diff --git a/changelog.d/fixes/13355-strip-internal-omniroute-markers.md b/changelog.d/fixes/13355-strip-internal-omniroute-markers.md
new file mode 100644
index 0000000000..da6e3d8a2f
--- /dev/null
+++ b/changelog.d/fixes/13355-strip-internal-omniroute-markers.md
@@ -0,0 +1 @@
+- **fix(dispatch):** strip every `_omniroute*` internal marker at the shared pre-serialization chokepoint (`cliFingerprints.ts`) instead of a hand-maintained per-key allowlist, so internal routing/handoff markers (e.g. `_omnirouteSkipContextRelay`, `_omnirouteResponsesStore`) can no longer leak into serialized upstream request bodies and draw `400 Extra inputs are not permitted` from strict Anthropic-compatible gateways ([#12729](https://github.com/diegosouzapw/OmniRoute/issues/12729), fixed in [#13355](https://github.com/diegosouzapw/OmniRoute/pull/13355))
diff --git a/open-sse/config/cliFingerprints.ts b/open-sse/config/cliFingerprints.ts
index 8891f73d9d..16c78c9d5f 100644
--- a/open-sse/config/cliFingerprints.ts
+++ b/open-sse/config/cliFingerprints.ts
@@ -262,18 +262,46 @@ export function orderHeaders(
 }
 
 /**
- * Apply a CLI fingerprint to headers and body.
- * Returns { headers, bodyString } with the correct ordering.
+ * Internal request-body markers that are NOT `_omniroute*`-prefixed and must be
+ * removed key-by-key. Everything else is caught by INTERNAL_BODY_FIELD_PREFIX.
+ */
+const INTERNAL_BODY_FIELDS: readonly string[] = [
+  "_claudeCodeRequiresLowercaseToolNames",
+  "_nativeCodexPassthrough",
+  "_nativeXaiResponsesPassthrough",
+  "_nativeOpenAICompatibleResponsesPassthrough",
+];
+
+/**
+ * Every omniroute-owned internal marker uses this prefix, so the strip is
+ * prefix-based rather than an allowlist. An allowlist silently leaks each newly
+ * added marker to the upstream, where strict gateways reject the whole request
+ * (observed live: `[400]: _omnirouteSkipContextRelay: Extra inputs are not
+ * permitted` on a claude hop, from the context/universal-handoff markers set in
+ * `open-sse/services/contextHandoff.ts`). Markers are consumed by routing before
+ * dispatch, so removing them at this chokepoint is always safe.
+ *
+ * Deliberately narrow: only omniroute-owned prefixes are ours. A caller-sent
+ * field that merely starts with `_` is client payload and passes through.
+ */
+const INTERNAL_BODY_FIELD_PREFIX = "_omniroute";
+
+/**
+ * Remove omniroute-internal markers from a request body before it is serialized
+ * for an upstream. Mutates and returns the same object.
  */
 export function stripInternalBodyFields(body: unknown): unknown {
   if (!body || typeof body !== "object" || Array.isArray(body)) return body;
 
   const record = body as Record<string, unknown>;
-  delete record._claudeCodeRequiresLowercaseToolNames;
-  delete record._nativeCodexPassthrough;
-  delete record._nativeXaiResponsesPassthrough;
-  delete record._nativeOpenAICompatibleResponsesPassthrough;
-  delete record._omnirouteResponsesStore;
+  for (const field of INTERNAL_BODY_FIELDS) {
+    delete record[field];
+  }
+  for (const key of Object.keys(record)) {
+    if (key.startsWith(INTERNAL_BODY_FIELD_PREFIX)) {
+      delete record[key];
+    }
+  }
   return body;
 }
 
