OpenClaw < 2026.2.23 - Twilio Webhook Replay Bypass via Randomized Event ID Normalization

@@ -10,6 +10,7 @@ Docs: https://docs.openclaw.ai
 
 ### Fixes
 
+- Security/Voice Call: harden Twilio webhook replay handling by preserving provider event IDs through normalization, adding bounded replay dedupe, and enforcing per-call turn-token matching for call-state transitions. This ships in the next npm release. Thanks @jiseoung for reporting.
 - Security/Commands: enforce sender-only matching for `commands.allowFrom` by blocking conversation-shaped `From` identities (`channel:`, `group:`, `thread:`, `@g.us`) while preserving direct-message fallback when sender fields are missing. Ships in the next npm release. Thanks @jiseoung.
 - Config/Kilo Gateway: Kilo provider flow now surfaces an updated list of models. (#24921) thanks @gumadeiras.
 - Security/Sandbox: enforce `tools.exec.applyPatch.workspaceOnly` and `tools.fs.workspaceOnly` for `apply_patch` in sandbox-mounted paths so writes/deletes cannot escape the workspace boundary via mounts like `/agent` unless explicitly opted out (`tools.exec.applyPatch.workspaceOnly=false`). This ships in the next npm release. Thanks @tdjackey for reporting.

@@ -177,6 +177,12 @@ headers are trusted.
 `webhookSecurity.trustedProxyIPs` only trusts forwarded headers when the request
 remote IP matches the list.
 
+Webhook replay protection is enabled for Twilio and Plivo. Replayed valid webhook
+requests are acknowledged but skipped for side effects.
+
+Twilio conversation turns include a per-turn token in `<Gather>` callbacks, so
+stale/replayed speech callbacks cannot satisfy a newer pending transcript turn.
+
 Example with a stable public host:
 
 ```json5

@@ -175,5 +175,7 @@ Actions:
 ## Notes
 
 - Uses webhook signature verification for Twilio/Telnyx/Plivo.
+- Adds replay protection for Twilio and Plivo webhooks (valid duplicate callbacks are ignored safely).
+- Twilio speech turns include a per-turn token so stale/replayed callbacks cannot complete a newer turn.
 - `responseModel` / `responseSystemPrompt` control AI auto-responses.
 - Media streaming requires `ws` and OpenAI Realtime API key.

@@ -6,6 +6,7 @@ export type TranscriptWaiter = {
   resolve: (text: string) => void;
   reject: (err: Error) => void;
   timeout: NodeJS.Timeout;
+  turnToken?: string;
 };
 
 export type CallManagerRuntimeState = {

@@ -234,4 +234,49 @@ describe("processEvent (functional)", () => {
     expect(() => processEvent(ctx, event)).not.toThrow();
     expect(ctx.activeCalls.size).toBe(0);
   });
+
+  it("deduplicates by dedupeKey even when event IDs differ", () => {
+    const now = Date.now();
+    const ctx = createContext();
+    ctx.activeCalls.set("call-dedupe", {
+      callId: "call-dedupe",
+      providerCallId: "provider-dedupe",
+      provider: "plivo",
+      direction: "outbound",
+      state: "answered",
+      from: "+15550000000",
+      to: "+15550000001",
+      startedAt: now,
+      transcript: [],
+      processedEventIds: [],
+      metadata: {},
+    });
+    ctx.providerCallIdMap.set("provider-dedupe", "call-dedupe");
+
+    processEvent(ctx, {
+      id: "evt-1",
+      dedupeKey: "stable-key-1",
+      type: "call.speech",
+      callId: "call-dedupe",
+      providerCallId: "provider-dedupe",
+      timestamp: now + 1,
+      transcript: "hello",
+      isFinal: true,
+    });
+
+    processEvent(ctx, {
+      id: "evt-2",
+      dedupeKey: "stable-key-1",
+      type: "call.speech",
+      callId: "call-dedupe",
+      providerCallId: "provider-dedupe",
+      timestamp: now + 2,
+      transcript: "hello",
+      isFinal: true,
+    });
+
+    const call = ctx.activeCalls.get("call-dedupe");
+    expect(call?.transcript).toHaveLength(1);
+    expect(Array.from(ctx.processedEventIds)).toEqual(["stable-key-1"]);
