diff --git a/crates/tool_parser/src/parsers/glm4_moe.rs b/crates/tool_parser/src/parsers/glm4_moe.rs
index dea54a8f28..ae092ad9aa 100644
--- a/crates/tool_parser/src/parsers/glm4_moe.rs
+++ b/crates/tool_parser/src/parsers/glm4_moe.rs
@@ -1,5 +1,3 @@
-use std::collections::HashMap;
-
 use async_trait::async_trait;
 use openai_protocol::common::Tool;
 use regex::Regex;
@@ -195,7 +193,7 @@ impl Glm4MoeParser {
     fn parse_arguments(
         &self,
         args_text: &str,
-        param_types: &HashMap<String, String>,
+        param_types: &helpers::ParamTypes<'_>,
     ) -> serde_json::Map<String, Value> {
         let mut arguments = serde_json::Map::new();
 
@@ -203,9 +201,8 @@ impl Glm4MoeParser {
             let key = capture.get(1).map_or("", |m| m.as_str()).trim();
             let value_str = capture.get(2).map_or("", |m| m.as_str());
 
-            let value =
-                helpers::coerce_by_schema_type(value_str, param_types.get(key).map(String::as_str))
-                    .unwrap_or_else(|| infer_value(value_str.trim()));
+            let value = helpers::coerce_by_schema_type(value_str, param_types.get(key))
+                .unwrap_or_else(|| infer_value(value_str.trim()));
 
             arguments.insert(key.to_string(), value);
         }
@@ -222,7 +219,7 @@ impl Glm4MoeParser {
             // Get arguments text
             let args_text = captures.get(2).map_or("", |m| m.as_str());
 
-            let param_types = helpers::param_types_for_function(tools, func_name);
+            let param_types = helpers::ParamTypes::for_function(tools, func_name);
             let arguments = self.parse_arguments(args_text, &param_types);
 
             let arguments_str = serde_json::to_string(&arguments)
diff --git a/crates/tool_parser/src/parsers/helpers.rs b/crates/tool_parser/src/parsers/helpers.rs
index ac34fd82e8..2967eea3f5 100644
--- a/crates/tool_parser/src/parsers/helpers.rs
+++ b/crates/tool_parser/src/parsers/helpers.rs
@@ -9,6 +9,43 @@ use crate::{
     types::{StreamingParseResult, ToolCallItem},
 };
 
+/// Declared types for XML arguments, including unlisted parameter names.
+pub(crate) struct ParamTypes<'a> {
+    schema: Option<&'a Value>,
+}
+
+impl<'a> ParamTypes<'a> {
+    pub(crate) fn for_function(tools: &'a [Tool], func_name: &str) -> Self {
+        Self {
+            schema: tools
+                .iter()
+                .find(|tool| tool.function.name == func_name)
+                .map(|tool| &tool.function.parameters),
+        }
+    }
+
+    /// Read a single declared type without changing union or unknown-type inference.
+    pub(crate) fn get(&self, name: &str) -> Option<&str> {
+        let root = self.schema?;
+        let schema = if let Some(property) = root.get("properties").and_then(|p| p.get(name)) {
+            property
+        } else {
+            // additionalProperties excludes keys matched by patternProperties.
+            // Patterns are not evaluated here; any nonempty patternProperties
+            // keeps unlisted keys on the existing inference path.
+            if root
+                .get("patternProperties")
+                .and_then(Value::as_object)
+                .is_some_and(|patterns| !patterns.is_empty())
+            {
+                return None;
+            }
+            root.get("additionalProperties")?
+        };
+        schema.get("type").and_then(Value::as_str)
+    }
+}
+
 /// `param_name -> declared JSON-schema type` for the named function (empty if the
 /// function or its `properties` are absent). Lets XML-style parsers coerce by the
 /// declared type instead of guessing from text (e.g. keep a numeric-looking
@@ -510,6 +547,49 @@ pub(crate) fn handle_json_tool_streaming(
 mod tests {
     use super::*;
 
