diff --git a/Makefile b/Makefile
index ea42652e9..4f06eaf12 100644
--- a/Makefile
+++ b/Makefile
@@ -8,7 +8,7 @@ ci-build: install proto http-api-docs
 install: grpc-install api-linter-install buf-install
 
 # Run all linters and compile proto files.
-proto: grpc http-api-docs nexus-rpc-yaml
+proto: grpc http-api-docs system-nexus-wit
 ########################################################################
 
 ##### Variables ######
@@ -17,7 +17,9 @@ GOPATH := $(shell go env GOPATH)
 endif
 
 GOBIN := $(if $(shell go env GOBIN),$(shell go env GOBIN),$(GOPATH)/bin)
-PATH := $(GOBIN):$(PATH)
+CARGO_HOME ?= $(HOME)/.cargo
+CARGO_BIN := $(CARGO_HOME)/bin
+PATH := $(GOBIN):$(CARGO_BIN):$(PATH)
 STAMPDIR := .stamp
 
 COLOR := "\e[1;36m%s\e[0m\n"
@@ -33,6 +35,10 @@ PROTO_PATHS = paths=source_relative:$(PROTO_OUT)
 OAPI_OUT := openapi
 OAPI3_PATH := .components.schemas.Payload
 
+SYSTEM_NEXUS_WIT := nexus/temporal-system.wit
+SYSTEM_NEXUS_SERVICE_PROTO_FILES := $(shell find temporal/api -name "service.proto" | sort)
+NEXUS_API_GEN ?= nexus-api-gen
+
 $(PROTO_OUT):
 	mkdir $(PROTO_OUT)
 
@@ -121,21 +127,22 @@ buf-breaking:
 	@printf $(COLOR) "Run buf breaking changes check against master branch..."	
 	@(cd $(PROTO_ROOT) && buf breaking --against 'https://github.com/temporalio/api.git#branch=master')
 
-nexus-rpc-yaml: nexus-rpc-yaml-install
-	printf $(COLOR) "Generate nexus/temporal-proto-models-nexusrpc.yaml..."
-	mkdir -p nexus
+##### Compile system Nexus WIT files #####
+system-nexus-wit: system-nexus-wit-install nexus-api-gen-install
+	printf $(COLOR) "Generate system Nexus WIT..."
 	protoc -I $(PROTO_ROOT) \
-		--nexus-rpc-yaml_opt=nexus-rpc_langs_out=nexus/temporal-proto-models-nexusrpc.yaml \
-		--nexus-rpc-yaml_opt=python_package_prefix=temporalio.api \
-		--nexus-rpc-yaml_opt=typescript_package_prefix=@temporalio/api \
-		--nexus-rpc-yaml_opt=include_operation_tags=exposed \
-		--nexus-rpc-yaml_out=. \
-		temporal/api/workflowservice/v1/* \
-		temporal/api/operatorservice/v1/*
-
-nexus-rpc-yaml-install:
-	printf $(COLOR) "Build and install protoc-gen-nexus-rpc-yaml..."
-	@cd cmd/protoc-gen-nexus-rpc-yaml && go install .
+		--system-nexus-wit_opt=output=$(SYSTEM_NEXUS_WIT) \
+		--system-nexus-wit_opt=nexus_api_gen=$(NEXUS_API_GEN) \
+		--system-nexus-wit_out=. \
+		$(SYSTEM_NEXUS_SERVICE_PROTO_FILES)
+
+system-nexus-wit-install:
+	printf $(COLOR) "Build and install protoc-gen-system-nexus-wit..."
+	@cd cmd/protoc-gen-system-nexus-wit && go install .
+
+nexus-api-gen-install:
+	printf $(COLOR) "Install nexus-api-gen if missing..."
+	command -v $(NEXUS_API_GEN) >/dev/null || CARGO_NET_GIT_FETCH_WITH_CLI=true cargo install --git https://github.com/temporalio/nexus-api-gen
 
 ##### Clean #####
 clean:
diff --git a/README.md b/README.md
index 788613ae8..b31d2f70d 100644
--- a/README.md
+++ b/README.md
@@ -9,6 +9,7 @@ Install as git submodule to the project.
