diff --git a/agent/auxiliary_client.py b/agent/auxiliary_client.py
index 44b11dfaa..c5fd9a20a 100644
--- a/agent/auxiliary_client.py
+++ b/agent/auxiliary_client.py
@@ -4725,24 +4725,23 @@ def _build_call_kwargs(
         kwargs["temperature"] = temperature
 
     if max_tokens is not None:
-        # Codex adapter handles max_tokens internally; OpenRouter/Nous use max_tokens.
-        # Direct OpenAI api.openai.com with newer models needs max_completion_tokens.
-        # ZAI vision models (glm-4v-flash, glm-4v-plus, etc.) reject max_tokens with
-        # error code 1210 ("API 调用参数有误") on multimodal requests — skip it.
-        _model_lower = (model or "").lower()
-        _skip_max_tokens = (
-            provider == "zai"
-            and ("4v" in _model_lower or "5v" in _model_lower or "-v" in _model_lower)
+        # We do NOT cap output by default. Most chat-completions providers treat
+        # an omitted max_tokens as "use the model's max output", which is what we
+        # want for auxiliary tasks (compression summaries, titles, vision, etc.) —
+        # an explicit cap only risks truncating a summary or 400-ing on providers
+        # that reject the parameter outright (e.g. GitHub Copilot / newer OpenAI
+        # GPT-5 models require max_completion_tokens, not max_tokens; ZAI vision
+        # models reject it entirely with error 1210). Omitting it sidesteps all of
+        # those wire-format quirks at once.
+        #
+        # The one exception is the Anthropic Messages wire (MiniMax and any
+        # ``/anthropic`` endpoint reached through the OpenAI SDK wrapper), where
+        # max_tokens is a MANDATORY field — omitting it is a hard 400. Keep it only
+        # there.
+        _effective_base = base_url or (
+            _current_custom_base_url() if provider == "custom" else ""
         )
-        if _skip_max_tokens:
-            pass  # ZAI vision models do not accept max_tokens
-        elif provider == "custom":
-            custom_base = base_url or _current_custom_base_url()
-            if base_url_hostname(custom_base) == "api.openai.com":
-                kwargs["max_completion_tokens"] = max_tokens
-            else:
-                kwargs["max_tokens"] = max_tokens
-        else:
+        if _is_anthropic_compat_endpoint(provider, _effective_base):
             kwargs["max_tokens"] = max_tokens
 
     if tools:
diff --git a/tests/agent/test_auxiliary_client.py b/tests/agent/test_auxiliary_client.py
index 66e52b6c1..97c3a7f6b 100644
--- a/tests/agent/test_auxiliary_client.py
+++ b/tests/agent/test_auxiliary_client.py
@@ -88,6 +88,62 @@ class TestAuxiliaryMaxTokensParam:
             assert auxiliary_max_tokens_param(2048) == {"max_completion_tokens": 2048}
 
 
+class TestBuildCallKwargsMaxTokens:
+    """_build_call_kwargs should not cap output by default (#34530).
+
+    Most chat-completions providers treat an omitted max_tokens as "use the
+    model max", which is what we want for auxiliary tasks. An explicit cap only
+    risks truncation or a wire-format 400 (GitHub Copilot / GPT-5 reject
+    max_tokens; ZAI vision rejects it entirely). The Anthropic Messages wire is
+    the one exception — max_tokens is a mandatory field there.
+    """
+
+    @pytest.mark.parametrize(
+        "provider,model,base_url",
+        [
+            ("copilot", "gpt-5.4", "https://api.githubcopilot.com"),
+            ("copilot", "gpt-5.5", "https://api.githubcopilot.com"),
+            ("custom", "gpt-5", "https://api.openai.com/v1"),
+            ("openrouter", "anthropic/claude-sonnet-4.6", "https://openrouter.ai/api/v1"),
+            ("nous", "hermes-4", "https://inference-api.nousresearch.com/v1"),
+            ("custom", "qwen", "http://localhost:8080/v1"),
+            ("zai", "glm-4v-flash", "https://open.bigmodel.cn/api/paas/v4"),
+        ],
+    )
+    def test_omits_max_tokens_for_openai_compatible(self, provider, model, base_url):
+        from agent.auxiliary_client import _build_call_kwargs
+
+        kwargs = _build_call_kwargs(
+            provider=provider,
+            model=model,
+            messages=[{"role": "user", "content": "hi"}],
+            max_tokens=1234,
+            base_url=base_url,
+        )
+        assert "max_tokens" not in kwargs
+        assert "max_completion_tokens" not in kwargs
+
+    @pytest.mark.parametrize(
+        "provider,model,base_url",
+        [
+            ("minimax", "minimax-m2", "https://api.minimax.io/v1"),
+            ("custom", "claude", "https://proxy.example.com/anthropic/v1"),
+        ],
+    )
+    def test_keeps_max_tokens_on_anthropic_wire(self, provider, model, base_url):
+        from agent.auxiliary_client import _build_call_kwargs
+
+        kwargs = _build_call_kwargs(
+            provider=provider,
+            model=model,
+            messages=[{"role": "user", "content": "hi"}],
+            max_tokens=1234,
+            base_url=base_url,
+        )
+        assert kwargs["max_tokens"] == 1234
+        assert "max_completion_tokens" not in kwargs
+
+
 class TestNormalizeAuxProvider:
     def test_maps_github_copilot_aliases(self):
         assert _normalize_aux_provider("github") == "copilot"
diff --git a/tests/agent/test_unsupported_temperature_retry.py b/tests/agent/test_unsupported_temperature_retry.py
index 82d8d3208..4d2ebb980 100644
--- a/tests/agent/test_unsupported_temperature_retry.py
+++ b/tests/agent/test_unsupported_temperature_retry.py
@@ -112,8 +112,13 @@ class TestCallLlmUnsupportedTemperatureRetry:
         retry_kwargs = client.chat.completions.create.call_args_list[1].kwargs
         assert first_kwargs["temperature"] == 0.3
         assert "temperature" not in retry_kwargs
-        # other kwargs preserved
-        assert retry_kwargs["max_tokens"] == 500
+        # max_tokens is intentionally omitted on OpenAI-compatible endpoints
+        # (#34530) — auxiliary calls let the model max out its own output — so
+        # it must be absent in BOTH the first and retry kwargs. Use a kwarg that
+        # actually survives (model) to prove the retry preserves the rest.
+        assert "max_tokens" not in first_kwargs
+        assert "max_tokens" not in retry_kwargs
+        assert retry_kwargs["model"] == first_kwargs["model"]
 
     def test_non_temperature_400_does_not_retry_as_temperature(self):
         """Unrelated 400s (e.g. bad tool role) must not silently drop temp."""
@@ -207,7 +212,11 @@ class TestAsyncCallLlmUnsupportedTemperatureRetry:
         retry_kwargs = client.chat.completions.create.call_args_list[1].kwargs
         assert first_kwargs["temperature"] == 0.3
         assert "temperature" not in retry_kwargs
-        assert retry_kwargs["max_tokens"] == 500
+        # max_tokens is intentionally omitted on OpenAI-compatible endpoints
+        # (#34530); assert it's absent and that model survives the retry.
+        assert "max_tokens" not in first_kwargs
+        assert "max_tokens" not in retry_kwargs
+        assert retry_kwargs["model"] == first_kwargs["model"]
 
     @pytest.mark.asyncio
     async def test_async_non_temperature_400_does_not_retry(self):