fix(coding-agent): warn when session-id creates a session

closes #6407

packages/coding-agent/CHANGELOG.md

@@ -22,6 +22,7 @@
 - Fixed auto-retry for Bun fetch socket-drop errors reported as `socket connection was closed`, so transient provider disconnects do not end headless runs without retrying ([#6431](https://github.com/earendil-works/pi/issues/6431)).
 - Fixed `models.json` `modelOverrides` to apply to extension-registered provider models ([#6367](https://github.com/earendil-works/pi/issues/6367)).
 - Fixed project context file discovery to use stable parent traversal on Windows so startup no longer hangs while loading AGENTS.md or CLAUDE.md ([#6369](https://github.com/earendil-works/pi/issues/6369)).
+- Fixed `--session-id` startup to warn when no existing project session has that id and pi creates a new session ([#6407](https://github.com/earendil-works/pi/issues/6407)).
 
 ### Removed
 

packages/coding-agent/src/main.ts

@@ -344,6 +344,11 @@ async function createSessionManager(
 		if (existingSession) {
 			return SessionManager.open(existingSession.path, sessionDir);
 		}
+		console.error(
+			chalk.yellow(
+				`Warning: No project session found with id '${parsed.sessionId}'; creating a new session with that id.`,
+			),
+		);
 	}
 
 	return SessionManager.create(cwd, sessionDir, { id: parsed.sessionId });

packages/coding-agent/test/session-id-readonly.test.ts

@@ -122,6 +122,46 @@ describe("--session-id read-only commands", () => {
 		expect(hasSessionWithId(join(result.agentDir, "sessions"), "read-only-models")).toBe(false);
 	});
 
+	it("warns when a missing --session-id creates a new session", async () => {
+		const result = await runCli((dirs) => [
+			"--session-dir",
+			dirs.sessionDir,
+			"--session-id",
+			"missing-session-id",
+			"--model",
+			"missing-model",
+			"-p",
+			"hi",
+		]);
+
+		expect(result.code).toBe(1);
+		expect(result.stderr).toContain(
+			"Warning: No project session found with id 'missing-session-id'; creating a new session with that id.",
+		);
+	});
+
+	it("does not warn when --session-id opens an existing session", async () => {
+		const result = await runCli(
+			(dirs) => [
+				"--session-dir",
+				dirs.sessionDir,
+				"--session-id",
+				"existing-session-id",
+				"--model",
+				"missing-model",
+				"-p",
+				"hi",
+			],
+			(dirs) => {
+				mkdirSync(dirs.sessionDir, { recursive: true });
+				writeSession(dirs.sessionDir, dirs.projectDir, "existing-session-id");
+			},
+		);
+
+		expect(result.code).toBe(1);
+		expect(result.stderr).not.toContain("No project session found with id 'existing-session-id'");
+	});
+
 	it("rejects an existing fork target session id", async () => {
 		const result = await runCli(
 			(dirs) => ["--session-dir", dirs.sessionDir, "--fork", "source-id", "--session-id", "existing-id", "-p", "hi"],