 ## Contribution
 
 Make your change to the temporal/proto files, and run `make` to update the openapi definitions.
+Rust is also required because `make` installs and runs `nexus-api-gen` when regenerating system Nexus WIT files.
 
 ## Breaking changes
 
diff --git a/cmd/protoc-gen-nexus-rpc-yaml/generator.go b/cmd/protoc-gen-nexus-rpc-yaml/generator.go
deleted file mode 100644
index 2e4a939c8..000000000
--- a/cmd/protoc-gen-nexus-rpc-yaml/generator.go
+++ /dev/null
@@ -1,272 +0,0 @@
-package main
-
-import (
-	"fmt"
-	"slices"
-	"sort"
-	"strings"
-
-	nexusannotationsv1 "github.com/nexus-rpc/nexus-proto-annotations/go/nexusannotations/v1"
-	"google.golang.org/protobuf/compiler/protogen"
-	"google.golang.org/protobuf/proto"
-	"google.golang.org/protobuf/reflect/protoreflect"
-	"google.golang.org/protobuf/types/descriptorpb"
-	"gopkg.in/yaml.v3"
-)
-
-// params holds the parsed protoc plugin options.
-// Passed via --nexus-rpc-yaml_opt=key=value (multiple opts are comma-joined by protoc).
-//
-//   - nexus-rpc_langs_out: optional. Output path for the langs YAML.
-//     If empty, nothing is written.
-//     Example: "nexus/temporal-proto-models-nexusrpc.yaml"
-//
-//   - python_package_prefix: optional. Dot-separated package prefix for $pythonRef.
-//     The last two path segments of the go_package ({service}/v{n}) are appended.
-//     Example: "temporalio.api" → "temporalio.api.workflowservice.v1.TypeName"
-//     If empty, $pythonRef is omitted.
-//
-//   - typescript_package_prefix: optional. Scoped package prefix for $typescriptRef.
-//     The last two path segments of the go_package ({service}/v{n}) are appended.
-//     Example: "@temporalio/api" → "@temporalio/api/workflowservice/v1.TypeName"
-//     If empty, $typescriptRef is omitted.
-//
-//   - include_operation_tags: optional, repeatable. Only include operations whose tags
-//     contain at least one of these values. If empty, all annotated operations are included
-//     (subject to exclude_operation_tags). Specify multiple times for multiple tags.
-//     Example: include_operation_tags=exposed
-//
-//   - exclude_operation_tags: optional, repeatable. Exclude operations whose tags contain
-//     any of these values. Applied after include_operation_tags.
-//     Example: exclude_operation_tags=internal
-type params struct {
-	nexusRpcLangsOut        string
-	pythonPackagePrefix     string
-	typescriptPackagePrefix string
-	includeOperationTags    []string
-	excludeOperationTags    []string
-}
-
-// parseParams parses the comma-separated key=value parameter string provided by protoc.
-func parseParams(raw string) (params, error) {
-	var p params
-	if raw == "" {
-		return p, nil
-	}
-	for kv := range strings.SplitSeq(raw, ",") {
-		key, value, ok := strings.Cut(kv, "=")
-		if !ok {
-			return p, fmt.Errorf("invalid parameter %q: expected key=value", kv)
-		}
-		switch key {
-		case "nexus-rpc_langs_out":
-			p.nexusRpcLangsOut = value
-		case "python_package_prefix":
-			p.pythonPackagePrefix = value
-		case "typescript_package_prefix":
-			p.typescriptPackagePrefix = value
-		case "include_operation_tags":
-			p.includeOperationTags = append(p.includeOperationTags, value)
-		case "exclude_operation_tags":
-			p.excludeOperationTags = append(p.excludeOperationTags, value)
-		default:
-			return p, fmt.Errorf("unknown parameter %q", key)
-		}
-	}
-	return p, nil
-}
-
-// shouldIncludeOperation returns true if the method's nexus operation tags pass
-// the include/exclude filters. Mirrors the logic from protoc-gen-go-nexus:
-//  1. Method must have the nexus operation extension set.
-//  2. If includeOperationTags is non-empty, at least one of the method's tags must match.
