diff --git a/packages/coding-agent/src/core/session-manager.ts b/packages/coding-agent/src/core/session-manager.ts
index 89431f2a3..784cd8e3b 100644
--- a/packages/coding-agent/src/core/session-manager.ts
+++ b/packages/coding-agent/src/core/session-manager.ts
@@ -1790,8 +1790,11 @@ export class SessionManager {
 			}
 		}
 		const cwd = cwdOverride ?? (header ? getSessionHeaderCwd(header) : undefined) ?? process.cwd();
-		// If no sessionDir provided, derive from file's parent directory
-		const dir = sessionDir ? normalizePath(sessionDir) : resolve(resolvedPath, "..");
+		// If no sessionDir provided, use the cwd's default directory for files in the sessions directory,
+		// otherwise the file's parent directory
+		const parentDir = resolve(resolvedPath, "..");
+		const inSessionsDir = resolve(parentDir, "..") === resolvePath(getSessionsDir());
+		const dir = sessionDir ? normalizePath(sessionDir) : inSessionsDir ? getDefaultSessionDirPath(cwd) : parentDir;
 		return new SessionManager(cwd, dir, resolvedPath, true, undefined, preloadedFileEntries);
 	}
 
diff --git a/packages/coding-agent/test/session-manager/file-operations.test.ts b/packages/coding-agent/test/session-manager/file-operations.test.ts
index ac6441490..fb9864643 100644
--- a/packages/coding-agent/test/session-manager/file-operations.test.ts
+++ b/packages/coding-agent/test/session-manager/file-operations.test.ts
@@ -15,6 +15,7 @@ import {
 import { tmpdir } from "os";
 import { join } from "path";
 import { afterEach, beforeEach, describe, expect, it } from "vitest";
+import { ENV_AGENT_DIR } from "../../src/config.ts";
 import { findMostRecentSession, loadEntriesFromFile, SessionManager } from "../../src/core/session-manager.ts";
 import { assistantMsg, readSessionFileRoles, userMsg } from "../utilities.ts";
 
@@ -356,6 +357,44 @@ describe("SessionManager custom flat session directory", () => {
 	});
 });
 
+describe("SessionManager.open session directory", () => {
+	let tempDir: string;
+	let savedAgentDir: string | undefined;
+
+	beforeEach(() => {
+		tempDir = mkdtempSync(join(tmpdir(), "session-default-dir-"));
+		savedAgentDir = process.env[ENV_AGENT_DIR];
+		process.env[ENV_AGENT_DIR] = join(tempDir, "agent");
+		mkdirSync(join(tempDir, "a"));
+		mkdirSync(join(tempDir, "b"));
+	});
+
+	afterEach(() => {
+		if (savedAgentDir === undefined) delete process.env[ENV_AGENT_DIR];
+		else process.env[ENV_AGENT_DIR] = savedAgentDir;
+		rmSync(tempDir, { recursive: true, force: true });
+	});
+
+	it("uses the default directory of the cwd for a session opened with another cwd", () => {
+		const session = SessionManager.create(join(tempDir, "a"));
+		session.appendMessage(userMsg("hello"));
+
+		const reopened = SessionManager.open(session.getSessionFile()!, undefined, join(tempDir, "b"));
+		expect(reopened.getSessionDir()).toBe(SessionManager.create(join(tempDir, "b")).getSessionDir());
+		expect(reopened.usesDefaultSessionDir()).toBe(true);
+	});
+
+	it("keeps a custom session directory", () => {
+		const customDir = join(tempDir, "custom");
+		const session = SessionManager.create(join(tempDir, "a"), customDir);
+		session.appendMessage(userMsg("hello"));
+
+		const reopened = SessionManager.open(session.getSessionFile()!, undefined, join(tempDir, "b"));
+		expect(reopened.getSessionDir()).toBe(customDir);
+		expect(reopened.usesDefaultSessionDir()).toBe(false);
+	});
+});
+
 describe("SessionManager.setSessionFile with corrupted files", () => {
 	let tempDir: string;