claude-tab-fix

claude-tab-fix

A Claude Code hook that fixes indentation mismatches in Edit tool calls and warns Claude about the tab-separator ambiguity in Read output.

Problem

Claude Code's Edit tool matches old_string against file content as a literal string. When the model generates old_string with spaces but the file uses tabs (or vice versa), the match fails silently and no edit is applied. This is a persistent issue with Go, .templ, and other tab-indented files.

The root cause is in the Read tool: it formats output as N\t<line content>, using a tab as the line-number separator. For tab-indented files this is visually identical to the file's own indentation — the model cannot distinguish the separator tab from content tabs, so it consistently produces old_string with one extra leading tab per nesting level.

Installation

prebuilt

go to the right side of this website and look for releases for a bin/exe download. download that

via go

go install github.com/WithHolm/claude-tab-fix@latest

build from source

git clone https://github.com/WithHolm/claude-tab-fix
cd claude-tab-fix
make install

Setup

As a plugin (recommended)

Install the binary first, then install the plugin so Claude Code picks up the hook automatically:

# 1. Install the binary
go install github.com/WithHolm/claude-tab-fix@latest

# 2. Install the plugin
/plugin marketplace add WithHolm/claude-tab-fix
/plugin install claude-tab-fix@WithHolm/claude-tab-fix

The plugin registers both hooks for you — no manual config editing needed.

Manually

Install the binary, then add the hooks to your Claude Code settings.

Per-project — commit .claude/settings.json to your repo so anyone who clones it gets the hooks automatically:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": "Edit",
        "hooks": [{ "type": "command", "command": "claude-tab-fix" }]
      }
    ],
    "PostToolUse": [
      {
        "matcher": "Read",
        "hooks": [{ "type": "command", "command": "claude-tab-fix" }]
      }
    ]
  }
}

Globally — add the same block to ~/.claude/settings.json to enable it for every project on your machine.

If the binary is not on your PATH, use the full path: $(go env GOPATH)/bin/claude-tab-fix.

How it works

PostToolUse / Read — after every Read call on a tab-indented file, the hook injects a context note reminding Claude that the N\t line-number prefix is a separator tab, not part of the file content, so old_string should have one fewer leading tab than the raw output suggests.

PreToolUse / Edit — before every Edit call, the hook:

1. Reads the target file and detects its dominant indent style (tabs or N-spaces)
2. Detects the indent style used in old_string
3. If they differ, re-indents both old_string and new_string to match the file
4. If the re-indented old_string isn't found verbatim, falls back to fuzzy line-similarity matching to handle minor content drift
5. Exits with code 2 and prints the corrected strings to stderr — Claude Code surfaces this as feedback, causing Claude to retry the Edit with the exact corrected strings

If the indentation already matches, the Edit hook passes through unchanged.

See FLOW.md for the full decision diagram.

Edge cases handled

Situation	Behaviour
File doesn't exist yet	Pass through (new file creation)
Binary file	Pass through
old_string has no indented lines	Pass through
Mixed indentation in file	Majority-wins detection
replace_all: true	Same normalization applies
Bash with sed/awk/perl on a tab-indented file	Allow + advisory warning injected into Claude's context
Write overwrites a tab-indented file with space-indented content	Allow + advisory warning noting the mixed indentation risk

The hook fires on Read (PostToolUse), Edit, Bash, and Write (all PreToolUse). Only Edit calls can be automatically corrected — Bash and Write bypass the normalization path, so the hook injects a strong suggestion to use Edit instead. A CLAUDE.md file is included in the release archive; placing it in your project root reinforces this at session start.

Development

make build   # build ./claude-tab-fix binary
make fmt     # run gofmt
make install # go install
go test ./...