diff --git a/agent/auxiliary_client.py b/agent/auxiliary_client.py
index c47c3a4a1d26..c6e00340e7e1 100644
--- a/agent/auxiliary_client.py
+++ b/agent/auxiliary_client.py
@@ -102,7 +102,7 @@ OpenAI = _OpenAIProxy()  # module-level name, resolves lazily on call/isinstance
 from agent.credential_pool import load_pool
 from hermes_cli.config import get_hermes_home
 from hermes_constants import OPENROUTER_BASE_URL
-from utils import base_url_host_matches, base_url_hostname, normalize_proxy_env_vars
+from utils import base_url_host_matches, base_url_hostname, model_forces_max_completion_tokens, normalize_proxy_env_vars
 
 logger = logging.getLogger(__name__)
 
@@ -4300,13 +4300,15 @@ def get_auxiliary_extra_body() -> dict:
     return _nous_extra_body() if auxiliary_is_nous else {}
 
 
-def auxiliary_max_tokens_param(value: int) -> dict:
+def auxiliary_max_tokens_param(value: int, *, model: Optional[str] = None) -> dict:
     """Return the correct max tokens kwarg for the auxiliary client's provider.
-    
+
     OpenRouter and local models use 'max_tokens'. Direct OpenAI with newer
-    models (gpt-4o, o-series, gpt-5+) requires 'max_completion_tokens'.
+    models (gpt-4o, gpt-4.1, gpt-5+, o-series) requires 'max_completion_tokens'.
     The Codex adapter translates max_tokens internally, so we use max_tokens
-    for it as well.
+    for it as well. Pass ``model`` so third-party OpenAI-compatible endpoints
+    fronting the newer families are also recognised — URL-only detection
+    misses the case where a custom base URL serves e.g. ``gpt-5.4``.
     """
     custom_base = _current_custom_base_url()
     or_key = os.getenv("OPENROUTER_API_KEY")
@@ -4316,6 +4318,9 @@ def auxiliary_max_tokens_param(value: int) -> dict:
             and _read_nous_auth() is None
             and base_url_hostname(custom_base) in {"api.openai.com", "api.githubcopilot.com"}):
         return {"max_completion_tokens": value}
+    # ...and for any caller serving a newer OpenAI-family model by name.
+    if model_forces_max_completion_tokens(model):
+        return {"max_completion_tokens": value}
     return {"max_tokens": value}
 
 
diff --git a/run_agent.py b/run_agent.py
index c717c66c178d..5465bb9ae2de 100644
--- a/run_agent.py
+++ b/run_agent.py
@@ -196,7 +196,7 @@ from agent.tool_dispatch_helpers import (
     _extract_error_preview,
     _trajectory_normalize_msg,  # noqa: F401  # re-exported for tests that `from run_agent import _trajectory_normalize_msg`
 )
-from utils import atomic_json_write, base_url_host_matches, base_url_hostname, is_truthy_value
+from utils import atomic_json_write, base_url_host_matches, base_url_hostname, is_truthy_value, model_forces_max_completion_tokens
 
 
 
@@ -1253,13 +1253,24 @@ class AIAgent:
     def _max_tokens_param(self, value: int) -> dict:
         """Return the correct max tokens kwarg for the current provider.
 
-        OpenAI's newer models (gpt-4o, o-series, gpt-5+) require
-        'max_completion_tokens'. Azure OpenAI also requires
-        'max_completion_tokens' for gpt-5.x models served via the
-        OpenAI-compatible endpoint. OpenRouter, local models, and older
+        OpenAI's newer models (gpt-4o, gpt-4.1, gpt-5+, o-series) require
+        'max_completion_tokens'. Azure OpenAI and GitHub Copilot also require
+        'max_completion_tokens' for those families served via their
+        OpenAI-compatible endpoints. OpenRouter, local models, and older
         OpenAI models use 'max_tokens'.
+
+        The check is URL-first (api.openai.com / Azure / Copilot all use the
+        new kwarg), then falls back to a model-name check so third-party
+        OpenAI-compatible endpoints fronting those models are recognised —
+        URL-only detection misses that case and silently sends the wrong
+        kwarg, which the upstream model rejects with a 400.
         """
-        if self._is_direct_openai_url() or self._is_azure_openai_url() or self._is_github_copilot_url():
+        if (
+            self._is_direct_openai_url()
+            or self._is_azure_openai_url()
+            or self._is_github_copilot_url()
+            or model_forces_max_completion_tokens(self.model)
+        ):
             return {"max_completion_tokens": value}
         return {"max_tokens": value}
 
