diff --git a/src/lib/workflow-budget.ts b/src/lib/workflow-budget.ts
new file mode 100644
index 0000000000..5cbe8e665f
--- /dev/null
+++ b/src/lib/workflow-budget.ts
@@ -0,0 +1,172 @@
+/**
+ * Root-workflow admission: a finite budget above the logical request (#4546).
+ *
+ * The per-request send budget bounds how many times ONE request reaches upstream. It cannot
+ * bound how many requests a fan-out makes. A worker that spawns seven hundred children, each
+ * of which sends exactly once, never violates a per-request cap and still spends the account.
+ * That is the second half of the #4546 incident and it needs a ceiling of its own.
+ *
+ * The unit is the root workflow -- the user-visible task -- identified by the parent thread
+ * header when the client supplies one. A retry is not a new user task and gets no new
+ * allowance; a genuinely new top-level request does.
+ *
+ * This ledger is process-local and in-memory. It bounds a single proxy process honestly and
+ * says nothing about a second process sharing the same account pool; that needs a shared
+ * durable store and is declared out of scope rather than implied.
+ */
+
+export interface WorkflowBudgetPolicy {
+  /** Children admitted concurrently under one root. */
+  readonly maxConcurrentChildren: number;
+  /** Physical model sends charged to one root across its whole life. */
+  readonly maxPhysicalSends: number;
+  /** Distinct children one root may ever create. */
+  readonly maxDistinctChildren: number;
+  /**
+   * Concurrency slots a fan-out may never take. An interactive turn arriving into a saturated
+   * root still gets admitted; without this a worker burst starves the conversation it serves.
+   */
+  readonly interactiveReserve: number;
+  /** Roots tracked at once. Bounded so a caller minting new ids cannot grow this forever. */
+  readonly maxTrackedRoots: number;
+}
+
+export const DEFAULT_WORKFLOW_BUDGET_POLICY: WorkflowBudgetPolicy = {
+  maxConcurrentChildren: 8,
+  maxPhysicalSends: 256,
+  maxDistinctChildren: 64,
+  interactiveReserve: 1,
+  maxTrackedRoots: 512,
+};
+
+export type WorkflowDenial =
+  | "workflow-concurrency-exhausted"
+  | "workflow-sends-exhausted"
+  | "workflow-children-exhausted";
+
+export type WorkflowLane = "interactive" | "worker";
+
+export interface WorkflowAdmission {
+  readonly rootId: string;
+  release(): void;
+}
+
+export type WorkflowDecision =
+  | { admitted: true; lease: WorkflowAdmission }
+  | { admitted: false; reason: WorkflowDenial; rootId: string };
+
+interface WorkflowState {
+  active: number;
+  sends: number;
+  children: Set<string>;
+  lastSeenMs: number;
+}
+
+const roots = new Map<string, WorkflowState>();
+
+function pruneOldestRoot(): void {
+  let oldestKey: string | undefined;
+  let oldestAt = Number.POSITIVE_INFINITY;
+  for (const [key, state] of roots) {
+    // An active root is never evicted: dropping it would hand its fan-out a fresh allowance,
+    // which is the exact laundering this ledger exists to prevent.
+    if (state.active > 0) continue;
+    if (state.lastSeenMs < oldestAt) { oldestAt = state.lastSeenMs; oldestKey = key; }
+  }
+  if (oldestKey !== undefined) roots.delete(oldestKey);
+}
+
+/**
+ * Admit one turn under a root workflow.
+ *
+ * `childId` distinguishes the members of a fan-out; omit it for the root's own turns.
+ * An interactive lane may use the reserved slots a worker lane may not.
+ */
+export function admitWorkflowTurn(
+  rootId: string | undefined,
+  lane: WorkflowLane,
+  policy: WorkflowBudgetPolicy = DEFAULT_WORKFLOW_BUDGET_POLICY,
+  childId?: string,
+  now: number = Date.now(),
+): WorkflowDecision | undefined {
+  if (!rootId) return undefined;
+  let state = roots.get(rootId);
+  if (!state) {
+    if (roots.size >= policy.maxTrackedRoots) pruneOldestRoot();
+    state = { active: 0, sends: 0, children: new Set(), lastSeenMs: now };
+    roots.set(rootId, state);
+  }
+  state.lastSeenMs = now;
+
+  if (state.sends >= policy.maxPhysicalSends) {
+    return { admitted: false, reason: "workflow-sends-exhausted", rootId };
+  }
+  if (childId !== undefined && !state.children.has(childId)
+    && state.children.size >= policy.maxDistinctChildren) {
+    return { admitted: false, reason: "workflow-children-exhausted", rootId };
+  }
+  const ceiling = lane === "worker"
+    ? Math.max(0, policy.maxConcurrentChildren - policy.interactiveReserve)
+    : policy.maxConcurrentChildren;
+  if (state.active >= ceiling) {
+    return { admitted: false, reason: "workflow-concurrency-exhausted", rootId };
+  }
+
+  state.active += 1;
+  if (childId !== undefined) state.children.add(childId);
+  let released = false;
+  return {
+    admitted: true,
+    lease: {
+      rootId,
+      release(): void {
+        if (released) return;
+        released = true;
+        const current = roots.get(rootId);
+        if (!current) return;
+        current.active = Math.max(0, current.active - 1);
+        current.lastSeenMs = Date.now();
+      },
+    },
+  };
+}
+
+/**
+ * Charge physical sends to a root. Called from the send budget's own accounting so a retry
+ * inside one request counts toward the workflow total, not only the request total.
