diff --git a/docs/provider-endpoint-constraints.md b/docs/provider-endpoint-constraints.md
index b3781f654..957476df5 100644
--- a/docs/provider-endpoint-constraints.md
+++ b/docs/provider-endpoint-constraints.md
@@ -257,8 +257,9 @@ session/provider path.
 ### Responses and Codex custom tools
 
 Responses and Codex both support freeform custom grammar tools for `apply_patch`.
-Both disable request-level parallel tool calls when any custom grammar tool is
-present. Responses additionally:
+Custom grammar tools do not force request-level `parallel_tool_calls`; Codex
+`responsesLite` separately disables request-level parallel tool calls whenever
+tools are present. Responses additionally:
 
 - sanitizes schemas differently
 - quarantines invalid enum/const schema contradictions
diff --git a/packages/ai/CHANGELOG.md b/packages/ai/CHANGELOG.md
index 83a4e51ae..681989fe4 100644
--- a/packages/ai/CHANGELOG.md
+++ b/packages/ai/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Changed
+
+- Changed OpenAI Responses and Codex Responses custom grammar tool requests to leave `parallel_tool_calls` unset instead of forcing serial tool calls; Codex `responsesLite` still disables parallel tool calls when tools are present.
+
 ### Fixed
 
 - Fixed Bedrock `/btw` and other no-tool ephemeral turns failing after prior tool calls by sending the required sentinel `toolConfig` whenever replayed history contains `toolUse`/`toolResult` blocks. ([#3124](https://github.com/can1357/oh-my-pi/issues/3124))
diff --git a/packages/ai/src/providers/openai-codex-responses.ts b/packages/ai/src/providers/openai-codex-responses.ts
index 37eafbefc..0164e4903 100644
--- a/packages/ai/src/providers/openai-codex-responses.ts
+++ b/packages/ai/src/providers/openai-codex-responses.ts
@@ -714,7 +714,8 @@ async function buildCodexRequestContext(
 	};
 }
 
-async function buildTransformedCodexRequestBody(
+/** @internal Exported for tests. */
+export async function buildTransformedCodexRequestBody(
 	model: Model<"openai-codex-responses">,
 	context: Context,
 	options: OpenAICodexResponsesOptions | undefined,
@@ -744,16 +745,6 @@ async function buildTransformedCodexRequestBody(
 				params.tool_choice = toolChoice;
 			}
 		}
-		// When a custom-tool is active, force serial tool-calling. OpenAI's
-		// `parallel_tool_calls` is request-scoped — disabling it here affects
-		// every tool in the turn, not just the custom one. That's coarser
-		// than spec §1's "supports_parallel_tool_calls = false" (which
-		// strictly targets `apply_patch`), but the platform API offers no
-		// per-tool flag.
-		const emittedTools = params.tools as CodexToolPayload[];
-		if (emittedTools.some(t => t.type === "custom")) {
-			params.parallel_tool_calls = false;
-		}
 	}
 
 	const systemPrompts = normalizeSystemPrompts(context.systemPrompt);
diff --git a/packages/ai/src/providers/openai-responses.ts b/packages/ai/src/providers/openai-responses.ts
index c9673926a..12315959b 100644
--- a/packages/ai/src/providers/openai-responses.ts
+++ b/packages/ai/src/providers/openai-responses.ts
@@ -862,15 +862,6 @@ export function buildParams(
 				params.tool_choice = toolChoice;
 			}
 		}
-		// The apply_patch spec §1 marks only `apply_patch` itself as
-		// `supports_parallel_tool_calls = false`. OpenAI's Responses API
-		// exposes `parallel_tool_calls` as a request-scoped flag, not a
-		// per-tool one, so when a custom grammar tool is in the list we
-		// disable parallelism for the whole turn. Slightly coarser than
-		// the spec requires — but the platform API offers no finer knob.
-		if (params.tools.some(t => (t as { type?: string }).type === "custom")) {
-			params.parallel_tool_calls = false;
-		}
 	}
 
 	const reasoningPolicy = resolveOpenAICompatPolicy(model, {
diff --git a/packages/ai/test/apply-patch-freeform.test.ts b/packages/ai/test/apply-patch-freeform.test.ts
index 7e539ff9c..10b8bb7ec 100644
--- a/packages/ai/test/apply-patch-freeform.test.ts
+++ b/packages/ai/test/apply-patch-freeform.test.ts
@@ -1,9 +1,11 @@
 import { describe, expect, test } from "bun:test";
 import {
+	buildTransformedCodexRequestBody,
 	convertOpenAICodexResponsesTools as convertCodexTools,
 	normalizeCodexToolChoice,
 } from "@oh-my-pi/pi-ai/providers/openai-codex-responses";
 import {
+	buildParams,
 	convertTools,
 	mapOpenAIResponsesToolChoiceForTools,
 	supportsFreeformApplyPatch,
@@ -74,6 +76,13 @@ const plainTool: Tool = {
 	parameters: type({ path: "string" }),
 };
 
+function hasCustomTool(tools: unknown): boolean {
+	return (
+		Array.isArray(tools) &&
+		tools.some(tool => typeof tool === "object" && tool !== null && (tool as { type?: unknown }).type === "custom")
+	);
+}
+
 const unionBranches = [
 	{
 		type: "object",
@@ -221,6 +230,31 @@ describe("tool choice mapping: freeform emission", () => {
 	});
 });
 
+describe("request params: freeform custom tools", () => {
+	test("openai responses leaves parallel tool calls unset", () => {
+		const { params } = buildParams(
+			makeModel({ applyPatchToolType: "freeform" }),
+			{ messages: [{ role: "user", content: "edit", timestamp: 0 }], tools: [editTool] },
+			undefined,
+			undefined,
+		);
+
+		expect(hasCustomTool(params.tools)).toBe(true);
+		expect(params.parallel_tool_calls).toBeUndefined();
+	});
+
+	test("codex responses leaves parallel tool calls unset for custom tools", async () => {
+		const params = await buildTransformedCodexRequestBody(
+			makeCodexModel({ applyPatchToolType: "freeform" }),
+			{ messages: [{ role: "user", content: "edit", timestamp: 0 }], tools: [editTool] },
+			undefined,
+		);
+
+		expect(hasCustomTool(params.tools)).toBe(true);
+		expect(params.parallel_tool_calls).toBeUndefined();
+	});
+});
+
 describe("custom_tool_call stream receive", () => {
 	async function* makeStream(events: unknown[]): AsyncIterable<ResponseStreamEvent> {
 		for (const e of events) yield e as ResponseStreamEvent;