Studio: surface Gemini 3.5/3.1/3 + Nano Banana 2/Pro and plumb thinking budget

`gemini-2.0-flash` / `gemini-2.0-flash-exp` were retired by Google in 2026
(`/v1beta/models/gemini-2.0-flash:streamGenerateContent` returns HTTP 404
"no longer available to new users"), and the picker had nothing past the
2.x family. Verified against the live ListModels catalog: drop the retired
ids from `default_models` + allowlist and surface the chat-capable
3.5 / 3.1 / 3 families plus the Nano Banana image trio.

Also plumb `enable_thinking` / `reasoning_effort` into Gemini's
`generationConfig.thinkingConfig`. Without this, Gemini 3.5 Flash,
gemini-pro-latest, and the 3.x previews silently spend the caller's
`max_tokens` budget on hidden "thoughts" before emitting any visible
answer -- the chat shows a truncated stub like "The capital of" and
streams stop. Mapping:
  - enable_thinking=False / reasoning_effort=none -> thinkingBudget=0
    (Flash tier; Pro tier coerces to a small positive budget because
    the API 400s on 0 with "This model only works in thinking mode")
  - minimal/low/medium/high -> 512/2048/8192/24576 budget tokens
  - max/xhigh -> -1 (dynamic)
  - default (neither knob set) -> thinkingConfig omitted, model decides

Frontend `getExternalReasoningCapabilities` now surfaces a
`reasoning_effort` picker for every Gemini chat id (Pro tier hides the
"none" option; image-tier ids stay knob-less). Adds 6 unit tests
covering Flash/Pro effort mapping, the off-toggle coercion on Pro,
default omission, and the nano-banana-pro-preview alias routing
through the image modalities path. 28 -> 34 tests in
`test_gemini_provider.py`, all green; full backend suite still passes
(1459/1460; the unrelated test_help_output flake is pre-existing and
not in any file this PR touches).

Live verification against generativelanguage.googleapis.com on
2026-05-24 with `_stream_gemini` directly:
  text   gemini-3.5-flash           single PASS  multi PASS
  text   gemini-3.1-pro-preview     single PASS  multi PASS
  text   gemini-3.1-flash-lite      single PASS  multi PASS
  text   gemini-3-pro-preview       single PASS  multi PASS
  text   gemini-3-flash-preview     single PASS  multi PASS
  text   gemini-2.5-pro             single PASS  multi PASS
  text   gemini-2.5-flash           single PASS  multi PASS
  text   gemini-2.5-flash-lite      single PASS  multi PASS
  text   gemini-flash-latest        single PASS  multi PASS
  text   gemini-flash-lite-latest   single PASS  multi PASS
  text   gemini-pro-latest          single PASS  multi PASS
  image  gemini-2.5-flash-image     PASS (1082 KB png returned)
  image  gemini-3.1-flash-image-preview  PASS (Nano Banana 2)
  image  gemini-3-pro-image-preview      PASS (Nano Banana Pro)
  tool   web_search                 PASS
  tool   code_execution             PASS
  -> 16/16 e2e through the actual ExternalProviderClient code path.

studio/backend/core/inference/external_provider.py

@@ -377,6 +377,8 @@ class ExternalProviderClient:
                     presence_penalty,
                     enabled_tools,
                     enable_prompt_caching,
+                    enable_thinking,
+                    reasoning_effort,
                 ):
                     yield line
                 return
@@ -2574,6 +2576,8 @@ class ExternalProviderClient:
         presence_penalty: float = 0.0,
         enabled_tools: Optional[list[str]] = None,
         enable_prompt_caching: Optional[Any] = None,
+        enable_thinking: Optional[bool] = None,
+        reasoning_effort: Optional[str] = None,
     ) -> AsyncGenerator[str, None]:
         """
         Call Google's native Gemini API and translate its streaming
@@ -2797,12 +2801,58 @@ class ExternalProviderClient:
         # we translate to a tool_end with image_b64/image_mime so the
         # chat UI renders the picture inline. See
         # https://ai.google.dev/gemini-api/docs/image-generation.
-        is_image_model = "-image" in model.lower() or bool(
+        is_image_model = "-image" in model.lower() or "nano-banana" in model.lower() or bool(
             enabled_tools and "image_generation" in enabled_tools
         )
         if is_image_model:
             gen_config["responseModalities"] = ["TEXT", "IMAGE"]
 
