diff --git a/agent/transports/chat_completions.py b/agent/transports/chat_completions.py
index c43611f85f..0da01fedf9 100644
--- a/agent/transports/chat_completions.py
+++ b/agent/transports/chat_completions.py
@@ -24,6 +24,18 @@ def _build_gemini_thinking_config(model: str, reasoning_config: dict | None) ->
     if reasoning_config is None or not isinstance(reasoning_config, dict):
         return None
 
+    normalized_model = (model or "").strip().lower()
+    if normalized_model.startswith("google/"):
+        normalized_model = normalized_model.split("/", 1)[1]
+
+    # ``thinking_config`` is a Gemini-only request parameter. The same
+    # ``gemini`` provider also serves Gemma (and historically PaLM/Bard);
+    # those reject the field with HTTP 400 "Unknown name 'thinking_config':
+    # Cannot find field" — including the polite ``{"includeThoughts": False}``
+    # form. Omit the field entirely on non-Gemini models. (#17426)
+    if not normalized_model.startswith("gemini"):
+        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.
@@ -34,9 +46,6 @@ def _build_gemini_thinking_config(model: str, reasoning_config: dict | 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
diff --git a/tests/agent/transports/test_chat_completions.py b/tests/agent/transports/test_chat_completions.py
index bec7dc58a0..66aa7e9058 100644
--- a/tests/agent/transports/test_chat_completions.py
+++ b/tests/agent/transports/test_chat_completions.py
@@ -244,6 +244,46 @@ class TestChatCompletionsBuildKwargs:
             "thinking_level": "low",
         }
 
+    def test_gemma_does_not_receive_thinking_config(self, transport):
+        # The `gemini` provider also serves Gemma (e.g. `gemma-4-31b-it`),
+        # but Gemma rejects `thinking_config` with HTTP 400 (#17426). Even
+        # when Hermes has reasoning enabled, the field must be omitted for
+        # non-Gemini models on this provider.
+        msgs = [{"role": "user", "content": "Hi"}]
+        kw = transport.build_kwargs(
+            model="gemma-4-31b-it",
+            messages=msgs,
+            provider_name="gemini",
+            reasoning_config={"enabled": True, "effort": "high"},
+        )
+        assert "thinking_config" not in kw.get("extra_body", {})
+
+    def test_gemma_disabled_reasoning_still_omits_thinking_config(self, transport):
+        # The `Unknown name 'thinking_config': Cannot find field` rejection
+        # fires even on `{"includeThoughts": False}` — the entire field must
+        # be absent, not just disabled. (#17426)
+        msgs = [{"role": "user", "content": "Hi"}]
+        kw = transport.build_kwargs(
+            model="gemma-4-31b-it",
+            messages=msgs,
+            provider_name="gemini",
+            reasoning_config={"enabled": False},
+        )
+        assert "thinking_config" not in kw.get("extra_body", {})
+
+    def test_google_prefixed_gemma_also_omits_thinking_config(self, transport):
+        # OpenRouter-style `google/gemma-...` IDs hit the same provider path
+        # and must also omit `thinking_config`. The existing `google/`
+        # prefix-stripping must not accidentally classify Gemma as Gemini.
+        msgs = [{"role": "user", "content": "Hi"}]
+        kw = transport.build_kwargs(
+            model="google/gemma-4-31b-it",
+            messages=msgs,
+            provider_name="gemini",
+            reasoning_config={"enabled": True, "effort": "medium"},
+        )
+        assert "thinking_config" not in kw.get("extra_body", {})
+
     def test_max_tokens_with_fn(self, transport):
         msgs = [{"role": "user", "content": "Hi"}]
         kw = transport.build_kwargs(