re-introduce partial fallback -> route upgrading by ztanner · Pull Request #89063 · vercel/next.js

ztanner · 2026-01-26T16:56:25Z

In #69282, we landed support for Partial Fallback Prerendering. The intention behind this feature was to be able to upgrade from a partial fallback (essentially the UI produced when await params is dynamic, ie the param was not part of generateStaticParams at build time) to a route shell (when the param values are filled in, and thus we can prerender more of the page).

It was reverted in #79258 because we didn't add first-class support to this in the build output API and the necessary infrastructure changes to perform similar behavior that this PR does to trigger a background revalidation with params filled in.

This re-lands the feature in Next.js now that upstream support landed in our public adapter/CLI (vercel/vercel#14703, vercel/vercel#15338, nextjs/adapter-vercel#39, and Vercel infra changes which are rolling out)

ztanner · 2026-01-26T16:56:51Z

re-introduce partial fallback -> route upgrading #89063 👈 (View in Graphite)
preserve allowQuery for partial fallback shells #90887
partial fallbacks: add adapter flag #90395
canary

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nextjs-bot · 2026-01-26T17:04:58Z

Tests Passed

nextjs-bot · 2026-01-26T17:15:15Z

Stats from current PR

✅ No significant changes detected

📊 All Metrics

📖 Metrics Glossary

Dev Server Metrics:

Listen = TCP port starts accepting connections
First Request = HTTP server returns successful response
Cold = Fresh build (no cache)
Warm = With cached build artifacts

Build Metrics:

Fresh = Clean build (no .next directory)
Cached = With existing .next directory

Change Thresholds:

Time: Changes < 50ms AND < 10%, OR < 2% are insignificant
Size: Changes < 1KB AND < 1% are insignificant
All other changes are flagged to catch regressions

⚡ Dev Server

Metric	Canary	PR	Change	Trend
Cold (Listen)	456ms	456ms	✓	▁▁▁▁▁
Cold (Ready in log)	436ms	437ms	✓	▁▁▁▁▁
Cold (First Request)	1.274s	1.250s	✓	▂▂▁▁▂
Warm (Listen)	456ms	457ms	✓	▁▁▁▁▁
Warm (Ready in log)	442ms	442ms	✓	▁▁▁▁▁
Warm (First Request)	350ms	349ms	✓	▁▁▁▁▁

📦 Dev Server (Webpack) (Legacy)

📦 Dev Server (Webpack)

Metric	Canary	PR	Change	Trend
Cold (Listen)	507ms	507ms	✓	▁▁▁▁▁
Cold (Ready in log)	499ms	503ms	✓	▆▆▅▅▁
Cold (First Request)	2.283s	2.278s	✓	▄▃▃▇▁
Warm (Listen)	507ms	508ms	✓	▁▁▁▁▁
Warm (Ready in log)	501ms	497ms	✓	▅▄▅▅▁
Warm (First Request)	2.305s	2.287s	✓	▄▄▄▇▁

⚡ Production Builds

Metric	Canary	PR	Change	Trend
Fresh Build	3.853s	3.803s	✓	▁▁▁▁▁
Cached Build	3.897s	3.839s	✓	▁▁▁▁▁

📦 Production Builds (Webpack) (Legacy)

📦 Production Builds (Webpack)

Metric	Canary	PR	Change	Trend
Fresh Build	16.566s	16.592s	✓	▁▁▂▅▁
Cached Build	16.651s	16.674s	✓	▁▁▁▅▁
node_modules Size	477 MB	477 MB	✓	▁▁▁▁▁

📦 Bundle Sizes

Bundle Sizes

⚡ Turbopack

Client

Main Bundles: **402 kB** → **402 kB** ⚠️ +31 B

80 files with content-based hashes (individual files not comparable between builds)

Server

Middleware

	Canary	PR	Change
middleware-b..fest.js gzip	766 B	767 B	✓
Total	766 B	767 B	⚠️ +1 B

