diff --git a/README.md b/README.md
index 61ef8a3be..f1b4f6145 100644
--- a/README.md
+++ b/README.md
@@ -72,7 +72,7 @@ Strix are autonomous AI agents that act just like real hackers - they run your c
 
 **Prerequisites:**
 - Docker (running)
-- An LLM API key from any [supported provider](https://docs.strix.ai/llm-providers/overview) (OpenAI, Anthropic, Google, etc.)
+- An LLM API key from any [supported provider](https://docs.strix.ai/llm-providers/overview) (OpenAI, Anthropic, Google, etc.), or a Codex CLI ChatGPT login
 
 ### Installation & First Scan
 
@@ -234,6 +234,14 @@ export PERPLEXITY_API_KEY="your-api-key"  # for search capabilities
 export STRIX_REASONING_EFFORT="high"  # control thinking effort (default: high, quick scan: medium)
 ```
 
+To use Codex OAuth instead of an API key, log in with the Codex CLI and select a
+`codex/` model:
+
+```bash
+codex login
+export STRIX_LLM="codex/gpt-5.5"
+```
+
 > [!NOTE]
 > Strix automatically saves your configuration to `~/.strix/cli-config.json`, so you don't have to re-enter it on every run.
 
diff --git a/strix/interface/main.py b/strix/interface/main.py
index bc88da673..7f0e586e8 100644
--- a/strix/interface/main.py
+++ b/strix/interface/main.py
@@ -20,6 +20,7 @@
 
 from strix.config import Config, apply_saved_config, save_current_config
 from strix.config.config import resolve_llm_config
+from strix.llm.codex_oauth import codex_model_name, complete_codex_oauth
 from strix.llm.utils import resolve_strix_model
 
 
@@ -57,11 +58,12 @@ def validate_environment() -> None:  # noqa: PLR0912, PLR0915
 
     strix_llm = Config.get("strix_llm")
     uses_strix_models = strix_llm and strix_llm.startswith("strix/")
+    uses_codex_oauth = strix_llm and strix_llm.startswith("codex/")
 
     if not strix_llm:
         missing_required_vars.append("STRIX_LLM")
 
-    has_base_url = uses_strix_models or any(
+    has_base_url = uses_strix_models or uses_codex_oauth or any(
         [
             Config.get("llm_api_base"),
             Config.get("openai_api_base"),
@@ -70,7 +72,7 @@ def validate_environment() -> None:  # noqa: PLR0912, PLR0915
         ]
     )
 
-    if not Config.get("llm_api_key"):
+    if not Config.get("llm_api_key") and not uses_codex_oauth:
         missing_optional_vars.append("LLM_API_KEY")
 
     if not has_base_url:
@@ -209,6 +211,21 @@ async def warm_up_llm() -> None:
 
     try:
         model_name, api_key, api_base = resolve_llm_config()
+
+        if model_name and model_name.startswith("codex/"):
+            test_messages = [
+                {"role": "system", "content": "You are a helpful assistant."},
+                {"role": "user", "content": "Reply with just 'OK'."},
+            ]
+            llm_timeout = int(Config.get("llm_timeout") or "300")
+            complete_codex_oauth(
+                codex_model_name(model_name),
+                test_messages,
+                Config.get("strix_reasoning_effort"),
+                llm_timeout,
+            )
+            return
+
         litellm_model, _ = resolve_strix_model(model_name)
         litellm_model = litellm_model or model_name
 
diff --git a/strix/llm/codex_oauth.py b/strix/llm/codex_oauth.py
new file mode 100644
index 000000000..80e24e72c
--- /dev/null
+++ b/strix/llm/codex_oauth.py
@@ -0,0 +1,357 @@
+import json
+import os
+from dataclasses import dataclass
+from pathlib import Path
+from typing import Any
+
+import requests
+
+
+CODEX_BACKEND_BASE_URL = "https://chatgpt.com/backend-api/codex"
+CODEX_OAUTH_TOKEN_URL = "https://auth.openai.com/oauth/token"  # noqa: S105
+CODEX_OAUTH_CLIENT_ID = "app_EMoamEEZ73f0CkXaXp7hrann"
+
+
+class CodexOAuthError(Exception):
+    """Raised when Codex OAuth credentials or backend calls fail."""
