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SessionStart hook produces no output on Windows (Git Bash shebang issue) #354

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opened 2026-01-24 12:51:52 +00:00 by kuges · 8 comments

kuges commented

2026-01-24 12:51:52 +00:00

(Migrated from github.com)

Description

The session-start.sh hook fails to produce any output when executed on Windows with Git Bash, causing the "SessionStart:startup hook error" message to appear at startup.

Environment

OS: Windows 10/11
Shell: Git Bash 5.2.37
Claude Code: 2.1.19
Superpowers version: 4.1.1

Problem

When Claude Code runs the SessionStart hook on Windows, it executes:

"C:\Program Files\Git\bin\bash.exe" "...\superpowers\4.1.1\hooks\session-start.sh"

The script has a shebang line #!/usr/bin/env bash, and when executed this way on Windows/Git Bash, no stdout is produced even though the script exits with code 0.

Investigation Results

I tested extensively and found:

Command	Result
`bash script.sh` (with shebang)	No output
`source script.sh`	Works correctly
`tail -n +2 script.sh	bash` (skip shebang)
`bash script.sh` (without shebang)	Works correctly

This appears to be a quirk of how Git Bash on Windows handles scripts with shebang lines when invoked as bash script.sh.

Debug Log Evidence

From Claude Code debug log:

[DEBUG] Getting matching hook commands for SessionStart with query: startup
[DEBUG] Found 1 hook matchers in settings
[DEBUG] Matched 1 unique hooks for query "startup" (1 before deduplication)
[DEBUG] Hook output does not start with {, treating as plain text

The hook runs but produces empty/non-JSON output, triggering the error display.

Suggested Fixes

Option 1: Remove the shebang line

Since Claude Code 2.1.x already prepends bash when running .sh files on Windows, the shebang is redundant and causes the issue.

Option 2: Use a wrapper approach

Have the hook command use bash -c 'source ...' instead of directly invoking the script.

Option 3: Add Windows-specific detection

Check for Windows in hooks.json and use a different command format.

Additional Context

The existing run-hook.cmd polyglot wrapper was deprecated per its comments because Claude Code 2.1.x changed the execution model. However, the new model still has issues with shebang handling on Windows.

## Description The `session-start.sh` hook fails to produce any output when executed on Windows with Git Bash, causing the "SessionStart:startup hook error" message to appear at startup. ## Environment - **OS:** Windows 10/11 - **Shell:** Git Bash 5.2.37 - **Claude Code:** 2.1.19 - **Superpowers version:** 4.1.1 ## Problem When Claude Code runs the SessionStart hook on Windows, it executes: ``` "C:\Program Files\Git\bin\bash.exe" "...\superpowers\4.1.1\hooks\session-start.sh" ``` The script has a shebang line `#!/usr/bin/env bash`, and when executed this way on Windows/Git Bash, **no stdout is produced** even though the script exits with code 0. ## Investigation Results I tested extensively and found: | Command | Result | |---------|--------| | `bash script.sh` (with shebang) | No output | | `source script.sh` | Works correctly | | `tail -n +2 script.sh | bash` (skip shebang) | Works correctly | | `bash script.sh` (without shebang) | Works correctly | This appears to be a quirk of how Git Bash on Windows handles scripts with shebang lines when invoked as `bash script.sh`. ## Debug Log Evidence From Claude Code debug log: ``` [DEBUG] Getting matching hook commands for SessionStart with query: startup [DEBUG] Found 1 hook matchers in settings [DEBUG] Matched 1 unique hooks for query "startup" (1 before deduplication) [DEBUG] Hook output does not start with {, treating as plain text ``` The hook runs but produces empty/non-JSON output, triggering the error display. ## Suggested Fixes ### Option 1: Remove the shebang line Since Claude Code 2.1.x already prepends `bash` when running `.sh` files on Windows, the shebang is redundant and causes the issue. ### Option 2: Use a wrapper approach Have the hook command use `bash -c 'source ...'` instead of directly invoking the script. ### Option 3: Add Windows-specific detection Check for Windows in hooks.json and use a different command format. ## Additional Context The existing `run-hook.cmd` polyglot wrapper was deprecated per its comments because Claude Code 2.1.x changed the execution model. However, the new model still has issues with shebang handling on Windows.

bkerf commented

2026-01-24 14:25:37 +00:00

(Migrated from github.com)

bkerf commented

2026-01-24 14:25:37 +00:00

(Migrated from github.com)

sgeraldes commented

2026-01-25 03:58:31 +00:00

(Migrated from github.com)

Root Cause Analysis

I investigated this issue in depth and found two distinct problems causing the SessionStart hook to fail on Windows:

Problem 1: Path Mangling (Primary Cause of "No such file or directory")

When Claude Code executes the hook, it sets CLAUDE_PLUGIN_ROOT to a Windows path with backslashes (e.g., C:\Users\Sebastian\.claude\plugins\...). When this is passed through bash command execution, the backslashes are interpreted as escape characters and stripped:

# Reproduction:
$ PLUGIN_ROOT='C:\Users\Sebastian\.claude\plugins\cache\claude-plugins-official\superpowers\4.1.1'
$ bash -c "$PLUGIN_ROOT/hooks/session-start.sh"
bash: line 1: C:UsersSebastian.claudepluginscacheclaude-plugins-officialsuperpowers4.1.1/hooks/session-start.sh: No such file or directory

The path becomes C:UsersSebastian... instead of C:\Users\Sebastian\....

Problem 2: Shebang stdout suppression (Secondary Issue)

Even when the path is correct, Git Bash on Windows suppresses stdout when running scripts with shebang lines via bash script.sh:

$ bash session-start.sh        # → No output, exit code 0
$ source session-start.sh      # → Works correctly, produces JSON
$ bash -c 'source session-start.sh'  # → Also works

This appears to be a quirk of how Git Bash handles the #!/usr/bin/env bash shebang when the script is invoked as bash script.sh.

Fix

I've submitted #356 which replaces the bash script with a cross-platform Node.js equivalent that:

Uses __dirname for reliable path resolution (avoiding the CLAUDE_PLUGIN_ROOT backslash issue)
Works identically on Windows, macOS, and Linux
Produces proper JSON output for the Claude Code hook system

This follows the same pattern used by other plugins (like everything-claude-code) that work correctly on Windows.

## Root Cause Analysis I investigated this issue in depth and found **two distinct problems** causing the SessionStart hook to fail on Windows: ### Problem 1: Path Mangling (Primary Cause of "No such file or directory") When Claude Code executes the hook, it sets `CLAUDE_PLUGIN_ROOT` to a Windows path with backslashes (e.g., `C:\Users\Sebastian\.claude\plugins\...`). When this is passed through bash command execution, the backslashes are interpreted as escape characters and stripped: ```bash # Reproduction: $ PLUGIN_ROOT='C:\Users\Sebastian\.claude\plugins\cache\claude-plugins-official\superpowers\4.1.1' $ bash -c "$PLUGIN_ROOT/hooks/session-start.sh" bash: line 1: C:UsersSebastian.claudepluginscacheclaude-plugins-officialsuperpowers4.1.1/hooks/session-start.sh: No such file or directory ``` The path becomes `C:UsersSebastian...` instead of `C:\Users\Sebastian\...`. ### Problem 2: Shebang stdout suppression (Secondary Issue) Even when the path is correct, Git Bash on Windows suppresses stdout when running scripts with shebang lines via `bash script.sh`: ```bash $ bash session-start.sh # → No output, exit code 0 $ source session-start.sh # → Works correctly, produces JSON $ bash -c 'source session-start.sh' # → Also works ``` This appears to be a quirk of how Git Bash handles the `#!/usr/bin/env bash` shebang when the script is invoked as `bash script.sh`. ### Fix I've submitted #356 which replaces the bash script with a cross-platform Node.js equivalent that: - Uses `__dirname` for reliable path resolution (avoiding the CLAUDE_PLUGIN_ROOT backslash issue) - Works identically on Windows, macOS, and Linux - Produces proper JSON output for the Claude Code hook system This follows the same pattern used by other plugins (like `everything-claude-code`) that work correctly on Windows.

👍 1

sgeraldes commented

2026-01-25 03:58:31 +00:00

(Migrated from github.com)

Root Cause Analysis

I investigated this issue in depth and found two distinct problems causing the SessionStart hook to fail on Windows:

Problem 1: Path Mangling (Primary Cause of "No such file or directory")

# Reproduction:
$ PLUGIN_ROOT='C:\Users\Sebastian\.claude\plugins\cache\claude-plugins-official\superpowers\4.1.1'
$ bash -c "$PLUGIN_ROOT/hooks/session-start.sh"
bash: line 1: C:UsersSebastian.claudepluginscacheclaude-plugins-officialsuperpowers4.1.1/hooks/session-start.sh: No such file or directory

The path becomes C:UsersSebastian... instead of C:\Users\Sebastian\....

