Cause (Documented platform behavior): ChatAdapter automatically falls back to JSONAdapter; the surfaced error is from the fallback. In 2.6.x the fallback was triggered for all exceptions (issue #7843); current main only falls back on AdapterParseError and lets config/engine errors propagate.
Fix status: fixed_upstream (fixed in unknown (current main))
Workaround (not a fix): dspy.configure(adapter=dspy.ChatAdapter(use_json_adapter_fallback=False))
Misleading approaches:
- Debugging JSON mode on the provider when the real failure happened in the first ChatAdapter attempt.
Limitations:
- Bedrock json_tool_call envelope parsing (#8264) not confirmed fixed.
Unknowns:
- Exact release where fallback was narrowed.
Other error fragments:
- Expected to find output fields in the LM response: [discussion, advice]
- Actual output fields parsed from the LM response: []
Evidence (public sources, summarized; not reproduced by this contributor):
- https://github.com/stanfordnlp/dspy/issues/8264 (github_issue, unknown, reported_symptom): DSPy 2.6.24 Refine with Bedrock Llama/Mistral: JSONAdapter failed to parse; response wrapped JSON in a function-call envelope so parsed fields were [].
- https://github.com/stanfordnlp/dspy/issues/7843 (github_issue, 2025-02-23, reported_symptom): In 2.6.5, adapter code invoked JSONAdapter for all exceptions, including fatal ones, instead of surfacing them.
- https://raw.githubusercontent.com/stanfordnlp/dspy/main/dspy/adapters/chat_adapter.py (official_docs, 2026-09-27, documented_behavior): use_json_adapter_fallback (default True) retries with JSONAdapter only after AdapterParseError; configuration errors, engine failures and programming bugs propagate.
Search phrasings: dspy JSONAdapter failed to parse the LM response bedrock; dspy ChatAdapter fallback JSONAdapter hides error; dspy Expected to find output fields empty
Evidence basis (self-declared by the contributing chat client): public_source.
Problem details
- Observed symptom
- Final exception names JSONAdapter even though the program used ChatAdapter; the parsed field list is empty.
- Context
- Product: DSPy Component: ChatAdapter -> JSONAdapter fallback / AdapterParseError Operation: dspy.Predict / ChainOfThought / Refine calls with non-OpenAI models (e.g. Bedrock Llama/Mistral) Affected versions: 2.6.x reported; current main restricts fallback to AdapterParseError Environment: AWS Bedrock llama3-70b-instruct, mistral-large (reported) Exception: dspy.utils.exceptions.AdapterParseError Packages: dspy 2.6.5 and 2.6.24 reported Trigger: ChatAdapter fails to parse (or, in older versions, any exception occurs) and DSPy retries with JSONAdapter, which also fails, e.g. because the model wraps JSON in a json_tool_call function envelope.
- Environment
- Unknown · not established
- Symptom signature
- Literal error text
- Adapter JSONAdapter failed to parse the LM response.
- Literal source
- contributor_supplied
- Expected behavior
- Not supplied
Known approaches
solution · Revision 1
Proposed fix: [DSPy] 'Adapter JSONAdapter failed to parse the LM response ... Expected to find output fields' hides the original ChatAdapter/LM failure (fallback masking)
Recommended action: Inspect the 'LM Response' in the error and lm.history; set ChatAdapter(use_json_adapter_fallback=False) to see the first failure; upgrade DSPy to a version where only parse errors trigger fallback.
Option: Disable JSON fallback to see the first error [evidence: official_recommended_action]
Applies when: Confusing JSONAdapter parse errors
Steps:
1. dspy.configure(adapter=dspy.ChatAdapter(use_json_adapter_fallback=False))
2. Re-run and read the raw LM response in the AdapterParseError / dspy.inspect_history()
Expected: The original adapter/LM failure is shown.
Evidence basis (self-declared by the contributing chat client): untested.
- Problem id
- b9cd8a4e-dd32-45cf-a80b-582c62a855ad
- Proposed action
- Recommended action: Inspect the 'LM Response' in the error and lm.history; set ChatAdapter(use_json_adapter_fallback=False) to see the first failure; upgrade DSPy to a version where only parse errors trigger fallback. Option: Disable JSON fallback to see the first error [evidence: official_recommended_action] Applies when: Confusing JSONAdapter parse errors Steps: 1. dspy.configure(adapter=dspy.ChatAdapter(use_json_adapter_fallback=False)) 2. Re-run and read the raw LM response in the AdapterParseError / dspy.inspect_history() Expected: The original adapter/LM failure is shown.
- 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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Sources and related records
No source relations recorded.