+
+
+@dataclass(frozen=True)
+class CodexOAuthCredentials:
+    access_token: str
+    account_id: str | None = None
+
+
+def default_codex_home() -> Path:
+    configured = os.getenv("CODEX_HOME")
+    if configured:
+        return Path(configured).expanduser()
+    return Path.home() / ".codex"
+
+
+def load_codex_oauth_credentials(codex_home: Path | None = None) -> CodexOAuthCredentials:
+    home = codex_home or default_codex_home()
+    raw = _read_codex_auth_json(home)
+    tokens = raw.get("tokens")
+    if not isinstance(tokens, dict):
+        raise CodexOAuthError("Codex auth file does not contain OAuth tokens.")
+
+    access_token = tokens.get("access_token")
+    if not isinstance(access_token, str) or not access_token.strip():
+        raise CodexOAuthError("Codex auth file does not contain an access token.")
+
+    account_id = tokens.get("account_id")
+    if not isinstance(account_id, str) or not account_id.strip():
+        account_id = raw.get("account_id")
+    if not isinstance(account_id, str) or not account_id.strip():
+        account_id = None
+
+    return CodexOAuthCredentials(access_token=access_token, account_id=account_id)
+
+
+def refresh_codex_oauth_credentials(
+    codex_home: Path | None = None,
+    post: Any = requests.post,
+) -> CodexOAuthCredentials:
+    home = codex_home or default_codex_home()
+    auth_file = home / "auth.json"
+    raw = _read_codex_auth_json(home)
+
+    tokens = raw.get("tokens")
+    if not isinstance(tokens, dict):
+        raise CodexOAuthError("Codex auth file does not contain OAuth tokens.")
+
+    refresh_token = tokens.get("refresh_token")
+    if not isinstance(refresh_token, str) or not refresh_token.strip():
+        raise CodexOAuthError("Codex auth file does not contain a refresh token.")
+
+    try:
+        response = post(
+            CODEX_OAUTH_TOKEN_URL,
+            headers={"Content-Type": "application/json"},
+            json={
+                "client_id": CODEX_OAUTH_CLIENT_ID,
+                "grant_type": "refresh_token",
+                "refresh_token": refresh_token,
+            },
+            timeout=30,
+        )
+    except requests.RequestException as exc:
+        raise CodexOAuthError(f"Codex OAuth token refresh failed: {exc}") from exc
+
+    if not response.ok:
+        body = response.text[:1000]
+        raise CodexOAuthError(
+            f"Codex OAuth token refresh failed: HTTP {response.status_code}: {body}"
+        )
+
+    try:
+        refreshed = response.json()
+    except ValueError as exc:
+        raise CodexOAuthError(f"Codex OAuth token refresh returned invalid JSON: {exc}") from exc
+
+    for token_name in ("id_token", "access_token", "refresh_token"):
+        token_value = refreshed.get(token_name)
+        if isinstance(token_value, str) and token_value.strip():
+            tokens[token_name] = token_value
+
+    try:
+        auth_file.write_text(json.dumps(raw, indent=2), encoding="utf-8")
+    except OSError as exc:
+        raise CodexOAuthError(f"Could not persist refreshed Codex OAuth tokens: {exc}") from exc
+
+    return load_codex_oauth_credentials(home)
+
+
+def codex_model_name(model_name: str) -> str:
+    return model_name.removeprefix("codex/")
+
+
+def build_codex_responses_payload(
+    model: str,
+    messages: list[dict[str, Any]],
+    reasoning_effort: str | None,
+) -> dict[str, Any]:
+    instructions: list[str] = []
+    input_items: list[dict[str, Any]] = []
+
+    for message in messages:
+        role = str(message.get("role", "user"))
+        content = message.get("content", "")
+        if role == "system":
+            text = _content_to_text(content)
+            if text:
+                instructions.append(text)
+            continue
+
+        response_role = "assistant" if role == "assistant" else "user"
+        input_items.append(
+            {
+                "type": "message",
+                "role": response_role,
+                "content": _content_to_responses_items(content, response_role),
+            }
+        )
+
+    payload: dict[str, Any] = {
+        "model": model,
+        "instructions": "\n\n".join(instructions),
+        "input": input_items,
+        "tools": [],
+        "tool_choice": "none",
+        "parallel_tool_calls": False,
+        "reasoning": {"effort": reasoning_effort} if reasoning_effort else None,
+        "store": False,
+        "stream": True,
+        "include": [],
+    }
+
+    return payload
+
+
+def complete_codex_oauth(
+    model: str,
+    messages: list[dict[str, Any]],
+    reasoning_effort: str | None,
+    timeout: int,
+    codex_home: Path | None = None,
+    base_url: str = CODEX_BACKEND_BASE_URL,
+) -> tuple[str, dict[str, int]]:
+    credentials = load_codex_oauth_credentials(codex_home)
+    payload = build_codex_responses_payload(model, messages, reasoning_effort)
+    response = _post_codex_responses(base_url, credentials, payload, timeout)
+
+    if response.status_code == 401:
+        credentials = refresh_codex_oauth_credentials(codex_home)
+        response = _post_codex_responses(base_url, credentials, payload, timeout)
+        if response.status_code == 401:
+            raise CodexOAuthError("Codex OAuth request was unauthorized. Run `codex login` again.")
