diff --git a/README.md b/README.md
index d3a6bc6..160a2ec 100644
--- a/README.md
+++ b/README.md
@@ -106,6 +106,41 @@ Type `@` in Claude Code to see available Resources.
 
 You can ask your LLM to add new Resources by providing an `https://` URI to a public webpage or a `beam://` URI to a file in Clay.
 
+## Contributing
+
+This repo requires commits to be signed with a [Groundwire](https://groundwire.network) identity. PRs with unsigned commits will be rejected by CI.
+
+### Setup commit signing
+
+You need an Urbit ship running the `%vitriol` agent.
+
+**Quick install:**
+
+```bash
+./hooks/install.sh <your-ship-url>/vitriol "<auth-cookie>"
+```
+
+**Manual install:**
+
+```bash
+git config gpg.program /path/to/hooks/groundwire-sign
+git config commit.gpgsign true
+git config groundwire.sign-endpoint <your-ship-url>/vitriol
+git config groundwire.sign-token "<auth-cookie>"
+```
+
+Once configured, all commits will be automatically signed with your ship's Ed25519 networking key. The CI verifies signatures against on-chain keys via [vitriol.bot](https://vitriol.bot).
+
+### Re-signing existing commits
+
+If you have unsigned commits on a branch:
+
+```bash
+git rebase --exec "true" HEAD~N
+```
+
+(where N is the number of commits to re-sign)
+
 ## Development
 
 ### Build Commands
diff --git a/hooks/groundwire-sign b/hooks/groundwire-sign
new file mode 100755
index 0000000..803ec70
--- /dev/null
+++ b/hooks/groundwire-sign
@@ -0,0 +1,196 @@
+#!/usr/bin/env bash
+#
+# groundwire-sign — custom gpg.program for signing git commits with a
+# Groundwire key via a remote signing service (an Urbit ship running %vitriol).
+#
+# Git calls gpg.program with:
+#   gpg.program --status-fd=2 -bsau <key-id>
+# and pipes the commit object (minus gpgsig) to stdin.
+#
+# This script:
+#   1. Reads the commit content from stdin
+#   2. Optionally fetches the maintainer's ecash pubkey and sats-per-pr price
+#   3. Sends content (+ sats_required if applicable) to the committer's ship
+#   4. Returns a structured signature block on stdout
+#   5. Emits GPG-compatible status on fd 2
+#
+# The signature block includes the signer's @p so the CI verifier can
+# look up the identity on-chain via its own ship, rather than trusting
+# the committer's self-reported pubkey.
+#
+# Configuration (git config or env vars):
+#   GROUNDWIRE_SIGN_ENDPOINT       — base URL of the committer's ship (e.g. http://localhost:8080/vitriol)
+#   GROUNDWIRE_SIGN_TOKEN          — auth cookie for the ship
+#   GROUNDWIRE_MAINTAINER_ENDPOINT — (optional) base URL of the maintainer's ship for ecash pubkey
+#   GROUNDWIRE_MAINTAINER_TOKEN    — (optional) auth cookie for the maintainer's ship
+#
+# Install:
+#   git config --global gpg.program /path/to/groundwire-sign
+#   git config --global commit.gpgsign true
+#
+
+set -euo pipefail
+
+# --- Configuration -----------------------------------------------------------
+
+ENDPOINT="${GROUNDWIRE_SIGN_ENDPOINT:-$(git config --get groundwire.sign-endpoint 2>/dev/null || echo "")}"
+TOKEN="${GROUNDWIRE_SIGN_TOKEN:-$(git config --get groundwire.sign-token 2>/dev/null || echo "")}"
+MAINTAINER_ENDPOINT="${GROUNDWIRE_MAINTAINER_ENDPOINT:-$(git config --get groundwire.maintainer-endpoint 2>/dev/null || echo "")}"
+MAINTAINER_TOKEN="${GROUNDWIRE_MAINTAINER_TOKEN:-$(git config --get groundwire.maintainer-token 2>/dev/null || echo "")}"
+
+if [ -z "$ENDPOINT" ]; then
+  echo "groundwire-sign: GROUNDWIRE_SIGN_ENDPOINT not set" >&2
+  echo "  Set via: git config --global groundwire.sign-endpoint <url>" >&2
+  echo "  Or env:  export GROUNDWIRE_SIGN_ENDPOINT=<url>" >&2
+  exit 1
+fi
+
+# --- Parse git's gpg arguments -----------------------------------------------
