diff --git a/plugins/trace/hooks/hook_utils.py b/plugins/trace/hooks/hook_utils.py
new file mode 100644
index 0000000..e5466eb
--- /dev/null
+++ b/plugins/trace/hooks/hook_utils.py
@@ -0,0 +1,38 @@
+#!/usr/bin/env python3
+
+import base64
+import binascii
+import sys
+import time
+
+
+def hex_to_base64(hex_value: str) -> int:
+    raw = binascii.unhexlify(hex_value)
+    print(base64.b64encode(raw).decode("ascii"))
+    return 0
+
+
+def now_ns() -> int:
+    print(time.time_ns())
+    return 0
+
+
+def main() -> int:
+    if len(sys.argv) < 2:
+        raise SystemExit("usage: hook_utils.py <command> [args]")
+
+    command = sys.argv[1]
+    if command == "hex_to_base64":
+        if len(sys.argv) != 3:
+            raise SystemExit("usage: hook_utils.py hex_to_base64 <hex>")
+        return hex_to_base64(sys.argv[2])
+    if command == "now_ns":
+        if len(sys.argv) != 2:
+            raise SystemExit("usage: hook_utils.py now_ns")
+        return now_ns()
+
+    raise SystemExit(f"unknown command: {command}")
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())
diff --git a/plugins/trace/hooks/lib.sh b/plugins/trace/hooks/lib.sh
index c53eecf..d936140 100755
--- a/plugins/trace/hooks/lib.sh
+++ b/plugins/trace/hooks/lib.sh
@@ -16,8 +16,11 @@ export PL_QUEUE_DRAIN_LIMIT="${PROMPTLAYER_QUEUE_DRAIN_LIMIT:-10}"
 export PL_OTLP_CONNECT_TIMEOUT="${PROMPTLAYER_OTLP_CONNECT_TIMEOUT:-5}"
 export PL_OTLP_MAX_TIME="${PROMPTLAYER_OTLP_MAX_TIME:-12}"
 export PL_PLUGIN_VERSION="1.0.0"
-export PL_CC_VERSION="$(claude --version 2>/dev/null || echo 'unknown')"
+PL_CC_VERSION="$(claude --version 2>/dev/null || echo 'unknown')"
+export PL_CC_VERSION
 export PL_USER_AGENT="promptlayer-claude-plugin/${PL_PLUGIN_VERSION} claude-code/${PL_CC_VERSION}"
+PL_HOOKS_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+export PL_HOOKS_DIR
 
 mkdir -p "$(dirname "$PL_LOG_FILE")"
 mkdir -p "$PL_SESSION_STATE_DIR"
@@ -66,6 +69,61 @@ generate_span_id() {
 	uuidgen | tr -d '-' | tr '[:upper:]' '[:lower:]' | cut -c1-16
 }
 
