Breaking for implementers: INatsJSStream and INatsJSConsumer gained members (ResetConsumerAsync, ResetAsync). Anything implementing these interfaces directly, such as a test double, needs updating. Callers are unaffected. Default implementations are not possible while the client targets netstandard2.0.

JetStream parity with server 2.14:

• Offline assets are now reported on the stream and consumer list responses, keyed by name with the reason, alongside the existing missing list (#1248)
• ConsumerConfig.Sourcing exposes the flag the server sets on consumers it creates for interest and work queue stream sourcing (#1249)
• Consumers can be reset from the stream and consumer objects, not just the context (#1250)
• Fix TlsPreferTest hang that aborts the Core2 test run (#1246)

Patch release on top of 3.1.0, promoting 3.1.1-preview.1 to stable. Most of it is the OpenTelemetry receive path. Receive spans for messages consumed internally by the Key/Value watcher, the ordered push consumer behind the Object Store and service endpoints were never ended, so they never reached a backend; shared-inbox request-reply produced disconnected traces with no request span; a JetStream direct get was traced under the write that stored the value rather than the read; and several span tags were wrong. Also fixes a null array passed on the netstandard2.0 object store put path, and a stall when TCP splits the trailing CRLF of an empty payload message.

• Fix receive activity leak when tracing is enabled (#1229)
• Fix inverted array check in object store put path (#1235)
• Fix OpenTelemetry receive span lifecycle, parenting and tags (#1236)
• Update OpenTelemetry documentation (#1242)
• Fix stall when TCP splits the trailing CRLF of an empty payload message (#1243)

Three changes in #1236 are visible in a tracing backend, so queries and dashboards built on the client's spans may need updating: messaging.operation on subscribe and request spans now reads subscribe and request rather than publish, the subscribe span is ActivityKind.Client rather than Producer, and a reply whose trace id differs from its request is linked rather than parented.

Thanks

• @ani-philips for reporting the receive activity leak (#1225)
• @fresh55 for the object store put fix (#1235) and for reporting the empty payload stall (#1237)

Full Changelog: v3.1.0...v3.1.1

Preview on the 3.1 patch line with one fix. Receive activities were created on the connection's read loop and left in Activity.Current, so a message without its own trace context took the previous message's activity as a parent; unrelated messages chained together and the chain stayed reachable for the life of the connection. Receive activities are now parented only by trace context carried in the message, and activities that were never ended before (messages evicted from a full subscription channel, messages that never reach a consumer) are ended. Worth testing if you run with OpenTelemetry tracing enabled.

• Fix receive activity leak when tracing is enabled (#1229)

Adjacent issues in the same receive path are not covered here: the remainder of #1227, plus #1228, #1230, #1232 and #1233.

Thanks to @ani-philips for reporting the leak (#1225).

Full Changelog: v3.1.0...v3.1.1-preview.1

Minor release on top of 3.0.1. Adds subscription events with an OnSubscribed callback, enables full-graph NuGet dependency auditing, and adds documentation examples.

• Add subscription events with OnSubscribed callback (#1217)
• Audit transitive dependencies (#1220)
• Add subject-dispatch serialization example (#1218)
• Add docs.nats.io examples to main (#1216)

The async enumerable returned by SubscribeAsync does not establish the subscription until it is iterated. When the enumerable is handed off to another task, the new OnSubscribed callback signals when it is safe to publish messages the subscription must observe:

var subscribed = new TaskCompletionSource(TaskCreationOptions.RunContinuationsAsynchronously);

var opts = new NatsSubOpts
{
    Events = new NatsSubEvents
    {
        OnSubscribed = _ =>
        {
            subscribed.TrySetResult();
            return default;
        },
    },
};

var consumer = Task.Run(async () =>
{
    await foreach (var msg in nats.SubscribeAsync<string>("greet", opts: opts))
    {
        // process messages
    }
});

await subscribed.Task;
await nats.PublishAsync("greet", "hello"); // subscription is established

OnSubscribed fires once the SUB protocol message has been queued on the subscribing connection, which is enough when publishing on the same connection. If the publisher uses a different connection, add a PingAsync round-trip on the subscribing connection after the callback to be sure the server has processed the subscription:

await subscribed.Task;
await nats.PingAsync(); // server has processed the SUB
await otherConnection.PublishAsync("greet", "hello");

Thanks

• @btzdnl for reporting the SubscribeAsync early-return issue and helping with the design (#1178)

Patch release on top of 3.0.0. Fixes a JetStream list enumeration cancellation bug and adds opt-in W3C Baggage propagation to the OpenTelemetry integration.

• Fix silent completion when JetStream list enumeration is cancelled (#1214)
• OpenTelemetry: opt-in W3C Baggage propagation (#1208)
• Add explicit permissions to CI workflows (#1213)
• Update README for 3.0 release (#1212)

Thanks

• @paagamelo2 for reporting the JetStream list cancellation issue (#1193)
• @iamadamreed for the opt-in W3C Baggage propagation support (#1208)

NATS .NET 3.0 is now stable. This release has been in the works since early this year and brings OpenTelemetry tracing and metrics, .NET 10 target, and a number of API and behavior changes refined over the preview series. Thanks to everyone who tried the previews and reported issues along the way. There are no changes since 3.0.0-preview.11.