+  });
 });

@@ -92,10 +92,11 @@ function createInboundCall(params: {
 }
 
 export function processEvent(ctx: EventContext, event: NormalizedEvent): void {
-  if (ctx.processedEventIds.has(event.id)) {
+  const dedupeKey = event.dedupeKey || event.id;
+  if (ctx.processedEventIds.has(dedupeKey)) {
     return;
   }
-  ctx.processedEventIds.add(event.id);
+  ctx.processedEventIds.add(dedupeKey);
 
   let call = findCall({
     activeCalls: ctx.activeCalls,
@@ -158,7 +159,7 @@ export function processEvent(ctx: EventContext, event: NormalizedEvent): void {
     }
   }
 
-  call.processedEventIds.push(event.id);
+  call.processedEventIds.push(dedupeKey);
 
   switch (event.type) {
     case "call.initiated":
@@ -192,8 +193,20 @@ export function processEvent(ctx: EventContext, event: NormalizedEvent): void {
 
     case "call.speech":
       if (event.isFinal) {
+        const hadWaiter = ctx.transcriptWaiters.has(call.callId);
+        const resolved = resolveTranscriptWaiter(
+          ctx,
+          call.callId,
+          event.transcript,
+          event.turnToken,
+        );
+        if (hadWaiter && !resolved) {
+          console.warn(
+            `[voice-call] Ignoring speech event with mismatched turn token for ${call.callId}`,
+          );
+          break;
+        }
         addTranscriptEntry(call, "user", event.transcript);
-        resolveTranscriptWaiter(ctx, call.callId, event.transcript);
       }
       transitionState(call, "listening");
       break;

@@ -291,6 +291,7 @@ export async function continueCall(
   ctx.activeTurnCalls.add(callId);
 
   const turnStartedAt = Date.now();
+  const turnToken = provider.name === "twilio" ? crypto.randomUUID() : undefined;
 
   try {
     await speak(ctx, callId, prompt);
@@ -299,9 +300,9 @@ export async function continueCall(
     persistCallRecord(ctx.storePath, call);
 
     const listenStartedAt = Date.now();
-    await provider.startListening({ callId, providerCallId });
+    await provider.startListening({ callId, providerCallId, turnToken });
 
-    const transcript = await waitForFinalTranscript(ctx, callId);
+    const transcript = await waitForFinalTranscript(ctx, callId, turnToken);
     const transcriptReceivedAt = Date.now();
 
     // Best-effort: stop listening after final transcript.

@@ -17,12 +17,16 @@ import type {
 } from "./types.js";
 
 class FakeProvider implements VoiceCallProvider {
-  readonly name = "plivo" as const;
+  readonly name: "plivo" | "twilio";
   readonly playTtsCalls: PlayTtsInput[] = [];
   readonly hangupCalls: HangupCallInput[] = [];
   readonly startListeningCalls: StartListeningInput[] = [];
   readonly stopListeningCalls: StopListeningInput[] = [];
 
+  constructor(name: "plivo" | "twilio" = "plivo") {
+    this.name = name;
+  }
+
   verifyWebhook(_ctx: WebhookContext): WebhookVerificationResult {
     return { ok: true };
   }
@@ -319,6 +323,61 @@ describe("CallManager", () => {
     expect(provider.stopListeningCalls).toHaveLength(1);
   });
 
+  it("ignores speech events with mismatched turnToken while waiting for transcript", async () => {
+    const { manager, provider } = createManagerHarness(
+      {
+        transcriptTimeoutMs: 5000,
+      },
+      new FakeProvider("twilio"),
+    );
+
+    const started = await manager.initiateCall("+15550000004");
+    expect(started.success).toBe(true);
+
+    markCallAnswered(manager, started.callId, "evt-turn-token-answered");
+
+    const turnPromise = manager.continueCall(started.callId, "Prompt");
+    await new Promise((resolve) => setTimeout(resolve, 0));
+
+    const expectedTurnToken = provider.startListeningCalls[0]?.turnToken;
+    expect(typeof expectedTurnToken).toBe("string");
+
+    manager.processEvent({
+      id: "evt-turn-token-bad",
+      type: "call.speech",
