diff --git a/.claude/hooks/session-start.sh b/.claude/hooks/session-start.sh
new file mode 100755
index 0000000..b7c2a49
--- /dev/null
+++ b/.claude/hooks/session-start.sh
@@ -0,0 +1,36 @@
+#!/bin/bash
+# Canonical Tap House SessionStart hook for Claude Code on the web.
+#
+# Fresh web-session containers clone the repo bare: no submodules, no
+# pre-commit hook installed — so `git commit` runs unformatted and the
+# clang-format CI gate fails on code a local clone would have fixed at commit
+# time. This hook closes that gap at session start:
+#
+#   1. init submodules recursively (kernels, SDKs, vendored test harnesses)
+#   2. install pre-commit and register the repo's canonical hook
+#      (.pre-commit-config.yaml — the Tap-wide pinned clang-format)
+#   3. warm the pinned clang-format binary so the first commit doesn't
+#      pay the download (the container snapshot caches it)
+#
+# Canonical copy: taphouse (distributed by scripts/sync.sh alongside
+# .clang-format / .clang-tidy / .pre-commit-config.yaml; drift-guarded where
+# present). Web-only; local clones are untouched.
+set -euo pipefail
+
+if [ "${CLAUDE_CODE_REMOTE:-}" != "true" ]; then
+    exit 0
+fi
+
+cd "$CLAUDE_PROJECT_DIR"
+
+echo "session-start: initializing submodules ..."
+git submodule update --init --recursive
+
+echo "session-start: installing pre-commit ..."
+if ! python3 -m pre_commit --version >/dev/null 2>&1; then
+    python3 -m pip install --quiet pre-commit
+fi
+python3 -m pre_commit install
+python3 -m pre_commit install-hooks
+
+echo "session-start: done."
diff --git a/.claude/settings.json b/.claude/settings.json
new file mode 100644
index 0000000..e06b033
--- /dev/null
+++ b/.claude/settings.json
@@ -0,0 +1,14 @@
+{
+  "hooks": {
+    "SessionStart": [
+      {
+        "hooks": [
+          {
+            "type": "command",
+            "command": "$CLAUDE_PROJECT_DIR/.claude/hooks/session-start.sh"
+          }
+        ]
+      }
+    ]
+  }
+}
diff --git a/.github/workflows/drift-check.yml b/.github/workflows/drift-check.yml
index 7681ffc..4f1454c 100644
--- a/.github/workflows/drift-check.yml
+++ b/.github/workflows/drift-check.yml
@@ -46,5 +46,16 @@ jobs:
               fail=1
             fi
           fi
+          # The Claude Code web SessionStart hook is guarded only when BOTH
+          # sides carry it: older pinned TapHouse refs predate the hook, and
+          # .claude/settings.json is deliberately unguarded (repos may extend
+          # their settings with repo-specific hooks).
+          if [ -f ".claude/hooks/session-start.sh" ] && [ -f ".taphouse/.claude/hooks/session-start.sh" ]; then
+            if ! diff -q ".taphouse/.claude/hooks/session-start.sh" ".claude/hooks/session-start.sh" >/dev/null 2>&1; then
+              echo "::error file=.claude/hooks/session-start.sh::session-start.sh differs from TapHouse canonical (ref ${{ inputs.ref }}). Re-run taphouse/scripts/sync.sh and commit."
+              diff -u ".taphouse/.claude/hooks/session-start.sh" ".claude/hooks/session-start.sh" || true
+              fail=1
+            fi
+          fi
           if [ "$fail" -eq 0 ]; then echo "All shared configs match TapHouse@${{ inputs.ref }}."; fi
           exit $fail
diff --git a/CLAUDE.md b/CLAUDE.md
new file mode 100644
index 0000000..14c35ae
--- /dev/null
+++ b/CLAUDE.md
@@ -0,0 +1,38 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## What this is
+
+**TapHouse** — the single source of truth for the Tap family's shared style and setup files.
+Nothing here is consumed at build time; the files are *distributed as copies* into each consumer
+repo's root by `scripts/sync.sh`, and consumer CI runs the reusable
+`.github/workflows/drift-check.yml` against a pinned tag to guarantee the copies never silently
+diverge. `README.md` is the authoritative description of the mechanism; read it before changing
+anything.
+
+## The rules that keep this working
+
+- **Never hand-edit a synced copy in a consumer repo.** Change the canonical file *here*, tag,
+  then re-run `scripts/sync.sh` into each consumer. A hand-edited consumer copy fails that repo's
+  drift job — that is the mechanism working, not a nuisance.
+- **Adding a file to the canonical set touches three places:** the file itself, the copy step in
+  `scripts/sync.sh`, and (if it should be guarded) a comparison in `drift-check.yml`. Files that
+  not every consumer carries — `scripts/tidy.sh`, `.claude/hooks/session-start.sh` — are guarded
+  *conditionally* (only when present; the hook additionally only when the pinned TapHouse ref
+  carries it, so older pins don't break). `.claude/settings.json` is deliberately unguarded and
+  synced only-if-missing: repos may extend their settings with repo-specific hooks.
+- **Two version knobs, both living here** (see README "Updating the rules"): the TapHouse tag
+  (consumers pin it in their `style.yml` `ref:`) and the clang-format version (the mirror `rev:`
+  inside `.pre-commit-config.yaml`). Tagging is a deliberate release act — bump consumer `ref:`s
+  in the same sweep as the re-sync, and don't move existing tags.
+- **Consumer rollout order matters:** merge the TapHouse change first, then sync + commit in
+  consumers. The conditional guards make the in-between state safe, but a guarded unconditional
+  file (the core four) must exist at the pinned ref before consumers reference it.
+
+## Conventions recorded here but not yet drift-enforced
+
+The family C++ namespace convention lives in README.md (one `tap::<library>` sub-namespace per
+repo) rather than in the drift-checked `STYLE.md`, so recording it didn't force a family-wide
+re-sync. Promote such conventions into `STYLE.md` (a tagged release) only once every consumer
+complies.
diff --git a/README.md b/README.md
index 0ba8658..f4a0c0b 100644
--- a/README.md
+++ b/README.md
@@ -4,7 +4,7 @@ Canonical **Tap House Rules** — the shared C++ style for the Tap family of
 libraries (AmbiTap, SampleRateTap, OscTap, AmbiTap-Pd, AmbiTap-Max, …).
 