+parse_traceparent() {
+	local raw="${1:-}"
+	[[ -z "$raw" ]] && return 1
+
+	raw="$(printf '%s' "$raw" | tr '[:upper:]' '[:lower:]')"
+
+	if [[ ! "$raw" =~ ^([0-9a-f]{2})-([0-9a-f]{32})-([0-9a-f]{16})-([0-9a-f]{2})(-.+)?$ ]]; then
+		return 1
+	fi
+
+	local version="${BASH_REMATCH[1]}"
+	local trace_id="${BASH_REMATCH[2]}"
+	local parent_span_id="${BASH_REMATCH[3]}"
+	local trace_flags="${BASH_REMATCH[4]}"
+	local suffix="${BASH_REMATCH[5]:-}"
+
+	if [[ "$version" == "ff" ]]; then
+		return 1
+	fi
+	if [[ "$version" == "00" && -n "$suffix" ]]; then
+		return 1
+	fi
+	if [[ "$trace_id" == "00000000000000000000000000000000" ]]; then
+		return 1
+	fi
+	if [[ "$parent_span_id" == "0000000000000000" ]]; then
+		return 1
+	fi
+
+	printf '%s %s %s %s\n' "$version" "$trace_id" "$parent_span_id" "$trace_flags"
+}
+
+load_initial_trace_context() {
+	PL_INITIAL_TRACEPARENT_VERSION=""
+	PL_INITIAL_TRACE_ID=""
+	PL_INITIAL_PARENT_SPAN_ID=""
+	PL_INITIAL_TRACE_FLAGS=""
+	PL_INITIAL_TRACE_CONTEXT_SOURCE="generated"
+
+	local raw="${PROMPTLAYER_TRACEPARENT:-}"
+	if [[ -z "$raw" ]]; then
+		return 0
+	fi
+
+	local parsed
+	if ! parsed="$(parse_traceparent "$raw")"; then
+		log "WARN" "Ignoring invalid PROMPTLAYER_TRACEPARENT"
+		return 1
+	fi
+
+	read -r PL_INITIAL_TRACEPARENT_VERSION PL_INITIAL_TRACE_ID PL_INITIAL_PARENT_SPAN_ID PL_INITIAL_TRACE_FLAGS <<<"$parsed"
+	PL_INITIAL_TRACE_CONTEXT_SOURCE="external_traceparent"
+	return 0
+}
+
 normalize_hex_id() {
 	local raw="$1"
 	local expected_len="$2"
@@ -93,22 +151,11 @@ normalize_hex_id() {
 
 hex_to_base64() {
 	local hex="$1"
-	python3 - "$hex" <<'PY'
-import base64
-import binascii
-import sys
-
-h = sys.argv[1]
-raw = binascii.unhexlify(h)
-print(base64.b64encode(raw).decode("ascii"))
-PY
+	python3 "$PL_HOOKS_DIR/hook_utils.py" hex_to_base64 "$hex"
 }
 
 now_ns() {
-	python3 - <<'PY'
-import time
-print(time.time_ns())
-PY
+	python3 "$PL_HOOKS_DIR/hook_utils.py" now_ns
 }
 
 session_state_file() {
@@ -197,9 +244,10 @@ ensure_session_initialized() {
 	local requested_start_ns="${2:-}"
 	[[ -z "$sid" ]] && return 1
 
-	local trace_id session_span_id session_start_ns init_source root_emitted pending_tool_calls
+	local trace_id session_span_id session_parent_span_id session_start_ns init_source root_emitted pending_tool_calls
 	trace_id="$(get_session_state "$sid" trace_id)"
 	session_span_id="$(get_session_state "$sid" session_span_id)"
+	session_parent_span_id="$(get_session_state "$sid" session_parent_span_id)"
 	session_start_ns="$(get_session_state "$sid" session_start_ns)"
 	init_source="$(get_session_state "$sid" session_init_source)"
 	root_emitted="$(get_session_state "$sid" session_root_emitted)"
@@ -220,6 +268,18 @@ ensure_session_initialized() {
 		if [[ -z "$pending_tool_calls" ]]; then
 			set_session_state "$sid" pending_tool_calls "[]"
 		fi
+		if [[ -z "$session_parent_span_id" ]]; then
+			set_session_state "$sid" session_parent_span_id ""
+		fi
+		if [[ -z "$(get_session_state "$sid" session_traceparent_version)" ]]; then
+			set_session_state "$sid" session_traceparent_version ""
+		fi
+		if [[ -z "$(get_session_state "$sid" session_trace_flags)" ]]; then
+			set_session_state "$sid" session_trace_flags ""
+		fi
+		if [[ -z "$(get_session_state "$sid" trace_context_source)" ]]; then
+			set_session_state "$sid" trace_context_source "generated"
+		fi
 		if [[ -z "$(get_session_state "$sid" session_end_requested)" ]]; then
 			set_session_state "$sid" session_end_requested "false"
 		fi
@@ -231,15 +291,21 @@ ensure_session_initialized() {
 