+      callId: started.callId,
+      providerCallId: "request-uuid",
+      timestamp: Date.now(),
+      transcript: "stale replay",
+      isFinal: true,
+      turnToken: "wrong-token",
+    });
+
+    const pendingState = await Promise.race([
+      turnPromise.then(() => "resolved"),
+      new Promise<"pending">((resolve) => setTimeout(() => resolve("pending"), 0)),
+    ]);
+    expect(pendingState).toBe("pending");
+
+    manager.processEvent({
+      id: "evt-turn-token-good",
+      type: "call.speech",
+      callId: started.callId,
+      providerCallId: "request-uuid",
+      timestamp: Date.now(),
+      transcript: "final answer",
+      isFinal: true,
+      turnToken: expectedTurnToken,
+    });
+
+    const turnResult = await turnPromise;
+    expect(turnResult.success).toBe(true);
+    expect(turnResult.transcript).toBe("final answer");
+
+    const call = manager.getCall(started.callId);
+    expect(call?.transcript.map((entry) => entry.text)).toEqual(["Prompt", "final answer"]);
+  });
+
   it("tracks latency metadata across multiple closed-loop turns", async () => {
     const { manager, provider } = createManagerHarness({
       transcriptTimeoutMs: 5000,

@@ -77,16 +77,25 @@ export function resolveTranscriptWaiter(
   ctx: TranscriptWaiterContext,
   callId: CallId,
   transcript: string,
-): void {
+  turnToken?: string,
+): boolean {
   const waiter = ctx.transcriptWaiters.get(callId);
   if (!waiter) {
-    return;
+    return false;
+  }
+  if (waiter.turnToken && waiter.turnToken !== turnToken) {
+    return false;
   }
   clearTranscriptWaiter(ctx, callId);
   waiter.resolve(transcript);
+  return true;
 }
 
-export function waitForFinalTranscript(ctx: TimerContext, callId: CallId): Promise<string> {
+export function waitForFinalTranscript(
+  ctx: TimerContext,
+  callId: CallId,
+  turnToken?: string,
+): Promise<string> {
   if (ctx.transcriptWaiters.has(callId)) {
     return Promise.reject(new Error("Already waiting for transcript"));
   }
@@ -98,6 +107,6 @@ export function waitForFinalTranscript(ctx: TimerContext, callId: CallId): Promi
       reject(new Error(`Timed out waiting for transcript after ${timeoutMs}ms`));
     }, timeoutMs);
 
-    ctx.transcriptWaiters.set(callId, { resolve, reject, timeout });
+    ctx.transcriptWaiters.set(callId, { resolve, reject, timeout, turnToken });
   });
 }

@@ -30,6 +30,29 @@ export interface PlivoProviderOptions {
 type PendingSpeak = { text: string; locale?: string };
 type PendingListen = { language?: string };
 
+function getHeader(
+  headers: Record<string, string | string[] | undefined>,
+  name: string,
+): string | undefined {
+  const value = headers[name.toLowerCase()];
+  if (Array.isArray(value)) {
+    return value[0];
+  }
+  return value;
+}
+
+function createPlivoRequestDedupeKey(ctx: WebhookContext): string {
+  const nonceV3 = getHeader(ctx.headers, "x-plivo-signature-v3-nonce");
+  if (nonceV3) {
+    return `plivo:v3:${nonceV3}`;
+  }
+  const nonceV2 = getHeader(ctx.headers, "x-plivo-signature-v2-nonce");
+  if (nonceV2) {
+    return `plivo:v2:${nonceV2}`;
+  }
+  return `plivo:fallback:${crypto.createHash("sha256").update(ctx.rawBody).digest("hex")}`;
+}
+
 export class PlivoProvider implements VoiceCallProvider {
   readonly name = "plivo" as const;
 
@@ -104,7 +127,7 @@ export class PlivoProvider implements VoiceCallProvider {
       console.warn(`[plivo] Webhook verification failed: ${result.reason}`);
     }
 
-    return { ok: result.ok, reason: result.reason };
+    return { ok: result.ok, reason: result.reason, isReplay: result.isReplay };
   }
 
   parseWebhookEvent(ctx: WebhookContext): ProviderWebhookParseResult {
@@ -173,7 +196,8 @@ export class PlivoProvider implements VoiceCallProvider {
 
     // Normal events.
