diff --git a/Makefile b/Makefile
index 5e599a01..dbde6695 100644
--- a/Makefile
+++ b/Makefile
@@ -1,4 +1,4 @@
-.PHONY: help install dev-install format lint type-check test test-cov clean pre-commit setup-dev
+.PHONY: help install dev-install format lint type-check test test-cov clean pre-commit setup-dev integration
 
 help:
 	@echo "Available commands:"
@@ -14,8 +14,10 @@ help:
 	@echo "  check-all     - Run all code quality checks"
 	@echo ""
 	@echo "Testing:"
-	@echo "  test          - Run tests with pytest"
-	@echo "  test-cov      - Run tests with coverage reporting"
+	@echo "  test               - Run tests with pytest"
+	@echo "  test-cov           - Run tests with coverage reporting"
+	@echo "  integration          - Run integration tests (verbose)"
+	@echo "  PRETTY=1 integration - Run integration tests (clean TUI)"
 	@echo ""
 	@echo "Development:"
 	@echo "  pre-commit    - Run pre-commit hooks on all files"
@@ -70,6 +72,15 @@ test-cov:
 	@echo "✅ Tests with coverage complete!"
 	@echo "📊 Coverage report generated in htmlcov/"
 
+integration:
+ifdef PRETTY
+	poetry run pytest tests/integration/ -m integration --no-cov --no-header -q --tb=no -p no:logging -s --pretty
+else
+	@echo "🧪 Running integration tests..."
+	poetry run pytest tests/integration/ -v -s --log-cli-level=INFO -m integration --no-cov
+	@echo "✅ Integration tests complete!"
+endif
+
 pre-commit:
 	@echo "🔧 Running pre-commit hooks..."
 	uv run pre-commit run --all-files
diff --git a/pyproject.toml b/pyproject.toml
index 7e8982a1..d1f52d36 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -359,6 +359,7 @@ asyncio_mode = "auto"
 asyncio_default_fixture_loop_scope = "session"
 markers = [
     "integration: end-to-end tests requiring Docker and network access",
+    "browsers(n): launch n isolated browser sessions for the test",
 ]
 
 [tool.coverage.run]
diff --git a/strix/tools/browser/browser_actions.py b/strix/tools/browser/browser_actions.py
index 69618542..4e689e31 100644
--- a/strix/tools/browser/browser_actions.py
+++ b/strix/tools/browser/browser_actions.py
@@ -156,7 +156,7 @@ async def _run_browser_tool(
     metadata: dict[str, Any] | None = None,
 ) -> Any:
     if not session.browser.is_cdp_connected:
-        await session.browser.start()
+        await session.start()
 
     llm, _ = _build_llm(metadata=metadata)
     tools = Tools()
diff --git a/strix/tools/browser/browser_manager.py b/strix/tools/browser/browser_manager.py
index 5b88d03d..c6a02a4a 100644
--- a/strix/tools/browser/browser_manager.py
+++ b/strix/tools/browser/browser_manager.py
@@ -60,6 +60,7 @@ class BrowserSession:
     __slots__ = (
         "auth_token",
         "browser",
+        "browser_context_id",
         "cdp_url",
         "local",
         "profile_directory",
@@ -83,9 +84,45 @@ class BrowserSession:
         self.auth_token = auth_token
         self.local = local
         self.profile_directory = profile_directory
+        self.browser_context_id: str | None = None
         self.task_count = 0
 
+    async def start(self) -> None:
+        from cdp_use.cdp.target.commands import CreateTargetParameters
+
+        await self.browser.start()
+
+        # [info] Really annoying discovery: despite being isolated in
+        #        different internal sessions, browser use does not
+        #        *actually* isolate the cookies and internals.
+        #
+        # [fix]  We use CDP browser contexts manually to make the
+        #        sessions unique per CDP connection. Nothing groundbreaking
+        cdp = self.browser.cdp_client
+        ctx = await cdp.send.Target.createBrowserContext(params={"disposeOnDetach": True})
+        self.browser_context_id = ctx["browserContextId"]
+
+        async def _scoped(
+            url: str = "about:blank", background: bool = False, new_window: bool = False
+        ) -> str:
+            params = CreateTargetParameters(
+                url=url, background=background, browserContextId=self.browser_context_id
+            )
+            if new_window:
+                params["newWindow"] = True
+            return (await cdp.send.Target.createTarget(params=params))["targetId"]  # type: ignore[no-any-return]
+
+        self.browser._cdp_create_new_page = _scoped
+        target_id = await _scoped(new_window=True)
+        await self.browser.get_or_create_cdp_session(target_id, focus=True)
+
     async def close(self) -> None:
+        if self.browser_context_id and self.browser and self.browser.is_cdp_connected:
+            with contextlib.suppress(Exception):
+                # [fix] properly dispose the session on closure.
+                await self.browser.cdp_client.send.Target.disposeBrowserContext(
+                    params={"browserContextId": self.browser_context_id},
+                )
         await _close_browser(self.browser)
         self.browser = None
 
