Python: make SerializationMixin.from_dict enforce the documented type check

[Yufeng He (he-yufeng)]

Motivation & Context

SerializationMixin.from_dict documents that a mismatched type field raises ValueError, but the guard could never fire: it resolved the expected identifier via cls._get_type_identifier(value), and that helper *prefers the payload's own type field when one is present. Any supplied type therefore "matched" itself, and a payload like {"type": "function_tool", "role": "user", "contents": []} silently deserialized into a Message, with the wrong type baked in until the next to_dict() rewrote it.

Description & Review Guide

• What are the major changes?
  Resolve the expected identifier from the class (no payload), which is also what to_dict() writes, so the mismatch guard now works as documented: type_id = cls._get_type_identifier().
• What is the impact of these changes?
  In every previously valid case the class-resolved identifier is the same string the payload carried, so well-formed payloads (including no-type payloads and dependencies= injection, which keys on the same id) are unchanged. Classes that do polymorphic deserialization (Content, the file-access harness entries) override from_dict themselves and are unaffected.
• What do you want reviewers to focus on?
  The two behavioral edges: payloads that omit type entirely, and the polymorphic subclasses that override from_dict.

Related Issue

Fixes #7255

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR title and description follow the guidelines
• Documentation has been updated if needed (not needed: docstring already documents the raised ValueError)

Two new tests in test_serializable_mixin.py: mismatched type raises on both from_dict and from_json. Full file passes (28/28). pytest packages/core/tests/core is green except test_feature_stage.py::test_feature_id_allows_lowercase_values, which fails identically on a clean main checkout (pre-existing, unrelated). The exact repro from the issue now raises Type mismatch: expected 'message', got 'function_tool'.

[Copilot]

Pull request overview

This PR fixes a correctness issue in the Python core serialization layer where SerializationMixin.from_dict() could not actually enforce its documented “type mismatch raises ValueError” behavior, allowing payloads with an incorrect "type" discriminator to deserialize into the wrong class (Fixes #7255).

Changes:

• Make SerializationMixin.from_dict() derive the expected type identifier from the target class rather than the incoming payload, so the mismatch guard can trigger.
• Add regression tests ensuring mismatched "type" is rejected for both from_dict() and from_json().

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
python/packages/core/agent_framework/_serialization.py	Fixes the mismatch check by resolving the expected type identifier from the class instead of the payload.
python/packages/core/tests/core/test_serializable_mixin.py	Adds tests verifying mismatched "type" raises for both dict and JSON inputs.

[github-actions]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
packages/core/agent_framework
_serialization.py	173	12	93%	355–356, 561, 571, 636, 639, 679–680, 684–685, 687, 689
TOTAL	46102	4479	90%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
9547	34 💤	0 ❌	0 🔥	2m 29s ⏱️

[Eduard van Valkenburg (eavanvalkenburg)]

please make sure to use the defined PR template Yufeng He (@he-yufeng)

[Yufeng He (he-yufeng)]

Done, body rewritten to the defined template (Motivation & Context / Description & Review Guide / Related Issue / Contribution Checklist), with the test evidence folded into the checklist section.