     const callIdFromQuery = this.getCallIdFromQuery(ctx);
-    const event = this.normalizeEvent(parsed, callIdFromQuery);
+    const dedupeKey = createPlivoRequestDedupeKey(ctx);
+    const event = this.normalizeEvent(parsed, callIdFromQuery, dedupeKey);
 
     return {
       events: event ? [event] : [],
@@ -186,7 +210,11 @@ export class PlivoProvider implements VoiceCallProvider {
     };
   }
 
-  private normalizeEvent(params: URLSearchParams, callIdOverride?: string): NormalizedEvent | null {
+  private normalizeEvent(
+    params: URLSearchParams,
+    callIdOverride?: string,
+    dedupeKey?: string,
+  ): NormalizedEvent | null {
     const callUuid = params.get("CallUUID") || "";
     const requestUuid = params.get("RequestUUID") || "";
 
@@ -201,6 +229,7 @@ export class PlivoProvider implements VoiceCallProvider {
 
     const baseEvent = {
       id: crypto.randomUUID(),
+      dedupeKey,
       callId: callIdOverride || callUuid || requestUuid,
       providerCallId: callUuid || requestUuid || undefined,
       timestamp: Date.now(),

@@ -59,4 +59,38 @@ describe("TwilioProvider", () => {
     expect(result.providerResponseBody).toContain('<Parameter name="token" value="');
     expect(result.providerResponseBody).toContain("<Connect>");
   });
+
+  it("uses a stable dedupeKey for identical request payloads", () => {
+    const provider = createProvider();
+    const rawBody = "CallSid=CA789&Direction=inbound&SpeechResult=hello";
+    const ctxA = {
+      ...createContext(rawBody, { callId: "call-1", turnToken: "turn-1" }),
+      headers: { "i-twilio-idempotency-token": "idem-123" },
+    };
+    const ctxB = {
+      ...createContext(rawBody, { callId: "call-1", turnToken: "turn-1" }),
+      headers: { "i-twilio-idempotency-token": "idem-123" },
+    };
+
+    const eventA = provider.parseWebhookEvent(ctxA).events[0];
+    const eventB = provider.parseWebhookEvent(ctxB).events[0];
+
+    expect(eventA).toBeDefined();
+    expect(eventB).toBeDefined();
+    expect(eventA?.id).not.toBe(eventB?.id);
+    expect(eventA?.dedupeKey).toBe("twilio:idempotency:idem-123");
+    expect(eventA?.dedupeKey).toBe(eventB?.dedupeKey);
+  });
+
+  it("keeps turnToken from query on speech events", () => {
+    const provider = createProvider();
+    const ctx = createContext("CallSid=CA222&Direction=inbound&SpeechResult=hello", {
+      callId: "call-2",
+      turnToken: "turn-xyz",
+    });
+
+    const event = provider.parseWebhookEvent(ctx).events[0];
+    expect(event?.type).toBe("call.speech");
+    expect(event?.turnToken).toBe("turn-xyz");
+  });
 });

@@ -20,6 +20,33 @@ import type { VoiceCallProvider } from "./base.js";
 import { twilioApiRequest } from "./twilio/api.js";
 import { verifyTwilioProviderWebhook } from "./twilio/webhook.js";
 
+function getHeader(
+  headers: Record<string, string | string[] | undefined>,
+  name: string,
+): string | undefined {
+  const value = headers[name.toLowerCase()];
+  if (Array.isArray(value)) {
+    return value[0];
+  }
+  return value;
+}
+
+function createTwilioRequestDedupeKey(ctx: WebhookContext): string {
+  const idempotencyToken = getHeader(ctx.headers, "i-twilio-idempotency-token");
+  if (idempotencyToken) {
+    return `twilio:idempotency:${idempotencyToken}`;
+  }
+
+  const signature = getHeader(ctx.headers, "x-twilio-signature") ?? "";
+  const callId = typeof ctx.query?.callId === "string" ? ctx.query.callId.trim() : "";
+  const flow = typeof ctx.query?.flow === "string" ? ctx.query.flow.trim() : "";
+  const turnToken = typeof ctx.query?.turnToken === "string" ? ctx.query.turnToken.trim() : "";
+  return `twilio:fallback:${crypto
+    .createHash("sha256")
+    .update(`${signature}\n${callId}\n${flow}\n${turnToken}\n${ctx.rawBody}`)
+    .digest("hex")}`;
+}
+
 /**
  * Twilio Voice API provider implementation.
