diff --git a/src/app/api/v1/responses/input_tokens/route.ts b/src/app/api/v1/responses/input_tokens/route.ts
new file mode 100644
index 0000000000..ed912c7a95
--- /dev/null
+++ b/src/app/api/v1/responses/input_tokens/route.ts
@@ -0,0 +1,215 @@
+import { CORS_HEADERS } from "@/shared/utils/cors";
+import { enforceApiKeyPolicy } from "@/shared/utils/apiKeyPolicy";
+import { isRequireApiKeyEnabled } from "@/shared/utils/featureFlags";
+import { withChatAdmission } from "@/shared/middleware/withChatAdmission";
+import {
+  countTextTokens,
+  tokenizerContextFromBody,
+  type TokenizerContext,
+} from "@/shared/utils/tiktokenCounter";
+import { extractApiKey, isValidApiKey } from "@/sse/services/auth";
+import { errorResponse } from "@omniroute/open-sse/utils/error.ts";
+import { HTTP_STATUS } from "@omniroute/open-sse/config/constants.ts";
+
+/**
+ * POST /v1/responses/input_tokens — local Responses token count.
+ *
+ * This static segment deliberately shadows the `[...path]` passthrough route.
+ * Forwarding this preflight upstream is never useful and is actively harmful:
+ *
+ *   - For Codex OAuth the upstream subpath is not served for the account
+ *     (`404 {"detail":"Not Found"}` once the request gets through), so the
+ *     round trip can only ever fail.
+ *   - The same request reaches OpenAI's Cloudflare edge, which answers a
+ *     managed challenge (`cf-mitigated: challenge`, HTTP 403). Before the
+ *     errorClassifier fix that 403 was terminalized into `banned` /
+ *     `isActive:false` and took the whole provider offline; even with the fix
+ *     it still burns a request, wastes the latency and forces a combo
+ *     fallback on every single preflight.
+ *
+ * Counting locally removes the upstream call entirely, so no challenge can be
+ * triggered from this path. The shape mirrors the OpenAI Responses contract
+ * (`object: "response.input_tokens"` plus an `input_tokens` integer), and the
+ * counter is the same offline tokenizer `/v1/messages/count_tokens` already
+ * falls back to — Codex/`cx` models resolve to `o200k_base` through
+ * `tokenizerContextFromBody`.
+ *
+ * The estimate is deliberately conservative: a client uses this number to
+ * decide when to compact, so over-counting is safe (it compacts slightly
+ * early) while under-counting risks sending a request past the context
+ * window. The public response stays byte-shape compatible with the OpenAI
+ * contract: no OmniRoute-only metadata fields are added.
+ */
+
+/** Protocol overhead per input item, mirroring Responses message framing. */
+const PER_ITEM_OVERHEAD_TOKENS = 4;
+/** Fixed Responses framing overhead measured against live Codex usage. */
+const BASE_REQUEST_OVERHEAD_TOKENS = 10;
+/** Safety margin for tool schemas, whose wire representation can vary by model. */
+const SAFETY_MARGIN = 1.05;
+
+export async function OPTIONS() {
+  return new Response(null, { headers: CORS_HEADERS });
+}
+
+function json(body: unknown, status = 200): Response {
+  return new Response(JSON.stringify(body), {
+    status,
+    headers: { "Content-Type": "application/json", ...CORS_HEADERS },
+  });
+}
+
+function asRecord(value: unknown): Record<string, unknown> | null {
+  return value && typeof value === "object" && !Array.isArray(value)
+    ? (value as Record<string, unknown>)
+    : null;
+}
+
+function stringify(value: unknown): string {
+  if (typeof value === "string") return value;
+  try {
+    return JSON.stringify(value) ?? "";
+  } catch {
+    return "";
+  }
+}
+
+/**
+ * Count one Responses content part. Text-bearing parts are tokenized; image
+ * and file parts are not text-estimable from the request alone and are left to
+ * the safety margin rather than guessed at.
+ */
+function countContentPart(part: unknown, ctx: TokenizerContext): number {
+  if (typeof part === "string") return countTextTokens(part, ctx);
+  const record = asRecord(part);
+  if (!record) return 0;
+
+  const type = typeof record.type === "string" ? record.type : "";
+  switch (type) {
+    case "input_text":
+    case "output_text":
+    case "summary_text":
+    case "text":
+      return countTextTokens(stringify(record.text), ctx);
+    case "refusal":
+      return countTextTokens(stringify(record.refusal), ctx);
+    case "input_image":
+    case "input_file":
+    case "computer_screenshot":
+      return 0;
+    default:
+      // Unknown part types still carry their payload into the prompt.
+      return countTextTokens(stringify(record), ctx);
+  }
+}
+
+/** Count one item of the `input` array (message, tool call, tool output, …). */
+function countInputItem(item: unknown, ctx: TokenizerContext): number {
+  if (typeof item === "string") return countTextTokens(item, ctx);
+  const record = asRecord(item);
+  if (!record) return 0;
+
+  let tokens = PER_ITEM_OVERHEAD_TOKENS;
+  if (typeof record.role === "string") tokens += countTextTokens(record.role, ctx);
+
+  const content = record.content;
+  if (typeof content === "string") {
+    tokens += countTextTokens(content, ctx);
+  } else if (Array.isArray(content)) {
+    for (const part of content) tokens += countContentPart(part, ctx);
+  }
+
+  // Function/tool call items carry their payload outside `content`.
+  for (const key of ["name", "arguments", "output", "call_id", "text", "summary"]) {
+    const value = record[key];
+    if (value !== undefined && value !== null && key !== "content") {
+      tokens += countTextTokens(stringify(value), ctx);
+    }
+  }
+  return tokens;
+}
+
+/** Tool definitions are serialized into the prompt and must be counted. */
+function countTools(tools: unknown, ctx: TokenizerContext): number {
+  if (!Array.isArray(tools)) return 0;
+  let tokens = 0;
+  for (const tool of tools)
+    tokens += PER_ITEM_OVERHEAD_TOKENS + countTextTokens(stringify(tool), ctx);
+  return tokens;
+}
+
+async function postHandler(request: Request): Promise<Response> {
+  let body: unknown;
+  try {
+    body = await request.json();
+  } catch {
+    return json({ error: { message: "Invalid JSON body", type: "invalid_request_error" } }, 400);
+  }
+
+  const record = asRecord(body);
+  if (!record) {
+    return json(
+      { error: { message: "Request body must be a JSON object", type: "invalid_request_error" } },
+      400
+    );
+  }
+
+  // Preserve the same API-key and model-policy boundary as the catch-all
+  // Responses route this static route shadows. Token counting is local, but it
+  // must not turn into an unauthenticated model-catalog/policy side channel.
+  const apiKey = extractApiKey(request);
+  if (isRequireApiKeyEnabled() && !apiKey) {
+    return errorResponse(HTTP_STATUS.UNAUTHORIZED, "Authentication required");
+  }
+  if (isRequireApiKeyEnabled() && apiKey && !(await isValidApiKey(apiKey))) {
+    return errorResponse(HTTP_STATUS.UNAUTHORIZED, "Invalid API key");
+  }
+
+  const model = typeof record.model === "string" ? record.model : "";
+  const policy = await enforceApiKeyPolicy(request, model);
+  if (policy.rejection) return policy.rejection;
+
+  const ctx = tokenizerContextFromBody(record);
+  let tokens = 0;
+
+  if (typeof record.instructions === "string") {
+    tokens += countTextTokens(record.instructions, ctx);
+  }
+
+  const input = record.input;
+  if (typeof input === "string") {
+    tokens += countTextTokens(input, ctx);
+  } else if (Array.isArray(input)) {
+    for (const item of input) tokens += countInputItem(item, ctx);
+  }
+
+  const hasTools = Array.isArray(record.tools) && record.tools.length > 0;
+  tokens += countTools(record.tools, ctx);
+
+  if (record.tool_choice !== undefined && typeof record.tool_choice !== "string") {
+    tokens += countTextTokens(stringify(record.tool_choice), ctx);
+  }
+  if (record.text !== undefined) tokens += countTextTokens(stringify(record.text), ctx);
+  if (record.reasoning !== undefined) tokens += countTextTokens(stringify(record.reasoning), ctx);
+
+  // Live A/B against Codex `usage.input_tokens` showed a stable ~9–10 token
+  // request-envelope overhead for requests without tools (short text through
+  // long/code inputs). Tool definitions already carry their own framing in
+  // `countTools`, so retain the percentage margin there rather than stacking
+  // the fixed base and systematically over-counting every tool request.
+  const inputTokens =
+    tokens === 0
+      ? 0
+      : hasTools
+        ? Math.ceil(tokens * SAFETY_MARGIN)
+        : tokens + BASE_REQUEST_OVERHEAD_TOKENS;
+
+  return json({
+    object: "response.input_tokens",
+    input_tokens: inputTokens,
+  });
+}
+
+// Preserve the same process-wide body-size / fairness admission boundary as
+// the catch-all Responses route this static segment shadows.