@@ -164,7 +201,9 @@ async def _launch_browser(api_url: str, agent_id: str, auth_token: str = "") ->
         task.add_done_callback(_manager.background_tasks.discard)
         return session
 
-    return _manager.create(agent_id, browser, api_url, ws_url, auth_token=auth_token)
+    session = _manager.create(agent_id, browser, api_url, ws_url, auth_token=auth_token)
+    await session.start()
+    return session
 
 
 async def _launch_local_browser(
diff --git a/tests/integration/conftest.py b/tests/integration/conftest.py
index ac5630a9..0bd2d96d 100644
--- a/tests/integration/conftest.py
+++ b/tests/integration/conftest.py
@@ -131,19 +131,32 @@ def agent_state(sandbox_info):
     )
 
 
-@pytest.fixture(scope="session")
-def browser_session(agent_state):
-    set_current_agent_id(_SESSION_AGENT_ID)
-    ui.status("Launching browser…")
-    result = _run_in_bg(browser_action(action="launch", agent_state=agent_state))
-    if "error" in result:
-        pytest.fail(f"Browser launch failed: {result}")
-    ui.log(f"Browser ready: mode={result.get('mode')}")
-    yield agent_state
+@pytest.fixture
+def browsers(agent_state, request):
     from strix.tools.browser.browser_manager import _manager
 
-    _manager.sessions.pop(_SESSION_AGENT_ID, None)
-    _bg_loop.call_soon_threadsafe(_bg_loop.stop)
+    from .helpers import Browser
+
+    marker = request.node.get_closest_marker("browsers")
+    count = marker.args[0] if marker else 1
+
+    agent_ids = [f"{_SESSION_AGENT_ID}-{i}" for i in range(count)]
+    for aid in agent_ids:
+        set_current_agent_id(aid)
+        result = _run_in_bg(browser_action(action="launch", agent_state=agent_state))
+        if "error" in result:
+            pytest.fail(f"Browser launch failed for {aid}: {result}")
+
+    yield [Browser(aid) for aid in agent_ids]
+
+    for aid in agent_ids:
+        _manager.sessions.pop(aid, None)
+    set_current_agent_id(_SESSION_AGENT_ID)
+
+
+@pytest.fixture
+def browser(browsers):
+    return browsers[0]
 
 
 def pytest_runtest_logstart(nodeid, location):
@@ -164,10 +177,3 @@ def pytest_runtest_logreport(report):
 @pytest.fixture(autouse=True)
 def _set_agent_context():
     set_current_agent_id(_SESSION_AGENT_ID)
-
-
-@pytest.fixture
-def browser(browser_session):
-    from .helpers import Browser
-
-    return Browser(browser_session)
diff --git a/tests/integration/helpers.py b/tests/integration/helpers.py
index f7fbc5c6..8c16b024 100644
--- a/tests/integration/helpers.py
+++ b/tests/integration/helpers.py
@@ -6,7 +6,6 @@ from pathlib import Path
 from pytest_check import check
 
 from . import console as ui
-from .conftest import _run_in_bg
 
 
 SCREENSHOTS_DIR = Path(__file__).parent / "screenshots"
@@ -19,20 +18,24 @@ def setup_screenshots_dir():
 
 
 class Browser:
-    def __init__(self, agent_state):
-        self._state = agent_state
+    def __init__(self, agent_id):
+        self._agent_id = agent_id
 
     def __getattr__(self, action):
+        import asyncio
+
         from strix.tools.browser.browser_actions import browser_action
+        from strix.tools.context import set_current_agent_id
+
+        from .conftest import _bg_loop
 
         def call(**kwargs):
-            result = _run_in_bg(
-                browser_action(
-                    action=action,
-                    agent_state=self._state,
-                    **kwargs,
-                )
-            )
+            async def _run():
+                set_current_agent_id(self._agent_id)
+                return await browser_action(agent_state=None, action=action, **kwargs)
+
+            future = asyncio.run_coroutine_threadsafe(_run(), _bg_loop)
+            result = future.result(timeout=120)
             if "error" in result:
                 Fail(result).error(result["error"])
             return result
@@ -40,6 +43,18 @@ class Browser:
         return call
 
