diff --git a/docs-site/src/content/docs/reference/configuration/providers.md b/docs-site/src/content/docs/reference/configuration/providers.md
index 9a0b822f52..90d6146e95 100644
--- a/docs-site/src/content/docs/reference/configuration/providers.md
+++ b/docs-site/src/content/docs/reference/configuration/providers.md
@@ -33,6 +33,17 @@ Bearer authentication; this does not change the inference URL or imply different
 quota consumption between adapters. Existing BigModel CN monitor selection remains
 separate. Full request URLs such as `/api/v1/responses` are not provider base URLs.
 
+## Responses client identity
+
+For `openai-responses` providers, OpenCodex preserves the incoming client's `User-Agent`
+in both API-key and forward modes. This lets upstream services select client-specific
+compatibility handling instead of seeing the Bun runtime's default identity.
+
+An explicit `User-Agent` in the provider's `headers` takes precedence, regardless of
+header-name casing. Responses-based search and vision helpers use the same precedence.
+When the incoming request has no User-Agent and no override is configured, OpenCodex
+does not invent a Codex identity; the transport keeps its normal default behavior.
+
 ## Provider-related top-level fields
 
 | Field | Type | Default | Meaning |
diff --git a/scripts/test-layout/layout.json b/scripts/test-layout/layout.json
index 885d2abcd4..9bc986e908 100644
--- a/scripts/test-layout/layout.json
+++ b/scripts/test-layout/layout.json
@@ -1200,6 +1200,7 @@
     "responses-tool-search-repair.test.ts": "responses",
     "responses-undeclared-tool-guard.test.ts": "responses",
     "responses-usage-passthrough.test.ts": "responses",
+    "responses-user-agent.test.ts": "responses",
     "restore-completes-shared-teardown.test.ts": "cli",
     "retry-after-429.test.ts": "server",
     "route-decision-trace.test.ts": "server",
diff --git a/src/adapters/openai-responses/passthrough.ts b/src/adapters/openai-responses/passthrough.ts
index 4cde7b8d3c..dfe6c1a281 100644
--- a/src/adapters/openai-responses/passthrough.ts
+++ b/src/adapters/openai-responses/passthrough.ts
@@ -41,9 +41,11 @@ import { applyTierDecisionToResponsesBody, normalizeCanonicalForwardContinuation
 import { normalizeImageGenClientTools, preferConfiguredHostedTools } from "./image-gen";
 import { stripMuseSparkUnsupportedWebSearchFields, stripOpenAiOnlyWebSearchFields } from "./web-search";
 
-// Headers relayed verbatim from the caller in OAuth-passthrough ("forward") mode.
-// Exported so the web-search sidecar reuses the exact same forwarded-auth set for its ChatGPT call.
+// Caller headers retained through auth materialization and internal request bridges.
+// Forward executors relay this set; an explicit provider User-Agent takes precedence.
 export const FORWARD_HEADERS = [
+  // Keep client identity through auth materialization and internal HTTP/WS bridges.
+  "user-agent",
   "authorization",
   "chatgpt-account-id",
   "openai-beta",
@@ -195,6 +197,8 @@ export function createResponsesPassthroughAdapter(provider: OcxProviderConfig):
         }
         if (mayForwardCallerCredentials) {
           for (const h of FORWARD_HEADERS) {
+            // User-Agent uses the provider-first fallback shared with API-key mode below.
+            if (h === "user-agent") continue;
             const v = incoming?.headers.get(h);
             if (v) {
               if (h === CODEX_RESPONSES_LITE_HEADER) {
@@ -222,6 +226,13 @@ export function createResponsesPassthroughAdapter(provider: OcxProviderConfig):
         if (provider.headers) Object.assign(headers, provider.headers);
       }
 
