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+# Code Patterns
+
+Cross-cutting patterns used throughout the codebase: exception handling, encoding utilities, compression, concurrent data structures, and batch processing. These patterns provide consistency, type safety, and correctness guarantees across all modules.
+
+For protocol-specific encoding details, see [transport.md](transport.md). For cryptographic operations, see the inline documentation in [Crypto.hs](../../src/Simplex/Messaging/Crypto.hs).
+
+- [Exception handling](#exception-handling)
+- [Binary encoding](#binary-encoding)
+- [String encoding](#string-encoding)
+- [Compression](#compression)
+- [Concurrent data structures](#concurrent-data-structures)
+- [Batch processing](#batch-processing)
+
+---
+
+## Exception handling
+
+**Source**: [Agent/Protocol.hs](../../src/Simplex/Messaging/Agent/Protocol.hs), [Agent/Client.hs](../../src/Simplex/Messaging/Agent/Client.hs), [Agent/Store/SQLite.hs](../../src/Simplex/Messaging/Agent/Store/SQLite.hs)
+
+### Error type hierarchy
+
+The codebase uses a hierarchical error type structure:
+
+**`AgentErrorType`** - top-level error type for agent client responses:
+- `CMD` - command/response errors with context string
+- `CONN` - connection errors with context (NOT_FOUND, DUPLICATE, SIMPLEX)
+- `SMP`/`NTF`/`XFTP` - protocol-specific errors with server address
+- `BROKER` - transport-level broker errors
+- `AGENT` - internal agent errors (A_DUPLICATE, A_PROHIBITED)
+- `INTERNAL` - implementation bugs (should never occur in production)
+- `CRITICAL` - critical errors with optional restart offer
+
+**`StoreError`** - database/storage layer errors:
+- `SEInternal` - IO exceptions during database operations
+- `SEDatabaseBusy` - database locked/busy (triggers CRITICAL with restart)
+- `SEConnNotFound`/`SEUserNotFound` - entity lookup failures
+- `SEBadConnType` - wrong connection type for operation
+
+**`AgentCryptoError`** - cryptographic failures:
+- `DECRYPT_AES`/`DECRYPT_CB` - decryption failures
+- `RATCHET_HEADER`/`RATCHET_EARLIER Word32`/`RATCHET_SKIPPED Word32`/`RATCHET_SYNC` - double ratchet state issues
+
+### Monad stack
+
+```
+AM a  = ExceptT AgentErrorType (ReaderT Env IO) a  -- full error handling
+AM' a = ReaderT Env IO a                            -- no error handling (for batch ops)
+```
+
+### Store access patterns
+
+**Basic operations** lift IO actions into the AM monad:
+
+```haskell
+withStore :: AgentClient -> (DB.Connection -> IO (Either StoreError a)) -> AM a
+withStore' :: AgentClient -> (DB.Connection -> IO a) -> AM a  -- wraps result in Right
+```
+
+Both wrap the action in a database transaction and convert `StoreError` to `AgentErrorType` via `storeError`.