-//  3. If excludeOperationTags is non-empty, none of the method's tags may match.
-func shouldIncludeOperation(p params, m *protogen.Method) bool {
-	opts, ok := m.Desc.Options().(*descriptorpb.MethodOptions)
-	if !ok || opts == nil {
-		return false
-	}
-	if !proto.HasExtension(opts, nexusannotationsv1.E_Operation) {
-		return false
-	}
-	tags := proto.GetExtension(opts, nexusannotationsv1.E_Operation).(*nexusannotationsv1.OperationOptions).GetTags()
-	if len(p.includeOperationTags) > 0 && !slices.ContainsFunc(p.includeOperationTags, func(t string) bool {
-		return slices.Contains(tags, t)
-	}) {
-		return false
-	}
-	return !slices.ContainsFunc(p.excludeOperationTags, func(t string) bool {
-		return slices.Contains(tags, t)
-	})
-}
-
-func generate(gen *protogen.Plugin) error {
-	p, err := parseParams(gen.Request.GetParameter())
-	if err != nil {
-		return err
-	}
-
-	langsDoc := newDoc()
-	hasOps := false
-
-	for _, f := range gen.Files {
-		if !f.Generate {
-			continue
-		}
-		for _, svc := range f.Services {
-			for _, m := range svc.Methods {
-				if !shouldIncludeOperation(p, m) {
-					continue
-				}
-				svcName := string(svc.Desc.Name())
-				methodName := string(m.Desc.Name())
-				hasOps = true
-				addOperation(langsDoc, svcName, methodName,
-					langRefs(p, f.Desc, m.Input.Desc),
-					langRefs(p, f.Desc, m.Output.Desc),
-				)
-			}
-		}
-	}
-
-	if !hasOps {
-		return nil
-	}
-	if p.nexusRpcLangsOut != "" {
-		return writeFile(gen, p.nexusRpcLangsOut, langsDoc)
-	}
-	return nil
-}
-
-// langRefs builds the map of language-specific type refs for a message.
-//
-// Go, Java, dotnet, and Ruby refs are derived from proto file-level package options.
-// Python and TypeScript refs require the corresponding prefix params to be set; if
-// empty they are omitted. Both use the last two path segments of go_package
-// ({service}/v{n}), dropping any intermediate grouping directory.
-func langRefs(p params, file protoreflect.FileDescriptor, msg protoreflect.MessageDescriptor) map[string]string {
-	opts, ok := file.Options().(*descriptorpb.FileOptions)
-	if !ok || opts == nil {
-		return nil
-	}
-	name := string(msg.Name())
-	refs := make(map[string]string)
-
-	if pkg := opts.GetGoPackage(); pkg != "" {
-		// strip the ";alias" suffix (e.g. "go.temporal.io/api/workflowservice/v1;workflowservice")
-		pkg = strings.SplitN(pkg, ";", 2)[0]
-		refs["$goRef"] = pkg + "." + name
-
-		segments := strings.Split(pkg, "/")
-		if len(segments) >= 2 {
-			tail := segments[len(segments)-2] + "/" + segments[len(segments)-1]
-			if p.pythonPackagePrefix != "" {
-				dotTail := strings.ReplaceAll(tail, "/", ".")
-				refs["$pythonRef"] = p.pythonPackagePrefix + "." + dotTail + "." + name
-			}
-			if p.typescriptPackagePrefix != "" {
-				refs["$typescriptRef"] = p.typescriptPackagePrefix + "/" + tail + "." + name
-			}
-		}
-	}
-	if pkg := opts.GetJavaPackage(); pkg != "" {
-		refs["$javaRef"] = pkg + "." + name
-	}
-	if pkg := opts.GetRubyPackage(); pkg != "" {
-		refs["$rubyRef"] = pkg + "::" + name
-	}
-	if pkg := opts.GetCsharpNamespace(); pkg != "" {
-		refs["$dotnetRef"] = pkg + "." + name
-	}
-	if len(refs) == 0 {
-		return nil
-	}
-	return refs
-}
-
-// newDoc creates a yaml.Node document with the "nexusrpc: 1.0.0" header
-// and an empty "services" mapping node.