  *
@@ -212,7 +239,16 @@ export class TwilioProvider implements VoiceCallProvider {
         typeof ctx.query?.callId === "string" && ctx.query.callId.trim()
           ? ctx.query.callId.trim()
           : undefined;
-      const event = this.normalizeEvent(params, callIdFromQuery);
+      const turnTokenFromQuery =
+        typeof ctx.query?.turnToken === "string" && ctx.query.turnToken.trim()
+          ? ctx.query.turnToken.trim()
+          : undefined;
+      const dedupeKey = createTwilioRequestDedupeKey(ctx);
+      const event = this.normalizeEvent(params, {
+        callIdOverride: callIdFromQuery,
+        dedupeKey,
+        turnToken: turnTokenFromQuery,
+      });
 
       // For Twilio, we must return TwiML. Most actions are driven by Calls API updates,
       // so the webhook response is typically a pause to keep the call alive.
@@ -245,14 +281,24 @@ export class TwilioProvider implements VoiceCallProvider {
   /**
    * Convert Twilio webhook params to normalized event format.
    */
-  private normalizeEvent(params: URLSearchParams, callIdOverride?: string): NormalizedEvent | null {
+  private normalizeEvent(
+    params: URLSearchParams,
+    options?: {
+      callIdOverride?: string;
+      dedupeKey?: string;
+      turnToken?: string;
+    },
+  ): NormalizedEvent | null {
     const callSid = params.get("CallSid") || "";
+    const callIdOverride = options?.callIdOverride;
 
     const baseEvent = {
       id: crypto.randomUUID(),
+      dedupeKey: options?.dedupeKey,
       callId: callIdOverride || callSid,
       providerCallId: callSid,
       timestamp: Date.now(),
+      turnToken: options?.turnToken,
       direction: TwilioProvider.parseDirection(params.get("Direction")),
       from: params.get("From") || undefined,
       to: params.get("To") || undefined,
@@ -603,9 +649,14 @@ export class TwilioProvider implements VoiceCallProvider {
       throw new Error("Missing webhook URL for this call (provider state not initialized)");
     }
 
+    const actionUrl = new URL(webhookUrl);
+    if (input.turnToken) {
+      actionUrl.searchParams.set("turnToken", input.turnToken);
+    }
+
     const twiml = `<?xml version="1.0" encoding="UTF-8"?>
 <Response>
-  <Gather input="speech" speechTimeout="auto" language="${input.language || "en-US"}" action="${escapeXml(webhookUrl)}" method="POST">
+  <Gather input="speech" speechTimeout="auto" language="${input.language || "en-US"}" action="${escapeXml(actionUrl.toString())}" method="POST">
   </Gather>
 </Response>`;
 

@@ -28,5 +28,6 @@ export function verifyTwilioProviderWebhook(params: {
   return {
     ok: result.ok,
     reason: result.reason,
+    isReplay: result.isReplay,
   };
 }

@@ -74,9 +74,13 @@ export type EndReason = z.infer<typeof EndReasonSchema>;
 
 const BaseEventSchema = z.object({
   id: z.string(),
+  // Stable provider-derived key for idempotency/replay dedupe.
+  dedupeKey: z.string().optional(),
   callId: z.string(),
   providerCallId: z.string().optional(),
   timestamp: z.number(),
+  // Optional per-turn nonce for speech events (Twilio <Gather> replay hardening).
