diff --git a/.github/workflows/validate-39951-subagent-fork.yml b/.github/workflows/validate-39951-subagent-fork.yml
index d6028adebd32..16cff47930c4 100644
--- a/.github/workflows/validate-39951-subagent-fork.yml
+++ b/.github/workflows/validate-39951-subagent-fork.yml
@@ -51,9 +51,14 @@ jobs:
                   "agent_type": "worker",
                   "fork_context": true,
               }))?;'''
-          if text.count(old) != 1:
-              raise SystemExit(f"expected one SubagentStart test anchor, found {text.count(old)}")
-          path.write_text(text.replace(old, new, 1))
+          start = text.index("async fn subagent_start_replaces_session_start_and_injects_context")
+          end = text.index("\n#[tokio::test", start + 1)
+          section = text[start:end]
+          if section.count(old) != 1:
+              raise SystemExit(
+                  f"expected one SubagentStart test anchor in target function, found {section.count(old)}"
+              )
+          path.write_text(text[:start] + section.replace(old, new, 1) + text[end:])
           PY
           cargo fmt --manifest-path codex-rs/Cargo.toml --all
           git diff --check
@@ -96,9 +101,14 @@ jobs:
                   "agent_type": "worker",
                   "fork_context": true,
               }))?;'''
-          if text.count(old) != 1:
-              raise SystemExit(f"expected one SubagentStart test anchor, found {text.count(old)}")
-          path.write_text(text.replace(old, new, 1))
+          start = text.index("async fn subagent_start_replaces_session_start_and_injects_context")
+          end = text.index("\n#[tokio::test", start + 1)
+          section = text[start:end]
+          if section.count(old) != 1:
+              raise SystemExit(
+                  f"expected one SubagentStart test anchor in target function, found {section.count(old)}"
+              )
+          path.write_text(text[:start] + section.replace(old, new, 1) + text[end:])
           PY
           cargo fmt --manifest-path codex-rs/Cargo.toml --all
           git diff --check
diff --git a/validation-subagent-fork.txt b/validation-subagent-fork.txt
new file mode 100644
index 000000000000..5ca13fdf8091
--- /dev/null
+++ b/validation-subagent-fork.txt
@@ -0,0 +1 @@
+trigger forked SubagentStart lifecycle validation