 	# Fallback path for SDK environments that do not surface SessionStart.
 	[[ -z "$requested_start_ns" ]] && requested_start_ns="$(now_ns)"
+	load_initial_trace_context || true
+	[[ -z "$trace_id" ]] && trace_id="${PL_INITIAL_TRACE_ID:-}"
 	[[ -z "$trace_id" ]] && trace_id="$(generate_trace_id)"
 	[[ -z "$session_span_id" ]] && session_span_id="$(generate_span_id)"
 
 	set_session_state "$sid" trace_id "$trace_id"
 	set_session_state "$sid" session_span_id "$session_span_id"
+	set_session_state "$sid" session_parent_span_id "${PL_INITIAL_PARENT_SPAN_ID:-}"
 	set_session_state "$sid" session_start_ns "$requested_start_ns"
 	set_session_state "$sid" current_turn_start_ns ""
 	set_session_state "$sid" pending_tool_calls "[]"
 	set_session_state "$sid" session_init_source "lazy_init"
+	set_session_state "$sid" session_traceparent_version "${PL_INITIAL_TRACEPARENT_VERSION:-}"
+	set_session_state "$sid" session_trace_flags "${PL_INITIAL_TRACE_FLAGS:-}"
+	set_session_state "$sid" trace_context_source "${PL_INITIAL_TRACE_CONTEXT_SOURCE:-generated}"
 	set_session_state "$sid" session_root_emitted "false"
 	set_session_state "$sid" session_end_requested "false"
 	set_session_state "$sid" stop_in_flight "false"
diff --git a/plugins/trace/hooks/session_end.sh b/plugins/trace/hooks/session_end.sh
index 21bbe5f..3c2dd2f 100755
--- a/plugins/trace/hooks/session_end.sh
+++ b/plugins/trace/hooks/session_end.sh
@@ -17,6 +17,7 @@ trap 'release_session_lock' EXIT
 
 trace_id="$(get_session_state "$session_id" trace_id)"
 session_span_id="$(get_session_state "$session_id" session_span_id)"
+session_parent_span_id="$(get_session_state "$session_id" session_parent_span_id)"
 session_start_ns="$(get_session_state "$session_id" session_start_ns)"
 stop_in_flight="$(get_session_state "$session_id" stop_in_flight)"
 current_turn_start_ns="$(get_session_state "$session_id" current_turn_start_ns)"
@@ -37,7 +38,7 @@ trap - EXIT
 # span_id conflict, so this safely updates the end time and lifecycle attribute.
 end_ns="$(now_ns)"
 attrs='{"source":"claude-code","hook":"SessionEnd","node_type":"WORKFLOW","session.lifecycle":"complete"}'
-emit_span "$trace_id" "$session_span_id" "" "Claude Code session" "1" "$session_start_ns" "$end_ns" "$attrs" || true
+emit_span "$trace_id" "$session_span_id" "$session_parent_span_id" "Claude Code session" "1" "$session_start_ns" "$end_ns" "$attrs" || true
 
 acquire_session_lock "$session_id" || exit 0
 trap 'release_session_lock' EXIT
diff --git a/plugins/trace/hooks/session_start.sh b/plugins/trace/hooks/session_start.sh
index 5bd612a..fe3d282 100755
--- a/plugins/trace/hooks/session_start.sh
+++ b/plugins/trace/hooks/session_start.sh
@@ -21,6 +21,18 @@ if [[ -n "$existing_trace_id" && -n "$existing_session_span_id" ]]; then
 	if [[ -z "$(get_session_state "$session_id" pending_tool_calls)" ]]; then
 		set_session_state "$session_id" pending_tool_calls "[]"
 	fi
+	if [[ -z "$(get_session_state "$session_id" session_parent_span_id)" ]]; then
+		set_session_state "$session_id" session_parent_span_id ""
+	fi
+	if [[ -z "$(get_session_state "$session_id" session_traceparent_version)" ]]; then
+		set_session_state "$session_id" session_traceparent_version ""
+	fi
+	if [[ -z "$(get_session_state "$session_id" session_trace_flags)" ]]; then
+		set_session_state "$session_id" session_trace_flags ""
+	fi
+	if [[ -z "$(get_session_state "$session_id" trace_context_source)" ]]; then
+		set_session_state "$session_id" trace_context_source "generated"
+	fi
 	if [[ -z "$(get_session_state "$session_id" session_end_requested)" ]]; then
 		set_session_state "$session_id" session_end_requested "false"
 	fi
@@ -31,16 +43,22 @@ if [[ -n "$existing_trace_id" && -n "$existing_session_span_id" ]]; then
 	exit 0
 fi
 