+  turnToken: z.string().optional(),
   // Optional fields for inbound call detection
   direction: z.enum(["inbound", "outbound"]).optional(),
   from: z.string().optional(),
@@ -171,6 +175,8 @@ export type CallRecord = z.infer<typeof CallRecordSchema>;
 export type WebhookVerificationResult = {
   ok: boolean;
   reason?: string;
+  /** Signature is valid, but request was seen before within replay window. */
+  isReplay?: boolean;
 };
 
 export type WebhookContext = {
@@ -226,6 +232,8 @@ export type StartListeningInput = {
   callId: CallId;
   providerCallId: ProviderCallId;
   language?: string;
+  /** Optional per-turn nonce for provider callbacks (replay hardening). */
+  turnToken?: string;
 };
 
 export type StopListeningInput = {

@@ -163,6 +163,40 @@ describe("verifyPlivoWebhook", () => {
     expect(result.ok).toBe(false);
     expect(result.reason).toMatch(/Missing Plivo signature headers/);
   });
+
+  it("marks replayed valid V3 requests as replay without failing auth", () => {
+    const authToken = "test-auth-token";
+    const nonce = "nonce-replay-v3";
+    const urlWithQuery = "https://example.com/voice/webhook?flow=answer&callId=abc";
+    const postBody = "CallUUID=uuid&CallStatus=in-progress&From=%2B15550000000";
+    const signature = plivoV3Signature({
+      authToken,
+      urlWithQuery,
+      postBody,
+      nonce,
+    });
+
+    const ctx = {
+      headers: {
+        host: "example.com",
+        "x-forwarded-proto": "https",
+        "x-plivo-signature-v3": signature,
+        "x-plivo-signature-v3-nonce": nonce,
+      },
+      rawBody: postBody,
+      url: urlWithQuery,
+      method: "POST" as const,
+      query: { flow: "answer", callId: "abc" },
+    };
+
+    const first = verifyPlivoWebhook(ctx, authToken);
+    const second = verifyPlivoWebhook(ctx, authToken);
+
+    expect(first.ok).toBe(true);
+    expect(first.isReplay).toBeFalsy();
+    expect(second.ok).toBe(true);
+    expect(second.isReplay).toBe(true);
+  });
 });
 
 describe("verifyTwilioWebhook", () => {
@@ -197,6 +231,48 @@ describe("verifyTwilioWebhook", () => {
     expect(result.ok).toBe(true);
   });
 
+  it("marks replayed valid requests as replay without failing auth", () => {
+    const authToken = "test-auth-token";
+    const publicUrl = "https://example.com/voice/webhook";
+    const urlWithQuery = `${publicUrl}?callId=abc`;
+    const postBody = "CallSid=CS777&CallStatus=completed&From=%2B15550000000";
+    const signature = twilioSignature({ authToken, url: urlWithQuery, postBody });
+    const headers = {
+      host: "example.com",
+      "x-forwarded-proto": "https",
+      "x-twilio-signature": signature,
+      "i-twilio-idempotency-token": "idem-replay-1",
+    };
+
+    const first = verifyTwilioWebhook(
+      {
+        headers,
+        rawBody: postBody,
+        url: "http://local/voice/webhook?callId=abc",
+        method: "POST",
+        query: { callId: "abc" },
+      },
+      authToken,
+      { publicUrl },
+    );
+    const second = verifyTwilioWebhook(
+      {
+        headers,
+        rawBody: postBody,
+        url: "http://local/voice/webhook?callId=abc",
+        method: "POST",
+        query: { callId: "abc" },
+      },
+      authToken,
+      { publicUrl },
+    );
+
+    expect(first.ok).toBe(true);
+    expect(first.isReplay).toBeFalsy();
+    expect(second.ok).toBe(true);
+    expect(second.isReplay).toBe(true);
+  });
+
   it("rejects invalid signatures even when attacker injects forwarded host", () => {
     const authToken = "test-auth-token";
     const postBody = "CallSid=CS123&CallStatus=completed&From=%2B15550000000";

@@ -1,6 +1,63 @@
 import crypto from "node:crypto";
 import type { WebhookContext } from "./types.js";
 
+const REPLAY_WINDOW_MS = 10 * 60 * 1000;
+const REPLAY_CACHE_MAX_ENTRIES = 10_000;
+const REPLAY_CACHE_PRUNE_INTERVAL = 64;