Build Details

Build Manifests

	Canary	PR	Change
_buildManifest.js gzip	446 B	450 B	✓
Total	446 B	450 B	⚠️ +4 B

📦 Webpack

Client

Main Bundles

	Canary	PR	Change
5528-HASH.js gzip	5.54 kB	N/A	-
6280-HASH.js gzip	59.4 kB	N/A	-
6335.HASH.js gzip	169 B	N/A	-
912-HASH.js gzip	4.59 kB	N/A	-
e8aec2e4-HASH.js gzip	62.6 kB	N/A	-
framework-HASH.js gzip	59.7 kB	59.7 kB	✓
main-app-HASH.js gzip	256 B	255 B	✓
main-HASH.js gzip	39.1 kB	39.1 kB	✓
webpack-HASH.js gzip	1.68 kB	1.68 kB	✓
262-HASH.js gzip	N/A	4.59 kB	-
2889.HASH.js gzip	N/A	169 B	-
5602-HASH.js gzip	N/A	5.55 kB	-
6948ada0-HASH.js gzip	N/A	62.6 kB	-
9544-HASH.js gzip	N/A	60.2 kB	-
Total	233 kB	234 kB	⚠️ +734 B

Polyfills

	Canary	PR	Change
polyfills-HASH.js gzip	39.4 kB	39.4 kB	✓
Total	39.4 kB	39.4 kB	✓

Pages

	Canary	PR	Change
_app-HASH.js gzip	194 B	194 B	✓
_error-HASH.js gzip	183 B	180 B	🟢 3 B (-2%)
css-HASH.js gzip	331 B	330 B	✓
dynamic-HASH.js gzip	1.81 kB	1.81 kB	✓
edge-ssr-HASH.js gzip	256 B	256 B	✓
head-HASH.js gzip	351 B	352 B	✓
hooks-HASH.js gzip	384 B	383 B	✓
image-HASH.js gzip	580 B	581 B	✓
index-HASH.js gzip	260 B	260 B	✓
link-HASH.js gzip	2.51 kB	2.51 kB	✓
routerDirect..HASH.js gzip	320 B	319 B	✓
script-HASH.js gzip	386 B	386 B	✓
withRouter-HASH.js gzip	315 B	315 B	✓
1afbb74e6ecf..834.css gzip	106 B	106 B	✓
Total	7.98 kB	7.98 kB	✅ -1 B

Server

Edge SSR

	Canary	PR	Change
edge-ssr.js gzip	125 kB	125 kB	✓
page.js gzip	256 kB	256 kB	✓
Total	380 kB	381 kB	⚠️ +1.05 kB

Middleware

	Canary	PR	Change
middleware-b..fest.js gzip	619 B	615 B	✓
middleware-r..fest.js gzip	156 B	155 B	✓
middleware.js gzip	43.8 kB	43.7 kB	✓
edge-runtime..pack.js gzip	842 B	842 B	✓
Total	45.4 kB	45.3 kB	✅ -88 B

Build Details

Build Manifests

	Canary	PR	Change
_buildManifest.js gzip	715 B	718 B	✓
Total	715 B	718 B	⚠️ +3 B

Build Cache

	Canary	PR	Change
0.pack gzip	4.06 MB	4.08 MB	🔴 +14.1 kB (+0%)
index.pack gzip	103 kB	103 kB	✓
index.pack.old gzip	103 kB	104 kB	✓
Total	4.27 MB	4.29 MB	⚠️ +15 kB

🔄 Shared (bundler-independent)

Runtimes

	Canary	PR	Change
app-page-exp...dev.js gzip	322 kB	322 kB	✓
app-page-exp..prod.js gzip	171 kB	171 kB	✓
app-page-tur...dev.js gzip	322 kB	322 kB	✓
app-page-tur..prod.js gzip	171 kB	171 kB	✓
app-page-tur...dev.js gzip	318 kB	318 kB	✓
app-page-tur..prod.js gzip	169 kB	169 kB	✓
app-page.run...dev.js gzip	319 kB	319 kB	✓
app-page.run..prod.js gzip	169 kB	169 kB	✓
app-route-ex...dev.js gzip	70.9 kB	70.9 kB	✓
app-route-ex..prod.js gzip	49.3 kB	49.3 kB	✓
app-route-tu...dev.js gzip	70.9 kB	70.9 kB	✓
app-route-tu..prod.js gzip	49.3 kB	49.3 kB	✓
app-route-tu...dev.js gzip	70.5 kB	70.5 kB	✓
app-route-tu..prod.js gzip	49 kB	49.1 kB	✓
app-route.ru...dev.js gzip	70.4 kB	70.5 kB	✓
app-route.ru..prod.js gzip	49 kB	49 kB	✓
dist_client_...dev.js gzip	324 B	324 B	✓
dist_client_...dev.js gzip	326 B	326 B	✓
dist_client_...dev.js gzip	318 B	318 B	✓
dist_client_...dev.js gzip	317 B	317 B	✓
pages-api-tu...dev.js gzip	43.2 kB	43.3 kB	✓
pages-api-tu..prod.js gzip	32.9 kB	32.9 kB	✓
pages-api.ru...dev.js gzip	43.2 kB	43.2 kB	✓
pages-api.ru..prod.js gzip	32.9 kB	32.9 kB	✓
pages-turbo....dev.js gzip	52.6 kB	52.6 kB	✓
pages-turbo...prod.js gzip	38.5 kB	38.5 kB	✓
pages.runtim...dev.js gzip	52.6 kB	52.6 kB	✓
pages.runtim..prod.js gzip	38.5 kB	38.5 kB	✓
server.runti..prod.js gzip	62 kB	62 kB	✓
Total	2.84 MB	2.84 MB	⚠️ +431 B

