[BUG] CallbackHandler not being invoked with `Agent.structured_output`

[sromoam]

Checks

• I have updated to the lastest minor and patch version of Strands
• I have checked the documentation and this is not expected behavior
• I have searched ./issues and there are no duplicates of my issue

Strands Version

1.7.1

Python Version

3.12.11

Operating System

15.6.1

Installation Method

pip

Steps to Reproduce

from pydantic import BaseModel, Field
from strands.models import BedrockModel
import json
from strands import Agent

class ContactCard(BaseModel):
    name: str = Field(description="The name of the person")
    age: int = Field(description="The age of the person")
    address: str = Field(description="The address of the person")

# Create BedrockModel with custom timeout configuration
bedrock_model = BedrockModel(
    model_id="us.anthropic.claude-sonnet-4-20250514-v1:0",
    region_name='us-west-2',
    additional_request_fields= {
        "thinking": {
            "type": "enabled", 
            "budget_tokens": 2048
        }
    }
)
# Let's prep the images for input to our models. 

system_prompt = "you are a helpful document understanding agent. You parse documetns into structured data"


events = []
## Let's createa a quick event callback so we can grab token usage. 
def grab_event_callback(**kwargs):
    # Print the values in kwargs so that we can see everything
    events.append(kwargs)

# Create an agent with initial messages
agent = Agent(
    model = bedrock_model, 
    system_prompt=system_prompt,
    callback_handler=grab_event_callback
    )


cc = agent.structured_output(ContactCard, "Convert the following data to a contact card <input> John, last name Smith, who resides at 123 anytown usa, Texas, 78756, and is of age 38</input>")


print(events) #Nothing populated here
print(cc.model_dump_json(indent = 2))

In the above example, the value of events, which should be populated with intermediate messages from the agent execution, is []

Expected Behavior

I expect the events object to be populated with all events from the execution. I want this so I can account for token usage when calling Agent.structued_output.
I found that I could see events when adding a print statement inthis for loop:

sdk-python/src/strands/agent/agent.py

Lines 516 to 518 in 9213bc5

	async for event in events:
	if "callback" in event:
	self.callback_handler(**cast(dict, event["callback"]))

None of the events have the callback key.

Actual Behavior

The events list is empty. This changed after upgrading from 1.5.x to 1.7.x

Additional Context

No response

Possible Solution

I believe that we should make the following enhancements to the structured_output functinality.

• Return the result as an AgentResult with an additional value that includes the object
• Do this so the user can access the token usage.
• Provide a backwards compatible API for users that want to continue to use the structured_output format as currently implemented
• Do enable the user callback to be enacted
• Enhance the testing to include checking for the circular reference.

Related Issues

#810, #695, #670

[zastrowm]

Can confirm that this broke when we switched over to TypedEvents

[zastrowm]

@sromoam how much do you care about the events here vs the metrics? We have a tracking issue for metrics specifically (#335) which I think solves that issue.

In re-implementing this I'm not 100% convinced the callback should be should be fired in this case - the implementation is very provider specific and is more of an implementation detail - for instance, if bedrock every added an api for structured_output, we'd most likely switch to that changing what events would be emitted.

[sromoam]

@zastrowm my main goal is simplifying figuring out how many tokens were consumed; that was the point of the callback in the first place, and if you've got a fix that will expose the metrics such as usage, and steps, then I'm ok to wait for the fix proposed in #335 .

[zastrowm]

Fixed as part of #857