The .NET 10 LTS Upgrade Playbook

.NET 10 LTS arrived on November 11, 2025, and it’s the release that finally makes the migration conversation easy. Between C# 14 quality-of-life features, noticeably faster runtime and JIT work, stronger ASP.NET Core primitives (including passkey support), and a clear three‑year support runway through November 2028, the business case is straightforward. This playbook covers what’s new, risk areas to watch, and a practical path to upgrade without breaking your quarter.

Why .NET 10 LTS matters right now

Shipping velocity and operating cost hinge on two things: developer time and runtime efficiency. .NET 10 pushes both forward. It’s a Long‑Term Support release with security fixes and servicing through November 2028, so you get stability without freezing innovation. And for teams still on .NET 8 LTS or .NET 9 STS, the calendar pressure is real—.NET 8 support ends in 2026, and short‑term support on 9 also wraps in 2026. Consolidating on .NET 10 this fiscal year removes a 2026 cliff while banking performance and tooling gains today.

What’s new in .NET 10 (the parts you’ll actually use)

C# 14: small changes, real impact

Most teams won’t rewrite code to chase syntax sugar, but C# 14 has a few features that reduce friction in everyday work. Field‑backed properties make it painless to add logic to properties you once left auto‑implemented. Extension members broaden how you add capability to types you don’t own. You also get cleaner span conversions and null‑safe assignment patterns that tidy up hot code paths. These are incremental wins that add up when you’re touching code daily.

ASP.NET Core 10: secure defaults and better observability

On the web stack, .NET 10 includes practical upgrades: built‑in passkey (WebAuthn/FIDO2) support in Identity; typed results for Server‑Sent Events for simple, resilient real‑time updates; and automatic memory pool eviction so long‑running services shed idle memory without manual tuning. Diagnostics also step up with new meters for critical areas like Identity and Blazor, plus better tracing for Blazor Server and new tooling for WebAssembly apps. Net effect: you detect issues faster, and many incidents simply never happen.

Runtime and JIT: measurable speed, not just vibes

Under the hood, JIT improvements, AVX10.2 vectorization on newer Intel chips, Arm64 tuning, Native AOT refinements, and reduced allocations across networking and collections translate into higher throughput and lower latency. If your APIs are CPU‑bound or you run high QPS on Kestrel, expect tangible wins. In mixed fleets, we’ve seen double‑digit reductions in p95 latency after recompiling and redeploying with zero application logic changes.

Post‑quantum cryptography and modern TLS

Security teams will appreciate expanded post‑quantum cryptography support (ML‑DSA/ML‑KEM options and Windows CNG integrations) and consistent TLS 1.3 behavior across platforms. Even if PQC adoption remains strategic for now, landing on a platform that understands the future cryptography story avoids another upgrade for crypto reasons alone.

Aspire 13: orchestration that grows with you

If you’re using Aspire to stitch together front ends, APIs, workers, and data services, the .NET 10 bundle makes that cleaner. The AppHost experience simplifies, container workflows accelerate, and the dashboard becomes easier to secure with OpenID Connect. For teams standardizing on a .NET‑first developer platform, these cuts in friction pay back quickly.

Is .NET 10 LTS safe to adopt before year‑end?

Yes—with a disciplined rollout. The base SDK and runtime have been exercised across previews and RCs, and the long‑term servicing promise means fixes keep coming. The keys are validating your package graph, upgrading toolchains together (Visual Studio 2026 or the updated C# Dev Kit), and running compatible CI images. Avoid half‑upgrades: pick an environment slice, move it fully to .NET 10, then expand.

People also ask: common upgrade questions

Do I need to move to C# 14 immediately?

No. The compiler defaults to C# 14 when you target net10.0, but you can opt your project to a prior language version while you upgrade. That lets you decouple runtime migration from language adoption.

Will .NET 8 LTS be supported long enough if we wait?

