diff --git a/src/auto-reply/get-reply-options.types.ts b/src/auto-reply/get-reply-options.types.ts
index e2868521f9f..8f907a6b666 100644
--- a/src/auto-reply/get-reply-options.types.ts
+++ b/src/auto-reply/get-reply-options.types.ts
@@ -49,6 +49,8 @@ export type PartialReplyPayload = Pick<ReplyPayload, "text" | "mediaUrls"> & {
 export type GetReplyOptions = {
   /** Override run id for agent events (defaults to random UUID). */
   runId?: string;
+  /** Stable provider prompt-cache affinity key; distinct from run id/idempotency. */
+  promptCacheKey?: string;
   /** Abort signal for the underlying agent run. */
   abortSignal?: AbortSignal;
   /** Optional inbound images (used for webchat attachments). */
diff --git a/src/auto-reply/reply/agent-runner-execution.ts b/src/auto-reply/reply/agent-runner-execution.ts
index 7d13bf3693b..37276f700d2 100644
--- a/src/auto-reply/reply/agent-runner-execution.ts
+++ b/src/auto-reply/reply/agent-runner-execution.ts
@@ -2231,6 +2231,7 @@ export async function runAgentTurnWithFallback(params: {
                 hasRepliedRef: params.opts?.hasRepliedRef,
                 provider,
                 runId,
+                promptCacheKey: params.opts?.promptCacheKey,
                 allowTransientCooldownProbe: runOptions?.allowTransientCooldownProbe,
                 model,
               });
diff --git a/src/auto-reply/reply/agent-runner-run-params.ts b/src/auto-reply/reply/agent-runner-run-params.ts
index f691e11d285..e38ae319bcb 100644
--- a/src/auto-reply/reply/agent-runner-run-params.ts
+++ b/src/auto-reply/reply/agent-runner-run-params.ts
@@ -55,6 +55,7 @@ export function buildEmbeddedRunBaseParams(params: {
   provider: string;
   model: string;
   runId: string;
+  promptCacheKey?: string;
   authProfile: ReturnType<typeof resolveProviderScopedAuthProfile>;
   allowTransientCooldownProbe?: boolean;
   isReasoningTagProvider?: ReasoningTagProviderResolver;
@@ -99,6 +100,7 @@ export function buildEmbeddedRunBaseParams(params: {
     bashElevated: params.run.bashElevated,
     timeoutMs: params.run.timeoutMs,
     runId: params.runId,
+    promptCacheKey: params.promptCacheKey,
     allowTransientCooldownProbe: params.allowTransientCooldownProbe,
   };
 }
diff --git a/src/auto-reply/reply/agent-runner-utils.test.ts b/src/auto-reply/reply/agent-runner-utils.test.ts
index 9847f21dd25..53eedd979d7 100644
--- a/src/auto-reply/reply/agent-runner-utils.test.ts
+++ b/src/auto-reply/reply/agent-runner-utils.test.ts
@@ -25,6 +25,7 @@ const {
   buildThreadingToolContext,
   buildEmbeddedRunBaseParams,
   buildEmbeddedRunContexts,
+  buildEmbeddedRunExecutionParams,
   resolveModelFallbackOptions,
   resolveEnforceFinalTag,
   resolveProviderScopedAuthProfile,
@@ -138,6 +139,7 @@ describe("agent-runner-utils", () => {
       provider: "openai",
       model: "gpt-4.1-mini",
       runId: "run-1",
+      promptCacheKey: "webchat-cache-key",
       authProfile,
     });
 
@@ -160,6 +162,24 @@ describe("agent-runner-utils", () => {
     expect(resolved.bashElevated).toBe(run.bashElevated);
     expect(resolved.timeoutMs).toBe(run.timeoutMs);
     expect(resolved.runId).toBe("run-1");
+    expect(resolved.promptCacheKey).toBe("webchat-cache-key");
+  });
+
+  it("threads prompt cache affinity through embedded execution params", () => {
+    const run = makeRun();
+
+    const resolved = buildEmbeddedRunExecutionParams({
+      run,
+      sessionCtx: { Provider: "webchat" },
+      hasRepliedRef: undefined,
+      provider: "openai",
+      model: "gpt-4.1-mini",
+      runId: "run-1",
+      promptCacheKey: "stable-session-cache-key",
+    });
+
+    expect(resolved.runBaseParams.runId).toBe("run-1");
+    expect(resolved.runBaseParams.promptCacheKey).toBe("stable-session-cache-key");
   });
 