+    #[test]
+    fn test_additional_parameter_types_preserve_explicit_properties() {
+        let schema = serde_json::json!({
+            "properties": {"count": {"type": "integer"}, "untyped": {}},
+            "additionalProperties": {"type": "string"}
+        });
+        let types = ParamTypes {
+            schema: Some(&schema),
+        };
+        assert_eq!(types.get("code"), Some("string"));
+        assert_eq!(types.get("count"), Some("integer"));
+        assert_eq!(types.get("untyped"), None);
+    }
+
+    #[test]
+    fn test_additional_parameter_types_keep_unknown_type_fallback() {
+        for additional in [
+            Value::Bool(true),
+            Value::Bool(false),
+            serde_json::json!({}),
+            serde_json::json!({"type": ["string", "null"]}),
+        ] {
+            let schema = serde_json::json!({"additionalProperties": additional});
+            assert_eq!(
+                ParamTypes {
+                    schema: Some(&schema)
+                }
+                .get("code"),
+                None
+            );
+        }
+        let schema = serde_json::json!({
+            "properties": {"count": {"type": "integer"}},
+            "patternProperties": {"^code": {"type": "number"}},
+            "additionalProperties": {"type": "string"}
+        });
+        let types = ParamTypes {
+            schema: Some(&schema),
+        };
+        assert_eq!(types.get("code"), None);
+        assert_eq!(types.get("count"), Some("integer"));
+    }
+
     #[test]
     fn test_ends_with_partial_token() {
         assert!(ends_with_partial_token("hello <|py", "<|python_tag|>").is_some());
diff --git a/crates/tool_parser/src/parsers/minimax_m2.rs b/crates/tool_parser/src/parsers/minimax_m2.rs
index fed31346fc..116af4cc36 100644
--- a/crates/tool_parser/src/parsers/minimax_m2.rs
+++ b/crates/tool_parser/src/parsers/minimax_m2.rs
@@ -112,7 +112,7 @@ impl MinimaxM2Parser {
     fn parse_parameters(
         &self,
         params_text: &str,
-        param_types: &HashMap<String, String>,
+        param_types: &helpers::ParamTypes<'_>,
     ) -> serde_json::Map<String, Value> {
         let mut parameters = serde_json::Map::new();
 
@@ -121,11 +121,8 @@ impl MinimaxM2Parser {
             let value_str = capture.get(2).map_or("", |m| m.as_str());
 
             let decoded_value = Self::decode_xml_entities(value_str);
-            let value = helpers::coerce_by_schema_type(
-                &decoded_value,
-                param_types.get(key).map(String::as_str),
-            )
-            .unwrap_or_else(|| Self::parse_value(&decoded_value));
+            let value = helpers::coerce_by_schema_type(&decoded_value, param_types.get(key))
+                .unwrap_or_else(|| Self::parse_value(&decoded_value));
 
             parameters.insert(key.to_string(), value);
         }
@@ -157,7 +154,7 @@ impl MinimaxM2Parser {
             let params_text = captures.get(2).map_or("", |m| m.as_str());
 
             // Parse parameters, coerced by this function's declared schema.
-            let param_types = helpers::param_types_for_function(tools, func_name);
+            let param_types = helpers::ParamTypes::for_function(tools, func_name);
             let parameters = self.parse_parameters(params_text, &param_types);
 
             match serde_json::to_string(&parameters) {
@@ -212,7 +209,7 @@ impl MinimaxM2Parser {
     /// Parse and stream parameters incrementally
     fn parse_and_stream_parameters(&mut self, text: &str, tools: &[Tool]) -> Vec<ToolCallItem> {
         let mut calls = Vec::new();
-        let param_types = helpers::param_types_for_function(tools, &self.current_function_name);
+        let param_types = helpers::ParamTypes::for_function(tools, &self.current_function_name);
 
         // Find all complete parameter patterns in the buffer
         let param_matches: Vec<_> = self
@@ -225,18 +222,15 @@ impl MinimaxM2Parser {
 
                 // Coerce by declared type when known; otherwise keep the prior
                 // JSON-first-then-infer behavior for nested objects/arrays.
-                let value = helpers::coerce_by_schema_type(
-                    &decoded,
-                    param_types.get(&name).map(String::as_str),
-                )
-                .unwrap_or_else(|| {
-                    if decoded.starts_with('{') || decoded.starts_with('[') {
-                        serde_json::from_str::<Value>(&decoded)
-                            .unwrap_or_else(|_| Self::parse_value(&decoded))
-                    } else {
-                        Self::parse_value(&decoded)
-                    }
-                });
+                let value = helpers::coerce_by_schema_type(&decoded, param_types.get(&name))
+                    .unwrap_or_else(|| {
+                        if decoded.starts_with('{') || decoded.starts_with('[') {
+                            serde_json::from_str::<Value>(&decoded)
+                                .unwrap_or_else(|_| Self::parse_value(&decoded))
+                        } else {
+                            Self::parse_value(&decoded)
+                        }
+                    });
 
                 (name, value)
             })
diff --git a/crates/tool_parser/src/parsers/qwen_xml.rs b/crates/tool_parser/src/parsers/qwen_xml.rs
index 9d4d339e90..3bd004f235 100644
--- a/crates/tool_parser/src/parsers/qwen_xml.rs
+++ b/crates/tool_parser/src/parsers/qwen_xml.rs
@@ -195,14 +195,14 @@ impl QwenXmlParser {
             return Ok(None);
         }
 
-        let param_types = helpers::param_types_for_function(tools, &function_name);
+        let param_types = helpers::ParamTypes::for_function(tools, &function_name);
         let mut parameters = serde_json::Map::new();
 
         for cap in self.xml_param_pattern.captures_iter(content) {
             if let (Some(key_match), Some(value_match)) = (cap.get(1), cap.get(2)) {
                 let key = key_match.as_str().trim().to_string();
                 let value = value_match.as_str();
-                let json_value = coerce_value(value, param_types.get(&key).map(String::as_str));
+                let json_value = coerce_value(value, param_types.get(&key));
                 parameters.insert(key, json_value);
             }
         }
@@ -226,7 +226,7 @@ impl QwenXmlParser {
         parameter_end: usize,
     ) -> Vec<ToolCallItem> {
         let mut calls: Vec<ToolCallItem> = vec![];
-        let param_types = helpers::param_types_for_function(tools, &self.current_function_name);
+        let param_types = helpers::ParamTypes::for_function(tools, &self.current_function_name);
 