+
+type ReplayCache = {
+  seenUntil: Map<string, number>;
+  calls: number;
+};
+
+const twilioReplayCache: ReplayCache = {
+  seenUntil: new Map<string, number>(),
+  calls: 0,
+};
+
+const plivoReplayCache: ReplayCache = {
+  seenUntil: new Map<string, number>(),
+  calls: 0,
+};
+
+function sha256Hex(input: string): string {
+  return crypto.createHash("sha256").update(input).digest("hex");
+}
+
+function pruneReplayCache(cache: ReplayCache, now: number): void {
+  for (const [key, expiresAt] of cache.seenUntil) {
+    if (expiresAt <= now) {
+      cache.seenUntil.delete(key);
+    }
+  }
+  while (cache.seenUntil.size > REPLAY_CACHE_MAX_ENTRIES) {
+    const oldest = cache.seenUntil.keys().next().value;
+    if (!oldest) {
+      break;
+    }
+    cache.seenUntil.delete(oldest);
+  }
+}
+
+function markReplay(cache: ReplayCache, replayKey: string): boolean {
+  const now = Date.now();
+  cache.calls += 1;
+  if (cache.calls % REPLAY_CACHE_PRUNE_INTERVAL === 0) {
+    pruneReplayCache(cache, now);
+  }
+
+  const existing = cache.seenUntil.get(replayKey);
+  if (existing && existing > now) {
+    return true;
+  }
+
+  cache.seenUntil.set(replayKey, now + REPLAY_WINDOW_MS);
+  if (cache.seenUntil.size > REPLAY_CACHE_MAX_ENTRIES) {
+    pruneReplayCache(cache, now);
+  }
+  return false;
+}
+
 /**
  * Validate Twilio webhook signature using HMAC-SHA1.
  *
@@ -328,13 +385,29 @@ export interface TwilioVerificationResult {
   verificationUrl?: string;
   /** Whether we're running behind ngrok free tier */
   isNgrokFreeTier?: boolean;
+  /** Request is cryptographically valid but was already processed recently. */
+  isReplay?: boolean;
 }
 
 export interface TelnyxVerificationResult {
   ok: boolean;
   reason?: string;
 }
 
+function createTwilioReplayKey(params: {
+  ctx: WebhookContext;
+  signature: string;
+  verificationUrl: string;
+}): string {
+  const idempotencyToken = getHeader(params.ctx.headers, "i-twilio-idempotency-token");
+  if (idempotencyToken) {
+    return `twilio:idempotency:${idempotencyToken}`;
+  }
+  return `twilio:fallback:${sha256Hex(
+    `${params.verificationUrl}\n${params.signature}\n${params.ctx.rawBody}`,
+  )}`;
+}
+
 function decodeBase64OrBase64Url(input: string): Buffer {
   // Telnyx docs say Base64; some tooling emits Base64URL. Accept both.
   const normalized = input.replace(/-/g, "+").replace(/_/g, "/");
@@ -505,7 +578,9 @@ export function verifyTwilioWebhook(
   const isValid = validateTwilioSignature(authToken, signature, verificationUrl, params);
 
   if (isValid) {
-    return { ok: true, verificationUrl };
+    const replayKey = createTwilioReplayKey({ ctx, signature, verificationUrl });
+    const isReplay = markReplay(twilioReplayCache, replayKey);
+    return { ok: true, verificationUrl, isReplay };
   }
 
   // Check if this is ngrok free tier - the URL might have different format
@@ -533,6 +608,8 @@ export interface PlivoVerificationResult {
   verificationUrl?: string;
   /** Signature version used for verification */
   version?: "v3" | "v2";
+  /** Request is cryptographically valid but was already processed recently. */
+  isReplay?: boolean;
 }
 
 function normalizeSignatureBase64(input: string): string {
@@ -753,14 +830,17 @@ export function verifyPlivoWebhook(
       url: verificationUrl,
       postParams,
     });
-    return ok
-      ? { ok: true, version: "v3", verificationUrl }
-      : {
-          ok: false,
-          version: "v3",
-          verificationUrl,
-          reason: "Invalid Plivo V3 signature",
-        };
+    if (!ok) {
+      return {
+        ok: false,
+        version: "v3",
+        verificationUrl,
+        reason: "Invalid Plivo V3 signature",
+      };
+    }
+    const replayKey = `plivo:v3:${sha256Hex(`${verificationUrl}\n${nonceV3}`)}`;
+    const isReplay = markReplay(plivoReplayCache, replayKey);
