fix(plugin): deterministic mem0 user_id resolution

Hooks previously fell back to $USER when MEM0_USER_ID wasn't set,
producing a different user_id on every machine for the same person.
Result: a single account with memories scattered across many user
buckets, none of which can see each other.

Resolution priority (same in bash and python):
  1. MEM0_USER_ID env var (explicit override)
  2. ~/.mem0/identity.json cache (pinned to MEM0_API_KEY fingerprint)
  3. Derived: "mem0-" + sha256(MEM0_API_KEY)[:12]
  4. Fallback: $USER, else "default"

Same MEM0_API_KEY across machines now yields the same user_id without
the user having to set MEM0_USER_ID by hand on every laptop.

Resolver shipped as two tiny files instead of a shared module:
  _identity.sh -- sourced by bash hooks, exports MEM0_RESOLVED_USER_ID
  _identity.py -- imported by on_pre_compact.py, exposes resolve_user_id()

Hook integration:
  on_user_prompt.sh -- sources resolver, USER_ID interpolated into rubric
  on_session_start.sh -- emits an "Active user_id: <X>" header before the
    bootstrap text, so the agent's MCP search_memories/add_memory calls
    use the same bucket the hooks write to (closes the agent-side half
    of the symptom)
  on_pre_compact.py -- replaces inline env lookup with resolve_user_id()

Existing memories under previous $USER values are not auto-migrated.
The cache file regenerates on key rotation (fingerprint mismatch).

mem0-plugin/scripts/_identity.py

@@ -0,0 +1,59 @@
+"""Resolve mem0 user_id with deterministic priority.
+
+Resolution priority:
+  1. MEM0_USER_ID env var (explicit override)
+  2. ~/.mem0/identity.json cache (pinned to current MEM0_API_KEY fingerprint)
+  3. Derived: "mem0-" + sha256(MEM0_API_KEY)[:12]
+  4. Fallback: $USER, else "default"
+
+Same MEM0_API_KEY across machines yields the same user_id, which fixes
+the "47 user buckets per account" symptom from running on multiple
+laptops with different $USER values.
+"""
+
+from __future__ import annotations
+
+import hashlib
+import json
+import os
+from datetime import datetime, timezone
+
+_CACHE_PATH = os.path.expanduser("~/.mem0/identity.json")
+
+
+def resolve_user_id() -> str:
+    explicit = os.environ.get("MEM0_USER_ID", "").strip()
+    if explicit:
+        return explicit
+
+    api_key = os.environ.get("MEM0_API_KEY", "").strip()
+    if api_key:
+        digest = hashlib.sha256(api_key.encode("utf-8")).hexdigest()
+        fingerprint = digest[:8]
+
+        try:
+            with open(_CACHE_PATH, "r") as f:
+                cached = json.load(f)
+            if cached.get("api_key_fingerprint") == fingerprint and cached.get("user_id"):
+                return cached["user_id"]
+        except (OSError, json.JSONDecodeError):
+            pass
+
+        derived = "mem0-" + digest[:12]
+        try:
+            os.makedirs(os.path.dirname(_CACHE_PATH), exist_ok=True)
+            with open(_CACHE_PATH, "w") as f:
+                json.dump(
+                    {
+                        "user_id": derived,
+                        "source": "api_key",
+                        "api_key_fingerprint": fingerprint,
+                        "resolved_at": datetime.now(timezone.utc).strftime("%Y-%m-%dT%H:%M:%SZ"),
+                    },
+                    f,
+                )
+        except OSError:
+            pass
+        return derived
+
+    return os.environ.get("USER") or "default"