-func newDoc() *yaml.Node {
-	doc := &yaml.Node{Kind: yaml.DocumentNode}
-	root := &yaml.Node{Kind: yaml.MappingNode, Tag: "!!map"}
-	doc.Content = []*yaml.Node{root}
-	root.Content = append(root.Content,
-		scalarNode("nexusrpc"),
-		scalarNode("1.0.0"),
-		scalarNode("services"),
-		&yaml.Node{Kind: yaml.MappingNode, Tag: "!!map"},
-	)
-	return doc
-}
-
-// servicesNode returns the "services" mapping node from a doc created by newDoc.
-func servicesNode(doc *yaml.Node) *yaml.Node {
-	root := doc.Content[0]
-	for i := 0; i < len(root.Content)-1; i += 2 {
-		if root.Content[i].Value == "services" {
-			return root.Content[i+1]
-		}
-	}
-	panic("services node not found")
-}
-
-// addOperation inserts a service → operation → {input, output} entry into doc.
-// Services and operations are inserted in the order first encountered.
-func addOperation(doc *yaml.Node, svcName, methodName string, input, output map[string]string) {
-	svcs := servicesNode(doc)
-
-	var svcOps *yaml.Node
-	for i := 0; i < len(svcs.Content)-1; i += 2 {
-		if svcs.Content[i].Value == svcName {
-			svcMap := svcs.Content[i+1]
-			for j := 0; j < len(svcMap.Content)-1; j += 2 {
-				if svcMap.Content[j].Value == "operations" {
-					svcOps = svcMap.Content[j+1]
-				}
-			}
-		}
-	}
-	if svcOps == nil {
-		svcMap := &yaml.Node{Kind: yaml.MappingNode, Tag: "!!map"}
-		svcOps = &yaml.Node{Kind: yaml.MappingNode, Tag: "!!map"}
-		svcMap.Content = append(svcMap.Content, scalarNode("operations"), svcOps)
-		svcs.Content = append(svcs.Content, scalarNode(svcName), svcMap)
-	}
-
-	opNode := &yaml.Node{Kind: yaml.MappingNode, Tag: "!!map"}
-	if len(input) > 0 {
-		opNode.Content = append(opNode.Content, scalarNode("input"), mapNode(input))
-	}
-	if len(output) > 0 {
-		opNode.Content = append(opNode.Content, scalarNode("output"), mapNode(output))
-	}
-	svcOps.Content = append(svcOps.Content, scalarNode(methodName), opNode)
-}
-
-// mapNode serializes a map[string]string as a yaml mapping node with keys in sorted order.
-func mapNode(m map[string]string) *yaml.Node {
-	keys := make([]string, 0, len(m))
-	for k := range m {
-		keys = append(keys, k)
-	}
-	sort.Strings(keys)
-	node := &yaml.Node{Kind: yaml.MappingNode, Tag: "!!map"}
-	for _, k := range keys {
-		node.Content = append(node.Content, scalarNode(k), scalarNode(m[k]))
-	}
-	return node
-}
-
-func scalarNode(value string) *yaml.Node {
-	return &yaml.Node{Kind: yaml.ScalarNode, Tag: "!!str", Value: value}
-}
-
-func writeFile(gen *protogen.Plugin, name string, doc *yaml.Node) error {
-	f := gen.NewGeneratedFile(name, "")
-	enc := yaml.NewEncoder(f)
-	enc.SetIndent(2)
-	if err := enc.Encode(doc); err != nil {
-		return err
-	}
-	return enc.Close()
-}
diff --git a/cmd/protoc-gen-nexus-rpc-yaml/go.mod b/cmd/protoc-gen-nexus-rpc-yaml/go.mod
deleted file mode 100644
index 863c771a6..000000000
--- a/cmd/protoc-gen-nexus-rpc-yaml/go.mod
+++ /dev/null
@@ -1,12 +0,0 @@
-module github.com/temporalio/api/cmd/protoc-gen-nexus-rpc-yaml
-
-go 1.25.4
-
-require (
-	google.golang.org/protobuf v1.36.1
