diff --git a/packages/ai/CHANGELOG.md b/packages/ai/CHANGELOG.md
index ac3d06d48..28093061d 100644
--- a/packages/ai/CHANGELOG.md
+++ b/packages/ai/CHANGELOG.md
@@ -8,7 +8,7 @@
 
 ### Fixed
 
-- Fixed Anthropic-compatible Umans requests escaping client `web_search` tools so Kimi answers normally instead of returning raw gateway search results.
+- Fixed Anthropic-compatible Umans requests escaping client tool names so Kimi answers normally instead of returning raw gateway search results.
 
 ## [16.0.1] - 2026-06-15
 
diff --git a/packages/ai/src/providers/anthropic.ts b/packages/ai/src/providers/anthropic.ts
index 44b9c7543..9e8680e9d 100644
--- a/packages/ai/src/providers/anthropic.ts
+++ b/packages/ai/src/providers/anthropic.ts
@@ -758,18 +758,6 @@ function isUmansGatewayWebSearchEnabled(
 	return isUmansAnthropicModel(model) && getUmansWebSearchProvider(headers) !== undefined;
 }
 
-function shouldEscapeAnthropicBuiltinToolName(
-	name: string,
-	escapeBuiltinToolNames: boolean,
-	useUmansGatewayWebSearch: boolean,
-): boolean {
-	return (
-		escapeBuiltinToolNames &&
-		ANTHROPIC_BUILTIN_TOOL_NAMES.has(name.toLowerCase()) &&
-		!shouldUseUmansGatewayWebSearch(name, useUmansGatewayWebSearch)
-	);
-}
-
 function encodeAnthropicToolName(
 	name: string,
 	isOAuthToken: boolean,
@@ -777,17 +765,13 @@ function encodeAnthropicToolName(
 	useUmansGatewayWebSearch = false,
 ): string {
 	if (shouldUseUmansGatewayWebSearch(name, useUmansGatewayWebSearch)) return name;
-	if (shouldEscapeAnthropicBuiltinToolName(name, escapeBuiltinToolNames, useUmansGatewayWebSearch)) {
-		return `${claudeToolPrefix}${name}`;
-	}
+	if (escapeBuiltinToolNames) return `${claudeToolPrefix}${name}`;
 	return isOAuthToken ? applyClaudeToolPrefix(name) : name;
 }
 
 function decodeAnthropicToolName(name: string, isOAuthToken: boolean, escapeBuiltinToolNames: boolean): string {
-	if (isOAuthToken) return stripClaudeToolPrefix(name);
-	if (!escapeBuiltinToolNames) return name;
-	const stripped = stripClaudeToolPrefix(name);
-	return ANTHROPIC_BUILTIN_TOOL_NAMES.has(stripped.toLowerCase()) ? stripped : name;
+	if (isOAuthToken || escapeBuiltinToolNames) return stripClaudeToolPrefix(name);
+	return name;
 }
 
 const ANTHROPIC_MANY_IMAGE_THRESHOLD = 20;
diff --git a/packages/ai/test/anthropic-alignment.test.ts b/packages/ai/test/anthropic-alignment.test.ts
index b3d0d526c..acd6b0645 100644
--- a/packages/ai/test/anthropic-alignment.test.ts
+++ b/packages/ai/test/anthropic-alignment.test.ts
@@ -823,7 +823,7 @@ describe("Anthropic request fingerprint alignment", () => {
 		expect(payload.metadata?.user_id).not.toBe("invalid-user-id");
 		expectClaudeMetadataUserId(payload.metadata?.user_id);
 	});
-	it("escapes Anthropic built-in tool names for compatible gateways", async () => {
+	it("escapes Umans client tool names for unambiguous round-trips", async () => {
 		const tools: Tool[] = [
 			{
 				name: "web_search",
@@ -843,6 +843,15 @@ describe("Anthropic request fingerprint alignment", () => {
 					required: ["path"],
 				},
 			},
+			{
+				name: "_web_search",
+				description: "literal prefixed search",
+				parameters: {
+					type: "object",
+					properties: { query: { type: "string" } },
+					required: ["query"],
+				},
+			},
 		];
 
 		const payload = (await captureAnthropicPayload(
@@ -858,7 +867,7 @@ describe("Anthropic request fingerprint alignment", () => {
 			tool_choice?: { type?: string; name?: string };
 		};
 
-		expect(payload.tools?.map(tool => tool.name)).toEqual(["_web_search", "read"]);
+		expect(payload.tools?.map(tool => tool.name)).toEqual(["_web_search", "_read", "__web_search"]);
 		expect(payload.tool_choice).toEqual({ type: "tool", name: "_web_search" });
 	});
 
@@ -901,7 +910,7 @@ describe("Anthropic request fingerprint alignment", () => {
 			tool_choice?: { type?: string; name?: string };
 		};
 
-		expect(payload.tools?.map(tool => tool.name)).toEqual(["web_search", "read"]);
+		expect(payload.tools?.map(tool => tool.name)).toEqual(["web_search", "_read"]);
 		expect(payload.tool_choice).toEqual({ type: "tool", name: "web_search" });
 	});
 
diff --git a/packages/ai/test/anthropic-stream-envelope.test.ts b/packages/ai/test/anthropic-stream-envelope.test.ts
index e445a2d9b..e0453a219 100644
--- a/packages/ai/test/anthropic-stream-envelope.test.ts
+++ b/packages/ai/test/anthropic-stream-envelope.test.ts
@@ -383,6 +383,66 @@ describe("anthropic stream envelope handling", () => {
 		]);
 	});
 
+	it("decodes escaped literal-prefixed Umans tool names", async () => {
+		vi.spyOn(AnthropicMessages.prototype, "create").mockImplementation(
+			() =>
+				createMockRequest([
+					{
+						type: "message_start",
+						message: {
+							id: "msg_literal_tool",
+							usage: {
+								input_tokens: 12,
+								output_tokens: 0,
+								cache_read_input_tokens: 0,
+								cache_creation_input_tokens: 0,
+							},
+						},
+					},
+					{
+						type: "content_block_start",
+						index: 0,
+						content_block: { type: "tool_use", id: "tool_1", name: "__web_search", input: {} },
+					},
+					{
+						type: "content_block_delta",
+						index: 0,
+						delta: { type: "input_json_delta", partial_json: '{"query":"literal"}' },
+					},
+					{ type: "content_block_stop", index: 0 },
+					{
+						type: "message_delta",
+						delta: { stop_reason: "tool_use" },
+						usage: {
+							input_tokens: 12,
+							output_tokens: 4,
+							cache_read_input_tokens: 0,
+							cache_creation_input_tokens: 0,
+						},
+					},
+					{ type: "message_stop" },
+				]) as never,
+		);
+
+		const stream = streamAnthropic(umansModel, context, { apiKey: "sk-ant-test" });
+		const events: AssistantMessageEvent[] = [];
+		for await (const event of stream) {
+			events.push(event);
+		}
+		const result = await stream.result();
+
+		expect(countEvents(events, "toolcall_start")).toBe(1);
+		expect(result.stopReason).toBe("toolUse");
+		expect(result.content).toEqual([
+			{
+				type: "toolCall",
+				id: "tool_1",
+				name: "_web_search",
+				arguments: { query: "literal" },
+			},
+		]);
+	});
+
 	it("ignores Umans gateway web search server blocks and keeps final text", async () => {
 		vi.spyOn(AnthropicMessages.prototype, "create").mockImplementation(
 			() =>