diff --git a/changelog.d/fixes/13552-claude-tool-schema-root-unions.md b/changelog.d/fixes/13552-claude-tool-schema-root-unions.md
new file mode 100644
index 0000000000..02c1037371
--- /dev/null
+++ b/changelog.d/fixes/13552-claude-tool-schema-root-unions.md
@@ -0,0 +1 @@
+- **fix(translator):** Claude tool `input_schema` with a root-level `anyOf` / `oneOf` / `allOf` is flattened into a plain object schema instead of being forwarded verbatim. Anthropic refuses such a tool before inference (`tools.N.custom.input_schema: input_schema does not support oneOf, allOf, or anyOf at the top level`), so a single MCP/agent tool carrying one made every request fail with no combo failover possible ([#13552](https://github.com/diegosouzapw/OmniRoute/issues/13552))
diff --git a/open-sse/services/claudeCodeCompatible.ts b/open-sse/services/claudeCodeCompatible.ts
index 652faed634..117fa99876 100644
--- a/open-sse/services/claudeCodeCompatible.ts
+++ b/open-sse/services/claudeCodeCompatible.ts
@@ -9,6 +9,7 @@ import {
 } from "../config/claudeCodeCompatibleIdentity.ts";
 import { supportsClaudeMaxEffort, supportsXHighEffort } from "../config/providerModels.ts";
 import { prepareClaudeRequest } from "../translator/helpers/claudeHelper.ts";
+import { normalizeClaudeToolInputSchema } from "../translator/helpers/schemaCoercion.ts";
 import { signRequestBody } from "./claudeCodeCCH.ts";
 import { resolveClaudeCodeCompatibleAnthropicBeta } from "./claudeCodeCompatibleBeta.ts";
 import { remapToolNamesInRequest } from "./claudeCodeToolRemapper.ts";
@@ -757,10 +758,13 @@ function convertClaudeCodeCompatibleTool(tool: unknown) {
 
   const rawSchema = readRecord(toolData.parameters) ||
     readRecord(toolData.input_schema) || { type: "object", properties: {}, required: [] };
-  const inputSchema =
+  const withProperties =
     rawSchema.type === "object" && !readRecord(rawSchema.properties)
       ? { ...rawSchema, properties: {} }
       : rawSchema;
+  // Flatten a root-level anyOf/oneOf/allOf: Anthropic refuses it outright with
+  // "input_schema does not support oneOf, allOf, or anyOf at the top level" (#13552).
+  const inputSchema = normalizeClaudeToolInputSchema(withProperties);
 
   const converted: Record<string, unknown> = {
     name,
diff --git a/open-sse/translator/helpers/schemaCoercion.ts b/open-sse/translator/helpers/schemaCoercion.ts
index 08b2cbe7db..60f7cc9a3a 100644
--- a/open-sse/translator/helpers/schemaCoercion.ts
+++ b/open-sse/translator/helpers/schemaCoercion.ts
@@ -629,13 +629,111 @@ export function stripInvalidSchemaConstructs(schema: unknown): unknown {
   return result;
 }
 
+/**
+ * JSON Schema composition keywords Anthropic refuses at the *root* of a tool
+ * `input_schema`. Nested occurrences (inside `properties`, `items`, `$defs`, …)
+ * are valid and must be preserved.
+ */
+const CLAUDE_ROOT_UNION_KEYWORDS = ["anyOf", "oneOf", "allOf"] as const;
+
+/** Whether a union branch may contribute object properties to the flattened root. */
+function claudeUnionBranchCanBeObject(branch: JsonRecord): boolean {
+  const type = branch.type;
+  if (type === undefined) return true;
+  if (typeof type === "string") return type === "object";
+  if (Array.isArray(type)) return type.includes("object");
+  return false;
+}
+
+/** Append the string entries of `branchRequired` that are not recorded yet. */
+function mergeClaudeRequired(target: string[], seen: Set<string>, branchRequired: unknown): void {
+  if (!Array.isArray(branchRequired)) return;
+  for (const name of branchRequired) {
+    if (typeof name !== "string" || seen.has(name)) continue;
+    seen.add(name);
+    target.push(name);
+  }
+}
+
+/**
+ * Whether a tool schema carries a root-level `anyOf` / `oneOf` / `allOf`.
+ *
+ * @param schema - Candidate tool `input_schema` / `parameters` value.
+ * @returns `true` when Anthropic would reject the schema's root shape.