diff --git a/tests/agent/test_auxiliary_client.py b/tests/agent/test_auxiliary_client.py
index 68355482fe09..86b310ac82c3 100644
--- a/tests/agent/test_auxiliary_client.py
+++ b/tests/agent/test_auxiliary_client.py
@@ -3791,3 +3791,82 @@ class TestAuxUnhealthyCache:
             )
             # After the 402, OpenRouter is in the unhealthy cache.
             assert _is_provider_unhealthy("openrouter") is True
+
+
+# ── auxiliary_max_tokens_param ──────────────────────────────────────────────
+
+
+class TestAuxiliaryMaxTokensParam:
+    """Verify the kwarg emitted by ``auxiliary_max_tokens_param`` across
+    URL / provider / model-name combinations. Regression cover: a custom
+    OpenAI-compatible endpoint serving ``gpt-5.x`` was silently getting
+    ``max_tokens`` and 400-ing on ``unsupported_parameter``."""
+
+    def test_direct_openai_returns_max_completion_tokens(self):
+        with (
+            patch("agent.auxiliary_client._current_custom_base_url",
+                  return_value="https://api.openai.com/v1"),
+            patch("agent.auxiliary_client._read_nous_auth", return_value=None),
+        ):
+            assert auxiliary_max_tokens_param(4096) == {"max_completion_tokens": 4096}
+
+    def test_local_endpoint_without_model_uses_max_tokens(self):
+        with (
+            patch("agent.auxiliary_client._current_custom_base_url",
+                  return_value="http://localhost:11434/v1"),
+            patch("agent.auxiliary_client._read_nous_auth", return_value=None),
+        ):
+            assert auxiliary_max_tokens_param(4096) == {"max_tokens": 4096}
+
+    def test_openrouter_api_key_present_keeps_max_tokens_without_model_hint(self, monkeypatch):
+        monkeypatch.setenv("OPENROUTER_API_KEY", "sk-or-v1-test")
+        with (
+            patch("agent.auxiliary_client._current_custom_base_url",
+                  return_value="https://openrouter.ai/api/v1"),
+            patch("agent.auxiliary_client._read_nous_auth", return_value=None),
+        ):
+            assert auxiliary_max_tokens_param(4096) == {"max_tokens": 4096}
+
+    # Model-name fallback — this is the regression guard.
+
+    def test_custom_endpoint_serving_gpt5_uses_max_completion_tokens(self):
+        """Third-party gateway + gpt-5.x: name-based detection must kick in."""
+        with (
+            patch("agent.auxiliary_client._current_custom_base_url",
+                  return_value="https://my-gateway.example.com/v1"),
+            patch("agent.auxiliary_client._read_nous_auth", return_value=None),
+        ):
+            assert auxiliary_max_tokens_param(4096, model="gpt-5.4") == {
+                "max_completion_tokens": 4096
+            }
+
+    def test_openrouter_serving_gpt4o_uses_max_completion_tokens(self, monkeypatch):
+        monkeypatch.setenv("OPENROUTER_API_KEY", "sk-or-v1-test")
+        with (
+            patch("agent.auxiliary_client._current_custom_base_url",
+                  return_value="https://openrouter.ai/api/v1"),
+            patch("agent.auxiliary_client._read_nous_auth", return_value=None),
+        ):
+            assert auxiliary_max_tokens_param(4096, model="openai/gpt-4o-mini") == {
+                "max_completion_tokens": 4096
+            }
+
+    def test_custom_endpoint_serving_classic_llama_keeps_max_tokens(self):
+        with (
+            patch("agent.auxiliary_client._current_custom_base_url",
+                  return_value="https://my-gateway.example.com/v1"),
+            patch("agent.auxiliary_client._read_nous_auth", return_value=None),
+        ):
+            assert auxiliary_max_tokens_param(4096, model="llama3-70b") == {
+                "max_tokens": 4096
+            }
+
+    def test_empty_model_falls_back_to_url_only(self):
+        """No model hint → only the URL-based rule applies."""
+        with (
+            patch("agent.auxiliary_client._current_custom_base_url",
+                  return_value="https://my-gateway.example.com/v1"),
+            patch("agent.auxiliary_client._read_nous_auth", return_value=None),
+        ):
+            assert auxiliary_max_tokens_param(4096, model="") == {"max_tokens": 4096}
+            assert auxiliary_max_tokens_param(4096, model=None) == {"max_tokens": 4096}
diff --git a/tests/run_agent/test_run_agent.py b/tests/run_agent/test_run_agent.py
index d215c7b193ad..9d97693aa7f6 100644
--- a/tests/run_agent/test_run_agent.py
+++ b/tests/run_agent/test_run_agent.py
@@ -5063,6 +5063,41 @@ class TestMaxTokensParam:
         result = agent._max_tokens_param(4096)
         assert result == {"max_completion_tokens": 4096}
 