.NET 8 LTS remains supported into late 2026. If you’re mid‑delivery and can’t risk change, hold your line—then set a date for .NET 10 cutover next quarter so you’re not compressing the schedule in 2026.

What breaks when targeting net10.0?

The usual suspects: older analyzers that pin to APIs that moved, packages with tight <TargetFramework> constraints, and any reflection‑heavy code that depends on non‑public framework internals. Most first‑party Microsoft packages are ready; the edge cases are niche libraries. Run your solution through a full dotnet build -warnaserror and sort warnings before smoke tests.

.NET 10 LTS upgrade checklist (works for monoliths and microservices)

Here’s the battle‑tested path we use with product teams. Treat it as a two‑to‑four‑week sprint, depending on system size.

1. Inventory and slice: enumerate all apps/services targeting .NET 6–9. Group by deploy unit. Pick one low‑risk API and one user‑facing app as your pilot slice.
2. Toolchain baseline: install .NET 10 SDK on dev boxes and build agents; align to Visual Studio 2026 or the latest VS Code C# Dev Kit. Update your base Dockerfiles to mcr.microsoft.com/dotnet/aspnet:10.0 and sdk:10.0.
3. Package graph: update first‑party Microsoft packages (Extensions, EF Core, ASP.NET Core) to their 10.x‑compatible lines. For third‑party libraries, bump to versions that declare net10.0 support.
4. Target framework bump: change <TargetFramework> to net10.0. If needed, set <LangVersion> to your current language version to avoid mixing changes.
5. Build, test, and benchmark: run unit/integration tests. Capture baseline p50/p95 latency and memory for a real workload. Compare after the upgrade—expect improvements without code changes.
6. Security pass: verify you’re on patched Kestrel and ASP.NET Core packages (request‑smuggling fixes landed in October). Confirm TLS settings and Identity changes—especially if you’re enabling passkeys.
7. Progressive rollout: deploy the pilot behind a feature flag or shard traffic (1% → 10% → 50% → 100%). Watch error budgets, GC metrics, and thread pool saturation. If clean for 48–72 hours, expand to the next slice.

Data points that justify the sprint

Two facts move stakeholders: the support timeline and the security posture. .NET 10 is supported into November 2028, while both .NET 8 LTS and 9 STS end service in 2026. Meanwhile, the ASP.NET Core ecosystem just dealt with a high‑severity HTTP request smuggling issue (CVE‑2025‑55315) in Kestrel that required broad patching across versions. Running the latest runtime reduces the number of moving parts and gets you defaults that match current guidance.

.NET 10 LTS for web backends: Kestrel, SSE, passkeys

For API and app teams, three capabilities are immediately useful. First, typed Server‑Sent Events simplify real‑time updates when WebSockets are overkill (think dashboards or long‑running jobs). Second, automatic memory pool eviction means fewer memory plateaus over days‑long uptimes. Third, ASP.NET Core Identity adds passkey support out of the box, so you can pilot passwordless auth without bolt‑on libraries. None of these require a redesign—just adopt them where they unlock reliability or UX wins.

Performance and cost: how to translate speed into savings

In cloud environments, throughput headroom often shows up as lower compute spend. If your p95 latency drops 10–20% after the upgrade, you can reduce container replicas while keeping SLOs intact. Combine that with Native AOT for tools or throughput‑critical services and you’ll see smaller images, faster cold starts, and less RAM pressure. If your edge is on a CDN, you can also push more cacheable responses via SSE‑friendly patterns, reducing origin load.

Related reads from our team if you’re tuning your platform while you upgrade:

• Evaluating your CI minutes and parallelism as pipelines get faster? Read our GitHub Actions billing playbook to adjust your runner strategy.
• Running static assets and APIs behind AWS? Factor in origin hits as you test—our take on CloudFront flat‑rate pricing helps teams decide whether to consolidate or stay pay‑as‑you‑go.
• Planning AI features on AWS in 2026? This context from AWS’s multi‑year AI buildout can shape your roadmap as .NET 10’s AI integrations mature.