+
+STATUS_FD=""
+KEY_ID=""
+
+while [ $# -gt 0 ]; do
+  case "$1" in
+    --status-fd=*)
+      STATUS_FD="${1#--status-fd=}"
+      ;;
+    --status-fd)
+      shift
+      STATUS_FD="$1"
+      ;;
+    -bsau)
+      shift
+      KEY_ID="$1"
+      ;;
+    *)
+      ;;
+  esac
+  shift
+done
+
+# --- Read commit content from stdin -------------------------------------------
+
+COMMIT_CONTENT=$(cat)
+
+# --- Fetch maintainer's ecash pubkey and price (optional) --------------------
+
+ECASH_PUBKEY=""
+SATS_REQUIRED=""
+
+if [ -n "$MAINTAINER_ENDPOINT" ]; then
+  # Fetch ecash pubkey
+  ECASH_RESPONSE=$(curl -s -f \
+    -H "Cookie: ${MAINTAINER_TOKEN}" \
+    "${MAINTAINER_ENDPOINT}/ecash-pubkey") || {
+    echo "groundwire-sign: warning: could not fetch maintainer ecash pubkey" >&2
+  }
+  if [ -n "$ECASH_RESPONSE" ]; then
+    ECASH_CONFIGURED=$(echo "$ECASH_RESPONSE" | jq -r '.configured // empty')
+    if [ "$ECASH_CONFIGURED" = "true" ]; then
+      ECASH_PUBKEY=$(echo "$ECASH_RESPONSE" | jq -r '.pubkey // empty')
+    else
+      echo "groundwire-sign: warning: maintainer ecash not configured" >&2
+    fi
+  fi
+
+  # Fetch sats-per-pr price
+  PRICE_RESPONSE=$(curl -s -f \
+    -H "Cookie: ${MAINTAINER_TOKEN}" \
+    "${MAINTAINER_ENDPOINT}/sats-per-pr") || {
+    echo "groundwire-sign: warning: could not fetch maintainer price" >&2
+  }
+  if [ -n "$PRICE_RESPONSE" ]; then
+    PRICE_CONFIGURED=$(echo "$PRICE_RESPONSE" | jq -r '.configured // empty')
+    if [ "$PRICE_CONFIGURED" = "true" ]; then
+      SATS_REQUIRED=$(echo "$PRICE_RESPONSE" | jq -r '.sats // empty')
+    fi
+  fi
+fi
+
+# --- Send to signing service --------------------------------------------------
+
+PAYLOAD_ARGS=(--arg content "$COMMIT_CONTENT" --arg key_id "$KEY_ID")
+PAYLOAD_EXPR='{content: $content, key_id: $key_id'
+
+if [ -n "$SATS_REQUIRED" ] && [ "$SATS_REQUIRED" != "0" ]; then
+  PAYLOAD_ARGS+=(--argjson sats "$SATS_REQUIRED")
+  PAYLOAD_EXPR+=', sats_required: $sats'
+fi
+
+if [ -n "$ECASH_PUBKEY" ]; then
+  PAYLOAD_ARGS+=(--arg ecash_pubkey "$ECASH_PUBKEY")
+  PAYLOAD_EXPR+=', ecash_pubkey: $ecash_pubkey'
+fi
+
+PAYLOAD_EXPR+='}'
+PAYLOAD=$(jq -n "${PAYLOAD_ARGS[@]}" "$PAYLOAD_EXPR")
+
+RESPONSE=$(curl -s -f \
+  -X POST \
+  -H "Content-Type: application/json" \
+  -H "Cookie: ${TOKEN}" \
+  "${ENDPOINT}/sign" \
+  -d "$PAYLOAD") || {
+  echo "groundwire-sign: signing service request failed" >&2
+  exit 1
+}
+
+# Check for error in response (e.g. insufficient balance)
+ERROR=$(echo "$RESPONSE" | jq -r '.error // empty')
+if [ -n "$ERROR" ]; then
+  echo "groundwire-sign: $ERROR" >&2
+  exit 1
+fi
+
+SIGNATURE=$(echo "$RESPONSE" | jq -r '.signature // empty')
+SIGNER_ID=$(echo "$RESPONSE" | jq -r '.signer_id // empty')
+PASS=$(echo "$RESPONSE" | jq -r '.pass // empty')
+ECASH_AMOUNT=$(echo "$RESPONSE" | jq -r '.ecash_amount // empty')
+ECASH_ENCRYPTED=$(echo "$RESPONSE" | jq -r '.ecash_encrypted // empty')
+ECASH_CIPHERTEXT=$(echo "$RESPONSE" | jq -r '.ecash_ciphertext // empty')
+ECASH_EPH_PUBKEY=$(echo "$RESPONSE" | jq -r '.ecash_ephemeral_pubkey // empty')
+ECASH_MAC=$(echo "$RESPONSE" | jq -r '.ecash_mac // empty')
+ECASH_TOKENS=$(echo "$RESPONSE" | jq -r '.ecash_tokens // empty')
+
+if [ -z "$SIGNATURE" ]; then
+  echo "groundwire-sign: no signature in response" >&2
+  exit 1
+fi
+
+if [ -z "$SIGNER_ID" ]; then
+  echo "groundwire-sign: no signer_id in response" >&2
+  exit 1
+fi
+
+# --- Output structured signature to stdout ------------------------------------
+# Git captures this verbatim as the gpgsig header.
+# The CI verifier parses this to extract the signer's @p and signature.