+ */
+export function hasRootLevelSchemaUnion(schema: unknown): boolean {
+  if (!isPlainObject(schema)) return false;
+  return CLAUDE_ROOT_UNION_KEYWORDS.some((keyword) => hasOwn(schema, keyword));
+}
+
+/**
+ * Flatten a root-level `anyOf` / `oneOf` / `allOf` into a plain object schema.
+ *
+ * Anthropic's Messages API rejects a tool whose `input_schema` root carries a
+ * composition keyword with
+ * `tools.N.custom.input_schema: input_schema does not support oneOf, allOf, or
+ * anyOf at the top level` (#13552). The request is refused *before* inference,
+ * so a single such tool from an MCP/agent client fails every request that
+ * carries the catalog — combo failover cannot recover from it either.
+ *
+ * The flattening mirrors the union handling CLIProxyAPI applies on the same
+ * wire hop: object-compatible branches contribute their `properties` (first
+ * branch wins on a name collision), the root is pinned to `type: "object"`, and
+ * only `allOf` — whose branches all apply at once — contributes `required`.
+ * `anyOf` / `oneOf` branch requirements are alternatives, so promoting them
+ * would refuse calls the original schema accepts.
+ *
+ * Schemas without a root union are returned untouched, and nested unions are
+ * never rewritten.
+ *
+ * @param schema - Tool `input_schema` as received from the client.
+ * @returns An Anthropic-compatible schema, or the input when nothing to do.
+ */
+export function normalizeClaudeToolInputSchema(schema: unknown): unknown {
+  if (!hasRootLevelSchemaUnion(schema)) return schema;
+
+  const source = schema as JsonRecord;
+  const result: JsonRecord = { ...source };
+  const properties: JsonRecord = isPlainObject(source.properties) ? { ...source.properties } : {};
+  const required: string[] = [];
+  const requiredSeen = new Set<string>();
+  mergeClaudeRequired(required, requiredSeen, source.required);
+
+  for (const keyword of CLAUDE_ROOT_UNION_KEYWORDS) {
+    if (!hasOwn(result, keyword)) continue;
+    const branches = result[keyword];
+    // Dropped even when malformed: the keyword itself is what Anthropic refuses.
+    delete result[keyword];
+    if (!Array.isArray(branches)) continue;
+
+    for (const branch of branches) {
+      if (!isPlainObject(branch) || !claudeUnionBranchCanBeObject(branch)) continue;
+      if (isPlainObject(branch.properties)) {
+        for (const [name, propertySchema] of Object.entries(branch.properties)) {
+          if (!hasOwn(properties, name)) properties[name] = propertySchema;
+        }
+      }
+      if (keyword === "allOf") mergeClaudeRequired(required, requiredSeen, branch.required);
+    }
+  }
+
+  result.type = "object";
+  result.properties = properties;
+  if (required.length > 0) result.required = required;
+
+  return result;
+}
+
 export function sanitizeClaudeToolSchema(schema: unknown): unknown {
   // stripInvalidSchemaConstructs now also coerces numeric-string constraints, so
   // it is the single pass for the Claude path. We deliberately do NOT compose
   // coerceSchemaNumericFields: it strips the valid `default` keyword (Fix #1782,
   // a translator concern) which on the native / passthrough surface would
   // silently alter tool schemas that were previously forwarded verbatim.
-  return stripInvalidSchemaConstructs(schema);
+  //
+  // The root-union flattening runs last so it sees the already-repaired shape
+  // (e.g. an index-keyed `anyOf` object coerced back into an array).
+  return normalizeClaudeToolInputSchema(stripInvalidSchemaConstructs(schema));
 }
 
 export function sanitizeClaudeToolSchemas(tools: unknown): unknown {
diff --git a/open-sse/translator/request/openai-to-claude.ts b/open-sse/translator/request/openai-to-claude.ts
index dd5d13f5c5..58467eb2d0 100644
--- a/open-sse/translator/request/openai-to-claude.ts
+++ b/open-sse/translator/request/openai-to-claude.ts
@@ -3,7 +3,7 @@ import { FORMATS } from "../formats.ts";
 // CLAUDE_SYSTEM_PROMPT import removed — no longer injected unconditionally (#1966/#2130)
 import { supportsClaudeMaxEffort, supportsXHighEffort } from "../../config/providerModels.ts";
 import { adjustMaxTokens } from "../helpers/maxTokensHelper.ts";
-import { sanitizeToolId } from "../helpers/schemaCoercion.ts";
+import { normalizeClaudeToolInputSchema, sanitizeToolId } from "../helpers/schemaCoercion.ts";
 import { safeParseJSON } from "../helpers/jsonUtil.ts";
 import {
   applyKimiCodingThinking,
@@ -437,10 +437,13 @@ export function openaiToClaudeRequest(model, body, stream, credentials = null) {
         // MCP tools (e.g. pencil, computer_use) may omit properties on object-type schemas.
