How to Make a Screeps Spawn Create a New Creep

What you will complete in this lesson

This lesson has one goal: make one owned Spawn begin creating a new Creep named Worker1.

You will use the three-part learning body from the previous lesson:

[WORK, CARRY, MOVE]

The request costs 200 Energy. You will first validate it with dryRun, then submit the real request only when the validation result is OK.

Lesson boundary: this lesson creates one fixed-name Creep. Dynamic names, population targets, role Memory, replacement timing, and spawn queues belong in later guides.

A spawnCreep() request needs three decisions

Before copying the code, run Object.keys(Game.spawns) in the Console. Replace Spawn1 with the exact case-sensitive name returned by your game.

Review WORK, CARRY, and MOVE before changing the body array.

Validate the request with dryRun

State impact: validation only. A dryRun request checks whether spawning is currently possible without starting the process or spending Energy.

const SPAWN_NAME = 'Spawn1';
const CREEP_NAME = 'Worker1';
const BODY = [WORK, CARRY, MOVE];

const spawn = Game.spawns[SPAWN_NAME];

if (!spawn) {
  console.log(
    SPAWN_NAME +
    ' was not found. Check the name and capitalization.'
  );
} else {
  const bodyCost = BODY.reduce(function (total, part) {
    return total + BODYPART_COST[part];
  }, 0);

  const dryRunResult = spawn.spawnCreep(
    BODY,
    CREEP_NAME,
    { dryRun: true }
  );

  console.log(JSON.stringify({
    spawnName: spawn.name,
    spawnBusy: Boolean(spawn.spawning),
    nameAlreadyExists: Boolean(Game.creeps[CREEP_NAME]),
    roomEnergyAvailable: spawn.room.energyAvailable,
    bodyCost: bodyCost,
    dryRunResult: dryRunResult
  }, null, 2));
}

Do not continue to the real call until the Spawn name is correct and dryRunResult is OK.

Start one real spawning process

State impact: this code can consume room Energy and start a real spawn process. Replace Spawn1 before saving it.

const SPAWN_NAME = 'Spawn1';
const CREEP_NAME = 'Worker1';
const BODY = [WORK, CARRY, MOVE];

module.exports.loop = function () {
  const spawn = Game.spawns[SPAWN_NAME];

  if (!spawn) {
    console.log(
      SPAWN_NAME +
      ' was not found. Check the name and capitalization.'
    );
    return;
  }

  if (Game.creeps[CREEP_NAME] || spawn.spawning) {
    return;
  }

  const dryRunResult = spawn.spawnCreep(
    BODY,
    CREEP_NAME,
    { dryRun: true }
  );

  if (dryRunResult !== OK) {
    console.log(
      'Cannot start ' +
      CREEP_NAME +
      ': spawnCreep() returned ' +
      dryRunResult +
      '.'
    );
    return;
  }

  const spawnResult = spawn.spawnCreep(
    BODY,
    CREEP_NAME
  );

  console.log(
    spawnResult === OK
      ? CREEP_NAME + ' started spawning.'
      : 'Real spawn request returned ' + spawnResult + '.'
  );
};

The two guards matter because module.exports.loop runs again on later ticks:

• Game.creeps[CREEP_NAME] prevents another request after the Creep exists.
• spawn.spawning prevents another request while the Spawn is already working.

Read the return code before changing the body

OK means the spawn operation was scheduled successfully. It does not mean the completed Creep already exists.

Use the focused spawnCreep() return-code guide when a different result needs deeper diagnosis.

Watch the Spawn and the finished Creep on later ticks

The official constant CREEP_SPAWN_TIME is 3 ticks per body part. The three-part tutorial body therefore needs 9 ticks to finish under the current constant.

During the process, inspect:

const spawn = Game.spawns['Spawn1'];

console.log(JSON.stringify({
  tick: Game.time,
  spawning: spawn ? spawn.spawning : null,
  workerExists: Boolean(Game.creeps['Worker1'])
}, null, 2));

Expected sequence:

1. The real call returns OK.
2. spawn.spawning describes the active process.
3. The remaining time decreases across later ticks.
4. After completion, Game.creeps['Worker1'] exists.
5. The finished Creep contains WORK, CARRY, and MOVE.

Fix the request, not an unrelated system

The Spawn object is undefined

The name is wrong or belongs to a different shard. Run Object.keys(Game.spawns) and copy the exact value.

The room never reaches 200 Energy

The body is valid, but the room economy is not refilling the Spawn or Extensions. Return to the Energy delivery lesson.

The Console prints the same failure every tick

This is temporary tutorial code. Use the return code to fix the request, then remove repeated diagnostic logging after the first successful spawn.

The real call returned OK, but Worker1 is still missing

Check spawn.spawning. A scheduled process still needs later ticks before the new Creep appears.

Completion check

You have completed this lesson when all of the following are true:

• the code uses your real Spawn name;
• dryRun returns OK before the real request;
• the real request returns OK once;
• spawn.spawning becomes active;
• Game.creeps['Worker1'] appears after completion;
• you can explain why OK means scheduled, not finished.

Give multiple Creeps simple responsibilities

You have created one fixed-name worker. The next lesson separates body abilities from player-defined jobs:

Understand Harvester, Upgrader, and Builder roles →

Return to the English beginner roadmap to review all twelve lessons in order.

Official sources

• Screeps API: Game.spawns
• Screeps API: StructureSpawn.spawnCreep()
• Screeps API: StructureSpawn.spawning
• Screeps API: constants and body-part costs
• Screeps Documentation: Creeps and body parts