diff --git a/libs/hackbot-client/hackbot_client/client.py b/libs/hackbot-client/hackbot_client/client.py
index e9db069f88..1e0e8e996d 100644
--- a/libs/hackbot-client/hackbot_client/client.py
+++ b/libs/hackbot-client/hackbot_client/client.py
@@ -2,24 +2,58 @@
 
 from __future__ import annotations
 
+import asyncio
 from collections.abc import Mapping
 from typing import Any
 
 import httpx
+from google.auth.transport import requests as google_requests
+from google.oauth2 import id_token
 
 from hackbot_client.models import RunRef
 
 
 class HackbotClient:
-    """Call the public, API-key-authenticated Hackbot endpoints."""
+    """Call the public Hackbot API with either API key or service account auth."""
 
     def __init__(
-        self, base_url: str, api_key: str, timeout_seconds: float = 30.0
+        self,
+        base_url: str,
+        api_key: str = "",
+        audience: str = "",
+        timeout_seconds: float = 30.0,
     ) -> None:
+        """Initialize the client.
+
+        Args:
+            base_url: Base URL of the Hackbot API
+            api_key: API key for legacy authentication (optional)
+            audience: Audience for service account OIDC token minting.
+                Required if api_key is not set.
+            timeout_seconds: HTTP request timeout
+        """
         self._base_url = base_url.rstrip("/")
         self._api_key = api_key
+        self._audience = audience
         self._timeout_seconds = timeout_seconds
 
+        if not api_key and not audience:
+            raise ValueError(
+                "Either api_key or audience must be provided for authentication"
+            )
+
+    async def _get_headers(self) -> dict[str, str]:
+        """Return auth headers: API key or service account token."""
+        if self._api_key:
+            return {"X-API-Key": self._api_key}
+
+        token = await asyncio.to_thread(
+            id_token.fetch_id_token,
+            google_requests.Request(),
+            self._audience,
+        )
+        return {"Authorization": f"Bearer {token}"}
+
     async def trigger_run(
         self,
         agent_name: str,
@@ -28,7 +62,7 @@ async def trigger_run(
         on_behalf_of: str | None = None,
     ) -> RunRef:
         """Create an agent run and return the API's typed run reference."""
-        headers = {"X-API-Key": self._api_key}
+        headers = await self._get_headers()
         if on_behalf_of is not None:
             headers["X-On-Behalf-Of"] = on_behalf_of
 
diff --git a/libs/hackbot-client/pyproject.toml b/libs/hackbot-client/pyproject.toml
index b63b8433b0..df2e8af07b 100644
--- a/libs/hackbot-client/pyproject.toml
+++ b/libs/hackbot-client/pyproject.toml
@@ -4,6 +4,7 @@ version = "0.1.0"
 description = "Small shared Hackbot API client (httpx-based)"
 requires-python = ">=3.12"
 dependencies = [
+    "google-auth[requests]>=2.25.0",
     "httpx>=0.26.0",
     "pydantic>=2.6.0",
 ]
diff --git a/libs/hackbot-client/tests/test_client.py b/libs/hackbot-client/tests/test_client.py
index 44d022f525..5551f89cd9 100644
--- a/libs/hackbot-client/tests/test_client.py
+++ b/libs/hackbot-client/tests/test_client.py
@@ -12,11 +12,12 @@
 
 
 def _client(**kwargs) -> HackbotClient:
-    return HackbotClient(
-        base_url=kwargs.pop("base_url", "https://hackbot.example"),
-        api_key=kwargs.pop("api_key", "secret"),
-        **kwargs,
-    )
+    defaults = {
+        "base_url": "https://hackbot.example",
+        "api_key": "secret",
+    }
+    defaults.update(kwargs)
+    return HackbotClient(**defaults)
 
 
 def _capture_post(monkeypatch, response: httpx.Response) -> dict:
@@ -95,3 +96,51 @@ async def test_trigger_run_rejects_an_invalid_success_response(monkeypatch):
 
