[Rust Frontend] Add error context in tool parser failures (#46512)

Signed-off-by: Maria Guevara <kawaiiplush14@gmail.com>

rust/src/parser/src/tool/deepseek_json/deepseek_v3.rs

@@ -238,6 +238,9 @@ mod tests {
 
         let error = parser.parse_chunk(&input).unwrap_err();
 
-        expect!["tool parser parsing failed: "].assert_eq(&error.to_report_string());
+        expect![[
+            r#"tool parser parsing failed: near "tool<｜tool▁sep｜>get_weather\n```json\n{}": "#
+        ]]
+        .assert_eq(&error.to_report_string());
     }
 }

rust/src/parser/src/tool/deepseek_json/deepseek_v31.rs

@@ -242,6 +242,7 @@ mod tests {
 
         let error = parser.parse_chunk(&input).unwrap_err();
 
-        expect!["tool parser parsing failed: "].assert_eq(&error.to_report_string());
+        expect![[r#"tool parser parsing failed: near "<｜tool▁sep｜>{}": "#]]
+            .assert_eq(&error.to_report_string());
     }
 }

rust/src/parser/src/tool/json/granite4.rs

@@ -540,8 +540,10 @@ mod tests {
             .parse_chunk(r#"<tool_call>{"name":"f","arguments":42}</tool_call>"#)
             .unwrap_err();
 
-        expect!["tool parser parsing failed: invalid Granite4 arguments"]
-            .assert_eq(&error.to_report_string());
+        expect![[
+            r#"tool parser parsing failed: near "42}</tool_call>": invalid Granite4 arguments"#
+        ]]
+        .assert_eq(&error.to_report_string());
     }
 
     #[test]

rust/src/parser/src/tool/json/internlm2.rs

@@ -335,7 +335,7 @@ mod tests {
         let error = parser.parse_chunk(&input).unwrap_err();
 
         expect![[r#"
-            tool parser parsing failed: invalid InternLM2
+            tool parser parsing failed: near "{\"name\":\"get_weather\",\"params\":{\"location\":\"Tokyo\"}}<|action_end|>": invalid InternLM2
             expected `parameters`, `arguments`"#]]
         .assert_eq(&error.to_report_string());
     }

rust/src/parser/src/tool/json/llama.rs

@@ -336,7 +336,7 @@ mod tests {
             .unwrap_err();
 
         expect![[r#"
-            tool parser parsing failed: invalid Llama JSON
+            tool parser parsing failed: near "{\"name\":\"get_weather\",\"arguments\":{\"location\":\"Tokyo\"}}": invalid Llama JSON
             expected `parameters`"#]]
         .assert_eq(&error.to_report_string());
     }
@@ -474,7 +474,7 @@ mod tests {
         let error = parser.parse_chunk(r#"{"parameters":{},"name":"get_weather"}"#).unwrap_err();
 
         expect![[r#"
-            tool parser parsing failed: invalid Llama JSON
+            tool parser parsing failed: near "{\"parameters\":{},\"name\":\"get_weather\"}": invalid Llama JSON
             expected `name`"#]]
         .assert_eq(&error.to_report_string());
     }
@@ -489,7 +489,7 @@ mod tests {
             ))
             .unwrap_err();
 
-        expect!["tool parser parsing failed: invalid Llama JSON"]
+        expect![[r#"tool parser parsing failed: near " trailing": invalid Llama JSON"#]]
             .assert_eq(&error.to_report_string());
     }
 }

rust/src/parser/src/tool/json/mistral.rs

@@ -240,7 +240,7 @@ mod tests {
             .unwrap_err();
 
         expect![[r#"
-            tool parser parsing failed: invalid Mistral
+            tool parser parsing failed: near "{\"arguments\":{},\"name\":\"get_weather\"}]": invalid Mistral
             expected `name`"#]]
         .assert_eq(&error.to_report_string());
     }

rust/src/parser/src/tool/json/qwen.rs

@@ -280,7 +280,7 @@ mod tests {
             .unwrap_err();
 
         expect![[r#"
-            tool parser parsing failed: invalid Qwen XML
+            tool parser parsing failed: near "{\"arguments\":{},\"name\":\"get_weather\"}\n</tool_call>": invalid Qwen XML
             expected `name`"#]]
         .assert_eq(&error.to_report_string());
     }

rust/src/parser/src/tool/kimi_k2.rs

@@ -594,6 +594,9 @@ mod tests {
 
         let error = parser.parse_chunk(&input).unwrap_err();
 
