Problem

The built-in websearch tool returns HTTP 400 errors, reported by the model as "Tavily API error: 400 Bad Request". The root cause is that the websearch tool uses MCP endpoints (Exa/Parallel) that return Server-Sent Events (text/event-stream) responses. The v1.17.9 code reads the response body via response.text from Effect's HttpClientResponse, which can fail to properly decode SSE-formatted bodies.

The same issue exists in both packages:

• packages/core/src/tool/websearch.ts (the core tool implementation)
• packages/opencode/src/tool/mcp-websearch.ts (the opencode package MCP wrapper)

Analysis

When response.text is called on a text/event-stream response, the charset detection may behave incorrectly depending on the runtime and the Effect library version. The SSE response format (event: message\ndata: {...}\n\n) can cause decoding failures or produce malformed strings that the downstream parseResponse function cannot handle. The HttpClient.filterStatusOk correctly passes the 200 status, but the subsequent body decoding fails, resulting in a ToolFailure that the model interprets as a 400 error — even though the actual HTTP status is 200.

Fix

Replace response.text with collectBoundedResponseBody, which:

1. Reads raw bytes from the response stream via response.stream
2. Manually tracks sizes with upper bounds (MAX_RESPONSE_BYTES = 256 KB)
3. Converts to UTF-8 explicitly with .toString("utf8")

This approach is already used by the webfetch tool (which has its own collectBody function) and was added upstream in this very commit 69f1ec22e ("fix(core): bound web tool failures").

Changes

Added

• packages/core/src/tool/http-body.ts — Shared collectBoundedResponseBody function that reads a bounded response body from a stream, checks content-length headers, and returns a Buffer. This file was referenced by the import in websearch.ts but was missing from the v1.17.9 tag.

Modified

• packages/core/src/tool/websearch.ts — Import and use collectBoundedResponseBody instead of response.text
• packages/opencode/src/tool/mcp-websearch.ts — Same fix applied to the opencode package's MCP websearch wrapper

Testing

• The existing test suite in packages/core/test/tool-websearch.test.ts covers provider selection, plain JSON and SSE response parsing, oversized body rejection, and API key isolation. All tests pass with this fix.
• The fix matches the upstream commit 69f1ec22e on the dev branch.

Related

• Upstream fix: commit 69f1ec22e ("fix(core): bound web tool failures")
• Closes issue related to websearch returning 400 Bad Request for SSE-formatted responses