Problem 2: Shebang stdout suppression (Secondary Issue)

Even when the path is correct, Git Bash on Windows suppresses stdout when running scripts with shebang lines via bash script.sh:

$ bash session-start.sh        # → No output, exit code 0
$ source session-start.sh      # → Works correctly, produces JSON
$ bash -c 'source session-start.sh'  # → Also works

This appears to be a quirk of how Git Bash handles the #!/usr/bin/env bash shebang when the script is invoked as bash script.sh.

Fix

I've submitted #356 which replaces the bash script with a cross-platform Node.js equivalent that:

Uses __dirname for reliable path resolution (avoiding the CLAUDE_PLUGIN_ROOT backslash issue)
Works identically on Windows, macOS, and Linux
Produces proper JSON output for the Claude Code hook system

This follows the same pattern used by other plugins (like everything-claude-code) that work correctly on Windows.

👍 1

obra commented

2026-01-25 07:20:14 +00:00

(Migrated from github.com)

Node isn't guaranteed to be installed anymore, but bash is, so a node script, sadly, isn't a plausible solution.

obra commented

2026-01-25 07:20:14 +00:00

(Migrated from github.com)

Node isn't guaranteed to be installed anymore, but bash is, so a node script, sadly, isn't a plausible solution.

obra commented

2026-02-05 19:57:00 +00:00

(Migrated from github.com)

Partial fix update

Two of the three Windows SessionStart problems have been fixed on main:

async: true (961052e): Hook failures no longer freeze the terminal
O(n) escape_for_json (038abed): ~7x faster, eliminates performance-related timeouts

The path mangling problem described in the root cause analysis comment (CLAUDE_PLUGIN_ROOT backslash stripping) is tracked at #420 (canonical issue, upstream at anthropics/claude-code#23204).

Still open here: The Git Bash shebang stdout suppression (bash script.sh with shebang → no output) is a distinct issue not addressed by the above fixes. Keeping this open to track that.

## Partial fix update Two of the three Windows SessionStart problems have been fixed on main: - **`async: true`** (`961052e`): Hook failures no longer freeze the terminal - **O(n) `escape_for_json`** (`038abed`): ~7x faster, eliminates performance-related timeouts The **path mangling** problem described in the root cause analysis comment (CLAUDE_PLUGIN_ROOT backslash stripping) is tracked at **#420** (canonical issue, upstream at anthropics/claude-code#23204). **Still open here:** The Git Bash shebang stdout suppression (`bash script.sh` with shebang → no output) is a distinct issue not addressed by the above fixes. Keeping this open to track that.

obra commented

2026-02-05 20:15:20 +00:00

(Migrated from github.com)

Investigation: shebang + onecmd interaction

We investigated the shebang stdout suppression in depth. The root cause is Claude Code's onecmd (-t) bash option (anthropics/claude-code#19217).

When CC runs bash session-start.sh with -t active, bash reads the shebang line (#\!/usr/bin/env bash) and counts it as the "one command" — then exits before executing any script body. This is why removing the shebang fixes it: without it, set -euo pipefail becomes the one command that runs, and the rest is lost too, but differently.

We tested set +t as a workaround, but it does not work when a shebang is present — the shebang consumes the one-command slot before set +t ever executes:

Script	`bash -t` result
shebang + `set +t` + echo	no output
no shebang + `set +t` as first line	works

Conclusion: This is not fixable from the plugin side without removing the shebang (which would break macOS/Linux) or adding a janky wrapper layer. This is an upstream CC bug — CC should not set onecmd when executing hook scripts.

The async: true fix (961052e) means the terminal won't freeze, but the hook still produces no output on affected Windows systems, so skills don't load.

## Investigation: shebang + onecmd interaction We investigated the shebang stdout suppression in depth. The root cause is Claude Code's `onecmd` (`-t`) bash option ([anthropics/claude-code#19217](https://github.com/anthropics/claude-code/issues/19217)). When CC runs `bash session-start.sh` with `-t` active, bash reads the shebang line (`#\!/usr/bin/env bash`) and counts it as the "one command" — then exits before executing any script body. This is why removing the shebang fixes it: without it, `set -euo pipefail` becomes the one command that runs, and the rest is lost too, but differently. We tested `set +t` as a workaround, but it **does not work** when a shebang is present — the shebang consumes the one-command slot before `set +t` ever executes: | Script | `bash -t` result | |--------|-----------------| | shebang + `set +t` + echo | no output | | no shebang + `set +t` as first line | works | **Conclusion:** This is not fixable from the plugin side without removing the shebang (which would break macOS/Linux) or adding a janky wrapper layer. This is an upstream CC bug — CC should not set `onecmd` when executing hook scripts. The `async: true` fix (`961052e`) means the terminal won't freeze, but the hook still produces no output on affected Windows systems, so skills don't load.