-trace_id="$(generate_trace_id)"
+load_initial_trace_context || true
+trace_id="${PL_INITIAL_TRACE_ID:-}"
+[[ -z "$trace_id" ]] && trace_id="$(generate_trace_id)"
 span_id="$(generate_span_id)"
 start_ns="$(now_ns)"
 
 set_session_state "$session_id" trace_id "$trace_id"
 set_session_state "$session_id" session_span_id "$span_id"
+set_session_state "$session_id" session_parent_span_id "${PL_INITIAL_PARENT_SPAN_ID:-}"
 set_session_state "$session_id" session_start_ns "$start_ns"
 set_session_state "$session_id" current_turn_start_ns ""
 set_session_state "$session_id" pending_tool_calls "[]"
 set_session_state "$session_id" session_init_source "session_start_hook"
+set_session_state "$session_id" session_traceparent_version "${PL_INITIAL_TRACEPARENT_VERSION:-}"
+set_session_state "$session_id" session_trace_flags "${PL_INITIAL_TRACE_FLAGS:-}"
+set_session_state "$session_id" trace_context_source "${PL_INITIAL_TRACE_CONTEXT_SOURCE:-generated}"
 set_session_state "$session_id" session_root_emitted "false"
 set_session_state "$session_id" session_end_requested "false"
 set_session_state "$session_id" stop_in_flight "false"
diff --git a/plugins/trace/hooks/stop_hook.sh b/plugins/trace/hooks/stop_hook.sh
index 8b0e68b..fe7c0eb 100755
--- a/plugins/trace/hooks/stop_hook.sh
+++ b/plugins/trace/hooks/stop_hook.sh
@@ -43,6 +43,7 @@ ensure_session_initialized "$session_id"
 
 trace_id="$(get_session_state "$session_id" trace_id)"
 session_span_id="$(get_session_state "$session_id" session_span_id)"
+session_parent_span_id="$(get_session_state "$session_id" session_parent_span_id)"
 turn_start_ns="$(get_session_state "$session_id" current_turn_start_ns)"
 pending_tool_calls="$(get_session_state "$session_id" pending_tool_calls)"
 session_end_requested="$(get_session_state "$session_id" session_end_requested)"
@@ -99,7 +100,7 @@ else
 		session_lifecycle_attr="in_progress"
 	fi
 	session_attrs="{\"source\":\"claude-code\",\"hook\":\"$session_hook_attr\",\"node_type\":\"WORKFLOW\",\"session.lifecycle\":\"$session_lifecycle_attr\"}"
-	add_span_to_batch "$trace_id" "$session_span_id" "" "Claude Code session" "1" "$session_start_ns" "$turn_end_ns" "$session_attrs" || true
+	add_span_to_batch "$trace_id" "$session_span_id" "$session_parent_span_id" "Claude Code session" "1" "$session_start_ns" "$turn_end_ns" "$session_attrs" || true
 	emitted_root="true"
 
 	while IFS= read -r tool; do
@@ -127,7 +128,7 @@ fi
 if [[ "$session_end_requested" == "true" ]]; then
 	end_ns="$(now_ns)"
 	session_end_attrs='{"source":"claude-code","hook":"SessionEnd","node_type":"WORKFLOW","session.lifecycle":"deferred_finalize"}'
-	add_span_to_batch "$trace_id" "$session_span_id" "" "Claude Code session" "1" "$session_start_ns" "$end_ns" "$session_end_attrs" || true
+	add_span_to_batch "$trace_id" "$session_span_id" "$session_parent_span_id" "Claude Code session" "1" "$session_start_ns" "$end_ns" "$session_end_attrs" || true
 	emitted_root="true"
 fi
 
