API Changes

• core: Delete the never-used io.grpc.internal.ReadableBuffer.readBytes(ByteBuffer) (#12580) (738782fb0). This is deeply internal and not accessible, so shouldn’t impact anything. However, Apache Arrow Java uses reflection to access private fields; GH-939: Remove reflection for gRPC buffers is swapping to gRPC’s public zero-copy APIs

• opentelemetry: Add target attribute filter for metrics (#12587). Introduce an optional Predicate targetAttributeFilter to control how grpc.target is recorded in OpenTelemetry client metrics. When a filter is provided, targets rejected by the predicate are normalized to "other" to reduce grpc.target metric cardinality, while accepted targets are recorded as-is. If no filter is set, existing behavior is preserved. This change adds a new Builder API on GrpcOpenTelemetry to allow applications to configure the filter. 

Behavior Changes

• core: Convert AutoConfiguredLB to an actual LB (4bbf8eee5). This is an internal refactoring, but it does improve how errors are handled for broken binaries. Previously, not being able to load pick_first would result in a channel panic. Now it is handled as a regular load balancing error

• okhttp: Assert no pending streams before transport READY (#12566) (ed6d175fc). No pending streams should exist when the transport transitions to READY. This PR adds an assertion to help verify this invariant.

Bug Fixes

• core: PickFirstLB should not return a subchannel during CONNECTING (228fc8ecd). Pick-first in grpc-java has behaved this way since it was created, and it was of no consequence. However, now there are some load balancing policies (mainly RLS) that will do a pick() and hope the result to be reasonably accurate for metrics.

Improvements

• core: Improve DEADLINE_EXCEEDED message for CallCreds delays (ead532b39). Previously the error message contained “buffered_nanos” and “waiting_for_connection” for connection delays. However, we discovered the same strings were also used if waiting on CallCredentials. Now you’ll see details like “connecting_and_lb_delay”, “call_credentials_delay”, and “was_still_waiting”.

• opentelemetry: Add Android API checking (a9f73f4c0). Previously we assumed OpenTelemetry support would not be used on Android. It did happen to be compatible with Android, but since OpenTelemetry does have some Android support, we now have a check that it remains compatible

• core: Catch Errors when calling complex config parsing code (a535ed799). Error (and any other Throwable) is now caught and handled when parsing configuration (e.g., service config, xds). This will cause such failures to be handled gracefully instead of panicking the channel

• core: Implement LoadBalancer.Helper.createOobChannel() with the internals of createResolvingOobChannel() (3915d029c). This API is only expected to be relevant to the gRPC-LB lookaside load balancer, and is not believed to have behavior changes. Out-of-band channel had been implemented with its own stripped-down Channel without load balancing. Reimplementing using the resolving oob channel makes it a full-fledged channel and reduces the burden when integrating new features and allows us to have a ManagedChannelBuilder to use with efforts like gRFC A110: Child Channel Options.

• xds: Implement the proactive connection logic in RingHashLoadBalancer as outlined in gRFC A61 (#12596). Previously, the Java implementation only initialized child balancers when a ring-chosen endpoint was in TRANSIENT_FAILURE during a picker's pickSubchannel call. This PR adds the missing logic: when a child balancer reports TRANSIENT_FAILURE, the LoadBalancer now proactively initializes the first available IDLE child if no other children are currently connecting or ready.

This ensures a backup subchannel starts warming up immediately outside the RPC flow, reducing failover latency and improving overall resilience. This behavior was previously present but was inadvertently lost after #10610.

• api: Add RFC 3986 support to DnsNameResolverProvider (#12602) (f65127cf7) Experimental RFC 3986 target URI parsing mode (disabled by default)

New Features

• opentelemetry: Actual reason for the disconnects in subchannel metrics(6b2f7580c), completing the remaining work in gRFC A96: OTel metrics for Subchannels

Dependencies 

• protobuf: Upgrade Bazel protobuf to 33.1 (#12553) (b61a8f49c) and load java_proto_library from the protobuf repo (c7f3cdbc3)

• protobuf: Fix build with Bazel 9 by upgrading bazel_jar_jar and grpc-proto versions (#12569)

• Upgrade dependencies (#12588) (6422092e3) Netty to 4.1.130, error-prone annotations to 2.45.0, google-auth-library to 1.41.0, tomcat-embed-core9 to 9.0.113, tomcat-embed-core to 10.1.50, opentelemetry to 1.57.0, jetty-ee10-servlet to 12.1.5, jetty-http2-server to 12.1.5, google-cloud-logging to 3.23.9, google-auth to 1.41.0, proto-google-common-protos to 2.63.2.

Thanks to

• @benjaminp
• @becomeStar
• @meteorcloudy