diff --git a/docs-site/src/content/docs/reference/proxy-formats.md b/docs-site/src/content/docs/reference/proxy-formats.md
index eba53cf8201..82b9208f78a 100644
--- a/docs-site/src/content/docs/reference/proxy-formats.md
+++ b/docs-site/src/content/docs/reference/proxy-formats.md
@@ -50,6 +50,11 @@ snapshot omits it. Auto/none, incomplete results and unknown contracts retain th
 
 ## Required tools on translated Chat routes
 
+Before sending, the proxy rejects a mandatory tool choice that the selected Chat destination cannot
+represent, such as `computer_use` or a required allowed-tool list containing MCP selections. A direct
+route returns HTTP 400 without an upstream request. A routing profile can continue to another
+originally eligible destination under its configured fallback policy, preserving the original choice.
+
 A translated OpenAI Chat response must contain a complete selected client tool call when
 `tool_choice` is required, names a tool, or uses a required allowed-tool selection. If the upstream
 finishes normally without one, the turn fails with `required_tool_not_called` and preserves usage.
diff --git a/scripts/test-layout/layout-responses.json b/scripts/test-layout/layout-responses.json
index 60f1e7075ea..df758d8a344 100644
--- a/scripts/test-layout/layout-responses.json
+++ b/scripts/test-layout/layout-responses.json
@@ -1,4 +1,5 @@
 {
+  "responses-direct-chat-tool-choice.test.ts": "responses",
   "responses-native-output-identity.test.ts": "responses",
   "graceful-max-output-tokens.test.ts": "responses",
   "graceful-max-output-tokens-exclusions.test.ts": "responses",
diff --git a/src/server/responses/combo-required-tool-contract.ts b/src/server/responses/combo-required-tool-contract.ts
index a5177b364d8..60e9f1b7091 100644
--- a/src/server/responses/combo-required-tool-contract.ts
+++ b/src/server/responses/combo-required-tool-contract.ts
@@ -45,7 +45,14 @@ export function hasMandatoryToolChoice(rawBody: unknown): boolean {
 export function chatCanPreserveMandatoryToolChoice(rawBody: unknown, provider: OcxProviderConfig, modelId: string): boolean {
   if (!hasMandatoryToolChoice(rawBody)) return true;
   if (modelInList(provider.autoToolChoiceOnlyModels, modelId)) return false;
-  const mapped = mapToolChoice((rawBody as Record<string, unknown>).tool_choice);
+  const choice = (rawBody as Record<string, unknown>).tool_choice;
+  if (choice && typeof choice === "object" && !Array.isArray(choice)) {
+    const selector = choice as Record<string, unknown>;
+    // Chat function names cannot preserve the selected MCP server's ownership.
+    if (selector.type === "allowed_tools" && Array.isArray(selector.tools)
+      && selector.tools.some(tool => tool && typeof tool === "object" && tool.type === "mcp")) return false;
+  }
+  const mapped = mapToolChoice(choice);
   return mapped === "required" || (!!mapped && typeof mapped === "object");
 }
 
diff --git a/src/server/responses/core-lifetime.ts b/src/server/responses/core-lifetime.ts
index 06e27a40c7b..14a24d51ee2 100644
--- a/src/server/responses/core-lifetime.ts
+++ b/src/server/responses/core-lifetime.ts
@@ -1,6 +1,7 @@
 import { carryProxyError } from "../../lib/client-error-body";
 import type { TranslatorBudget } from "../../lib/translator-budget";
 import { carryResponseToolOwnership } from "./response-tool-ownership";
+import { carryPolicyCandidateRefusal } from "./policy-request-scope";
 import { finalizeTranslatorBudgetResponse } from "../../lib/translator-budget";
 import { carryReplayRefusal, isNonReplayableResponse, markResponseNonReplayable } from "../../lib/upstream-retry";
 import { isVerifiedModelNotInPlanResponse, markVerifiedModelNotInPlanResponse } from "../../lib/errors";
@@ -57,6 +58,7 @@ export function finalizeOwnedTranslatorBudget(response: Response, budget: Transl
   if (isVerifiedModelNotInPlanResponse(response)) markVerifiedModelNotInPlanResponse(finalizedResponse);
   if (runTurnAdapterSseResponses.has(response)) runTurnAdapterSseResponses.add(finalizedResponse);
   carryCodexAccountCooldownResponse(response, finalizedResponse);
+  carryPolicyCandidateRefusal(response, finalizedResponse);
   return carryResponseToolOwnership(response, carryFormattedQuotaSource(response, finalizedResponse));
 }
 