-	gopkg.in/yaml.v3 v3.0.1
-)
-
-require github.com/nexus-rpc/nexus-proto-annotations v0.0.0-20260330194009-e558d6edaf84
-
-require github.com/google/go-cmp v0.6.0 // indirect
diff --git a/cmd/protoc-gen-nexus-rpc-yaml/go.sum b/cmd/protoc-gen-nexus-rpc-yaml/go.sum
deleted file mode 100644
index cbc5252ff..000000000
--- a/cmd/protoc-gen-nexus-rpc-yaml/go.sum
+++ /dev/null
@@ -1,10 +0,0 @@
-github.com/google/go-cmp v0.6.0 h1:ofyhxvXcZhMsU5ulbFiLKl/XBFqE1GSq7atu8tAmTRI=
-github.com/google/go-cmp v0.6.0/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=
-github.com/nexus-rpc/nexus-proto-annotations v0.0.0-20260330194009-e558d6edaf84 h1:SWHt3Coj0VvF0Km1A0wlY+IjnHKsjQLgO29io84r3wY=
-github.com/nexus-rpc/nexus-proto-annotations v0.0.0-20260330194009-e558d6edaf84/go.mod h1:n3UjF1bPCW8llR8tHvbxJ+27yPWrhpo8w/Yg1IOuY0Y=
-google.golang.org/protobuf v1.36.1 h1:yBPeRvTftaleIgM3PZ/WBIZ7XM/eEYAaEyCwvyjq/gk=
-google.golang.org/protobuf v1.36.1/go.mod h1:9fA7Ob0pmnwhb644+1+CVWFRbNajQ6iRojtC/QF5bRE=
-gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
-gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
-gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
-gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=
diff --git a/cmd/protoc-gen-nexus-rpc-yaml/main.go b/cmd/protoc-gen-nexus-rpc-yaml/main.go
deleted file mode 100644
index 31cdca92f..000000000
--- a/cmd/protoc-gen-nexus-rpc-yaml/main.go
+++ /dev/null
@@ -1,13 +0,0 @@
-// protoc-gen-nexus-rpc-yaml is a protoc plugin that generates nexus/temporal-proto-models-nexusrpc.yaml
-// from proto service methods annotated with option (nexusannotations.v1.operation).tags = "exposed".
-package main
-
-import (
-	"google.golang.org/protobuf/compiler/protogen"
-)
-
-func main() {
-	protogen.Options{}.Run(func(gen *protogen.Plugin) error {
-		return generate(gen)
-	})
-}
diff --git a/cmd/protoc-gen-system-nexus-wit/generator.go b/cmd/protoc-gen-system-nexus-wit/generator.go
new file mode 100644
index 000000000..31fdfbb43
--- /dev/null
+++ b/cmd/protoc-gen-system-nexus-wit/generator.go
@@ -0,0 +1,179 @@
+package main
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"slices"
+	"strings"
+
+	nexusannotationsv1 "github.com/nexus-rpc/nexus-proto-annotations/go/nexusannotations/v1"
+	"google.golang.org/protobuf/compiler/protogen"
+	"google.golang.org/protobuf/proto"
+	"google.golang.org/protobuf/types/descriptorpb"
+)
+
+type params struct {
+	nexusAPIGen string
+	output      string
+	input       string
+}
+
+// parseParams parses the comma-separated key=value parameter string provided by protoc.
+//
+//   - output: required. Path to the WIT file to generate, relative to the
+//     --system-nexus-wit_out directory. Example: "nexus/temporal-system.wit".
+//
+//   - nexus_api_gen: optional. Path to the nexus-api-gen binary. Defaults to
+//     "nexus-api-gen".
+//
+//   - input: optional. Existing WIT file to update. Defaults to output, so
+//     existing handwritten annotations and type refinements are preserved when
+//     regenerating in place.
