Just Writing “HERMES.md” Could Cost You $200!? The Terrifying “String Charge Bug” Hidden in Claude Code

📰 News Overview

• Accidental Charges Triggered by Specific Strings: A bug has been reported where if the string “HERMES.md” (case-sensitive) is included in the git commit history, API requests to Claude Code are erroneously processed as “extra usage” rather than under the “Max plan” (flat-rate plan).
• High Financial Impact: The reporter, who subscribed to the Max 20x plan ($200/month), found themselves unknowingly consuming $200.98 in extra credits despite having over 86% of their plan’s capacity remaining.
• Identifying the Cause: It appears that the issue stems not from the project files themselves, but from the strings included in the git commit messages, suggesting a problem with the server-side routing logic.

💡 Key Points

• Specific Reproduction Conditions: The string “HERMES.md” triggers the bug, but lowercase “hermes.md,” the name “HERMES” without an extension, or the alternative name “AGENTS.md” work without issues.
• Diagnosis Difficulty: The error message only displays “Insufficient extra usage,” making it incredibly challenging to pinpoint that the content-based routing is the root cause.
• The Trap of System Prompts: The design of Claude Code includes recent commit history in the system prompts, inadvertently leading to this unexpected billing route.

🦈 Shark’s Eye (Curator’s Perspective)

What an unbelievable bug! Just having that eight-character string in your commit history can hit your wallet without mercy, truly reminiscent of a “great white shark lurking in the ocean of code”!

Particularly noteworthy is the reporter’s tenacity in using “binary search” to track down this specific string. Cloning the repository, branching out, and meticulously checking each commit message to identify “HERMES.md” as the culprit is a debugging masterpiece. However, if the server-side logic is altering billing routes based on “specific project names or file names,” this is a significant issue that goes beyond a mere bug—it’s a case of “opaque routing”! Developers should check their commit messages for any hidden “landmines” immediately!

🚀 What’s Next?

There’s an urgent need for Anthropic to fix the routing logic. This incident has exposed the risk of “billing structures changing based on prompt content,” likely amplifying user demands for greater billing transparency in the future. Additionally, attention is turning toward whether refunds will be offered.

💬 A Word from Haru Shark

“If ‘HERMES.md’ is a no-go, I’ll fill my commits with ‘SHARK.md’ instead! Everyone, be wary of accidental charges and debug with a heart as vast as the ocean!”

📚 Terminology Explained

• Claude Code: A CLI tool provided by Anthropic that allows direct interaction with AI to generate and modify code straight from the terminal.

• Max 20x plan: One of the high-end subscription plans for Claude, which includes a large number of inference requests.

• System Prompt: The instruction text that is input in the background, serving as context or prerequisites for the AI when generating responses. In Claude Code, this includes the git history automatically.

• Source: HERMES.md: Anthropic bug causes $200 extra charge, refuses refund