fix(coding-agent): routed custom images as user content

Split image-bearing custom messages into developer text plus user image content before provider conversion so queued skill images stay out of developer slots.

packages/coding-agent/src/session/messages.ts

@@ -481,6 +481,30 @@ export function sanitizeRehydratedOpenAIResponsesAssistantMessage(message: Assis
 	};
 }
 
+function convertImageBearingCustomMessage(message: CustomMessage | HookMessage): Message[] | undefined {
+	if (typeof message.content === "string") return undefined;
+	const textBlocks = message.content.filter((content): content is TextContent => content.type === "text");
+	const imageBlocks = message.content.filter((content): content is ImageContent => content.type === "image");
+	if (imageBlocks.length === 0) return undefined;
+
+	const converted: Message[] = [];
+	if (textBlocks.length > 0) {
+		converted.push({
+			role: "developer",
+			content: textBlocks,
+			attribution: message.attribution,
+			timestamp: message.timestamp,
+		});
+	}
+	converted.push({
+		role: "user",
+		content: [{ type: "text", text: `Images attached to ${message.customType}.` }, ...imageBlocks],
+		attribution: message.attribution,
+		timestamp: message.timestamp,
+	});
+	return converted;
+}
+
 /**
  * Transform AgentMessages (including custom types) to LLM-compatible Messages.
  *
@@ -556,7 +580,12 @@ export function convertToLlm(messages: AgentMessage[]): Message[] {
 				return out;
 			}
 			case "custom":
-			case "hookMessage":
+			case "hookMessage": {
+				const split = convertImageBearingCustomMessage(m);
+				if (split) return split;
+				const converted = convertMessageToLlm(m);
+				return converted ? [converted] : [];
+			}
 			case "branchSummary":
 			case "compactionSummary":
 			case "user":

packages/coding-agent/test/session-messages.test.ts

@@ -259,6 +259,34 @@ describe("convertToLlm custom message mapping", () => {
 		expectAttribution(converted[0], "user");
 		expect(inferCopilotInitiator(converted)).toBe("user");
 	});
+
+	it("routes custom-message images through a user message", () => {
+		const image: ImageContent = { type: "image", data: "c2tpbGw=", mimeType: "image/png" };
+		const messages: AgentMessage[] = [
+			{
+				role: "custom",
+				customType: "skill-prompt",
+				content: [{ type: "text", text: "Skill body" }, image],
+				display: true,
+				attribution: "user",
+				timestamp: Date.now(),
+			},
+		];
+
+		const converted = convertToLlm(messages);
+
+		expect(converted.map(message => message.role)).toEqual(["developer", "user"]);
+		expectAttribution(converted[0], "user");
+		expectAttribution(converted[1], "user");
+		if (converted[0]?.role !== "developer" || !Array.isArray(converted[0].content)) {
+			throw new Error("Expected developer skill text");
+		}
+		expect(converted[0].content).toEqual([{ type: "text", text: "Skill body" }]);
+		if (converted[1]?.role !== "user" || !Array.isArray(converted[1].content)) {
+			throw new Error("Expected user skill images");
+		}
+		expect(converted[1].content.filter(content => content.type === "image")).toEqual([image]);
+	});
 });
 
 function getUserText(message: AgentMessage | undefined): string {