+    return { ok: true, version: "v3", verificationUrl, isReplay };
   }
 
   if (signatureV2 && nonceV2) {
@@ -770,14 +850,17 @@ export function verifyPlivoWebhook(
       nonce: nonceV2,
       url: verificationUrl,
     });
-    return ok
-      ? { ok: true, version: "v2", verificationUrl }
-      : {
-          ok: false,
-          version: "v2",
-          verificationUrl,
-          reason: "Invalid Plivo V2 signature",
-        };
+    if (!ok) {
+      return {
+        ok: false,
+        version: "v2",
+        verificationUrl,
+        reason: "Invalid Plivo V2 signature",
+      };
+    }
+    const replayKey = `plivo:v2:${sha256Hex(`${verificationUrl}\n${nonceV2}`)}`;
+    const isReplay = markReplay(plivoReplayCache, replayKey);
+    return { ok: true, version: "v2", verificationUrl, isReplay };
   }
 
   return {

@@ -45,12 +45,14 @@ const createCall = (startedAt: number): CallRecord => ({
 
 const createManager = (calls: CallRecord[]) => {
   const endCall = vi.fn(async () => ({ success: true }));
+  const processEvent = vi.fn();
   const manager = {
     getActiveCalls: () => calls,
     endCall,
+    processEvent,
   } as unknown as CallManager;
 
-  return { manager, endCall };
+  return { manager, endCall, processEvent };
 };
 
 describe("VoiceCallWebhookServer stale call reaper", () => {
@@ -116,3 +118,51 @@ describe("VoiceCallWebhookServer stale call reaper", () => {
     }
   });
 });
+
+describe("VoiceCallWebhookServer replay handling", () => {
+  it("acknowledges replayed webhook requests and skips event side effects", async () => {
+    const replayProvider: VoiceCallProvider = {
+      ...provider,
+      verifyWebhook: () => ({ ok: true, isReplay: true }),
+      parseWebhookEvent: () => ({
+        events: [
+          {
+            id: "evt-replay",
+            dedupeKey: "stable-replay",
+            type: "call.speech",
+            callId: "call-1",
+            providerCallId: "provider-call-1",
+            timestamp: Date.now(),
+            transcript: "hello",
+            isFinal: true,
+          },
+        ],
+        statusCode: 200,
+      }),
+    };
+    const { manager, processEvent } = createManager([]);
+    const config = createConfig({ serve: { port: 0, bind: "127.0.0.1", path: "/voice/webhook" } });
+    const server = new VoiceCallWebhookServer(config, manager, replayProvider);
+
+    try {
+      const baseUrl = await server.start();
+      const address = (
+        server as unknown as { server?: { address?: () => unknown } }
+      ).server?.address?.();
+      const requestUrl = new URL(baseUrl);
+      if (address && typeof address === "object" && "port" in address && address.port) {
+        requestUrl.port = String(address.port);
+      }
+      const response = await fetch(requestUrl.toString(), {
+        method: "POST",
+        headers: { "content-type": "application/x-www-form-urlencoded" },
+        body: "CallSid=CA123&SpeechResult=hello",
+      });
+
+      expect(response.status).toBe(200);
+      expect(processEvent).not.toHaveBeenCalled();
+    } finally {
+      await server.stop();
+    }
+  });
+});

@@ -346,11 +346,15 @@ export class VoiceCallWebhookServer {
     const result = this.provider.parseWebhookEvent(ctx);
 
     // Process each event
-    for (const event of result.events) {
-      try {
-        this.manager.processEvent(event);
-      } catch (err) {
-        console.error(`[voice-call] Error processing event ${event.type}:`, err);
+    if (verification.isReplay) {
+      console.warn("[voice-call] Replay detected; skipping event side effects");
+    } else {
+      for (const event of result.events) {
+        try {
+          this.manager.processEvent(event);
+        } catch (err) {
+          console.error(`[voice-call] Error processing event ${event.type}:`, err);
+        }
       }
     }