   it("passes through recovered auto fallback provenance for embedded run params", () => {
diff --git a/src/auto-reply/reply/agent-runner-utils.ts b/src/auto-reply/reply/agent-runner-utils.ts
index bf28684ac80..572c214468f 100644
--- a/src/auto-reply/reply/agent-runner-utils.ts
+++ b/src/auto-reply/reply/agent-runner-utils.ts
@@ -273,6 +273,7 @@ export function buildEmbeddedRunExecutionParams(params: {
   provider: string;
   model: string;
   runId: string;
+  promptCacheKey?: string;
   allowTransientCooldownProbe?: boolean;
 }) {
   const { authProfile, embeddedContext, senderContext } = buildEmbeddedRunContexts(params);
@@ -281,6 +282,7 @@ export function buildEmbeddedRunExecutionParams(params: {
     provider: params.provider,
     model: params.model,
     runId: params.runId,
+    promptCacheKey: params.promptCacheKey,
     authProfile,
     allowTransientCooldownProbe: params.allowTransientCooldownProbe,
   });
diff --git a/src/gateway/server-methods/chat.ts b/src/gateway/server-methods/chat.ts
index a886176b922..52144eb6aec 100644
--- a/src/gateway/server-methods/chat.ts
+++ b/src/gateway/server-methods/chat.ts
@@ -333,6 +333,28 @@ function buildMediaOnlyTtsSupplementTranscriptMarker(
   return buildTtsSupplementTranscriptMarker(payload);
 }
 
+function resolveWebchatPromptCacheKey(params: {
+  agentId: string;
+  model: string;
+  provider: string;
+  sessionKey: string;
+}): string {
+  const digest = createHash("sha256")
+    .update(
+      [
+        "v1",
+        params.provider.trim().toLowerCase(),
+        params.model.trim(),
+        normalizeAgentId(params.agentId),
+        params.sessionKey,
+      ].join("\0"),
+      "utf8",
+    )
+    .digest("hex")
+    .slice(0, 32);
+  return `openclaw-webchat-${digest}`;
+}
+
 async function buildWebchatAssistantMediaMessage(
   payloads: ReplyPayload[],
   options?: {
@@ -3633,6 +3655,16 @@ export const chatHandlers: GatewayRequestHandlers = {
             dispatcher,
             replyOptions: {
               runId: clientRunId,
+              ...(isOperatorUiClient(clientInfo)
+                ? {
+                    promptCacheKey: resolveWebchatPromptCacheKey({
+                      agentId,
+                      provider: resolvedSessionModel.provider,
+                      model: resolvedSessionModel.model,
+                      sessionKey: activeRunScopeKey,
+                    }),
+                  }
+                : {}),
               abortSignal: activeRunAbort.controller.signal,
               images: replyOptionImages,
               imageOrder: imageOrder.length > 0 ? imageOrder : undefined,
diff --git a/src/gateway/server.chat.gateway-server-chat-b.test.ts b/src/gateway/server.chat.gateway-server-chat-b.test.ts
index 93a1251df7a..eca4569c158 100644
--- a/src/gateway/server.chat.gateway-server-chat-b.test.ts
+++ b/src/gateway/server.chat.gateway-server-chat-b.test.ts
@@ -1004,6 +1004,17 @@ describe("gateway server chat", () => {
         },
       ]);
       expect(dispatchInboundMessageMock).toHaveBeenCalledTimes(2);
+      const dispatchOptions = dispatchInboundMessageMock.mock.calls.map(([params]) => {
+        return (params as { replyOptions?: GetReplyOptions }).replyOptions;
+      });
+      expect(dispatchOptions[0]?.runId).toBe("idem-sequential-a");
+      expect(dispatchOptions[1]?.runId).toBe("idem-sequential-b");
+      expect(dispatchOptions[0]?.promptCacheKey).toEqual(
+        expect.stringMatching(/^openclaw-webchat-[a-f0-9]{32}$/u),
+      );
+      expect(dispatchOptions[1]?.promptCacheKey).toBe(dispatchOptions[0]?.promptCacheKey);
+      expect(dispatchOptions[0]?.promptCacheKey).not.toContain("main");
+      expect(dispatchOptions[0]?.promptCacheKey).not.toContain("sess-main");
       expect(context.addChatRun).toHaveBeenCalledTimes(2);
     } finally {
       dispatchInboundMessageMock.mockReset();