+        # Thinking budget plumbing. Gemini 3.x, 3.5 Flash, gemini-pro-latest
+        # and gemini-flash-latest spend hidden "thoughts" tokens before they
+        # produce the streamed answer. With a tight `max_tokens` budget the
+        # answer can be entirely consumed by thoughts, so the caller's
+        # `enable_thinking` / `reasoning_effort` knobs need to actually
+        # reach `generationConfig.thinkingConfig`. Per
+        # https://ai.google.dev/gemini-api/docs/thinking:
+        #   thinkingBudget = 0  -> disable thinking (Flash-tier only;
+        #                          Pro-tier 400s with "only works in
+        #                          thinking mode")
+        #   thinkingBudget = -1 -> dynamic / let the model decide
+        #   thinkingBudget = N>0 -> hard cap of N thought tokens
+        # Pro-tier models cannot be turned off; we silently coerce
+        # an "off" request to a small budget on those so they stay
+        # responsive instead of 400ing the whole turn.
+        _PRO_THINKING_ONLY = (
+            "gemini-pro-latest",
+            "gemini-3.1-pro",
+            "gemini-3-pro",
+            "gemini-2.5-pro",
+        )
+        _is_pro_thinking_only = any(p in model for p in _PRO_THINKING_ONLY)
+        # Effort -> budget tokens. Mirrors the OpenAI ladder so the
+        # frontend's existing "minimal/low/medium/high/max" picker maps
+        # to sensible Gemini budgets. Match the keys the rest of Studio
+        # uses (see `stream_chat_completion`'s reasoning_effort docstring
+        # and the OpenAI / Anthropic helpers).
+        _EFFORT_TO_BUDGET: dict[str, int] = {
+            "minimal": 512,
+            "low": 2048,
+            "medium": 8192,
+            "high": 24576,
+            "xhigh": -1,  # dynamic
+            "max": -1,    # dynamic
+        }
+        thinking_budget: Optional[int] = None
+        effort_lc = (reasoning_effort or "").strip().lower()
+        if effort_lc == "none" or enable_thinking is False:
+            thinking_budget = 128 if _is_pro_thinking_only else 0
+        elif effort_lc in _EFFORT_TO_BUDGET:
+            thinking_budget = _EFFORT_TO_BUDGET[effort_lc]
+        elif enable_thinking is True:
+            thinking_budget = -1
+        if thinking_budget is not None:
+            gen_config["thinkingConfig"] = {"thinkingBudget": thinking_budget}
+
         if gen_config:
             body["generationConfig"] = gen_config
 

studio/backend/core/inference/providers.py

@@ -72,14 +72,23 @@ PROVIDER_REGISTRY: dict[str, dict[str, Any]] = {
         "base_url": "https://generativelanguage.googleapis.com/v1beta",
         # Curated lineup -- the live ListModels response returns dozens
         # of historical / experimental / embedding ids. Cap to the
-        # current 2.5/2.0 family plus the Nano Banana image model and
+        # current chat-capable Gemini families (3.5 / 3.1 / 3 / 2.5)
-        # the rolling `*-latest` aliases.
+        # plus the Nano Banana image trio and the rolling `*-latest`
+        # aliases. `gemini-2.0-flash*` were retired by Google in 2026
+        # and are intentionally excluded; the allowlist below blocks
+        # them from re-appearing through the live ListModels fetch.
+        # Verified against the live `/v1beta/models` catalog 2026-05-24.
         "default_models": [
+            "gemini-3.5-flash",
+            "gemini-3.1-pro-preview",
+            "gemini-3.1-flash-lite",
+            "gemini-3-pro-preview",
+            "gemini-3-flash-preview",
             "gemini-2.5-pro",
             "gemini-2.5-flash",
             "gemini-2.5-flash-lite",
-            "gemini-2.0-flash",
+            "gemini-3-pro-image-preview",
-            "gemini-2.0-flash-exp",
+            "gemini-3.1-flash-image-preview",
             "gemini-2.5-flash-image",
         ],
         "supports_streaming": True,
@@ -95,11 +104,25 @@ PROVIDER_REGISTRY: dict[str, dict[str, Any]] = {
             "API key from https://aistudio.google.com/apikey. "
             "See https://ai.google.dev/gemini-api/docs for endpoint shapes."
         ),
+        # Matches the chat-capable 3.5 / 3.1 / 3 / 2.5 families plus the
+        # rolling *-latest aliases (which Google rolls forward as new
+        # generations ship). Image-tier ids (`-image`, `-image-preview`,
+        # `nano-banana-pro-preview`) flow through the Nano Banana
+        # `responseModalities` path in `_stream_gemini`. Retired 2.0
+        # ids ARE NOT in this regex on purpose -- Google's ListModels
+        # would otherwise re-surface them and they 404 on use.
         "model_id_allowlist": re.compile(
-            r"^(gemini-2\.5-pro|gemini-2\.5-flash|gemini-2\.5-flash-lite|"
+            r"^("
-            r"gemini-2\.5-flash-image|gemini-2\.0-flash|"
+            r"gemini-3\.5-(?:flash|pro)(?:-preview)?|"
-            r"gemini-2\.0-flash-exp|gemini-pro-latest|"
+            r"gemini-3\.1-(?:flash|pro|flash-lite)(?:-preview)?(?:-customtools)?|"
-            r"gemini-flash-latest|gemini-flash-lite-latest)$"
+            r"gemini-3\.1-flash-image-preview|"
+            r"gemini-3-(?:flash|pro)(?:-preview)?|"
+            r"gemini-3-pro-image-preview|"
+            r"nano-banana-pro-preview|"
+            r"gemini-2\.5-pro|gemini-2\.5-flash|gemini-2\.5-flash-lite|"
+            r"gemini-2\.5-flash-image|"
+            r"gemini-pro-latest|gemini-flash-latest|gemini-flash-lite-latest"
+            r")$"
         ),
     },
     "deepseek": {

