diff --git a/AGENTS.md b/AGENTS.md
index 869c99a..4645e9c 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -121,10 +121,14 @@ nix develop -c cargo test -p grasp-audit --lib specific_test_name -- --nocapture
 ### Troubleshooting
 
 **Buffer Size Errors:**
-If you see mpsc channel buffer size panics on first test run, this is usually transient. Simply run the tests again.
+Capture the failing test and panic before retrying. Diagnose the fixture or
+capacity assumption; a successful rerun alone does not establish correctness.
 
 **Port Conflicts:**
-Both `TestRelay` and `test-ngit-relay.sh` use random ports to avoid conflicts. If you see port errors, ensure no stale processes are running.
+`TestRelay` transfers an owned loopback listener into the subprocess and
+retains it across restarts. Do not release a reservation and rebind its port.
+See [test fixture guidance](docs/how-to/test-fixtures.md) for readiness,
+shutdown, and timing rules. Audit external server scripts separately.
 
 ## Code Patterns
 
diff --git a/docs/how-to/test-fixtures.md b/docs/how-to/test-fixtures.md
new file mode 100644
index 0000000..1f693d3
--- /dev/null
+++ b/docs/how-to/test-fixtures.md
@@ -0,0 +1,41 @@
+# Reliable test fixtures
+
+Run tests through the repository development shell. Keep parallel checks
+enabled: each fixture owns its socket, tasks, subprocesses and temporary data.
+
+## Socket ownership
+
+Allocate loopback port zero and transfer the bound listener into the server.
+Do not bind, inspect the port, drop the socket and bind that port again.
+Subprocess fixtures pass `NGIT_TEST_LISTENER_FD` with `NGIT_TEST=1` on Unix.
+The binary advertises this private protocol through
+`--internal-test-listener-support`; it is not deployment configuration and is
+not exposed through the NixOS module or example service environment.
+
+`TestRelay` retains a parent copy across same-address restarts. Offline relay
+scenarios use `UnavailableEndpoint`, which accepts and closes connections
+while retaining the socket, then transfers that socket to the recovered
+server. This prevents another parallel test from taking the offline address.
+
+## Observable readiness and shutdown
+
+Wait for HTTP readiness, connected-state metrics, or the expected event with
+a bounded deadline. A successful TCP connection, a connection attempt counter,
+and an arbitrary grace period do not prove that a server is ready.
+
+Connection tasks belong to their fixture's accept loop. Stopping the fixture
+cancels and joins them; dropping it cancels the owner. Git fixture subprocesses
+are cancelled with their request, and all three pipes are driven concurrently.
+
+## Time and streaming assertions
+
+Age private cache timestamps explicitly and bracket wall-clock timestamps
+before and after the operation. Streaming tests coordinate fake Git with an
+owned loopback gate, collect bytes independently of HTTP frame boundaries,
+and retain assertions about terminal flush and post-push promotion ordering.
+Fixed sleeps remain appropriate only when elapsed time is itself under test;
+polling an observable condition must always have a bounded deadline.
+
+Targeted regressions include `fixture_lifecycle`, `relay_identity`,
+`git_response_streaming`, and the shared Git-server tests in `sync`.
+Full package validation remains necessary after these scoped checks.