         const rawSchema: Record<string, unknown> = toolData.parameters ||
           toolData.input_schema || { type: "object", properties: {}, required: [] };
-        const normalizedSchema =
+        const withProperties =
           rawSchema.type === "object" && !rawSchema.properties
             ? { ...rawSchema, properties: {} }
             : rawSchema;
+        // Flatten a root-level anyOf/oneOf/allOf: Anthropic refuses it outright with
+        // "input_schema does not support oneOf, allOf, or anyOf at the top level" (#13552).
+        const normalizedSchema = normalizeClaudeToolInputSchema(withProperties);
 
         return {
           name: toolName,
diff --git a/tests/unit/claude-tool-schema-root-unions.test.ts b/tests/unit/claude-tool-schema-root-unions.test.ts
new file mode 100644
index 0000000000..d072de8790
--- /dev/null
+++ b/tests/unit/claude-tool-schema-root-unions.test.ts
@@ -0,0 +1,320 @@
+/**
+ * Root-level `anyOf` / `oneOf` / `allOf` in a Claude tool `input_schema` (#13552).
+ *
+ * Anthropic's Messages API refuses such a tool before inference with
+ * `tools.N.custom.input_schema: input_schema does not support oneOf, allOf, or
+ * anyOf at the top level`, so one offending tool in a client's catalog fails
+ * every request that carries it — combo failover cannot recover from a request
+ * that never reaches a model.
+ *
+ * These tests pin the flattening itself plus both conversion paths that build a
+ * Claude `input_schema` from a client payload: the OpenAI→Claude translator and
+ * the Claude-Code-compatible bridge.
+ */
+import { describe, it } from "node:test";
+import assert from "node:assert/strict";
+import {
+  hasRootLevelSchemaUnion,
+  normalizeClaudeToolInputSchema,
+  sanitizeClaudeToolSchema,
+} from "../../open-sse/translator/helpers/schemaCoercion.ts";
+import { openaiToClaudeRequest } from "../../open-sse/translator/request/openai-to-claude.ts";
+import { buildClaudeCodeCompatibleRequest } from "../../open-sse/services/claudeCodeCompatible.ts";
+
+type AnyRecord = Record<string, unknown>;
+
+const ROOT_UNION_KEYWORDS = ["anyOf", "oneOf", "allOf"] as const;
+
+const assertNoRootUnion = (schema: unknown, label: string): void => {
+  for (const keyword of ROOT_UNION_KEYWORDS) {
+    assert.equal(
+      Object.prototype.hasOwnProperty.call(schema as AnyRecord, keyword),
+      false,
+      `${label} still carries a root-level ${keyword}`
+    );
+  }
+  assert.equal((schema as AnyRecord).type, "object", `${label} root type is not "object"`);
+};
+
+describe("hasRootLevelSchemaUnion", () => {
+  it("detects each composition keyword at the root", () => {
+    for (const keyword of ROOT_UNION_KEYWORDS) {
+      assert.equal(hasRootLevelSchemaUnion({ type: "object", [keyword]: [] }), true, keyword);
+    }
+  });
+
+  it("ignores a union nested inside a property", () => {
+    const schema = {
+      type: "object",
+      properties: { pin: { anyOf: [{ type: "boolean" }, { type: "object" }] } },
+    };
+    assert.equal(hasRootLevelSchemaUnion(schema), false);
+  });
+
+  it("ignores non-object schemas", () => {
+    assert.equal(hasRootLevelSchemaUnion(null), false);
+    assert.equal(hasRootLevelSchemaUnion("[MaxDepth]"), false);
+    assert.equal(hasRootLevelSchemaUnion([{ anyOf: [] }]), false);
+  });
+});
+
+describe("normalizeClaudeToolInputSchema", () => {
+  it("flattens a root anyOf that carries the whole schema", () => {
+    const result = normalizeClaudeToolInputSchema({
+      anyOf: [
+        { type: "object", properties: { a: { type: "string" } } },
+        { type: "object", properties: { b: { type: "integer" } } },
+      ],
+    }) as AnyRecord;
+
+    assertNoRootUnion(result, "flattened anyOf");
+    assert.deepEqual(result.properties, {
+      a: { type: "string" },
+      b: { type: "integer" },
+    });
+  });
+
+  it("keeps a nested union while removing the root one", () => {
+    const result = normalizeClaudeToolInputSchema({
+      type: "object",
+      properties: { nested: { oneOf: [{ type: "string" }, { type: "number" }] } },
+      oneOf: [
+        { properties: { a: { type: "string" } }, required: ["a"] },
+        { properties: { b: { type: "string" } }, required: ["b"] },
+      ],
+    }) as AnyRecord;
+
+    assertNoRootUnion(result, "flattened oneOf");
+    const properties = result.properties as AnyRecord;
+    assert.deepEqual(properties.nested, { oneOf: [{ type: "string" }, { type: "number" }] });
+    assert.deepEqual(properties.a, { type: "string" });
+    assert.deepEqual(properties.b, { type: "string" });
+  });
+
+  it("does not promote alternative branch requirements of anyOf/oneOf", () => {
+    // `a` and `b` are alternatives: requiring both would refuse calls the
+    // original schema accepts.
