[Bug] Parallel invocations to the same Agent instance corrupt message history

[zastrowm]

Problem

Currently, if an invocation occurs while another one is being processed, the internal global state of the agent becomes corrupted, causing not only the concurrent invocations to fail, but also any subsequent invocations to fail.

A common scenario affected by this behavior is when a retry occurs due to client timeout, which triggers another invocation with the same session ID while the previous request is still being processed.

Proposed Solution

Throw a concurrency exception instead of handling the request and corrupting the agent's state, which causes all subsequent calls to fail.

Implementation Requirements

Based on the clarification discussion and repository analysis, this fix requires the following changes:

1. Add ConcurrencyException

File: src/strands/types/exceptions.py

Add a new exception class:

class ConcurrencyException(Exception):
    """Exception raised when concurrent invocations are attempted on an agent instance.
    
    Agent instances maintain internal state that cannot be safely accessed concurrently.
    This exception is raised when an invocation is attempted while another invocation
    is already in progress on the same agent instance.
    """
    pass

2. Guard Agent Invocation Methods

File: src/strands/agent/agent.py

Changes required:

1. Add import asyncio at the top

2. Add lock instance variable in __init__():

   self._invocation_lock = asyncio.Lock()

3. Guard stream_async() method (line ~521):

   • Check if lock is already acquired BEFORE modifying any state
   • Raise ConcurrencyException if locked
   • Acquire lock using async context manager

   Pattern:

   async def stream_async(self, ...):
       if self._invocation_lock.locked():
           raise ConcurrencyException("Agent is already processing a request. Concurrent invocations are not supported.")
       
       async with self._invocation_lock:
           # existing implementation

Rationale:

• Guarding stream_async() protects all invocation paths:
  • __call__() → invoke_async() → stream_async()
  • invoke_async() → stream_async()
  • structured_output() → structured_output_async() → invoke_async() → stream_async()
  • Direct calls to stream_async()

Important: Consider the event loop concern raised in PR #1205 comments: __call__() uses run_async() which creates an isolated event loop. The asyncio.Lock instance created in __init__ may need special handling to work across different event loops. Consider lazy initialization or thread-local storage if issues arise.

3. Guard Direct Tool Invocations

File: src/strands/tools/_caller.py

Changes required:
Guard the caller() function inside __getattr__() (around line 54-116):

• Check if agent's invocation lock is acquired
• Raise ConcurrencyException if locked
• Acquire lock during tool execution

Pattern:

def caller(user_message_override: str | None = None, ...):
    if self._agent._interrupt_state.activated:
        raise RuntimeError("cannot directly call tool during interrupt")
    
    # Add concurrency check
    async def check_and_execute():
        if self._agent._invocation_lock.locked():
            raise ConcurrencyException("Agent is already processing a request. Concurrent invocations are not supported.")
        
        async with self._agent._invocation_lock:
            # existing acall() implementation
            ...
    
    tool_result = run_async(check_and_execute)
    ...

4. Testing Requirements

Unit Tests

File: tests/strands/agent/test_agent.py

Add tests for concurrent invocations on all methods:

1. Test invoke_async() concurrent calls (can reference PR Concurrent invocations protection strands-agents/sdk-python#1205's test)
2. Test stream_async() concurrent calls
3. Test structured_output_async() concurrent calls
4. Test __call__() concurrent calls (sync interface)
5. Test structured_output() concurrent calls (sync interface)

Pattern from PR strands-agents#1205:

@pytest.mark.asyncio
async def test_agent_parallel_invocations():
    model = MockedModelProvider([{"role": "assistant", "content": [{"text": "hello!"}]}])
    agent = Agent(model=model)

    async with agent._invocation_lock:
        with pytest.raises(ConcurrencyException, match="Concurrent invocations are not supported"):
            await agent.invoke_async("test")

Direct Tool Call Tests

File: tests/strands/tools/test_caller.py (create if needed) or add to existing caller tests

Add test for concurrent direct tool calls:

def test_direct_tool_call_concurrent_invocations():
    # Similar pattern to agent tests but for agent.tool.tool_name()

Integration Test

File: tests_integ/test_stream_agent.py

Add integration test with real threading (can reference PR strands-agents#1205's integration test):

• Use threading to invoke agent concurrently
• First thread should succeed
• Second thread should raise ConcurrencyException
• Includes retry scenario mentioned in issue

5. Files to Modify Summary

1. src/strands/types/exceptions.py - Add ConcurrencyException
2. src/strands/agent/agent.py - Add lock and guard stream_async()
3. src/strands/tools/_caller.py - Guard direct tool calls
4. tests/strands/agent/test_agent.py - Unit tests for all invocation methods
5. tests/strands/tools/test_caller.py - Unit tests for direct tool calls (may need to create)
6. tests_integ/test_stream_agent.py - Integration test with threading

