diff --git a/docs/CAPTURES/22-py-sql-error.socat.log b/docs/CAPTURES/22-py-sql-error.socat.log
new file mode 100644
index 0000000..5fcdce4
--- /dev/null
+++ b/docs/CAPTURES/22-py-sql-error.socat.log
@@ -0,0 +1,38 @@
+2026/05/04 11:54:54 socat[941309] N listening on AF=2 0.0.0.0:9090
+2026/05/04 11:54:55 socat[941309] N accepting connection from AF=2 127.0.0.1:46066 on AF=2 127.0.0.1:9090
+2026/05/04 11:54:55 socat[941309] N opening connection to 127.0.0.1:9088
+2026/05/04 11:54:55 socat[941309] N opening connection to AF=2 127.0.0.1:9088
+2026/05/04 11:54:55 socat[941309] N successfully connected from local address AF=2 127.0.0.1:37838
+2026/05/04 11:54:55 socat[941309] N successfully connected to 127.0.0.1:9088
+2026/05/04 11:54:55 socat[941309] N starting data transfer loop with FDs [6,6] and [5,5]
+> 2026/05/04 11:54:55.272845  length=384 from=0 to=383
+ 01 80 01 3c 00 00 00 64 00 65 00 00 00 3d 00 06 49 45 45 45 4d 00 00 6c 73 71 6c 65 78 65 63 00 00 00 00 00 00 06 39 2e 32 38 30 00 00 0c 52 44 53 23 52 30 30 30 30 30 30 00 00 05 73 71 6c 69 00 00 00 01 3c 00 00 00 00 00 00 00 00 00 01 00 09 69 6e 66 6f 72 6d 69 78 00 00 07 69 6e 34 6d 69 78 00 6f 6c 00 00 00 00 00 00 00 00 00 3d 74 6c 69 74 63 70 00 00 00 00 00 01 00 68 00 0b 00 00 00 03 00 09 69 6e 66 6f 72 6d 69 78 00 00 00 00 00 00 00 00 00 00 00 00 6a 00 06 00 07 44 42 50 41 54 48 00 00 02 2e 00 00 0e 43 4c 49 45 4e 54 5f 4c 4f 43 41 4c 45 00 00 0d 65 6e 5f 55 53 2e 38 38 35 39 2d 31 00 00 11 43 4c 4e 54 5f 50 41 4d 5f 43 41 50 41 42 4c 45 00 00 02 31 00 00 07 44 42 44 41 54 45 00 00 06 59 34 4d 44 2d 00 00 0c 49 46 58 5f 55 50 44 44 45 53 43 00 00 02 31 00 00 09 4e 4f 44 45 46 44 41 43 00 00 03 6e 6f 00 00 6b 00 00 00 00 00 0e 5d 0a 00 00 00 00 00 0b 72 70 6d 2d 62 75 6c 6c 65 74 00 00 00 00 29 2f 68 6f 6d 65 2f 72 70 6d 2f 63 6c 61 75 64 65 2f 69 6e 66 6f 72 6d 69 78 2f 70 79 74 68 6f 6e 2d 6c 69 62 72 61 72 79 00 00 74 00 20 00 00 00 00 00 00 00 00 00 16 69 6e 66 6f 72 6d 69 78 2d 64 62 40 70 69 64 39 34 31 33 32 32 00 00 7f
+< 2026/05/04 11:54:55.276844  length=276 from=0 to=275
+ 01 14 02 3c 10 00 00 64 00 65 00 00 00 3d 00 06 49 45 45 45 49 00 00 6c 73 72 76 69 6e 66 78 00 00 00 00 00 00 2f 49 42 4d 20 49 6e 66 6f 72 6d 69 78 20 44 79 6e 61 6d 69 63 20 53 65 72 76 65 72 20 56 65 72 73 69 6f 6e 20 31 35 2e 30 2e 31 2e 30 2e 33 00 00 07 73 65 72 69 61 6c 00 00 09 69 6e 66 6f 72 6d 69 78 00 00 00 01 3c 00 00 00 00 00 00 00 00 00 00 00 00 00 00 6f 6e 00 00 00 00 00 00 00 00 00 3d 73 6f 63 74 63 70 00 00 00 00 00 00 00 66 00 00 00 00 00 00 00 00 00 00 00 14 00 00 00 6b 00 00 00 00 00 00 03 1a 00 00 00 00 00 0d 32 33 32 37 63 34 33 35 34 65 61 38 00 00 00 00 0f 2f 68 6f 6d 65 2f 69 6e 66 6f 72 6d 69 78 00 00 6e 00 04 00 00 00 00 00 74 00 33 00 00 00 c8 00 00 00 c8 00 29 2f 6f 70 74 2f 69 62 6d 2f 69 6e 66 6f 72 6d 69 78 2f 76 31 35 2e 30 2e 31 2e 30 2e 33 2f 62 69 6e 2f 6f 6e 69 6e 69 74 00 00 7f