     with pytest.raises(ValidationError):
         await _client().trigger_run("bug-fix", {"bug_id": 1234})
+
+
+def test_client_requires_either_api_key_or_audience():
+    """Client must have at least one auth method configured."""
+    with pytest.raises(ValueError, match="Either api_key or audience must be provided"):
+        HackbotClient(base_url="https://example.com")
+
+
+async def test_trigger_run_with_service_account_auth(monkeypatch):
+    """Client uses Bearer token when api_key is not set but audience is."""
+    captured = _capture_post(
+        monkeypatch,
+        httpx.Response(
+            201,
+            json={"run_id": RUN_ID, "agent": "bug-fix", "status": "pending"},
+        ),
+    )
+
+    def mock_fetch_id_token(request, audience):
+        return "fake.jwt.token"
+
+    monkeypatch.setattr(client_module.id_token, "fetch_id_token", mock_fetch_id_token)
+
+    client = _client(api_key="", audience="https://hackbot.example")
+    await client.trigger_run("bug-fix", {"bug_id": 1234})
+
+    assert captured["headers"] == {"Authorization": "Bearer fake.jwt.token"}
+
+
+async def test_trigger_run_prefers_api_key_over_service_account(monkeypatch):
+    """When both api_key and audience are set, use the API key."""
+    captured = _capture_post(
+        monkeypatch,
+        httpx.Response(
+            201,
+            json={"run_id": RUN_ID, "agent": "bug-fix", "status": "pending"},
+        ),
+    )
+
+    def mock_fetch_id_token(request, audience, credentials=None):
+        raise AssertionError("Should not call fetch_id_token when api_key is set")
+
+    monkeypatch.setattr(client_module.id_token, "fetch_id_token", mock_fetch_id_token)
+
+    client = _client(api_key="secret", audience="https://hackbot.example")
+    await client.trigger_run("bug-fix", {"bug_id": 1234})
+
+    assert captured["headers"] == {"X-API-Key": "secret"}
diff --git a/services/hackbot-api/app/auth.py b/services/hackbot-api/app/auth.py
index 903b4f4e66..2ae9fc16c6 100644
--- a/services/hackbot-api/app/auth.py
+++ b/services/hackbot-api/app/auth.py
@@ -3,6 +3,7 @@
 import logging
 
 from fastapi import Header, HTTPException, Request, status
+from google.auth import exceptions as google_auth_exceptions
 from google.auth.transport import requests as google_requests
 from google.oauth2 import id_token
 from slack_sdk.signature import SignatureVerifier
@@ -79,18 +80,44 @@ async def require_slack_signature(
         )
 
 
-async def require_api_key(x_api_key: str | None = Header(default=None)) -> None:
-    if not settings.external_api_key:
+def require_api_key(
+    x_api_key: str | None = Header(default=None),
+    authorization: str | None = Header(default=None),
+) -> None:
+    """Accept either API key (X-API-Key) or service account token (Bearer)."""
+    if x_api_key is not None:
+        if settings.external_api_key and hmac.compare_digest(
+            x_api_key, settings.external_api_key
+        ):
+            return
         raise HTTPException(
-            status_code=status.HTTP_500_INTERNAL_SERVER_ERROR,
-            detail="API key not configured",
+            status_code=status.HTTP_401_UNAUTHORIZED,
+            detail="Invalid X-API-Key",
         )
-    if x_api_key is None or not hmac.compare_digest(
-        x_api_key, settings.external_api_key
-    ):
+
+    if not authorization or not authorization.startswith("Bearer "):
+        raise HTTPException(
+            status_code=status.HTTP_401_UNAUTHORIZED,
+            detail="Invalid or missing credentials",
+        )
+
+    try:
+        claims = id_token.verify_oauth2_token(
+            authorization.removeprefix("Bearer "),
+            google_requests.Request(),
+            audience=settings.api_audience,
+        )
+    except (ValueError, google_auth_exceptions.GoogleAuthError):
+        log.warning("Rejected request with invalid OIDC credentials")
+        raise HTTPException(
+            status_code=status.HTTP_401_UNAUTHORIZED,
+            detail="Invalid credentials",
+        ) from None
+
+    if claims.get("email") not in settings.allowed_service_accounts:
         raise HTTPException(
             status_code=status.HTTP_401_UNAUTHORIZED,
-            detail="Invalid or missing X-API-Key",
+            detail="Service account not authorized",
         )
 
