diff --git a/agent/transports/chat_completions.py b/agent/transports/chat_completions.py
index 0e497c9ec3..ab7075aa18 100644
--- a/agent/transports/chat_completions.py
+++ b/agent/transports/chat_completions.py
@@ -18,6 +18,52 @@ from agent.transports.base import ProviderTransport
 from agent.transports.types import NormalizedResponse, ToolCall, Usage
 
 
+def _build_gemini_thinking_config(model: str, reasoning_config: dict | None) -> dict | None:
+    """Translate Hermes/OpenRouter-style reasoning config to Gemini thinkingConfig.
+
+    Gemini native/cloud-code adapters do not read ``extra_body.reasoning``.
+    They only inspect ``extra_body.thinking_config`` / ``thinkingConfig`` and
+    then request thought parts with ``includeThoughts`` enabled.
+    """
+    if reasoning_config is None or not isinstance(reasoning_config, dict):
+        return None
+
+    if reasoning_config.get("enabled") is False:
+        # Gemini can hide thought parts even when internal thinking still
+        # happens; omit thinkingLevel to avoid model-specific validation quirks.
+        return {"includeThoughts": False}
+
+    effort = str(reasoning_config.get("effort", "medium") or "medium").strip().lower()
+    if effort == "none":
+        return {"includeThoughts": False}
+
+    thinking_config: Dict[str, Any] = {"includeThoughts": True}
+    normalized_model = (model or "").strip().lower()
+    if normalized_model.startswith("google/"):
+        normalized_model = normalized_model.split("/", 1)[1]
+
+    # Gemini 2.5 accepts thinkingBudget; don't guess a budget from Hermes'
+    # coarse effort levels. ``includeThoughts`` alone is enough to surface
+    # thought parts without risking request validation errors.
+    if normalized_model.startswith("gemini-2.5-"):
+        return thinking_config
+
+    if effort not in {"minimal", "low", "medium", "high", "xhigh"}:
+        effort = "medium"
+
+    # Gemini 3 Flash supports the wider level set; Gemini 3 Pro is stricter,
+    # so collapse low-end efforts to ``low`` and high-end efforts to ``high``.
+    if normalized_model.startswith(("gemini-3", "gemini-3.1")):
+        if "flash" in normalized_model:
+            thinking_config["thinkingLevel"] = "high" if effort == "xhigh" else effort
+        elif "pro" in normalized_model:
+            thinking_config["thinkingLevel"] = (
+                "high" if effort in {"high", "xhigh"} else "low"
+            )
+
+    return thinking_config
+
+
 class ChatCompletionsTransport(ProviderTransport):
     """Transport for api_mode='chat_completions'.
 
@@ -241,6 +287,7 @@ class ChatCompletionsTransport(ProviderTransport):
         is_openrouter = params.get("is_openrouter", False)
         is_nous = params.get("is_nous", False)
         is_github_models = params.get("is_github_models", False)
+        provider_name = str(params.get("provider_name") or "").strip().lower()
 
         provider_prefs = params.get("provider_preferences")
         if provider_prefs and is_openrouter:
@@ -293,6 +340,11 @@ class ChatCompletionsTransport(ProviderTransport):
         if is_qwen:
             extra_body["vl_high_resolution_images"] = True
 
+        if provider_name in {"gemini", "google-gemini-cli"}:
+            thinking_config = _build_gemini_thinking_config(model, reasoning_config)
+            if thinking_config:
+                extra_body["thinking_config"] = thinking_config
+
         # Merge any pre-built extra_body additions
         additions = params.get("extra_body_additions")
         if additions:
diff --git a/run_agent.py b/run_agent.py
index b4cf706257..9156b39a6b 100644
--- a/run_agent.py
+++ b/run_agent.py
@@ -8094,6 +8094,7 @@ class AIAgent:
             supports_reasoning=self._supports_reasoning_extra_body(),
             github_reasoning_extra=self._github_models_reasoning_extra_body() if _is_gh else None,
             anthropic_max_output=_ant_max,
+            provider_name=self.provider,
         )
 
     def _supports_reasoning_extra_body(self) -> bool:
diff --git a/tests/agent/transports/test_chat_completions.py b/tests/agent/transports/test_chat_completions.py
index 4adf9f72e5..0110f893ca 100644
--- a/tests/agent/transports/test_chat_completions.py
+++ b/tests/agent/transports/test_chat_completions.py
@@ -4,7 +4,7 @@ import pytest
 from types import SimpleNamespace
 
 from agent.transports import get_transport
-from agent.transports.types import NormalizedResponse, ToolCall
+from agent.transports.types import NormalizedResponse
 
 
 @pytest.fixture
@@ -122,6 +122,75 @@ class TestChatCompletionsBuildKwargs:
         )
         assert kw["extra_body"]["think"] is False
 
+    def test_gemini_without_explicit_reasoning_config_keeps_existing_behavior(self, transport):
+        msgs = [{"role": "user", "content": "Hi"}]
+        kw = transport.build_kwargs(
+            model="gemini-3-flash-preview",
+            messages=msgs,
+            provider_name="gemini",
+        )
+        assert "thinking_config" not in kw.get("extra_body", {})
+
+    def test_gemini_flash_reasoning_maps_to_thinking_config(self, transport):
+        msgs = [{"role": "user", "content": "Hi"}]
+        kw = transport.build_kwargs(
+            model="gemini-3-flash-preview",
+            messages=msgs,
+            provider_name="gemini",
+            reasoning_config={"enabled": True, "effort": "high"},
+        )
+        assert kw["extra_body"]["thinking_config"] == {
+            "includeThoughts": True,
+            "thinkingLevel": "high",
+        }
+
+    def test_gemini_25_reasoning_only_enables_visible_thoughts(self, transport):
+        msgs = [{"role": "user", "content": "Hi"}]
+        kw = transport.build_kwargs(
+            model="gemini-2.5-flash",
+            messages=msgs,
+            provider_name="gemini",
+            reasoning_config={"enabled": True, "effort": "high"},
+        )
+        assert kw["extra_body"]["thinking_config"] == {
+            "includeThoughts": True,
+        }
+
+    def test_gemini_pro_reasoning_clamps_to_supported_levels(self, transport):
+        msgs = [{"role": "user", "content": "Hi"}]
+        kw = transport.build_kwargs(
+            model="google/gemini-3.1-pro-preview",
+            messages=msgs,
+            provider_name="gemini",
+            reasoning_config={"enabled": True, "effort": "medium"},
+        )
+        assert kw["extra_body"]["thinking_config"] == {
+            "includeThoughts": True,
+            "thinkingLevel": "low",
+        }
+
+    def test_gemini_disabled_reasoning_hides_thoughts(self, transport):
+        msgs = [{"role": "user", "content": "Hi"}]
+        kw = transport.build_kwargs(
+            model="gemini-3-flash-preview",
+            messages=msgs,
+            provider_name="gemini",
+            reasoning_config={"enabled": False},
+        )
+        assert kw["extra_body"]["thinking_config"] == {
+            "includeThoughts": False,
+        }
+
+    def test_gemini_xhigh_clamps_to_high(self, transport):
+        msgs = [{"role": "user", "content": "Hi"}]
+        kw = transport.build_kwargs(
+            model="gemini-3-flash-preview",
+            messages=msgs,
+            provider_name="gemini",
+            reasoning_config={"enabled": True, "effort": "xhigh"},
+        )
+        assert kw["extra_body"]["thinking_config"]["thinkingLevel"] == "high"
+
     def test_max_tokens_with_fn(self, transport):
         msgs = [{"role": "user", "content": "Hi"}]
         kw = transport.build_kwargs(