+      // Some Responses providers select Codex compatibility by User-Agent. Preserve the
+      // caller identity unless the provider explicitly overrides it, regardless of casing.
+      const callerUserAgent = incoming.headers.get("user-agent");
+      if (callerUserAgent !== null && !Object.keys(headers).some(name => name.toLowerCase() === "user-agent")) {
+        headers["User-Agent"] = callerUserAgent;
+      }
+
       const forward = provider.authMode === "forward";
       let convertedRoutedCustomToolNames: Set<string> | undefined;
       let routedCustomToolRepairNames: Set<string> | undefined;
diff --git a/src/vision/describe.ts b/src/vision/describe.ts
index 83c51afaeb..1715a77d0f 100644
--- a/src/vision/describe.ts
+++ b/src/vision/describe.ts
@@ -70,6 +70,8 @@ export async function describeImage(
   const headers: Record<string, string> = { "Content-Type": "application/json" };
   if (forwardProvider.headers) Object.assign(headers, forwardProvider.headers);
   for (const h of FORWARD_HEADERS) {
+    // Explicit provider identity wins over the caller's fallback, including mixed casing.
+    if (h === "user-agent" && Object.keys(headers).some(name => name.toLowerCase() === h)) continue;
     const v = selectedForwardHeaders.get(h);
     if (v) headers[h] = v;
   }
diff --git a/src/web-search/executor.ts b/src/web-search/executor.ts
index 489fa8f399..7655972a5e 100644
--- a/src/web-search/executor.ts
+++ b/src/web-search/executor.ts
@@ -58,6 +58,8 @@ export async function runWebSearch(
   const headers: Record<string, string> = { "Content-Type": "application/json" };
   if (forwardProvider.headers) Object.assign(headers, forwardProvider.headers);
   for (const h of FORWARD_HEADERS) {
+    // Explicit provider identity wins over the caller's fallback, including mixed casing.
+    if (h === "user-agent" && Object.keys(headers).some(name => name.toLowerCase() === h)) continue;
     const v = selectedForwardHeaders.get(h);
     if (v) headers[h] = v;
   }
diff --git a/structure/adapters/registry.md b/structure/adapters/registry.md
index 83e8f4f466..1b28faf31e 100644
--- a/structure/adapters/registry.md
+++ b/structure/adapters/registry.md
@@ -1,5 +1,8 @@
 # Adapter Registry Authority
 
+Responses-derived adapters inherit the
+[client User-Agent forwarding contract](../transports/responses.md#client-user-agent-forwarding).
+
 Request-local adapter bindings are separate from registry authority in the Responses
 [core module ownership](../transports/responses.md#core-module-ownership). This surface retains its existing behavior.
 
diff --git a/structure/data-planes/inbound-compat.md b/structure/data-planes/inbound-compat.md
index 33ca8c76b0..15cf2a0277 100644
--- a/structure/data-planes/inbound-compat.md
+++ b/structure/data-planes/inbound-compat.md
@@ -1,5 +1,8 @@
 # Inbound Compatibility Surfaces
 
+Internal request bridges retain User-Agent for the
+[Responses client identity contract](../transports/responses.md#client-user-agent-forwarding).
+
 Compatibility callers retain the public Responses ingress described by the
 [core module ownership](../transports/responses.md#core-module-ownership). This surface retains its existing behavior.
 
diff --git a/structure/gui-and-management-api.md b/structure/gui-and-management-api.md
index 4628ca5e50..a0d0a9f711 100644
--- a/structure/gui-and-management-api.md
+++ b/structure/gui-and-management-api.md
@@ -1,5 +1,8 @@
 # GUI And Management API
 
+Configured provider headers take precedence over caller identity under the
+[Responses User-Agent contract](transports/responses.md#client-user-agent-forwarding).
+
 The shared server request path follows the Responses
 [core module ownership](transports/responses.md#core-module-ownership). This surface retains its existing behavior.
 
diff --git a/structure/ops/docs-and-release.md b/structure/ops/docs-and-release.md
index 0c0ffe38d0..909748c910 100644
--- a/structure/ops/docs-and-release.md
+++ b/structure/ops/docs-and-release.md
@@ -1,5 +1,9 @@
 # Docs And Release
 
+The provider configuration reference documents the
+[Responses User-Agent contract](../transports/responses.md#client-user-agent-forwarding),
+with regression coverage registered in the Responses test domain.
+
 The configuration-only [plaintext V2 contract](../subagents.md#plaintext-v2-agent-messages)
 is scoped to canonical ChatGPT Responses forwarding; other source-area behavior described here is unchanged.
 
diff --git a/structure/overview.md b/structure/overview.md
index 0151115adc..9dc9e8c6d5 100644
--- a/structure/overview.md
+++ b/structure/overview.md
@@ -1,5 +1,8 @@
 # Overview
 
+Responses and its search/vision helpers preserve client identity according to the
+[User-Agent forwarding contract](transports/responses.md#client-user-agent-forwarding).
+
 The configuration-only [plaintext V2 contract](subagents.md#plaintext-v2-agent-messages)
 is scoped to canonical ChatGPT Responses forwarding; other source-area behavior described here is unchanged.
 