diff --git a/tests/unit/strip-internal-omniroute-markers.test.ts b/tests/unit/strip-internal-omniroute-markers.test.ts
new file mode 100644
index 0000000000..5c89f03eb4
--- /dev/null
+++ b/tests/unit/strip-internal-omniroute-markers.test.ts
@@ -0,0 +1,167 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+
+import { stripInternalBodyFields } from "../../open-sse/config/cliFingerprints.ts";
+import { SKIP_UNIVERSAL_HANDOFF_FLAG } from "../../open-sse/services/contextHandoff.ts";
+
+// Live 400 on the claude/claude-opus-5 hop of best-reasoning-paid:
+//   [400]: _omnirouteSkipContextRelay: Extra inputs are not permitted
+//
+// contextHandoff.ts stamps `_omnirouteSkipContextRelay` / `_omnirouteInternalRequest`
+// onto the internal summary body so the context-relay injection in chat.ts skips
+// its own request. Those keys are consumed by routing BEFORE dispatch and must
+// never reach an upstream. stripInternalBodyFields() was a hand-maintained
+// allowlist of 5 unrelated markers, so the handoff markers leaked into the
+// serialized upstream payload and strict Anthropic-compatible gateways rejected
+// the whole request. Every `_omniroute*` marker is internal by construction, so
+// the strip is prefix-based, not per-key.
+
+test("stripInternalBodyFields removes the context-handoff markers", () => {
+  const body: Record<string, unknown> = {
+    model: "claude-opus-5",
+    messages: [{ role: "user", content: "hi" }],
+    max_tokens: 64,
+    _omnirouteSkipContextRelay: true,
+    _omnirouteInternalRequest: "context-handoff",
+  };
+
+  stripInternalBodyFields(body);
+
+  assert.equal(body._omnirouteSkipContextRelay, undefined);
+  assert.equal(body._omnirouteInternalRequest, undefined);
+  assert.ok(!("_omnirouteSkipContextRelay" in body));
+  assert.ok(!("_omnirouteInternalRequest" in body));
+  // Real payload must survive.
+  assert.equal(body.model, "claude-opus-5");
+  assert.equal(body.max_tokens, 64);
+  assert.ok(Array.isArray(body.messages));
+});
+
+test("stripInternalBodyFields removes the universal-handoff markers", () => {
+  const body: Record<string, unknown> = {
+    model: "claude-opus-5",
+    messages: [{ role: "user", content: "hi" }],
+    _omnirouteSkipContextRelay: true,
+    _omnirouteInternalRequest: "universal-handoff",
+    [SKIP_UNIVERSAL_HANDOFF_FLAG]: true,
+  };
+
+  stripInternalBodyFields(body);
+
+  assert.equal(body[SKIP_UNIVERSAL_HANDOFF_FLAG], undefined);
+  assert.equal(body._omnirouteInternalRequest, undefined);
+});
+
+test("stripInternalBodyFields strips every _omniroute* marker by prefix", () => {
+  // Guards the whole class: any future internal `_omniroute*` marker is stripped
+  // without editing an allowlist, so it cannot become the next upstream 400.
+  const body: Record<string, unknown> = {
+    model: "m",
+    _omnirouteReasoningRouteTrace: { decision: "x" },
+    _omnirouteReasoningRule: { rule: "y" },
+    _omnirouteResponsesStore: "auto",
+    _omnirouteCopilotReasoningSummary: "summarized",
+    _omnirouteSomeFutureMarkerNotYetWritten: true,
+  };
+
+  stripInternalBodyFields(body);
+
+  for (const key of Object.keys(body)) {
+    assert.ok(!key.startsWith("_omniroute"), `leaked internal marker: ${key}`);
+  }
+  assert.equal(body.model, "m");
+});
+
+test("stripInternalBodyFields keeps the pre-existing non-_omniroute markers stripped", () => {
+  const body: Record<string, unknown> = {
+    model: "m",
+    _claudeCodeRequiresLowercaseToolNames: true,
+    _nativeCodexPassthrough: true,
+    _nativeXaiResponsesPassthrough: true,
+    _nativeOpenAICompatibleResponsesPassthrough: true,
+  };
+
+  stripInternalBodyFields(body);
+
+  assert.deepEqual(Object.keys(body), ["model"]);
+});
+
+test("stripInternalBodyFields leaves client fields with a leading underscore alone", () => {
+  // Only omniroute-owned prefixes are internal. A client field that merely
+  // starts with `_` is part of the caller's payload and must pass through.
+  const body: Record<string, unknown> = {
+    model: "m",
+    _id: "caller-owned",
+    _meta: { trace: 1 },
+  };
+
+  stripInternalBodyFields(body);
+
+  assert.equal(body._id, "caller-owned");
+  assert.deepEqual(body._meta, { trace: 1 });
+});
+
+test("stripInternalBodyFields tolerates non-object input", () => {
+  assert.equal(stripInternalBodyFields(null), null);
+  assert.equal(stripInternalBodyFields(undefined), undefined);
+  assert.equal(stripInternalBodyFields("str"), "str");
+  const arr = [1, 2];
+  assert.equal(stripInternalBodyFields(arr), arr);
+});
+
+// End-to-end guard at the real boundary: the helper above is only correct if the
+// dispatch path actually calls it. This drives DefaultExecutor.execute() with a
+// stubbed fetch and asserts the SERIALIZED upstream body -- the exact bytes that
+// produced the live 400 -- carries no internal markers.
+test("DefaultExecutor.execute never serializes _omniroute* markers into the upstream body", async () => {
+  const { DefaultExecutor } = await import("../../open-sse/executors/default.ts");
+
+  const originalFetch = globalThis.fetch;
+  const sentBodies: string[] = [];
+  globalThis.fetch = (async (_url: unknown, init: { body?: unknown } = {}) => {
+    if (typeof init.body === "string") sentBodies.push(init.body);
+    return new Response(JSON.stringify({ ok: true }), {
+      status: 200,
+      headers: { "Content-Type": "application/json" },
+    });
+  }) as typeof globalThis.fetch;
+
+  try {
+    const executor = new DefaultExecutor("anthropic-compatible-cc-test");
+    await executor.execute({
+      model: "claude-opus-5",
+      body: {
+        model: "claude-opus-5",
+        messages: [{ role: "user", content: "hi" }],
+        max_tokens: 1,
+        _omnirouteSkipContextRelay: true,
+        _omnirouteInternalRequest: "context-handoff",
+      },
+      stream: false,
+      credentials: {
+        apiKey: "test-key",
+        // #13452/#13798: `*-compatible-*` nodes must carry an explicit baseUrl or
+        // buildUrl() throws rather than silently defaulting to the real Anthropic
+        // API. The stubbed fetch below intercepts this URL; nothing leaves the box.
+        providerSpecificData: {
+          ccSessionId: "session-1",
+          baseUrl: "http://127.0.0.1:1/v1",
+        },
+      },
+      extendedContext: false,
+    } as never);
+  } finally {
+    globalThis.fetch = originalFetch;
+  }
+
+  assert.ok(sentBodies.length > 0, "executor must have dispatched a request");
+  for (const raw of sentBodies) {
+    assert.ok(
+      !raw.includes("_omniroute"),
+      `internal marker leaked into serialized upstream body: ${raw.slice(0, 300)}`
+    );
+    // The real payload must still be there -- proving the assertion above is not
+    // passing because the body was emptied.
+    assert.ok(raw.includes("claude-opus-5"), "real payload must survive the strip");
+  }
+});