Security spotlight: CVE‑2025‑55315 and your upgrade

October’s Kestrel request smuggling vulnerability (CVSS 9.9) was a wake‑up call for anyone running internet‑facing .NET services. Even if your exact setup wasn’t exploitable, the response work—base image rebuilds, runtime redeploys, package pinning—was real. Landing on .NET 10 with current ASP.NET Core bits reduces patch lag and lines you up with the latest mitigations and parsing hardening. Don’t skip the basics: rebuild all images from patched base layers, verify WAF/proxy normalization for Transfer‑Encoding/Content‑Length, and test request framing in your canaries.

Tooling and ecosystem: what to upgrade with .NET 10

Your upgrade will go smoother if you move the ecosystem together:

• IDE: Visual Studio 2026 or the latest C# Dev Kit for VS Code. Align the team to eliminate “works on my machine.”
• Build images/runners: adopt the 10.0 SDK images; ensure your CI caches are invalidated so Roslyn and analyzers aren’t fighting you. If you’re adjusting spend while speeding builds, our Actions billing guide has practical knobs to turn.
• UI stacks: Blazor enhancements and vendor libraries with day‑zero support are available—verify your component vendor’s .NET 10 updates before you flip the switch.
• Security scanning: add a rule to flag Dockerfiles that reference pre‑October 2025 base images for ASP.NET/SDK.

API gotchas and edge cases worth testing

Three areas deserve extra scrutiny:

• Serialization: stricter JSON handling helps security, but double‑check if you were relying on permissive behavior (duplicate keys, ignored properties).
• Auth flows: if you introduce passkeys, plan for account recovery and device loss; keep passwords or TOTP as a backup during the pilot.
• Streaming: SSE is simpler than WebSockets, but load test intermediaries (proxies, gateways) for connection lifetime limits and buffering quirks.

Let’s get practical: a two‑week cutover plan

Assuming a medium‑size product (6–12 services, one public web app):

1. Day 1–2: Pilot repo prep. Update SDK, target framework, and packages. Green‑bar tests; fix warnings as errors. Capture baseline perf from production before you touch traffic.
2. Day 3–4: Security bake. Rebuild images from patched base, confirm Kestrel package versions, turn on enhanced metrics. Run dynamic scanning focused on request framing.
3. Day 5–7: Canary. Route 1% traffic. Watch p95/p99, socket exhaustion, and memory. Exercise passkey auth on a subset of accounts.
4. Day 8–10: Ramp. 10% → 50% → 100% with rollback ready. Compare spend before/after. If you gain headroom, reduce replicas to keep SLOs constant.
5. Day 11–14: Roll the pattern across the rest of the fleet, starting with internal services and finishing with the main web app.

FAQ for executives and program managers

What’s the ROI timeline?

You’ll see developer productivity gains immediately (fewer toolchain mismatches, simpler code reviews), with runtime savings the moment traffic hits the upgraded services. Expect a cleaner security posture and fewer emergency patch cycles.

Do we need a big‑bang migration?

No. The slice‑and‑expand model balances risk and speed. The only “big‑bang” you want is on your build agents and Docker base images—those should be upgraded upfront.

What to do next

• Pick two candidates for the pilot (one API, one user‑facing app) and schedule the sprint.
• Standardize developer environments and CI runners on the .NET 10 SDK now.
• Rebuild all base images to include the October security fixes and retest with a proxy in the loop.
• Enable the new ASP.NET Core meters and set alerts for error budgets and GC pause time.
• If you need outside help to plan or execute the migration, see our services overview or contact our team.

Here’s the thing: most migrations stall because teams try to refactor and upgrade at the same time. Don’t. Take the platform win first—.NET 10 LTS gives you speed, security, and a stable runway. Then, with the house in order, you can safely invest in features like passkeys, SSE feeds, or Native AOT where they matter. That’s how you ship faster without adding 3 a.m. pages to your on‑call rotation.