diff --git a/structure/providers/chat-compat.md b/structure/providers/chat-compat.md
index 5ee17ed807..d0634975d1 100644
--- a/structure/providers/chat-compat.md
+++ b/structure/providers/chat-compat.md
@@ -1,5 +1,8 @@
 # Chat Provider Compatibility
 
+Requests translated onto the Responses adapter follow its
+[client User-Agent forwarding contract](../transports/responses.md#client-user-agent-forwarding).
+
 The configuration-only [plaintext V2 contract](../subagents.md#plaintext-v2-agent-messages)
 is scoped to canonical ChatGPT Responses forwarding; other source-area behavior described here is unchanged.
 
diff --git a/structure/providers/cursor.md b/structure/providers/cursor.md
index 5ae38028d8..cf359a3199 100644
--- a/structure/providers/cursor.md
+++ b/structure/providers/cursor.md
@@ -1,5 +1,8 @@
 # Cursor Provider
 
+The [client User-Agent forwarding contract](../transports/responses.md#client-user-agent-forwarding)
+belongs to Responses-based sends; Cursor's direct transport retains its own identity policy.
+
 The configuration-only [plaintext V2 contract](../subagents.md#plaintext-v2-agent-messages)
 is scoped to canonical ChatGPT Responses forwarding; other source-area behavior described here is unchanged.
 
diff --git a/structure/runtime.md b/structure/runtime.md
index bd9ebbd561..d9955d80c0 100644
--- a/structure/runtime.md
+++ b/structure/runtime.md
@@ -1,5 +1,8 @@
 # Runtime
 
+Responses adapters and auxiliary executors follow the
+[client User-Agent forwarding contract](transports/responses.md#client-user-agent-forwarding).
+
 Responses admission and finalization are composed through the
 [core module ownership](transports/responses.md#core-module-ownership). This surface retains its existing behavior.
 
diff --git a/structure/transports/byte-accounting.md b/structure/transports/byte-accounting.md
index 7f01dee197..caaa05699a 100644
--- a/structure/transports/byte-accounting.md
+++ b/structure/transports/byte-accounting.md
@@ -1,5 +1,8 @@
 # Byte Accounting
 
+Adapter request headers follow the [client User-Agent forwarding contract](responses.md#client-user-agent-forwarding);
+this header-only policy does not allocate or reserialize request bodies.
+
 Responses body-reader limits and lifetime handling follow the
 [core module ownership](responses.md#core-module-ownership). This surface retains its existing behavior.
 
diff --git a/structure/transports/inventory.md b/structure/transports/inventory.md
index ca80373a8e..9894f6a4f0 100644
--- a/structure/transports/inventory.md
+++ b/structure/transports/inventory.md
@@ -1,5 +1,8 @@
 # Transport Inventory
 
+Responses and its auxiliary HTTP sends follow the
+[client User-Agent forwarding contract](responses.md#client-user-agent-forwarding).
+
 The existing Responses transport is divided by responsibility in the
 [core module ownership](responses.md#core-module-ownership). This surface retains its existing behavior.
 
diff --git a/structure/transports/responses.md b/structure/transports/responses.md
index 4a6a664cad..601838629c 100644
--- a/structure/transports/responses.md
+++ b/structure/transports/responses.md
@@ -15,6 +15,20 @@ Retired Codex Spark has no model-specific tool or Responses Lite override; gener
 namespace scrubbing remain shared compatibility behavior. Codex quota/reset evidence follows the
 [shared/Reserve policy](../providers/openai-tiers.md#public-provider-contract), including suppression of retired model-derived evidence before shared recovery.
 
+### Client User-Agent forwarding
+
+`src/adapters/openai-responses/passthrough.ts` includes `user-agent` in the shared
+forward-header allowlist so auth materialization and internal HTTP/WebSocket bridges
+retain caller identity. The Responses adapter uses that identity in both API-key and
+forward modes only when the provider has no explicit User-Agent header, matched
+case-insensitively. A caller with no User-Agent does not acquire a fabricated Codex identity.
+Credential selection and the other allowlisted fields keep their existing precedence.
+
+The Responses-based search and vision executors apply the same provider-first User-Agent
+precedence when merging the shared allowlist. `tests/responses/responses-user-agent.test.ts`
+covers adapter modes, internal WebSocket header selection, absent UA, mixed-case overrides,
+and the headers received over real loopback HTTP by Responses and sidecar upstreams.
+
 ### Credential-bearing HTTP redirects
 