+    # ── Model-name fallback for non-openai.com endpoints serving newer families ──
+
+    def test_returns_max_completion_tokens_for_gpt5_on_custom_endpoint(self, agent):
+        """Custom OpenAI-compatible endpoint serving gpt-5.x must also use
+        max_completion_tokens — otherwise the server 400s on max_tokens."""
+        agent.base_url = "https://my-gateway.example.com/v1"
+        agent.model = "gpt-5.4"
+        result = agent._max_tokens_param(4096)
+        assert result == {"max_completion_tokens": 4096}
+
+    def test_returns_max_completion_tokens_for_gpt4o_on_openrouter(self, agent):
+        agent.base_url = "https://openrouter.ai/api/v1"
+        agent.model = "openai/gpt-4o-mini"
+        result = agent._max_tokens_param(4096)
+        assert result == {"max_completion_tokens": 4096}
+
+    def test_returns_max_completion_tokens_for_o1_on_custom_endpoint(self, agent):
+        agent.base_url = "https://custom.example.com/v1"
+        agent.model = "o1-preview"
+        result = agent._max_tokens_param(4096)
+        assert result == {"max_completion_tokens": 4096}
+
+    def test_returns_max_tokens_for_classic_gpt4_on_openrouter(self, agent):
+        """Classic gpt-4 (non-omni) still uses max_tokens. Don't over-match."""
+        agent.base_url = "https://openrouter.ai/api/v1"
+        agent.model = "openai/gpt-4-turbo"
+        result = agent._max_tokens_param(4096)
+        assert result == {"max_tokens": 4096}
+
+    def test_returns_max_tokens_for_llama_on_local(self, agent):
+        agent.base_url = "http://localhost:11434/v1"
+        agent.model = "llama3"
+        result = agent._max_tokens_param(4096)
+        assert result == {"max_tokens": 4096}
+
 