@@ -86,6 +88,7 @@ export function finalizeAccountLease(response: Response, release: () => void, on
   if (runTurnAdapterSseResponses.has(response)) runTurnAdapterSseResponses.add(wrapped);
   // Retained compaction delivers through this lease wrapper; keep a local cooldown refusal local (#7850).
   carryCodexAccountCooldownResponse(response, wrapped);
+  carryPolicyCandidateRefusal(response, wrapped);
   return carryResponseToolOwnership(response, carryReplayRefusal(response, carryFormattedQuotaSource(response, wrapped)));
 }
 
diff --git a/src/server/responses/policy-request-scope.ts b/src/server/responses/policy-request-scope.ts
index 3504fcd7419..d954bd7c374 100644
--- a/src/server/responses/policy-request-scope.ts
+++ b/src/server/responses/policy-request-scope.ts
@@ -41,6 +41,12 @@ export function isPolicyCandidateRefusal(response: Response): boolean {
   return localRefusals.has(response);
 }
 
+/** Preserve local refusal authority when response lifetime owners wrap the body. */
+export function carryPolicyCandidateRefusal(source: Response, target: Response): Response {
+  if (localRefusals.has(source)) localRefusals.add(target);
+  return target;
+}
+
 export function capturePolicyRequestRoute(scope: PolicyRequestScope, route: RouteResult): RouteResult {
   if (!scope.eligibility && route.policyEligibility && route.routeDecision?.routeKind === "policy") {
     scope.eligibility = new Set(route.policyEligibility);
diff --git a/src/server/responses/request-prepare.ts b/src/server/responses/request-prepare.ts
index 9d373415c04..1027249cc78 100644
--- a/src/server/responses/request-prepare.ts
+++ b/src/server/responses/request-prepare.ts
@@ -8,7 +8,7 @@ import type { PolicyRequestEvidence } from "../../routing/evaluator";
 import type { RequestLogContext } from "../request-log";
 import { markProtocolEntry, protocolMarkFeatures } from "../../protocols/trace";
 import { isAntigravityRoute } from "../antigravity-direct-eligibility";
-import { capturePolicyRequestRoute, assertPolicyNormalizedRoute, recordPolicyPreparedDestination, policyCandidateRefusalResponse } from "./policy-request-scope";
+import { capturePolicyRequestRoute, assertPolicyNormalizedRoute, recordPolicyPreparedDestination, policyCandidateRefusalResponse, PolicyCandidateUnavailableError } from "./policy-request-scope";
 import {
   agentTaskRecoveryConfig,
   restoreCachedEncryptedAgentTasks,
@@ -74,6 +74,7 @@ import {
   previousResponseProviderState,
 } from "../../responses/state";
 import { formatErrorResponse } from "../../bridge";
+import { chatCanPreserveMandatoryToolChoice } from "./combo-required-tool-contract";
 import { mapCodexAuthContextErrorToResponse } from "./codex-auth-error";
 import type { OcxParsedRequest } from "../../types";
 import { buildToolBridgeMaps } from "./collaboration";
@@ -1459,6 +1460,12 @@ export async function prepareResponsesRequest(
   });
   try {
     assertPolicyNormalizedRoute(policyScope, admittedPolicyRoute, route);
+    if (route.provider.adapter === "openai-chat"
+      && !chatCanPreserveMandatoryToolChoice(parsed._rawBody, route.provider, route.modelId)) {
+      if (policyScope.eligibility) throw new PolicyCandidateUnavailableError();
+      return formatErrorResponse(400, "invalid_request_error",
+        "This mandatory tool_choice cannot be preserved for a translated Chat destination.");
+    }
   } catch (error) {
     const policyRefusal = policyCandidateRefusalResponse(error);
     if (policyRefusal) return policyRefusal;
diff --git a/structure/transports/policy-fallback.md b/structure/transports/policy-fallback.md
index 196d0c90a96..9ee70b16e76 100644
--- a/structure/transports/policy-fallback.md
+++ b/structure/transports/policy-fallback.md
@@ -16,6 +16,11 @@ When a main-account hard-lock fallback selects a profile, `src/server/responses/
 checks its first ranked destination against the original model under the
 [fallback policy](../fallback-policy.md#sites), before authentication or upstream dispatch.
 