Acceptance Criteria

• ConcurrencyException defined in src/strands/types/exceptions.py
• stream_async() guarded with lock check and acquisition
• Direct tool calls guarded with lock check and acquisition
• Unit tests pass for all invocation methods (5 methods)
• Unit tests pass for direct tool calls
• Integration test passes with real threading
• Exception is raised BEFORE any state modification
• Lock is released properly even on exceptions
• All existing tests continue to pass
• Code follows repository style guidelines (type hints, docstrings, logging)

Reference Material

• PR Concurrent invocations protection strands-agents/sdk-python#1205: Concurrent invocations protection strands-agents/sdk-python#1205
  • Shows initial implementation approach (asyncio.Lock in invoke_async)
  • Includes test patterns
  • Has outstanding comments about incomplete coverage
• Issue [BUG] Agent permanent stuck in ValidationException with Bedrock Converse : The number of toolResult blocks at messages.{id}.content exceeds the number of toolUse blocks of previous turn strands-agents/sdk-python#1435: [BUG] Agent permanent stuck in ValidationException with Bedrock Converse : The number of toolResult blocks at messages.{id}.content exceeds the number of toolUse blocks of previous turn strands-agents/sdk-python#1435
  • Mentioned the need to guard direct method invocations

Notes

• This is a bug fix, no backwards compatibility needed
• Exception should be thrown BEFORE modifying any internal state, so no cleanup is necessary
• The guard is per-agent instance, not per-session ID
• No need for waiting/locking behavior now - fail-fast only
• Consider event loop safety when using asyncio.Lock across run_async() calls

[zastrowm]

/strands

[github-actions]

Repository Analysis & Clarifying Questions

Repository Analysis

I've analyzed the codebase to understand the concurrent invocation issue. Here's what I found:

Affected Methods

Based on the code structure in src/strands/agent/agent.py, the following methods use shared instance state and are vulnerable to concurrent invocation corruption:

1. __call__() (line 317) - Synchronous invocation via run_async()
2. invoke_async() (line 358) - Async invocation
3. stream_async() (line 521) - Core async streaming method (called by others)
4. structured_output() (line 400) - Sync structured output
5. structured_output_async() (line 431) - Async structured output
6. Direct tool calls via agent.tool.tool_name() (in src/strands/tools/_caller.py)

Vulnerable Shared State

All these methods share the following instance variables:

• self.messages - Conversation history
• self.trace_span - OpenTelemetry tracing span
• self.event_loop_metrics - Metrics collection
• self._interrupt_state - Interrupt handling state
• self.state - Agent state object
• self.conversation_manager - Conversation history manager

Current Implementation

• No concurrency guards or locks exist in agent.py
• No ConcurrencyException or similar exception type defined
• No existing tests for concurrent invocations
• The __call__() method wraps async operations using run_async()

---

Clarifying Questions

1. Approach Confirmation

The proposed solution is to throw a concurrency exception. Should we:

• a) Throw exception immediately (fail-fast)
• b) Use a lock and wait (could cause client timeouts)
• c) Provide both options via configuration flag?

My recommendation is (a) based on the issue description, but wanted to confirm.

2. Method Coverage

Which methods need guarding? Should we guard:

• All 5 invocation methods listed above?
• Only the core stream_async() method (which others call)?
• Direct tool calls via agent.tool.tool_name() (as suggested by issue [BUG] Agent permanent stuck in ValidationException with Bedrock Converse : The number of toolResult blocks at messages.{id}.content exceeds the number of toolUse blocks of previous turn strands-agents/sdk-python#1435)?

3. Guard Implementation Level

Where should the guard be implemented:

• a) Only in stream_async() (core method)?
• b) In each public method individually?
• c) Both levels for defense in depth?

4. Direct Tool Invocations

The issue mentions guarding direct method invocations based on issue strands-agents#1435:

• Should agent.tool.tool_name() calls be guarded?
• Do they share the same concurrency protection semantics?
• Should they throw the same exception type?

5. Exception Design

For the new exception:

• What should it be named? (ConcurrencyException, AgentBusyException, ConcurrentInvocationException)?
• Should it be added to src/strands/types/exceptions.py?
• What information should it include in the error message?
  • Session ID?
  • Duration of the blocking invocation?
  • Whether it's a retry scenario?

6. Session ID Semantics

The issue mentions "same session ID" - should the guard be:

• a) Per-agent instance (any concurrent call fails)?
• b) Per-session ID (only calls with same session fail)?
• c) Configurable?

7. Testing Requirements

What tests should be included:

• Unit tests for concurrent invocations throwing exceptions?
• Integration tests?
• Tests for direct tool call concurrency?
• Tests for the retry scenario mentioned in the issue?
• Tests for all guarded methods?

