Add PostToolUse hook to format files with Prettier on write

Runs the workspace Prettier on each file Claude Code writes or edits inside
the repo, using --ignore-unknown so it's a no-op on unsupported files and
honors .prettierignore. Keeps the tree formatted without a separate pass;
CI's prettier:check stays the source of truth.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NvxEaPhyEq2HCZ6XFjEAde

Author: claude

.claude/hooks/format-file.sh

@@ -0,0 +1,32 @@
+#!/usr/bin/env bash
+#
+# PostToolUse hook: formats a file with Prettier right after Claude Code writes
+# or edits it, so formatting never has to be fixed up later (or caught by CI).
+#
+# It runs the workspace's pinned Prettier with `--ignore-unknown`, so it is a
+# no-op on files Prettier can't format and it honors .prettierignore — meaning
+# it is safe to run on every write regardless of file type.
+set -euo pipefail
+
+ROOT="${CLAUDE_PROJECT_DIR:-$(git rev-parse --show-toplevel 2>/dev/null || pwd)}"
+
+# The hook payload arrives as JSON on stdin; the edited path is tool_input.file_path.
+FILE_PATH="$(node -e 'let s="";process.stdin.on("data",d=>s+=d).on("end",()=>{try{const j=JSON.parse(s);process.stdout.write(String((j.tool_input&&j.tool_input.file_path)||""))}catch{process.stdout.write("")}})' 2>/dev/null || true)"
+
+# Nothing to do without a real, existing file.
+[ -n "$FILE_PATH" ] && [ -f "$FILE_PATH" ] || exit 0
+
+# Only format files inside the repo (skip scratchpad/other locations).
+ABS_FILE="$(cd "$(dirname "$FILE_PATH")" 2>/dev/null && pwd)/$(basename "$FILE_PATH")" || exit 0
+case "$ABS_FILE" in
+  "$ROOT"/*) ;;
+  *) exit 0 ;;
+esac
+
+# Use the workspace Prettier if dependencies are installed; otherwise skip.
+PRETTIER="$ROOT/node_modules/.bin/prettier"
+[ -x "$PRETTIER" ] || exit 0
+
+# Never fail the tool call over formatting.
+"$PRETTIER" --ignore-unknown --write "$ABS_FILE" >/dev/null 2>&1 || true
+exit 0

.claude/settings.json

@@ -9,6 +9,17 @@
           }
         ]
       }
+    ],
+    "PostToolUse": [
+      {
+        "matcher": "Write|Edit|MultiEdit",
+        "hooks": [
+          {
+            "type": "command",
+            "command": "$CLAUDE_PROJECT_DIR/.claude/hooks/format-file.sh"
+          }
+        ]
+      }
     ]
   }
 }

CLAUDE.md

@@ -19,3 +19,11 @@ ready to build, lint, and test the Node.js tooling packages.
 
 See the "Environment & Bootstrap" section of `AGENTS.md` for the manual steps and
 for why native iOS/Android builds are not part of the default bootstrap.
+
+## Formatting
+
+A `PostToolUse` hook (`.claude/hooks/format-file.sh`) runs the workspace's
+Prettier on every file Claude Code writes or edits inside the repo. It uses
+`--ignore-unknown`, so it is a no-op on files Prettier can't format and it honors
+`.prettierignore` — keeping the tree formatted without a separate `prettier:write`
+pass. It's a convenience only; CI's `prettier:check` remains the source of truth.