+
+{
+  echo "-----BEGIN GROUNDWIRE SIGNATURE-----"
+  echo "signer:${SIGNER_ID}"
+  echo "pass:${PASS}"
+  echo "sig:${SIGNATURE}"
+  [ -n "$ECASH_PUBKEY" ] && echo "ecash-pubkey:${ECASH_PUBKEY}"
+  if [ -n "$ECASH_AMOUNT" ] && [ "$ECASH_AMOUNT" != "0" ]; then
+    echo "ecash-amount:${ECASH_AMOUNT}"
+    if [ "$ECASH_ENCRYPTED" = "true" ]; then
+      echo "ecash-ciphertext:${ECASH_CIPHERTEXT}"
+      echo "ecash-ephemeral-pubkey:${ECASH_EPH_PUBKEY}"
+      echo "ecash-mac:${ECASH_MAC}"
+    elif [ -n "$ECASH_TOKENS" ] && [ "$ECASH_TOKENS" != "null" ]; then
+      echo "ecash-tokens:${ECASH_TOKENS}"
+    fi
+  fi
+  echo "-----END GROUNDWIRE SIGNATURE-----"
+}
+
+# --- Emit GPG-compatible status on status-fd ----------------------------------
+
+if [ -n "$STATUS_FD" ]; then
+  {
+    echo "[GNUPG:] SIG_CREATED D"
+    echo "[GNUPG:] BEGIN_SIGNING"
+  } >&"$STATUS_FD"
+fi
diff --git a/hooks/install.sh b/hooks/install.sh
new file mode 100755
index 0000000..31fda1d
--- /dev/null
+++ b/hooks/install.sh
@@ -0,0 +1,85 @@
+#!/usr/bin/env bash
+#
+# Install Groundwire commit signing.
+#
+# Usage:
+#   ./install.sh <ship-url> [auth-cookie] [--maintainer <url> [cookie]]
+#
+# This configures git globally to sign all commits with your Groundwire key.
+# Your ship must be running the %vitriol agent with a signing key configured.
+#
+# When --maintainer is provided, the signing hook will fetch the maintainer's
+# ecash pubkey and sats-per-PR price. If a price is set, ecash tokens from
+# the committer's wallet are automatically included in the signature.
+#
+# Example:
+#   ./install.sh http://localhost:8080/vitriol "urbauth-~zod=0v5.abc..."
+#   ./install.sh http://localhost:8080/vitriol "urbauth-~zod=0v5.abc..." \
+#     --maintainer http://maintainer:8080/vitriol "urbauth-~nec=0v5.xyz..."
+#
+
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+SIGN_PROGRAM="${SCRIPT_DIR}/groundwire-sign"
+
+if [ $# -lt 1 ]; then
+  echo "Usage: $0 <ship-url> [auth-cookie]"
+  echo ""
+  echo "  ship-url     Base URL of your ship's vitriol endpoint (e.g. http://localhost:8080/vitriol)"
+  echo "  auth-cookie  Auth cookie for your ship (e.g. urbauth-~zod=0v5.abc...)"
+  exit 1
+fi
+
+ENDPOINT="$1"
+TOKEN="${2:-}"
+
+# Parse optional --maintainer flag
+MAINTAINER_ENDPOINT=""
+MAINTAINER_TOKEN=""
+shift; shift 2>/dev/null || true
+while [ $# -gt 0 ]; do
+  case "$1" in
+    --maintainer)
+      shift
+      MAINTAINER_ENDPOINT="${1:-}"
+      shift 2>/dev/null || true
+      # Next arg is maintainer token if it doesn't start with --
+      if [ $# -gt 0 ] && [[ "$1" != --* ]]; then
+        MAINTAINER_TOKEN="$1"
+        shift
+      fi
+      ;;
+    *)
+      shift
+      ;;
+  esac
+done
+
+chmod +x "$SIGN_PROGRAM"
+
+git config --global gpg.program "$SIGN_PROGRAM"
+git config --global commit.gpgsign true
+git config --global groundwire.sign-endpoint "$ENDPOINT"
+
+if [ -n "$TOKEN" ]; then
+  git config --global groundwire.sign-token "$TOKEN"
+fi
+
+if [ -n "$MAINTAINER_ENDPOINT" ]; then
+  git config --global groundwire.maintainer-endpoint "$MAINTAINER_ENDPOINT"
+  echo "  groundwire.maintainer-endpoint: $MAINTAINER_ENDPOINT"
+  if [ -n "$MAINTAINER_TOKEN" ]; then
+    git config --global groundwire.maintainer-token "$MAINTAINER_TOKEN"
+  fi
+fi
+
+echo "Groundwire commit signing configured."
+echo "  gpg.program:              $SIGN_PROGRAM"
+echo "  groundwire.sign-endpoint: $ENDPOINT"
+echo "  commit.gpgsign:           true"
+if [ -n "$MAINTAINER_ENDPOINT" ]; then
+  echo "  groundwire.maintainer-endpoint: $MAINTAINER_ENDPOINT"
+fi
+echo ""
+echo "All future commits will be signed with your Groundwire key."