+func parseParams(raw string) (params, error) {
+	p := params{
+		nexusAPIGen: "nexus-api-gen",
+	}
+	if raw == "" {
+		return p, nil
+	}
+	for kv := range strings.SplitSeq(raw, ",") {
+		key, value, ok := strings.Cut(kv, "=")
+		if !ok {
+			return p, fmt.Errorf("invalid parameter %q: expected key=value", kv)
+		}
+		switch key {
+		case "nexus_api_gen":
+			p.nexusAPIGen = value
+		case "output":
+			p.output = value
+		case "input":
+			p.input = value
+		default:
+			return p, fmt.Errorf("unknown parameter %q", key)
+		}
+	}
+	return p, nil
+}
+
+func generate(gen *protogen.Plugin) error {
+	p, err := parseParams(gen.Request.GetParameter())
+	if err != nil {
+		return err
+	}
+	if p.output == "" {
+		return fmt.Errorf("missing required output parameter")
+	}
+	if p.input == "" {
+		p.input = p.output
+	}
+
+	rpcs := exposedRPCs(gen)
+	if len(rpcs) == 0 {
+		return fmt.Errorf("no proto RPCs are marked as exposed Nexus operations")
+	}
+
+	tempDir, err := os.MkdirTemp("", "system-nexus-wit-*")
+	if err != nil {
+		return err
+	}
+	defer os.RemoveAll(tempDir)
+
+	descriptorPath := filepath.Join(tempDir, "temporal_api.bin")
+	if err := writeDescriptorSet(gen, descriptorPath); err != nil {
+		return err
+	}
+
+	tempOutput := filepath.Join(tempDir, "system-nexus.wit")
+	input := ""
+	if _, err := os.Stat(p.input); err == nil {
+		if err := copyFile(p.input, tempOutput); err != nil {
+			return err
+		}
+		input = tempOutput
+	} else if !os.IsNotExist(err) {
+		return err
+	}
+
+	for _, rpc := range rpcs {
+		if err := runAddRPC(p.nexusAPIGen, descriptorPath, rpc, tempOutput, input); err != nil {
+			return err
+		}
+		input = tempOutput
+	}
+
+	content, err := os.ReadFile(tempOutput)
+	if err != nil {
+		return err
+	}
+	_, err = gen.NewGeneratedFile(p.output, "").Write(content)
+	return err
+}
+
+func exposedRPCs(gen *protogen.Plugin) []string {
+	var rpcs []string
+	for _, f := range gen.Files {
+		if !f.Generate {
+			continue
+		}
+		for _, svc := range f.Services {
+			for _, m := range svc.Methods {
+				if isExposedOperation(m) {
+					rpcs = append(rpcs, string(m.Desc.FullName()))
+				}
+			}
+		}
+	}
+	return rpcs
+}
+
+func isExposedOperation(m *protogen.Method) bool {
+	opts, ok := m.Desc.Options().(*descriptorpb.MethodOptions)
+	if !ok || opts == nil {
+		return false
+	}
+	if !proto.HasExtension(opts, nexusannotationsv1.E_Operation) {
+		return false
+	}
+	tags := proto.GetExtension(opts, nexusannotationsv1.E_Operation).(*nexusannotationsv1.OperationOptions).GetTags()
+	return slices.Contains(tags, "exposed")
+}
+
+func writeDescriptorSet(gen *protogen.Plugin, descriptorPath string) error {
+	data, err := proto.Marshal(&descriptorpb.FileDescriptorSet{
+		File: gen.Request.GetProtoFile(),
+	})
+	if err != nil {
+		return err
+	}
+	return os.WriteFile(descriptorPath, data, 0o644)
+}
+
+func runAddRPC(nexusAPIGen string, descriptors string, rpc string, output string, input string) error {
+	args := []string{
+		"add-rpc",
+		"--descriptors", descriptors,
+		"--rpc", rpc,
+		"--output", output,
+	}
+	if input != "" {
+		args = append(args, "--input", input)
+	}
+
+	command := exec.Command(nexusAPIGen, args...)