 Credential/body-bearing HTTP sends use `redirect: "manual"` at the final executor boundary,
diff --git a/tests/fixtures/test-layout-expected.json b/tests/fixtures/test-layout-expected.json
index d2eb5d244b..4dc5493360 100644
--- a/tests/fixtures/test-layout-expected.json
+++ b/tests/fixtures/test-layout-expected.json
@@ -1028,6 +1028,7 @@
   "responses-tool-search-repair.test.ts": "responses",
   "responses-undeclared-tool-guard.test.ts": "responses",
   "responses-usage-passthrough.test.ts": "responses",
+  "responses-user-agent.test.ts": "responses",
   "restore-completes-shared-teardown.test.ts": "cli",
   "retry-after-429.test.ts": "server",
   "route-decision-trace.test.ts": "server",
diff --git a/tests/responses/responses-user-agent.test.ts b/tests/responses/responses-user-agent.test.ts
new file mode 100644
index 0000000000..0244e04e7d
--- /dev/null
+++ b/tests/responses/responses-user-agent.test.ts
@@ -0,0 +1,147 @@
+import { describe, expect, test } from "bun:test";
+import { createResponsesPassthroughAdapter } from "../../src/adapters/openai-responses";
+import { parseRequest } from "../../src/responses/parser";
+import { handleResponses } from "../../src/server/responses";
+import { selectForwardHeaders } from "../../src/server/ws-bridge";
+import { describeImage } from "../../src/vision/describe";
+import { runWebSearch } from "../../src/web-search/executor";
+import type { OcxConfig, OcxProviderConfig } from "../../src/types";
+import { withTestTranslatorBudget } from "../helpers/translator-budget";
+
+const callerUserAgent = "codex_cli_rs/0.153.0 (Windows 11; x86_64)";
+const providerUserAgent = "provider-client/1.0";
+const routes: Array<[string, OcxProviderConfig]> = [
+  ["API key", { adapter: "openai-responses", baseUrl: "https://fixture.test/v1", authMode: "key", apiKey: "fixture-key" }],
+  ["canonical forward", { adapter: "openai-responses", baseUrl: "https://chatgpt.com/backend-api/codex", authMode: "forward" }],
+  ["custom forward", { adapter: "openai-responses", baseUrl: "https://fixture.test/v1", authMode: "forward" }],
+];
+
+/** Exercise the production adapter with the shared test budget lifecycle. */
+function buildRequest(provider: OcxProviderConfig, incoming: Headers) {
+  return withTestTranslatorBudget(createResponsesPassthroughAdapter(provider)).buildRequest(
+    parseRequest({ model: "fixture-model", input: "ping", stream: false }),
+    { headers: incoming },
+  );
+}
+
+describe("Responses upstream User-Agent", () => {
+  test("WebSocket bridge retains the caller UA without forwarding unrelated headers", () => {
+    const selected = selectForwardHeaders(new Headers({
+      "User-Agent": callerUserAgent, "x-unrelated": "private", cookie: "fixture=value",
+    }));
+    expect(selected.get("user-agent")).toBe(callerUserAgent);
+    expect(selected.has("x-unrelated")).toBe(false);
+    expect(selected.has("cookie")).toBe(false);
+  });
+
+  for (const [name, provider] of routes) {
+    test(`${name}: preserves the caller identity without forwarding unrelated headers`, () => {
+      const incoming = new Headers({ "User-Agent": callerUserAgent, cookie: "fixture=value", "x-unrelated": "private" });
+      const result = new Headers(buildRequest(provider, incoming).headers);
+      expect(result.get("user-agent")).toBe(callerUserAgent);
+      expect(result.has("cookie")).toBe(false);
+      expect(result.has("x-unrelated")).toBe(false);
+      expect(incoming.get("user-agent")).toBe(callerUserAgent);
+    });
+
+    // Header casing must not produce two identities when the transport normalizes names.
+    for (const headerName of ["User-Agent", "user-agent", "USER-AGENT"]) {
+      test(`${name}: explicit ${headerName} wins without a duplicate`, () => {
+        const configured = { [headerName]: providerUserAgent, "x-provider-option": "enabled" };
+        const request = buildRequest({ ...provider, headers: configured }, new Headers({ "user-agent": callerUserAgent }));
+        const result = new Headers(request.headers);
+        expect(result.get("user-agent")).toBe(providerUserAgent);
+        expect(Object.keys(request.headers).filter(key => key.toLowerCase() === "user-agent")).toHaveLength(1);
+        expect(result.get("x-provider-option")).toBe("enabled");
+        expect(configured).toEqual({ [headerName]: providerUserAgent, "x-provider-option": "enabled" });
