fix: validate agent implements stream() in AgentNode constructor

[jsamuel1]

Problem

Passing an object that doesn't implement InvokableAgent.stream() to AgentNode (or indirectly via Graph/Swarm node definitions) produces a cryptic runtime error:

_ is not a function

This happens because _resolveNodes() casts unrecognized objects to InvokableAgent without validation, and the error only surfaces later when AgentNode.handle() calls this._agent.stream().

Solution

Add an early check in AgentNode constructor that verifies the agent has a stream() method. The error message includes the node id for easier debugging:

node_id=<marge> | agent does not implement stream(). Ensure the object passed as an agent has a stream() method.

Testing

• Added unit test for the validation
• All existing tests pass (1633 tests, 0 failures)

[JiwaniZakir]

The validation in nodes.ts checks typeof agent.stream !== 'function' before calling super(), which is correct, but it then references agent.id in the error message without first verifying that agent.id is defined. If a caller passes an object with neither stream nor id, the error reads node_id=<undefined>, which is less helpful than it could be. A simple fallback like agent.id ?? 'unknown' would make the message more robust.

The test in nodes.test.ts is placed before the beforeEach block, which is unconventional — it inverts the typical setup-then-test structure and could confuse readers scanning the file. Moving it after beforeEach (or into a dedicated describe('constructor validation', ...) block) would be cleaner.

The test itself only covers the case where id is present but stream is absent. Adding a case where neither property exists would exercise the agent.id undefined path mentioned above and ensure the error message degrades gracefully in that scenario.

[jsamuel1]

Thanks @JiwaniZakir — all three points addressed:

• Added agent.id ?? 'unknown' fallback so the error message degrades gracefully
• Moved the test into a describe('constructor') block after beforeEach
• Added a test case for when neither stream nor id exists

[JiwaniZakir]

The fix is straightforward and the error message including node_id is a nice touch for debuggability. One consideration: since stream could technically be a non-function property, the check should use typeof this._agent.stream === 'function' rather than just checking existence — worth confirming that's what the implementation does. Otherwise this is a clean improvement over the silent runtime failure.

[zastrowm]

@jsamuel1 out of curiosity, how did you encounter this? Are you using JS directly?

[github-actions]

Assessment: Approve

Clean, focused bug fix that improves developer experience by catching invalid AgentNode inputs early with a clear error message instead of the cryptic "_ is not a function" error.

Review Summary

• Code Quality: Implementation correctly uses typeof agent.stream !== 'function' and error message follows the SDK logging format
• Testing: Two test cases properly cover both scenarios (with/without agent id), following repository patterns
• Documentation: Not required for this bug fix

All prior review feedback has been addressed. Nice improvement for debuggability 👍

[JiwaniZakir]

The duplicate PR (#524) being stuck in review for 1.5 months is worth resolving before merging this one — either close #524 in favor of this or consolidate. On the implementation, the typeof this._agent.stream === 'function' check @JiwaniZakir flagged is the correct approach and worth confirming is in place, since a property named stream with a non-function value would otherwise pass the guard and still produce a runtime error.

[zastrowm]

The duplicate PR (#524) being stuck in review for 1.5 months is worth resolving before merging this one — either close #524 in favor of this or consolidate.

That PR does not seem like a duplicate of this one

... @JiwaniZakir ...

That that you're calling out your own comment in the third person make me feel like this is an automated response. While we appreciate feedback from the community, we do have our own review agents and would like human feedback instead of agentic; please keep that in mind.

[strands-agent]

This repository has been merged into the strands-agents/harness-sdk monorepo and will be archived shortly. All new development happens there.

If this PR is still relevant, please recreate it against the monorepo. The code now lives under strands-ts/. Full commit history was preserved, so your base should be findable.

Apologies for the disruption, and thank you for contributing!