Anthropic MCP tools fail when input schema uses root-level anyOf/oneOf/allOf #47543

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1. github-actions

   assigned

   nexxeln

   on Sep 5, 2026
2. github-actions commented on Sep 5, 2026

   

   github-actionsBot

   on Sep 5, 2026 – with GitHub Actions

   Contributor

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   This issue might be a duplicate of existing issues. Please check:

   • MCP tool schemas are not sanitized for Anthropic: root-level anyOf/oneOf/allOf 400s, and tool.definition never sees MCP tools #46628: MCP tool schemas are not sanitized for Anthropic: root-level anyOf/oneOf/allOf 400s (open, very similar)
   • Bug: Anthropic models error Input_schema does not support oneOf, allOf, or anyOf at the top level #37916: Bug: Anthropic models error Input_schema does not support oneOf, allOf, or anyOf at the top level (open, same core issue)
   • Anthropic API rejects MCP tools with anyOf in input_schema #35516: Anthropic API rejects MCP tools with anyOf in input_schema (closed)
3. iceteaSA commented on Oct 24, 2026

   

   iceteaSA

   on Oct 24, 2026

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   This problem also occurs on OpenCode 2. I examined 2.1.3 and dev at 17b79f4e0c.

   The problem is not only for MCP tools. A plugin tool that a plugin adds with ctx.tool.transform and a Zod discriminatedUnion input causes the same failure. When the tool list contains one such tool, all requests to an Anthropic model stop with this error before the model runs:

   tools.26.custom.input_schema: input_schema does not support oneOf, allOf, or anyOf at the top level
   

   Cause on dev:

   • packages/core/src/tool/runtime.ts:157-172 changes a Zod input to JSON Schema with toJSONSchema(schema, { target: "draft-2020-12", io }). A discriminatedUnion gives a root anyOf and no root type.
   • objectRoot in packages/ai/src/protocols/utils/tool-schema.ts:57-72 adds type: "object" but keeps the root anyOf. It removes anyOf only for the empty-struct marker (lines 66-70).
   • modelCompatibility (lines 78-90) applies a sanitizer only for Gemini and Moonshot.
   • packages/ai/src/protocols/anthropic-messages.ts:550 sends input_schema: tool.inputSchema with no change.

   Minimal check with the same objectRoot code as dev:

   const input = z.discriminatedUnion("action", [
     z.object({ action: z.literal("list") }),
     z.object({ action: z.literal("get"), id: z.string() }),
   ])
   const sent = objectRoot(toJSONSchema(input, { target: "draft-2020-12", io: "input" }))
   // root keys: type, $schema, anyOf (2 branches)

   A plugin author can prevent the error with an object root. An MCP server cannot change its schema for one provider. Two possible repairs are an Anthropic sanitizer for the root, or an error at tool registration that names the tool.
4. opencodejr commented on Oct 28, 2026

   

   opencodejrBot

   on Oct 28, 2026

   More actions

   Thanks for the report. I reproduced this on v2.2.0 (the latest release; the issue doesn't name a version), and it still happens on the latest dev (b6c43a5).

   Steps:

   • A stdio MCP server exposes one tool whose inputSchema has a root-level anyOf: [{required:["route"]},{required:["file"]}]. The server is configured with codemode: false, so the tool goes on the native tool list.
   • anthropic is pointed at a local mock of the Messages API through settings.baseURL. The mock applies Anthropic's top-level combinator rule.
   • Running opencode run -m anthropic/claude-sonnet-4-5 "reply only: ok" sends the tool's schema to Anthropic unchanged, root anyOf included, and the run fails with:

     tools.5.custom.input_schema: input_schema does not support oneOf, allOf, or anyOf at the top level
     

   The script below reproduces it end to end. I had no real Anthropic account, so the 400 comes from the mock. What I saw directly is that OpenCode forwards the root combinator as is on the Anthropic path; the error is the one reported in #46628, #37916 and #35516.

   In the meantime, leaving codemode at its default (Code Mode) avoids the error. The MCP tools are then reached through execute, and their schemas aren't sent to Anthropic directly.

