diff --git a/src/main/java/com/google/devtools/build/lib/actions/SpawnResult.java b/src/main/java/com/google/devtools/build/lib/actions/SpawnResult.java
index 9b97352..8d09f96 100644
--- a/src/main/java/com/google/devtools/build/lib/actions/SpawnResult.java
+++ b/src/main/java/com/google/devtools/build/lib/actions/SpawnResult.java
@@ -26,17 +26,10 @@
 import java.util.Optional;
 import javax.annotation.Nullable;
 
-/**
- * The result of a spawn execution.
- *
- * <p>DO NOT IMPLEMENT THIS INTERFACE! Use {@link SpawnResult.Builder} to create instances instead.
- * This is a temporary situation as long as we still have separate internal and external
- * implementations - the plan is to merge the two into a single immutable, final class.
- */
-// TODO(ulfjack): Change this from an interface to an immutable, final class.
+/** The result of a {@link Spawn}'s execution. */
 public interface SpawnResult {
   /** The status of the attempted Spawn execution. */
-  public enum Status {
+  enum Status {
     /** Subprocess executed successfully, and returned a zero exit code. */
     SUCCESS,
 
@@ -53,26 +46,27 @@
     OUT_OF_MEMORY(true),
 
     /**
-     * Subprocess did not execute. This error is not catastrophic - Bazel will try to continue the
-     * build if keep_going is enabled, possibly attempt to rerun the same spawn, and attempt to run
-     * other actions.
+     * Subprocess did not execute, it's not the user's fault, and the error is not catastrophic. If
+     * keep_going is enabled then Bazel will try to continue the build, possibly will attempt to
+     * rerun the same spawn, and possibly will attempt to run other actions.
      */
     EXECUTION_FAILED,
 
     /**
-     * Subprocess did not execute. Do not retry, and exit immediately even if keep_going is enabled.
+     * Subprocess did not execute, it's not the user's fault, and the error is catastrophic. Bazel
+     * will not rerun this spawn. Bazel will attempt to not run other actions (regardless of whether
+     * keep_going is enabled).
      */
     EXECUTION_FAILED_CATASTROPHICALLY,
 
     /**
-     * Subprocess did not execute in a way that indicates that the user can fix it. For example, a
-     * remote system may have denied the execution due to too many inputs or too large inputs.
+     * Subprocess did not execute, and it may be the user's fault. The user may be able to fix it.
+     * For example, a remote system may have denied the execution due to too many inputs or too
+     * large inputs.
      */
     EXECUTION_DENIED(true),
 
-    /**
-     * The remote execution system is overloaded and had to refuse execution for this Spawn.
-     */
+    /** The remote execution system is overloaded and had to refuse execution for this Spawn. */
     REMOTE_EXECUTOR_OVERLOADED,
 
     /**
@@ -83,11 +77,11 @@
 
     private final boolean isUserError;
 
-    private Status(boolean isUserError) {
+    Status(boolean isUserError) {
       this.isUserError = isUserError;
     }
 
-    private Status() {
+    Status() {
       this(false);
     }
 
@@ -109,31 +103,33 @@
   /** Returns true if the status was {@link Status#EXECUTION_FAILED_CATASTROPHICALLY}. */
   boolean isCatastrophe();
 
-  /** The status of the attempted Spawn execution. */
+  /** Returns the status of the attempted Spawn execution. */
   Status status();
 
   /**
-   * The exit code of the subprocess if the subprocess was executed. Should only be called if
-   * {@link #status} returns {@link Status#SUCCESS} or {@link Status#NON_ZERO_EXIT}.
+   * Returns the exit code of the subprocess if the subprocess was executed.
    *
-   * <p>This method must return a non-zero exit code if the status is {@link Status#TIMEOUT} or
-   * {@link Status#OUT_OF_MEMORY}. It is recommended to return 128 + 14 when the status is
-   * {@link Status#TIMEOUT}, which corresponds to the Unix signal SIGALRM.
+   * <p>Returns zero if {@link #status} returns {@link Status#SUCCESS}.
    *
-   * <p>This method may throw {@link IllegalStateException} if called for any other status.
+   * <p>Returns non-zero if {@link #status} returns {@link Status#NON_ZERO_EXIT} or {@link
+   * Status#OUT_OF_MEMORY}.
+   *
+   * <p>Returns 128 + 14 (corresponding to the Unix signal SIGALRM) if {@link #status} returns
+   * {@link Status#TIMEOUT}.
    */
   int exitCode();
 
