[Bug]:Unstable computeHash causes unnecessary MysqlSession rewrites

[luowanghaoyun]

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Describe the bug
MysqlSession.save(SessionKey, String, List<? extends State>) may trigger unnecessary full list rewrites for List<Msg> even when messages are logically identical.
ListHashUtil.computeHash() relies on element hashCode(). For nested message content, semantically equivalent lists can still produce different hashes, causing needsFullRewrite(...) to evaluate to true.

To Reproduce
Steps to reproduce the behavior:

1. Use two newly created List<Msg> instances with logically identical messages:
   • user: "hello"
   • assistant: "hello"
2. Ensure all visible fields are set identically (e.g., same id, timestamp, role, and text content), then call:
   • ListHashUtil.computeHash(firstList)
   • ListHashUtil.computeHash(secondList)
3. Compare the two hash strings.

Minimal example:

List<Msg> first = List.of(
    Msg.builder().id("m-user-1").timestamp("2026-05-08 14:00:00.000")
        .role(MsgRole.USER).content(TextBlock.builder().text("hello").build()).build(),
    Msg.builder().id("m-assistant-1").timestamp("2026-05-08 14:00:01.000")
        .role(MsgRole.ASSISTANT).content(TextBlock.builder().text("hello").build()).build()
);

List<Msg> second = List.of(
    Msg.builder().id("m-user-1").timestamp("2026-05-08 14:00:00.000")
        .role(MsgRole.USER).content(TextBlock.builder().text("hello").build()).build(),
    Msg.builder().id("m-assistant-1").timestamp("2026-05-08 14:00:01.000")
        .role(MsgRole.ASSISTANT).content(TextBlock.builder().text("hello").build()).build()
);

String h1 = ListHashUtil.computeHash(first);
String h2 = ListHashUtil.computeHash(second);

Expected behavior
For logically equivalent message lists, computeHash should be stable and equal, so needsFullRewrite(...) does not force delete + rewrite.

Error messages
No runtime exception in normal flow.
Observed behavioral mismatch in hash values for equivalent lists, e.g.:

• h1 = de9291ea
• h2 = 68bb1c12

This leads to unnecessary full rewrites (DELETE + INSERT all) in session persistence paths.

Environment (please complete the following information):

• AgentScope-Java Version: 1.0.13-SNAPSHOT
• Java Version: 17
• OS: macOS

Additional context

• Related files:
  • agentscope-core/src/main/java/io/agentscope/core/session/ListHashUtil.java
  • agentscope-extensions/agentscope-extensions-session-mysql/src/main/java/io/agentscope/core/session/mysql/MysqlSession.java
  • agentscope-core/src/main/java/io/agentscope/core/state/State.java
  • agentscope-core/src/main/java/io/agentscope/core/message/Msg.java

[Alexxigang]

I traced this to ListHashUtil.computeHash() relying on element hashCode() for nested session state. I'm going to work on a stable content-based hashing path here and add regression coverage around logically equivalent Msg lists so unnecessary MysqlSession rewrites stop happening.

[Alexxigang]

Opened PR #1395 with the fix and regression test: #1395