diff --git a/CHANGELOG.md b/CHANGELOG.md
index 73736e9cb..5b5311121 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -592,6 +592,16 @@ _ALL_DATA_COLUMNS + ") " "SELECT " + _ALL_DATA_COLUMNS + " FROM v3_plans"`.
   it to a single batch call after all docs PRs are created per cycle, eliminating
   merge conflicts on `examples.json` when parallel UAT workers run simultaneously.
 
+- **Subprocess resource leak on failed initialization in StdioTransport.start()** (#11160):
+  The ``start()`` method now verifies the spawned subprocess is still alive immediately
+  after ``subprocess.Popen()`` returns. If the process died before or during initialization
+  (e.g., binary missing shared libraries, corrupted executable, or permission denied), the
+  transport calls ``stop()`` to terminate and release the process handle, then raises
+  ``LspError`` with the exit code. This prevents zombie process accumulation, ensures
+  ``is_alive`` returns ``False`` for dead processes after failed start, and eliminates leaks
+  in error paths where ``_process`` was previously left pointing to a terminated Popen object.
+  Includes BDD test coverage verifying cleanup on immediate process death.
+
 - **`invariant_enforced` decisions not propagated to child plans on subplan spawn** (#9131):
   Fixed `SubplanService.spawn()` to propagate all `invariant_enforced` decisions from the
   parent plan's decision tree to each child plan's decision tree. Previously, child plans
diff --git a/CONTRIBUTORS.md b/CONTRIBUTORS.md
index 2c474f369..44dd3e8bf 100644
--- a/CONTRIBUTORS.md
+++ b/CONTRIBUTORS.md
@@ -120,3 +120,4 @@ Below are some specific details of individual PR contributions.
 * Jeffrey Phillips Freeman has contributed the `--format`/`-f` flag to `agents session tell` (issue #10466): adds JSON envelope output for machine-readable workflows alongside existing Rich console output, with Behave BDD test coverage verifying all four non-rich format paths (JSON, YAML, plain, table) and the short `-f` flag alias.
 * HAL 9000 has contributed the Semgrep guard for broad exception suppression (PR #9185 / issue #9103): added two new Semgrep rules (`python-no-suppressed-exception` and `python-no-suppress-exception`) to automate enforcement of error propagation guidelines, integrated Semgrep into `nox -s lint` in audit mode with migration plan for ~337 existing violations, and comprehensive BDD test coverage across all rule patterns and escape hatch scenarios.
 * HAL 9000 has contributed the `ProviderRegistry.FALLBACK_ORDER` fix (#10906): added the missing `ProviderType.GEMINI` to the fallback provider order list so that when only a Gemini API key is configured, the registry correctly selects it as the default provider. Includes BDD regression scenarios in `features/fallback_gemini_provider.feature`.
+* HAL 9000 has contributed the StdioTransport subprocess cleanup on failed initialization fix (PR #11185 / issue #11160): after ``start()`` spawns a process with ``subprocess.Popen()``, the method now verifies the process is still alive via ``poll()``. If it died immediately during initialization, ``stop()`` is called to release the handle and an ``LspError`` with exit code is raised, preventing zombie processes and resource leaks. Includes BDD test coverage in ``features/lsp_transport_subprocess_cleanup.feature``.
diff --git a/features/lsp_transport_subprocess_cleanup.feature b/features/lsp_transport_subprocess_cleanup.feature
new file mode 100644
index 000000000..32cdb82fa
--- /dev/null
+++ b/features/lsp_transport_subprocess_cleanup.feature
@@ -0,0 +1,41 @@
+@tdd_issue @tdd_issue_11160
+Feature: StdioTransport subprocess cleanup on failed initialization
+  As a developer maintaining the LSP transport layer
+  I need proper cleanup of subprocess handles when the process dies during start()
+  So that resource leaks are prevented and is_alive returns False for dead processes
+
+  # ── start() successful spawn paths ─────────────────────────────────
+
+  Scenario: start succeeds when process stays alive after Popen
+    Given ltcov I create a StdioTransport for command "echo"
+    And ltcov Popen is mocked to return an alive process with pid 9999
+    When ltcov I start the transport
+    Then ltcov the transport should be alive
+    And ltcov no error should have been raised
+
+  # ── start() — process dies immediately on spawn (the new fix) ───────
+
+  Scenario: start raises LspError and cleans up when process exits before init completes
+    Given ltcov I create a StdioTransport for command "bad_binary"
+    And ltcov Popen is mocked to return a process that exited with code 127
+    When ltcov I try to start the transport
+    Then ltcov the error should be an LspError with message "died on spawn"
+    And ltcov the internal process should be None
+
+  Scenario: start raises LspError with exit_code detail when process dies with code -1
+    Given ltcov I create a StdioTransport for command "sigkill_binary"
+    And ltcov Popen is mocked to return a process that exited with code -1
+    When ltcov I try to start the transport
+    Then ltcov the error should be an LspError with message "died on spawn"
+
+  Scenario: start raises LspError when process returncode is zero (clean exit)
+    Given ltcov I create a StdioTransport for command "quick_exit"
+    And ltcov Popen is mocked to return a process that exited with code 0
+    When ltcov I try to start the transport
+    Then ltcov the error should be an LspError with message "died on spawn"
+
+  Scenario: stop() handles already-exited process and cleans up _process
+    Given ltcov I create a StdioTransport for command "die_on_start"
+    And ltcov Popen is mocked to return a process that exited with code 42
+    When ltcov I try to start the transport
+    Then ltcov the stop handles already-exited processes correctly
diff --git a/features/steps/lsp_transport_coverage_steps.py b/features/steps/lsp_transport_coverage_steps.py
index ce90e33a2..5bb7fa672 100644
--- a/features/steps/lsp_transport_coverage_steps.py
+++ b/features/steps/lsp_transport_coverage_steps.py
@@ -315,6 +315,37 @@ def step_ltcov_invalid_json_body(context: Context) -> None:
     context.add_cleanup(patcher.stop)
 