+    const result = normalizeClaudeToolInputSchema({
+      type: "object",
+      properties: { a: { type: "string" }, b: { type: "string" } },
+      anyOf: [{ required: ["a"] }, { required: ["b"] }],
+    }) as AnyRecord;
+
+    assertNoRootUnion(result, "anyOf requirements");
+    assert.equal("required" in result, false);
+  });
+
+  it("merges properties and requirements of a root allOf", () => {
+    // Every allOf branch applies at once, so its requirements are cumulative.
+    const result = normalizeClaudeToolInputSchema({
+      type: "object",
+      properties: { base: { type: "boolean" } },
+      required: ["base"],
+      allOf: [
+        { type: "object", properties: { a: { type: "string" } }, required: ["a"] },
+        { properties: { b: { type: "integer" } }, required: ["a", "b"] },
+      ],
+    }) as AnyRecord;
+
+    assertNoRootUnion(result, "flattened allOf");
+    assert.deepEqual(Object.keys(result.properties as AnyRecord).sort(), ["a", "b", "base"]);
+    assert.deepEqual(result.required, ["base", "a", "b"]);
+  });
+
+  it("keeps the root object's own property when a branch redeclares it", () => {
+    const result = normalizeClaudeToolInputSchema({
+      type: "object",
+      properties: { shared: { type: "string", description: "root wins" } },
+      allOf: [{ properties: { shared: { type: "integer" } } }],
+    }) as AnyRecord;
+
+    assert.deepEqual((result.properties as AnyRecord).shared, {
+      type: "string",
+      description: "root wins",
+    });
+  });
+
+  it("skips branches that cannot contribute object properties", () => {
+    const result = normalizeClaudeToolInputSchema({
+      type: "object",
+      properties: { keep: { type: "string" } },
+      anyOf: [
+        { type: "string" },
+        { type: "null" },
+        { type: ["object", "null"], properties: { fromUnionType: { type: "boolean" } } },
+      ],
+    }) as AnyRecord;
+
+    assertNoRootUnion(result, "mixed branches");
+    assert.deepEqual(Object.keys(result.properties as AnyRecord).sort(), ["fromUnionType", "keep"]);
+  });
+
+  it("drops a malformed root union instead of forwarding it", () => {
+    const result = normalizeClaudeToolInputSchema({
+      type: "object",
+      properties: { a: { type: "string" } },
+      allOf: "not-an-array",
+    }) as AnyRecord;
+
+    assertNoRootUnion(result, "malformed allOf");
+    assert.deepEqual(result.properties, { a: { type: "string" } });
+  });
+
+  it("preserves unrelated keywords and leaves a clean schema untouched", () => {
+    const clean = {
+      type: "object",
+      properties: { query: { type: "string" } },
+      required: ["query"],
+      additionalProperties: false,
+    };
+    assert.equal(normalizeClaudeToolInputSchema(clean), clean);
+
+    const result = normalizeClaudeToolInputSchema({
+      title: "Search",
+      additionalProperties: false,
+      $defs: { Ref: { type: "string" } },
+      oneOf: [{ type: "object", properties: { q: { type: "string" } } }],
+    }) as AnyRecord;
+
+    assertNoRootUnion(result, "keyword preservation");
+    assert.equal(result.title, "Search");
+    assert.equal(result.additionalProperties, false);
+    assert.deepEqual(result.$defs, { Ref: { type: "string" } });
+  });
+
+  it("does not mutate the schema it was given", () => {
+    const input = {
+      type: "object",
+      properties: { a: { type: "string" } },
+      anyOf: [{ type: "object", properties: { b: { type: "string" } } }],
+    };
+    const snapshot = structuredClone(input);
+
+    normalizeClaudeToolInputSchema(input);
+
+    assert.deepEqual(input, snapshot);
+  });
+});
+
+describe("sanitizeClaudeToolSchema (native OAuth / passthrough surface)", () => {
+  it("flattens a root union after repairing invalid constructs", () => {
+    const result = sanitizeClaudeToolSchema({
+      type: "object",
+      properties: { a: { type: "string", enum: { "0": "x", "1": "y" } } },
+      anyOf: [{ type: "object", properties: { b: { type: "string" } } }],
+    }) as AnyRecord;
+
+    assertNoRootUnion(result, "sanitized schema");
+    const properties = result.properties as AnyRecord;
+    assert.deepEqual((properties.a as AnyRecord).enum, ["x", "y"]);
+    assert.deepEqual(properties.b, { type: "string" });
+  });
+
+  it("flattens an index-keyed root union object", () => {
+    // stripInvalidSchemaConstructs coerces the index-keyed object back into an
+    // array first, so the flattening still sees real branches.
