diff --git a/agent/auxiliary_client.py b/agent/auxiliary_client.py
index 807ed076875..56b2d5c1bfb 100644
--- a/agent/auxiliary_client.py
+++ b/agent/auxiliary_client.py
@@ -711,8 +711,12 @@ class _CodexCompletionsAdapter:
             # keywords (HTTP 400). Strip them here to match the parity guarantee that
             # chat_completion_helpers.py provides for the main-agent xAI path.
             try:
-                from tools.schema_sanitizer import strip_pattern_and_format
+                from tools.schema_sanitizer import (
+                    strip_pattern_and_format,
+                    strip_slash_enum,
+                )
                 tools, _ = strip_pattern_and_format(list(tools))
+                tools, _ = strip_slash_enum(tools)
             except Exception as exc:
                 logger.warning(
                     "Auxiliary client: failed to sanitize tool schemas for "
diff --git a/agent/chat_completion_helpers.py b/agent/chat_completion_helpers.py
index 350a54e4066..2e0caebcbe3 100644
--- a/agent/chat_completion_helpers.py
+++ b/agent/chat_completion_helpers.py
@@ -291,10 +291,17 @@ def build_api_kwargs(agent, api_messages: list) -> dict:
         # in tool schemas (HTTP 400 "Invalid arguments passed to the model").
         # Most commonly hit when MCP-derived tools carry JSON Schema validation
         # keywords through. Strip them before building kwargs. See #27197.
+        # It also rejects ``enum`` values containing ``/`` (HuggingFace IDs
+        # like ``Qwen/Qwen3.5-0.8B`` shipped by MCP servers) — same 400 with
+        # the same opaque message; strip those enums too.
         if is_xai_responses:
             try:
-                from tools.schema_sanitizer import strip_pattern_and_format
+                from tools.schema_sanitizer import (
+                    strip_pattern_and_format,
+                    strip_slash_enum,
+                )
                 tools_for_api, _ = strip_pattern_and_format(tools_for_api)
+                tools_for_api, _ = strip_slash_enum(tools_for_api)
             except Exception as exc:
                 logger.warning(
                     "%s⚠️ Failed to sanitize tool schemas for xAI: %s",
diff --git a/tests/tools/test_schema_sanitizer.py b/tests/tools/test_schema_sanitizer.py
index 8c865e87b8d..b856440ef40 100644
--- a/tests/tools/test_schema_sanitizer.py
+++ b/tests/tools/test_schema_sanitizer.py
@@ -9,7 +9,11 @@ from __future__ import annotations
 
 import copy
 
-from tools.schema_sanitizer import sanitize_tool_schemas, strip_pattern_and_format
+from tools.schema_sanitizer import (
+    sanitize_tool_schemas,
+    strip_pattern_and_format,
+    strip_slash_enum,
+)
 