studio/backend/tests/test_gemini_provider.py

@@ -249,6 +249,90 @@ def test_presence_penalty_forwarded_to_generation_config(monkeypatch):
     assert "presencePenalty" not in captured["body"]["generationConfig"]
 
 
+# ── thinkingConfig translation ────────────────────────────────────────
+
+
+def test_thinking_disabled_sets_budget_zero_on_flash(monkeypatch):
+    """enable_thinking=False on Flash-tier sets thinkingBudget=0."""
+    captured = _capture_body(
+        monkeypatch,
+        model = "gemini-3.5-flash",
+        enable_thinking = False,
+    )
+    tc = captured["body"]["generationConfig"].get("thinkingConfig")
+    assert tc == {"thinkingBudget": 0}, tc
+
+
+def test_thinking_disabled_pro_tier_uses_small_budget(monkeypatch):
+    """Pro-tier ids 400 on thinkingBudget=0 ("only works in thinking mode");
+    a small positive budget is forwarded instead so the turn doesn't fail.
+    """
+    for model in (
+        "gemini-3.1-pro-preview",
+        "gemini-3-pro-preview",
+        "gemini-2.5-pro",
+        "gemini-pro-latest",
+    ):
+        captured = _capture_body(
+            monkeypatch,
+            model = model,
+            enable_thinking = False,
+        )
+        tc = captured["body"]["generationConfig"].get("thinkingConfig")
+        assert tc is not None, f"missing thinkingConfig for {model}: {captured}"
+        assert tc["thinkingBudget"] > 0, (model, tc)
+
+
+def test_reasoning_effort_levels_map_to_budgets(monkeypatch):
+    """The OpenAI/Anthropic effort ladder must translate to Gemini budgets."""
+    cases = {
+        "minimal": 512,
+        "low": 2048,
+        "medium": 8192,
+        "high": 24576,
+        "max": -1,    # dynamic
+        "xhigh": -1,
+    }
+    for effort, expected in cases.items():
+        captured = _capture_body(
+            monkeypatch,
+            model = "gemini-3.5-flash",
+            reasoning_effort = effort,
+        )
+        tc = captured["body"]["generationConfig"].get("thinkingConfig")
+        assert tc == {"thinkingBudget": expected}, (effort, tc)
+
+
+def test_reasoning_effort_none_disables_on_flash(monkeypatch):
+    """`reasoning_effort='none'` is shorthand for thinking off (Flash)."""
+    captured = _capture_body(
+        monkeypatch,
+        model = "gemini-3.5-flash",
+        reasoning_effort = "none",
+    )
+    tc = captured["body"]["generationConfig"].get("thinkingConfig")
+    assert tc == {"thinkingBudget": 0}, tc
+
+
+def test_thinking_default_omits_thinking_config(monkeypatch):
+    """When neither knob is supplied, thinkingConfig is omitted entirely
+    (Google's server-side default applies)."""
+    captured = _capture_body(monkeypatch, model = "gemini-3.5-flash")
+    gc = captured["body"]["generationConfig"]
+    assert "thinkingConfig" not in gc, gc
+
+
+def test_nano_banana_alias_routes_through_image_modalities(monkeypatch):
+    """`nano-banana-pro-preview` is an alias for the Pro image model and
+    must set responseModalities=[TEXT,IMAGE] same as the `*-image` ids."""
+    captured = _capture_body(
+        monkeypatch,
+        model = "nano-banana-pro-preview",
+    )
+    gc = captured["body"]["generationConfig"]
+    assert gc.get("responseModalities") == ["TEXT", "IMAGE"], gc
+
+
 # ── web_search forwarded as googleSearch tool ────────────────────────
 
