Bug Report: JSON Decode Error in Claude Code SDK

[lunartown]

Summary

Claude Code SDK crashes with CLIJSONDecodeError: Unterminated string due to truncated JSON messages in subprocess communication.

Error

json.decoder.JSONDecodeError: Unterminated string starting at: line 1 column 131 (char 130)

Location: claude_code_sdk/_internal/transport/subprocess_cli.py:192

Root Cause

The SDK's json.loads(line_str) fails when receiving partial/truncated JSON from subprocess. The message gets cut off at character 130,
causing parsing to fail.

Issue

• SDK doesn't handle incomplete JSON messages
• No buffering or retry for truncated data
• Crashes entire TaskGroup on JSON parse error

Fix Needed

Add proper JSON validation and buffering in subprocess_cli.py before calling json.loads().

  ExceptionGroup: unhandled errors in a TaskGroup (1 sub-exception)
  +-+---------------- 1 ----------------
    | Traceback (most recent call last):
    |   File "/usr/local/lib/python3.11/site-packages/claude_code_sdk/_internal/transport/subprocess_cli.py", line 192, in
  receive_messages
    |     data = json.loads(line_str)
    |            ^^^^^^^^^^^^^^^^^^^^
    |   File "/usr/local/lib/python3.11/json/__init__.py", line 346, in loads
    |     return _default_decoder.decode(s)
    |            ^^^^^^^^^^^^^^^^^^^^^^^^^^
    |   File "/usr/local/lib/python3.11/json/decoder.py", line 337, in decode
    |     obj, end = self.raw_decode(s, idx=_w(s, 0).end())
    |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |   File "/usr/local/lib/python3.11/json/decoder.py", line 353, in raw_decode
    |     obj, end = self.scan_once(s, idx)
    |                ^^^^^^^^^^^^^^^^^^^^^^
    | json.decoder.JSONDecodeError: Unterminated string starting at: line 1 column 131 (char 130)
    |
    | The above exception was the direct cause of the following exception:
    |
    | Traceback (most recent call last):
    |   File "/usr/local/lib/python3.11/site-packages/claude_code_sdk/_internal/transport/subprocess_cli.py", line 196, in
  receive_messages
    |     raise SDKJSONDecodeError(line_str, e) from e
    | claude_code_sdk._errors.CLIJSONDecodeError: Failed to decode JSON: 
  {"type":"user","message":{"role":"user","content":[{"tool_use_id":"toolu_01Fkq16zVZJG6jNpd1qWVvmt","...

[S1N4X]

I experience the same issue.

  | During handling of the above exception, another exception occurred:
  |
  | Traceback (most recent call last):
  |   File "/home/user/.local/lib/python3.10/site-packages/claude_code_sdk/_internal/transport/subprocess_cli.py", line 192, in receive_messages
  |     data = json.loads(line_str)
  |   File "/usr/lib/python3.10/json/__init__.py", line 346, in loads
  |     return _default_decoder.decode(s)
  |   File "/usr/lib/python3.10/json/decoder.py", line 337, in decode
  |     obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  |   File "/usr/lib/python3.10/json/decoder.py", line 353, in raw_decode
  |     obj, end = self.scan_once(s, idx)
  | json.decoder.JSONDecodeError: Unterminated string starting at: line 1 column 131 (char 130)
  |
  | The above exception was the direct cause of the following exception:
  |
  | Traceback (most recent call last):
  |   File "/home/user/.local/lib/python3.10/site-packages/claude_code_sdk/_internal/transport/subprocess_cli.py", line 200, in receive_messages
  |     raise SDKJSONDecodeError(line_str, e) from e
  | claude_code_sdk._errors.CLIJSONDecodeError: Failed to decode JSON: {"type":"user","message":{"role":"user","content":[{"tool_use_id":"toolu_01FGHdXWyEw9K1Y6piSK8Frx","...
|
| During handling of the above exception, another exception occurred:
|
| Traceback (most recent call last):
|   File "/home/user/.local/lib/python3.10/site-packages/claude_code_sdk/_internal/transport/subprocess_cli.py", line 192, in receive_messages
|     data = json.loads(line_str)
|   File "/usr/lib/python3.10/json/__init__.py", line 346, in loads
|     return _default_decoder.decode(s)
|   File "/usr/lib/python3.10/json/decoder.py", line 337, in decode
|     obj, end = self.raw_decode(s, idx=_w(s, 0).end())
|   File "/usr/lib/python3.10/json/decoder.py", line 353, in raw_decode
|     obj, end = self.scan_once(s, idx)
| json.decoder.JSONDecodeError: Unterminated string starting at: line 1 column 131 (char 130)
|
| The above exception was the direct cause of the following exception:
|
| Traceback (most recent call last):
|   File "/home/user/.local/lib/python3.10/site-packages/claude_code_sdk/_internal/transport/subprocess_cli.py", line 200, in receive_messages
|     raise SDKJSONDecodeError(line_str, e) from e
| claude_code_sdk._errors.CLIJSONDecodeError: Failed to decode JSON: {"type":"user","message":{"role":"user","content":[{"tool_use_id":"toolu_01JbVfcvsvZpBP64xr8nyhVE","...

[NickGuAI]

This is simple: the original subprocess_cli read message has a fixed buffer size and is not reading the entire stream before parsing it as json.

I have it fixed already locally. maybe today I'll send a PR

[ali-coplanet]

same here
@ltawfik please fix it

[ltawfik]

fixed with latest release

[Pyiner]

@ltawfik It seems the problem still persists after the fix. The issue with truncated json_line within the loop still exists:

for json_line in json_lines:
    json_line = json_line.strip()