fix(agent): preserved clamped tool image results

Added a textual omission marker when provider image clamping removes every block from a successful tool result, keeping the serialized tool_result meaningful and protocol-safe.\n\nFixes #3230

packages/coding-agent/src/session/provider-image-budget.ts

@@ -9,6 +9,11 @@ import type {
 } from "@oh-my-pi/pi-ai";
 import { providerImageBudget } from "@oh-my-pi/snapcompact";
 
+const TOOL_RESULT_IMAGE_OMISSION: TextContent = {
+	type: "text",
+	text: "[image omitted: provider image limit]",
+};
+
 function countImages(context: Context): number {
 	let count = 0;
 	for (const message of context.messages) {
@@ -52,7 +57,8 @@ function clampDeveloperMessage(message: DeveloperMessage, state: { remainingDrop
 function clampToolResultMessage(message: ToolResultMessage, state: { remainingDrops: number }): ToolResultMessage {
 	if (state.remainingDrops <= 0) return message;
 	const content = clampContent(message.content, state);
-	return content ? { ...message, content } : message;
+	if (!content) return message;
+	return { ...message, content: content.length > 0 ? content : [TOOL_RESULT_IMAGE_OMISSION] };
 }
 
 /** Drops oldest transient image blocks so outgoing vision requests fit the active provider's image cap. */

packages/coding-agent/test/session/provider-image-budget.test.ts

@@ -69,6 +69,28 @@ describe("provider context image budgets", () => {
 		expect(imageData(context)).toEqual(Array.from({ length: 31 }, (_, index) => `image-${index}`));
 	});
 
+	it("keeps image-only tool results meaningful when every image block is dropped", () => {
+		const context: Context = {
+			systemPrompt: [],
+			tools: [],
+			messages: Array.from({ length: 11 }, (_, index) => ({
+				role: "toolResult",
+				toolCallId: `call-${index}`,
+				toolName: "inspect_image",
+				content: [image(`image-${index}`)],
+				isError: false,
+				timestamp: index,
+			})),
+		};
+
+		const clamped = clampProviderContextImages(context, UMANS_MODEL);
+		const firstMessage = clamped.messages[0];
+
+		expect(imageData(clamped)).toEqual(Array.from({ length: 10 }, (_, index) => `image-${index + 1}`));
+		expect(firstMessage?.role).toBe("toolResult");
+		expect(firstMessage?.content).toEqual([text("[image omitted: provider image limit]")]);
+	});
+
 	it("preserves context identity when the provider cap is not exceeded", () => {
 		const context: Context = {
 			systemPrompt: [],