Apple Mail MCP Performance Patterns

The Core Insight

The bottleneck is per-subprocess overhead. Each osascript call costs 100-300ms regardless of what it does. All performance work reduces the number of subprocess calls.

Known Operation Timings

Pattern 1: Use whose Clauses for Server-Side Filtering

-- GOOD: Server-side filter (fast, Mail.app evaluates internally)
set msgs to (messages of mbox whose sender contains "user@example.com")

-- BAD: Fetch all then filter in Python (slow, transfers all data)
set msgs to every message of mbox
-- then filter in Python loop

whose clauses let Mail.app filter internally without transferring unmatched messages over IPC. This is 10-50x faster for large mailboxes.

Pattern 2: Single Script Per Batch Operation

# GOOD: One osascript call for N messages (near-constant time)
script = """
tell application "Mail"
    repeat with msgId in {id1, id2, id3}
        set read status of (first message whose id is msgId) to true
    end repeat
end tell
"""
self._run_applescript(script)

# BAD: N osascript calls for N messages (linear time)
for msg_id in message_ids:
    self._run_applescript(f'tell application "Mail" ...')

Batch operations should always build a single AppleScript that handles all items.

Pattern 3: Use limit for Pagination

The search_messages tool accepts a limit parameter (default: 50). AppleScript uses items 1 thru N of for server-side limiting. Always pass a reasonable limit — fetching 10,000 messages when the user wants the latest 10 wastes time.

Pattern 4: Accept Optional Account/Mailbox Parameters

Message ID lookup is O(accounts x mailboxes) when searching globally. Always accept optional account and mailbox parameters to narrow the search scope. If the caller knows which account, the search is dramatically faster.

Anti-Patterns

• Repeated subprocess calls in loops — Build one script, execute once
• Fetching all properties when only some are needed — Scripts currently fetch a fixed set of fields; when adding a new field, consider whether every caller needs it
• Not using whose clauses — Manual filtering in Python transfers all data first
• Default timeout too low for large operations — Increase from 60s for bulk operations on large mailboxes

Gmail Performance Notes

Gmail operations are inherently slower than IMAP because:

• Move requires copy + delete (two operations instead of one)
• Label operations don't map cleanly to folder operations
• Search across Gmail labels may scan differently than IMAP folders

Threading cost scales with folder count (not account size). IMAP SEARCH runs against one selected mailbox at a time — there is no cross-folder search — so get_thread's member collection probes each candidate folder in turn. Gmail exposes every label as an IMAP folder and copies each message into every label it carries, so a thread's members are scattered across many folders. The mitigation is All Mail: enable Gmail Settings → Labels → "All Mail" → Show in IMAP, and _find_all_mail_folder (\All SPECIAL-USE, RFC 6154) collapses the probe to that single mailbox holding one copy of everything. Without it, threading falls back to per-label probing. This is not Gmail-specific in principle: any IMAP account with dozens of folders pays the same per-folder cost — non-Gmail accounts are usually fast only because their mail is concentrated in a handful of folders (INBOX/Sent). See #415 and the get_thread performance callout in docs/reference/TOOLS.md.

Bound the mailbox set on the AppleScript side too — with the unified mailboxes. The same "don't visit every mailbox" rule applies to AppleScript lookups, and Mail.app gives you ready-made bounds: the application-level inbox, sent mailbox, and drafts mailbox aggregate the corresponding folder across every account, so probing them needs no per-account mailbox iteration and no name matching (which would break on localized names). whose id is N works against them, and account of (mailbox of msg) walks back to the owning account. _resolve_numeric_anchor_fast uses this to replace a repeat with acc in accounts / repeat with mb in mailboxes of acc scan: ~1.3s vs ~3.0s on a 33k-message Gmail account (#419). Keep the full walk as a not-found backstop.

Don't assume IMAP is the fast side. Indexed does not mean instant at scale: on that same account SEARCH HEADER Message-ID over a 33k-message All Mail measured ~17s, so replacing the AppleScript anchor above with an IMAP lookup would have been ~13× slower, not faster. Measure per-phase against a real large account before moving work across the AppleScript/IMAP boundary — and re-measure, because Gmail's server-side latency swings by tens of seconds with throttling.

Profiling

No formal benchmarking infrastructure yet (see issue #31). When added:

• Use 5 iterations per operation
• Calculate mean, stdev, CV%
• Set thresholds at 5x documented baseline
• Detect cold starts (first run > 2x median of remaining)