+    if not response.ok:
+        body = response.text[:1000]
+        raise CodexOAuthError(f"Codex OAuth request failed: HTTP {response.status_code}: {body}")
+
+    content_type = response.headers.get("content-type", "")
+    if "application/json" in content_type:
+        try:
+            return parse_responses_json_response(response.json())
+        except ValueError as exc:
+            raise CodexOAuthError(f"Codex OAuth JSON response could not be parsed: {exc}") from exc
+
+    return parse_responses_sse_events(response.iter_lines(decode_unicode=True))
+
+
+def _read_codex_auth_json(codex_home: Path) -> dict[str, Any]:
+    auth_file = codex_home / "auth.json"
+    if not auth_file.exists():
+        raise CodexOAuthError(
+            f"Codex auth file not found at {auth_file}. Run `codex login` first."
+        )
+
+    try:
+        raw = json.loads(auth_file.read_text(encoding="utf-8"))
+    except (OSError, json.JSONDecodeError) as exc:
+        raise CodexOAuthError(f"Could not read Codex auth file at {auth_file}: {exc}") from exc
+    if not isinstance(raw, dict):
+        raise CodexOAuthError("Codex auth file is not a JSON object.")
+    return raw
+
+
+def _post_codex_responses(
+    base_url: str,
+    credentials: CodexOAuthCredentials,
+    payload: dict[str, Any],
+    timeout: int,
+) -> requests.Response:
+    headers = {
+        "Authorization": f"Bearer {credentials.access_token}",
+        "Accept": "text/event-stream",
+        "Content-Type": "application/json",
+        "User-Agent": "strix-codex-oauth",
+    }
+    if credentials.account_id:
+        headers["ChatGPT-Account-ID"] = credentials.account_id
+
+    try:
+        return requests.post(
+            f"{base_url.rstrip('/')}/responses",
+            headers=headers,
+            json=payload,
+            stream=True,
+            timeout=timeout,
+        )
+    except requests.RequestException as exc:
+        raise CodexOAuthError(f"Codex OAuth request failed: {exc}") from exc
+
+
+def parse_responses_sse_events(lines: Any) -> tuple[str, dict[str, int]]:
+    deltas: list[str] = []
+    completed_parts: list[str] = []
+    usage: dict[str, int] = {}
+
+    for raw_line in lines:
+        event = _parse_sse_data_line(raw_line)
+        if event is None:
+            continue
+
+        event_type = event.get("type")
+        if event_type == "response.output_text.delta":
+            delta = event.get("delta")
+            if isinstance(delta, str):
+                deltas.append(delta)
+        elif event_type == "response.output_item.done" and not deltas:
+            completed_parts.append(_extract_output_text(event.get("item")))
+        elif event_type == "response.completed":
+            response = event.get("response")
+            if isinstance(response, dict):
+                usage = _extract_usage(response.get("usage"))
+                if not deltas and not completed_parts:
+                    completed_parts.append(_extract_output_text(response))
+
+    content = "".join(deltas) if deltas else "".join(completed_parts)
+    return content, usage
+
+
+def parse_responses_json_response(response: dict[str, Any]) -> tuple[str, dict[str, int]]:
+    return _extract_output_text(response), _extract_usage(response.get("usage"))
+
+
+def _parse_sse_data_line(raw_line: Any) -> dict[str, Any] | None:
+    if isinstance(raw_line, bytes):
+        parsed_line = raw_line.decode("utf-8", errors="replace")
+    else:
+        parsed_line = raw_line
+    if not isinstance(parsed_line, str):
+        return None
+
+    line = parsed_line.strip()
+    if not line.startswith("data:"):
+        return None
+
+    data = line[5:].strip()
+    if not data or data == "[DONE]":
+        return None
+
+    try:
+        event = json.loads(data)
+    except json.JSONDecodeError:
+        return None
+    return event if isinstance(event, dict) else None
+
+
+def _content_to_text(content: Any) -> str:
+    if isinstance(content, str):
+        return content
+    if isinstance(content, list):
+        parts = []
+        for item in content:
+            if isinstance(item, dict) and item.get("type") == "text":
+                text = item.get("text")
+                if isinstance(text, str):
+                    parts.append(text)
+        return "\n".join(parts)
+    return str(content)
