diff --git a/addon/manifest.json b/addon/manifest.json
index 251f06e..853c5f5 100644
--- a/addon/manifest.json
+++ b/addon/manifest.json
@@ -1,7 +1,7 @@
 {
   "manifest_version": 2,
   "name": "Thunderbird MCP Bridge",
-  "version": "1.2.0",
+  "version": "1.2.1",
   "description": "Lets a local MCP server drive this Thunderbird: mail, folders, contacts, calendar, filters and settings.",
   "author": "thunderbird-mcp",
   "browser_specific_settings": {
diff --git a/pyproject.toml b/pyproject.toml
index 5aa7a93..e59c0ce 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "thunderbird-mcp"
-version = "1.2.0"
+version = "1.2.1"
 description = "MCP server that drives Thunderbird — mail, folders, contacts, calendar, filters, and settings"
 readme = "README.md"
 requires-python = ">=3.11"
diff --git a/src/tbmcp/addon_install.py b/src/tbmcp/addon_install.py
index 09920f7..613f88a 100644
--- a/src/tbmcp/addon_install.py
+++ b/src/tbmcp/addon_install.py
@@ -177,7 +177,18 @@ def _launch(exe: pathlib.Path, extra_args: list[str], profile: ThunderbirdProfil
     argv = [str(exe), *extra_args]
     if profile is not None:
         argv += ["-profile", str(profile.path)]
-    kwargs: dict = {"stdin": subprocess.DEVNULL, "close_fds": True}
+    # All three streams, not just stdin. Thunderbird outlives us on purpose, so an
+    # inherited stdout keeps the caller's pipe open long after `install-addon` has
+    # finished — an agent or CI step capturing our output waits for the mail client
+    # to be closed, and its own timeout cannot rescue it, because killing us leaves
+    # the grandchild holding the pipe. `close_fds` does not cover the std handles;
+    # they have to be redirected explicitly, as the daemon spawn in bridge.py does.
+    kwargs: dict = {
+        "stdin": subprocess.DEVNULL,
+        "stdout": subprocess.DEVNULL,
+        "stderr": subprocess.DEVNULL,
+        "close_fds": True,
+    }
     if sys.platform == "win32":
         kwargs["creationflags"] = 0x00000008 | 0x00000200  # DETACHED | NEW_GROUP
     else:
diff --git a/src/tbmcp/server.py b/src/tbmcp/server.py
index 230cd8d..5585564 100644
--- a/src/tbmcp/server.py
+++ b/src/tbmcp/server.py
@@ -175,4 +175,4 @@ def _version() -> str:
 
         return version("thunderbird-mcp")
     except Exception:
-        return "1.2.0"
+        return "1.2.1"
diff --git a/tests/test_addon_install_launch.py b/tests/test_addon_install_launch.py
new file mode 100644
index 0000000..4e58949
--- /dev/null
+++ b/tests/test_addon_install_launch.py
@@ -0,0 +1,59 @@
+"""Launching Thunderbird must not hold the caller's stdout open.
+
+`_launch` starts a process that outlives us by design. If it lets that process
+inherit our stdout, every caller that captures output — an agent, a CI step, a
+shell pipeline — blocks until Thunderbird is closed, long after `install-addon`
+has finished its work. A timeout does not rescue them: killing the direct child
+leaves the grandchild holding the pipe.
+
+The daemon spawn in `bridge.py` already redirects all three streams; this is the
+same requirement one file over.
+"""
+
+from __future__ import annotations
+
+import subprocess
+import sys
+import time
+
+# Long enough that a regression is unmistakable against the deadline below, short
+# enough that a failing run still ends on its own.
+SLEEP_SECONDS = 25
+DEADLINE_SECONDS = 10
+
+HELPER = '''
+import sys
+from tbmcp.addon_install import _launch
+
+
+class Exe:
+    """`_launch` stringifies the exe and appends the extra args."""
+
+    def __str__(self):
+        return sys.executable
+
+
+_launch(Exe(), ["-c", "import time; time.sleep({sleep})"], None)
+print("launched", flush=True)
+'''
+
+
+def test_launch_does_not_hold_the_callers_stdout(tmp_path):
+    helper = tmp_path / "launch_helper.py"
+    helper.write_text(HELPER.format(sleep=SLEEP_SECONDS), encoding="utf-8")
+
+    started = time.monotonic()
+    done = subprocess.run(
+        [sys.executable, str(helper)],
+        capture_output=True,
+        text=True,
+        timeout=SLEEP_SECONDS + 60,
+    )
+    elapsed = time.monotonic() - started
+
+    assert done.returncode == 0, done.stderr
+    assert "launched" in done.stdout
+    assert elapsed < DEADLINE_SECONDS, (
+        f"the caller was held for {elapsed:.1f}s after the helper exited: the launched "
+        "process inherited its stdout"
+    )
diff --git a/tests/test_version.py b/tests/test_version.py
index e3f74d4..b4b4bf4 100644
--- a/tests/test_version.py
+++ b/tests/test_version.py
@@ -9,7 +9,7 @@
 from tbmcp import server
 
 ROOT = pathlib.Path(__file__).resolve().parents[1]
-EXPECTED = "1.2.0"
+EXPECTED = "1.2.1"
 
 
 def _pyproject_version() -> str: