diff --git a/src/adapters/openai-chat.ts b/src/adapters/openai-chat.ts
index 792d4034b3..ac486f5b8a 100644
--- a/src/adapters/openai-chat.ts
+++ b/src/adapters/openai-chat.ts
@@ -968,8 +968,11 @@ export function createOpenAIChatAdapter(provider: OcxProviderConfig): ProviderAd
         if (provider.parallelToolCalls === false) {
           // NIM documents the Boolean defaulting to false and kimi rejects true; pin the
           // wire bit so Codex cannot opt in via request.options. Other opted-out providers
-          // omit the field so strict OpenAI-compatible hosts never see an unsupported knob.
-          if (provider.baseUrl === "https://integrate.api.nvidia.com/v1") {
+          // omit the field by default so strict OpenAI-compatible hosts never see an
+          // unsupported knob, but a self-hosted gateway that DOES honor the field and keeps
+          // emitting parallel calls without it can opt in via pinParallelToolCallsFalse.
+          if (provider.baseUrl === "https://integrate.api.nvidia.com/v1"
+              || provider.pinParallelToolCallsFalse === true) {
             body.parallel_tool_calls = false;
           }
         } else if (provider.parallelToolCalls === true) {
diff --git a/src/types.ts b/src/types.ts
index d24811f430..6d0e84affb 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -1450,6 +1450,16 @@ export interface OcxProviderConfig {
    * only on explicit `true`. See devlog/_plan/260709_parallel_tool_calls.
    */
   parallelToolCalls?: boolean;
+  /**
+   * Opt-in: when `parallelToolCalls` is `false`, actually send `parallel_tool_calls: false`
+   * on the `/chat/completions` wire for this provider. By default an opted-out provider only
+   * OMITS the field (strict OpenAI-compatible hosts reject unknown knobs), and the NVIDIA NIM
+   * baseUrl is the sole built-in exception that pins the wire bit. Some self-hosted gateways
+   * (Kimi/GLM-family, vLLM, etc.) do honor `parallel_tool_calls` and keep emitting concurrent
+   * tool calls unless it is present; enable this to pin the bit without hardcoding their URL.
+   * No effect unless `parallelToolCalls === false`; ignored by non-`openai-chat` adapters.
+   */
+  pinParallelToolCallsFalse?: boolean;
   /**
    * Opt-in: forward `prompt_cache_key` to the upstream `/chat/completions` body.
    * OpenAI-specific extension; strict backends (Groq, Cerebras, etc.) reject unknown
diff --git a/tests/parallel-tool-calls-optin.test.ts b/tests/parallel-tool-calls-optin.test.ts
index 2252220cff..5b154e6ba8 100644
--- a/tests/parallel-tool-calls-optin.test.ts
+++ b/tests/parallel-tool-calls-optin.test.ts
@@ -42,6 +42,24 @@ describe("parallel tool calls provider opt-in (request body)", () => {
     const body = JSON.parse(adapter.buildRequest(parsedRequest({ parallelToolCalls: true })).body) as Record<string, unknown>;
     expect(body).not.toHaveProperty("parallel_tool_calls");
   });
+
+  test("pinParallelToolCallsFalse pins the wire bit for an opted-out non-NVIDIA provider", () => {
+    const adapter = createOpenAIChatAdapter({ adapter: "openai-chat", baseUrl: "https://llm.example.internal/v1", apiKey: "k", parallelToolCalls: false, pinParallelToolCallsFalse: true });
+    const body = JSON.parse(adapter.buildRequest(parsedRequest()).body) as Record<string, unknown>;
+    expect(body.parallel_tool_calls).toBe(false);
+  });
+
+  test("pinParallelToolCallsFalse keeps sending false even under a permissive request bit", () => {
+    const adapter = createOpenAIChatAdapter({ adapter: "openai-chat", baseUrl: "https://llm.example.internal/v1", apiKey: "k", parallelToolCalls: false, pinParallelToolCallsFalse: true });
+    const body = JSON.parse(adapter.buildRequest(parsedRequest({ parallelToolCalls: true })).body) as Record<string, unknown>;
+    expect(body.parallel_tool_calls).toBe(false);
+  });
+
+  test("pinParallelToolCallsFalse has no effect without parallelToolCalls:false", () => {
+    const adapter = createOpenAIChatAdapter({ adapter: "openai-chat", baseUrl: "https://llm.example.internal/v1", apiKey: "k", pinParallelToolCallsFalse: true });
+    const body = JSON.parse(adapter.buildRequest(parsedRequest()).body) as Record<string, unknown>;
+    expect(body).not.toHaveProperty("parallel_tool_calls");
+  });
 });
 
 describe("stale persisted config backfill (router)", () => {