 
 def _tool(name: str, parameters: dict) -> dict:
@@ -491,3 +495,142 @@ def test_strip_responses_mixed_formats():
     # Verify structure preserved
     assert result[0]["function"]["parameters"]["type"] == "object"
     assert result[1]["parameters"]["type"] == "object"
+
+
+# ─────────────────────────────────────────────────────────────────────────
+# strip_slash_enum — reactive recovery when xAI's /v1/responses (and
+# /v1/chat/completions) grammar-compiler rejects enum values containing
+# a forward slash. Symptom: HTTP 400 "Invalid arguments passed to the
+# model" before any token is emitted. Most commonly hit by MCP-derived
+# tools whose enum lists HuggingFace IDs like "Qwen/Qwen3.5-0.8B".
+# ─────────────────────────────────────────────────────────────────────────
+
+
+def test_strip_slash_enum_removes_huggingface_id_enum():
+    """enum containing HF-style 'owner/name' IDs → stripped."""
+    tools = [_tool("train", {
+        "type": "object",
+        "properties": {
+            "model": {
+                "type": "string",
+                "enum": ["Qwen/Qwen3.5-0.8B", "openai/gpt-oss-20b"],
+            },
+        },
+    })]
+    _, stripped = strip_slash_enum(tools)
+    assert stripped == 1
+    prop = tools[0]["function"]["parameters"]["properties"]["model"]
+    assert "enum" not in prop
+    # Type + description survive so the model still gets the prompting hint.
+    assert prop["type"] == "string"
+
+
+def test_strip_slash_enum_preserves_slashless_enum():
+    """enum without any '/' → preserved."""
+    tools = [_tool("pick", {
+        "type": "object",
+        "properties": {
+            "mode": {"type": "string", "enum": ["fast", "slow"]},
+        },
+    })]
+    _, stripped = strip_slash_enum(tools)
+    assert stripped == 0
+    assert tools[0]["function"]["parameters"]["properties"]["mode"]["enum"] == ["fast", "slow"]
+
+
+def test_strip_slash_enum_partial_match_strips_whole_enum():
+    """Any single value containing '/' triggers removal of the entire enum.
+
+    Rationale: if we kept the slashless values, the model could still pick
+    them, but xAI's grammar-compile failure is all-or-nothing on the enum
+    keyword — keeping a mixed-content enum would still 400. Drop it whole.
+    """
+    tools = [_tool("pick", {
+        "type": "object",
+        "properties": {
+            "target": {"type": "string", "enum": ["local", "hf://Qwen/Qwen3"]},
+        },
+    })]
+    _, stripped = strip_slash_enum(tools)
+    assert stripped == 1
+    assert "enum" not in tools[0]["function"]["parameters"]["properties"]["target"]
+
+
+def test_strip_slash_enum_responses_format():
+    """Responses-format tools (no `function` wrapper) are also handled."""
+    tools = [{
+        "type": "function",
+        "name": "mcp_prime_lab_train_model",
+        "parameters": {
+            "type": "object",
+            "properties": {
+                "model": {
+                    "type": "string",
+                    "enum": ["Qwen/Qwen3.5-0.8B", "meta-llama/Llama-3.2-1B-Instruct"],
+                },
+            },
+        },
+    }]
+    _, stripped = strip_slash_enum(tools)
+    assert stripped == 1
+    assert "enum" not in tools[0]["parameters"]["properties"]["model"]
+
+
+def test_strip_slash_enum_recurses_into_anyof():
+    """enum-with-slash inside an anyOf variant is also stripped."""
+    tools = [_tool("t", {
+        "type": "object",
+        "properties": {
+            "value": {
+                "anyOf": [
+                    {"type": "string", "enum": ["owner/repo"]},
+                    {"type": "null"},
+                ],
+            },
+        },
+    })]
+    _, stripped = strip_slash_enum(tools)
+    assert stripped == 1
+    variants = tools[0]["function"]["parameters"]["properties"]["value"]["anyOf"]
+    assert "enum" not in variants[0]
+    assert variants[0]["type"] == "string"
+
+
+def test_strip_slash_enum_is_idempotent():
+    """Second call on already-stripped tools is a no-op."""
+    tools = [_tool("t", {
+        "type": "object",
+        "properties": {"m": {"type": "string", "enum": ["a/b"]}},
+    })]
+    _, first = strip_slash_enum(tools)
+    _, second = strip_slash_enum(tools)
+    assert first == 1
+    assert second == 0
+
+
+def test_strip_slash_enum_empty_returns_zero():
+    tools, stripped = strip_slash_enum([])
+    assert tools == []
+    assert stripped == 0
+
+
+def test_strip_slash_enum_none_returns_zero():
+    tools, stripped = strip_slash_enum(None)
+    assert tools is None
+    assert stripped == 0
+
+
+def test_strip_slash_enum_ignores_non_string_enum_values():
+    """Integer/boolean enum values can't contain '/' — leave them alone."""
+    tools = [_tool("t", {
+        "type": "object",
+        "properties": {
+            "level": {"type": "integer", "enum": [1, 2, 3]},
+            "flag": {"type": "boolean", "enum": [True, False]},
+        },
+    })]
+    _, stripped = strip_slash_enum(tools)
+    assert stripped == 0
+    props = tools[0]["function"]["parameters"]["properties"]
+    assert props["level"]["enum"] == [1, 2, 3]
+    assert props["flag"]["enum"] == [True, False]
diff --git a/tools/schema_sanitizer.py b/tools/schema_sanitizer.py
index 0d03998d366..e9677ac4a1b 100644
--- a/tools/schema_sanitizer.py
+++ b/tools/schema_sanitizer.py
@@ -380,3 +380,66 @@ def strip_pattern_and_format(tools: list[dict]) -> tuple[list[dict], int]:
             stripped,
         )
     return tools, stripped
+
+
+def strip_slash_enum(tools: list[dict]) -> tuple[list[dict], int]:
+    """Strip ``enum`` keywords whose string values contain a forward slash.
+
+    xAI's ``/v1/responses`` and ``/v1/chat/completions`` endpoints compile
+    tool schemas to a grammar that rejects ``enum`` values containing ``/``
+    (the request fails with HTTP 400 "Invalid arguments passed to the
+    model" before any token is emitted). Most commonly hit by MCP-derived
+    tools whose enum lists HuggingFace model IDs (``Qwen/Qwen3.5-0.8B``,
+    ``openai/gpt-oss-20b``) or owner/name environment IDs. The constraint
+    is purely a prompting hint; dropping it lets the model still see the
+    field description and pick a value, without xAI tripping on the slash.
+
+    Args:
+        tools: OpenAI-format or Responses-format tool list, mutated in
+            place. Callers that need to preserve the original should
+            deep-copy first.
+
+    Returns:
+        ``(tools, stripped_count)`` — same list reference plus a count of
+        how many ``enum`` keywords were removed.
+    """
+    if not tools:
+        return tools, 0
+
+    stripped = 0
+
+    def _walk(node: Any) -> None:
+        nonlocal stripped
+        if isinstance(node, dict):
+            enum_val = node.get("enum")
+            if isinstance(enum_val, list) and any(
+                isinstance(v, str) and "/" in v for v in enum_val
+            ):
+                node.pop("enum", None)
+                stripped += 1
+            for v in node.values():
+                _walk(v)
+        elif isinstance(node, list):
+            for item in node:
+                _walk(item)
+
+    for tool in tools:
+        if not isinstance(tool, dict):
+            continue
+        fn = tool.get("function")
+        if isinstance(fn, dict):
+            params = fn.get("parameters")
+            if isinstance(params, dict):
+                _walk(params)
+                continue
+        params = tool.get("parameters")
+        if isinstance(params, dict):
+            _walk(params)
+
+    if stripped:
+        logger.info(
+            "schema_sanitizer: stripped %d enum keyword(s) containing '/' "
+            "from tool schemas (xAI Responses grammar-compile recovery)",
+            stripped,
+        )
+    return tools, stripped