+
+**Error mapping** (key cases from `storeError`):
+- `SEConnNotFound`/`SERatchetNotFound` -> `CONN NOT_FOUND`
+- `SEConnDuplicate` -> `CONN DUPLICATE`
+- `SEBadConnType` -> `CONN SIMPLEX` with context
+- `SEUserNotFound` -> `NO_USER`
+- `SEAgentError e` -> `e` (propagates wrapped error)
+- `SEDatabaseBusy` -> `CRITICAL True` (offers restart)
+- Other errors -> `INTERNAL` with error message
+
+### Error recovery patterns
+
+**tryError** - attempt an operation, handle failure without throwing:
+```haskell
+tryError (deleteQueue c NRMBackground rq') >>= \case
+  Left e -> logError e >> continue
+  Right () -> success
+```
+
+**catchAllErrors** - catch errors and run cleanup:
+```haskell
+getQueueMessage c rq `catchAllErrors` \e ->
+  atomically (releaseGetLock c rq) >> throwError e
+```
+
+**catchAll_** - catch all exceptions, return default on failure:
+```haskell
+notices <- liftIO $ withTransaction store (`getClientNotices` servers) `catchAll_` pure []
+```
+
+---
+
+## Binary encoding
+
+**Source**: [Encoding.hs](../../src/Simplex/Messaging/Encoding.hs)
+
+### Encoding typeclass
+
+```haskell
+class Encoding a where
+  smpEncode :: a -> ByteString      -- encode to binary
+  smpP :: Parser a                   -- attoparsec parser
+  smpDecode :: ByteString -> Either String a  -- default via parseAll smpP
+```
+
+### Primitive encoding
+
+| Type | Wire format |
+|------|-------------|
+| `Char` | Single byte |
+| `Bool` | `'T'` or `'F'` |
+| `Word16` | 2-byte big-endian |
+| `Word32` | 4-byte big-endian |
+| `Int64` | Two `Word32`s combined |
+| `ByteString` | 1-byte length prefix + data (max 255 bytes) |
+| `Maybe a` | `'0'` (Nothing) or `'1'` + encoded value |
+| `(a, b)` | Concatenated encodings (no separator) |
+
+### Special wrappers
+
+**`Tail`** - takes remaining bytes without length prefix:
+```haskell
+newtype Tail = Tail {unTail :: ByteString}
+-- smpEncode = unTail (no prefix)
+-- smpP = takeByteString
+```
+
+**`Large`** - for ByteStrings > 255 bytes:
+```haskell
+newtype Large = Large {unLarge :: ByteString}
+-- smpEncode = Word16 length prefix + data
+-- smpP = read Word16, take that many bytes
+```
+
+### List encoding
+
+```haskell
+smpEncodeList :: Encoding a => [a] -> ByteString
+-- 1-byte count prefix + concatenated encoded items (max 255 items)
+
+instance Encoding (NonEmpty a)
+-- Same format, fails on empty input during parsing
+```
+
+---
+
+## String encoding
+
+**Source**: [Encoding/String.hs](../../src/Simplex/Messaging/Encoding/String.hs)
+
+### StrEncoding typeclass
+
+```haskell
+class StrEncoding a where
+  strEncode :: a -> ByteString      -- human-readable encoding
+  strP :: Parser a                   -- parser (defaults to base64url)
+  strDecode :: ByteString -> Either String a
+```
+
+Used for addresses, keys, and values displayed to users or in URIs.
+
+### Base64 URL encoding
+
+`ByteString` instances use base64url encoding (RFC 4648):
+- Alphabet: A-Z, a-z, 0-9, `-`, `_`
+- No padding by default in output
+- Accepts optional `=` padding on input
+
+### Tuple and list encoding
+
+**Tuples** use space separation (via `B.unwords`):
+```haskell
+strEncode (a, b) = B.unwords [strEncode a, strEncode b]
+```
+
+**Lists** use comma separation:
+```haskell
+strEncodeList :: StrEncoding a => [a] -> ByteString
+strEncodeList = B.intercalate "," . map strEncode
+```
+
+### Numeric types
+
+`Int`, `Word16`, `Word32`, `Int64` encode as decimal strings (not binary).
+
+### JSON conversion utilities
+
+```haskell
+strToJSON :: StrEncoding a => a -> J.Value
+strParseJSON :: StrEncoding a => String -> J.Value -> JT.Parser a
+```
+
+Convert between `StrEncoding` and JSON string values for API serialization.
+
+---
+
+## Compression
+
+**Source**: [Compression.hs](../../src/Simplex/Messaging/Compression.hs)
+
+### Algorithm and thresholds
+
+Uses Zstandard (zstd) compression at level 3 (moderate compression/speed tradeoff).