 This repo is the single source of truth for four root-level config files, plus
-one distributed helper script:
+two distributed helper scripts:
 
 | File | Enforces |
 |------|----------|
@@ -13,6 +13,7 @@ one distributed helper script:
 | `STYLE.md` | The human-readable rules and rationale |
 | `.pre-commit-config.yaml` | Local git-hook wiring: formats staged C/C++ **before** each commit, so the clang-format CI gate can't fail on something a developer could have caught locally |
 | `scripts/tidy.sh` | Local mirror of the CI **clang-tidy** gate (naming + mandatory braces) over a repo's own TUs. Distributed to C++ repos that run the clang-tidy gate; kept a single copy here so it can't fork per-repo (it was, briefly). Repo-agnostic — no project name is baked in. |
+| `.claude/hooks/session-start.sh` | Claude Code **web** sessions: fresh containers clone bare (no submodules, no pre-commit hook), so agent commits could bypass the local format layer entirely. The hook initializes submodules, installs the pinned pre-commit hook, and warms its clang-format at session start. Registered per-repo by `.claude/settings.json` (created by sync only if missing — repos may extend their settings, so only the hook *script* is drift-guarded). |
 
 `clang-format` and `clang-tidy` discover their config by walking **up** the
 directory tree from each source file, so those config files (and the
@@ -75,7 +76,9 @@ consumer has migrated and it can be enforced.
    ```
    Now `git commit` reformats staged C/C++ with TapHouse's pinned clang-format
    before the commit lands. First run fetches the pinned hook; thereafter it is
-   cached and offline.
+   cached and offline. (Claude Code **web** sessions do this automatically via
+   the synced SessionStart hook, once it's merged on the repo's default
+   branch — no per-clone step there.)
 
 3. **Run the same hook in CI** instead of a hand-installed clang-format, so the
    version can never skew from local:
diff --git a/scripts/sync.sh b/scripts/sync.sh
index ba2e7bd..b0227e1 100755
--- a/scripts/sync.sh
+++ b/scripts/sync.sh
@@ -27,4 +27,19 @@ cp "$here/scripts/tidy.sh" "$target/scripts/tidy.sh"
 chmod +x "$target/scripts/tidy.sh"
 echo "synced scripts/tidy.sh -> $target/scripts/tidy.sh"
 
+# Claude Code web SessionStart hook — initializes submodules and installs the
+# pinned pre-commit hook in fresh web-session containers, so the clang-format
+# gate can never fail on code a local clone would have formatted at commit
+# time. The hook script is canonical (drift-guarded where present); the
+# .claude/settings.json registration is created only if missing, because repos
+# may legitimately extend their settings with repo-specific hooks.
+mkdir -p "$target/.claude/hooks"
+cp "$here/.claude/hooks/session-start.sh" "$target/.claude/hooks/session-start.sh"
+chmod +x "$target/.claude/hooks/session-start.sh"
+echo "synced .claude/hooks/session-start.sh -> $target/.claude/hooks/session-start.sh"
+if [ ! -f "$target/.claude/settings.json" ]; then
+    cp "$here/.claude/settings.json" "$target/.claude/settings.json"
+    echo "created $target/.claude/settings.json (registers the hook; repo-specific settings merge there)"
+fi
+
 echo "Done. Review and commit the updated files in: $target"