1.38.0 (Pending)

Incompatible behavior changes

Changes that are expected to cause an incompatibility if applicable; deployment changes are likely required

  • boringssl/fips: The previous flag for specifying FIPS builds (ie --define=boringssl=fips) will no longer work and has been replaced by --config=boringssl-fips. This change will allow us to better support custom SSL libraries, and will allow FIPS-compliant Envoy to be built with the imminent switch to Bazel bzlmod.

  • on_demand: The on_demand filter, when doing On Demand CDS, will no longer do internal redirects after CDS is successful. Filters appearing in the filter chain before the On Demand Filter will no longer be invoked twice. This behavior can be temporarily reverted by setting the runtime guard envoy.reloadable_features.on_demand_cluster_no_recreate_stream to false.

  • tcp_proxy: The TCP proxy filter now requires max_early_data_bytes to be explicitly set when using upstream_connect_mode modes other than IMMEDIATE (i.e., ON_DOWNSTREAM_DATA or ON_DOWNSTREAM_TLS_HANDSHAKE). The field can be set to 0 to disable early data buffering while still using delayed connection modes. Configurations using these modes without max_early_data_bytes will now fail validation at startup.

Minor behavior changes

Changes that may cause incompatibilities for some users, but should not for most

  • ext_authz: Changed the behavior of timeout: 0s in the HTTP ext_authz filter to mean “no timeout” (infinite) instead of immediate timeout. Previously, timeout: 0s would cause requests to fail immediately. This aligns with other Envoy timeout configurations where 0 means disabled or infinite.

  • histograms: Updated libcircllhist to 0.3.2, which changes how bucket bounds are interpreted. This should not impact production monitoring if the number of samples in the histograms is high. Affected tests were adjusted to account for histogram changes.

  • mcp: Changed the default metadata namespace for the MCP filter from mcp_proxy to envoy.filters.http.mcp. This change can be reverted by setting the runtime guard envoy.reloadable_features.mcp_filter_use_new_metadata_namespace to false.

  • ratelimit: Changed the behavior of timeout: 0s in the HTTP rate limit filter to mean “no timeout” (infinite) instead of immediate timeout. Previously, timeout: 0s would cause requests to fail immediately. This aligns with other Envoy timeout configurations where 0 means disabled or infinite.

  • stat_sinks: OpenTelemetry SinkConfig stopped reporting empty delta counters and histograms.

Bug fixes

Changes expected to improve the state of the world and are unlikely to have negative effects

  • drop_overload: Fixed a bug where drop_overload failed to use cached EDS resources.

  • ext_authz: Fixed a bug where headers from a denied authorization response (non-200) were not properly propagated to the client.

  • ext_authz: Fixed the HTTP ext_authz client to respect status_on_error configuration when the authorization server returns a 5xx error or when HTTP call failures occur. Previously, these error scenarios always returned 403 Forbidden regardless of the configured error status.

  • formatter: Fixed the log formatter in HTTP router upstream logs by correctly setting the downstream connection’s ConnectionInfoProvider in the StreamInfo.

  • http: Fixed a potential file descriptor leak where HTTP/1.1 connections with zombie streams (waiting for codec completion) would not be properly closed when in draining state. This could occur when a response was sent before the request was fully received, causing connections to remain open indefinitely. This change can be reverted by setting the runtime guard envoy.reloadable_features.http1_close_connection_on_zombie_stream_complete to false.

  • http: Fixed upstream client to not close connection when idle timeout fires before the connection is established. This change can be reverted by setting the runtime guard envoy.reloadable_features.codec_client_enable_idle_timer_only_when_connected to false.

  • load_report: Fixed an issue upon load-report shutdown race with ADS stream. Introduced proper cleanup of the gRPC stream.

  • oauth2: Fixed OAuth2 refresh requests so host rewriting no longer overrides the original Host header value.

  • odcds: Fixed a crash (SIGABRT) when destroying OdCDS handles on worker threads. The handle no longer holds a direct reference to the subscription, preventing thread-safety issues during destruction. The subscription now persists in ClusterManagerImpl and is looked up by a config source key.

  • overload_manager: Fixed a resource leak in global connection limit tracking that caused permanent connection rejections when using load shedding (e.g., envoy.load_shed_points.tcp_listener_accept). When connections were rejected due to load shedding after passing the global connection limit check, the allocated connection limit resource was not released, causing the connection counter to become incorrect and leading to failed_updates in the resource monitor. This resulted in permanent connection rejections even after load subsided. The fix ensures that connection limit resources are properly released when connections are rejected due to load shedding. Also added defensive resource cleanup for edge cases where address processing fails (e.g., localAddress() or peerAddress() errors).

  • release: Published contrib binaries now include the -contrib suffix in their version string.

  • spiffe: Reduce the number of the file watches needed by trust_bundles when the validator is used in multiple places. Add support for watched_directory to support Kubernetes environments that rely on atomic symbolic file updates. Add content hashing during file watching to handle excessive watch notifications.

  • tls: Fix on-demand TLS selector to enforce session resumption settings.

New features

  • config: Added support for set_node_on_first_message_only to Delta-xDS. Guarded by runtime flag envoy.reloadable_features.xds_legacy_delta_skip_subsequent_node.

  • dynamic modules: Added support for dynamic modules authors to register any combination of http, network, listener, udp listener, and bootstrap filters in the rust SDK.

  • dynamic modules: Introduced the extended ABI forward compatibility mechanism for dynamic modules where modules built with a SDK version can be loaded by Envoy binaries of the next Envoy version. For example, A module built with the v1.38 SDK can now be loaded by an Envoy binary of v1.39.

  • dynamic_modules: Added load balancing policies support for dynamic modules, enabling custom load balancing algorithms to be implemented in dynamic modules.

  • dynamic_modules: Added a process-wide function registry to the dynamic modules ABI. Modules can register functions by name via envoy_dynamic_module_callback_register_function and other modules can resolve them via envoy_dynamic_module_callback_get_function, enabling zero-copy cross-module interactions analogous to dlsym.

  • dynamic_modules: Added configurable metrics_namespace field to DynamicModuleConfig. This allows users to customize the prefix used for all metrics created by dynamic modules. Metrics now appear with the standard envoy_ prefix followed by the namespace in prometheus output (e.g. envoy_myapp_requests_total). The legacy behavior (stripping the namespace prefix from prometheus output) can be restored by setting the runtime guard envoy.reloadable_features.dynamic_modules_strip_custom_stat_prefix to true.

  • ext_authz: Added support for the append_action enum in gRPC ext_authz OkHttpResponse.headers for upstream request header mutations. Previously, only the deprecated append boolean was checked. Now APPEND_IF_EXISTS_OR_ADD, ADD_IF_ABSENT, OVERWRITE_IF_EXISTS, and OVERWRITE_IF_EXISTS_OR_ADD actions are fully supported, providing parity with response_headers_to_add handling.

  • formatter: Added %UPSTREAM_DETECTED_CLOSE_TYPE% and %DOWNSTREAM_DETECTED_CLOSE_TYPE% to expose the detected close type of downstream and upstream connections. The possible values are Normal, LocalReset, and RemoteReset.

  • formatter: Added QUERY_PARAMS support for substitution formatter to log all query params. They can either be logged in their original form or decoded.

  • formatter: Added the new access log formatter DOWNSTREAM_LOCAL_CLOSE_REASON.

  • formatter: Extended *_WITHOUT_PORT address formatters to accept an optional MASK_PREFIX_LEN parameter that masks IP addresses and returns them in CIDR notation (e.g., %DOWNSTREAM_REMOTE_ADDRESS_WITHOUT_PORT(16)% returns 10.1.0.0/16 for client IP 10.1.10.23).

  • mcp_router: Added SSE (Server-Sent Events) streaming support for MCP backend responses. The router now handles SSE responses from backends for tools/call with direct pass-through streaming, and supports SSE aggregation for fanout operations (tools/list, initialize) with incremental event parsing.

  • mcp_router: Added support for MCP client-to-server notification methods notifications/cancelled and notifications/roots/list_changed.

  • mcp_router: Added support for MCP completion method completion/complete with routing based on ref/prompt or ref/resource.

  • mcp_router: Added support for MCP logging method logging/setLevel.

  • mcp_router: Added support for MCP prompt methods prompts/list and prompts/get.

  • mcp_router: Added support for MCP resource methods resources/list, resources/read, resources/subscribe, and resources/unsubscribe.

  • network: Added support access logging in network filters like http filters, by allowing network filters to register as access logger instances.

  • ratelimit: Added per-descriptor x-ratelimit-* headers support. See the x_ratelimit_option field documentation for more details.

  • ratelimit: Added support for shadow mode in the local rate limit filter.

  • resource_monitors: Added cgroup v2 support to the CPU utilization resource monitor. The monitor now automatically detects and selects between cgroup v1 and v2 at runtime by checking available cgroup files on the system. This enables the resource monitor to work correctly in both cgroup v1 and v2 environments without configuration changes.

  • sse_parser: Extended the SSE parser utility to support all standard SSE fields: id, event (as event_type), and retry, in addition to the existing data field. The retry field is parsed as a uint32_t and only accepts values consisting of ASCII digits per the SSE specification.

  • stat_sinks: Added support for exporting OpenTelemetry metrics via HTTP. The new http_service field in SinkConfig enables direct OTLP metrics export to backends that only accept HTTP (Dynatrace, Datadog, Elastic), without requiring an intermediate collector sidecar.

  • stats: Added support to limit the number of stats stored in each stats scope in the stats library.

  • stats: The admin prometheus stats endpoint now supports the protobuf exposition format, and will automatically use it if the request contains the correct Accept header, or if query parameter prom_protobuf=1 is set. In a prometheus scrape configuration, add PrometheusProto to scrape_protocols to use the protobuf format.

  • tcp_proxy: Added an option to emit a log entry when the connection is accepted.

  • tls: Added support for fetching certificates on-demand via SDS in the upstream TLS transport socket using the extension on-demand certificate selector.

  • tracers: Add log events to spans created by OpenTelemetry tracer.