+
+
+def _content_to_responses_items(content: Any, role: str) -> list[dict[str, Any]]:
+    text_type = "output_text" if role == "assistant" else "input_text"
+
+    if isinstance(content, str):
+        return [{"type": text_type, "text": content}]
+
+    if not isinstance(content, list):
+        return [{"type": text_type, "text": str(content)}]
+
+    result: list[dict[str, Any]] = []
+    for item in content:
+        if not isinstance(item, dict):
+            continue
+        if item.get("type") == "text":
+            text = item.get("text")
+            if isinstance(text, str):
+                result.append({"type": text_type, "text": text})
+        elif role == "user" and item.get("type") == "image_url":
+            image_url = item.get("image_url")
+            if isinstance(image_url, dict):
+                image_url = image_url.get("url")
+            if isinstance(image_url, str):
+                result.append({"type": "input_image", "image_url": image_url})
+
+    return result or [{"type": text_type, "text": ""}]
+
+
+def _extract_output_text(value: Any) -> str:
+    if not isinstance(value, dict):
+        return ""
+
+    output_text = value.get("output_text")
+    if isinstance(output_text, str):
+        return output_text
+
+    parts: list[str] = []
+    output = value.get("output")
+    if isinstance(output, list):
+        parts.extend(_extract_output_text(item) for item in output)
+
+    content = value.get("content")
+    if isinstance(content, list):
+        for item in content:
+            if isinstance(item, dict):
+                text = item.get("text")
+                if isinstance(text, str):
+                    parts.append(text)
+
+    return "".join(parts)
+
+
+def _extract_usage(value: Any) -> dict[str, int]:
+    if not isinstance(value, dict):
+        return {}
+
+    usage: dict[str, int] = {}
+    input_tokens = value.get("input_tokens") or value.get("prompt_tokens")
+    output_tokens = value.get("output_tokens") or value.get("completion_tokens")
+    if isinstance(input_tokens, int):
+        usage["input_tokens"] = input_tokens
+    if isinstance(output_tokens, int):
+        usage["output_tokens"] = output_tokens
+    return usage
diff --git a/strix/llm/config.py b/strix/llm/config.py
index 017c77662..429c8160e 100644
--- a/strix/llm/config.py
+++ b/strix/llm/config.py
@@ -2,6 +2,7 @@
 
 from strix.config import Config
 from strix.config.config import resolve_llm_config
+from strix.llm.codex_oauth import codex_model_name
 from strix.llm.utils import resolve_strix_model
 
 
@@ -24,6 +25,9 @@ def __init__(
         if not self.model_name:
             raise ValueError("STRIX_LLM environment variable must be set and not empty")
 
+        self.uses_codex_oauth = self.model_name.startswith("codex/")
+        self.codex_model = codex_model_name(self.model_name) if self.uses_codex_oauth else None
+
         api_model, canonical = resolve_strix_model(self.model_name)
         self.litellm_model: str = api_model or self.model_name
         self.canonical_model: str = canonical or self.model_name
diff --git a/strix/llm/llm.py b/strix/llm/llm.py
index 6fd727d5f..4f70c0138 100644
--- a/strix/llm/llm.py
+++ b/strix/llm/llm.py
@@ -10,6 +10,7 @@
 from litellm.utils import supports_prompt_caching, supports_vision
 
 from strix.config import Config
+from strix.llm.codex_oauth import CodexOAuthError, complete_codex_oauth
 from strix.llm.config import LLMConfig
 from strix.llm.memory_compressor import MemoryCompressor, get_message_tokens
 from strix.llm.utils import (
@@ -187,6 +188,11 @@ async def generate(
                 await asyncio.sleep(wait)
 
     async def _stream(self, messages: list[dict[str, Any]]) -> AsyncIterator[LLMResponse]:
+        if self.config.uses_codex_oauth:
+            async for response in self._stream_codex_oauth(messages):
+                yield response
+            return
+
         accumulated = ""
         chunks: list[Any] = []
         done_streaming = 0
@@ -291,6 +297,33 @@ def _build_completion_args(self, messages: list[dict[str, Any]]) -> dict[str, An
 
         return args
 
+    async def _stream_codex_oauth(
+        self, messages: list[dict[str, Any]]
+    ) -> AsyncIterator[LLMResponse]:
+        self._total_stats.requests += 1
+        model = self.config.codex_model or self.config.model_name