8. Backwards Compatibility

This changes behavior from silent corruption to explicit exception:

• Should there be a deprecation period or feature flag?
• Should this be configurable (opt-in/opt-out)?
• What's the communication plan for users?

9. Edge Cases

How should we handle:

• Nested invocations (agent calling another agent)?
• Multi-agent scenarios (swarm, graph patterns)?
• Cleanup when exception is thrown?
• Stream interruption mid-execution?

10. PR strands-agents#1205 Context

The issue references PR #1205 from a different repository:

• Can you provide context on what was addressed there?
• What specific comments need to be addressed?
• Were there specific implementation patterns suggested?
• Is that PR's code available for reference?

---

Please let me know your preferences on these questions, and I'll update the issue with a comprehensive implementation plan.

[github-actions]

I've completed my analysis of issue #22 regarding concurrent invocations corrupting agent message history.

What I've Done

I've analyzed the repository and identified:

1. All affected methods: __call__(), invoke_async(), stream_async(), structured_output(), structured_output_async(), and direct tool calls via agent.tool.tool_name()

2. Vulnerable shared state: Including messages, trace_span, event_loop_metrics, _interrupt_state, and more

3. Current gaps: No concurrency guards, no relevant exception types, no tests for concurrent scenarios

Next Steps

I've posted a detailed comment on issue #22 with:

• My complete repository analysis
• 10 clarifying questions covering:
  • Implementation approach (fail-fast vs locking)
  • Method coverage scope
  • Exception design
  • Session ID semantics
  • Testing requirements
  • Backwards compatibility
  • Edge cases
  • Context from PR Concurrent invocations protection strands-agents/sdk-python#1205

Please review and respond to the clarifying questions on the issue so I can finalize the implementation requirements and update the issue description with a comprehensive task specification.

The questions are grouped and wrapped in a collapsible <details> block for easy navigation.

[zastrowm]

/strands

Clarifying Questions

1. Approach Confirmation

The proposed solution is to throw a concurrency exception. Should we:

• a) Throw exception immediately (fail-fast)
• b) Use a lock and wait (could cause client timeouts)
• c) Provide both options via configuration flag?

My recommendation is (a) based on the issue description, but wanted to confirm.

Throw for now; we can introduce an option for B later

2. Method Coverage

Which methods need guarding? Should we guard:

• All 5 invocation methods listed above?
• Only the core stream_async() method (which others call)?
• Direct tool calls via agent.tool.tool_name() (as suggested by issue [[BUG] Agent permanent stuck in ValidationException with Bedrock Converse : The number of toolResult blocks at messages.{id}.content exceeds the number of toolUse blocks of previous turn strands-agents/sdk-python#1435]([BUG] Agent permanent stuck in ValidationException with Bedrock Converse : The number of toolResult blocks at messages.{id}.content exceeds the number of toolUse blocks of previous turn strands-agents/sdk-python#1435))?

We need all methods guarded, but if we can do that just by calling stream_async, that's fine

3. Guard Implementation Level

Where should the guard be implemented:

• a) Only in stream_async() (core method)?
• b) In each public method individually?
• c) Both levels for defense in depth?

If we're sure stream_async is called everywhere, then fine. If not, then we we should guard each invidivudal. Start with tests for each and if the implementation can be solved in stream_async , then fine.

Ensure that we're locked before modifying an internal state though

4. Direct Tool Invocations

The issue mentions guarding direct method invocations based on issue [strands-agents#1435](strands-agents#1435):

• Should agent.tool.tool_name() calls be guarded?
• Do they share the same concurrency protection semantics?
• Should they throw the same exception type?

Throw the same type. Yes

5. Exception Design

For the new exception:

• What should it be named? (ConcurrencyException, AgentBusyException, ConcurrentInvocationException)?

• Should it be added to src/strands/types/exceptions.py?

• What information should it include in the error message?

  • Session ID?
  • Duration of the blocking invocation?
  • Whether it's a retry scenario?

ConcurrencyException

6. Session ID Semantics

The issue mentions "same session ID" - should the guard be:

• a) Per-agent instance (any concurrent call fails)?
• b) Per-session ID (only calls with same session fail)?
• c) Configurable?

Per-agent instance

7. Testing Requirements

What tests should be included:

• Unit tests for concurrent invocations throwing exceptions?
• Integration tests?
• Tests for direct tool call concurrency?
• Tests for the retry scenario mentioned in the issue?
• Tests for all guarded methods?

Yes:

• Unit tests for concurrent invocations throwing exceptions?
• Tests for direct tool call concurrency?
• Tests for all guarded methods?

8. Backwards Compatibility

This changes behavior from silent corruption to explicit exception:

• Should there be a deprecation period or feature flag?
• Should this be configurable (opt-in/opt-out)?
• What's the communication plan for users?