+> 2026/05/04 11:54:55.277117  length=14 from=384 to=397
+ 00 7e 00 08 ff fc 7f fc 3c 8c aa 97 00 0c
+< 2026/05/04 11:54:55.283165  length=16 from=276 to=291
+ 00 7e 00 09 bd be 9f fe 7f b7 ff ef ff 00 00 0c
+> 2026/05/04 11:54:55.283254  length=48 from=398 to=445
+ 00 51 00 06 00 26 00 0c 00 04 00 06 44 42 54 45 4d 50 00 04 2f 74 6d 70 00 0b 53 55 42 51 43 41 43 48 45 53 5a 00 00 02 31 30 00 00 00 00 00 0c
+< 2026/05/04 11:54:55.283343  length=2 from=292 to=293
+ 00 0c
+> 2026/05/04 11:54:55.283368  length=18 from=446 to=463
+ 00 24 00 09 73 79 73 6d 61 73 74 65 72 00 00 00 00 0c
+< 2026/05/04 11:54:55.283546  length=28 from=294 to=321
+ 00 0f 00 15 00 00 00 00 00 00 00 00 00 00 00 00 00 37 00 00 00 01 00 00 00 01 00 0c
+> 2026/05/04 11:54:55.283595  length=32 from=464 to=495
+ 00 02 00 00 00 00 00 11 53 45 4c 45 4b 54 20 42 41 44 20 53 59 4e 54 41 58 00 00 16 00 31 00 0c
+< 2026/05/04 11:54:55.283670  length=14 from=322 to=335
+ 00 0d ff 37 00 00 00 00 00 01 00 00 00 0c
+> 2026/05/04 11:54:55.283739  length=42 from=496 to=537
+ 00 02 00 00 00 00 00 1b 53 45 4c 45 43 54 20 2a 20 46 52 4f 4d 20 6e 6f 5f 73 75 63 68 5f 74 61 62 6c 65 00 00 16 00 31 00 0c
+< 2026/05/04 11:54:55.283852  length=28 from=336 to=363
+ 00 0d ff 32 ff 91 00 00 00 1b 00 0d 6e 6f 5f 73 75 63 68 5f 74 61 62 6c 65 00 00 0c
+> 2026/05/04 11:54:55.283918  length=2 from=538 to=539
+ 00 38
+< 2026/05/04 11:54:55.283967  length=2 from=364 to=365
+ 00 38
+2026/05/04 11:54:55 socat[941309] N socket 1 (fd 6) is at EOF
+2026/05/04 11:54:55 socat[941309] N socket 2 (fd 5) is at EOF
+2026/05/04 11:54:55 socat[941309] N exiting with status 0
diff --git a/src/informix_db/_errcodes.py b/src/informix_db/_errcodes.py
new file mode 100644
index 0000000..74b7054
--- /dev/null
+++ b/src/informix_db/_errcodes.py
@@ -0,0 +1,141 @@
+"""Informix sqlcode → exception class + human-readable text.
+
+Informix error codes (negative numbers) are documented in IBM/HCL's
+"Error Messages" reference. The JDBC driver looks up text from a
+locale-keyed message catalog shipped with the CSDK; we don't have access
+to that here, so we ship a small curated catalog of the ~50 most common
+errors. Anything not in the catalog gets a generic "Informix error <N>".
+
+Users can extend the catalog at runtime via ``register_error_text(code, text)``.
