diff --git a/changelog.d/fixes/9545-gpt56-effort-tools.md b/changelog.d/fixes/9545-gpt56-effort-tools.md
new file mode 100644
index 0000000000..7cc1a77bb3
--- /dev/null
+++ b/changelog.d/fixes/9545-gpt56-effort-tools.md
@@ -0,0 +1 @@
+- fix(providers): strip provider prefix in getModelTargetFormat to route GPT-5.6 models to /v1/responses (#9545)
diff --git a/open-sse/config/providerModels.ts b/open-sse/config/providerModels.ts
index 4051e00e93..75099bad83 100644
--- a/open-sse/config/providerModels.ts
+++ b/open-sse/config/providerModels.ts
@@ -12,27 +12,27 @@ export const PROVIDER_MODELS: Record<string, RegistryModel[]> = new Proxy(
   {} as Record<string, RegistryModel[]>,
   {
     get(_, prop) {
-      if (typeof prop === 'symbol') return undefined;
+      if (typeof prop === "symbol") return undefined;
       return Reflect.get(initModels(), prop, _models);
     },
     has(_, prop) {
-      if (typeof prop === 'symbol') return false;
+      if (typeof prop === "symbol") return false;
       return Reflect.has(initModels(), prop);
     },
     ownKeys() {
       return Reflect.ownKeys(initModels());
     },
     getOwnPropertyDescriptor(_, prop) {
-      if (typeof prop === 'symbol') return undefined;
+      if (typeof prop === "symbol") return undefined;
       return Object.getOwnPropertyDescriptor(initModels(), prop);
     },
     set(_, prop, value) {
-      if (typeof prop === 'symbol') return false;
+      if (typeof prop === "symbol") return false;
       (initModels() as Record<string, RegistryModel[]>)[prop] = value;
       return true;
     },
     deleteProperty(_, prop) {
-      if (typeof prop === 'symbol') return false;
+      if (typeof prop === "symbol") return false;
       return Reflect.deleteProperty(initModels(), prop);
     },
   }
@@ -41,27 +41,27 @@ export const PROVIDER_ID_TO_ALIAS: Record<string, string> = new Proxy(
   {} as Record<string, string>,
   {
     get(_, prop) {
-      if (typeof prop === 'symbol') return undefined;
+      if (typeof prop === "symbol") return undefined;
       return Reflect.get(initAliases(), prop, _aliases);
     },
     has(_, prop) {
-      if (typeof prop === 'symbol') return false;
+      if (typeof prop === "symbol") return false;
       return Reflect.has(initAliases(), prop);
     },
     ownKeys() {
       return Reflect.ownKeys(initAliases());
     },
     getOwnPropertyDescriptor(_, prop) {
-      if (typeof prop === 'symbol') return undefined;
+      if (typeof prop === "symbol") return undefined;
       return Object.getOwnPropertyDescriptor(initAliases(), prop);
     },
     set(_, prop, value) {
-      if (typeof prop === 'symbol') return false;
+      if (typeof prop === "symbol") return false;
       (initAliases() as Record<string, string>)[prop] = value;
       return true;
     },
     deleteProperty(_, prop) {
-      if (typeof prop === 'symbol') return false;
+      if (typeof prop === "symbol") return false;
       return Reflect.deleteProperty(initAliases(), prop);
     },
   }
@@ -116,7 +116,13 @@ export function findModelName(aliasOrId: string, modelId: string): string {
 
 export function getModelTargetFormat(aliasOrId: string, modelId: string): string | null {
   const models = PROVIDER_MODELS[aliasOrId];
-  const found = models?.find((m) => m.id === modelId);
+  // Strip provider prefix if present: "openai/gpt-5.6-luna" → "gpt-5.6-luna"
+  const prefix = aliasOrId + "/";
+  const bareModelId =
+    typeof modelId === "string" && modelId.startsWith(prefix)
+      ? modelId.slice(prefix.length)
+      : modelId;
+  const found = models?.find((m) => m.id === bareModelId);
   if (found?.targetFormat) return found.targetFormat;
   // #5842: OpenAI "*-pro" reasoning models (o1-pro, gpt-5.x-pro) are only served by
   // the native /v1/responses endpoint — /v1/chat/completions 404s ("only supported
@@ -124,7 +130,7 @@ export function getModelTargetFormat(aliasOrId: string, modelId: string): string
   // covers dynamically-synced ids that post-date the catalog (same spirit as the gh
   // executor's /codex/i routing, 9router#102). Scoped to the openai alias so other
   // providers shipping *-pro ids keep their own endpoint semantics.
-  if (aliasOrId === "openai" && /-pro$/i.test(modelId)) return "openai-responses";
+  if (aliasOrId === "openai" && /-pro$/i.test(bareModelId)) return "openai-responses";
   return null;
 }
 
diff --git a/tests/unit/9545-gpt56-reasoning-tools.test.ts b/tests/unit/9545-gpt56-reasoning-tools.test.ts
new file mode 100644
index 0000000000..4899e59f1f
--- /dev/null
+++ b/tests/unit/9545-gpt56-reasoning-tools.test.ts
@@ -0,0 +1,29 @@
+import { describe, it } from "node:test";
+import assert from "node:assert";
+
+describe("Issue #9545 — GPT-5.6 URL routing + reasoning_effort with tools", () => {
+  it("getModelTargetFormat should resolve gpt-5.6-luna with and without provider prefix", async () => {
+    const { getModelTargetFormat } = await import("../../open-sse/config/providerModels.ts");
+    assert.strictEqual(getModelTargetFormat("openai", "gpt-5.6-luna"), "openai-responses");
+    assert.strictEqual(getModelTargetFormat("openai", "openai/gpt-5.6-luna"), "openai-responses");
+  });
+
+  it("getModelTargetFormat should resolve non-prefixed models unchanged", async () => {
+    const { getModelTargetFormat } = await import("../../open-sse/config/providerModels.ts");
+    assert.strictEqual(getModelTargetFormat("openai", "gpt-4o"), null);
+    assert.strictEqual(getModelTargetFormat("openai", "gpt-5.5-pro"), "openai-responses");
+    assert.strictEqual(getModelTargetFormat("openai", "openai/gpt-5.5-pro"), "openai-responses");
+  });
+
+  it("stripGpt5ReasoningWhenTools should not strip when targetFormat=openai-responses", async () => {
+    const { stripGpt5ReasoningWhenTools } =
+      await import("../../open-sse/services/gpt5SamplingGuard.ts");
+    const body = {
+      model: "gpt-5.6-luna",
+      tools: [{ type: "function", function: { name: "test" } }],
+      reasoning_effort: "high",
+    };
+    const r = stripGpt5ReasoningWhenTools(body, "openai", "gpt-5.6-luna", "openai-responses", null);
+    assert.strictEqual(r.reasoning_effort, "high");
+  });
+});