fix(workflow-engine): replay completed void steps on restart

Author: NathanFlurry

rivetkit-typescript/packages/workflow-engine/src/context.ts

@@ -380,12 +380,27 @@ export class WorkflowContextImpl implements WorkflowContextInterface {
 				return stepData.output as T;
 			}
 
-			// Check if we should retry
+			// Check if we should retry.
+			//
+			// Important: steps that *return undefined* (i.e. "void" steps) are valid and
+			// must still be treated as completed on restart. Since JSON omits properties
+			// with value `undefined`, we cannot rely on `stepData.output !== undefined`
+			// to determine completion. Prefer metadata.status for completion tracking.
 			const metadata = await loadMetadata(
 				this.storage,
 				this.driver,
 				existing.id,
 			);
+
+			if (metadata.status === "completed") {
+				this.log("debug", {
+					msg: "replaying completed step from metadata",
+					step: config.name,
+					key,
+				});
+				return stepData.output as T;
+			}
+
 			const maxRetries = config.maxRetries ?? DEFAULT_MAX_RETRIES;
 
 			if (metadata.attempts >= maxRetries) {

rivetkit-typescript/packages/workflow-engine/tests/steps.test.ts

@@ -72,6 +72,40 @@ for (const mode of modes) {
 			expect(callCount).toBe(1);
 		});
 
+		it("should treat void step outputs as completed on restart", async () => {
+			let callCount = 0;
+
+			const workflow = async (ctx: WorkflowContextInterface) => {
+				await ctx.step("void-step", async () => {
+					callCount += 1;
+					// return undefined (void)
+				});
+				return "done";
+			};
+
+			const first = await runWorkflow(
+				"wf-void",
+				workflow,
+				undefined,
+				driver,
+				{ mode },
+			).result;
+			expect(first.state).toBe("completed");
+			expect(first.output).toBe("done");
+			expect(callCount).toBe(1);
+
+			const second = await runWorkflow(
+				"wf-void",
+				workflow,
+				undefined,
+				driver,
+				{ mode },
+			).result;
+			expect(second.state).toBe("completed");
+			expect(second.output).toBe("done");
+			expect(callCount).toBe(1);
+		});
+
 		it("should execute multiple steps in sequence", async () => {
 			const workflow = async (ctx: WorkflowContextInterface) => {
 				const a = await ctx.step("step-a", async () => 1);