+
+PEP 249 exception classification follows JDBC conventions:
+  syntax / semantic errors  → ProgrammingError
+  constraint violations     → IntegrityError
+  transaction/connection    → OperationalError
+  everything else           → DatabaseError
+"""
+
+from __future__ import annotations
+
+from .exceptions import (
+    DatabaseError,
+    Error,
+    IntegrityError,
+    NotSupportedError,
+    OperationalError,
+    ProgrammingError,
+)
+
+# Curated catalog of common Informix sqlcodes. Texts are short and
+# user-facing — the canonical IBM "Error Messages" doc has more detail.
+_ERROR_TEXT: dict[int, str] = {
+    # --- SQL syntax / semantic errors (ProgrammingError) ---
+    -201: "A syntax error has occurred",
+    -202: "An illegal character has been found in the statement",
+    -206: "The specified table is not in the database",
+    -239: "Could not insert new row - duplicate value in a UNIQUE INDEX column",
+    -253: "Statement size exceeds language buffer size",
+    -255: "Not in transaction",
+    -256: "Transaction not available",
+    -258: "Invalid statement identifier",
+    -259: "Cursor not open",
+    -260: "Cursor name already in use",
+    -262: "No such cursor",
+    -263: "Cannot open file for LOAD or UNLOAD",
+    -264: "Cannot read from input file for LOAD",
+    -267: "The transaction has been rolled back, all locks released",
+    -268: "Cannot insert duplicate value - violates UNIQUE constraint",
+    -269: "Default value not specified, cannot be NULL",
+    -271: "Could not insert new row into the table",
+    -272: "No SELECT permission",
+    -273: "No UPDATE permission",
+    -274: "No DELETE permission",
+    -286: "The number of values does not match number of columns",
+    -291: "Cannot insert duplicate row - violates unique index constraint",
+    -292: "NOT NULL constraint violated",
+    -297: "Lock timeout expired",
+    -217: "Column not found in any table in the query",
+    -310: "The result of an aggregate function returned NULL",
+    -311: "Column not found in named tables in the query",
+    -316: "Index already exists",
+    -329: "Database not found or no system permission",
+    -349: "Database not selected yet",
+    -390: "Synonym already exists in database",
+    -391: "Cannot insert NULL value into a NOT NULL column",
+    -407: "Begin transaction failed - already in a transaction",
+    -408: "Commit transaction failed - not in a transaction",
+    -440: "Connection not established",
+    -451: "Cannot retrieve column data",
+    -456: "Indicator variable required",
+    -465: "BYTE or TEXT subscript out of range",
+    -510: "Specified database does not exist",
+    -517: "Statement is too long",
+    -526: "Cursor is not on a SELECT statement",
+    -567: "Cannot select column from view (no underlying table)",
+    -608: "Number truncated converting from one type to another",
+    -703: "Primary key on table has a field with a NULL key value",
+    -722: "Out of memory",
+    -748: "Server has rejected the connection",
+    -751: "User not known to the database",
+    -759: "Database not available",
+    -908: "Attempt to connect to database server failed",
+    -930: "Cannot connect to database server",
+    -956: "Client/server unable to communicate",
+    -25553: "Connection not authorized",
+    -25563: "Server name unrecognized",
+}
+
+
+def text_for(sqlcode: int) -> str:
+    """Return a human-readable description for the sqlcode.
+
+    Falls back to a generic message for unknown codes.
+    """
+    if sqlcode in _ERROR_TEXT:
+        return _ERROR_TEXT[sqlcode]
+    return f"Informix error {sqlcode}"
+
+
+def register_error_text(sqlcode: int, text: str) -> None:
+    """Add or override a sqlcode → text mapping at runtime."""
+    _ERROR_TEXT[sqlcode] = text
+
+
+# PEP 249 exception class by Informix sqlcode. The general approach:
+# - Constraint violations (duplicate, NOT NULL, FK) → IntegrityError
+# - SQL syntax / object-not-found / permission → ProgrammingError
+# - Transaction / connection / network → OperationalError
+# - Everything else → DatabaseError (the safe fallback)
+
+_INTEGRITY_CODES = frozenset({-239, -268, -269, -291, -292, -391, -703})
+
+_PROGRAMMING_CODES = frozenset({
+    -201, -202, -206, -217, -253, -258, -259, -260, -262, -286,
+    -310, -311, -316, -390, -451, -456, -465, -517, -526, -567, -608,
+    -272, -273, -274,  # permission errors are programmer's responsibility
+})
+
+_OPERATIONAL_CODES = frozenset({
+    -255, -256, -267, -297, -407, -408,
+    -440, -748, -751, -759, -908, -930, -956,
+    -25553, -25563,
+})
+
+_NOT_SUPPORTED_CODES = frozenset({
+    -329,  # missing system permission — caller can't fix
+    -349,  # database not selected
+    -510,  # database not found
+})
+
+
+def exception_for(sqlcode: int) -> type[Error]:
+    """Pick the PEP 249 exception class for a given sqlcode."""