+ */
+export function chargeWorkflowSends(rootId: string | undefined, sends: number): void {
+  if (!rootId || sends <= 0) return;
+  const state = roots.get(rootId);
+  if (!state) return;
+  state.sends += sends;
+  state.lastSeenMs = Date.now();
+}
+
+/**
+ * Whether this root has already spent its whole physical-send ceiling.
+ *
+ * Separate from `admitWorkflowTurn` so a caller can refuse before dispatch without taking a
+ * concurrency slot it would have to remember to release.
+ */
+export function workflowSendCeilingReached(
+  rootId: string | undefined,
+  policy: WorkflowBudgetPolicy = DEFAULT_WORKFLOW_BUDGET_POLICY,
+): boolean {
+  if (!rootId) return false;
+  const state = roots.get(rootId);
+  return state !== undefined && state.sends >= policy.maxPhysicalSends;
+}
+
+export function workflowBudgetSnapshot(rootId: string): {
+
+  active: number; sends: number; children: number;
+} | undefined {
+  const state = roots.get(rootId);
+  return state ? { active: state.active, sends: state.sends, children: state.children.size } : undefined;
+}
+
+/** Test seam. Production never clears a live ledger: that would reset a spent budget. */
+export function resetWorkflowBudgetsForTest(): void {
+  roots.clear();
+}
diff --git a/src/server/responses/core.ts b/src/server/responses/core.ts
index 2f694105eb..908c46f7f2 100644
--- a/src/server/responses/core.ts
+++ b/src/server/responses/core.ts
@@ -233,6 +233,10 @@ import {
   type SendClass,
   type SingleUseDispatchPermit,
 } from "../../lib/request-execution-budget";
+import {
+  chargeWorkflowSends,
+  workflowSendCeilingReached,
+} from "../../lib/workflow-budget";
 import {
   ForwardAdmissionCredentialError,
   hasForwardableCodexBearer,
@@ -1297,6 +1301,8 @@ interface CodexPoolAccountRetryArgs {
     resolveCodexModelEntitlements?: typeof resolveCodexModelEntitlements;
     /** The logical request's execution budget: the account move is its fourth send. */
     sendBudget?: TransientSendBudget;
+    /** Root workflow this turn belongs to, so the move is charged there as well. */
+    workflowRootId?: string;
   };
   firstAuthCtx: Extract<CodexAuthContext, { kind: "pool" | "main-pool" }>;
   firstResponse: Response;
@@ -1659,6 +1665,8 @@ async function retryCodexPoolOnAlternateAccount(
           recordUnmovedTransientOutcome();
           return { kind: "no-alternate" };
         }
+        // The move is a physical send like any other, so the root workflow is charged too.
+        chargeWorkflowSends(args.options.workflowRootId, 1);
       }
       noteAttemptSend(logCtx.activeAttempt, passthroughEstimate);
       try {
@@ -5029,7 +5037,26 @@ async function handleResponsesInner(
   // parent's spend instead of starting over per target -- both halves of the measured
   // amplification in #4546.
   const sendBudget = options.sendBudget ?? createRequestExecutionBudget();
-  const noteTransientSends = (used: number): void => { sendBudget.used += Math.max(0, used); };
+  // The root workflow is the user-visible task. A per-request cap cannot bound a fan-out that
+  // sends once per child seven hundred times, so every send charged to the request is charged
+  // to the root as well (#4546).
+  const workflowRootId = req.headers.get("x-codex-parent-thread-id")?.trim() || undefined;
+  const noteTransientSends = (used: number): void => {
+    const charged = Math.max(0, used);
+    sendBudget.used += charged;
+    chargeWorkflowSends(workflowRootId, charged);
+  };
+  // Refused before any dispatch, and deliberately not by evicting the root's ledger entry:
+  // dropping the record to make room would hand the fan-out a fresh allowance, which is the
+  // laundering this ceiling exists to stop. The client is told the task needs a new grant
+  // rather than being given a synthetic upstream error.
+  if (workflowSendCeilingReached(workflowRootId)) {
+    return formatErrorResponse(
+      429,
+      "workflow_budget_exhausted",
+      "This task has used its whole send budget, so no further upstream request was made. Requests already in flight settle as they finish.",
+    );
+  }
   // No floor. Math.max(1, ...) meant an exhausted request still funded one send on every
   // recovery leg, so a bounded per-leg allowance never became a bounded per-request one.
   const remainingTransientSendBudget = (budget: number): number =>
@@ -6102,7 +6129,7 @@ async function handleResponsesInner(
           route,
           parsed,
           logCtx,
-          options,
+          options: { ...options, workflowRootId },
           firstAuthCtx: authCtx,
           firstResponse: upstreamResponse,
           outcomeStatus: poolRetryOutcome,