fix(acp): make session/load fail for an unknown session ID (#1907)

session/load registered an empty session and succeeded when the ID was neither in memory nor on disk, so a client could not tell that a stored session was gone. It now throws the same 'Session not found on disk' error as session/resume. Closes #1894

Author: addyCooks

.changeset/acp-load-unknown-session.md

@@ -0,0 +1,5 @@
+---
+"@nanocollective/nanocoder": patch
+---
+
+ACP `session/load` now fails with `Session not found on disk: <id>` for a session ID that is neither in memory nor on disk, the same error `session/resume` already returned. It used to register an empty session and succeed, so a client could not tell that a stored session was gone, and a prompt sent to that ID saved a new session under an ID the client had invented. Loading a session that exists, in memory or on disk, is unchanged.

docs/features/acp.md

@@ -34,7 +34,7 @@ With ACP the **client is the UI**: the agent runs headless and everything (strea
 - **Model display and switching** — the editor shows the current model and lets you switch between the models configured for your active provider.
 - **`ask_user`** — when the agent asks a clarifying question, the options appear as selectable buttons in the editor. (Selection only; a free-form typed answer is not available over ACP.)
 - **`@`-mentioned files** — files you reference in the editor are read and included in the prompt, using the editor's live buffer (including unsaved edits) when available.
-- **Session persistence** - ACP sessions are saved to disk like TUI sessions (see [Session Management](session-management.md)). Loading or resuming a thread replays its history, even after the editor and agent restart.
+- **Session persistence** - ACP sessions are saved to disk like TUI sessions (see [Session Management](session-management.md)). Loading or resuming a thread replays its history, even after the editor and agent restart. Loading or resuming a session ID that is neither in memory nor on disk fails with `Session not found on disk: <id>` instead of creating an empty session; only `session/new` creates one.
 - **Session management** - clients can list, resume, close and delete sessions. Nanocoder also answers a `renameSession` extension method (`sessionId`, `title`), and new sessions get a generated title from the first message.
 - **Images** - PNG, JPEG, GIF and WebP attachments are sent to the model as images.
 

source/acp/acp-agent.spec.ts

@@ -223,21 +223,32 @@ test('AcpAgent.initialize - advertises loadSession capability', async t => {
 	t.true(result.agentCapabilities?.loadSession);
 });
 
-test('AcpAgent.loadSession - creates a usable session for an unknown id', async t => {
+test('AcpAgent.loadSession - rejects an unknown id without creating a session', async t => {
 	const {agent} = createAgent();
-	const result = await agent.loadSession({
-		sessionId: 'persisted-123',
-		cwd: '/tmp',
-		mcpServers: [],
-	});
-	t.truthy(result.modes);
-	t.truthy(result.configOptions);
-	// The loaded session must accept prompts (no "session not found").
-	const prompt = await agent.prompt({
-		sessionId: 'persisted-123',
-		prompt: [{type: 'text', text: 'hi'}],
-	});
-	t.truthy(prompt.stopReason);
+	const sessionId = randomUUID();
+	await t.throwsAsync(
+		agent.loadSession({sessionId, cwd: '/tmp', mcpServers: []}),
+		{message: `Session not found on disk: ${sessionId}`},
+	);
+	t.false((agent as any).sessions.has(sessionId));
+	// Nothing was registered, so a following prompt fails and nothing is saved.
+	await t.throwsAsync(
+		agent.prompt({sessionId, prompt: [{type: 'text', text: 'hi'}]}),
+		{message: `Session not found: ${sessionId}`},
+	);
+	t.is(await sessionManager.loadSession(sessionId), null);
+});
+
+test('AcpAgent - loadSession and resumeSession reject an unknown id with the same error', async t => {
+	const {agent} = createAgent();
+	const sessionId = randomUUID();
+	const loadError = await t.throwsAsync(
+		agent.loadSession({sessionId, cwd: '/tmp', mcpServers: []}),
+	);
+	const resumeError = await t.throwsAsync(
+		agent.resumeSession({sessionId, cwd: '/tmp'}),
+	);
+	t.is(loadError?.message, resumeError?.message);
 });
 
 test('AcpAgent.loadSession - replays in-memory history for a known session', async t => {

source/acp/acp-agent.ts

@@ -172,11 +172,13 @@ export class AcpAgent implements Agent {
 			// Try loading from disk first so history persists across process restarts.
 			await sessionManager.initialize();
 			const persisted = await sessionManager.loadSession(params.sessionId);
+			if (!persisted) {
+				// Same error as resumeSession so clients can tell a missing session apart.
+				throw new Error(`Session not found on disk: ${params.sessionId}`);
+			}
 			session = this.registerSession(params.sessionId, params.cwd);
 			clearAcpReadStubs(params.sessionId);
-			if (persisted) {
-				session.messages = persisted.messages;
-			}
+			session.messages = persisted.messages;
 		}
 
 		logger.info(