diff --git a/build_defs/go.build_defs b/build_defs/go.build_defs
index 8de215fd..e79bafed 100644
--- a/build_defs/go.build_defs
+++ b/build_defs/go.build_defs
@@ -850,7 +850,7 @@ def go_test(name:str, srcs:list, resources:list=None, data:list|dict=None, deps:
     cmds, tools = _go_binary_cmds(name, static=static, definitions=definitions, gcov=cgo, test=True)
 
 
-    test_cmd = f'$TEST {flags} 2>&1 | tee $TMP_DIR/test.results'
+    test_cmd = f'$TEST {flags} 2>&1 | tee -- "$TMP_DIR/test.results"'
     worker_cmd = f'$(worker {worker})' if worker else ""
     if worker_cmd:
         test_cmd = f'{worker_cmd} && {test_cmd} '
@@ -1747,7 +1747,12 @@ def _go_binary_cmds(name, static=False, ldflags='', pkg_config='', definitions=N
     elif isinstance(definitions, dict):
         linkerdefs = [k if v is None else f'{k}={v}' for k, v in sorted(definitions.items())]
     if test:
-        linkerdefs += ["testing.testBinary=1"]
+        # testing.testBinary and testing.testing are required by the Go test runtime.
+        # Match go test: testing/testing.go:692 (Go 1.25.3) relies on testing.testing=true when linking tests.
+        if "testing.testBinary=1" not in linkerdefs:
+            linkerdefs += ["testing.testBinary=1"]
+        if "testing.testing=true" not in linkerdefs:
+            linkerdefs += ["testing.testing=true"]
 
     defs = ' '.join([f'-X "{linkerdef}"' for linkerdef in linkerdefs])
 
diff --git a/test/build_defs/e2e.build_defs b/test/build_defs/e2e.build_defs
index a68ed0f9..e43ae9d9 100644
--- a/test/build_defs/e2e.build_defs
+++ b/test/build_defs/e2e.build_defs
@@ -2,11 +2,22 @@ subinclude("///e2e//build_defs:e2e")
 
 _please_repo_e2e_test = please_repo_e2e_test
 
-def please_repo_e2e_test(name, expected_output=None, plz_command, repo, labels=[]):
+def please_repo_e2e_test(name, expected_output=None, plz_command, repo, labels=[], expect_stdout_contains=None, expect_stderr_contains=None):
+    plz_cmd = plz_command.replace("plz ", "plz -o please.PluginRepo:file://$TMP_DIR/$DATA_PLUGIN_REPO -o plugin.go.gotool:$TOOLS_GO -o plugin.go.pleasegotool:$TOOLS_PLEASE_GO ")
+    # Tee stdout and stderr to separate files while preserving the exit code.
+    wrapped_plz_command = f"( {plz_cmd} > >(tee stdout.log) 2> >(tee stderr.log >&2) ); STATUS=$?; exit $STATUS"
+
+    expect_output_contains = {}
+    if expect_stdout_contains:
+        expect_output_contains["stdout.log"] = expect_stdout_contains
+    if expect_stderr_contains:
+        expect_output_contains["stderr.log"] = expect_stderr_contains
+
     return _please_repo_e2e_test(
         name = name,
         expected_output = expected_output,
-        plz_command = plz_command.replace("plz ", "plz -o please.PluginRepo:file://$TMP_DIR/$DATA_PLUGIN_REPO -o plugin.go.gotool:$TOOLS_GO -o plugin.go.pleasegotool:$TOOLS_PLEASE_GO "),
+        expect_output_contains = expect_output_contains if expect_output_contains else None,
+        plz_command = wrapped_plz_command,
         repo = repo,
         data = {
             "PLUGIN_REPO": ["//test:export"],
diff --git a/test/testing_definition/BUILD b/test/testing_definition/BUILD
index 4bdd6436..2ea4549f 100644
--- a/test/testing_definition/BUILD
+++ b/test/testing_definition/BUILD
@@ -1,4 +1,5 @@
 subinclude("//build_defs:go")
+subinclude("//test/build_defs:e2e")
 
 go_test(
     name = "testing_definition_test",
@@ -7,3 +8,10 @@ go_test(
         "///third_party/go/github.com_stretchr_testify//assert",
     ],
 )
+
+please_repo_e2e_test(
+    name = "testing_definition_integration_test",
+    plz_command = "plz test --show_all_output //sample:testing_flag_test -- -test.v",
+    repo = "test_repo",
+    expect_stdout_contains = "testing.Testing() = true",
+)
diff --git a/test/testing_definition/test_repo/.plzconfig b/test/testing_definition/test_repo/.plzconfig
new file mode 100644
index 00000000..349fc964
--- /dev/null
+++ b/test/testing_definition/test_repo/.plzconfig
@@ -0,0 +1,7 @@
+[Parse]
+BuildFileName = BUILD_FILE
+PreloadSubincludes = ///go//build_defs:go
+
+[Plugin "go"]
+Target = //plugins:go
+Stdlib = //third_party/go:std
diff --git a/test/testing_definition/test_repo/plugins/BUILD_FILE b/test/testing_definition/test_repo/plugins/BUILD_FILE
new file mode 100644
index 00000000..763befc7
--- /dev/null
+++ b/test/testing_definition/test_repo/plugins/BUILD_FILE
@@ -0,0 +1,4 @@
+plugin_repo(
+    name = "go",
+    revision = "master",
+)
diff --git a/test/testing_definition/test_repo/sample/BUILD_FILE b/test/testing_definition/test_repo/sample/BUILD_FILE
new file mode 100644
index 00000000..ea5b4980
--- /dev/null
+++ b/test/testing_definition/test_repo/sample/BUILD_FILE
@@ -0,0 +1,4 @@
+go_test(
+    name = "testing_flag_test",
+    srcs = ["testing_flag_test.go"],
+)
diff --git a/test/testing_definition/test_repo/sample/testing_flag_test.go b/test/testing_definition/test_repo/sample/testing_flag_test.go
new file mode 100644
index 00000000..45afddfc
--- /dev/null
+++ b/test/testing_definition/test_repo/sample/testing_flag_test.go
@@ -0,0 +1,13 @@
+package sample
+
+import (
+	"fmt"
+	"testing"
+)
+
+func TestTestingFlag(t *testing.T) {
+	fmt.Printf("testing.Testing() = %v\n", testing.Testing())
+	if !testing.Testing() {
+		t.Fatalf("expected testing.Testing() to be true")
+	}
+}
diff --git a/test/testing_definition/test_repo/third_party/go/BUILD_FILE b/test/testing_definition/test_repo/third_party/go/BUILD_FILE
new file mode 100644
index 00000000..f6d6b9a1
--- /dev/null
+++ b/test/testing_definition/test_repo/third_party/go/BUILD_FILE
@@ -0,0 +1,3 @@
+go_stdlib(
+    name = "std",
+)