📝 Changed Files (25 files)

Files with changes:

app-page-exp..ntime.dev.js
app-page-exp..time.prod.js
app-page-tur..ntime.dev.js
app-page-tur..time.prod.js
app-page-tur..ntime.dev.js
app-page-tur..time.prod.js
app-page.runtime.dev.js
app-page.runtime.prod.js
app-route-ex..ntime.dev.js
app-route-ex..time.prod.js
app-route-tu..ntime.dev.js
app-route-tu..time.prod.js
app-route-tu..ntime.dev.js
app-route-tu..time.prod.js
app-route.runtime.dev.js
app-route.ru..time.prod.js
pages-api-tu..ntime.dev.js
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pages-api.runtime.dev.js
pages-api.ru..time.prod.js
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📎 Tarball URL

https://vercel-packages.vercel.app/next/commits/e7da475d6553d348bac8d450c92f8d1c969569a3/next

This PR adds the `partialFallback` property onto the Prerender document. This will signal different fallback semantics: namely, the expectation with a partial fallback is that it will immediately kick off a background revalidation to produce a more complete static document. See vercel/next.js#89063 corresponding Next.js changes.

codspeed-hq · 2026-03-04T20:50:02Z

Merging this PR will not alter performance

✅ 17 untouched benchmarks
⏩ 3 skipped benchmarks¹

_{Comparing 01-26-re-introduce_partial_fallback_-_route_upgrading (22c0c6f) with canary (8bd3a2a)²}

3 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩
No successful run was found on graphite-base/89063 (8bd3a2a) during the generation of this report, so canary (8bd3a2a) was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

gnoff · 2026-03-06T15:58:02Z

packages/next/src/build/templates/app-page.ts

+              // request itself.
+              !isPrefetchRSCRequest
+            ) {
+              scheduleOnNextTick(async () => {


feels like responseCache shoud own the concept of background revalidating. or we should abstract this into a shared helper.

ztanner · 2026-03-06T16:16:08Z

Merge activity

Mar 6, 4:16 PM UTC: A user started a stack merge that includes this pull request via Graphite.
Mar 6, 4:18 PM UTC: Graphite rebased this pull request as part of a merge.
Mar 6, 4:39 PM UTC: Graphite couldn't merge this PR because it was not satisfying all requirements (Failed CI: 'thank you, next', 'test integration windows / build', 'test prod (7/10) / build').

In #69282, we landed support for Partial Fallback Prerendering. The intention behind this feature was to be able to upgrade from a partial fallback (essentially the UI produced when `await params` is dynamic, ie the param was not part of `generateStaticParams` at build time) to a route shell (when the param values are filled in, and thus we can prerender more of the page). It was reverted in #79258 because we didn't add first-class support to this in the build output API and the necessary infrastructure changes to perform similar behavior that this PR does to trigger a background revalidation with params filled in. This re-lands the feature in Next.js now that upstream support landed (vercel/vercel#14703, vercel/vercel#15338, and Vercel infra changes which are rolling out)

This puts the partial fallback upgrading feature introduced in #89063 behind a flag to make it easier to test the rollout/eventual stabilization in isolation.