.NET 10 Target

3.0 targets netstandard2.0, netstandard2.1, net8.0, and net10.0. net6.0 has been dropped.

OpenTelemetry

• Add OTel metrics support #1154
• Add OpenTelemetry package (tracing) #1172
• Add custom span destination name formatter #1201
• Add OTel ack/dropped metrics and collapse inbox trace tags #1194
• Exclude NATS status frames from consumed metrics #1195
• Fix server.port type and trace tag source #1175
• Fix null-key tag in OpenTelemetry receive fallback #1205
• Match OpenTelemetry subject filters without per-message allocation #1206
• Make shared OpenTelemetry instrumentation options thread-visible #1207

API and behavior changes

• Add message context to serialization interfaces #1082
• Move socket connection interfaces to NATS.Client.Abstractions #1192
• Default request-reply to Direct mode #1182
• Add explicit subscription and consumer drain #1177
• Deprecate SkipSubjectValidation #1180
• Unify subscription channel overflow defaults #1181
• Use System.Threading.Lock on NET9_0_OR_GREATER #1118
• Clean up NET6 and optimize NETSTANDARD #1072
• Update DI package dependencies and documentation #1075

Performance and internals

• Optimize header handling with SearchValues #1203
• Simplify object store base64url encoder #1199

Tests and docs

• Add abstractions package boundary test #1197
• Gate positive-path test connections with ConnectRetryAsync #1198
• Fix flaky CI tests #1179
• Remove stale net6.0 references #1196
• Clarify AddNats vs AddNatsClient #1210

Thanks

Thanks to the community for the contributions and issue reports behind this release:

• @colprog for the header handling optimization (#1203), the custom span destination name formatter (#1201), and requesting the explicit drain API (#1176)
• @to11mtm for the object store base64url encoding work (#557, #565) behind the encoder simplification (#1199)
• @thompson-tomo for the abstractions and package consolidation proposals (#851, #866) behind moving the socket interfaces to NATS.Client.Abstractions (#1192)

Upgrade notes

Details of the API and behavior changes, with the preview each first shipped in.

Target frameworks (since preview.1)

net6.0 is dropped and net10.0 added; the full set is netstandard2.0, netstandard2.1, net8.0, net10.0. Apps targeting .NET 6 or 7 keep working through the netstandard2.1 build, whose encoding hot paths were optimized in 3.0 (#1072), but .NET 8+ gets the fastest code paths.

Request-reply defaults to Direct mode (since preview.9)

NatsOpts.RequestReplyMode now defaults to NatsRequestReplyMode.Direct: replies are correlated through the connection's existing inbox subscription instead of setting up a subscription and channel per request. Semantics are unchanged, including ThrowIfNoResponders. To restore the previous behavior:

var opts = new NatsOpts { RequestReplyMode = NatsRequestReplyMode.SharedInbox };

Subscription channel overflow defaults unified (since preview.9)

All entry points (NatsConnection, NatsClient, DI builders) now share the NatsOpts defaults: pending channel capacity 16384 (up from 1024) and BoundedChannelFullMode.DropNewest. Previously NatsClient and the DI builders forced Wait, which can stall the socket read loop and get the client disconnected as a slow consumer. If a subscriber now falls behind by more than 16K messages, the newest messages are dropped and surfaced through MessageDropped instead of blocking. To restore blocking, accepting the slow consumer risk:

var opts = new NatsOpts { SubPendingChannelFullMode = BoundedChannelFullMode.Wait };

SkipSubjectValidation is obsolete (since preview.9)

The option still works but produces a compiler warning. Validation costs 0-5% on a publish microbenchmark and prevents silently misrouted messages: a subject containing a space splits into subject and reply-to tokens on the wire with no error.

Serializers can opt into message context (since preview.3)

New opt-in INatsSerializeWithContext<T>, INatsDeserializeWithContext<T>, and INatsSerializerWithContext<T> interfaces receive a NatsMsgContext (subject, reply-to, headers) during (de)serialization. Existing serializers work unchanged. One side effect: NatsHeaders no longer becomes read-only after publish, so a single NatsHeaders instance should not be shared across concurrent publishes.

Socket interfaces moved to NATS.Client.Abstractions (since preview.11)

INatsSocketConnection and INatsTlsUpgradeableSocketConnection moved to the NATS.Client.Abstractions package so custom transports can implement them without referencing Core. The namespace is unchanged and the types are forwarded, so existing code is source and binary compatible.

