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Three defects decided what the model received after a tool call: 1. A tools/call the server rejected with a JSON-RPC error ended the query in the Python, TypeScript, Go and Ruby clients, so the model never learned its call failed. It now goes back as a tool_result with is_error set, as the Rust client already did. 2. A result with structuredContent and no text block reached the model as an empty tool_result in the same four clients. Its JSON is now sent. 3. Non-text blocks were dropped in all five clients. Images in a type the Messages API accepts become base64 image blocks, text resources become their text, and anything else becomes a placeholder such as "[unsupported result type: audio]". All five clients use the same mapping. Python, TypeScript and Ruby catch only the SDK's server-error class (MCPError, ProtocolError, MCP::Client::ServerError); Go and Rust catch any error from the call. The tool-loop test gains a fourth query, "tool result contents", backed by three new mock-server tools (rejected_call, structured_only, non_text). The unfixed Python client fails it; all five fixed clients pass. Closes #273 Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01NkL34oWhufSSqj9FbPx5dr
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🟡 Changes recommended
All five clients forward empty MCP text blocks, including those from empty text resources, which the Messages API rejects and causes follow-up requests to fail.
5 open issues
What changed in this PR
This PR addresses #273 by aligning how all five quickstart clients translate MCP tool results for the Anthropic Messages API.
Changes:
- Returns rejected tool calls to the model as failed tool results instead of ending the query.
- Forwards structured-only results as JSON and converts non-text content into supported blocks or explicit placeholders.
- Adds a fourth tool-loop scenario, shared fixtures, and documentation for the new behavior.
| File | Description |
|---|---|
tests/smoke-test.sh |
Updates comments to describe the fourth tool-loop query. |
tests/README.md |
Documents the new scenarios and mock tools. |
tests/helpers/tool-loop-test.ts |
Checks rejected calls and translated result content. |
tests/helpers/mock-values.ts |
Defines shared expected result values. |
tests/helpers/mock-mcp-server.ts |
Adds three tools exercising result translation. |
mcp-client-typescript/README.md |
Documents result mapping and rejected-call handling. |
mcp-client-typescript/index.ts |
Adds content conversion and protocol-error forwarding. |
mcp-client-rust/src/main.rs |
Extends result conversion to non-text blocks. |
mcp-client-rust/README.md |
Documents the unified result mapping. |
mcp-client-ruby/README.md |
Documents result mapping and rejected-call handling. |
mcp-client-ruby/client.rb |
Adds content conversion and server-error forwarding. |
mcp-client-python/README.md |
Documents result mapping and rejected-call handling. |
mcp-client-python/client.py |
Adds content conversion and MCP-error forwarding. |
mcp-client-go/README.md |
Documents result mapping and rejected-call handling. |
mcp-client-go/main.go |
Adds content conversion and failed-call forwarding. |
Review effort: Balanced
| func toolResultContent(result *mcp.CallToolResult) []anthropic.ToolResultBlockParamContentUnion { | ||
| var content []anthropic.ToolResultBlockParamContentUnion | ||
| text := func(s string) { | ||
| content = append(content, anthropic.ToolResultBlockParamContentUnion{OfText: &anthropic.TextBlockParam{Text: s}}) |
| if result.structured_content is not None and not any(isinstance(block, TextContent) for block in result.content): | ||
| content.append({"type": "text", "text": json.dumps(result.structured_content)}) | ||
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| return content |
| content << { type: "text", text: structured.to_json } | ||
| end | ||
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| content |
| content.push(json!({"type": "text", "text": structured.to_string()})); | ||
| } | ||
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| content |
| content.push({ type: "text", text: JSON.stringify(result.structuredContent) }); | ||
| } | ||
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| return content; |
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Motivation and Context
Closes #273.
Three defects decided what the model received after a tool call:
tools/callthe server rejected with a JSON-RPC error ended the query in the Python, TypeScript, Go and Ruby clients, so the model never learned its call failed. It now goes back as atool_resultwithis_errorset, as the Rust client already did.structuredContentand no text block reached the model as an emptytool_resultin the same four clients. Its JSON is now sent.All five clients use the same mapping. Python, TypeScript and Ruby catch only the SDK's server-error class (
MCPError,ProtocolError,MCP::Client::ServerError); Go and Rust catch any error from the call.The tool-loop test gains a fourth query, "tool result contents", backed by three new mock-server tools (
rejected_call,structured_only,non_text). The unfixed Python client fails it; all five fixed clients pass.How Has This Been Tested?
Locally.
Breaking Changes
N/a.
Types of changes
Checklist
Additional context
Assisted by Claude Code, reviewed by me.