+	command.Stdout = os.Stdout
+	command.Stderr = os.Stderr
+	if err := command.Run(); err != nil {
+		return fmt.Errorf("%s %s: %w", nexusAPIGen, strings.Join(args, " "), err)
+	}
+	return nil
+}
+
+func copyFile(source string, destination string) error {
+	content, err := os.ReadFile(source)
+	if err != nil {
+		return err
+	}
+	return os.WriteFile(destination, content, 0o644)
+}
diff --git a/cmd/protoc-gen-system-nexus-wit/go.mod b/cmd/protoc-gen-system-nexus-wit/go.mod
new file mode 100644
index 000000000..222e32187
--- /dev/null
+++ b/cmd/protoc-gen-system-nexus-wit/go.mod
@@ -0,0 +1,8 @@
+module github.com/temporalio/api/cmd/protoc-gen-system-nexus-wit
+
+go 1.25.4
+
+require (
+	github.com/nexus-rpc/nexus-proto-annotations v0.0.0-20260330194009-e558d6edaf84
+	google.golang.org/protobuf v1.36.1
+)
diff --git a/cmd/protoc-gen-system-nexus-wit/go.sum b/cmd/protoc-gen-system-nexus-wit/go.sum
new file mode 100644
index 000000000..c96f6a1c5
--- /dev/null
+++ b/cmd/protoc-gen-system-nexus-wit/go.sum
@@ -0,0 +1,8 @@
+github.com/google/go-cmp v0.5.5 h1:Khx7svrCpmxxtHBq5j2mp/xVjsi8hQMfNLvJFAlrGgU=
+github.com/google/go-cmp v0.5.5/go.mod h1:v8dTdLbMG2kIc/vJvl+f65V22dbkXbowE6jgT/gNBxE=
+github.com/nexus-rpc/nexus-proto-annotations v0.0.0-20260330194009-e558d6edaf84 h1:SWHt3Coj0VvF0Km1A0wlY+IjnHKsjQLgO29io84r3wY=
+github.com/nexus-rpc/nexus-proto-annotations v0.0.0-20260330194009-e558d6edaf84/go.mod h1:n3UjF1bPCW8llR8tHvbxJ+27yPWrhpo8w/Yg1IOuY0Y=
+golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543 h1:E7g+9GITq07hpfrRu66IVDexMakfv52eLZ2CXBWiKr4=
+golang.org/x/xerrors v0.0.0-20191204190536-9bdfabe68543/go.mod h1:I/5z698sn9Ka8TeJc9MKroUUfqBBauWjQqLJ2OPfmY0=
+google.golang.org/protobuf v1.36.1 h1:yBPeRvTftaleIgM3PZ/WBIZ7XM/eEYAaEyCwvyjq/gk=
+google.golang.org/protobuf v1.36.1/go.mod h1:9fA7Ob0pmnwhb644+1+CVWFRbNajQ6iRojtC/QF5bRE=
diff --git a/cmd/protoc-gen-system-nexus-wit/main.go b/cmd/protoc-gen-system-nexus-wit/main.go
new file mode 100644
index 000000000..0e9fe3182
--- /dev/null
+++ b/cmd/protoc-gen-system-nexus-wit/main.go
@@ -0,0 +1,11 @@
+// protoc-gen-system-nexus-wit generates nexus/temporal-system.wit from proto service
+// methods annotated with option (nexusannotations.v1.operation).tags = "exposed".
