Daniel StrobuschandBennet Bo Fenner
d83d9d3e8c
When a tool's JSON response fails to parse, the system would:
1. Create a `LanguageModelToolResult` with the error
2. Add it to `pending_message.tool_results`
3. **Never add the corresponding `ToolUse` to
`pending_message.content`**
This left an orphaned `tool_result` that would be sent to the LLM API
without a matching `tool_use` block, causing the provider to reject the
entire request with an error like:
```
messages: Assistant message must contain at least one content block, if
immediately followed by a user message with tool_result
```
The issue was in `handle_tool_use_json_parse_error_event()`. It created
and returned a `LanguageModelToolResult` (which gets added to
`tool_results`), but **failed to add the corresponding `ToolUse` to the
message `content`**.
This asymmetry meant:
- `pending_message.content`: [] (empty - no ToolUse!)
- `pending_message.tool_results`: {id: result}
When `AgentMessage::to_request()` converted this to API messages, it
would create:
- Assistant message: no tool_use blocks ❌
- User message: tool_result block ✅
APIs require tool_use and tool_result to be paired, so this would fail.
**Without this fix, the conversation becomes permanently broken** -
every subsequent message in the thread fails with the same API error
because the orphaned tool_result remains in the message history. The
only recovery is to start a completely new conversation, making this a
particularly annoying bug for users.
Modified `handle_tool_use_json_parse_error_event()` to:
1. **Add the `ToolUse` to `pending_message.content`** before returning
the result
2. Parse the raw_input JSON (falling back to `{}` if invalid, as the API
requires an object)
3. Send the `tool_call` event to update the UI
4. Check for duplicates to avoid adding the same `tool_use` twice
This ensures `tool_use` and `tool_result` are always properly paired.
Added comprehensive test coverage for
`handle_tool_use_json_parse_error_event()`:
- ✅ Verifies tool_use is added to message content
- ✅ Confirms tool_use has correct metadata and JSON fallback
- ✅ Tests deduplication logic to prevent duplicates
- ✅ Validates JSON parsing for valid input
## Manual Testing
To reproduce and test the fix:
1. Install the test MCP server:
```bash
cargo install --git https://github.com/dastrobu/mcp-fail-server
```
3. Add to Zed settings to enable the server:
```json
{
"context_servers": {
"mcp-fail-server": {
"command": "mcp-fail-server",
"args":[]
}
}
}
```
4. Open the assistant panel and ask it to use the `fail` tool
5. Without the fix: The conversation breaks permanently - every
subsequent message fails with the same API error, forcing you to start a
new thread
<img width="399" height="531" alt="image"
src="https://github.com/user-attachments/assets/533bdf40-80d3-4726-a9d9-dbabbe7379e5"
/>
7. With the fix: The error is handled gracefully, displayed in the UI,
and the conversation remains usable
<img width="391" height="512" alt="image"
src="https://github.com/user-attachments/assets/73aa6767-eeac-4d5d-bf6f-1beccca1d5cb"
/>
The mcp-fail-server always returns an error, triggering the JSON parse
error path that previously caused orphaned tool_result blocks.
Release Notes:
- Fixed an issue where errors could occur in the agent panel if an LLM
emitted a tool call with an invalid JSON payload
---------
Co-authored-by: Bennet Bo Fenner <bennetbo@gmx.de>
Zed
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