Fixing Log Stream Errors In Claude Code

Introduction: Understanding the Bug

Encountering the “bufio.Scanner: token too long” error while running Claude Code can be frustrating. This error often surfaces in non-interactive environments, especially when using specific command-line parameters. It interrupts the intended task execution, leading to incomplete logs and hindering the debugging process. The primary cause of this issue is the bufio.Scanner exceeding its buffer size while processing the standard output (STDOUT) from the Claude Code process. This often happens when the output stream contains very long tokens or lines of text, exceeding the scanner’s capacity. This article dives deep into the problem, its causes, and potential solutions. We will explore the specifics of this bug report, its implications, and how to mitigate its effects. The focus will be on providing a clear understanding of the problem and offering practical insights for developers and users of Claude Code.

Let's clarify the bug's context. The issue arises when claude is run with specific flags: --dangerously-skip-permissions, --print, --output-format=stream-json, --include-partial-messages, and --verbose. These flags configure Claude Code to operate in a particular mode, which involves streaming JSON output and detailed logging. The error itself indicates that the bufio.Scanner, a tool used to read input from the STDOUT, has encountered a token (a sequence of characters) that is larger than its maximum allowed size. This truncation prevents the complete processing of the log output, which is critical for understanding task execution. The bug report highlights that the issue is not always reproducible, making it challenging to debug. However, it appears to be more prevalent in subsequent runs of the same task, adding to the complexity of the problem. The fact that the problem arises in non-interactive environments suggests that automated scripts and CI/CD pipelines could be significantly affected.

The error’s implications extend beyond mere inconvenience. In automated CI/CD environments, the failure to completely log the process can lead to incomplete debugging information and potentially misdiagnosed errors. For example, if a task fails, but the error message is truncated due to this bug, it might be hard to determine the root cause of the failure. This can lead to wasted time, as developers may spend hours trying to decipher what went wrong. It's also important to note that this error is not just a cosmetic issue; it can directly impact the reliability of the Claude Code’s output and its ability to correctly execute tasks. Thus, solving this bug is critical for ensuring the smooth and efficient operation of Claude Code, particularly in automated and non-interactive environments.

The Root Cause: Why the Error Occurs

The bufio.Scanner: token too long error is fundamentally a limitation of Go's bufio.Scanner. The scanner reads data from an io.Reader (in this case, STDOUT) and splits it into tokens based on some delimiter (usually newline characters). The bufio.Scanner has a default maximum buffer size. When the scanner encounters a token (a piece of data, like a line of text) that exceeds this buffer size, it returns an error. This error is exactly what is happening when Claude Code is run in the specified configuration. Several factors can contribute to tokens becoming excessively long:

First, the --output-format=stream-json flag. This setting tells Claude Code to output data in JSON format, which is often verbose, especially when partial messages and detailed information are included. Long JSON strings can easily exceed the bufio.Scanner's buffer limit, particularly if the data includes long strings or detailed object representations. Second, the --include-partial-messages flag. This flag tells Claude Code to include intermediate and partial messages in the output, which significantly increases the amount of data being streamed. Partial messages can be extensive, which further contributes to the potential for long tokens. Third, the --verbose flag, which can lead to more detailed logging and debugging information. This additional information increases the probability of long log lines. Finally, the interaction of the aforementioned flags: when all these flags are combined, they create an environment that is highly prone to generating large output tokens. This makes it more likely that the bufio.Scanner will encounter a token too large to handle. When running the application, the output stream may contain very large log entries, which are common in complex operations.

In the context of the provided bug report, the issue arises more frequently during subsequent runs. This is likely due to accumulated data or the specific nature of the task, which causes larger output to be generated over time. Moreover, the non-interactive environment plays a crucial role, as the lack of user interaction implies that the entire process relies on the automated handling of STDOUT. This increases the importance of the error-free operation of the bufio.Scanner. A deeper dive into the source code of the claude-code program would likely reveal the specific points where these large tokens are generated. The challenge is to balance the need for detailed logging with the limitations of the bufio.Scanner. The goal is to ensure that the output remains both informative and within the buffer limits of the scanner, without sacrificing the critical information required for effective debugging.

Potential Solutions and Workarounds

Several strategies can be applied to mitigate the bufio.Scanner: token too long error and improve the reliability of Claude Code's output. These solutions can be broadly categorized into adjusting the program's output, modifying the scanner's behavior, and implementing workarounds for the specific environment.

One of the primary solutions involves adjusting the output of Claude Code. This can be accomplished by reducing the verbosity of the logs or by limiting the size of the output tokens. For instance, disabling the --verbose flag or removing --include-partial-messages might reduce the amount of data being outputted, thereby reducing the likelihood of the error. Alternatively, it may be possible to implement a mechanism within the code to truncate long tokens or split them into smaller pieces. This approach is more complicated but ensures that no token exceeds the bufio.Scanner’s buffer size. Another related solution is to carefully format the JSON output to avoid overly long lines. For example, by including line breaks and indentations in the JSON, the code can make it easier to read and parse.

Modifying the scanner's behavior is another potential solution. The bufio.Scanner's buffer size can be adjusted to accommodate larger tokens. In Go, the bufio.Scanner’s MaxScanTokenSize can be increased. By increasing this value, the scanner can handle larger input tokens without returning an error. However, the downside of this approach is that it requires modifying the source code of Claude Code or using a fork of the project. Changing the MaxScanTokenSize can increase the memory usage, so it is important to balance the need to process large tokens with the need to keep the program memory-efficient. This approach requires careful consideration to avoid introducing other memory-related issues.

Implementing workarounds can also provide a short-term fix. One possible workaround is to capture the output from Claude Code and manually parse the STDOUT. If a token is too long, it could be manually split into smaller chunks before being processed. In the command line environment, this could involve using grep or sed to post-process the output. The code can also be modified to write logs to a file. This would allow for larger log entries than can be processed by the bufio.Scanner, but at the cost of needing an additional tool to read the file. Another option is to introduce error handling. The script can catch the