feat(coding-agent): add fork position and duplicate session option (#3431)

packages/coding-agent/test/agent-session-runtime-events.test.ts

@@ -176,7 +176,7 @@ describe("AgentSessionRuntime session lifecycle events", () => {
 		expect(successResult.selectedText).toBe("hello");
 		await runtimeHost.session.bindExtensions({});
 		expect(events).toEqual([
-			{ type: "session_before_fork", entryId: userMessage.entryId },
+			{ type: "session_before_fork", entryId: userMessage.entryId, position: "before" },
 			{ type: "session_start", reason: "fork", previousSessionFile },
 		]);
 
@@ -184,6 +184,12 @@ describe("AgentSessionRuntime session lifecycle events", () => {
 		cancelNextFork = true;
 		const cancelResult = await runtimeHost.fork(userMessage.entryId);
 		expect(cancelResult).toEqual({ cancelled: true });
-		expect(events).toEqual([{ type: "session_before_fork", entryId: userMessage.entryId }]);
+		expect(events).toEqual([{ type: "session_before_fork", entryId: userMessage.entryId, position: "before" }]);
+
+		events.length = 0;
+		cancelNextFork = true;
+		const cancelAtResult = await runtimeHost.fork("missing-entry", { position: "at" });
+		expect(cancelAtResult).toEqual({ cancelled: true });
+		expect(events).toEqual([{ type: "session_before_fork", entryId: "missing-entry", position: "at" }]);
 	});
 });

packages/coding-agent/test/extensions-runner.test.ts

@@ -695,6 +695,30 @@ describe("ExtensionRunner", () => {
 		});
 	});
 
+	describe("command context", () => {
+		it("passes fork options through to the bound handler", async () => {
+			const runtime = createExtensionRuntime();
+			const runner = new ExtensionRunner([], runtime, tempDir, sessionManager, modelRegistry);
+			const fork = vi.fn(async () => ({ cancelled: false }));
+
+			runner.bindCommandContext({
+				waitForIdle: async () => {},
+				newSession: async () => ({ cancelled: false }),
+				fork,
+				navigateTree: async () => ({ cancelled: false }),
+				switchSession: async () => ({ cancelled: false }),
+				reload: async () => {},
+			});
+
+			const commandContext = runner.createCommandContext();
+			await commandContext.fork("entry-1");
+			expect(fork).toHaveBeenCalledWith("entry-1", undefined);
+
+			await commandContext.fork("entry-2", { position: "at" });
+			expect(fork).toHaveBeenLastCalledWith("entry-2", { position: "at" });
+		});
+	});
+
 	describe("hasHandlers", () => {
 		it("returns true when handlers exist for event type", async () => {
 			const extCode = `

packages/coding-agent/test/suite/agent-session-runtime.test.ts

@@ -217,7 +217,7 @@ describe("AgentSessionRuntime characterization", () => {
 		expect(successResult.selectedText).toBe("hello");
 		await runtime.session.bindExtensions({});
 		expect(events).toEqual([
-			{ type: "session_before_fork", entryId: userMessage.entryId },
+			{ type: "session_before_fork", entryId: userMessage.entryId, position: "before" },
 			{ type: "session_start", reason: "fork", previousSessionFile },
 		]);
 
@@ -225,7 +225,13 @@ describe("AgentSessionRuntime characterization", () => {
 		cancelNextFork = true;
 		const cancelResult = await runtime.fork(userMessage.entryId);
 		expect(cancelResult).toEqual({ cancelled: true });
-		expect(events).toEqual([{ type: "session_before_fork", entryId: userMessage.entryId }]);
+		expect(events).toEqual([{ type: "session_before_fork", entryId: userMessage.entryId, position: "before" }]);
+
+		events.length = 0;
+		cancelNextFork = true;
+		const cancelAtResult = await runtime.fork("missing-entry", { position: "at" });
+		expect(cancelAtResult).toEqual({ cancelled: true });
+		expect(events).toEqual([{ type: "session_before_fork", entryId: "missing-entry", position: "at" }]);
 	});
 
 	it("throws when forking with an invalid entry id", async () => {