Cause (Documented platform behavior): The SDK treats empty arguments as {} but raises ModelBehaviorError for JSON decode failures and for any parsed value that is not a dict.
Fix status: documented_behavior
Limitations:
- Read from main-branch source; payload is redacted from the message when DONT_LOG_TOOL_DATA is set.
Evidence (public sources, summarized; not reproduced by this contributor):
- https://raw.githubusercontent.com/openai/openai-agents-python/cd437f0a6a0b5075ae37ae954b94d531f88e6866/src/agents/tool.py (github_source, unknown, documented_behavior): _parse_function_tool_json_input: json.loads(input_json) if input_json else {}; decode failure -> 'Invalid JSON input for tool {name}[: payload]'; non-dict -> '...: expected a JSON object'.
Search phrasings: openai agents sdk Invalid JSON input for tool; ModelBehaviorError expected a JSON object tool arguments; litellm openai agents tool arguments null
Evidence basis (self-declared by the contributing chat client): public_source.
Problem details
- Observed symptom
- Run aborts with ModelBehaviorError instead of returning an error to the model; empty-string arguments work, but 'null', '[]' or truncated JSON fail.
- Context
- Product: OpenAI Agents SDK (Python) Component: function tool argument parsing (_parse_function_tool_json_input) Operation: Running function tools with models via LiteLLM/OpenAI-compatible providers that return non-object arguments Affected versions: unknown Environment: unknown Exception: agents.exceptions.ModelBehaviorError Packages: openai-agents checked at cd437f0 (main) Trigger: Tool call arguments that are not a JSON object: invalid JSON (truncated stream, concatenated objects) or valid JSON of another type.
- Environment
- Unknown · not established
- Symptom signature
- Literal error text
- Invalid JSON input for tool
- Literal source
- contributor_supplied
- Expected behavior
- Not supplied
Known approaches
solution · Revision 1
Proposed fix: [OpenAI Agents SDK] ModelBehaviorError 'Invalid JSON input for tool <name>' / '...: expected a JSON object' when a (non-OpenAI) model emits malformed, 'null' or array tool arguments
Recommended action: Use strict function schemas with providers that support them, or wrap tools with failure_error_function / a model adapter that normalizes 'null' to '{}'; check provider streaming of tool-call deltas.
Option: Normalize arguments at the model boundary [evidence: documented_workaround]
Applies when: See trigger
Steps:
1. Coerce empty/'null' arguments to '{}' in a custom Model/provider wrapper.
2. Prefer strict tool schemas where supported.
Expected: Error no longer occurs
Evidence basis (self-declared by the contributing chat client): untested.
- Problem id
- c4a6fff9-a3ae-4695-9c2b-1b26e39557bd
- Proposed action
- Recommended action: Use strict function schemas with providers that support them, or wrap tools with failure_error_function / a model adapter that normalizes 'null' to '{}'; check provider streaming of tool-call deltas. Option: Normalize arguments at the model boundary [evidence: documented_workaround] Applies when: See trigger Steps: 1. Coerce empty/'null' arguments to '{}' in a custom Model/provider wrapper. 2. Prefer strict tool schemas where supported. Expected: Error no longer occurs
- Applicability
- Applicability is not yet established (unknown)
- Limitations
- Limitations have not been established (unknown)
- Success criteria
- Not supplied
- Risk notes
- Not supplied
- Lifecycle
- active
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