@@ -145,6 +146,7 @@ latest_end_requested="$(get_session_state "$session_id" session_end_requested)"
 latest_turn_start_ns="$(get_session_state "$session_id" current_turn_start_ns)"
 latest_trace_id="$(get_session_state "$session_id" trace_id)"
 latest_session_span_id="$(get_session_state "$session_id" session_span_id)"
+latest_session_parent_span_id="$(get_session_state "$session_id" session_parent_span_id)"
 latest_session_start_ns="$(get_session_state "$session_id" session_start_ns)"
 [[ -z "$latest_end_requested" ]] && latest_end_requested="false"
 [[ -z "$latest_session_start_ns" ]] && latest_session_start_ns="$(now_ns)"
@@ -158,7 +160,7 @@ if [[ "$need_finalize_root" == "true" && -n "$latest_trace_id" && -n "$latest_se
 	release_session_lock
 	end_ns="$(now_ns)"
 	finalize_attrs='{"source":"claude-code","hook":"SessionEnd","node_type":"WORKFLOW","session.lifecycle":"deferred_finalize"}'
-	emit_span "$latest_trace_id" "$latest_session_span_id" "" "Claude Code session" "1" "$latest_session_start_ns" "$end_ns" "$finalize_attrs" || true
+	emit_span "$latest_trace_id" "$latest_session_span_id" "$latest_session_parent_span_id" "Claude Code session" "1" "$latest_session_start_ns" "$end_ns" "$finalize_attrs" || true
 	acquire_session_lock "$session_id" || exit 0
 	set_session_state "$session_id" stop_in_flight "false"
 	set_session_state "$session_id" session_root_emitted "true"
diff --git a/plugins/trace/testdata/session_end_input.json b/plugins/trace/testdata/session_end_input.json
new file mode 100644
index 0000000..b4a612e
--- /dev/null
+++ b/plugins/trace/testdata/session_end_input.json
@@ -0,0 +1 @@
+{"session_id":"example-session-id"}
diff --git a/plugins/trace/testdata/stop_input.json b/plugins/trace/testdata/stop_input.json
new file mode 100644
index 0000000..6ffd845
--- /dev/null
+++ b/plugins/trace/testdata/stop_input.json
@@ -0,0 +1 @@
+{"session_id":"example-session-id","transcript_path":"plugins/trace/testdata/stop_transcript.jsonl"}
diff --git a/plugins/trace/testdata/stop_transcript.jsonl b/plugins/trace/testdata/stop_transcript.jsonl
new file mode 100644
index 0000000..cd399f6
--- /dev/null
+++ b/plugins/trace/testdata/stop_transcript.jsonl
@@ -0,0 +1,2 @@
+{"sessionId":"example-session-id","type":"user","timestamp":"2026-03-16T12:00:00Z","message":{"content":[{"type":"text","text":"hello"}]}}
+{"sessionId":"example-session-id","type":"assistant","timestamp":"2026-03-16T12:00:01Z","message":{"model":"claude-sonnet-4-6","id":"msg_example_1","stop_reason":"end_turn","usage":{"input_tokens":12,"output_tokens":6},"content":[{"type":"text","text":"Hi there"}]}}
diff --git a/scripts/assert_session_span_payload.py b/scripts/assert_session_span_payload.py
new file mode 100644
index 0000000..d2fcc5c
--- /dev/null
+++ b/scripts/assert_session_span_payload.py
@@ -0,0 +1,40 @@
+#!/usr/bin/env python3
+
+import base64
+import json
+import sys
+
+
+def main() -> int:
+    queue_file, expected_trace_id, expected_parent_span_id = sys.argv[1:4]
+
+    with open(queue_file, encoding="utf-8") as f:
+        lines = [line.strip() for line in f if line.strip()]
+
+    if not lines:
+        raise SystemExit("Assertion failed: expected queued OTLP payload")
+
+    payload = json.loads(lines[-1])
+    spans = payload["resourceSpans"][0]["scopeSpans"][0]["spans"]
+    session_span = next((span for span in spans if span.get("name") == "Claude Code session"), None)
+    if session_span is None:
+        raise SystemExit("Assertion failed: missing Claude Code session span")
+
+    trace_id = base64.b64decode(session_span["traceId"]).hex()
+    if trace_id != expected_trace_id:
+        raise SystemExit(
+            f"Assertion failed: session span trace ID mismatch\n  expected: {expected_trace_id}\n  actual:   {trace_id}"
+        )
+
+    parent = session_span.get("parentSpanId")
+    parent_hex = base64.b64decode(parent).hex() if parent else ""
+    if parent_hex != expected_parent_span_id:
+        raise SystemExit(
+            f"Assertion failed: session span parent mismatch\n  expected: {expected_parent_span_id}\n  actual:   {parent_hex}"
+        )
+
+    return 0
+
+
+if __name__ == "__main__":
+    raise SystemExit(main())
diff --git a/scripts/replay-fixtures.sh b/scripts/replay-fixtures.sh
index 8e87fd9..6ca04ef 100755
--- a/scripts/replay-fixtures.sh
+++ b/scripts/replay-fixtures.sh
@@ -9,11 +9,185 @@ if ! command -v jq >/dev/null 2>&1; then
 	exit 0
 fi
 