 
+def act_parallel(tasks):
+    from concurrent.futures import ThreadPoolExecutor
+
+    def _run_one(browser, kwargs):
+        action = kwargs.pop("action")
+        return getattr(browser, action)(**kwargs)
+
+    with ThreadPoolExecutor(max_workers=len(tasks)) as pool:
+        futures = [pool.submit(_run_one, b, dict(kw)) for b, kw in tasks]
+        return [f.result(timeout=120) for f in futures]
+
+
 def _caller_test_name():
     for frame in inspect.stack():
         if frame.function.startswith("test_"):
diff --git a/tests/integration/test_browser_actions.py b/tests/integration/test_browser_actions.py
index 0aa27dd7..9bc733a3 100644
--- a/tests/integration/test_browser_actions.py
+++ b/tests/integration/test_browser_actions.py
@@ -2,7 +2,6 @@ import base64
 import binascii
 
 import pytest
-from pytest_check import check
 
 from . import console as ui
 from .helpers import Fail, setup_screenshots_dir
@@ -18,9 +17,10 @@ def test_navigate(browser):
     result = browser.navigate(url="https://example.com")
     ui.log(f"navigate → url={result.get('url')} title={result.get('title')}")
 
-    with check:
-        check.is_in("example.com", result.get("url", "").lower())
-        check.is_in("example", result.get("title", "").lower())
+    if "example.com" not in result.get("url", "").lower():
+        Fail(result).expected("url containing 'example.com'").got(result.get("url"))
+    if "example" not in result.get("title", "").lower():
+        Fail(result).expected("title containing 'example'").got(result.get("title"))
 
 
 def test_click(browser):
diff --git a/tests/integration/test_browser_isolation.py b/tests/integration/test_browser_isolation.py
new file mode 100644
index 00000000..ed00674c
--- /dev/null
+++ b/tests/integration/test_browser_isolation.py
@@ -0,0 +1,96 @@
+import pytest
+
+from . import console as ui
+from .helpers import Fail, act_parallel
+
+
+pytestmark = [pytest.mark.integration, pytest.mark.browsers(2)]
+
+
+def test_session_objects_are_distinct(browsers):
+    from strix.tools.browser.browser_manager import _manager
+
+    a, b = browsers
+    session_a = _manager.sessions.get(a._agent_id)
+    session_b = _manager.sessions.get(b._agent_id)
+
+    if not session_a or not session_b:
+        Fail().error("one or both sessions missing from manager")
+        return
+    if session_a is session_b:
+        Fail().error("both agents share the same session object")
+    if session_a.browser_context_id == session_b.browser_context_id:
+        Fail().expected("different browser_context_ids").got(
+            f"A={session_a.browser_context_id}, B={session_b.browser_context_id}"
+        )
+
+
+def test_parallel_navigate_isolated(browsers):
+    a, b = browsers
+    ui.status("isolation → navigating both agents concurrently")
+    result_a, result_b = act_parallel(
+        [
+            (a, {"action": "navigate", "url": "https://example.com"}),
+            (b, {"action": "navigate", "url": "https://www.iana.org"}),
+        ]
+    )
+    ui.log(f"agent A → {result_a.get('url')}")
+    ui.log(f"agent B → {result_b.get('url')}")
+
+    if "example.com" not in result_a.get("url", ""):
+        Fail(result_a).expected("url containing 'example.com'").got(result_a.get("url"))
+    if "iana.org" not in result_b.get("url", ""):
+        Fail(result_b).expected("url containing 'iana.org'").got(result_b.get("url"))
+
+    state_a, state_b = act_parallel(
+        [
+            (a, {"action": "screenshot"}),
+            (b, {"action": "screenshot"}),
+        ]
+    )
+
+    if "example.com" not in state_a.get("url", ""):
+        Fail(state_a).expected("agent A still on example.com").got(state_a.get("url"))
+    if "iana.org" not in state_b.get("url", ""):
+        Fail(state_b).expected("agent B still on iana.org").got(state_b.get("url"))
+
+
+def test_cookie_isolation(browsers):
+    a, b = browsers
+    a.navigate(url="https://example.com")
+    a.evaluate(code="document.cookie = 'agent=A; path=/'")
+
+    b.navigate(url="https://example.com")
+    result_b = b.evaluate(code="document.cookie")
+    cookie_b = str(result_b.get("result", ""))
+    ui.log(f"agent B cookies: {cookie_b}")
+
+    if "agent=A" in cookie_b:
+        Fail(result_b).expected("no cookie leakage").got(cookie_b)
+
+    result_a = a.evaluate(code="document.cookie")
+    cookie_a = str(result_a.get("result", ""))
+    ui.log(f"agent A cookies: {cookie_a}")
+
+    if "agent=A" not in cookie_a:
+        Fail(result_a).expected("cookie 'agent=A' present").got(cookie_a)
+
+
+def test_concurrent_mixed_actions(browsers):
+    a, b = browsers
+    a.navigate(url="https://example.com")
+    b.navigate(url="https://www.iana.org")
+
+    ui.status("isolation → mixed actions concurrently")
+    results = act_parallel(
+        [
+            (a, {"action": "scroll", "direction": "down", "amount": 3}),
+            (b, {"action": "screenshot"}),
+            (a, {"action": "screenshot"}),
+            (b, {"action": "evaluate", "code": "document.title"}),
+        ]
+    )
+
+    for r in results:
+        if "error" in r:
+            Fail(r).error(r["error"])