diff --git a/tests/control_plane_ts/production_scale_coordination_fixture.test.ts b/tests/control_plane_ts/production_scale_coordination_fixture.test.ts
index b3cdd1871e..2ee4366a83 100644
--- a/tests/control_plane_ts/production_scale_coordination_fixture.test.ts
+++ b/tests/control_plane_ts/production_scale_coordination_fixture.test.ts
@@ -9,7 +9,13 @@ import {
   TODO_ITEM_SCHEMA,
 } from "../../loopx/control_plane/coordination/coordination_state_contract.ts";
 import {validateCoordinationTodoReadModel} from "../../loopx/control_plane/coordination/coordination_projection.ts";
-import {productionScaleCoordinationFixture} from "./production_scale_coordination_fixture.ts";
+import {
+  PRODUCTION_SCALE_AGENT_TODO_COUNT,
+  PRODUCTION_SCALE_COMPLETION_TARGET_INDEX,
+  PRODUCTION_SCALE_SUPERSEDE_TARGET_INDEX,
+  productionScaleCoordinationFixture,
+  requireProductionScaleTargetIndex,
+} from "./production_scale_coordination_fixture.ts";
 
 test("production-scale fixture is deterministic and explicitly ordered", () => {
   const first = productionScaleCoordinationFixture("fixture-goal", "legacy");
@@ -75,3 +81,42 @@ test("fixture rejects silent status-count drift at construction time", () => {
   );
   assert.notEqual(TODO_ITEM_SCHEMA, TODO_DOMAIN_ITEM_SCHEMA);
 });
+
+test("fixture rejects a target index that addresses no generated agent Todo", () => {
+  // Invariant (typescript-control-plane-migration-v0 §2.5): parsed JSON enters
+  // the system as unknown, and a type annotation does not prove the bytes
+  // satisfy the contract. The envelope's two target indices are the only
+  // envelope fields consumed as positional lookups, so they are validated at
+  // the boundary. The cases below are derived from that rule, not from the
+  // generated fixture: an index either addresses a generated agent Todo or it
+  // is rejected, whatever the fixture happens to contain.
+  const agentTodoCount = PRODUCTION_SCALE_AGENT_TODO_COUNT;
+  assert.ok(agentTodoCount > 0, "the envelope must generate at least one agent Todo");
+
+  assert.equal(requireProductionScaleTargetIndex(0, "test target index", agentTodoCount), 0);
+  assert.equal(
+    requireProductionScaleTargetIndex(agentTodoCount - 1, "test target index", agentTodoCount),
+    agentTodoCount - 1,
+  );
+
+  // Mutation: the boundary one past the last generated Todo. Before the
+  // validation existed this value reached `agents[value]` and produced
+  // `undefined` instead of a diagnosis.
+  assert.throws(
+    () => requireProductionScaleTargetIndex(agentTodoCount, "test target index", agentTodoCount),
+    /does not address a generated agent Todo/u,
+  );
+  assert.throws(
+    () => requireProductionScaleTargetIndex(-1, "test target index", agentTodoCount),
+    /does not address a generated agent Todo/u,
+  );
+  assert.throws(
+    () => requireProductionScaleTargetIndex(1.5, "test target index", agentTodoCount),
+    /does not address a generated agent Todo/u,
+  );
+
+  // The checked-in envelope must satisfy the rule its consumers rely on.
