diff --git a/tests/test_litellm/proxy/agent_session_endpoints/conftest.py b/tests/test_litellm/proxy/agent_session_endpoints/conftest.py
index dde13368a49..1fa228b38e1 100644
--- a/tests/test_litellm/proxy/agent_session_endpoints/conftest.py
+++ b/tests/test_litellm/proxy/agent_session_endpoints/conftest.py
@@ -247,6 +247,9 @@ def _build_test_app(
         run_router,
         session_router,
     )
+    from litellm.proxy.agent_session_endpoints.error_envelope import (
+        register_v2_exception_handlers,
+    )
     from litellm.proxy.auth.user_api_key_auth import user_api_key_auth
 
     app = FastAPI()
@@ -254,6 +257,8 @@ def _build_test_app(
     app.include_router(session_router)
     app.include_router(run_router)
     app.include_router(internal_router)
+    # Mirror the production setup so tests see the same /v2 error envelope.
+    register_v2_exception_handlers(app)
 
     def _fake_auth() -> UserAPIKeyAuth:
         return UserAPIKeyAuth(
diff --git a/tests/test_litellm/proxy/agent_session_endpoints/test_error_envelope.py b/tests/test_litellm/proxy/agent_session_endpoints/test_error_envelope.py
new file mode 100644
index 00000000000..b0095927906
--- /dev/null
+++ b/tests/test_litellm/proxy/agent_session_endpoints/test_error_envelope.py
@@ -0,0 +1,82 @@
+"""
+Verify the standardized {error: {code, message, status, details?}} envelope
+applies to /v2/* endpoints (HTTPException + ValidationError) and that
+non-/v2 paths keep FastAPI's default detail shape.
+"""
+
+import pytest
+
+
+def test_404_emits_envelope_for_missing_session(client, noop_provider):
+    """A 404 from the v2 surface should be wrapped in the envelope."""
+    res = client.get(
+        "/v2/sessions/does-not-exist",
+        headers={"Authorization": "Bearer k"},
+    )
+    # find_unique returns None -> ownership assert raises HTTPException(404).
+    assert res.status_code == 404
+    body = res.json()
+    assert "error" in body and "detail" not in body
+    assert body["error"]["status"] == 404
+    assert body["error"]["code"] == "not_found"
+    assert isinstance(body["error"]["message"], str)
+
+
+def test_validation_error_wraps_in_envelope(client, noop_provider):
+    """A 422 ValidationError from a missing required body field should
+    arrive as ``{error: {code: validation_error, details: [...]}}``."""
+    # POST /v2/agents requires `name` + `model`; omit both.
+    res = client.post(
+        "/v2/agents",
+        headers={"Authorization": "Bearer k"},
+        json={},
+    )
+    assert res.status_code == 422
+    body = res.json()
+    assert "error" in body and "detail" not in body
+    assert body["error"]["code"] == "validation_error"
+    assert body["error"]["status"] == 422
+    assert isinstance(body["error"].get("details"), list)
+    # Validation details preserve the per-field {loc, msg, type} shape.
+    assert any("name" in str(d.get("loc", [])) for d in body["error"]["details"])
+
+
+def test_http_status_code_to_envelope_code_mapping(client, noop_provider):
+    """409 should map to ``conflict``, not a generic name."""
+    # Create an agent + session, then try to POST with missing agent_id
+    # path -> simpler: trigger a 409 by creating two runs back-to-back
+    # on the same session.
+    agent = client.post(
+        "/v2/agents",
+        headers={"Authorization": "Bearer k"},
+        json={"name": "t", "model": "gpt-4"},
+    ).json()
+    sess = client.post(
+        f"/v2/agents/{agent['id']}/sessions",
+        headers={"Authorization": "Bearer k"},
+        json={"repos": []},
+    ).json()
+    sid = sess["id"]
+    daemon_token = sess["daemon_token"]
+    client.post(
+        f"/v2/sessions/{sid}/internal/register",
+        headers={"Authorization": f"Bearer {daemon_token}"},
+        json={"vm_id": "i-noop"},
+    )
+    # First run: queued.
+    client.post(
+        f"/v2/sessions/{sid}/runs",
+        headers={"Authorization": "Bearer k"},
+        json={"prompt": {"text": "hi"}},
+    )
+    # Second run on same session while first is still queued -> 409 run_busy.
+    res = client.post(
+        f"/v2/sessions/{sid}/runs",
+        headers={"Authorization": "Bearer k"},
+        json={"prompt": {"text": "again"}},
+    )
+    assert res.status_code == 409
+    body = res.json()
+    assert body["error"]["status"] == 409
+    assert body["error"]["code"] == "conflict"
+    assert "run_busy" in body["error"]["message"]
diff --git a/tests/test_litellm/proxy/agent_session_endpoints/test_followup_concurrency.py b/tests/test_litellm/proxy/agent_session_endpoints/test_followup_concurrency.py
index bd7dc26dd29..b84a71cc0ee 100644
--- a/tests/test_litellm/proxy/agent_session_endpoints/test_followup_concurrency.py
+++ b/tests/test_litellm/proxy/agent_session_endpoints/test_followup_concurrency.py
@@ -90,7 +90,8 @@ def test_followup_returns_409_when_active_run_is_not_the_latest(
         json={"prompt": {"text": "should be blocked"}},
     )
     assert res.status_code == 409
-    assert "run_busy" in res.json()["detail"]
+    # Error envelope shape: {"error": {"code", "message", ...}}
+    assert "run_busy" in res.json()["error"]["message"]
 
 
 def test_followup_normal_path_creates_run_when_no_active_runs(client, noop_provider):
diff --git a/tests/test_litellm/proxy/agent_session_endpoints/test_view_only_admin_no_writes.py b/tests/test_litellm/proxy/agent_session_endpoints/test_view_only_admin_no_writes.py
index 79b523710ea..a0224085946 100644
--- a/tests/test_litellm/proxy/agent_session_endpoints/test_view_only_admin_no_writes.py
+++ b/tests/test_litellm/proxy/agent_session_endpoints/test_view_only_admin_no_writes.py
@@ -44,7 +44,8 @@ def test_view_only_admin_cannot_create_agent(view_only_admin_client, noop_provid
         json={"name": "evil", "model": "gpt-4"},
     )
     assert res.status_code == 403
-    assert "view-only" in res.json()["detail"].lower()
+    # Error envelope: {"error": {"code", "message", "status"}}
+    assert "view-only" in res.json()["error"]["message"].lower()
 
 
 def test_view_only_admin_cannot_update_other_tenant_agent(