fix(adk): return modified message from AfterChatModel #717

[hi-pender]

Summary

Enable AfterChatModel middleware modifications to affect the ReAct Agent processing flow, while keeping AgentEvent consistent with ChatModel's original output.

Problem

In ReAct Agent scenario, when AfterChatModel callback modifies state.Messages (e.g., adding/removing ToolCalls), it should affect the subsequent processing flow (like whether to call tools), but the AgentEvent should still reflect the original ChatModel output.

Solution

NoTools Scenario (chatmodel.go)

• AgentEvent is sent via callback with original ChatModel output
• StatePostHandler returns st.Messages[len(st.Messages)-1] (the potentially modified message)

ReAct Scenario (react.go)

• StatePostHandler returns st.Messages[len(st.Messages)-1] (the potentially modified message)
• This affects subsequent flow - e.g., if AfterChatModel removes ToolCalls, the tool node won't be called
• AgentEvent is still sent via callback with original ChatModel output
• If user wants to emit custom events for modifications, they can call SendEvent method

Changes

react.go

• Changed modelPostHandle return value from input to st.Messages[len(st.Messages)-1]
• This ensures AfterChatModel modifications affect subsequent nodes (tools, etc.)

chatmodel.go

• StatePostHandler returns original in (no change to output)
• Restored callback-based event emission for NoTools scenario

chatmodel_test.go

Added test cases:

• AfterChatModel_NoTools_ModifyDoesNotAffectEvent: Verifies NoTools scenario returns original ChatModel output
• AfterChatModel_ReAct_ModifyAffectsFlow: Verifies removing ToolCalls in AfterChatModel stops tool execution
• AfterChatModel_ReAct_AppendToolCall_AffectsFlow: Verifies adding ToolCalls in AfterChatModel triggers tool execution

Testing

All tests pass:

go test ./adk/...
ok      github.com/cloudwego/eino/adk
ok      github.com/cloudwego/eino/adk/filesystem
ok      github.com/cloudwego/eino/adk/middlewares/filesystem
ok      github.com/cloudwego/eino/adk/middlewares/reduction
ok      github.com/cloudwego/eino/adk/middlewares/skill
ok      github.com/cloudwego/eino/adk/prebuilt
ok      github.com/cloudwego/eino/adk/prebuilt/deep
ok      github.com/cloudwego/eino/adk/prebuilt/planexecute
ok      github.com/cloudwego/eino/adk/prebuilt/supervisor

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.44%. Comparing base (e685d86) to head (88cf943).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #792      +/-   ##
==========================================
- Coverage   80.47%   80.44%   -0.04%     
==========================================
  Files         129      129              
  Lines       13047    13047              
==========================================
- Hits        10500    10496       -4     
- Misses       1745     1748       +3     
- Partials      802      803       +1     

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