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Auto-Enabled Experimental.RuntimeServerDeploymentId Overrides Custom Build-Time Deployment ID And Breaks Skew Protection On Prebuilt Vercel Deployments

When building with vercel build and a custom NEXT_DEPLOYMENT_ID, Next.js 16.2.6 auto-enables experimental.runtimeServerDeploymentId. At runtime, base-server.ts overwrites the deploymentId with Vercel's platform-injected dpl_ ID. Skew Protection pinning for prebuilt deployments relies on the build-time custom ID, so clients receive a mismatched ID and cross-deployment Server Actions fail with UnrecognizedActionError.

highConfidence 95%Next.js Affected V16.2.6

Origin Analysis

The auto-enable logic in packages/next/src/server/config.ts activates runtimeServerDeploymentId whenever NOW_BUILDER=1, without checking whether a custom build-time deployment ID is already set. This causes base-server.ts to trust the runtime process.env.NEXT_DEPLOYMENT_ID (dpl_...) at request time, overriding the build-time custom ID that Vercel's prebuilt Skew Protection expects.

1. Create a Vercel Pro project with Skew Protection enabled. 2. Build the Next.js app with NEXT_DEPLOYMENT_ID=<custom-id> and vercel build. 3. Deploy with vercel deploy --prebuilt --archive=tgz. 4. Inspect the served HTML and observe data-dpl-id="dpl_..." instead of the custom ID. 5. Trigger a Server Action from a stale tab (or curl with x-deployment-id: <custom-id> vs dpl_...) and observe that dpl_ ID is not pinned and the request fails with UnrecognizedActionError.

Fixing Code Block

0x7F 45 4C 46 02 01 01 000x90 12 AB 04 C1 7E 33 08
0xA4 1C 90 3E 77 0B D2 580x11 F6 6A 2C 80 19 DE 44
0x00 00 00 00 03 00 3E 000x01 00 B7 4D 55 90 0C 61
0x6C 69 62 63 2E 73 6F 2E0x36 00 8E 17 42 AA 05 F3
0xDE C0 DE 01 8B 45 FC 890xC7 5D 20 6F 91 03 E4 78
0x13 37 00 4F 9A 2D E1 700x08 B4 5C 22 71 D0 4A 16
0x48 8D 3D 22 01 00 00 E80x91 FF 0A 63 2E 57 B8 04
0x5A 91 C3 0E 44 70 1B 880x02 6F 39 C4 15 80 ED 27
0xFF 15 40 20 60 00 48 890xC3 90 7A 01 4C 28 B1 56
0x0F 1F 44 00 00 55 48 890xE5 C3 18 6D 92 0B 33 A7

Edge Case Audit

Disabling runtimeServerDeploymentId may affect non-prebuilt or future deployment modes that rely on the runtime override. Before applying, verify that your deployment pipeline is strictly prebuilt with a custom build-time NEXT_DEPLOYMENT_ID. If the upstream Next.js fix is released, remove this config override and upgrade to the patched version. Rollback: delete the experimental block or set runtimeServerDeploymentId back to true after confirming the upstream fix.