MessageParseError: missing signature in thinking block with claude-agent-sdk 0.1.79 and Anthropic-compatible backend

[googs1025]

Summary

claude-agent-sdk crashes with:

claude_agent_sdk._errors.MessageParseError:
Missing required field in assistant message: 'signature'

This is similar to #339, but I can still reproduce it with claude-agent-sdk==0.1.79.

The issue appears when the model/backend emits a thinking content block without a signature field. The SDK parser currently assumes every thinking block has signature.

Environment

• OS: Linux / Docker
• Python: 3.11
• claude-agent-sdk: 0.1.79
• Runtime: Docker container
• Model: gpt-5.4 via Anthropic-compatible backend
• SDK entrypoint: claude_agent_sdk.query(...)
• ClaudeAgentOptions includes:
  • model="gpt-5.4"
  • allowed_tools=[...]
  • permission_mode="dontAsk"
  • skills=[...]
  • resume=<session_id> sometimes true

SDK inspection output:

sdk_version = 0.1.79
parser_file = /usr/local/lib/python3.11/site-packages/claude_agent_sdk/_internal/message_parser.py
100: case "thinking":
103:     thinking=block["thinking"],
104:     signature=block["signature"]

Reproduction

This is reproducible in our bot when the user sends a repository URL, for example:

• https://github.com/anthropics/claude-agent-sdk-python
• or a GitLab repository URL

The request runs for ~30 seconds, then fails before any final assistant result is returned.

Actual behavior

The SDK raises MessageParseError while parsing an assistant message:

Traceback (most recent call last):
  File "/usr/local/lib/python3.11/site-packages/claude_agent_sdk/_internal/message_parser.py", line 137, in parse_message
    signature=block["signature"],
KeyError: 'signature'

The above exception was the direct cause of the following exception:

claude_agent_sdk._errors.MessageParseError:
Missing required field in assistant message: 'signature'

Service logs:

claude sdk event type=SystemMessage resume=True
...
MessageParseError: Missing required field in assistant message: 'signature'

Expected behavior

The SDK should not crash the whole query when a thinking block is missing signature.

Possible acceptable behaviors:

1. If signature is required only for Anthropic native thinking blocks, document that non-Anthropic-compatible backends must disable thinking.
2. Provide a clearer error message explaining that the backend emitted an invalid/incompatible thinking block.
3. Allow ClaudeAgentOptions(thinking={"type": "disabled"}) to reliably prevent such blocks from being emitted/parsed.
4. Optionally parse unsigned thinking blocks as a best-effort compatibility mode, or expose a config flag for strict vs permissive parsing.

Related

• claude_agent_sdk._errors.MessageParseError: Missing required field in assistant message #339 — closed not planned, but reproducible on the latest version (0.1.79).
• signature Key not found for Thinking Block from CLI #473 — similar KeyError: signature from the CLI side.
• fix(parser): handle missing signature field in ThinkingBlock #786 — proposes block.get("signature", ""); would silence the crash but produce empty-signature blocks that may still fail on multi-turn round-trips.