+
+```haskell
+maxLengthPassthrough :: Int
+maxLengthPassthrough = 180  -- messages <= 180 bytes are not compressed
+```
+
+### Wire format
+
+```haskell
+data Compressed
+  = Passthrough ByteString  -- tag '0' + 1-byte length + data
+  | Compressed Large        -- tag '1' + 2-byte length + zstd data
+```
+
+### Decompression bomb protection
+
+`decompress1` requires the compressed data to declare its decompressed size upfront:
+
+```haskell
+decompress1 :: Int -> Compressed -> Either String ByteString
+```
+
+The function checks `Z1.decompressedSize` before decompressing. If the declared size exceeds the `limit` parameter (or is not specified), decompression is rejected. This prevents zip-bomb attacks where a small compressed payload would expand to exhaust memory.
+
+Zstd's `decompress` can return `Error`, `Skip` (empty result), or `Decompress bs'` - all cases are handled explicitly.
+
+---
+
+## Concurrent data structures
+
+**Source**: [TMap.hs](../../src/Simplex/Messaging/TMap.hs)
+
+### TMap
+
+A `TVar`-wrapped immutable `Data.Map`, providing atomic read-modify-write operations via STM:
+
+```haskell
+type TMap k a = TVar (Map k a)
+```
+
+### STM operations (atomic)
+
+| Operation | Description |
+|-----------|-------------|
+| `lookup k m` | Read value for key |
+| `member k m` | Check key existence |
+| `insert k v m` | Insert/update value |
+| `delete k m` | Remove key |
+| `lookupInsert k v m` | Atomic lookup-then-insert, returns old value |
+| `lookupDelete k m` | Atomic lookup-then-delete, returns deleted value |
+
+### IO operations (non-transactional)
+
+```haskell
+lookupIO :: Ord k => k -> TMap k a -> IO (Maybe a)
+memberIO :: Ord k => k -> TMap k a -> IO Bool
+```
+
+These bypass STM for read-only access when atomicity with other operations is not needed.
+
+### Usage pattern
+
+```haskell
+-- Within STM transaction (atomic with other STM ops)
+atomically $ do
+  existing <- TM.lookup key map
+  case existing of
+    Nothing -> TM.insert key newValue map
+    Just _ -> pure ()
+
+-- Outside transaction (simple read)
+value <- TM.lookupIO key map
+```
+
+---
+
+## Batch processing
+
+**Source**: [Agent/Client.hs](../../src/Simplex/Messaging/Agent/Client.hs)
+
+### withStoreBatch
+
+Executes multiple database operations in a single transaction:
+
+```haskell
+withStoreBatch :: Traversable t
+  => AgentClient
+  -> (DB.Connection -> t (IO (Either AgentErrorType a)))
+  -> AM' (t (Either AgentErrorType a))
+```
+
+All operations run within one database transaction, ensuring:
+- **Atomicity**: All operations succeed or all fail together
+- **Isolation**: No partial updates visible to other readers
+- **Efficiency**: Single transaction overhead instead of per-operation
+
+### Result semantics
+
+Each batched operation produces an individual `Either AgentErrorType a`:
+- Partial success is possible (some `Right`, some `Left`)
+- If the transaction itself fails, all results become errors
+- Fine-grained error handling per operation
+
+### Common patterns
+
+**Store multiple items**:
+```haskell
+void $ withStoreBatch' c $ \db ->
+  map (storeDelivery db) deliveries
+```
+
+**Fetch multiple items**:
+```haskell
+results <- withStoreBatch c $ \db ->
+  map (getConnection db) connIds
+```
+
+**Update multiple items**:
+```haskell
+void $ withStoreBatch' c $ \db ->
+  map (\connId -> setConnPQSupport db connId PQSupportOn) connIds
+```
+
+### withStoreBatch'
+
+Convenience variant that wraps results in `Right`:
+
+```haskell
+withStoreBatch' :: Traversable t
+  => AgentClient
+  -> (DB.Connection -> t (IO a))
+  -> AM' (t (Either AgentErrorType a))
+```
+
+Use when operations cannot fail (or failures should become `INTERNAL` errors).