         // Leave parameters from subsequent coalesced calls for their own iteration.
         let mut new_params = serde_json::Map::new();
@@ -237,7 +237,7 @@ impl QwenXmlParser {
             if let (Some(key_match), Some(value_match)) = (cap.get(1), cap.get(2)) {
                 let key = key_match.as_str().trim().to_string();
                 let value = value_match.as_str();
-                let json_value = coerce_value(value, param_types.get(&key).map(String::as_str));
+                let json_value = coerce_value(value, param_types.get(&key));
                 new_params.insert(key, json_value);
             }
         }
diff --git a/crates/tool_parser/tests/tool_parser_additional_properties.rs b/crates/tool_parser/tests/tool_parser_additional_properties.rs
new file mode 100644
index 0000000000..e7999f5d6e
--- /dev/null
+++ b/crates/tool_parser/tests/tool_parser_additional_properties.rs
@@ -0,0 +1,89 @@
+use openai_protocol::common::Tool;
+use serde_json::{json, Value};
+use tool_parser::{parsers::QwenXmlParser, Glm4MoeParser, MinimaxM2Parser, ToolParser};
+
+#[tokio::test]
+async fn test_additional_properties_precedence_complete_and_streaming() {
+    let inputs = [
+        "<tool_call>save_config<arg_key>code</arg_key><arg_value>42</arg_value><arg_key>empty</arg_key><arg_value>null</arg_value></tool_call>",
+        "<tool_call><function=save_config><parameter=code>42</parameter><parameter=empty>null</parameter></function></tool_call>",
+        "<minimax:tool_call><invoke name=\"save_config\"><parameter name=\"code\">42</parameter><parameter name=\"empty\">null</parameter></invoke></minimax:tool_call>",
+    ];
+    let cases = [
+        (
+            json!({"type": "object", "additionalProperties": {"type": "string"}}),
+            json!({"code": "42", "empty": "null"}),
+        ),
+        (
+            json!({"type": "object", "properties": {"code": {"type": "integer"}}, "additionalProperties": {"type": "string"}}),
+            json!({"code": 42, "empty": "null"}),
+        ),
+        (
+            json!({"type": "object", "properties": {"code": {}}, "additionalProperties": {"type": "string"}}),
+            json!({"code": 42, "empty": "null"}),
+        ),
+    ];
+    for (schema, expected) in cases {
+        let tools: Vec<Tool> = serde_json::from_value(json!([
+            {
+                "type": "function",
+                "function": {
+                    "name": "other_tool",
+                    "parameters": {"type": "object", "additionalProperties": {"type": "integer"}}
+                }
+            },
+            {
+                "type": "function",
+                "function": {"name": "save_config", "parameters": schema}
+            }
+        ]))
+        .unwrap();
+        for (dialect, input) in inputs.iter().enumerate() {
+            for streaming in [false, true] {
+                let mut parser: Box<dyn ToolParser> = match dialect {
+                    0 => Box::new(Glm4MoeParser::glm47()),
+                    1 => Box::new(QwenXmlParser::new()),
+                    _ => Box::new(MinimaxM2Parser::new()),
+                };
+                let arguments = if streaming {
+                    let mut arguments = String::new();
+                    let prefix = if dialect == 2 {
+                        "<minimax:tool_call>"
+                    } else {
+                        "<tool_call>"
+                    };
+                    let chunks = std::iter::once(&input[..prefix.len()]).chain(
+                        input.as_bytes()[prefix.len()..]
+                            .chunks(7)
+                            .map(|chunk| std::str::from_utf8(chunk).unwrap()),
+                    );
+                    for chunk in chunks {
+                        for call in parser.parse_incremental(chunk, &tools).await.unwrap().calls {
+                            assert_eq!(call.tool_index, 0);
+                            arguments.push_str(&call.parameters);
+                        }
+                    }
+                    if let Some(calls) = parser.get_unstreamed_tool_args() {
+                        for call in calls {
+                            arguments.push_str(&call.parameters);
+                        }
+                    }
+                    arguments
+                } else {
+                    let (_, calls) = parser
+                        .parse_complete_with_tools(input, &tools)
+                        .await
+                        .unwrap();
+                    assert_eq!(calls.len(), 1);
+                    assert_eq!(calls[0].function.name, "save_config");
+                    calls[0].function.arguments.clone()
+                };
+                assert_eq!(
+                    serde_json::from_str::<Value>(&arguments).unwrap(),
+                    expected,
+                    "schema={schema}, dialect={dialect}, streaming={streaming}"
+                );
+            }
+        }
+    }
+}