+    if sqlcode in _INTEGRITY_CODES:
+        return IntegrityError
+    if sqlcode in _PROGRAMMING_CODES:
+        return ProgrammingError
+    if sqlcode in _OPERATIONAL_CODES:
+        return OperationalError
+    if sqlcode in _NOT_SUPPORTED_CODES:
+        return NotSupportedError
+    return DatabaseError
diff --git a/src/informix_db/connections.py b/src/informix_db/connections.py
index d16702f..bbf6ec6 100644
--- a/src/informix_db/connections.py
+++ b/src/informix_db/connections.py
@@ -301,16 +301,27 @@ class Connection:
                 )
 
     def _raise_sq_err(self) -> None:
-        """Decode a SQ_ERR payload and raise OperationalError.
+        """Decode a SQ_ERR payload and raise the appropriate PEP 249 exception.
 
-        Per ``IfxSqli.receiveError``:
-          ``[short sqlcode][short isamcode][int statementOffset][...]``
+        Wire format (per IfxSqli.receiveError, line 2717):
+          [short sqlcode][short isamcode][int offset]
+          [short near_token_len][bytes name][optional pad][short SQ_EOT]
         """
+        from . import _errcodes
+
         sqlcode = struct.unpack("!h", self._sock.read_exact(2))[0]
         isamcode = struct.unpack("!h", self._sock.read_exact(2))[0]
-        offset = struct.unpack("!i", self._sock.read_exact(4))[0]  # noqa: F841
-        # Drain any remaining error payload (varies by sqlcode) until SQ_EOT.
-        # Best-effort: read shorts and discard until we hit 0x000c.
+        offset = struct.unpack("!i", self._sock.read_exact(4))[0]
+        near_token = ""
+        try:
+            name_len = struct.unpack("!h", self._sock.read_exact(2))[0]
+            if name_len > 0:
+                raw = self._sock.read_exact(name_len)
+                if name_len & 1:
+                    self._sock.read_exact(1)
+                near_token = raw.rstrip(b"\x00").decode("iso-8859-1", errors="replace")
+        except Exception:
+            pass
         try:
             while True:
                 next_tag = struct.unpack("!h", self._sock.read_exact(2))[0]
@@ -318,7 +329,20 @@ class Connection:
                     break
         except OperationalError:
             pass
-        raise OperationalError(f"server returned SQ_ERR sqlcode={sqlcode} isamcode={isamcode}")
+
+        text = _errcodes.text_for(sqlcode)
+        exc_class = _errcodes.exception_for(sqlcode)
+        msg = f"[{sqlcode}] {text} (near {near_token!r})" if near_token else f"[{sqlcode}] {text}"
+        if isamcode != 0:
+            msg += f" [ISAM {isamcode}]"
+        if offset > 0:
+            msg += f" (offset {offset})"
+        exc = exc_class(msg)
+        exc.sqlcode = sqlcode
+        exc.isamcode = isamcode
+        exc.offset = offset
+        exc.near = near_token
+        raise exc
 
     # -- login PDU assembly ------------------------------------------------
 
diff --git a/src/informix_db/cursors.py b/src/informix_db/cursors.py
index 3df68e5..a5ec7ee 100644
--- a/src/informix_db/cursors.py
+++ b/src/informix_db/cursors.py
@@ -24,6 +24,7 @@ import struct
 from collections.abc import Iterator
 from typing import TYPE_CHECKING, Any
 
+from . import _errcodes
 from ._messages import MessageType
 from ._protocol import IfxStreamReader, make_pdu_writer
 from ._resultset import ColumnInfo, parse_describe, parse_tuple_payload
@@ -568,11 +569,30 @@ class Cursor:
             self._rowcount = rows
 
     def _raise_sq_err(self, reader: IfxStreamReader) -> None:
-        """Decode SQ_ERR per IfxSqli.receiveError and raise."""