-export TRACE_TO_PROMPTLAYER="true"
-export PROMPTLAYER_API_KEY="pl_test_key"
-export PROMPTLAYER_OTLP_ENDPOINT="http://127.0.0.1:9/v1/traces"
+TRACEPARENT_VALID="00-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-01"
+TRACEPARENT_FUTURE="01-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-03"
+TRACEPARENT_FUTURE_SUFFIXED="02-4bf92f3577b34da6a3ce929d0e0e4736-00f067aa0ba902b7-05-deadbeef"
+TRACE_ID_VALID="4bf92f3577b34da6a3ce929d0e0e4736"
+PARENT_SPAN_ID_VALID="00f067aa0ba902b7"
+SESSION_ID="example-session-id"
 
-# Expect failure to send (no server), but script should still run.
-cat plugins/trace/testdata/session_start_input.json | bash plugins/trace/hooks/session_start.sh || true
+assert_eq() {
+	local actual="$1"
+	local expected="$2"
+	local message="$3"
+	if [[ "$actual" != "$expected" ]]; then
+		echo "Assertion failed: $message"
+		echo "  expected: $expected"
+		echo "  actual:   $actual"
+		exit 1
+	fi
+}
+
+assert_matches() {
+	local actual="$1"
+	local pattern="$2"
+	local message="$3"
+	if [[ ! "$actual" =~ $pattern ]]; then
+		echo "Assertion failed: $message"
+		echo "  pattern: $pattern"
+		echo "  actual:  $actual"
+		exit 1
+	fi
+}
+
+state_value() {
+	local home="$1"
+	local sid="$2"
+	local key="$3"
+	jq -r ".${key} // empty" "$home/.claude/state/promptlayer_sessions/$sid.json"
+}
+
+run_hook() {
+	local home="$1"
+	local hook="$2"
+	local input_file="$3"
+	shift 3
+
+	env \
+		HOME="$home" \
+		TRACE_TO_PROMPTLAYER="true" \
+		PROMPTLAYER_API_KEY="pl_test_key" \
+		PROMPTLAYER_OTLP_ENDPOINT="http://127.0.0.1:9/v1/traces" \
+		PROMPTLAYER_OTLP_CONNECT_TIMEOUT="1" \
+		PROMPTLAYER_OTLP_MAX_TIME="1" \
+		"$@" \
+		bash "$hook" <"$input_file"
+}
+
+assert_session_span_payload() {
+	local queue_file="$1"
+	local expected_trace_id="$2"
+	local expected_parent_span_id="$3"
+
+	python3 scripts/assert_session_span_payload.py "$queue_file" "$expected_trace_id" "$expected_parent_span_id"
+}
+
+new_home() {
+	local dir
+	dir="$(mktemp -d "${TMPDIR:-/tmp}/pl-fixture-home.XXXXXX")"
+	mkdir -p "$dir/.claude/state"
+	echo "$dir"
+}
+
+cleanup_home() {
+	local home="$1"
+	rm -rf "$home"
+}
+
+test_valid_traceparent_session_end() {
+	local home
+	home="$(new_home)"
+	trap 'cleanup_home "$home"' RETURN
+
+	run_hook "$home" "plugins/trace/hooks/session_start.sh" "plugins/trace/testdata/session_start_input.json" \
+		"PROMPTLAYER_TRACEPARENT=$TRACEPARENT_VALID"
+
+	assert_eq "$(state_value "$home" "$SESSION_ID" trace_id)" "$TRACE_ID_VALID" "valid traceparent should reuse upstream trace ID"
+	assert_eq "$(state_value "$home" "$SESSION_ID" session_parent_span_id)" "$PARENT_SPAN_ID_VALID" "valid traceparent should store upstream parent span ID"