+        content, usage = await asyncio.to_thread(
+            complete_codex_oauth,
+            model,
+            messages,
+            self._reasoning_effort,
+            self.config.timeout,
+        )
+
+        if usage:
+            self._total_stats.input_tokens += usage.get("input_tokens", 0)
+            self._total_stats.output_tokens += usage.get("output_tokens", 0)
+
+        content = _THINKING_BLOCK_RE.sub("", content)
+        content = normalize_tool_format(content)
+        content = fix_incomplete_tool_call(_truncate_to_first_function(content))
+
+        yield LLMResponse(
+            content=content,
+            tool_invocations=parse_tool_invocations(content),
+            thinking_blocks=None,
+        )
+
     def _get_chunk_content(self, chunk: Any) -> str:
         if chunk.choices and hasattr(chunk.choices[0], "delta"):
             return getattr(chunk.choices[0].delta, "content", "") or ""
@@ -348,6 +381,8 @@ def _extract_cost(self, response: Any) -> float:
             return 0.0
 
     def _should_retry(self, e: Exception) -> bool:
+        if isinstance(e, CodexOAuthError):
+            return False
         code = getattr(e, "status_code", None) or getattr(
             getattr(e, "response", None), "status_code", None
         )
diff --git a/tests/llm/test_codex_oauth.py b/tests/llm/test_codex_oauth.py
new file mode 100644
index 000000000..c848f22b9
--- /dev/null
+++ b/tests/llm/test_codex_oauth.py
@@ -0,0 +1,221 @@
+import json
+from pathlib import Path
+
+import pytest
+
+from strix.llm.codex_oauth import (
+    CodexOAuthCredentials,
+    CodexOAuthError,
+    _post_codex_responses,
+    build_codex_responses_payload,
+    load_codex_oauth_credentials,
+    parse_responses_sse_events,
+    refresh_codex_oauth_credentials,
+)
+from strix.llm.config import LLMConfig
+from strix.llm.llm import LLM
+
+
+def test_llm_config_detects_codex_oauth_provider(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setenv("STRIX_LLM", "codex/gpt-5.5")
+
+    config = LLMConfig()
+
+    assert config.uses_codex_oauth is True
+    assert config.codex_model == "gpt-5.5"
+
+
+def test_llm_does_not_retry_codex_oauth_errors(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setenv("STRIX_LLM", "codex/gpt-5.5")
+
+    llm = LLM(LLMConfig())
+
+    assert llm._should_retry(CodexOAuthError("Run `codex login` first.")) is False
+
+
+@pytest.mark.asyncio
+async def test_codex_oauth_stream_yields_single_processed_response(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    monkeypatch.setenv("STRIX_LLM", "codex/gpt-5.5")
+
+    def fake_complete_codex_oauth(*args, **kwargs):
+        return (
+            "<thinking>hidden</thinking><function=finish><parameter=summary>done</parameter></function>",
+            {"input_tokens": 1, "output_tokens": 2},
+        )
+
+    monkeypatch.setattr("strix.llm.llm.complete_codex_oauth", fake_complete_codex_oauth)
+
+    llm = LLM(LLMConfig())
+    responses = [
+        response
+        async for response in llm._stream_codex_oauth([{"role": "user", "content": "Hi"}])
+    ]
+
+    assert len(responses) == 1
+    assert "<thinking>" not in responses[0].content
+    assert responses[0].tool_invocations
+    assert llm._total_stats.input_tokens == 1
+    assert llm._total_stats.output_tokens == 2
+
+
+def test_load_codex_oauth_credentials_reads_codex_auth_json(tmp_path: Path) -> None:
+    auth_file = tmp_path / "auth.json"
+    auth_file.write_text(
+        json.dumps(
+            {
+                "auth_mode": "chatgpt",
+                "tokens": {
+                    "access_token": "access-token",
+                    "account_id": "account-id",
+                },
+            }
+        ),
+        encoding="utf-8",
+    )
+
+    credentials = load_codex_oauth_credentials(tmp_path)
+
+    assert credentials.access_token == "access-token"  # noqa: S105
+    assert credentials.account_id == "account-id"
+
+
+def test_load_codex_oauth_credentials_errors_when_not_logged_in(tmp_path: Path) -> None:
+    with pytest.raises(CodexOAuthError, match="Codex auth file not found"):
+        load_codex_oauth_credentials(tmp_path)
+
+
+def test_refresh_codex_oauth_credentials_persists_new_tokens(tmp_path: Path) -> None:
+    auth_file = tmp_path / "auth.json"
