diff --git a/src/index.ts b/src/index.ts
index c47d4f5..ef80466 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1391,6 +1391,10 @@ 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/index.test.ts b/test/index.test.ts
index c8e22be..4759881 100644
--- a/test/index.test.ts
+++ b/test/index.test.ts
@@ -112,11 +112,13 @@ describe("pi-apply-patch", () => {
 		let capturedToolName: string | undefined;
 		let capturedDescription: string | undefined;
 		let capturedFreeform: FreeformToolFormat | undefined;
+		let capturedConstrainedSampling: ReturnType<typeof createApplyPatchTool>["constrainedSampling"];
 		const extensionApi = {
 			registerTool(tool: ReturnType<typeof createApplyPatchTool>) {
 				capturedToolName = tool.name;
 				capturedDescription = tool.description;
 				capturedFreeform = tool.freeform;
+				capturedConstrainedSampling = tool.constrainedSampling;
 			},
 			on() {
 				return () => {};
@@ -138,6 +140,10 @@ describe("pi-apply-patch", () => {
 			syntax: "lark",
 			definition: APPLY_PATCH_LARK_GRAMMAR,
 		});
+		expect(capturedConstrainedSampling).toEqual({
+			type: "grammar",
+			variants: { openai_lark: APPLY_PATCH_LARK_GRAMMAR },
+		});
 	});
 
 	it("#given GPT model after reload with apply_patch already active #when session starts #then keeps apply_patch active", async () => {