   Related (all v1 and closed):

   • MCP tool schemas are not sanitized for Anthropic: root-level anyOf/oneOf/allOf 400s, and tool.definition never sees MCP tools #46628: same root cause, with a detailed repro
   • Bug: Anthropic models error Input_schema does not support oneOf, allOf, or anyOf at the top level #37916: same error
   • Anthropic API rejects MCP tools with anyOf in input_schema #35516: same error
   • fix(opencode): sanitize MCP tool schemas for Anthropic root combinators #47542: the earlier fix PR, closed by the automated PR cleanup without being merged

   Reproduction script (repro.sh)

   #!/usr/bin/env bash
   # Repro for #47543: an MCP tool whose inputSchema has a root-level anyOf is sent to Anthropic unchanged.
   # Needs bun, node and `opencode` (v2) on PATH. No real Anthropic key: a local mock Messages endpoint
   # logs the tools it receives and applies Anthropic's documented rule (no anyOf/oneOf/allOf at the
   # root of input_schema), returning Anthropic's error text.
   set -euo pipefail
   W=$(mktemp -d); mkdir -p "$W/proj" "$W/home"
   cat > "$W/mcp.mjs" <<'JS'
   let buf = ""; const send = (m) => process.stdout.write(JSON.stringify(m) + "\n");
   process.stdin.on("data", (d) => { buf += d; let i;
     while ((i = buf.indexOf("\n")) >= 0) { const line = buf.slice(0, i).trim(); buf = buf.slice(i + 1);
       if (!line) continue; const msg = JSON.parse(line); if (msg.id === undefined) continue;
       if (msg.method === "initialize") send({ jsonrpc: "2.0", id: msg.id, result: { protocolVersion: msg.params?.protocolVersion ?? "2025-06-18", capabilities: { tools: {} }, serverInfo: { name: "repro", version: "1.0.0" } } });
       else if (msg.method === "tools/list") send({ jsonrpc: "2.0", id: msg.id, result: { tools: [{ name: "api_impact", description: "Impact of a route or file",
         inputSchema: { type: "object", properties: { route: { type: "string" }, file: { type: "string" } }, anyOf: [{ required: ["route"] }, { required: ["file"] }] } }] } });
       else send({ jsonrpc: "2.0", id: msg.id, result: {} }); } });
   JS
   cat > "$W/mock.ts" <<'TS'
   Bun.serve({ port: 8787, async fetch(req) {
     const body: any = await req.json().catch(() => ({})); const tools = body.tools ?? [];
     for (const [i, t] of tools.entries()) { const s = t.input_schema ?? {};
       if (["anyOf", "oneOf", "allOf"].some((k) => k in s)) {
         console.log(`received tool ${i} ${t.name} input_schema:`, JSON.stringify(s));
         return Response.json({ type: "error", error: { type: "invalid_request_error", message: `tools.${i}.custom.input_schema: input_schema does not support oneOf, allOf, or anyOf at the top level` } }, { status: 400 }); } }
     const ev = (e: string, d: any) => `event: ${e}\ndata: ${JSON.stringify(d)}\n\n`;
     return new Response(ev("message_start", { type: "message_start", message: { id: "m", type: "message", role: "assistant", model: body.model, content: [], stop_reason: null, usage: { input_tokens: 1, output_tokens: 0 } } })
       + ev("content_block_start", { type: "content_block_start", index: 0, content_block: { type: "text", text: "" } })
       + ev("content_block_delta", { type: "content_block_delta", index: 0, delta: { type: "text_delta", text: "ok" } })
       + ev("content_block_stop", { type: "content_block_stop", index: 0 })
       + ev("message_delta", { type: "message_delta", delta: { stop_reason: "end_turn" }, usage: { output_tokens: 1 } })
       + ev("message_stop", { type: "message_stop" }), { headers: { "content-type": "text/event-stream" } }); } });
   TS
   cat > "$W/proj/opencode.json" <<JSON
   { "providers": { "anthropic": { "settings": { "baseURL": "http://127.0.0.1:8787/v1", "apiKey": "sk-ant-fake" } } },
     "mcp": { "servers": { "repro": { "type": "local", "command": ["node", "$W/mcp.mjs"], "codemode": false } } } }
   JSON
   bun "$W/mock.ts" & MOCK=$!; trap 'kill $MOCK; HOME=$W/home opencode service stop >/dev/null 2>&1 || true' EXIT
   cd "$W/proj"; export HOME="$W/home" XDG_CONFIG_HOME="$W/home/.config" XDG_DATA_HOME="$W/home/.local/share"
   opencode --version
   opencode mcp list >/dev/null 2>&1 || true   # start the background service so the MCP server is connected
   sleep 2
   opencode run -m anthropic/claude-sonnet-4-5 "reply only: ok" || true

   Run it: bash repro.sh
5. opencodejr

   added

   reproduced

   on Oct 28, 2026