   /**
-   * The host name of the executor or {@code null}. This information is intended for debugging
-   * purposes, especially for remote execution systems. Remote caches usually do not store the
-   * original host name, so this is generally {@code null} for cache hits.
+   * Returns the host name of the executor or {@code null}. This information is intended for
+   * debugging purposes, especially for remote execution systems. Remote caches usually do not store
+   * the original host name, so this is generally {@code null} for cache hits.
    */
-  @Nullable String getExecutorHostName();
+  @Nullable
+  String getExecutorHostName();
 
   /**
-   * The name of the SpawnRunner that executed the spawn. It should always be defined, unless
-   * isCacheHit is true, in which case the spawn was not actually run.
+   * Returns the name of the SpawnRunner that executed the spawn. It should always be defined,
+   * unless isCacheHit is true, in which case the spawn was not actually run.
    */
   String getRunnerName();
 
@@ -187,7 +183,7 @@
 
   SpawnMetrics getMetrics();
 
-  /** Whether the spawn result was a cache hit. */
+  /** Returns whether the spawn result was a cache hit. */
   boolean isCacheHit();
 
   /** Returns an optional custom failure message for the result. */
@@ -196,9 +192,10 @@
   }
 
   /**
-   * SpawnResults can optionally support returning outputs in-memory. Such outputs can be obtained
-   * from this method if so. This behavior is optional, and can be triggered with
-   * {@link ExecutionRequirements#REMOTE_EXECUTION_INLINE_OUTPUTS}.
+   * Returns a {@link Spawn}'s output in-memory, if supported and available.
+   *
+   * <p>This behavior may be triggered with {@link
+   * ExecutionRequirements#REMOTE_EXECUTION_INLINE_OUTPUTS}.
    */
   @Nullable
   default InputStream getInMemoryOutput(ActionInput output) {
@@ -215,11 +212,10 @@
   /** Returns a file path to the action metadata log. */
   Optional<MetadataLog> getActionMetadataLog();
 
-  /**
-   * Basic implementation of {@link SpawnResult}.
-   */
-  @Immutable @ThreadSafe
-  public static final class SimpleSpawnResult implements SpawnResult {
+  /** Basic implementation of {@link SpawnResult}. */
+  @Immutable
+  @ThreadSafe
+  final class SimpleSpawnResult implements SpawnResult {
     private final int exitCode;
     private final Status status;
     private final String executorHostName;
@@ -231,11 +227,13 @@
     private final Optional<Long> numBlockOutputOperations;
     private final Optional<Long> numBlockInputOperations;
     private final Optional<Long> numInvoluntaryContextSwitches;
+    private final Optional<MetadataLog> actionMetadataLog;
     private final boolean cacheHit;
     private final String failureMessage;
-    private final ActionInput inMemoryOutputFile;
-    private final ByteString inMemoryContents;
-    private final Optional<MetadataLog> actionMetadataLog;
+
+    // Invariant: Either both have a value or both are null.
+    @Nullable private final ActionInput inMemoryOutputFile;
+    @Nullable private final ByteString inMemoryContents;
 
     SimpleSpawnResult(Builder builder) {
       this.exitCode = builder.exitCode;
@@ -393,10 +391,8 @@
     }
   }
 
-  /**
-   * Builder class for {@link SpawnResult}.
-   */
-  public static final class Builder {
+  /** Builder class for {@link SpawnResult}. */
+  final class Builder {
     private int exitCode;
     private Status status;
     private String executorHostName;
@@ -411,9 +407,10 @@
     private Optional<MetadataLog> actionMetadataLog = Optional.empty();
     private boolean cacheHit;
     private String failureMessage = "";
-    /* Invariant: Either both have a value or both are null. */
-    private ActionInput inMemoryOutputFile;
-    private ByteString inMemoryContents;
+
+    // Invariant: Either both have a value or both are null.
+    @Nullable private ActionInput inMemoryOutputFile;
+    @Nullable private ByteString inMemoryContents;
 
     public SpawnResult build() {
       Preconditions.checkArgument(!runnerName.isEmpty());
@@ -499,14 +496,14 @@
       return this;
     }
 
-    public Builder setActionMetadataLog(MetadataLog actionMetadataLog) {
+    Builder setActionMetadataLog(MetadataLog actionMetadataLog) {
       this.actionMetadataLog = Optional.of(actionMetadataLog);
       return this;
     }
   }
 
-  /** A tuple containing the name reference to the metadata and the file path to the Metadata */
-  public static final class MetadataLog {
+  /** A {@link Spawn}'s metadata name and {@link Path}. */
+  final class MetadataLog {
     private final String name;
     private final Path filePath;