+      });
+    }
+
+    test(`${name}: missing caller UA does not fabricate a client identity`, () => {
+      expect(new Headers(buildRequest(provider, new Headers()).headers).has("user-agent")).toBe(false);
+    });
+
+    test(`${name}: an explicitly empty provider UA still takes precedence`, () => {
+      const result = new Headers(buildRequest({ ...provider, headers: { "User-Agent": "" } },
+        new Headers({ "user-agent": callerUserAgent })).headers);
+      expect(result.get("user-agent")).toBe("");
+    });
+  }
+
+  for (const override of [false, true]) {
+    test(`HTTP upstream receives ${override ? "the configured override" : "the original Codex UA"}`, async () => {
+      const expected = override ? providerUserAgent : callerUserAgent;
+      const received: Array<string | null> = [];
+      // Model a UA-gated upstream on loopback, without real provider credentials.
+      const upstream = Bun.serve({
+        hostname: "127.0.0.1",
+        port: 0,
+        async fetch(request) {
+          await request.text();
+          const ua = request.headers.get("user-agent");
+          received.push(ua);
+          if (ua !== expected) return Response.json({ error: { message: "client compatibility was not selected" } }, { status: 400 });
+          return Response.json({ id: "resp_fixture", object: "response", status: "completed", output: [
+            { type: "message", role: "assistant", content: [{ type: "output_text", text: "ok" }] },
+          ] });
+        },
+      });
+      try {
+        const config = {
+          port: 0,
+          defaultProvider: "fixture",
+          providers: { fixture: {
+            adapter: "openai-responses", baseUrl: `${upstream.url.origin}/v1`,
+            authMode: "key", apiKey: "fixture-key", allowPrivateNetwork: true,
+            ...(override ? { headers: { "USER-AGENT": providerUserAgent } } : {}),
+          } },
+        } as OcxConfig;
+        const response = await handleResponses(new Request("http://localhost/v1/responses", {
+          method: "POST",
+          headers: { "content-type": "application/json", "user-agent": callerUserAgent },
+          body: JSON.stringify({ model: "fixture/model", input: "ping", stream: false }),
+          signal: AbortSignal.timeout(5000),
+        }), config, { model: "", provider: "" });
+        expect(response.status).toBe(200);
+        expect(await response.text()).toContain("ok");
+        expect(received).toEqual([expected]);
+      } finally {
+        await upstream.stop(true);
+      }
+    });
+  }
+
+  for (const kind of ["search", "vision"] as const) {
+    for (const headerName of [undefined, "User-Agent", "user-agent", "USER-AGENT"]) {
+      test(`${kind} sidecar preserves ${headerName ?? "caller UA"} on the wire`, async () => {
+        const received: Array<string | null> = [];
+        const upstream = Bun.serve({
+          hostname: "127.0.0.1",
+          port: 0,
+          async fetch(request) {
+            await request.text();
+            received.push(request.headers.get("user-agent"));
+            return new Response('data: {"type":"response.output_text.delta","delta":"ok"}\n\n'
+              + 'data: {"type":"response.completed","response":{"status":"completed"}}\n\n',
+            { headers: { "content-type": "text/event-stream" } });
+          },
+        });
+        try {
+          const provider: OcxProviderConfig = {
+            adapter: "openai-responses", baseUrl: upstream.url.origin, authMode: "forward",
+            ...(headerName ? { headers: { [headerName]: providerUserAgent } } : {}),
+          };
+          const incoming = new Headers({ "user-agent": callerUserAgent });
+          const settings = { model: "fixture-model", reasoning: "low" as const, timeoutMs: 5000 };
+          const result = kind === "search"
+            ? await runWebSearch("ping", { type: "web_search" }, provider, incoming, settings)
+            : await describeImage("data:image/png;base64,AA==", "low", "ping", provider, incoming, settings);
+          expect(result.error).toBeUndefined();
+          expect(received).toEqual([headerName ? providerUserAgent : callerUserAgent]);
+        } finally {
+          await upstream.stop(true);
+        }
+      });
+    }
+  }
+});