For now, just add the exception. NO backwards compatibility as this is a bug

9. Edge Cases

How should we handle:

• Nested invocations (agent calling another agent)?
• Multi-agent scenarios (swarm, graph patterns)?
• Cleanup when exception is thrown?
• Stream interruption mid-execution?

We don't have to worry. An exception should be thrown before we modify anything so no clean up should be necesary

10. PR [strands-agents#1205](strands-agents#1205) Context

The issue references [PR #1205](strands-agents#1205) from a different repository:

• Can you provide context on what was addressed there?
• What specific comments need to be addressed?
• Were there specific implementation patterns suggested?
• Is that PR's code available for reference?

Please let me know your preferences on these questions, and I'll update the issue with a comprehensive implementation plan.

Take a look at https://github.com/strands-agents/sdk-python/pull/1205/changes

[github-actions]

Task Refinement Complete - Ready for Implementation

Summary

This issue has been refined and is now ready for implementation. The issue description has been updated with comprehensive implementation requirements based on:

1. Repository code analysis
2. Clarifying questions and your responses
3. Review of PR Concurrent invocations protection strands-agents/sdk-python#1205 and its outstanding comments
4. Identification of all affected code paths

What Was Accomplished

Code Analysis

• Identified all invocation methods that need guarding
• Analyzed call graph to determine optimal guard placement
• Found that guarding stream_async() protects most paths
• Identified that direct tool calls need separate guarding
• Discovered event loop safety concern from PR Concurrent invocations protection strands-agents/sdk-python#1205 comments

Implementation Strategy

• Exception: Add ConcurrencyException to src/strands/types/exceptions.py
• Guard Location: stream_async() in agent.py (protects all invocation methods)
• Direct Tool Calls: Guard in _ToolCaller in _caller.py
• Behavior: Fail-fast (check if locked, throw exception before state modification)
• Scope: Per-agent instance (not per-session)

Testing Plan

• Unit tests for all 5 invocation methods
• Unit tests for direct tool calls
• Integration test with real threading (retry scenario)
• Can reference PR Concurrent invocations protection strands-agents/sdk-python#1205's test patterns

Files to Modify

1. src/strands/types/exceptions.py - New exception
2. src/strands/agent/agent.py - Add lock, guard stream_async
3. src/strands/tools/_caller.py - Guard tool calls
4. tests/strands/agent/test_agent.py - Unit tests
5. tests/strands/tools/test_caller.py - Tool call tests
6. tests_integ/test_stream_agent.py - Integration test

Key Implementation Notes

1. Event Loop Safety: PR Concurrent invocations protection strands-agents/sdk-python#1205 comment raised concern about asyncio.Lock usage across different event loops created by run_async(). This needs to be considered in the implementation.

2. Guard Before State Modification: The lock check must happen BEFORE any internal state is modified to prevent corruption.

3. Outstanding from PR Concurrent invocations protection strands-agents/sdk-python#1205: The original PR only guarded invoke_async(), missing stream_async(), structured_output*(), and direct tool calls. This implementation addresses all paths.

4. No Cleanup Needed: Since exception is thrown before state modification, no cleanup is required on exception.

Ready for Implementation

The issue now contains:

• ✅ Clear problem statement
• ✅ Comprehensive implementation requirements
• ✅ Code patterns and examples
• ✅ Complete testing strategy
• ✅ Acceptance criteria
• ✅ Reference materials (PR Concurrent invocations protection strands-agents/sdk-python#1205, Issue [BUG] Agent permanent stuck in ValidationException with Bedrock Converse : The number of toolResult blocks at messages.{id}.content exceeds the number of toolUse blocks of previous turn strands-agents/sdk-python#1435)

A developer can now implement this fix with clear guidance on what needs to be changed, where, and how to test it.

[zastrowm]

/strands implement

[github-actions]

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[github-actions]

Implementation Complete ✅

I've successfully implemented concurrency protection for the Agent class to prevent parallel invocations from corrupting message history.

What Was Done

Core Changes:

• Added ConcurrencyException to handle concurrent invocation detection
• Protected Agent.stream_async() with threading.Lock (chosen over asyncio.Lock due to run_async() creating isolated event loops)
• Protected direct tool calls via agent.tool.tool_name()
• Exception raised immediately before any state modification

Testing:

• 8 unit tests covering all invocation methods (async and sync paths)
• 2 integration tests with real threading scenarios
• All 229 agent tests passing ✅
• Code formatted with ruff, type-checked with mypy

Git Status:

• Commit created: 6194f24 with conventional commit message
• Pull request prepared (deferred - will be created when sandbox completes)
• Branch: agent-tasks/22

Ready for Review

The implementation is complete and all tests pass. The PR will be created automatically with a detailed description following the repository's PR guidelines.

Would you like me to make any adjustments before the PR is finalized?