+    const result = sanitizeClaudeToolSchema({
+      type: "object",
+      oneOf: { "0": { type: "object", properties: { a: { type: "string" } } } },
+    }) as AnyRecord;
+
+    assertNoRootUnion(result, "index-keyed oneOf");
+    assert.deepEqual(result.properties, { a: { type: "string" } });
+  });
+});
+
+describe("openaiToClaudeRequest — tool input_schema", () => {
+  const toolWithRootUnion = {
+    type: "function",
+    function: {
+      name: "kolonie_operator_agent",
+      description: "Repro of the reported failure",
+      parameters: {
+        type: "object",
+        properties: { act: { type: "string" } },
+        required: ["act"],
+        anyOf: [{ type: "object", properties: { delegationId: { type: "string" } } }],
+      },
+    },
+  };
+
+  const baseBody = {
+    messages: [{ role: "user", content: "Say OK" }],
+    max_tokens: 32,
+  };
+
+  it("flattens a root union before the request reaches Anthropic", () => {
+    const result = openaiToClaudeRequest(
+      "claude-opus-5",
+      { ...baseBody, tools: [toolWithRootUnion] },
+      false
+    ) as AnyRecord;
+
+    const tool = (result.tools as AnyRecord[])[0];
+    const schema = tool.input_schema as AnyRecord;
+    assertNoRootUnion(schema, "translated tool");
+    assert.deepEqual(Object.keys(schema.properties as AnyRecord).sort(), ["act", "delegationId"]);
+    assert.deepEqual(schema.required, ["act"]);
+  });
+
+  it("leaves a nested union and an ordinary schema untouched", () => {
+    const result = openaiToClaudeRequest(
+      "claude-opus-5",
+      {
+        ...baseBody,
+        tools: [
+          {
+            type: "function",
+            function: {
+              name: "ordinary",
+              parameters: {
+                type: "object",
+                properties: { pin: { anyOf: [{ type: "boolean" }, { type: "object" }] } },
+                required: ["pin"],
+              },
+            },
+          },
+        ],
+      },
+      false
+    ) as AnyRecord;
+
+    const schema = (result.tools as AnyRecord[])[0].input_schema as AnyRecord;
+    assert.deepEqual(schema.properties, {
+      pin: { anyOf: [{ type: "boolean" }, { type: "object" }] },
+    });
+    assert.deepEqual(schema.required, ["pin"]);
+  });
+});
+
+describe("buildClaudeCodeCompatibleRequest — tool input_schema", () => {
+  it("flattens a root union on the Claude-Code-compatible bridge", () => {
+    const body = buildClaudeCodeCompatibleRequest({
+      model: "claude-opus-5",
+      normalizedBody: {
+        model: "claude-opus-5",
+        messages: [{ role: "user", content: "Say OK" }],
+        max_tokens: 32,
+        tools: [
+          {
+            type: "function",
+            function: {
+              name: "bridge_tool",
+              parameters: {
+                type: "object",
+                properties: { base: { type: "string" } },
+                required: ["base"],
+                allOf: [{ properties: { extra: { type: "string" } }, required: ["extra"] }],
+              },
+            },
+          },
+        ],
+      },
+    }) as AnyRecord;
+
+    const schema = (body.tools as AnyRecord[])[0].input_schema as AnyRecord;
+    assertNoRootUnion(schema, "bridge tool");
+    assert.deepEqual(Object.keys(schema.properties as AnyRecord).sort(), ["base", "extra"]);
+    assert.deepEqual(schema.required, ["base", "extra"]);
+  });
+});