diff --git a/packages/pi-plugin/src/issue-785-request-controls.test.ts b/packages/pi-plugin/src/issue-785-request-controls.test.ts
new file mode 100644
index 000000000..a2a48406a
--- /dev/null
+++ b/packages/pi-plugin/src/issue-785-request-controls.test.ts
@@ -0,0 +1,97 @@
+import { expect, test } from "bun:test";
+import {
+	piPreparedStructure,
+	piPreparedStructureMismatch,
+} from "./pi-dispatch-structure";
+
+// Issue 785: Oh My Pi's Anthropic provider appends request-control system
+// messages (tool_addition/tool_removal blocks, or an empty effort-only control)
+// after the context pass. They are not prepared turns, so the dispatch check
+// must not count them as conversation rows.
+
+const anthropic = {
+	api: "anthropic-messages",
+	compat: { supportsMidConversationSystem: true },
+	input: ["text", "image"],
+};
+const head = { role: "user", content: "managed history", timestamp: 1 };
+const prompt = { role: "user", content: "current request", timestamp: 4 };
+const prepared = piPreparedStructure(
+	[
+		head,
+		{
+			role: "assistant",
+			content: [{ type: "toolCall", id: "one", name: "read", arguments: {} }],
+			timestamp: 2,
+		},
+		{
+			role: "toolResult",
+			toolCallId: "one",
+			toolName: "read",
+			content: [{ type: "text", text: "body" }],
+			timestamp: 3,
+		},
+		prompt,
+	],
+	anthropic,
+);
+const wire = [
+	head,
+	{
+		role: "assistant",
+		content: [{ type: "tool_use", id: "one", name: "read", input: {} }],
+	},
+	{
+		role: "user",
+		content: [{ type: "tool_result", tool_use_id: "one", content: "body" }],
+	},
+	prompt,
+];
+const control = (type: string, name: string) => ({
+	type,
+	tool: { type: "tool_reference", name },
+});
+
+test("issue 785: trailing request-control system rows are not prepared turns", () => {
+	expect(
+		piPreparedStructureMismatch({ messages: wire }, prepared),
+	).toBeUndefined();
+	for (const content of [
+		[control("tool_addition", "x")],
+		[control("tool_addition", "x"), control("tool_removal", "y")],
+		[],
+	])
+		expect(
+			piPreparedStructureMismatch(
+				{ messages: [...wire, { role: "system", content }] },
+				prepared,
+			),
+		).toBeUndefined();
+});
+
+test("issue 785: unprepared system text rows still refuse", () => {
+	for (const content of [
+		"unprepared instructions",
+		[{ type: "text", text: "unprepared instructions" }],
+		[control("tool_addition", "x"), { type: "text", text: "unprepared" }],
+	]) {
+		expect(
+			piPreparedStructureMismatch(
+				{ messages: [...wire, { role: "system", content }] },
+				prepared,
+			),
+		).toContain("comparison=count");
+		expect(
+			piPreparedStructureMismatch(
+				{
+					messages: [
+						...wire.slice(0, 3),
+						{ role: "system", content },
+						...wire.slice(3),
+					],
+				},
+				prepared,
+			),
+		).toContain("comparison=");
+	}
+});
diff --git a/packages/pi-plugin/src/pi-dispatch-structure.ts b/packages/pi-plugin/src/pi-dispatch-structure.ts
index 978d4f11c..dbd8c6099 100644
--- a/packages/pi-plugin/src/pi-dispatch-structure.ts
+++ b/packages/pi-plugin/src/pi-dispatch-structure.ts
@@ -971,6 +971,19 @@ function payloadStructure(
 			(message.role === "system" || message.role === "developer")
 		)
 			continue;
+		// Oh My Pi's Anthropic provider appends request-control system messages
+		// (only tool_addition/tool_removal blocks, or empty effort-only controls)
+		// while building the request, after the context pass. They are not
+		// prepared turns. @oh-my-pi/pi-ai/src/providers/anthropic.ts
+		// insertAnthropicControlMarkers (18.12.6).
+		if (
+			message.role === "system" &&
+			Array.isArray(message.content) &&
+			message.content.every((block) =>
+				["tool_addition", "tool_removal"].includes(String(object(block)?.type)),
+			)
+		)
+			continue;
 		// Responses emits reasoning/text/calls as separate items for one assistant
 		// turn. Only its call/result IDs survive every supported replay mode.
 		if (responses && message.type === "reasoning") continue;