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Comprehensive guide for debugging Claude Agent SDK errors, TypedDict contracts, and internal SDK processing issues
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| name | Claude SDK Debugging |
| description | Comprehensive guide for debugging Claude Agent SDK errors, TypedDict contracts, and internal SDK processing issues |
| keywords | ["claude-sdk","sdk-error","mcp","typeddict","serialization","json-error","sdk-debugging"] |
CRITICAL: User-facing errors don't show root cause. Always check log files.
# Check error logs
tail -50 logs/errors.log
# Grep for specific error
grep "JSON serializable" logs/errors.log | tail -5
# Check recent errors
tail -100 logs/errors.log | grep -A 20 "ERROR"
Why: SDK errors surface deep in internal code. Full tracebacks show:
Once you have the traceback, find the SDK source file:
# Find SDK files in virtual environment
find .venv/lib -name "subprocess_cli.py" -path "*/claude_agent_sdk/*"
# Or in global Python packages
find /Users/$USER/.local/lib -name "*sdk*"
Read the SDK code at the failure point to understand:
Error: Object of type Server is not JSON serializable
Traceback shows: subprocess_cli.py:169 - json.dumps({"mcpServers": servers_for_cli})
Read SDK source at line 154-169:
if isinstance(config, dict) and config.get("type") == "sdk":
# For SDK servers, pass everything except the instance field
sdk_config = {k: v for k, v in config.items() if k != "instance"}
servers_for_cli[name] = sdk_config
else:
servers_for_cli[name] = config
# Line 169: JSON serialization
json.dumps({"mcpServers": servers_for_cli})
Root cause: SDK strips instance field but only if:
config.get("type") == "sdk" (must have type key)k != "instance" (must use instance key, not server)# Correct structure
McpSdkServerConfig(
type="sdk", # SDK checks: config.get("type") == "sdk"
name="server_name", # Server identifier
instance=server # SDK strips before JSON: k != "instance"
)
# Wrong - causes JSON serialization error
McpSdkServerConfig(
server=server, # Wrong key! SDK can't strip this
name="name" # Missing type="sdk"
)
Why it matters:
# SDK expects dict[str, McpSdkServerConfig | McpStdioServerConfig | ...]
options = ClaudeAgentOptions(
mcp_servers={
"server_name": create_sdk_mcp_server(
name="server_name",
version="1.0.0",
tools=[tool1, tool2]
)
}
)
When patching SDK bugs:
def patched_create_sdk_mcp_server(
name: str,
version: str = "1.0.0", # Bug: SDK passes this to Server() which doesn't accept it
tools: list[SdkMcpTool] | None = None,
) -> McpSdkServerConfig:
"""Patched version fixing bug #323."""
from mcp.server import Server
# FIX 1: Don't pass version to Server.__init__()
# Original SDK bug: Server(name, version=version)
server = Server(name) # Correct: Server only accepts name
# ... tool registration ...
# FIX 2: Return with correct TypedDict keys
# SDK subprocess_cli.py:154-159 expects these exact keys:
# - type="sdk" for identification
# - name for server name
# - instance for Server object (stripped before JSON serialization)
return McpSdkServerConfig(
type="sdk", # SDK checks config.get("type") == "sdk"
name=name, # Identifier
instance=server # SDK strips: k != "instance"
)
After fixing SDK bugs:
# 1. Verify immediate return
result = create_sdk_mcp_server(name="test", tools=[tool1])
assert result["type"] == "sdk"
assert result["name"] == "test"
assert "instance" in result
# 2. Verify SDK processing (simulate SDK's internal logic)
sdk_config = {k: v for k, v in result.items() if k != "instance"}
import json
json.dumps({"mcpServers": {"test": sdk_config}}) # Should not error
# 3. Test full workflow
options = ClaudeAgentOptions(mcp_servers={"test": result})
async with ClaudeSDKClient(options=options) as client:
# Should successfully connect without JSON serialization errors
pass
Symptom: Error occurs in SDK internal code, not at return statement
Cause: Wrong TypedDict keys - runtime doesn't validate, fails downstream
Solution: Read SDK source to find exact keys it expects
Symptom: Object of type X is not JSON serializable
Cause: SDK failed to strip non-serializable fields
Solution:
k != "instance")type="sdk")Symptom: TypeError: __init__() got unexpected keyword argument
Cause: SDK passes parameters that underlying library doesn't accept
Solution:
# Find error in logs
tail -50 logs/errors.log
# Find SDK source
find .venv/lib -name "*subprocess_cli*"
# Verify TypedDict structure
python -c "from claude_agent_sdk import McpSdkServerConfig; print(McpSdkServerConfig.__required_keys__)"
.quibbler/rules.md - "Check Full Error Tracebacks for SDK Failures"