+After final normalization, preparation refuses a Chat destination that cannot preserve a mandatory
+tool choice using `PolicyCandidateUnavailableError`. The local response identity allows traversal
+to a compatible originally eligible candidate without sending to the incompatible destination.
+Ordinary direct routes keep the client-facing 400; upstream errors do not acquire this identity.
+
 Normal virtual-model wire mapping remains attached to its admitted selector and does not add a new
 eligible model. Policy traversal records settled physical destinations and skips a destination already
 attempted through another candidate or redirect; same-target credential and transient retries retain
@@ -27,6 +32,8 @@ The initial decision remains the selection evidence while physical attempts reco
 Cancellation, non-replayable responses and committed output keep the
 [shared replay boundaries](responses-failover.md). A local policy refusal is identified by its
 in-process response identity; matching upstream prose or error codes cannot authorize another attempt.
+The translator-budget and account-lease wrappers in `src/server/responses/core-lifetime.ts` carry
+that identity from their source response without deriving it from serialized error contents.
 Formatted quota errors retain [their upstream-code provenance](responses-combo-boundaries.md#upstream-account-quota-exhaustion-failover)
 through lifecycle wrappers; a reflected quota message on a 400/422 response does not authorize a
 candidate hop merely because formatting serialized it as `insufficient_quota` or its text mentions a free-tier single-request cap.
@@ -35,3 +42,7 @@ Coverage: `tests/routing/routing-policy-dispatch-scope.test.ts` exercises real r
 normalization and configured subagent selection with synthetic dispatch. It also covers recovery,
 alias collisions, redirects, bounded traces and virtual models. `tests/routing/routing-policy-fallback.test.ts`
 pins request snapshots, terminal responses, cancellation, credential handoff and attempt accounting.
+`tests/responses/responses-direct-chat-tool-choice.test.ts` exercises the real endpoint's pre-send
+Chat refusal and preserved selector on a compatible Responses fallback.
+`tests/adapters/translator-budget-lifetime.test.ts` distinguishes a local refusal from identical
+upstream wire bytes through both lifetime wrappers.
diff --git a/structure/transports/responses-combo-boundaries.md b/structure/transports/responses-combo-boundaries.md
index d333e07a5c1..80f48158957 100644
--- a/structure/transports/responses-combo-boundaries.md
+++ b/structure/transports/responses-combo-boundaries.md
@@ -39,6 +39,13 @@ tool-search lowering, still applies after parsing. Chat translation maps a direc
 choice to its existing synthetic function name. MCP null names also parse inside required allowed-tools lists; function/custom null names remain invalid. A Chat destination is skipped before send when translation would degrade a mandatory choice to auto/none, including the resolved model's auto-only tool-choice policy; exhaustion reports the tool-choice constraint. Eligibility resolves synthetic Fast/effort selectors before model policy, matching child request preparation while preserving exact configured IDs.
 Registered OpenAI API virtual models resolve to their wire model before adapter and auto-only policy checks, using the same registry resolver as child normalization. Other providers and unregistered IDs remain literal. Coverage: `tests/server/server-combo-required-tool-contract.test.ts`.
 
+Required allowed-tools lists containing MCP entries also exclude Chat: a function name cannot retain
+the MCP server's ownership, even when a client function has the same name. The shared predicate in
+`src/server/responses/combo-required-tool-contract.ts` also guards direct Chat destinations after final
+normalization in `src/server/responses/request-prepare.ts`. An incompatible direct route returns 400
+before dispatch; an authorized policy candidate uses the [local policy refusal](policy-fallback.md)
+so another originally eligible destination can serve the unchanged choice.
+
 Before combo success bookkeeping, native delivery in `src/server/responses/passthrough-delivery.ts`
 uses `src/server/responses/combo-required-tool-contract.ts` to check successful native
 `openai-responses` JSON children with explicit required or named function/custom choices.
diff --git a/tests/adapters/translator-budget-lifetime.test.ts b/tests/adapters/translator-budget-lifetime.test.ts
index dc6de8e7c53..910f56cb097 100644
--- a/tests/adapters/translator-budget-lifetime.test.ts
+++ b/tests/adapters/translator-budget-lifetime.test.ts
@@ -7,6 +7,9 @@ import {
   finalizeAccountLease, finalizeOwnedTranslatorBudget, runTurnAdapterSseResponses,
 } from "../../src/server/responses/core-lifetime";
 import { isNonReplayableResponse, markResponseNonReplayable } from "../../src/lib/upstream-retry";
+import {
+  isPolicyCandidateRefusal, PolicyCandidateUnavailableError, policyCandidateRefusalResponse,
+} from "../../src/server/responses/policy-request-scope";
 import {
   markNativePassthroughSseResponse, isNativePassthroughSseResponse,
   markEagerRelaySseResponse, isEagerRelaySseResponse,
@@ -176,6 +179,20 @@ for (const outerLease of [false, true]) {
   }
 }
 