+	assert_eq "$(state_value "$home" "$SESSION_ID" session_traceparent_version)" "00" "v00 traceparent should store version"
+	assert_eq "$(state_value "$home" "$SESSION_ID" session_trace_flags)" "01" "v00 traceparent should store flags"
+	assert_eq "$(state_value "$home" "$SESSION_ID" trace_context_source)" "external_traceparent" "trace context source should record external parent"
+
+	run_hook "$home" "plugins/trace/hooks/session_end.sh" "plugins/trace/testdata/session_end_input.json" \
+		"PROMPTLAYER_TRACEPARENT=$TRACEPARENT_VALID"
+
+	assert_session_span_payload "$home/.claude/state/promptlayer_otlp_queue.ndjson" "$TRACE_ID_VALID" "$PARENT_SPAN_ID_VALID"
+}
+
+test_valid_traceparent_stop_hook() {
+	local home
+	home="$(new_home)"
+	trap 'cleanup_home "$home"' RETURN
+
+	run_hook "$home" "plugins/trace/hooks/session_start.sh" "plugins/trace/testdata/session_start_input.json" \
+		"PROMPTLAYER_TRACEPARENT=$TRACEPARENT_VALID"
+	run_hook "$home" "plugins/trace/hooks/user_prompt_submit.sh" "plugins/trace/testdata/session_end_input.json" \
+		"PROMPTLAYER_TRACEPARENT=$TRACEPARENT_VALID"
+	run_hook "$home" "plugins/trace/hooks/stop_hook.sh" "plugins/trace/testdata/stop_input.json" \
+		"PROMPTLAYER_TRACEPARENT=$TRACEPARENT_VALID"
+
+	assert_session_span_payload "$home/.claude/state/promptlayer_otlp_queue.ndjson" "$TRACE_ID_VALID" "$PARENT_SPAN_ID_VALID"
+}
+
+test_missing_traceparent_fallback() {
+	local home trace_id
+	home="$(new_home)"
+	trap 'cleanup_home "$home"' RETURN
+
+	run_hook "$home" "plugins/trace/hooks/session_start.sh" "plugins/trace/testdata/session_start_input.json"
+
+	trace_id="$(state_value "$home" "$SESSION_ID" trace_id)"
+	assert_matches "$trace_id" '^[0-9a-f]{32}$' "missing traceparent should generate a trace ID"
+	assert_eq "$(state_value "$home" "$SESSION_ID" session_parent_span_id)" "" "missing traceparent should not set a parent span ID"
+	assert_eq "$(state_value "$home" "$SESSION_ID" session_traceparent_version)" "" "missing traceparent should not set a version"
+	assert_eq "$(state_value "$home" "$SESSION_ID" session_trace_flags)" "" "missing traceparent should not set flags"
+	assert_eq "$(state_value "$home" "$SESSION_ID" trace_context_source)" "generated" "missing traceparent should record generated trace context"
+
+	run_hook "$home" "plugins/trace/hooks/session_end.sh" "plugins/trace/testdata/session_end_input.json"
+
+	assert_session_span_payload "$home/.claude/state/promptlayer_otlp_queue.ndjson" "$trace_id" ""
+}
+
+test_invalid_traceparent_fallback() {
+	local home trace_id
+	home="$(new_home)"
+	trap 'cleanup_home "$home"' RETURN
+
+	run_hook "$home" "plugins/trace/hooks/session_start.sh" "plugins/trace/testdata/session_start_input.json" \
+		"PROMPTLAYER_TRACEPARENT=bogus-value"
+
+	trace_id="$(state_value "$home" "$SESSION_ID" trace_id)"