nextjs-bot added created-by: Next.js team PRs by the Next.js team. tests type: next labels Jan 26, 2026

ztanner force-pushed the 01-26-re-introduce_partial_fallback_-_route_upgrading branch 3 times, most recently from d75a8eb to 69eae77 Compare January 30, 2026 23:17

ztanner force-pushed the 01-26-re-introduce_partial_fallback_-_route_upgrading branch from 69eae77 to 3c112e2 Compare February 23, 2026 23:03

This was referenced Feb 23, 2026

partial fallbacks: add adapter flag #90395

Merged

[next]: add partial fallback support vercel/vercel#14703

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ztanner force-pushed the 01-26-re-introduce_partial_fallback_-_route_upgrading branch from 3c112e2 to b849252 Compare February 25, 2026 19:25

ztanner changed the base branch from canary to graphite-base/89063 March 4, 2026 20:10

ztanner force-pushed the 01-26-re-introduce_partial_fallback_-_route_upgrading branch from b849252 to 22c0c6f Compare March 4, 2026 20:10

ztanner changed the base branch from graphite-base/89063 to 02-23-partial_fallbacks_add_adapter_flag_tweak_allowquery March 4, 2026 20:11

ztanner changed the base branch from 02-23-partial_fallbacks_add_adapter_flag_tweak_allowquery to graphite-base/89063 March 4, 2026 20:27

ztanner force-pushed the graphite-base/89063 branch from 22e217d to 8bfc33f Compare March 4, 2026 20:27

ztanner force-pushed the 01-26-re-introduce_partial_fallback_-_route_upgrading branch from 22c0c6f to 219a7ff Compare March 4, 2026 20:27

ztanner mentioned this pull request Mar 4, 2026

preserve allowQuery for partial fallback shells #90887

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ztanner force-pushed the graphite-base/89063 branch from 8bfc33f to ed3bb76 Compare March 4, 2026 20:38

ztanner force-pushed the 01-26-re-introduce_partial_fallback_-_route_upgrading branch from 219a7ff to 0d04c66 Compare March 4, 2026 20:38

ztanner force-pushed the graphite-base/89063 branch from ed3bb76 to 82ddf97 Compare March 4, 2026 22:56

ztanner force-pushed the 01-26-re-introduce_partial_fallback_-_route_upgrading branch from 0d04c66 to 52bac7b Compare March 4, 2026 22:56

ztanner changed the base branch from graphite-base/89063 to ztanner/partial-fallback-allowquery March 4, 2026 22:57

ztanner force-pushed the 01-26-re-introduce_partial_fallback_-_route_upgrading branch from 52bac7b to 9972012 Compare March 5, 2026 20:21

ztanner force-pushed the ztanner/partial-fallback-allowquery branch from 82ddf97 to 05758b2 Compare March 5, 2026 20:21

ztanner force-pushed the 01-26-re-introduce_partial_fallback_-_route_upgrading branch from 9972012 to 8bf4590 Compare March 5, 2026 21:14

ztanner marked this pull request as ready for review March 5, 2026 21:32

ztanner requested review from gnoff and ijjk March 5, 2026 21:32

ijjk approved these changes Mar 5, 2026

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gnoff approved these changes Mar 6, 2026

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ztanner changed the base branch from ztanner/partial-fallback-allowquery to graphite-base/89063 March 6, 2026 16:16

ztanner changed the base branch from graphite-base/89063 to canary March 6, 2026 16:16

ztanner added 2 commits March 6, 2026 16:17

re-introduce partial fallback -> route upgrading

538ccd6

test fixes

e7da475

ztanner force-pushed the 01-26-re-introduce_partial_fallback_-_route_upgrading branch from 8bf4590 to e7da475 Compare March 6, 2026 16:17

ztanner merged commit af48a18 into canary Mar 6, 2026
280 of 283 checks passed

ztanner deleted the 01-26-re-introduce_partial_fallback_-_route_upgrading branch March 6, 2026 16:53

ztanner mentioned this pull request Mar 9, 2026

partial fallbacks: gate behind experimental flag #91123

Merged

ztanner added a commit that referenced this pull request Mar 9, 2026

partial fallbacks: gate behind experimental flag (#91123)

c0bf4b6

This puts the partial fallback upgrading feature introduced in #89063 behind a flag to make it easier to test the rollout/eventual stabilization in isolation.

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re-introduce partial fallback -> route upgrading#89063

re-introduce partial fallback -> route upgrading#89063
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