 
+@given("ltcov Popen is mocked to return an alive process with pid {pid:d}")
+def step_ltcov_popen_alive(context: Context, pid: int) -> None:
+    """Mock subprocess.Popen to return a MagicMock that represents an alive process."""
+    proc = _make_mock_process(poll_return=None, pid=pid)
+    patcher = patch(
+        "cleveragents.lsp.transport.subprocess.Popen",
+        return_value=proc,
+    )
+    patcher.start()
+    context.add_cleanup(patcher.stop)
+    context.ltcov_mock_process = proc
+
+
+@given("ltcov Popen is mocked to return a process that exited with code {code:d}")
+def step_ltcov_popen_exited(context: Context, code: int) -> None:
+    """Mock subprocess.Popen to return a MagicMock that represents a process that already exited."""
+    proc = _make_mock_process(poll_return=code, returncode=code, pid=77777)
+    context._lsp_cleanup_exit_code = code
+
+    def side_effect(*args, **kwargs):
+        return proc
+
+    patcher = patch(
+        "cleveragents.lsp.transport.subprocess.Popen",
+        side_effect=side_effect,
+    )
+    patcher.start()
+    context.add_cleanup(patcher.stop)
+    context.ltcov_mock_process = proc
+
+
 # ---------------------------------------------------------------------------
 # When steps
 # ---------------------------------------------------------------------------
@@ -328,6 +359,16 @@ def step_ltcov_try_start(context: Context) -> None:
         context.ltcov_error = exc
 
 
+@when("ltcov I start the transport")
+def step_ltcov_start(context: Context) -> None:
+    """Start the transport (should succeed)."""
+    try:
+        context.ltcov_transport.start()
+        context.ltcov_error = None
+    except Exception as exc:
+        context.ltcov_error = exc
+
+
 @when("ltcov I stop the transport")
 def step_ltcov_stop(context: Context) -> None:
     context.ltcov_result = context.ltcov_transport.stop()
@@ -495,3 +536,36 @@ def step_ltcov_process_none_after_failed_start(context: Context) -> None:
         f"_process should be None after failed start, but got "
         f"{context.ltcov_transport._process}"
     )
+
+
+# ---------------------------------------------------------------------------
+# Cleanup-on-failed-init step definitions (lsp_transport_subprocess_cleanup.feature)
+# ---------------------------------------------------------------------------
+
+
+@then("ltcov no error should have been raised")
+def step_ltcov_no_error(context: Context) -> None:
+    assert context.ltcov_error is None, (
+        f"Unexpected error was raised: {context.ltcov_error}"
+    )
+
+
+@then("ltcov the stop handles already-exited processes correctly")
+def step_ltcov_stop_handles_exited(context: Context) -> None:
+    """Verify that stop() properly handled the already-dead mock process."""
+    assert context.ltcov_transport._process is None, (
+        "Expected _process to be None after failed start cleanup"
+    )
+
+
+@then("ltcov the error should have exit_code in details")
+def step_ltcov_error_has_exit_code_detail(context: Context) -> None:
+    """Verify the LspError contains an 'exit_code' detail."""
+    assert context.ltcov_error is not None, "Expected an error but none was raised"
+    assert isinstance(context.ltcov_error, LspError), (
+        f"Expected LspError, got {type(context.ltcov_error).__name__}"
+    )
+    assert context.ltcov_error.details is not None, "LspError should have details"
+    assert "exit_code" in context.ltcov_error.details, (
+        "LspError details should contain 'exit_code'"
+    )
diff --git a/src/cleveragents/lsp/transport.py b/src/cleveragents/lsp/transport.py
index d90c31069..c34219757 100644
--- a/src/cleveragents/lsp/transport.py
+++ b/src/cleveragents/lsp/transport.py
@@ -98,7 +98,9 @@ class StdioTransport:
         callers always receive proper error semantics.
 
         Raises:
-            LspError: If the process cannot be started.
+            LspError: If the process cannot be started, or if the spawned
+                process exits immediately during initialization (e.g.,
+                corrupted binary, missing shared libraries).
             RuntimeError: If already started or if a post-spawn init
                 step fails (the subprocess is cleaned up before re-raising).
         """
@@ -135,6 +137,23 @@ class StdioTransport:
                 details={"command": self._command, "error": str(exc)},
             ) from exc
 
+        # Verify the process is still alive after Popen succeeds.  If it has
+        # died immediately during initialization (common when the binary is
+        # corrupted, missing shared libraries, etc.), clean up the leaked
+        # subprocess handle to prevent resource leaks and ensure is_alive
+        # returns False for dead processes.
+        if self._process.poll() is not None:
+            logger.warning(
+                "lsp.transport.process_died_on_spawn",
+                pid=self._process.pid,
+                exit_code=self._process.returncode,
+            )
+            code = self.stop()
+            raise LspError(
+                f"LSP server process died on spawn with exit code {code}",
+                details={"command": self._command, "exit_code": code},
+            )
+
         # All post-Popen initialization logic must be placed within the
         # guarded block below to prevent orphaned subprocesses if init
         # steps fail after a successful spawn.