diff --git a/nexus-sdk/src/main/java/io/nexusrpc/client/CompleteOperationOptions.java b/nexus-sdk/src/main/java/io/nexusrpc/client/CompleteOperationOptions.java
new file mode 100644
index 0000000..92a2b9c
--- /dev/null
+++ b/nexus-sdk/src/main/java/io/nexusrpc/client/CompleteOperationOptions.java
@@ -0,0 +1,61 @@
+package io.nexusrpc.client;
+
+import io.nexusrpc.Link;
+import java.time.Instant;
+import java.util.List;
+import java.util.Map;
+
+/** CompleteOperationOptions are options for the CompleteOperation client and server APIs. */
+public class CompleteOperationOptions {
+  public static Builder newBuilder() {
+    return new Builder();
+  }
+
+  private final Map<String, String> headers;
+  private final Instant startTime;
+  private final List<Link> links;
+
+  private CompleteOperationOptions(
+      Map<String, String> headers, Instant startTime, List<Link> links) {
+    this.headers = headers;
+    this.startTime = startTime;
+    this.links = links;
+  }
+
+  public List<Link> getLinks() {
+    return links;
+  }
+
+  public Instant getStartTime() {
+    return startTime;
+  }
+
+  public Map<String, String> getHeaders() {
+    return headers;
+  }
+
+  public static class Builder {
+    private Map<String, String> headers;
+    private Instant startTime;
+    private List<Link> links;
+
+    public Builder setHeaders(Map<String, String> headers) {
+      this.headers = headers;
+      return this;
+    }
+
+    public Builder setStartTime(Instant startTime) {
+      this.startTime = startTime;
+      return this;
+    }
+
+    public Builder setLinks(List<Link> links) {
+      this.links = links;
+      return this;
+    }
+
+    public CompleteOperationOptions build() {
+      return new CompleteOperationOptions(headers, startTime, links);
+    }
+  }
+}
diff --git a/nexus-sdk/src/main/java/io/nexusrpc/client/CompletionClient.java b/nexus-sdk/src/main/java/io/nexusrpc/client/CompletionClient.java
new file mode 100644
index 0000000..6db3945
--- /dev/null
+++ b/nexus-sdk/src/main/java/io/nexusrpc/client/CompletionClient.java
@@ -0,0 +1,70 @@
+package io.nexusrpc.client;
+
+import io.nexusrpc.OperationException;
+import io.nexusrpc.client.transport.Transport;
+
+/** CompletionClient is used to complete or fail async operations. */
+public class CompletionClient {
+  private final Transport transport;
+
+  // TODO take CompleteOperationOptions as a parameter?
+  public CompletionClient(Transport transport) {
+    this.transport = transport;
+  }
+
+  /**
+   * Complete an operation identified with the given token and result.
+   *
+   * @param token the operation token
+   * @param result the result of the operation
+   */
+  void completeOperation(String token, Object result) {
+    completeOperation(token, result, CompleteOperationOptions.newBuilder().build());
+  }
+
+  /**
+   * Complete an operation identified with the given token and result.
+   *
+   * @param token the operation token
+   * @param result the result of the operation
+   * @param options additional options for completing the operation
+   */
+  void completeOperation(String token, Object result, CompleteOperationOptions options) {
+    transport.completeOperation(
+        token,
+        io.nexusrpc.client.transport.CompleteOperationOptions.newBuilder()
+            .setResult(result)
+            .setOperationToken(token)
+            .setLinks(options.getLinks())
+            .setStartTime(options.getStartTime())
+            .build());
+  }
+
+  /**
+   * Fail an operation identified with the given token and result.
+   *
+   * @param token the operation token
+   * @param error the error that caused the operation to fail
+   */
+  void failOperation(String token, OperationException error) {
+    failOperation(token, error, CompleteOperationOptions.newBuilder().build());
+  }
+
+  /**
+   * Fail an operation identified with the given token and result.
+   *
+   * @param token the operation token
+   * @param error the error that caused the operation to fail
+   * @param options additional options for failing the operation
+   */
+  void failOperation(String token, OperationException error, CompleteOperationOptions options) {
+    transport.completeOperation(
+        token,
+        io.nexusrpc.client.transport.CompleteOperationOptions.newBuilder()
+            .setError(error)
+            .setOperationToken(token)
+            .setLinks(options.getLinks())
+            .setStartTime(options.getStartTime())
+            .build());
+  }
+}
diff --git a/nexus-sdk/src/main/java/io/nexusrpc/client/transport/CompleteOperationOptions.java b/nexus-sdk/src/main/java/io/nexusrpc/client/transport/CompleteOperationOptions.java
new file mode 100644
index 0000000..b462c73
--- /dev/null
+++ b/nexus-sdk/src/main/java/io/nexusrpc/client/transport/CompleteOperationOptions.java
@@ -0,0 +1,104 @@
+package io.nexusrpc.client.transport;
+
+import io.nexusrpc.Experimental;
+import io.nexusrpc.Link;
+import io.nexusrpc.OperationException;
+import java.time.Instant;
+import java.util.List;
+import java.util.Map;
+
+@Experimental
+public class CompleteOperationOptions {
+  public static Builder newBuilder() {
+    return new Builder();
+  }
+
+  private final Map<String, String> headers;
+  private final Object result;
+  private final OperationException error;
+  private final String operationToken;
+  private final Instant startTime;
+  private final List<Link> links;
+
+  private CompleteOperationOptions(
+      Map<String, String> headers,
+      Object result,
+      OperationException error,
+      String operationToken,
+      Instant startTime,
+      List<Link> links) {
+    this.headers = headers;
+    this.result = result;
+    this.error = error;
+    this.operationToken = operationToken;
+    this.startTime = startTime;
+    this.links = links;
+  }
+
+  public String getOperationToken() {
+    return operationToken;
+  }
+
+  public List<Link> getLinks() {
+    return links;
+  }
+
+  public Instant getStartTime() {
+    return startTime;
+  }
+
+  public OperationException getError() {
+    return error;
+  }
+
+  public Object getResult() {
+    return result;
+  }
+
+  public Map<String, String> getHeaders() {
+    return headers;
+  }
+
+  public static class Builder {
+    private Map<String, String> headers;
+    private Object result;
+    private OperationException error;
+    private String operationToken;
+    private Instant startTime;
+    private List<Link> links;
+
+    public Builder setHeaders(Map<String, String> headers) {
+      this.headers = headers;
+      return this;
+    }
+
+    public Builder setResult(Object result) {
+      this.result = result;
+      return this;
+    }
+
+    public Builder setError(OperationException error) {
+      this.error = error;
+      return this;
+    }
+
+    public Builder setOperationToken(String operationToken) {
+      this.operationToken = operationToken;
+      return this;
+    }
+
+    public Builder setStartTime(Instant startTime) {
+      this.startTime = startTime;
+      return this;
+    }
+
+    public Builder setLinks(List<Link> links) {
+      this.links = links;
+      return this;
+    }
+
+    public CompleteOperationOptions build() {
+      return new CompleteOperationOptions(headers, result, error, operationToken, startTime, links);
+    }
+  }
+}
diff --git a/nexus-sdk/src/main/java/io/nexusrpc/client/transport/CompleteOperationResponse.java b/nexus-sdk/src/main/java/io/nexusrpc/client/transport/CompleteOperationResponse.java
new file mode 100644
index 0000000..a1acfd5
--- /dev/null
+++ b/nexus-sdk/src/main/java/io/nexusrpc/client/transport/CompleteOperationResponse.java
@@ -0,0 +1,6 @@
+package io.nexusrpc.client.transport;
+
+import io.nexusrpc.Experimental;
+
+@Experimental
+public class CompleteOperationResponse {}
diff --git a/nexus-sdk/src/main/java/io/nexusrpc/client/transport/Transport.java b/nexus-sdk/src/main/java/io/nexusrpc/client/transport/Transport.java
index 1f245a6..19b6094 100644
--- a/nexus-sdk/src/main/java/io/nexusrpc/client/transport/Transport.java
+++ b/nexus-sdk/src/main/java/io/nexusrpc/client/transport/Transport.java
@@ -1,16 +1,23 @@
 package io.nexusrpc.client.transport;
 
+import io.nexusrpc.Experimental;
+import io.nexusrpc.OperationException;
+import io.nexusrpc.OperationStillRunningException;
 import java.util.concurrent.CompletableFuture;
 
+/** Transport interface for Nexus RPC operations. */
+@Experimental
 public interface Transport {
   StartOperationResponse startOperation(
-      String operationName, String serviceName, StartOperationOptions options);
+      String operationName, String serviceName, Object input, StartOperationOptions options)
+      throws OperationException;
 
   GetOperationResultResponse getOperationResult(
       String operationName,
       String serviceName,
       String operationToken,
-      GetOperationResultOptions options);
+      GetOperationResultOptions options)
+      throws OperationException, OperationStillRunningException;
 
   GetOperationInfoResponse getOperationInfo(
       String operationName,
@@ -24,6 +31,9 @@ CancelOperationResponse cancelOperation(
       String operationToken,
       CancelOperationOptions options);
 
+  CompleteOperationResponse completeOperation(
+      String operationToken, CompleteOperationOptions options);
+
   CompletableFuture<StartOperationResponse> startOperationAsync(
       String operationName,
       String serviceName,
@@ -47,4 +57,7 @@ CompletableFuture<CancelOperationResponse> cancelOperationAsync(
       String serviceName,
       String operationToken,
       CancelOperationOptions options);
+
+  CompletableFuture<CompleteOperationResponse> completeOperationAsync(
+      String operationToken, CompleteOperationOptions options);
 }