diff --git a/src/lib/onboard.ts b/src/lib/onboard.ts
index a05c9f16da..98a49eea55 100644
--- a/src/lib/onboard.ts
+++ b/src/lib/onboard.ts
@@ -6389,6 +6389,7 @@ const recordStepSkipped = onboardRuntimeBoundary.recordStepSkipped.bind(onboardR
 const recordStepFailed = onboardRuntimeBoundary.recordStepFailed.bind(onboardRuntimeBoundary);
 const recordStateSkipped = onboardRuntimeBoundary.recordStateSkipped.bind(onboardRuntimeBoundary);
 const recordRepairEvent = onboardRuntimeBoundary.recordRepairEvent.bind(onboardRuntimeBoundary);
+const recordResumeConflict = onboardRuntimeBoundary.recordResumeConflict.bind(onboardRuntimeBoundary);
 const recordPostVerifyStarted = onboardRuntimeBoundary.recordPostVerifyStarted.bind(onboardRuntimeBoundary);
 const recordSessionComplete = onboardRuntimeBoundary.recordSessionComplete.bind(onboardRuntimeBoundary);
 
@@ -6598,6 +6599,7 @@ async function onboard(opts: OnboardOptions = {}): Promise<void> {
       });
       if (resumeConflicts.length > 0) {
         for (const conflict of resumeConflicts) {
+          await recordResumeConflict(conflict);
           if (conflict.field === "sandbox") {
             console.error(
               `  Resumable state belongs to sandbox '${conflict.recorded}', not '${conflict.requested}'.`,
diff --git a/src/lib/onboard/machine/runtime.test.ts b/src/lib/onboard/machine/runtime.test.ts
index f098ba0dc3..d48da85e0a 100644
--- a/src/lib/onboard/machine/runtime.test.ts
+++ b/src/lib/onboard/machine/runtime.test.ts
@@ -209,6 +209,24 @@ describe("OnboardRuntime", () => {
     expect(events[1]).toMatchObject({ state: "post_verify" });
   });
 
+  it("emits redacted resume conflict events without mutating durable state", async () => {
+    const { runtime, events, getSession } = createHarness(sessionInState("provider_selection"));
+
+    await runtime.emitResumeConflict({
+      field: "fromDockerfile",
+      recorded: "/workspace/Dockerfile",
+      requested: "/tmp/Dockerfile",
+      metadata: { endpoint: "https://alice:secret@example.com/v1?token=super-secret" },
+    });
+
+    expect(getSession().machine.state).toBe("provider_selection");
+    expect(events).toHaveLength(1);
+    expect(events[0]).toMatchObject({ type: "resume.conflict", state: "provider_selection" });
+    expect(events[0].metadata.field).toBe("fromDockerfile");
+    expect(JSON.stringify(events)).not.toContain("super-secret");
+    expect(JSON.stringify(events)).not.toContain("alice:secret");
+  });
+
   it("emits skipped and repair events without mutating durable state", async () => {
     const { runtime, events, getSession } = createHarness(sessionInState("provider_selection"));
 
diff --git a/src/lib/onboard/machine/runtime.ts b/src/lib/onboard/machine/runtime.ts
index 2e5d584f3b..65516c3212 100644
--- a/src/lib/onboard/machine/runtime.ts
+++ b/src/lib/onboard/machine/runtime.ts
@@ -243,6 +243,25 @@ export class OnboardRuntime {
     return session;
   }
 
+  async emitResumeConflict(options: {
+    field: string;
+    recorded?: unknown;
+    requested?: unknown;
+    metadata?: Record<string, unknown> | null;
+  }): Promise<Session> {
+    const session = this.ensureSession();
+    this.emit("resume.conflict", session, {
+      state: session.machine.state,
+      metadata: {
+        ...eventMetadata(options.metadata),
+        field: options.field,
+        recorded: options.recorded ?? null,
+        requested: options.requested ?? null,
+      },
+    });
+    return session;
+  }
+
   async emitRepairEvent(
     type: Extract<
       OnboardMachineEventType,
diff --git a/src/lib/onboard/runtime-boundary.test.ts b/src/lib/onboard/runtime-boundary.test.ts
index d81116ed86..21d6f1083e 100644
--- a/src/lib/onboard/runtime-boundary.test.ts
+++ b/src/lib/onboard/runtime-boundary.test.ts
@@ -91,4 +91,25 @@ describe("OnboardRuntimeBoundary", () => {
     expect(harness.events[0]).toMatchObject({ state: "init" });
     expect(harness.events[1]).toMatchObject({ state: "init" });
   });
+
+  it("records resume conflict diagnostics through the runtime", async () => {
+    const harness = createRuntimeHarness();
+    const boundary = new OnboardRuntimeBoundary({
+      toSessionUpdates: (updates) => filterSafeUpdates(updates as SessionUpdates) as SessionUpdates,
+      maybeForceE2eStepFailure: () => undefined,
+      createRuntime: harness.createRuntime,
+    });
+
+    await boundary.recordResumeConflict({
+      field: "sandbox",
+      recorded: "old-sandbox",
+      requested: "new-sandbox",
+    });
+
+    expect(harness.events).toHaveLength(1);
+    expect(harness.events[0]).toMatchObject({
+      type: "resume.conflict",
+      metadata: { field: "sandbox", recorded: "old-sandbox", requested: "new-sandbox" },
+    });
+  });
 });
diff --git a/src/lib/onboard/runtime-boundary.ts b/src/lib/onboard/runtime-boundary.ts
index e90166e17b..e2306e3ce5 100644
--- a/src/lib/onboard/runtime-boundary.ts
+++ b/src/lib/onboard/runtime-boundary.ts
@@ -37,6 +37,7 @@ export class OnboardRuntimeBoundary {
       recordStepSkipped: this.recordStepSkipped.bind(this),
       recordStateSkipped: this.recordStateSkipped.bind(this),
       recordRepairEvent: this.recordRepairEvent.bind(this),
+      recordResumeConflict: this.recordResumeConflict.bind(this),
       recordStepFailed: this.recordStepFailed.bind(this),
       recordPostVerifyStarted: this.recordPostVerifyStarted.bind(this),
       recordSessionComplete: this.recordSessionComplete.bind(this),
@@ -83,6 +84,15 @@ export class OnboardRuntimeBoundary {
     return this.getRuntime().markSkipped(state, metadata);
   }
 
+  async recordResumeConflict(conflict: {
+    field: string;
+    recorded?: unknown;
+    requested?: unknown;
+    metadata?: Record<string, unknown> | null;
+  }): Promise<Session> {
+    return this.getRuntime().emitResumeConflict(conflict);
+  }
+
   async recordRepairEvent(
     type: Extract<
       OnboardMachineEventType,