fix: classify OpenAI developer messages as system context

CHANGELOG.md

@@ -4,6 +4,13 @@ All notable changes to this project are documented here. The format is based on
 [Keep a Changelog](https://keepachangelog.com/en/1.1.0/) and this project
 adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## [Unreleased]
+
+### Fixed
+
+- Attribute OpenAI chat `developer` messages to the system segment instead of
+  the user segment.
+
 ## [0.1.1] - 2026-08-06
 
 ### Changed

src/ctxlens/parsers/openai_chat.py

@@ -9,6 +9,8 @@
 
 or an object ``{"messages": [...], "tools": [...]}`` where ``tools`` is the
 function/tool schema list sent to the model.
+
+Both ``system`` and ``developer`` messages contribute to the system segment.
 """
 
 from __future__ import annotations
@@ -41,7 +43,7 @@ def sniff(cls, raw: str, path: Path | None = None) -> float:
             return 0.0
         roles = {m.get("role") for m in messages if isinstance(m, dict)}
         score = 0.5
-        if roles & {"system", "user", "assistant", "tool"}:
+        if roles & {"system", "developer", "user", "assistant", "tool"}:
             score += 0.3
         # a bare array is the strongest generic signal
         if isinstance(obj, list):
@@ -98,7 +100,7 @@ def parse(self, raw: str, path: Path | None = None) -> Session:
         )
 
     def _emit_message(self, entry, role, content, turn, out):
-        if role == "system":
+        if role in {"system", "developer"}:
             out.append(Message(Segment.SYSTEM, stringify(content), turn, role=role))
             return
         if role == "tool":

tests/test_parsers.py

@@ -1,5 +1,7 @@
 from __future__ import annotations
 
+import json
+
 import pytest
 
 from ctxlens.models import Segment
@@ -83,6 +85,41 @@ def test_parse_openai_array_roles(openai_array):
     assert any(m.segment == Segment.ASSISTANT for m in s.messages)
 
 
+@pytest.mark.parametrize("wrapped", [False, True], ids=["array", "object"])
+def test_detect_openai_developer_only(wrapped):
+    def transcript(role):
+        messages = [{"role": role, "content": "Answer concisely."}]
+        return json.dumps({"messages": messages} if wrapped else messages)
+
+    raw = transcript("developer")
+    assert detect_parser(raw) is OpenAIChatParser
+    assert OpenAIChatParser.sniff(raw) == OpenAIChatParser.sniff(transcript("system"))
+
+
+@pytest.mark.parametrize("wrapped", [False, True], ids=["array", "object"])
+def test_parse_openai_developer_role(wrapped):
+    messages = [
+        {"role": "developer", "content": "Answer concisely."},
+        {"role": "user", "content": "Hello!"},
+        {"role": "assistant", "content": "Hi!"},
+        {"role": "developer", "content": "Use bullet points now."},
+    ]
+    raw = json.dumps({"messages": messages} if wrapped else messages)
+
+    session = parse_text(raw)
+
+    assert session.source_format == "openai-chat"
+    assert [m.segment for m in session.messages] == [
+        Segment.SYSTEM,
+        Segment.USER,
+        Segment.ASSISTANT,
+        Segment.SYSTEM,
+    ]
+    assert [m.role for m in session.messages] == [m["role"] for m in messages]
+    assert [m.text for m in session.messages] == [m["content"] for m in messages]
+    assert [m.turn for m in session.messages] == [1, 1, 1, 2]
+
+
 def test_force_format_override(openai_array):
     raw = openai_array.read_text()
     s = parse_text(raw, fmt="openai-chat")
@@ -96,7 +133,9 @@ def test_unknown_format_raises():
 
 def test_bad_json_line_raises():
     with pytest.raises(ParseError):
-        ClaudeCodeParser().parse('{"type":"user","message":{"role":"user","content":"hi"}}\n{bad json')
+        ClaudeCodeParser().parse(
+            '{"type":"user","message":{"role":"user","content":"hi"}}\n{bad json'
+        )
 
 
 def test_turns_are_monotonic(claude_jsonl, codex_session, openai_chat):