 

studio/frontend/src/features/chat/provider-capabilities.ts

@@ -577,6 +577,67 @@ function resolveKimiReasoningCapabilities(modelId: string): ExternalReasoningCap
   return withEnableThinkingStyle();
 }
 
+// Gemini's thinking ladder.
+//   - 3.5 / 3.1 / 3 Flash + Flash-Lite + 2.5 Flash + *-latest aliases:
+//     toggleable thinking with effort levels (backend maps to
+//     `thinkingConfig.thinkingBudget`).
+//   - 3.x Pro + gemini-pro-latest + 2.5 Pro: "thinking only" -- the
+//     API 400s on `thinkingBudget=0` ("This model only works in
+//     thinking mode"), so the UI hides the off switch.
+//   - 2.5 Flash-Lite: no native thinking surfaced; leave it off.
+//   - Image-tier ids (`*-image*`, `nano-banana-pro-preview`): image
+//     generation path -- no reasoning controls.
+const GEMINI_THINKING_PRO_PREFIXES = [
+  "gemini-3.5-pro",
+  "gemini-3.1-pro",
+  "gemini-3-pro",
+  "gemini-2.5-pro",
+  "gemini-pro-latest",
+];
+const GEMINI_THINKING_FLASH_PREFIXES = [
+  "gemini-3.5-flash",
+  "gemini-3.1-flash",
+  "gemini-3-flash",
+  "gemini-2.5-flash",
+  "gemini-flash-latest",
+  "gemini-flash-lite-latest",
+];
+const GEMINI_IMAGE_HINTS = [
+  "-image",
+  "nano-banana",
+];
+function resolveGeminiReasoningCapabilities(
+  modelId: string,
+): ExternalReasoningCapabilities {
+  const m = modelId.toLowerCase();
+  if (GEMINI_IMAGE_HINTS.some((h) => m.includes(h))) {
+    // Image generation; no thinking knob.
+    return withEnableThinkingStyle();
+  }
+  if (GEMINI_THINKING_PRO_PREFIXES.some((p) => m.startsWith(p))) {
+    return withReasoningEffortStyle({
+      supportsReasoning: true,
+      // Pro tier: cannot turn thinking fully off.
+      supportsReasoningOff: false,
+      reasoningEffortLevels: ["low", "medium", "high", "max"] as const,
+    });
+  }
+  if (GEMINI_THINKING_FLASH_PREFIXES.some((p) => m.startsWith(p))) {
+    return withReasoningEffortStyle({
+      supportsReasoning: true,
+      supportsReasoningOff: true,
+      reasoningEffortLevels: [
+        "none",
+        "low",
+        "medium",
+        "high",
+        "max",
+      ] as const,
+    });
+  }
+  return withEnableThinkingStyle();
+}
+
 function resolveMistralReasoningCapabilities(modelId: string): ExternalReasoningCapabilities {
   if (modelId === "magistral-medium-latest") {
     return withReasoningEffortStyle({
@@ -674,6 +735,9 @@ export function getExternalReasoningCapabilities(
   }
   if (isKimiProvider) return resolveKimiReasoningCapabilities(modelForMatching);
   if (isMistralProvider) return resolveMistralReasoningCapabilities(modelForMatching);
+  if (normalizedProvider === "gemini") {
+    return resolveGeminiReasoningCapabilities(modelForMatching);
+  }
   if (!isOpenAIProvider && !isAnthropicProvider) {
     return withEnableThinkingStyle();
   }