
 
-const repoUrl = (
-  import.meta.env.PUBLIC_REPO_URL ||
-  import.meta.env.REPO_URL ||
-  'https://github.com/microsoft/aspire.dev'
-).replace(/\/$/, '');
-const commit = import.meta.env.PUBLIC_GIT_COMMIT_ID || import.meta.env.GIT_COMMIT_ID || '';
+const { commit, repoUrl } = deploymentMetadata;
 
 const route = Astro.locals?.starlightRoute;
 const entryId = route?.entry?.id;
diff --git a/src/frontend/src/env.d.ts b/src/frontend/src/env.d.ts
index 341130a95..e3e8366ca 100644
--- a/src/frontend/src/env.d.ts
+++ b/src/frontend/src/env.d.ts
@@ -1 +1,8 @@
 declare const __ASPIRE_REDIRECT_PATHS__: readonly string[];
+
+interface ImportMetaEnv {
+  readonly PUBLIC_GIT_COMMIT_ID?: string;
+  readonly GIT_COMMIT_ID?: string;
+  readonly PUBLIC_REPO_URL?: string;
+  readonly REPO_URL?: string;
+}
diff --git a/src/frontend/src/utils/api-cache-key.ts b/src/frontend/src/utils/api-cache-key.ts
index b36f405c3..e02e2b90b 100644
--- a/src/frontend/src/utils/api-cache-key.ts
+++ b/src/frontend/src/utils/api-cache-key.ts
@@ -1,4 +1,5 @@
 import { createHash } from 'node:crypto';
+import { deploymentMetadata } from './deployment-metadata';
 
 const digests = new WeakMap<object, string>();
 
@@ -7,13 +8,16 @@ const digests = new WeakMap<object, string>();
  *
  * Every page generated from a package or module document returns this value
  * from `getStaticPaths()`, so Astro can reuse a page from the previous build
- * as long as the document it was rendered from (and the template code) is
- * unchanged. The digest is memoized per document object.
+ * as long as its document, deployment metadata and template code are unchanged.
+ * The digest is memoized per document object.
  */
 export function apiDocumentCacheKey(document: object): string {
   let digest = digests.get(document);
   if (!digest) {
-    digest = createHash('sha1').update(JSON.stringify(document)).digest('base64url');
+    // Bun reads import.meta.env at render time, outside Astro's dependency hash.
+    digest = createHash('sha1')
+      .update(JSON.stringify([document, deploymentMetadata]))
+      .digest('base64url');
     digests.set(document, digest);
   }
   return digest;
diff --git a/src/frontend/src/utils/deployment-metadata.ts b/src/frontend/src/utils/deployment-metadata.ts
new file mode 100644
index 000000000..f1c051faa
--- /dev/null
+++ b/src/frontend/src/utils/deployment-metadata.ts
@@ -0,0 +1,8 @@
+export const deploymentMetadata = {
+  commit: import.meta.env.PUBLIC_GIT_COMMIT_ID || import.meta.env.GIT_COMMIT_ID || '',
+  repoUrl: (
+    import.meta.env.PUBLIC_REPO_URL ||
+    import.meta.env.REPO_URL ||
+    'https://github.com/microsoft/aspire.dev'
+  ).replace(/\/$/, ''),
+};
diff --git a/src/frontend/tests/unit/api-cache-key.vitest.test.ts b/src/frontend/tests/unit/api-cache-key.vitest.test.ts
new file mode 100644
index 000000000..3a4c99b43
--- /dev/null
+++ b/src/frontend/tests/unit/api-cache-key.vitest.test.ts
@@ -0,0 +1,69 @@
+import { afterEach, describe, expect, test, vi } from 'vitest';
+
+afterEach(() => {
+  vi.unstubAllEnvs();
+  vi.resetModules();
+});
+
+async function loadCacheKeys(commit: string, repoUrl = 'https://github.com/microsoft/aspire.dev') {
+  vi.stubEnv('PUBLIC_GIT_COMMIT_ID', commit);
+  vi.stubEnv('PUBLIC_REPO_URL', repoUrl);
+  vi.resetModules();
+  return import('../../src/utils/api-cache-key');
+}
+
+describe('API incremental cache deployment identity', () => {
+  const document = { package: { name: 'Aspire.Hosting.Redis' }, types: [] };
+
+  test('retains the same key for unchanged data and deployment', async () => {
+    const first = await loadCacheKeys('build-a');
+    const key = first.apiDocumentCacheKey(document);
+    expect(first.apiDocumentCacheKey(document)).toBe(key);
+    expect(first.apiDocumentCacheKey(structuredClone(document))).toBe(key);
+    const second = await loadCacheKeys('build-a');
+    expect(second.apiDocumentCacheKey(document)).toBe(key);
+    expect(second.apiDocumentCacheKey({ ...document, types: ['RedisResource'] })).not.toBe(key);
+  });
+
+  test('invalidates unchanged API data when the deployment changes', async () => {
+    const first = await loadCacheKeys('build-a');
+    const oldKey = first.apiDocumentCacheKey(document);
+    const second = await loadCacheKeys('build-b');
+    expect(second.apiDocumentCacheKey(document)).not.toBe(oldKey);
+  });
+
+  test('invalidates source links when the repository changes, ignoring a trailing slash', async () => {
+    const first = await loadCacheKeys('build-a');
+    const oldKey = first.apiDocumentCacheKey(document);
+    const equivalent = await loadCacheKeys('build-a', 'https://github.com/microsoft/aspire.dev/');
+    expect(equivalent.apiDocumentCacheKey(document)).toBe(oldKey);
+    const changed = await loadCacheKeys('build-a', 'https://github.com/example/docs');
+    expect(changed.apiDocumentCacheKey(document)).not.toBe(oldKey);
+  });
+
+  test('applies deployment-aware keys to every path for the owning package', async () => {
+    const { apiDocumentCacheKey, withApiCacheKeys } = await loadCacheKeys('build-a');
+    const paths = ['type', 'member'].map((name) => ({
+      params: { name },
+      props: { packageName: document.package.name },
+    }));
+    const result = withApiCacheKeys(paths, [{ data: document }]);
+    expect(result).toEqual(paths.map((path) => ({
+      ...path,
+      cacheKey: apiDocumentCacheKey(document),
+    })));
+  });
+
+  test('shares the head metadata fallback values', async () => {
+    await loadCacheKeys('');
+    vi.stubEnv('GIT_COMMIT_ID', 'fallback-build');
+    vi.stubEnv('PUBLIC_REPO_URL', '');
+    vi.stubEnv('REPO_URL', 'https://github.com/example/docs/');
+    vi.resetModules();
+    const { deploymentMetadata } = await import('../../src/utils/deployment-metadata');
+    expect(deploymentMetadata).toEqual({
+      commit: 'fallback-build',
+      repoUrl: 'https://github.com/example/docs',
+    });
+  });
+});