+test.each([true, false])("policy refusal authority survives lifetime wrappers only for a local response (%s)", async local => {
+  const refusal = policyCandidateRefusalResponse(new PolicyCandidateUnavailableError())!;
+  // An upstream can reproduce every wire byte, but cannot mint the local response identity.
+  const source = local ? refusal : new Response(await refusal.text(), { status: refusal.status, headers: refusal.headers });
+  let released = 0;
+  const budget = createTranslatorBudget();
+  const wrapped = finalizeAccountLease(finalizeOwnedTranslatorBudget(source, budget), () => { released++; });
+  expect(isPolicyCandidateRefusal(wrapped)).toBe(local);
+  expect(wrapped.status).toBe(409);
+  expect((await wrapped.json()).error.code).toBe("policy_candidate_unavailable");
+  expect(released).toBe(1);
+  expect(budget.snapshot().currentBytes).toBe(0);
+});
+
 for (const surface of ["responses", "chat-completions", "claude-messages"] as const) {
   test(`${surface}: HTTP owner links request abort to its unread response budget`, async () => {
     const handle = surface === "responses"
diff --git a/tests/fixtures/test-layout-expected.json b/tests/fixtures/test-layout-expected.json
index 37927c69741..b695f16a86a 100644
--- a/tests/fixtures/test-layout-expected.json
+++ b/tests/fixtures/test-layout-expected.json
@@ -305,7 +305,8 @@
   "openai-chat-tool-call-id-remint.test.ts": "adapters/openai", "coding-agent-json-lines-scaling.test.ts": "providers",
   "usage-snapshot-digest-reuse.test.ts": "usage", "worker-embed.test.ts": "lib",
   "worker-embed-coverage.test.ts": "lib", "jev-stats.test.ts": "usage",
-  "responses-devin-401-replay.test.ts": "responses", "responses-oauth-refresh-capacity.test.ts": "responses", "responses-grok-devin-preflight.test.ts": "responses",
+  "responses-devin-401-replay.test.ts": "responses",
+  "responses-direct-chat-tool-choice.test.ts": "responses", "responses-oauth-refresh-capacity.test.ts": "responses", "responses-grok-devin-preflight.test.ts": "responses",
   "abort-idle-deadline.test.ts": "lib", "tool-envelope-echo-whole-line.test.ts": "adapters",
   "abort-race.test.ts": "adapters", "account-import.test.ts": "server",
   "account-import-project-fallback.test.ts": "server",
diff --git a/tests/responses/responses-direct-chat-tool-choice.test.ts b/tests/responses/responses-direct-chat-tool-choice.test.ts
new file mode 100644
index 00000000000..8da303ddb03
--- /dev/null
+++ b/tests/responses/responses-direct-chat-tool-choice.test.ts
@@ -0,0 +1,107 @@
+import { afterEach, beforeEach, expect, test } from "bun:test";
+import { clearResponseStateForTests, flushResponseState } from "../../src/responses/state";
+import { closeRequestHistoryIndex } from "../../src/routing/history/indexer";
+import { handleResponses } from "../../src/server/responses";
+import type { RequestLogContext } from "../../src/server/request-log";
+import type { OcxConfig } from "../../src/types";
+import { acquireOwnedSpendHome } from "../helpers/owned-spend-home";
+import { createTempHome } from "../helpers/temp-home";
+
+const clientTool = { type: "function", name: "shell", parameters: { type: "object", properties: {} } };
+const mcpTool = { type: "mcp", server_label: "remote", server_url: "https://mcp.fixture.invalid/mcp" };
+const mcpChoice = { type: "allowed_tools", mode: "required", tools: [{ type: "mcp", server_label: "remote", name: "shell" }] };
+const originalFetch = globalThis.fetch;
+let home: ReturnType<typeof createTempHome>;
+let releaseHome: () => void;
+
+beforeEach(() => {
+  home = createTempHome("ocx-direct-chat-choice-");
+  releaseHome = acquireOwnedSpendHome();
+  clearResponseStateForTests();
+  globalThis.fetch = (() => { throw new Error("unexpected network access in tool-choice fixture"); }) as typeof fetch;
+});
+
+afterEach(async () => {
+  globalThis.fetch = originalFetch;
+  await flushResponseState();
+  clearResponseStateForTests();
+  closeRequestHistoryIndex();
+  releaseHome();
+  home.remove();
+});
+
+function fixture() {