OpenTelemetry package (metrics since preview.8, package since preview.11)

The new NATS.Client.OpenTelemetry package adds AddNatsClientInstrumentation() extensions for both TracerProviderBuilder and MeterProviderBuilder, with options for subject filtering and custom span destination names.

using NATS.Client.OpenTelemetry;

services.AddOpenTelemetry()
    .WithTracing(tracing => tracing
        .AddNatsClientInstrumentation(opts => opts.FilterSubjects(exclude: ["_INBOX.>"])))
    .WithMetrics(metrics => metrics
        .AddNatsClientInstrumentation());

Explicit drain (since preview.9)

INatsSub<T>.DrainAsync() drains a single subscription without disposing the connection: no new deliveries, in-flight messages fenced with a PING/PONG, channel completed.

var sub = await nats.SubscribeCoreAsync<Order>("orders.>");
// ... read from sub.Msgs ...
await sub.DrainAsync(); // in-flight messages still delivered, connection stays open

For JetStream consume loops, the opt-in DrainOnCancel consume option delivers buffered messages after cancellation so handlers can still ack; the default keeps the previous stop-immediately behavior.

var opts = new NatsJSConsumeOpts { DrainOnCancel = true };
await foreach (var msg in consumer.ConsumeAsync<Order>(opts: opts, cancellationToken: ct))
{
    await msg.AckAsync();
}
// on cancellation the loop drains: stops pulling, delivers buffered messages, then completes

DI package dependencies (since preview.2)

NATS.Extensions.Microsoft.DependencyInjection now depends on NATS.Client.Simplified instead of the all-inclusive NATS.Net, and NATS.Net now includes the DI package. If you referenced only the DI package and used JetStream, Key-Value, Object Store, or Services through its transitive dependency, add a direct NATS.Net reference (or the specific packages you use).

Full Changelog: v2.8.2...v3.0.0

Last planned preview on the 3.0 line centered on OpenTelemetry: a new NATS.Client.OpenTelemetry package, richer metrics, and lower-overhead tracing. Also moves the socket connection interfaces into NATS.Client.Abstractions and tightens the header-parsing hot path.

OpenTelemetry:

• Add OpenTelemetry package (#1172)
• Add custom span destination name formatter (#1201)
• Add ack/dropped metrics and collapse inbox trace tags (#1194)
• Exclude NATS status frames from consumed metrics (#1195)
• Make shared instrumentation options thread-visible (#1207)
• Match subject filters without per-message allocation (#1206)
• Fix null-key tag in receive fallback (#1205)

Core & packaging:

• Move socket connection interfaces to NATS.Client.Abstractions (#1192)
• Add abstractions package boundary test (#1197)
• Optimize header handling with SearchValues (#1203)
• Simplify object store base64url encoder (#1199)
• Gate positive-path test connections with ConnectRetryAsync (#1198)
• Remove stale net6.0 references (#1196)

Thanks to @colprog for the custom span destination name formatter (#1201) and the SearchValues header optimization (#1203).

Thanks to everyone testing the previews so far. This should be the last one; please give it one more round of testing, and if no bugs turn up we'll cut the stable 3.0 release in about a week.

Full Changelog: v3.0.0-preview.10...v3.0.0-preview.11

Bug-fix preview on the 3.0 line. Carries the ordered push consumer subscription teardown fixes from the 2.x line; no 3.0-only changes this round.

From the 2.x line:

• jetstream: simplify ordered push consumer sub teardown (#1191)
• jetstream: dispose ordered push consumer sub on teardown (#1188)

Full Changelog: v3.0.0-preview.9...v3.0.0-preview.10

Patch release on the 2.8 line. Fixes ordered push consumer subscription teardown. Thanks to @haoguanjun for the fix.

What's Changed

• Bump MessagePack from 3.1.1 to 3.1.7 #1183
• Clarify DI package descriptions #1163
• Fix ordered push consumer subscription leak on teardown #1188
• Simplify ordered push consumer sub teardown #1191
• Release 2.8.2 #1189

Full Changelog: v2.8.1...v2.8.2

Ninth preview of NATS .NET 3.0. Request-reply now defaults to Direct mode, and subscriptions and consumers can be drained explicitly.

Heads-up: the default request-reply mode is now NatsRequestReplyMode.Direct (previously SharedInbox). Direct uses the same inbox prefix but skips per-reply muxer processing, making RequestAsync more resource-efficient. At the default it still throws NatsNoRespondersException on a no-responders reply, so existing RequestAsync behavior is preserved. If you already set RequestReplyMode explicitly to either mode, nothing changes for you. To keep the old default mechanism, set NatsOpts.RequestReplyMode = NatsRequestReplyMode.SharedInbox.

3.0-only changes:

• Default request-reply to Direct mode (#1182)
• Add explicit subscription and consumer drain (#1177)
• Unify subscription channel overflow defaults (#1181)
• Deprecate SkipSubjectValidation (#1180)
• Fix OTel server.port tag type and source (#1175)

Also from the 2.x line:

• Bump MessagePack 3.1.1 -> 3.1.7 (#1183)

Full Changelog: v3.0.0-preview.8...v3.0.0-preview.9