 
diff --git a/services/hackbot-api/app/config.py b/services/hackbot-api/app/config.py
index b7673dc339..70b14ed681 100644
--- a/services/hackbot-api/app/config.py
+++ b/services/hackbot-api/app/config.py
@@ -70,6 +70,8 @@ class Settings(BaseSettings):
 
     # API auth
     external_api_key: str = ""
+    api_audience: str = ""
+    allowed_service_accounts: list[str] = []
 
     phabricator: PhabricatorSettings
 
diff --git a/services/hackbot-api/app/routers/webhooks.py b/services/hackbot-api/app/routers/webhooks.py
index f096babcff..37638a6886 100644
--- a/services/hackbot-api/app/routers/webhooks.py
+++ b/services/hackbot-api/app/routers/webhooks.py
@@ -37,6 +37,7 @@ def get_hackbot_client() -> HackbotClient:
     return HackbotClient(
         base_url=settings.hackbot_api_url,
         api_key=settings.external_api_key,
+        audience=settings.api_audience,
     )
 
 
diff --git a/services/hackbot-api/tests/test_auth.py b/services/hackbot-api/tests/test_auth.py
new file mode 100644
index 0000000000..8e504ac7c7
--- /dev/null
+++ b/services/hackbot-api/tests/test_auth.py
@@ -0,0 +1,37 @@
+import inspect
+
+import pytest
+from app import auth
+from fastapi import HTTPException
+from google.auth import exceptions as google_auth_exceptions
+from google.oauth2 import id_token
+
+
+@pytest.mark.parametrize(
+    "error",
+    [
+        ValueError(),
+        google_auth_exceptions.GoogleAuthError(),
+        google_auth_exceptions.TransportError(),
+    ],
+)
+def test_google_token_errors_return_401(monkeypatch, error):
+    def fail(*_args, **_kwargs):
+        raise error
+
+    monkeypatch.setattr(id_token, "verify_oauth2_token", fail)
+    monkeypatch.setattr(auth.settings, "api_audience", "https://hackbot.example")
+    monkeypatch.setattr(
+        auth.settings,
+        "allowed_service_accounts",
+        ["listener@example.iam.gserviceaccount.com"],
+    )
+
+    with pytest.raises(HTTPException) as exc:
+        auth.require_api_key(x_api_key=None, authorization="Bearer bad-token")
+
+    assert exc.value.status_code == 401
+
+
+def test_google_auth_dependencies_are_synchronous():
+    assert not inspect.iscoroutinefunction(auth.require_api_key)
diff --git a/uv.lock b/uv.lock
index c4b7e1f0a7..26d3fd796a 100644
--- a/uv.lock
+++ b/uv.lock
@@ -2766,6 +2766,7 @@ name = "hackbot-client"
 version = "0.1.0"
 source = { editable = "libs/hackbot-client" }
 dependencies = [
+    { name = "google-auth", extra = ["requests"] },
     { name = "httpx" },
     { name = "pydantic" },
 ]
@@ -2778,6 +2779,7 @@ dev = [
 
 [package.metadata]
 requires-dist = [
+    { name = "google-auth", extras = ["requests"], specifier = ">=2.25.0" },
     { name = "httpx", specifier = ">=0.26.0" },
     { name = "pydantic", specifier = ">=2.6.0" },
     { name = "pytest", marker = "extra == 'dev'", specifier = ">=8.0.0" },