+  const sent: Array<{ provider: string; body: Record<string, unknown> }> = [];
+  const send = (provider: string) => (async (_input, init) => {
+    sent.push({ provider, body: JSON.parse(String(init?.body)) });
+    return Response.json(provider === "chat" ? {
+      id: "chat_fixture", object: "chat.completion", model: "model",
+      choices: [{ index: 0, finish_reason: "tool_calls", message: { role: "assistant", content: null,
+        tool_calls: [{ id: "call_fixture", type: "function", function: { name: "shell", arguments: "{}" } }],
+      } }],
+      usage: { prompt_tokens: 2, completion_tokens: 1, total_tokens: 3 },
+    } : {
+      id: "resp_fixture", object: "response", status: "completed", model: "model",
+      output: [{ id: "computer_fixture", type: "computer_call", status: "completed" }],
+      usage: { input_tokens: 2, output_tokens: 1, total_tokens: 3 },
+    });
+  }) as typeof fetch;
+  const config: OcxConfig = {
+    port: 0, defaultProvider: "chat", multiAgentGuidanceEnabled: false,
+    providers: {
+      chat: { adapter: "openai-chat", baseUrl: "https://api.openai.com/v1", authMode: "key",
+        apiKey: "fixture-key", models: ["model"], liveModels: false, fetch: send("chat") },
+      responses: { adapter: "openai-responses", baseUrl: "https://responses.fixture.invalid/v1", authMode: "key",
+        apiKey: "fixture-key", models: ["model"], liveModels: false, fetch: send("responses") },
+    },
+    routingProfiles: { tools: { candidates: [{ provider: "chat", model: "model" }, { provider: "responses", model: "model" }] } },
+  };
+  return { config, sent };
+}
+
+function request(toolChoice: unknown, tools: unknown[], model = "chat/model"): Request {
+  return new Request("http://localhost/v1/responses", {
+    method: "POST",
+    headers: { "content-type": "application/json" },
+    body: JSON.stringify({ model, input: "Use the selected tool.", stream: false, store: false, tools, tool_choice: toolChoice }),
+  });
+}
+
+test("direct translated Chat rejects an unrepresentable computer_use choice before send", async () => {
+  const { config, sent } = fixture();
+  const response = await handleResponses(request({ type: "computer_use" }, [{ type: "computer_use" }, clientTool]), config, {});
+  expect(response.status).toBe(400);
+  expect(await response.text()).toContain("mandatory tool_choice cannot be preserved");
+  expect(sent).toEqual([]);
+});
+
+test("native Chat cannot replace a required named MCP tool with a same-named client function", async () => {
+  const { config, sent } = fixture();
+  const response = await handleResponses(request(mcpChoice, [mcpTool, clientTool]), config, {});
+  expect(response.status, await response.text()).toBe(400);
+  expect(sent).toEqual([]);
+});
+
+test.each(["required", { type: "function", name: "shell" }, {
+  type: "allowed_tools", mode: "required", tools: [{ type: "function", name: "shell" }],
+}])("representable client tool choice %j reaches Chat unchanged", async choice => {
+  const { config, sent } = fixture();
+  const response = await handleResponses(request(choice, [clientTool]), config, {});
+  const body = await response.json();
+  expect(response.status).toBe(200);
+  expect(body.status).toBe("completed");
+  expect(sent).toHaveLength(1);
+  expect(sent[0]!.provider).toBe("chat");
+  expect(sent[0]!.body.tool_choice).toEqual(choice === "required" ? "required" : { type: "function", function: { name: "shell" } });
+});
+
+test("a policy skips incompatible Chat and preserves the mandatory selector for Responses", async () => {
+  const { config, sent } = fixture();
+  const choice = { type: "computer_use" };
+  const log: RequestLogContext = {};
+  const response = await handleResponses(request(choice, [{ type: "computer_use" }, clientTool], "policy/tools"), config, log);
+  expect(response.status, await response.text()).toBe(200);
+  expect(log.routeDecision?.selected.provider).toBe("chat");
+  expect(sent.map(row => row.provider)).toEqual(["responses"]);
+  expect(sent[0]!.body.tool_choice).toEqual(choice);
+});