-        expect!["tool parser parsing failed: "].assert_eq(&error.to_report_string());
+        expect![[
+            r#"tool parser parsing failed: near "get_weather<|tool_call_argument_begin|>{}": "#
+        ]]
+        .assert_eq(&error.to_report_string());
     }
 }

rust/src/parser/src/tool/minimax_m2.rs

@@ -593,6 +593,7 @@ mod tests {
         let mut parser = MinimaxM2ToolParser::new(&test_tools());
         let error = parser.parse_chunk("<minimax:tool_call><bad></minimax:tool_call>").unwrap_err();
 
-        expect!["tool parser parsing failed: "].assert_eq(&error.to_report_string());
+        expect![[r#"tool parser parsing failed: near "<bad></minimax:tool_call>": "#]]
+            .assert_eq(&error.to_report_string());
     }
 }

rust/src/parser/src/tool/minimax_m3.rs

@@ -878,7 +878,10 @@ mod tests {
             ))
             .unwrap_err();
 
-        expect!["tool parser parsing failed: "].assert_eq(&error.to_report_string());
+        expect![[
+            r#"tool parser parsing failed: near "]<]minimax[>[<bad>]<]minimax[>[</tool_call>": "#
+        ]]
+        .assert_eq(&error.to_report_string());
     }
 
     #[test]

rust/src/parser/src/tool/qwen_coder.rs

@@ -685,7 +685,8 @@ mod tests {
         let mut parser = Qwen3CoderToolParser::new(&test_tools());
         let error = parser.parse_chunk("<tool_call>\n<bad>\n</tool_call>").unwrap_err();
 
-        expect!["tool parser parsing failed: "].assert_eq(&error.to_report_string());
+        expect![[r#"tool parser parsing failed: near "\n<bad>\n</tool_call>": "#]]
+            .assert_eq(&error.to_report_string());
     }
 
     #[test]
@@ -697,7 +698,7 @@ mod tests {
             )
             .unwrap_err();
 
-        expect!["tool parser parsing failed: "].assert_eq(&error.to_report_string());
+        expect![[r#"tool parser parsing failed: near "\n<function=get_weather>\n<parameter=location>SF</function>\n</tool_call>": "#]].assert_eq(&error.to_report_string());
     }
 
     #[test]

rust/src/parser/src/utils.rs

@@ -395,9 +395,9 @@ pub fn parse_buffered_event<E>(
         Ok(event) => event,
         Err(ErrMode::Incomplete(_)) => return Ok(None),
         Err(ErrMode::Backtrack(e) | ErrMode::Cut(e)) => {
-            // TODO: enrich context for error reporting
+            let snippet = buffer.char_indices().nth(80).map_or(buffer, |(i, _)| &buffer[..i]);
             return Err(ToolParserError::ParsingFailed {
-                message: e.to_string(),
+                message: format!("near {snippet:?}: {e}"),
             });
         }
     };
@@ -423,8 +423,9 @@ mod tests {
     use winnow::stream::{Offset, Partial, Stream};
 
     use super::{
-        JsonObjectScanState, JsonStringScanState, MarkerScanState, json_str, partial_prefix_len,
-        safe_text_len, safe_text_len_mul, take_json_object, take_json_string, take_until_marker,
+        JsonObjectScanState, JsonStringScanState, MarkerScanState, json_str, parse_buffered_event,
+        partial_prefix_len, safe_text_len, safe_text_len_mul, take_json_object, take_json_string,
+        take_until_marker,
     };
 
     #[test]
@@ -832,4 +833,27 @@ mod tests {
 
         assert!(matches!(error, ErrMode::Incomplete(_)));
     }
+
+    #[test]
+    fn parse_buffered_event_error_includes_input_snippet() {
+        let result = parse_buffered_event(" {\"x\":1}", |input| {
+            take_json_object(input, &mut JsonObjectScanState::default())
+        });
+        let err = result.unwrap_err().to_string();
+        assert!(err.contains("near \""), "error must include snippet");
+    }
+
+    #[test]
+    fn parse_buffered_event_error_truncates_long_input() {
+        let long_input = format!(" {}", "x".repeat(100));
+        let result = parse_buffered_event(&long_input, |input| {
+            take_json_object(input, &mut JsonObjectScanState::default())
+        });
+        let err = result.unwrap_err().to_string();
+        assert!(err.contains("near \""), "error must include snippet");
+        assert!(
+            !err.contains(&long_input),
+            "snippet must be truncated for long input"
+        );
+    }
 }