Fix send-side `MsgTypeError` rendering

Once `pformat_caller_frame()` renders successfully, the default
`_mk_send_mte()` path still fails while formatting the valid IPC
msg spec and then constructs its error message as a one-element
tuple.

Pass `MsgCodec` to `pformat_msgspec()`, keep the assembled message
a string and exercise the complete path through a printable
`MsgTypeError` regression.

Prompt-IO: ai/prompt-io/opencode/20260820T150250Z_9afda1c6_prompt_io.md

(this patch was generated in some part by `opencode` using
`gpt-5.6-sol` (`openai`))

ai/prompt-io/opencode/20260820T150250Z_9afda1c6_prompt_io.md

@@ -0,0 +1,33 @@
+---
+model: openai/gpt-5.6-sol
+service: opencode
+session: 7b9c97c4-fff7-4ac4-97fb-35720453308e
+timestamp: 2026-08-20T15:02:50Z
+git_ref: pformat_caller_frame_render_guard
+scope: code
+substantive: true
+raw_file: 20260820T150250Z_9afda1c6_prompt_io.raw.md
+---
+
+## Prompt
+
+Fix both newly exposed send-side `MsgTypeError` formatting failures
+and pin them with an end-to-end regression in PR #503.
+
+## Response summary
+
+Corrected codec-spec formatting and default error-message assembly so
+`_mk_send_mte()` returns a printable error instead of raising another
+formatter exception.
+
+## Files changed
+
+- `tractor/msg/_codec.py` - pass the codec to its supported formatter.
+- `tractor/_exceptions.py` - assemble the default message as `str`.
+- `tests/devx/test_pformat.py` - render the complete default error.
+
+## Human edits
+
+The human selected both one-line fixes and the single end-to-end test
+as coherent additions to PR #503, while leaving broader formatter
+cleanup out of scope.

ai/prompt-io/opencode/20260820T150250Z_9afda1c6_prompt_io.raw.md

@@ -0,0 +1,25 @@
+---
+model: openai/gpt-5.6-sol
+service: opencode
+timestamp: 2026-08-20T15:02:50Z
+git_ref: pformat_caller_frame_render_guard
+diff_cmd: git diff HEAD~1..HEAD
+---
+
+## Prompt
+
+After reviewing additional `tractor.devx.pformat` work suitable for
+PR #503, the user approved fixing both send-side `MsgTypeError`
+formatting failures and adding an end-to-end regression.
+
+## Response
+
+The generated code corrects the `MsgCodec.msg_spec_str` formatter
+input, keeps `_mk_send_mte()`'s assembled default message a string,
+and tests that the resulting `MsgTypeError` can be rendered:
+
+> `git diff HEAD~1..HEAD -- tractor/msg/_codec.py tractor/_exceptions.py tests/devx/test_pformat.py`
+
+These failures were hidden behind the original
+`pformat_caller_frame()` keyword error addressed by the first two
+commits on the branch.

tests/devx/test_pformat.py

@@ -6,10 +6,12 @@ from __future__ import annotations
 
 import pytest
 
+from tractor._exceptions import _mk_send_mte
 from tractor.devx.pformat import (
     pformat_boxed_tb,
     pformat_caller_frame,
 )
+from tractor.msg._codec import _def_tractor_codec
 
 
 @pytest.mark.parametrize(
@@ -50,3 +52,33 @@ def test_pformat_boxed_tb_rejects_unknown_kwargs():
             tb_str='doggy\n',
             indent='',
         )
+
+
+def test_send_mte_default_message_renders():
+    '''
+    The default send-side `MsgTypeError` must remain printable.
+
+    Once `pformat_caller_frame()` stopped failing first, this path
+    exposed two more formatter errors: `MsgCodec.msg_spec_str` passed
+    a type union where `pformat_msgspec()` requires a codec/decoder,
+    then `_mk_send_mte()` wrapped its message in a one-element tuple.
+
+    Construct the error without an override message to execute that
+    complete default path. Requiring a `str` message with the bad
+    value and valid spec, then rendering the exception, proves the
+    original IPC violation survives every formatter layer.
+
+    '''
+    bad_msg: dict[str, bool] = {'bad': True}
+    mte = _mk_send_mte(
+        msg=bad_msg,
+        codec=_def_tractor_codec,
+    )
+
+    assert isinstance(mte.message, str)
+    assert f'invalid msg -> {bad_msg}' in mte.message
+    assert 'Valid IPC msgs are:' in mte.message
+
+    report: str = repr(mte)
+    assert 'MsgTypeError' in report
+    assert f'invalid msg -> {bad_msg}' in report

tractor/_exceptions.py

@@ -1509,7 +1509,7 @@ def _mk_send_mte(
             f'invalid msg -> {msg}: {type(msg)}\n\n'
             f'{tb_fmt}\n'
             f'Valid IPC msgs are:\n\n'
-            f'{codec.msg_spec_str}\n',
+            f'{codec.msg_spec_str}\n'
         )
     elif src_type_error:
         src_message: str = str(src_type_error)

tractor/msg/_codec.py

@@ -430,7 +430,7 @@ class MsgCodec(Struct):
     # wrapped field over the `.msg_spec` one?
     @property
     def msg_spec_str(self) -> str:
-        return pformat_msgspec(self.msg_spec)
+        return pformat_msgspec(self)
 
     lib: ModuleType = msgspec