 class TestGpt5ApiModeRouting:
     """Verify provider-specific GPT-5 API-mode routing."""
diff --git a/tests/test_model_forces_max_completion_tokens.py b/tests/test_model_forces_max_completion_tokens.py
new file mode 100644
index 000000000000..196dcc5095f7
--- /dev/null
+++ b/tests/test_model_forces_max_completion_tokens.py
@@ -0,0 +1,137 @@
+"""Targeted tests for ``utils.model_forces_max_completion_tokens``.
+
+This helper decides whether a given model name requires the newer
+``max_completion_tokens`` kwarg (rather than the legacy ``max_tokens``) on
+``/v1/chat/completions``. It protects against the 400 ``unsupported_parameter``
+error seen when third-party OpenAI-compatible endpoints serve gpt-4o / 4.1 /
+5.x / o-series models by name and the caller only checks the URL host.
+"""
+
+from __future__ import annotations
+
+from utils import model_forces_max_completion_tokens
+
+
+# ─── Positive cases: families that require max_completion_tokens ────────────
+
+
+class TestPositiveCases:
+    def test_gpt_5_bare(self):
+        assert model_forces_max_completion_tokens("gpt-5") is True
+
+    def test_gpt_5_point_release(self):
+        # The case the user actually hit — gpt-5.4 on a custom OpenAI-compatible
+        # endpoint was being sent max_tokens and getting 400 back.
+        assert model_forces_max_completion_tokens("gpt-5.4") is True
+
+    def test_gpt_5_mini(self):
+        assert model_forces_max_completion_tokens("gpt-5-mini") is True
+
+    def test_gpt_5_nano(self):
+        assert model_forces_max_completion_tokens("gpt-5-nano") is True
+
+    def test_gpt_4o(self):
+        assert model_forces_max_completion_tokens("gpt-4o") is True
+
+    def test_gpt_4o_mini(self):
+        assert model_forces_max_completion_tokens("gpt-4o-mini") is True
+
+    def test_gpt_4_1(self):
+        assert model_forces_max_completion_tokens("gpt-4.1") is True
+
+    def test_gpt_4_1_mini(self):
+        assert model_forces_max_completion_tokens("gpt-4.1-mini") is True
+
+    def test_o1(self):
+        assert model_forces_max_completion_tokens("o1") is True
+
+    def test_o1_preview(self):
+        assert model_forces_max_completion_tokens("o1-preview") is True
+
+    def test_o1_mini(self):
+        assert model_forces_max_completion_tokens("o1-mini") is True
+
+    def test_o3(self):
+        assert model_forces_max_completion_tokens("o3") is True
+
+    def test_o3_mini(self):
+        assert model_forces_max_completion_tokens("o3-mini") is True
+
+    def test_o4_mini(self):
+        # Future-proofing — o4 is already listed publicly.
+        assert model_forces_max_completion_tokens("o4-mini") is True
+
+
+# ─── Negative cases: older or non-OpenAI families still use max_tokens ──────
+
+
+class TestNegativeCases:
+    def test_gpt_3_5_turbo(self):
+        assert model_forces_max_completion_tokens("gpt-3.5-turbo") is False
+
+    def test_gpt_4(self):
+        # Classic gpt-4 (non-omni) still uses max_tokens on chat completions.
+        assert model_forces_max_completion_tokens("gpt-4") is False
+
+    def test_gpt_4_turbo(self):
+        assert model_forces_max_completion_tokens("gpt-4-turbo") is False
+
+    def test_claude_family(self):
+        assert model_forces_max_completion_tokens("claude-3-opus") is False
+        assert model_forces_max_completion_tokens("claude-sonnet-4-6") is False
+
+    def test_llama_family(self):
+        assert model_forces_max_completion_tokens("llama3") is False
+        assert model_forces_max_completion_tokens("llama-3-70b-instruct") is False
+
+    def test_mistral_family(self):
+        assert model_forces_max_completion_tokens("mistral-7b-instruct") is False
+
+    def test_qwen_family(self):
+        assert model_forces_max_completion_tokens("qwen2.5-72b") is False
+
+    def test_deepseek_family(self):
+        assert model_forces_max_completion_tokens("deepseek-chat") is False
+
+
+# ─── Edge cases ─────────────────────────────────────────────────────────────
+
+
+class TestEdgeCases:
+    def test_empty_string(self):
+        assert model_forces_max_completion_tokens("") is False
+
+    def test_none(self):
+        assert model_forces_max_completion_tokens(None) is False  # type: ignore[arg-type]
+
+    def test_whitespace_only(self):
+        assert model_forces_max_completion_tokens("   ") is False
+
+    def test_case_insensitive(self):
+        assert model_forces_max_completion_tokens("GPT-5.4") is True
+        assert model_forces_max_completion_tokens("Gpt-4o-Mini") is True
+        assert model_forces_max_completion_tokens("O3-MINI") is True
+
+    def test_leading_trailing_whitespace(self):
+        assert model_forces_max_completion_tokens("  gpt-5  ") is True
+
+    def test_vendor_prefix_stripped(self):
+        # OpenRouter-style "vendor/model" names should match the tail.
+        assert model_forces_max_completion_tokens("openai/gpt-5.4") is True
+        assert model_forces_max_completion_tokens("openai/gpt-4o-mini") is True
+        assert model_forces_max_completion_tokens("openai/o3-mini") is True
+
+    def test_vendor_prefix_with_non_matching_tail(self):
+        assert model_forces_max_completion_tokens("openai/gpt-3.5-turbo") is False
+        assert model_forces_max_completion_tokens("anthropic/claude-3-opus") is False
+
+    def test_fake_prefix_not_matched(self):
+        # "o-series-but-not-really" doesn't start with o1/o3/o4.
+        assert model_forces_max_completion_tokens("omni-chat") is False
+        # "ox" isn't an o-series model, and "olive" / "opus" shouldn't collide.
+        assert model_forces_max_completion_tokens("ox-large") is False
+        assert model_forces_max_completion_tokens("opus-3") is False
+
+    def test_gpt_5_substring_in_middle_not_matched(self):
+        # Only a prefix should match — "local-gpt-5-clone" is a different model.
+        assert model_forces_max_completion_tokens("local-gpt-5-clone") is False
diff --git a/utils.py b/utils.py
index b2c9f1e83ff2..4ab94f18612a 100644
--- a/utils.py
+++ b/utils.py
@@ -355,6 +355,44 @@ def base_url_hostname(base_url: str) -> str:
     return (parsed.hostname or "").lower().rstrip(".")
 
