diff --git a/src/lib/onboard/machine/runtime.test.ts b/src/lib/onboard/machine/runtime.test.ts
index d48da85e0a..512ee7f56b 100644
--- a/src/lib/onboard/machine/runtime.test.ts
+++ b/src/lib/onboard/machine/runtime.test.ts
@@ -12,6 +12,13 @@ import {
   type SessionUpdates,
 } from "../../state/onboard-session";
 import type { OnboardMachineEvent } from "./events";
+import {
+  advanceTo,
+  branchTo,
+  completeOnboardMachine,
+  failOnboardMachine,
+  retryTo,
+} from "./result";
 import { OnboardRuntime, type OnboardRuntimeDeps } from "./runtime";
 import { InvalidOnboardMachineTransitionError } from "./transitions";
 
@@ -159,6 +166,65 @@ describe("OnboardRuntime", () => {
     expect(JSON.stringify(events)).not.toContain("super-secret");
   });
 
+  it("applies explicit advance results through validated runtime transitions", async () => {
+    const { runtime, events, getSession } = createHarness();
+
+    await runtime.applyResult(
+      advanceTo("preflight", {
+        updates: { sandboxName: "my-assistant" },
+        metadata: { source: "handler" },
+      }),
+    );
+
+    expect(getSession()).toMatchObject({
+      sandboxName: "my-assistant",
+      machine: { state: "preflight", revision: 1 },
+    });
+    expect(events.map((event) => event.type)).toEqual([
+      "context.updated",
+      "state.exited",
+      "state.entered",
+    ]);
+    expect(events[0].metadata.fields).toEqual(["sandboxName"]);
+    expect(events[1]).toMatchObject({ state: "init", metadata: { source: "handler" } });
+    expect(events[2]).toMatchObject({ state: "preflight", metadata: { source: "handler" } });
+  });
+
+  it("applies explicit retry, branch, completion, and failure results", async () => {
+    const retryHarness = createHarness(sessionInState("inference"));
+    await retryHarness.runtime.applyResult(retryTo("provider_selection"));
+    expect(retryHarness.getSession().machine).toMatchObject({ state: "provider_selection" });
+
+    const branchHarness = createHarness(sessionInState("sandbox"));
+    await branchHarness.runtime.applyResult(branchTo("agent_setup"));
+    expect(branchHarness.getSession().machine).toMatchObject({ state: "agent_setup" });
+
+    const completeHarness = createHarness(sessionInState("post_verify"));
+    await completeHarness.runtime.applyResult(completeOnboardMachine({ sandboxName: "done" }));
+    expect(completeHarness.getSession()).toMatchObject({
+      status: "complete",
+      sandboxName: "done",
+      machine: { state: "complete" },
+    });
+
+    const failedHarness = createHarness(sessionInState("gateway"));
+    await failedHarness.runtime.applyResult(failOnboardMachine("boom", { step: "gateway" }));
+    expect(failedHarness.getSession()).toMatchObject({
+      status: "failed",
+      failure: { step: "gateway", message: "boom" },
+      machine: { state: "failed" },
+    });
+  });
+
+  it("rejects invalid explicit transition kinds before mutating context", async () => {
+    const { runtime, getSession } = createHarness(sessionInState("inference"));
+
+    await expect(
+      runtime.applyResult(advanceTo("provider_selection", { updates: { sandboxName: "mutated" } })),
+    ).rejects.toThrow("expected advance, got retry");
+    expect(getSession()).toMatchObject({ sandboxName: null, machine: { state: "inference" } });
+  });
+
   it("fails non-terminal sessions with redacted failure events", async () => {
     const { runtime, events, getSession } = createHarness(sessionInState("gateway"));
 
diff --git a/src/lib/onboard/machine/runtime.ts b/src/lib/onboard/machine/runtime.ts
index 65516c3212..47cee9f0d2 100644
--- a/src/lib/onboard/machine/runtime.ts
+++ b/src/lib/onboard/machine/runtime.ts
@@ -9,6 +9,7 @@ import {
   emitOnboardMachineEvent,
   type OnboardMachineEvent,
 } from "./events";
+import type { OnboardStateResult } from "./result";
 import {
   assertValidOnboardMachineTransition,
   canTransitionOnboardMachineState,
@@ -197,6 +198,33 @@ export class OnboardRuntime {
     return updated;
   }
 
+  async applyResult(result: OnboardStateResult): Promise<Session> {
+    if (result.type === "complete") {
+      return this.complete(result.updates ?? {});
+    }
+    if (result.type === "failed") {
+      return this.fail(result.error, {
+        step: result.step,
+        metadata: result.metadata,
+      });
+    }
+
+    const current = this.ensureSession();
+    const transition = assertValidOnboardMachineTransition(current.machine.state, result.next);
+    if (result.transitionKind && transition.kind !== result.transitionKind) {
+      throw new Error(
+        `Invalid onboarding machine transition kind: ${current.machine.state} -> ${result.next} expected ${result.transitionKind}, got ${transition.kind}`,
+      );
+    }
+    if (result.updates && Object.keys(this.deps.filterSafeUpdates(result.updates)).length > 0) {
+      await this.updateContext(result.updates, {
+        state: current.machine.state,
+        metadata: result.metadata,
+      });
+    }
+    return this.transition(result.next, { metadata: result.metadata });
+  }
+
   async fail(message: string | null, options: OnboardRuntimeFailureOptions = {}): Promise<Session> {
     const current = this.ensureSession();
     const from = current.machine.state;