+	assert_matches "$trace_id" '^[0-9a-f]{32}$' "invalid traceparent should fall back to a generated trace ID"
+	assert_eq "$(state_value "$home" "$SESSION_ID" session_parent_span_id)" "" "invalid traceparent should not set a parent span ID"
+	assert_eq "$(state_value "$home" "$SESSION_ID" session_traceparent_version)" "" "invalid traceparent should not set a version"
+	assert_eq "$(state_value "$home" "$SESSION_ID" session_trace_flags)" "" "invalid traceparent should not set flags"
+	assert_eq "$(state_value "$home" "$SESSION_ID" trace_context_source)" "generated" "invalid traceparent should record generated trace context"
+}
+
+test_non_zero_zero_version_traceparent() {
+	local home
+	home="$(new_home)"
+	trap 'cleanup_home "$home"' RETURN
+
+	run_hook "$home" "plugins/trace/hooks/session_start.sh" "plugins/trace/testdata/session_start_input.json" \
+		"PROMPTLAYER_TRACEPARENT=$TRACEPARENT_FUTURE"
+
+	assert_eq "$(state_value "$home" "$SESSION_ID" trace_id)" "$TRACE_ID_VALID" "non-00 traceparent should reuse upstream trace ID"
+	assert_eq "$(state_value "$home" "$SESSION_ID" session_parent_span_id)" "$PARENT_SPAN_ID_VALID" "non-00 traceparent should store upstream parent span ID"
+	assert_eq "$(state_value "$home" "$SESSION_ID" session_traceparent_version)" "01" "non-00 traceparent should store version"
+	assert_eq "$(state_value "$home" "$SESSION_ID" session_trace_flags)" "03" "non-00 traceparent should store flags"
+	assert_eq "$(state_value "$home" "$SESSION_ID" trace_context_source)" "external_traceparent" "non-00 traceparent should record external parent"
+}
+
+test_future_version_traceparent_with_suffix() {
+	local home
+	home="$(new_home)"
+	trap 'cleanup_home "$home"' RETURN
+
+	run_hook "$home" "plugins/trace/hooks/session_start.sh" "plugins/trace/testdata/session_start_input.json" \
+		"PROMPTLAYER_TRACEPARENT=$TRACEPARENT_FUTURE_SUFFIXED"
+
+	assert_eq "$(state_value "$home" "$SESSION_ID" trace_id)" "$TRACE_ID_VALID" "future traceparent with suffix should reuse upstream trace ID"
+	assert_eq "$(state_value "$home" "$SESSION_ID" session_parent_span_id)" "$PARENT_SPAN_ID_VALID" "future traceparent with suffix should store upstream parent span ID"
+	assert_eq "$(state_value "$home" "$SESSION_ID" session_traceparent_version)" "02" "future traceparent with suffix should store version"
+	assert_eq "$(state_value "$home" "$SESSION_ID" session_trace_flags)" "05" "future traceparent with suffix should store flags"
+}
+
+test_valid_traceparent_session_end
+test_valid_traceparent_stop_hook
+test_missing_traceparent_fallback
+test_invalid_traceparent_fallback
+test_non_zero_zero_version_traceparent
+test_future_version_traceparent_with_suffix
 
 echo "Fixture replay completed"