+    auth_file.write_text(
+        json.dumps(
+            {
+                "auth_mode": "chatgpt",
+                "tokens": {
+                    "id_token": "old-id",
+                    "access_token": "old-access",
+                    "refresh_token": "old-refresh",
+                    "account_id": "account-id",
+                },
+            }
+        ),
+        encoding="utf-8",
+    )
+
+    class FakeResponse:
+        status_code = 200
+        ok = True
+        text = ""
+
+        def json(self) -> dict[str, str]:
+            return {
+                "id_token": "new-id",
+                "access_token": "new-access",
+                "refresh_token": "new-refresh",
+            }
+
+    calls = []
+
+    def fake_post(url: str, **kwargs):
+        calls.append((url, kwargs))
+        return FakeResponse()
+
+    credentials = refresh_codex_oauth_credentials(tmp_path, post=fake_post)
+
+    updated = json.loads(auth_file.read_text(encoding="utf-8"))
+    assert credentials.access_token == "new-access"  # noqa: S105
+    assert updated["tokens"]["id_token"] == "new-id"  # noqa: S105
+    assert updated["tokens"]["access_token"] == "new-access"  # noqa: S105
+    assert updated["tokens"]["refresh_token"] == "new-refresh"  # noqa: S105
+    assert updated["tokens"]["account_id"] == "account-id"
+    assert calls[0][0] == "https://auth.openai.com/oauth/token"
+    assert calls[0][1]["json"]["grant_type"] == "refresh_token"
+
+
+def test_post_codex_responses_uses_user_agent_header(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    calls = []
+
+    class FakeResponse:
+        pass
+
+    def fake_post(url: str, **kwargs):
+        calls.append((url, kwargs))
+        return FakeResponse()
+
+    monkeypatch.setattr("strix.llm.codex_oauth.requests.post", fake_post)
+
+    response = _post_codex_responses(
+        "https://example.test/codex",
+        CodexOAuthCredentials(access_token="access-token", account_id="account-id"),
+        {"model": "gpt-5.5"},
+        60,
+    )
+
+    assert isinstance(response, FakeResponse)
+    headers = calls[0][1]["headers"]
+    assert headers["User-Agent"] == "strix-codex-oauth"
+    assert "version" not in headers
+
+
+def test_build_codex_responses_payload_converts_chat_messages() -> None:
+    payload = build_codex_responses_payload(
+        model="gpt-5.5",
+        messages=[
+            {"role": "system", "content": "System prompt."},
+            {"role": "user", "content": "Find bugs."},
+            {"role": "assistant", "content": "I will inspect the code."},
+            {
+                "role": "user",
+                "content": [
+                    {"type": "text", "text": "Tool Results:"},
+                    {"type": "image_url", "image_url": {"url": "data:image/png;base64,abc"}},
+                ],
+            },
+        ],
+        reasoning_effort="high",
+    )
+
+    assert payload["model"] == "gpt-5.5"
+    assert payload["instructions"] == "System prompt."
+    assert payload["stream"] is True
+    assert payload["reasoning"] == {"effort": "high"}
+    assert payload["input"] == [
+        {
+            "type": "message",
+            "role": "user",
+            "content": [{"type": "input_text", "text": "Find bugs."}],
+        },
+        {
+            "type": "message",
+            "role": "assistant",
+            "content": [{"type": "output_text", "text": "I will inspect the code."}],
+        },
+        {
+            "type": "message",
+            "role": "user",
+            "content": [
+                {"type": "input_text", "text": "Tool Results:"},
+                {"type": "input_image", "image_url": "data:image/png;base64,abc"},
+            ],
+        },
+    ]
+
+
+def test_parse_responses_sse_events_extracts_text_deltas_and_usage() -> None:
+    content, usage = parse_responses_sse_events(
+        [
+            'data: {"type":"response.output_text.delta","delta":"hel"}',
+            'data: {"type":"response.output_text.delta","delta":"lo"}',
+            (
+                'data: {"type":"response.completed","response":'
+                '{"usage":{"input_tokens":3,"output_tokens":2}}}'
+            ),
+            "data: [DONE]",
+        ]
+    )
+
+    assert content == "hello"
+    assert usage == {"input_tokens": 3, "output_tokens": 2}