diff --git a/src/app/api/internal/codex-responses-ws/modelResolution.ts b/src/app/api/internal/codex-responses-ws/modelResolution.ts
index 244e63d22d..3011c8adbf 100644
--- a/src/app/api/internal/codex-responses-ws/modelResolution.ts
+++ b/src/app/api/internal/codex-responses-ws/modelResolution.ts
@@ -46,3 +46,38 @@ export async function resolveCodexWsModelInfo(
   const codexInfo = await resolve(`codex/${requestedModel}`);
   return codexInfo?.provider === "codex" ? codexInfo : info;
 }
+
+/**
+ * Resolve a model ID for the HTTP Responses path, applying codex preference
+ * for bare ChatGPT-style model IDs (those without a provider prefix).
+ *
+ * When the Codex CLI falls back from WebSocket to HTTP (#15492), it sends bare
+ * model IDs like "gpt-5.5" to /v1/responses. Without this resolution, OmniRoute
+ * routes them to openrouter/openai instead of the configured codex OAuth
+ * connections, producing "No credentials for provider: openrouter".
+ *
+ * @param requestedModel the model id from the Responses API request body
+ * @param resolve a getModelInfo-style resolver
+ * @returns { model, changed } — model is the (possibly rewritten) id;
+ *          changed=true means a codex/ prefix was applied.
+ */
+export async function resolveResponsesApiModel(
+  requestedModel: string,
+  resolve: ModelResolver
+): Promise<{ model: string; changed: boolean }> {
+  if (!requestedModel || requestedModel.includes("/")) {
+    return { model: requestedModel, changed: false };
+  }
+
+  try {
+    const resolved = await resolveCodexWsModelInfo(requestedModel, resolve);
+    if (resolved?.provider !== "codex") {
+      return { model: requestedModel, changed: false };
+    }
+
+    const prefixed = `codex/${resolved.model || requestedModel}`;
+    return { model: prefixed, changed: true };
+  } catch {
+    return { model: requestedModel, changed: false };
+  }
+}
diff --git a/src/app/api/v1/responses/route.ts b/src/app/api/v1/responses/route.ts
index 0dbb556449..163471cace 100644
--- a/src/app/api/v1/responses/route.ts
+++ b/src/app/api/v1/responses/route.ts
@@ -1,5 +1,7 @@
 import { handleChat } from "@/sse/handlers/chat";
 import { withEarlyStreamKeepalive } from "@omniroute/open-sse/utils/earlyStreamKeepalive";
+import { resolveResponsesApiModel } from "../internal/codex-responses-ws/modelResolution";
+import { getModelInfo } from "@/sse/services/model";
 
 // NOTE: We do NOT call initTranslators() here — the translator registry is
 // bootstrapped at module level inside open-sse/translator/index.ts when it
@@ -19,6 +21,36 @@ export async function OPTIONS() {
   });
 }
 
+/**
+ * Rewrite a bare ChatGPT-style model id to the codex/ prefix when the model
+ * resolves to a codex provider. This fixes the Codex CLI WS→HTTP fallback path:
+ * the CLI sends bare "gpt-5.5" over HTTP after WS closes (1008 Policy), and
+ * without this rewrite OmniRoute routes it to openrouter instead of codex.
+ *
+ * Safe: only rewrites when codex/model is genuinely registered; all other models
+ * pass through unchanged. Errors are caught and the original request is returned.
+ */
+async function withCodexPreferredModel(request: Request): Promise<Request> {
+  try {
+    const clone = request.clone();
+    const body = await clone.json().catch(() => null);
+    if (!body || typeof body !== "object" || typeof body.model !== "string") {
+      return request;
+    }
+    const { model, changed } = await resolveResponsesApiModel(body.model, getModelInfo);
+    if (!changed) return request;
+
+    return new Request(request.url, {
+      method: request.method,
+      headers: request.headers,
+      body: JSON.stringify({ ...body, model }),
+      signal: request.signal,
+    });
+  } catch {
+    return request;
+  }
+}
+
 /**
  * POST /v1/responses - OpenAI Responses API format
  * Handled by the unified chat handler (openai-responses format auto-detected).
@@ -28,9 +60,10 @@ export async function POST(request) {
   // client drops the connection if no bytes arrive within ~5s. Keep the connection
   // warm with early keepalives while the upstream produces its first token (#2544).
   // Non-streaming callers (JSON) keep the original verbatim path untouched.