mem0-plugin/scripts/_identity.sh

@@ -0,0 +1,57 @@
+# Source this file. Sets MEM0_RESOLVED_USER_ID.
+#
+# Resolution priority:
+#   1. MEM0_USER_ID env var (explicit override)
+#   2. ~/.mem0/identity.json cache (pinned to current MEM0_API_KEY fingerprint)
+#   3. Derived: "mem0-" + sha256(MEM0_API_KEY)[:12]
+#   4. Fallback: $USER, else "default"
+#
+# Same MEM0_API_KEY across machines yields the same user_id, which fixes
+# the "47 user buckets per account" symptom from running on multiple
+# laptops with different $USER values.
+
+_mem0_sha256() {
+  if command -v sha256sum >/dev/null 2>&1; then
+    sha256sum | cut -d' ' -f1
+  else
+    shasum -a 256 | cut -d' ' -f1
+  fi
+}
+
+_mem0_resolve_identity() {
+  if [ -n "${MEM0_USER_ID:-}" ]; then
+    printf '%s' "$MEM0_USER_ID"
+    return
+  fi
+
+  local api_key="${MEM0_API_KEY:-}"
+  local cache="$HOME/.mem0/identity.json"
+
+  if [ -n "$api_key" ]; then
+    local digest
+    digest=$(printf '%s' "$api_key" | _mem0_sha256)
+    local fp="${digest:0:8}"
+
+    if [ -f "$cache" ]; then
+      local cached_fp cached_id
+      cached_fp=$(jq -r '.api_key_fingerprint // ""' "$cache" 2>/dev/null)
+      cached_id=$(jq -r '.user_id // ""' "$cache" 2>/dev/null)
+      if [ "$cached_fp" = "$fp" ] && [ -n "$cached_id" ]; then
+        printf '%s' "$cached_id"
+        return
+      fi
+    fi
+
+    local derived="mem0-${digest:0:12}"
+    mkdir -p "$HOME/.mem0" 2>/dev/null && \
+      printf '{"user_id":"%s","source":"api_key","api_key_fingerprint":"%s","resolved_at":"%s"}\n' \
+        "$derived" "$fp" "$(date -u +%FT%TZ)" > "$cache" 2>/dev/null
+    printf '%s' "$derived"
+    return
+  fi
+
+  printf '%s' "${USER:-default}"
+}
+
+MEM0_RESOLVED_USER_ID="$(_mem0_resolve_identity)"
+export MEM0_RESOLVED_USER_ID

mem0-plugin/scripts/on_pre_compact.py

@@ -18,8 +18,11 @@ import json
 import logging
 import os
 import sys
-import urllib.request
 import urllib.error
+import urllib.request
+
+sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
+from _identity import resolve_user_id
 
 log = logging.getLogger("mem0-capture")
 log.setLevel(logging.DEBUG)
@@ -219,7 +222,7 @@ def main():
         return
 
     session_id = hook_input.get("session_id", "")
-    user_id = os.environ.get("MEM0_USER_ID", os.environ.get("USER", "default"))
+    user_id = resolve_user_id()
 
     lines = tail_lines(transcript_path, MAX_TAIL_LINES)
     if not lines:

mem0-plugin/scripts/on_session_start.sh

@@ -21,9 +21,24 @@ if [ -z "${MEM0_API_KEY:-}" ]; then
   exit 0
 fi
 
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+# shellcheck source=_identity.sh
+. "$SCRIPT_DIR/_identity.sh"
+
 INPUT=$(cat)
 SOURCE=$(echo "$INPUT" | jq -r '.source // "startup"' 2>/dev/null || echo "startup")
 
+# Identity line is emitted before every bootstrap variant so the agent
+# uses the same user_id the hooks resolved. Without this, the agent's
+# search_memories/add_memory MCP calls may bind to a different bucket
+# than what the hooks write to.
+echo "## Mem0 Identity"
+echo ""
+echo "Active user_id: \`$MEM0_RESOLVED_USER_ID\`"
+echo ""
+echo "Always include \`{\"user_id\": \"$MEM0_RESOLVED_USER_ID\"}\` (wrapped in an \`AND\` clause) in every \`search_memories\` filter and as \`user_id\` on every \`add_memory\` call. This keeps memories under one bucket regardless of which machine you're on."
+echo ""
+
 if [ "$SOURCE" = "startup" ]; then
   cat <<'EOF'
 ## Mem0 Session Bootstrap

mem0-plugin/scripts/on_user_prompt.sh

@@ -30,7 +30,10 @@ if [ -z "${MEM0_API_KEY:-}" ]; then
   exit 0
 fi
 
-USER_ID="${MEM0_USER_ID:-${USER:-default}}"
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+# shellcheck source=_identity.sh
+. "$SCRIPT_DIR/_identity.sh"
+USER_ID="$MEM0_RESOLVED_USER_ID"
 
 cat <<EOF
 ## Memory check