diff --git a/CHANGELOG.md b/CHANGELOG.md
index 1debc8a..3641ee7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,7 @@
 
 - Recognize indented file headers like Codex, and reject any other line between file sections instead of silently dropping that section while reporting success (#45).
 - Preserve each line's ending on update: CRLF and mixed-ending files are no longer rewritten to LF, inserted lines take the file's line ending, and context lines keep their exact text (#47).
+- Declare the Codex Lark grammar through pi's `constrainedSampling` hook, so GPT models on providers with OpenAI grammar tools get `apply_patch` as a native grammar tool instead of a plain JSON function (#43, thanks @zidou-kiyn).
 
 ### Changed
 
diff --git a/src/index.ts b/src/index.ts
index 3e98659..c38a056 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1415,6 +1415,10 @@ export function createApplyPatchTool(): ApplyPatchToolDefinition {
 		label: "ApplyPatch",
 		description: APPLY_PATCH_FREEFORM_DESCRIPTION,
 		parameters: APPLY_PATCH_PARAMS,
+		// Pi's public grammar hook: models whose provider sets `compat.supportsOpenAIGrammarTools`
+		// receive apply_patch as a native OpenAI custom tool constrained by the Codex Lark grammar.
+		// Other providers fall back to the plain function tool.
+		constrainedSampling: { type: "grammar", variants: { openai_lark: APPLY_PATCH_LARK_GRAMMAR } },
 		prepareArguments: normalizeApplyPatchArguments,
 		promptSnippet: "Apply Codex-format file patches with apply_patch",
 		promptGuidelines: [
diff --git a/test/grammar-tool.test.ts b/test/grammar-tool.test.ts
new file mode 100644
index 0000000..1f3b843
--- /dev/null
+++ b/test/grammar-tool.test.ts
@@ -0,0 +1,29 @@
+import { resolveGrammarConstrainedSampling } from "@earendil-works/pi-ai/api/constrained-sampling";
+import { describe, expect, it } from "vitest";
+import { APPLY_PATCH_LARK_GRAMMAR, createApplyPatchTool } from "../src/index.js";
+
+describe("apply_patch grammar tool", () => {
+	it("#given a provider with OpenAI grammar tools #when pi prepares apply_patch #then it is sent as the Codex Lark grammar tool", () => {
+		// given
+		const tool = createApplyPatchTool();
+
+		// when
+		const grammar = resolveGrammarConstrainedSampling(tool, true);
+
+		// then
+		expect(grammar?.format).toBe("lark");
+		expect(grammar?.definition).toBe(APPLY_PATCH_LARK_GRAMMAR);
+		expect(grammar?.inputProperty).toBe("input");
+	});
+
+	it("#given a provider without grammar tools #when pi prepares apply_patch #then it stays a plain function tool", () => {
+		// given
+		const tool = createApplyPatchTool();
+
+		// when
+		const grammar = resolveGrammarConstrainedSampling(tool, false);
+
+		// then
+		expect(grammar).toBeUndefined();
+	});
+});