 
+# ─── Model Capability Detection ──────────────────────────────────────────────
+
+
+def model_forces_max_completion_tokens(model: str) -> bool:
+    """Return True for model families that require ``max_completion_tokens``.
+
+    OpenAI's newer families reject ``max_tokens`` on /v1/chat/completions with
+    HTTP 400 ``unsupported_parameter`` — the caller must send
+    ``max_completion_tokens`` instead. This covers:
+
+    - ``gpt-4o`` / ``gpt-4o-mini`` / ``gpt-4o-*``
+    - ``gpt-4.1`` / ``gpt-4.1-*``
+    - ``gpt-5`` / ``gpt-5.x`` / ``gpt-5-*``
+    - ``o1`` / ``o1-*``
+    - ``o3`` / ``o3-*``
+    - ``o4`` / ``o4-*``
+
+    Handles vendor prefixes like ``openai/gpt-5.4`` by stripping to the tail.
+    The URL-based check (``base_url_hostname == "api.openai.com"``) misses
+    third-party OpenAI-compatible endpoints (custom OpenAI gateways,
+    OpenRouter) that front these models and enforce the same parameter
+    constraint, so name-based detection is required as a fallback.
+    """
+    m = (model or "").strip().lower()
+    if not m:
+        return False
+    if "/" in m:
+        m = m.rsplit("/", 1)[-1]
+    return (
+        m.startswith("gpt-4o")
+        or m.startswith("gpt-4.1")
+        or m.startswith("gpt-5")
+        or m.startswith("o1")
+        or m.startswith("o3")
+        or m.startswith("o4")
+    )
+
+
 def base_url_host_matches(base_url: str, domain: str) -> bool:
     """Return True when the base URL's hostname is ``domain`` or a subdomain.