diff --git a/open-sse/config/providerRegistry.ts b/open-sse/config/providerRegistry.ts
index b030d9c498bb..a13cdde1306d 100644
--- a/open-sse/config/providerRegistry.ts
+++ b/open-sse/config/providerRegistry.ts
@@ -112,6 +112,12 @@ export interface RegistryEntry {
   modelsUrl?: string;
   /** Prefix to prepend to model IDs before upstream API calls (e.g. "accounts/fireworks/models/") */
   modelIdPrefix?: string;
+  /**
+   * Additional already-qualified model ID prefixes that must NOT receive `modelIdPrefix`
+   * (e.g. Fireworks router IDs "accounts/fireworks/routers/"). Prevents double-prefixing
+   * fully-qualified IDs that legitimately differ from `modelIdPrefix`. See issue #3133.
+   */
+  acceptedModelIdPrefixes?: string[];
   chatPath?: string;
   clientVersion?: string;
   timeoutMs?: number;
@@ -3037,6 +3043,7 @@ const _REGISTRY_EAGER: Record<string, RegistryEntry> = {
     modelsUrl:
       "https://api.fireworks.ai/v1/accounts/fireworks/models?filter=supports_serverless=true",
     modelIdPrefix: "accounts/fireworks/models/",
+    acceptedModelIdPrefixes: ["accounts/fireworks/models/", "accounts/fireworks/routers/"],
     authType: "apikey",
     authHeader: "bearer",
     models: [
diff --git a/open-sse/executors/default.ts b/open-sse/executors/default.ts
index 8595e82d39eb..91cfe9df6f56 100644
--- a/open-sse/executors/default.ts
+++ b/open-sse/executors/default.ts
@@ -510,8 +510,16 @@ export class DefaultExecutor extends BaseExecutor {
       const entry = getRegistryEntry(this.provider);
       if (entry?.modelIdPrefix) {
         const body = withDefaults as Record<string, unknown>;
-        if (typeof body.model === "string" && !body.model.startsWith(entry.modelIdPrefix)) {
-          body.model = `${entry.modelIdPrefix}${body.model}`;
+        if (typeof body.model === "string") {
+          // Skip prepending when the model already carries the canonical prefix OR any
+          // other accepted fully-qualified prefix (e.g. Fireworks router IDs). #3133.
+          const acceptedPrefixes = [entry.modelIdPrefix, ...(entry.acceptedModelIdPrefixes ?? [])];
+          const alreadyQualified = acceptedPrefixes.some((prefix) =>
+            (body.model as string).startsWith(prefix)
+          );
+          if (!alreadyQualified) {
+            body.model = `${entry.modelIdPrefix}${body.model}`;
+          }
         }
       }
     }
diff --git a/tests/unit/fireworks-model-id-prefix.test.ts b/tests/unit/fireworks-model-id-prefix.test.ts
index c16203b9cda8..24acb2afee7b 100644
--- a/tests/unit/fireworks-model-id-prefix.test.ts
+++ b/tests/unit/fireworks-model-id-prefix.test.ts
@@ -27,6 +27,26 @@ test("DefaultExecutor.transformRequest does not double-prepend modelIdPrefix", (
   assert.equal((result as Record<string, unknown>).model, "accounts/fireworks/models/kimi-k2p6");
 });
 
+test("DefaultExecutor.transformRequest preserves fully-qualified fireworks router IDs (#3133)", () => {
+  const executor = new DefaultExecutor("fireworks");
+  const body = {
+    model: "accounts/fireworks/routers/kimi-k2p6-turbo",
+    messages: [{ role: "user", content: "hi" }],
+  };
+
+  const result = executor.transformRequest(
+    "accounts/fireworks/routers/kimi-k2p6-turbo",
+    body,
+    false,
+    {}
+  );
+
+  assert.equal(
+    (result as Record<string, unknown>).model,
+    "accounts/fireworks/routers/kimi-k2p6-turbo"
+  );
+});
+
 test("DefaultExecutor.transformRequest does not modify model for providers without modelIdPrefix", () => {
   const executor = new DefaultExecutor("openai");
   const body = { model: "gpt-4.1", messages: [{ role: "user", content: "hi" }] };