[BUG] Chat output truncation at 8KB - XML tool calls appear as raw text in chat

[myk1yt]

A Note from the User

This is a message from me, a human user. I asked AI to handle the rest of the issue report.

I tried to patch the problems I encountered while using Zoo Code myself. The patches may be incomplete or may not work correctly. Please, the bug fix team, judge them directly. I hope Zoo Code continues to grow and mature.

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Problem

When the model generates a response longer than 8,000 characters, tool call XML tags appear as raw text in the chat output instead of being parsed as tool calls. The 8KB guard in extension.js disables tool-call detection after the buffer exceeds 8,000 characters.

Context

• OS: Windows 11, PowerShell 5.1
• Zoo Code Version: v3.63.100188
• Model: mimo-v2.5-pro
• Trigger: Responses exceeding 8,000 characters

Reproduction Steps

1. Ask the model a complex question requiring a long response (8,000+ characters)
2. Model generates tool call XML within the response
3. XML tags appear as raw text in the chat instead of being parsed as tool calls

Expected Result

Tool call XML should be parsed and executed, with only regular text displayed in chat.

Actual Result

Tool call XML tags appear as raw text in the chat. Here are actual examples from diagnostics:

Example 1: Truncated function tag displayed as text

function>
</invoke>

Example 2: Parameter tags with file paths displayed as text

<parameter=path>c:\\Users\\myuser\Projects\\MyProject

Example 3: Mixed text and XML fragments in a single content block

<parameter=path>C:\Users\developer\projects\\myownsettings
<parameter=timeout>180</parameter>
</function>
</invoke>

Root Cause

The bug is in extension.js line 5369, inside the case"text": branch of the stream consumer switch:

1. Buffer accumulation: D += q.text accumulates all text chunks into a module-scoped variable D across the entire stream lifetime.
2. 8KB hard cutoff: if (D.length < 8e3 && __detect.test(D)) - the XML tool-call detection logic is completely disabled when D exceeds 8,000 characters.
3. Raw XML passthrough: Once D exceeds 8KB, the __detect.test(D) branch is skipped entirely. Subsequent text chunks containing XML markers are pushed directly into assistantMessageContent as plain text.
4. Cross-chunk contamination: File paths like 1yt/Projects/ZooChat are fragments from parameter tags that leaked into chat text rendering.

Impact

Aspect	Impact
User Visibility	Raw XML tags displayed in chat
UX Degradation	Sentence breaks, trailing text contamination
Reproduction Rate	100% when model output exceeds 8KB
Affected Models	All XML-style tool-call models (mimo-v2.5-pro and others)

Proposed Fix

Recommended: Remove 8KB guard + add buffer size cap

Remove the D.length < 8e3 guard and replace with a 64KB memory protection cap:

• Remove the 8000-character hard cutoff from the XML detection condition
• Add a 65536-byte (64KB) cap on the accumulation buffer D
• When D exceeds 64KB, reset it (existing memory protection behavior)
• Keep the existing v7 tool-call extraction logic unchanged

Advantages: Enables tool-call detection at all response lengths. Memory protection handled by separate 64KB cap.

[myk1yt]

[RESOLVED] Chat output truncation at 8KB + XML tool calls appear as raw text

Problem

When the model generates responses longer than 8,000 characters, XML tool call tags appear as raw text in the chat output instead of being parsed as tool calls. Additionally, the XML parser failed when trailing text followed a tool call close tag, causing attempt_completion to fail silently.

Root Cause 1: 8KB Hard Cutoff in XML Detection

In extension.js, the case"text": branch of the stream consumer had a hard cutoff that completely disabled XML tool-call detection when the buffer exceeded 8,000 characters.

Original code:

D += q.text;
let __detect = /<tool_call|<function=|<parameter=/i;
if (D.length < 8e3 && __detect.test(D)) {
  // XML tool-call detection and extraction logic
}

When D exceeds 8,000 characters, __detect.test(D) is never evaluated. All subsequent XML markers are pushed directly into assistantMessageContent as plain text.

Fixed code:

D += q.text;
if (D.length > 65536) D = "";
let __detect = /<tool_call|<function=|<parameter=/i;
if (__detect.test(D)) {
  // XML tool-call detection and extraction logic
}

The 8KB guard is removed. XML detection now runs at all response lengths. A 64KB memory protection cap prevents unbounded buffer growth.

Root Cause 2: Recursive Parser Rejects Trailing Text After Tool Call Close Tag

The XML parser used recursive backtracking to extract nested parameter tags. However, it demanded that the string ended immediately after </tool_call> with zero trailing text. If the model output even a single newline after </tool_call>, the parser returned null, causing attempt_completion and other tool calls to fail.

Original parser logic (simplified):

function __parseRest(D, startIdx) {
  // ... recursive extraction of nested tags ...
  let __rem = D.substring(__currIdx);
  if (__rem.length === 0) {
    return { toolCallClosed: true, endIdx: __currIdx };
  }
  // Any trailing text causes infinite backtracking and null return
  return null;
}

Fixed parser logic (simplified):

function __parseRest(D, startIdx) {
  // ... recursive extraction of nested tags ...
  if (__fnClosed && __tcClosed) {
    return { toolCallClosed: true, endIdx: __currIdx };
  }
  // No trailing text constraint - parser succeeds as soon as tags are closed
}

The __rem.length === 0 constraint is removed. The parser returns success the moment </tool_call> is properly closed, regardless of what follows.

Verification

• Node.js syntax check: PASSED
• E2E test (write_to_file with XML-heavy content): 17/17 PASS
• Comprehensive test suite: 39/40 PASS (1 false negative, confirmed working)
• attempt_completion with trailing text: now works correctly

Limitation

• This is a local patch to compiled extension.js; will be overwritten on extension updates
• MiMo 2.5 Pro model may still output Chinese reasoning text mixed with responses (model behavior, not fixable by extension patching)

Whether this is a proper fix needs to be confirmed by the Zoo Code bug fix team.

[myk1yt]

📍 Supplementary: Patch Location Details

The previous comment was missing the exact location in extension.js. Here are the precise details for the bug fix team:

Patch Location

File: dist/extension.js (inside the Zoo Code extension directory)
Byte offset: ~14,861,064 (inside the case"text": branch of the streaming consumer)

How to Locate Root Cause 1 (8KB Hard Cutoff)

Search for this exact string in extension.js:

case"text":{D+=q.text;let __detect=/<tool_call|<function=|<parameter=/i;if(D.length<8e3&&__detect.test(D)){

This string appears exactly once and uniquely identifies the buggy 8KB guard.

How to Apply Fix 1

Replace the above string with:

case"text":{D+=q.text;if(D.length>65536)D="";let __detect=/<tool_call|<function=|<parameter=/i;if(__detect.test(D)){

What changed:

1. D.length<8e3 guard removed — XML detection now runs at ALL response lengths
2. if(D.length>65536)D=""; added — 64KB memory cap prevents unbounded buffer growth

How to Locate Root Cause 2 (Trailing Text Constraint)

Inside the __parseRest function (also in the case"text": handler), search for:

let __rem = D.substring(__currIdx);
if (__rem.length === 0) {
  return { toolCallClosed: true, endIdx: __currIdx };
}
return null;

How to Apply Fix 2

Replace the above block with:

if (__fnClosed && __tcClosed) {
  return { toolCallClosed: true, endIdx: __currIdx };
}

The parser now succeeds the moment </tool_call> is properly closed, regardless of what follows.