+package main
+
+import "google.golang.org/protobuf/compiler/protogen"
+
+func main() {
+	protogen.Options{}.Run(func(gen *protogen.Plugin) error {
+		return generate(gen)
+	})
+}
diff --git a/nexus/temporal-proto-models-nexusrpc.yaml b/nexus/temporal-proto-models-nexusrpc.yaml
deleted file mode 100644
index e0761fd15..000000000
--- a/nexus/temporal-proto-models-nexusrpc.yaml
+++ /dev/null
@@ -1,19 +0,0 @@
-nexusrpc: 1.0.0
-services:
-  WorkflowService:
-    operations:
-      SignalWithStartWorkflowExecution:
-        input:
-          $dotnetRef: Temporalio.Api.WorkflowService.V1.SignalWithStartWorkflowExecutionRequest
-          $goRef: go.temporal.io/api/workflowservice/v1.SignalWithStartWorkflowExecutionRequest
-          $javaRef: io.temporal.api.workflowservice.v1.SignalWithStartWorkflowExecutionRequest
-          $pythonRef: temporalio.api.workflowservice.v1.SignalWithStartWorkflowExecutionRequest
-          $rubyRef: Temporalio::Api::WorkflowService::V1::SignalWithStartWorkflowExecutionRequest
-          $typescriptRef: '@temporalio/api/workflowservice/v1.SignalWithStartWorkflowExecutionRequest'
-        output:
-          $dotnetRef: Temporalio.Api.WorkflowService.V1.SignalWithStartWorkflowExecutionResponse
-          $goRef: go.temporal.io/api/workflowservice/v1.SignalWithStartWorkflowExecutionResponse
-          $javaRef: io.temporal.api.workflowservice.v1.SignalWithStartWorkflowExecutionResponse
-          $pythonRef: temporalio.api.workflowservice.v1.SignalWithStartWorkflowExecutionResponse
-          $rubyRef: Temporalio::Api::WorkflowService::V1::SignalWithStartWorkflowExecutionResponse
-          $typescriptRef: '@temporalio/api/workflowservice/v1.SignalWithStartWorkflowExecutionResponse'
diff --git a/nexus/temporal-system.wit b/nexus/temporal-system.wit
new file mode 100644
index 000000000..e410c64d7
--- /dev/null
+++ b/nexus/temporal-system.wit
@@ -0,0 +1,78 @@
+package temporal:nexus@1.0.0;
+
+world system {
+  export workflow-service;
+}
+
+/// @nexus.endpoint "__temporal_system"
+interface workflow-service {
+  use nexus:temporal-types/model@1.0.0.{
+    duration,
+    memo,
+    payloads,
+    placeholder,
+    priority,
+    retry-policy,
+    search-attributes,
+    signal-function,
+    task-queue,
+    user-metadata,
+    versioning-override,
+    workflow-function,
+    workflow-id-conflict-policy,
+    workflow-id-reuse-policy,
+  };
+
+  /// @nexus.proto "temporal.api.workflowservice.v1.SignalWithStartWorkflowExecutionRequest"
+  record signal-with-start-workflow-request {
+    /// @nexus.proto-field "workflow_type"
+    workflow: workflow-function,
+    workflow-id: string,
+    task-queue: task-queue,
+    /// @nexus.proto-field "signal_name"
+    signal: signal-function,
+    workflow-execution-timeout: option<duration>,
+    workflow-run-timeout: option<duration>,
+    workflow-task-timeout: option<duration>,
+    identity: option<string>,
+    request-id: option<string>,
+    workflow-id-reuse-policy: option<workflow-id-reuse-policy>,
+    workflow-id-conflict-policy: option<workflow-id-conflict-policy>,
+    retry-policy: option<retry-policy>,
+    cron-schedule: option<string>,
+    memo: option<memo>,
+    search-attributes: option<search-attributes>,
+    priority: option<priority>,
+    versioning-override: option<versioning-override>,
+    workflow-start-delay: option<duration>,
+    user-metadata: option<user-metadata>,
+    /// @nexus.source python="workflow_namespace" typescript="workflowNamespace"
+    namespace: string,
+    /// @nexus.omit
+    control: placeholder,
+    /// @nexus.omit
+    header: placeholder,
+    /// @nexus.omit
+    links: placeholder,
+    /// @nexus.omit
+    time-skipping-config: placeholder,
+  }
+
+  /// @nexus.proto "temporal.api.workflowservice.v1.SignalWithStartWorkflowExecutionResponse"
+  record signal-with-start-workflow-response {
+    run-id: option<string>,
+    started: option<bool>,
+    /// @nexus.omit
+    signal-link: placeholder,
+  }
+
+  /// @nexus.output-transform
+  ///   python-type="workflow.ExternalWorkflowHandle[typing.Any]"
+  ///   python="workflow.get_external_workflow_handle(request.workflow_id, run_id=result.run_id)"
+  ///   typescript-type="workflow.ExternalWorkflowHandle"
+  ///   typescript="workflow.getExternalWorkflowHandle(request.workflowId, result.runId ?? undefined)"
+  /// @nexus.operation name="SignalWithStartWorkflowExecution"
+  signal-with-start-workflow: func(
+    request: signal-with-start-workflow-request,
+  ) -> signal-with-start-workflow-response;
+}