+        """Decode SQ_ERR and raise the appropriate PEP 249 exception class.
+
+        Wire format (per IfxSqli.receiveError, line 2717):
+          [short sqlcode][short isamcode][int offset]
+          [short near_token_len][bytes name][optional pad][short SQ_EOT]
+
+        The "near" token is the object name where the error occurred
+        (e.g. table or column name for "not found" errors). Empty for
+        most syntax errors.
+        """
         sqlcode = reader.read_short()
         isamcode = reader.read_short()
-        reader.read_int()  # offset into statement
-        # Drain remaining error bytes until SQ_EOT.
+        offset = reader.read_int()
+        near_token = ""
+        try:
+            name_len = reader.read_short()
+            if name_len > 0:
+                raw = reader.read_exact(name_len)
+                if name_len & 1:
+                    reader.read_exact(1)  # pad to even
+                near_token = raw.rstrip(b"\x00").decode("iso-8859-1", errors="replace")
+        except Exception:
+            pass
+        # Drain remaining bytes until SQ_EOT.
         try:
             while True:
                 t = reader.read_short()
@@ -580,7 +600,21 @@ class Cursor:
                     break
         except Exception:
             pass
-        raise ProgrammingError(f"server returned SQ_ERR sqlcode={sqlcode} isamcode={isamcode}")
+
+        text = _errcodes.text_for(sqlcode)
+        exc_class = _errcodes.exception_for(sqlcode)
+        msg = f"[{sqlcode}] {text} (near {near_token!r})" if near_token else f"[{sqlcode}] {text}"
+        if isamcode != 0:
+            msg += f" [ISAM {isamcode}]"
+        if offset > 0:
+            msg += f" (offset {offset})"
+        exc = exc_class(msg)
+        # Attach structured fields for programmatic inspection.
+        exc.sqlcode = sqlcode
+        exc.isamcode = isamcode
+        exc.offset = offset
+        exc.near = near_token
+        raise exc
 
 
 class _SocketReader(IfxStreamReader):
diff --git a/tests/test_decimal.py b/tests/test_decimal.py
index 80dfb1e..ac6c43c 100644
--- a/tests/test_decimal.py
+++ b/tests/test_decimal.py
@@ -98,4 +98,4 @@ def test_decimal_param_binding_raises_for_now(conn_params: ConnParams) -> None:
         cur = conn.cursor()
         cur.execute("CREATE TEMP TABLE td2 (n DECIMAL(10,2))")
         with pytest.raises(NotImplementedError, match="Decimal"):
-            cur.execute("INSERT INTO td2 VALUES (?)", (Decimal("3.14"),))
\ No newline at end of file
+            cur.execute("INSERT INTO td2 VALUES (?)", (Decimal("3.14"),))
diff --git a/tests/test_errors.py b/tests/test_errors.py
new file mode 100644
index 0000000..d92c7c2
--- /dev/null
+++ b/tests/test_errors.py
@@ -0,0 +1,124 @@
+"""Phase 5.a integration tests — error message decoding + PEP 249 classification.
+
+Server SQ_ERR responses get decoded to:
+  - the appropriate PEP 249 exception class (Programming/Integrity/...)
+  - a human-readable message with sqlcode, near-token, ISAM code, offset
+  - structured ``sqlcode``, ``isamcode``, ``offset``, ``near`` attributes
+
+The connection survives errors — subsequent queries work normally.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+import informix_db
+from tests.conftest import ConnParams
+
+pytestmark = pytest.mark.integration
+
+
+def _connect(conn_params: ConnParams) -> informix_db.Connection:
+    return informix_db.connect(
+        host=conn_params.host,
+        port=conn_params.port,
+        user=conn_params.user,
+        password=conn_params.password,
+        database=conn_params.database,
+        server=conn_params.server,
+        connect_timeout=10.0,
+        read_timeout=10.0,
+    )
+
+
+def test_syntax_error_is_programming_error(conn_params: ConnParams) -> None:
+    """sqlcode -201 (syntax error) → ProgrammingError."""
