[Bugfix][Rust Frontend][Renderer] Align DeepSeek tool-call arguments with deepseek-recipe (#56260)

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>
Co-authored-by: Bugen Zhao <i@bugenzhao.com>
Co-authored-by: Codex <noreply@openai.com>
Signed-off-by: JinYan Su <751080330@qq.com>
Signed-off-by: Bugen Zhao <i@bugenzhao.com>

rust/src/chat/src/renderer/deepseek.rs

@@ -11,7 +11,7 @@ use std::collections::HashMap;
 use std::fmt::Write as _;
 
 use serde::Serialize;
-use serde_json::Value;
+use serde_json::{Map, Value};
 use serde_json_fmt::JsonFormat;
 
 use llm_multimodal::DEEPSEEK_V41_IMAGE_PLACEHOLDER;
@@ -685,19 +685,18 @@ fn encode_arguments_to_dsml(
     dialect: DsDialect,
 ) -> Result<()> {
     let parameter_tag = dialect.parameter_tag();
-    let arguments: Value = serde_json::from_str(&tool_call.arguments).map_err(|error| {
-        Error::ChatTemplate(format!(
-            "assistant tool call has invalid JSON arguments for DeepSeek: {error}"
-        ))
-    })?;
-    let Some(arguments) = arguments.as_object() else {
-        return Err(Error::ChatTemplate(
-            "assistant tool call arguments for DeepSeek must be a JSON object".to_string(),
-        ));
-    };
+    // Match deepseek-recipe's render_tool_arguments for both V4 and V4.1:
+    // https://github.com/deepseek-ai/deepseek-recipe/blob/8cadfede7063c896b944e7bae05daa3549ae97ea/deepseek-recipe-encoding/src/v4/mod.rs#L48-L58
+    let arguments = serde_json::from_str::<Map<String, Value>>(&tool_call.arguments)
+        .unwrap_or_else(|_| {
+            Map::from_iter([(
+                "arguments".to_owned(),
+                Value::String(tool_call.arguments.clone()),
+            )])
+        });
 
     let mut wrote_parameter = false;
-    for (key, value) in arguments {
+    for (key, value) in &arguments {
         if wrote_parameter {
             out.push('\n');
         }

rust/src/chat/src/renderer/deepseek_v4/tests.rs

@@ -9,6 +9,7 @@ use serde_json::Value;
 use super::DeepSeekV4ChatRenderer;
 use crate::ChatRenderer;
 use crate::event::{AssistantContentBlock, AssistantToolCall};
+use crate::renderer::deepseek_v41::DeepSeekV41ChatRenderer;
 use crate::renderer::test_utils::{FixtureRequestOptions, fixture_chat_request};
 use crate::request::{ChatMessage, ChatRequest, ChatTool, GenerationPromptMode, ReasoningEffort};
 
@@ -399,6 +400,51 @@ fn reasoning_effort_template_kwarg_is_ignored() {
     assert!(rendered.ends_with("<｜Assistant｜><think>"));
 }
 
+#[test]
+fn tool_call_arguments_preserve_non_objects_like_recipe() {
+    let renderers: [(&dyn ChatRenderer, &str); 2] = [
+        (&DeepSeekV4ChatRenderer::new(), "parameter"),
+        (&DeepSeekV41ChatRenderer::new(), " parameter"),
+    ];
+    for arguments in [
+        "not json",
+        "[1, 2]",
+        "null",
+        "42",
+        "true",
+        "",
+        r#""text""#,
+        r#""{\"query\":\"double\"}""#,
+    ] {
+        let request = ChatRequest {
+            messages: vec![
+                ChatMessage::assistant_blocks(vec![AssistantContentBlock::ToolCall(
+                    AssistantToolCall {
+                        id: "call".to_string(),
+                        name: "search".to_string(),
+                        arguments: arguments.to_string(),
+                    },
+                )]),
+                ChatMessage::tool_response("result", "call"),
+            ],
+            ..ChatRequest::for_test()
+        };
+
+        for (renderer, tag) in renderers {
+            let prompt = renderer.render(&request).unwrap().prompt.into_text().unwrap();
+            let parameters: Vec<_> =
+                prompt.lines().filter(|line| line.contains("parameter name=")).collect();
+            assert_eq!(
+                parameters,
+                [format!(
+                    "<｜DSML｜{tag} name=\"arguments\" string=\"true\">{arguments}</｜DSML｜{tag}>"
+                )],
+                "arguments: {arguments:?}, tag: {tag:?}",
+            );
+        }
+    }
+}
+
 #[test]
 fn tool_results_are_sorted_by_previous_assistant_tool_call_order() {
     let mut request = ChatRequest {