+  const resolved = await withCodexPreferredModel(request);
   const accept = String(request.headers?.get?.("accept") || "").toLowerCase();
   if (accept.includes("text/event-stream")) {
-    return await withEarlyStreamKeepalive(handleChat(request), { signal: request.signal });
+    return await withEarlyStreamKeepalive(handleChat(resolved), { signal: request.signal });
   }
-  return await handleChat(request);
+  return await handleChat(resolved);
 }
diff --git a/tests/unit/codex-ws-http-fallback.test.ts b/tests/unit/codex-ws-http-fallback.test.ts
new file mode 100644
index 0000000000..5151dc0928
--- /dev/null
+++ b/tests/unit/codex-ws-http-fallback.test.ts
@@ -0,0 +1,91 @@
+/**
+ * Tests for resolveResponsesApiModel — ensures bare ChatGPT model IDs are
+ * codex-preferred when the Codex CLI falls back from WebSocket to HTTP and
+ * hits /v1/responses with a bare model id (e.g. "gpt-5.5").
+ *
+ * Root cause: WS transport requires bare ids (codex/ prefix rejected client-side),
+ * but HTTP routing resolves bare "gpt-5.5" → openrouter, not codex.
+ * Fix: /v1/responses pre-resolves bare ids with codex preference.
+ */
+import test from "node:test";
+import assert from "node:assert/strict";
+import { resolveResponsesApiModel } from "../../src/app/api/internal/codex-responses-ws/modelResolution.ts";
+import type { ModelResolver } from "../../src/app/api/internal/codex-responses-ws/modelResolution.ts";
+
+/** Resolver that maps model id to a { provider, model } pair. */
+function makeResolver(map: Record<string, { provider: string; model: string }>): ModelResolver {
+  return async (id: string) => map[id] ?? {};
+}
+
+test("bare gpt-5.5 that resolves to codex is rewritten to codex/gpt-5.5", async () => {
+  const resolve = makeResolver({
+    "gpt-5.5": { provider: "openrouter", model: "gpt-5.5" },
+    "codex/gpt-5.5": { provider: "codex", model: "gpt-5.5" },
+  });
+  const result = await resolveResponsesApiModel("gpt-5.5", resolve);
+  assert.equal(result.model, "codex/gpt-5.5");
+  assert.equal(result.changed, true);
+});
+
+test("bare gpt-4o that has no codex mapping is passed through unchanged", async () => {
+  const resolve = makeResolver({
+    "gpt-4o": { provider: "openai", model: "gpt-4o" },
+    // codex/gpt-4o is NOT in the registry
+  });
+  const result = await resolveResponsesApiModel("gpt-4o", resolve);
+  assert.equal(result.model, "gpt-4o");
+  assert.equal(result.changed, false);
+});
+
+test("already-prefixed codex/gpt-5.5 is passed through unchanged", async () => {
+  const resolve = makeResolver({
+    "codex/gpt-5.5": { provider: "codex", model: "gpt-5.5" },
+  });
+  const result = await resolveResponsesApiModel("codex/gpt-5.5", resolve);
+  assert.equal(result.model, "codex/gpt-5.5");
+  assert.equal(result.changed, false);
+});
+
+test("bare model that resolves to openrouter AND has no codex equivalent is passed through", async () => {
+  const resolve = makeResolver({
+    "llama-3.1": { provider: "openrouter", model: "llama-3.1" },
+    "codex/llama-3.1": { provider: "openrouter", model: "llama-3.1" }, // no codex match
+  });
+  const result = await resolveResponsesApiModel("llama-3.1", resolve);
+  assert.equal(result.model, "llama-3.1");
+  assert.equal(result.changed, false);
+});
+
+test("empty model string is passed through unchanged", async () => {
+  const resolve = makeResolver({});
+  const result = await resolveResponsesApiModel("", resolve);
+  assert.equal(result.model, "");
+  assert.equal(result.changed, false);
+});
+
+test("bare gpt-5.5 that directly resolves to codex (without prefix retry) is rewritten", async () => {
+  const resolve = makeResolver({
+    "gpt-5.5": { provider: "codex", model: "gpt-5.5" },
+  });
+  const result = await resolveResponsesApiModel("gpt-5.5", resolve);
+  assert.equal(result.model, "codex/gpt-5.5");
+  assert.equal(result.changed, true);
+});
+
+test("other-provider/ prefix passes through unchanged (not a bare model)", async () => {
+  const resolve = makeResolver({
+    "anthropic/claude-opus-4": { provider: "anthropic", model: "claude-opus-4" },
+  });
+  const result = await resolveResponsesApiModel("anthropic/claude-opus-4", resolve);
+  assert.equal(result.model, "anthropic/claude-opus-4");
+  assert.equal(result.changed, false);
+});
+
+test("resolver throwing is handled gracefully — model passes through unchanged", async () => {
+  const resolve: ModelResolver = async () => {
+    throw new Error("resolver unavailable");
+  };
+  const result = await resolveResponsesApiModel("gpt-5.5", resolve);
+  assert.equal(result.model, "gpt-5.5");
+  assert.equal(result.changed, false);
+});