+export const POST = withChatAdmission(postHandler);
diff --git a/tests/unit/responses-input-tokens-local-route.test.ts b/tests/unit/responses-input-tokens-local-route.test.ts
new file mode 100644
index 0000000000..b7f6c295bd
--- /dev/null
+++ b/tests/unit/responses-input-tokens-local-route.test.ts
@@ -0,0 +1,138 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+const { POST, OPTIONS } = await import("../../src/app/api/v1/responses/input_tokens/route.ts");
+
+function post(body: unknown): Request {
+  return new Request("http://localhost/v1/responses/input_tokens", {
+    method: "POST",
+    headers: { "content-type": "application/json" },
+    body: typeof body === "string" ? body : JSON.stringify(body),
+  });
+}
+
+async function count(body: unknown) {
+  const response = await POST(post(body));
+  return { status: response.status, json: await response.json() };
+}
+
+test("returns the OpenAI Responses contract shape", async () => {
+  const { status, json } = await count({ model: "cx/gpt-5.6-sol", input: "hello world" });
+  assert.equal(status, 200);
+  assert.equal(json.object, "response.input_tokens");
+  assert.equal(typeof json.input_tokens, "number");
+  assert.equal(Number.isInteger(json.input_tokens), true);
+  assert.ok(json.input_tokens > 0);
+  // No OmniRoute-only fields may leak into the public contract.
+  assert.deepEqual(Object.keys(json).sort(), ["input_tokens", "object"]);
+});
+
+test("never performs an upstream request (no fetch from this route)", async () => {
+  const originalFetch = globalThis.fetch;
+  let calls = 0;
+  globalThis.fetch = (async (...args: unknown[]) => {
+    calls += 1;
+    void args;
+    throw new Error("the local token count route must not call upstream");
+  }) as typeof globalThis.fetch;
+  try {
+    const { status } = await count({ model: "cx/gpt-5.6-sol", input: "hello world" });
+    assert.equal(status, 200);
+    assert.equal(calls, 0, "no upstream call may be issued — this is what avoids the CF challenge");
+  } finally {
+    globalThis.fetch = originalFetch;
+  }
+});
+
+test("counts instructions, structured input and tools", async () => {
+  const bare = await count({ model: "cx/gpt-5.6-sol", input: "hello" });
+  const withInstructions = await count({
+    model: "cx/gpt-5.6-sol",
+    input: "hello",
+    instructions: "You are a careful assistant that always explains its reasoning.",
+  });
+  assert.ok(withInstructions.json.input_tokens > bare.json.input_tokens);
+
+  const withTools = await count({
+    model: "cx/gpt-5.6-sol",
+    input: "hello",
+    tools: [
+      {
+        type: "function",
+        name: "search_documents",
+        description: "Search the corpus for matching documents",
+        parameters: { type: "object", properties: { query: { type: "string" } } },
+      },
+    ],
+  });
+  assert.ok(
+    withTools.json.input_tokens > bare.json.input_tokens,
+    "tool definitions are serialized into the prompt and must be counted"
+  );
+
+  const structured = await count({
+    model: "cx/gpt-5.6-sol",
+    input: [
+      { role: "user", content: [{ type: "input_text", text: "first message" }] },
+      { role: "assistant", content: [{ type: "output_text", text: "second message" }] },
+    ],
+  });
+  assert.ok(structured.json.input_tokens > 0);
+});
+
+test("estimate is conservative — never under-counts the raw text", async () => {
+  // Under-counting is the dangerous direction: a client would send a request
+  // past the context window. Over-counting only compacts slightly early.
+  const text = "The quick brown fox jumps over the lazy dog. ".repeat(40);
+  const { json } = await count({ model: "cx/gpt-5.6-sol", input: text });
+  const naiveLowerBound = Math.ceil(text.length / 6);
+  assert.ok(
+    json.input_tokens >= naiveLowerBound,
+    `expected >= ${naiveLowerBound}, got ${json.input_tokens}`
+  );
+});
+
+test("non-text parts do not crash the counter", async () => {
+  const { status, json } = await count({
+    model: "cx/gpt-5.6-sol",
+    input: [
+      {
+        role: "user",
+        content: [
+          { type: "input_text", text: "describe this" },
+          { type: "input_image", image_url: "data:image/png;base64,AAAABBBBCCCC" },
+          { type: "input_file", filename: "a.pdf" },
+        ],
+      },
+    ],
+  });
+  assert.equal(status, 200);
+  assert.ok(json.input_tokens > 0);
+});
+
+test("server-held context references stay in the plain contract shape", async () => {
+  const { status, json } = await count({
+    model: "cx/gpt-5.6-sol",
+    input: "follow up",
+    previous_response_id: "resp_abc123",
+  });
+  assert.equal(status, 200);
+  assert.deepEqual(Object.keys(json).sort(), ["input_tokens", "object"]);
+});
+
+test("an empty request still answers the contract with zero", async () => {
+  const { status, json } = await count({ model: "cx/gpt-5.6-sol" });
+  assert.equal(status, 200);
+  assert.equal(json.object, "response.input_tokens");
+  assert.equal(json.input_tokens, 0);
+});
+
+test("invalid JSON is a 400, not a 500", async () => {
+  const response = await POST(post("not json at all"));
+  assert.equal(response.status, 400);
+});
+
+test("OPTIONS preflight is answered", async () => {
+  const response = await OPTIONS();
+  assert.equal(response.status, 200);
+});