+  assert.ok(PRODUCTION_SCALE_COMPLETION_TARGET_INDEX < agentTodoCount);
+  assert.ok(PRODUCTION_SCALE_SUPERSEDE_TARGET_INDEX < agentTodoCount);
+  assert.notEqual(PRODUCTION_SCALE_COMPLETION_TARGET_INDEX, PRODUCTION_SCALE_SUPERSEDE_TARGET_INDEX);
+});
diff --git a/tests/control_plane_ts/production_scale_coordination_fixture.ts b/tests/control_plane_ts/production_scale_coordination_fixture.ts
index f3cbc68d0f..41bf57a013 100644
--- a/tests/control_plane_ts/production_scale_coordination_fixture.ts
+++ b/tests/control_plane_ts/production_scale_coordination_fixture.ts
@@ -199,7 +199,7 @@ function todoRecords(
         ...(index % 8 === 0 ? {watch_only: "true"} : {max_no_change_before_replan: "5"})});
     }
     if (role === "agent" && status === "done" && index < 3) {
-      record.successor_todo_ids = [todoId("agent", envelope.completion_target_index + index)];
+      record.successor_todo_ids = [todoId("agent", PRODUCTION_SCALE_COMPLETION_TARGET_INDEX + index)];
       record.completion_continuation = "successor";
     }
     if (role === "user" && index < envelope.standing_user_decision_count) {
@@ -219,7 +219,7 @@ function todoRecords(
       record.decision_scope = {kind: "direction", granularity: "goal", scope_key: goalId};
       if (index >= partialEnd) {
         record.decision_outcome = "approve";
-        record.unblocks_todo_id = todoId("agent", envelope.completion_target_index);
+        record.unblocks_todo_id = todoId("agent", PRODUCTION_SCALE_COMPLETION_TARGET_INDEX);
       }
     }
     // An explicit rejection is a recorded decision, not absent authority: the
@@ -255,8 +255,8 @@ export function productionScaleCoordinationFixture(
   const archiveDependent = [...agents].reverse().find(item => item.status === "open")!;
   archiveDependent.task_class = "advancement_task";
   archiveDependent.resume_when = `todo_done:${todoId("agent", 3)}`;
-  const completionTodo = agents[envelope.completion_target_index]!;
-  const supersedeTodo = agents[envelope.supersede_target_index]!;
+  const completionTodo = agents[PRODUCTION_SCALE_COMPLETION_TARGET_INDEX]!;
+  const supersedeTodo = agents[PRODUCTION_SCALE_SUPERSEDE_TARGET_INDEX]!;
   completionTodo.task_class = "advancement_task";
   completionTodo.claimed_by = "agent-a";
   completionTodo.completion_validation_required = true;
@@ -355,6 +355,38 @@ function requireSafeCount(value: number, label: string): number {
   return value;
 }
 
+/**
+ * Validate the two envelope fields that address a generated agent Todo.
+ *
+ * The envelope is parsed JSON, so `as` proves nothing about its numbers: the
+ * two target indices below are the only envelope fields a consumer turns into a
+ * positional lookup. They are validated once here, at the boundary, and every
+ * consumer reads the validated constant instead of the raw field, so a drifted
+ * index fails the fixture instead of yielding `undefined` mid-construction.
+ */
+export function requireProductionScaleTargetIndex(
+  value: number,
+  label: string,
+  agentTodoCount: number,
+): number {
+  if (!Number.isSafeInteger(value) || value < 0 || value >= agentTodoCount) {
+    throw new Error(`production fixture ${label} does not address a generated agent Todo`);
+  }
+  return value;
+}
+
+export const PRODUCTION_SCALE_AGENT_TODO_COUNT = statusSeries(
+  envelope.agent_status_counts, envelope.agent_status_order, "agent",
+).length;
+
+export const PRODUCTION_SCALE_COMPLETION_TARGET_INDEX = requireProductionScaleTargetIndex(
+  envelope.completion_target_index, "completion target index", PRODUCTION_SCALE_AGENT_TODO_COUNT,
+);
+
+export const PRODUCTION_SCALE_SUPERSEDE_TARGET_INDEX = requireProductionScaleTargetIndex(
+  envelope.supersede_target_index, "supersede target index", PRODUCTION_SCALE_AGENT_TODO_COUNT,
+);
+
 /**
  * Checked while the module loads, so a drifted history envelope fails every
  * consumer instead of quietly shrinking a provider's retained history.