+    with _connect(conn_params) as conn:
+        cur = conn.cursor()
+        with pytest.raises(informix_db.ProgrammingError) as excinfo:
+            cur.execute("SELEKT * FROM systables")
+        e = excinfo.value
+        assert e.sqlcode == -201
+        assert "syntax error" in str(e).lower()
+
+
+def test_table_not_found_is_programming_error(conn_params: ConnParams) -> None:
+    """sqlcode -206 (table not found) → ProgrammingError; near-token has table name."""
+    with _connect(conn_params) as conn:
+        cur = conn.cursor()
+        with pytest.raises(informix_db.ProgrammingError) as excinfo:
+            cur.execute("SELECT * FROM no_such_table_xyz")
+        e = excinfo.value
+        assert e.sqlcode == -206
+        assert e.near == "no_such_table_xyz"
+        assert "not in the database" in str(e)
+
+
+def test_column_not_found_is_programming_error(conn_params: ConnParams) -> None:
+    """sqlcode -217 (column not found) → ProgrammingError; near-token has column name."""
+    with _connect(conn_params) as conn:
+        cur = conn.cursor()
+        with pytest.raises(informix_db.ProgrammingError) as excinfo:
+            cur.execute("SELECT no_such_column FROM systables")
+        e = excinfo.value
+        assert e.sqlcode == -217
+        assert e.near == "no_such_column"
+
+
+def test_unique_violation_is_integrity_error(conn_params: ConnParams) -> None:
+    """sqlcode -268 (UNIQUE violation) → IntegrityError."""
+    with _connect(conn_params) as conn:
+        cur = conn.cursor()
+        cur.execute("CREATE TEMP TABLE t_uniq (id INTEGER PRIMARY KEY)")
+        cur.execute("INSERT INTO t_uniq VALUES (?)", (1,))
+        with pytest.raises(informix_db.IntegrityError) as excinfo:
+            cur.execute("INSERT INTO t_uniq VALUES (?)", (1,))
+        e = excinfo.value
+        assert e.sqlcode == -268
+        assert "duplicate" in str(e).lower()
+
+
+def test_not_null_violation_is_integrity_error(conn_params: ConnParams) -> None:
+    """sqlcode -391 (NOT NULL violation) → IntegrityError."""
+    with _connect(conn_params) as conn:
+        cur = conn.cursor()
+        cur.execute("CREATE TEMP TABLE t_nn (id INTEGER NOT NULL)")
+        with pytest.raises(informix_db.IntegrityError) as excinfo:
+            cur.execute("INSERT INTO t_nn VALUES (?)", (None,))
+        assert excinfo.value.sqlcode == -391
+
+
+def test_commit_in_unlogged_db_is_operational_error(conn_params: ConnParams) -> None:
+    """sqlcode -255 (no transaction) → OperationalError."""
+    with _connect(conn_params) as conn:
+        with pytest.raises(informix_db.OperationalError) as excinfo:
+            conn.commit()
+        assert excinfo.value.sqlcode == -255
+
+
+def test_connection_survives_query_error(conn_params: ConnParams) -> None:
+    """Critical: a failed query must not poison the connection — subsequent queries must work."""
+    with _connect(conn_params) as conn:
+        cur = conn.cursor()
+        # First query fails
+        with pytest.raises(informix_db.ProgrammingError):
+            cur.execute("SELECT * FROM no_such_table")
+        # Second query works
+        cur.execute("SELECT 1 FROM systables WHERE tabid = 1")
+        assert cur.fetchone() == (1,)
+
+
+def test_error_has_structured_fields(conn_params: ConnParams) -> None:
+    """``sqlcode``, ``isamcode``, ``offset``, ``near`` are attached for inspection."""
+    with _connect(conn_params) as conn:
+        cur = conn.cursor()
+        with pytest.raises(informix_db.Error) as excinfo:
+            cur.execute("SELECT * FROM no_such_table")
+        e = excinfo.value
+        # All four structured attributes present
+        assert hasattr(e, "sqlcode")
+        assert hasattr(e, "isamcode")
+        assert hasattr(e, "offset")
+        assert hasattr(e, "near")
+        assert e.sqlcode == -206
+        assert e.near == "no_such_table"