fix(llm_agent): add duck typing fallback for BaseLlm in bundled environments

[Alex980102]

Summary

This PR fixes an issue where custom BaseLlm subclasses fail to be recognized in bundled environments.

Fixes #36

Problem

When packages are bundled (e.g., with adk-devtools, webpack, esbuild), the BaseLlm class identity differs between the bundled code and external packages. This causes instanceof BaseLlm to return false for valid subclasses, resulting in:

Error: No model found for {agent_name}

Solution

Add a duck typing fallback in canonicalModel getter that checks for required BaseLlm properties when instanceof fails.

Testing

• Build passes
• All 150 tests pass

Related

• Fixes Bug: Custom BaseLlm subclasses fail with "No model found" in bundled environments #36
• Relates to Support for other LLM Models #23 (Support for other LLM Models)

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[kalenkevich]

I was thinking of another a bit more robust approach here using the symbols.

in the BaseLlm:

const BASE_MODEL_SYMBOL = Symbol('baseModel');

export function isBaseLlm(obj: unknown): obj is BaseLlm {
  return typeof obj === 'object' && obj !== null && BASE_MODEL_SYMBOL in obj && obj[BASE_MODEL_SYMBOL] === true;
}

/**
 * The BaseLLM class.
 */
export abstract class BaseLlm {
  readonly [BASE_MODEL_SYMBOL] = true;
  // ...
}

Then use the isBaseLlm function here:

Suggested change

	if (this.model instanceof BaseLlm) {
	return this.model;
	}
	// Duck typing fallback for bundled environments where instanceof fails.
	if (this.model && typeof this.model === 'object' &&
	'model' in this.model &&
	'generateContentAsync' in this.model &&
	typeof (this.model as BaseLlm).generateContentAsync === 'function') {
	return this.model as BaseLlm;
	}
	if (isBaseLlm(this.model)) {
	return this.model;
	}

[Alex980102]

Thanks for the feedback, @kalenkevich! I see the Symbol approach with isBaseLlm() is already implemented in main. Should I close this PR since the fix is already in place?

[kalenkevich]

Hello, sorry forgot to mention it here.

Yes please, close the PR. Thanks for your issue.