diff --git a/api/routes.py b/api/routes.py
index a014860e..e4375b70 100644
--- a/api/routes.py
+++ b/api/routes.py
@@ -268,6 +268,18 @@ def _normalize_session_model_in_place(session) -> str:
     return effective_model
 
 
+def _resolve_effective_session_model_for_display(session) -> str:
+    """Resolve the model a session should display without mutating persisted state.
+
+    `GET /api/session` should stay side-effect free. If a stale persisted model
+    needs normalization for the current provider configuration, return the
+    effective model for the response payload only and leave disk state alone.
+    """
+    original_model = getattr(session, "model", None) or ""
+    effective_model, _changed = _resolve_compatible_session_model(original_model or None)
+    return effective_model or original_model
+
+
 from api.models import (
     Session,
     get_session,
@@ -604,7 +616,7 @@ def handle_get(handler, parsed) -> bool:
             return j(handler, {"error": "session_id is required"}, status=400)
         try:
             s = get_session(sid)
-            _normalize_session_model_in_place(s)
+            effective_model = _resolve_effective_session_model_for_display(s)
             raw = s.compact() | {
                 "messages": s.messages,
                 "tool_calls": getattr(s, "tool_calls", []),
@@ -613,6 +625,8 @@ def handle_get(handler, parsed) -> bool:
                 "pending_attachments": getattr(s, "pending_attachments", []),
                 "pending_started_at": getattr(s, "pending_started_at", None),
             }
+            if effective_model:
+                raw["model"] = effective_model
             return j(handler, {"session": redact_session_data(raw)})
         except KeyError:
             # Not a WebUI session -- try CLI store
diff --git a/tests/test_provider_mismatch.py b/tests/test_provider_mismatch.py
index b4407032..eb989785 100644
--- a/tests/test_provider_mismatch.py
+++ b/tests/test_provider_mismatch.py
@@ -14,6 +14,7 @@ import json
 import pathlib
 import re
 import urllib.request
+from tests.conftest import TEST_STATE_DIR
 
 REPO_ROOT = pathlib.Path(__file__).parent.parent.resolve()
 from tests._pytest_port import BASE
@@ -23,6 +24,15 @@ def _read(rel_path: str) -> str:
     return (REPO_ROOT / rel_path).read_text(encoding="utf-8")
 
 
+def _post(path, body=None):
+    data = json.dumps(body or {}).encode()
+    req = urllib.request.Request(
+        BASE + path, data=data, headers={"Content-Type": "application/json"}
+    )
+    with urllib.request.urlopen(req, timeout=10) as r:
+        return json.loads(r.read()), r.status
+
+
 # ── 1. streaming.py: auth error detection ───────────────────────────────────
 
 class TestStreamingAuthErrorDetection:
@@ -348,7 +358,7 @@ def test_google_active_provider_keeps_valid_gemini_session_model(monkeypatch):
 
 
 def test_session_model_normalizer_persists_corrected_model(monkeypatch):
-    """GET /api/session should persist the corrected model back to disk/state."""
+    """Write-path normalization should still persist corrected models."""
     import api.routes as routes
 
     monkeypatch.setattr(
@@ -377,6 +387,65 @@ def test_session_model_normalizer_persists_corrected_model(monkeypatch):
     assert save_calls == [False]
 
 
+def test_session_model_display_resolver_is_read_only(monkeypatch):
+    """Read-path model resolution must not mutate or save the session."""
+    import api.routes as routes
+
+    monkeypatch.setattr(
+        routes,
+        "get_available_models",
+        lambda: {
+            "active_provider": "openai-codex",
+            "default_model": "gpt-5.4-mini",
+        },
+    )
+
+    save_calls = []
+
+    class DummySession:
+        def __init__(self):
+            self.model = "gemini-3.1-pro-preview"
+
+        def save(self, touch_updated_at=True):
+            save_calls.append(touch_updated_at)
+
+    session = DummySession()
+    effective = routes._resolve_effective_session_model_for_display(session)
+
+    assert effective == "gpt-5.4-mini"
+    assert session.model == "gemini-3.1-pro-preview"
+    assert save_calls == []
+
+
+def test_api_session_is_side_effect_free_for_stale_models():
+    """GET /api/session must not rewrite the session file on first open (#845)."""
+    created, status = _post("/api/session/new", {})
+    assert status == 200
+    sid = created["session"]["session_id"]
+
+    session_path = TEST_STATE_DIR / "sessions" / f"{sid}.json"
+    session_data = json.loads(session_path.read_text(encoding="utf-8"))
+    stale_model = "google/gemini-3.1-pro-preview"
+    session_data["model"] = stale_model
+    before = json.dumps(session_data, ensure_ascii=False, indent=2)
+    session_path.write_text(before, encoding="utf-8")
+
+    with urllib.request.urlopen(
+        BASE + f"/api/session?session_id={sid}", timeout=10
+    ) as r:
+        payload = json.loads(r.read())
+
+    after = session_path.read_text(encoding="utf-8")
+    assert payload["session"]["model"], "response should still expose an effective display model"
+    assert payload["session"]["model"] != stale_model, (
+        "response model should be compatibility-normalized on the read path"
+    )
+    assert after == before, (
+        "GET /api/session must return an effective model for display without "
+        "rewriting the session file on disk"
+    )
+
+
 # ── Model switch toast (#419) ─────────────────────────────────────────────────
 
 class TestModelSwitchToast: