diff --git a/livekit-plugins/livekit-plugins-ultravox/livekit/plugins/ultravox/realtime/realtime_model.py b/livekit-plugins/livekit-plugins-ultravox/livekit/plugins/ultravox/realtime/realtime_model.py
index 39d535f9ba..744501a197 100644
--- a/livekit-plugins/livekit-plugins-ultravox/livekit/plugins/ultravox/realtime/realtime_model.py
+++ b/livekit-plugins/livekit-plugins-ultravox/livekit/plugins/ultravox/realtime/realtime_model.py
@@ -28,7 +28,7 @@
 
 from ..log import logger
 from ..models import UltravoxModel, UltravoxVoice
-from ..utils import parse_tools
+from ..utils import coerce_parameters_to_schema, parse_tools
 from .events import (
     CallStartedEvent,
     ClientToolInvocationEvent,
@@ -1052,11 +1052,14 @@ def _handle_tool_invocation_event(self, event: ClientToolInvocationEvent) -> Non
                 f"invocationId={event.invocation_id} params_keys={list(event.parameters.keys())}"
             )
 
+        # Coerce parameters to match the tool schema so downstream consumers get correct types
+        coerced_params = coerce_parameters_to_schema(self._tools, event.tool_name, event.parameters)
+
         # Emit FunctionCall to maintain framework compatibility
         function_call = llm.FunctionCall(
             call_id=event.invocation_id,
             name=event.tool_name,
-            arguments=json.dumps(event.parameters),
+            arguments=json.dumps(coerced_params),
         )
 
         if self._current_generation is None:
diff --git a/livekit-plugins/livekit-plugins-ultravox/livekit/plugins/ultravox/utils.py b/livekit-plugins/livekit-plugins-ultravox/livekit/plugins/ultravox/utils.py
index ec5d9e685e..8ea9548c67 100644
--- a/livekit-plugins/livekit-plugins-ultravox/livekit/plugins/ultravox/utils.py
+++ b/livekit-plugins/livekit-plugins-ultravox/livekit/plugins/ultravox/utils.py
@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import json
 from typing import Any
 
 from livekit.agents import llm
@@ -77,3 +78,199 @@ def _extract_type(prop: dict[str, Any]) -> str:
             }
         )
     return results
+
+
+def coerce_parameters_to_schema(
+    tools_ctx: llm.ToolContext,
+    tool_name: str,
+    params: dict[str, Any] | None,
+) -> dict[str, Any]:
+    """Coerce Ultravox tool parameters to match the declared tool schema.
+
+    This mitigates cases where Ultravox delivers stringified values (e.g., "10", "true",
+    "[\"x\"]") by converting them to the correct JSON types expected by
+    downstream tool executors (e.g., MCP servers).
+    """
+
+    def _best_effort_coerce(raw: dict[str, Any]) -> dict[str, Any]:
+        def try_parse_json(s: str) -> Any:
+            try:
+                return json.loads(s)
+            except Exception:
+                return s
+
+        out: dict[str, Any] = {}
+        for k, v in (raw or {}).items():
+            if isinstance(v, str):
+                vv = v.strip()
+                lower = vv.lower()
+                if lower in ("true", "false"):
+                    out[k] = lower == "true"
+                elif lower == "null":
+                    out[k] = None
+                elif vv and (vv[0] in "[{" and vv[-1] in "]}"):
+                    out[k] = try_parse_json(vv)
+                else:
+                    try:
+                        out[k] = float(vv) if "." in vv or "e" in lower else int(vv)
+                    except Exception:
+                        out[k] = v
+            else:
+                out[k] = v
+        return out
+
+    def _coerce_value(value: Any, schema: dict[str, Any]) -> Any:
+        t = schema.get("type")
+        if t is None:
+            for key in ("anyOf", "oneOf", "allOf"):
+                if key in schema and isinstance(schema[key], list) and schema[key]:
+                    for variant in schema[key]:
+                        if isinstance(variant, dict):
+                            try:
+                                vt = variant.get("type")
+                                if (
+                                    isinstance(vt, (str, list))
+                                    and isinstance(value, str)
+                                    and value.strip().lower() in ("null", "none")
+                                ):
+                                    allowed = [vt] if isinstance(vt, str) else list(vt)
+                                    if "null" in allowed:
+                                        return None
+                                return _coerce_value(value, variant)
+                            except Exception:
+                                continue
+                    break
+            return value
+
+        # Normalize types and detect nullability
+        allowed_types: list[str] = [t] if isinstance(t, str) else list(t)
+        is_nullable = "null" in allowed_types
+
+        # If schema allows null and value is a string "null"/"None", convert to None
+        if is_nullable and isinstance(value, str) and value.strip().lower() in ("null", "none"):
+            return None
+
+        # Pick the effective type to coerce to (prefer first non-null)
+        t = next((tt for tt in allowed_types if tt != "null"), allowed_types[0])
+
+        if t == "null":
+            return None if value in (None, "null") else value
+
+        if t == "boolean":
+            if isinstance(value, bool):
+                return value
+            if isinstance(value, (int, float)):
+                return bool(value)
+            if isinstance(value, str):
+                v = value.strip().lower()
+                if v in ("true", "1", "yes", "y", "on"):
+                    return True
+                if v in ("false", "0", "no", "n", "off"):
+                    return False
+            return value
+
+        if t in ("number", "integer"):
+            if isinstance(value, (int, float)):
+                return int(value) if t == "integer" else float(value)
+            if isinstance(value, str):
+                v = value.strip()
+                vl = v.lower()
+                if vl == "nan":
+                    return float("nan")
+                if vl in ("inf", "+inf", "infinity", "+infinity"):
+                    return float("inf")
+                if vl in ("-inf", "-infinity"):
+                    return float("-inf")
+                try:
+                    num = float(v) if "." in v or "e" in vl else int(v)
+                    return int(num) if t == "integer" else float(num)
+                except Exception:
+                    return value
+            return value
+
+        if t == "array":
+            if isinstance(value, list):
+                items_schema = schema.get("items")
+                if isinstance(items_schema, dict):
+                    return [_coerce_value(it, items_schema) for it in value]
+                return value
+            if isinstance(value, str):
+                try:
+                    parsed = json.loads(value)
+                    if isinstance(parsed, list):
+                        items_schema = schema.get("items")
+                        if isinstance(items_schema, dict):
+                            return [_coerce_value(it, items_schema) for it in parsed]
+                        return parsed
+                except Exception:
+                    return [value]
+            return [value]
+
+        if t == "object":
+            if isinstance(value, dict):
+                props = schema.get("properties") or {}
+                coerced = {}
+                for k, v in value.items():
+                    if k in props and isinstance(props[k], dict):
+                        coerced[k] = _coerce_value(v, props[k])
+                    else:
+                        coerced[k] = v
+                return coerced
+            if isinstance(value, str):
+                try:
+                    parsed = json.loads(value)
+                    if isinstance(parsed, dict):
+                        return _coerce_value(parsed, schema)
+                except Exception:
+                    return value
+            return value
+
+        if t == "string" and isinstance(value, str):
+            if schema.get("enum") is None:
+                v = value.strip()
+                if (v.startswith("{") and v.endswith("}")) or (
+                    v.startswith("[") and v.endswith("]")
+                ):
+                    try:
+                        return json.loads(v)
+                    except Exception:
+                        pass
+        return value
+
+    params = params or {}
+    tool = tools_ctx.function_tools.get(tool_name)
+    if not tool:
+        return _best_effort_coerce(params)
+
+    schema: dict[str, Any] | None = None
+    if is_function_tool(tool):
+        try:
+            model = function_arguments_to_pydantic_model(tool)
+            schema = model.model_json_schema()
+        except Exception:
+            schema = None
+    elif is_raw_function_tool(tool):
+        try:
+            info = get_raw_function_info(tool)
+            schema = info.raw_schema.get("parameters")
+        except Exception:
+            schema = None
+
+    if not schema:
+        return _best_effort_coerce(params)
+
+    properties = schema.get("properties") or {}
+    out: dict[str, Any] = {}
+    for k, v in params.items():
+        prop_schema = properties.get(k)
+        if isinstance(prop_schema, dict):
+            out[k] = _coerce_value(v, prop_schema)
+        else:
+            out[k] = v
+
+    required = schema.get("required") or []
+    for k in required:
+        if k in out and isinstance(out[k], str) and out[k].strip().lower() == "null":
+            out[k] = None
+
+    return out