diff --git a/backend/src/adapters/request/anthropic.ts b/backend/src/adapters/request/anthropic.ts
index ffe9e57..10e1e99 100644
--- a/backend/src/adapters/request/anthropic.ts
+++ b/backend/src/adapters/request/anthropic.ts
@@ -38,6 +38,7 @@ interface AnthropicContentBlock {
   content?: string | AnthropicContentBlock[];
   is_error?: boolean;
   cache_control?: { type: "ephemeral" };
+  signature?: string;
 }
 
 interface AnthropicMessage {
@@ -109,6 +110,7 @@ function convertContentBlock(
       return {
         type: "thinking",
         thinking: block.thinking || "",
+        signature: block.signature,
       } as ThinkingContentBlock;
 
     case "tool_use":
@@ -249,6 +251,7 @@ function convertTools(
     name: tool.name,
     description: tool.description,
     inputSchema: tool.input_schema,
+    cacheControl: tool.cache_control,
   }));
 }
 
diff --git a/backend/src/adapters/response/anthropic.ts b/backend/src/adapters/response/anthropic.ts
index cf323af..88a3fb7 100644
--- a/backend/src/adapters/response/anthropic.ts
+++ b/backend/src/adapters/response/anthropic.ts
@@ -191,6 +191,11 @@ export const anthropicResponseAdapter: ResponseAdapter<AnthropicMessage> = {
             type: "thinking_delta",
             thinking: chunk.delta.thinking || "",
           };
+        } else if (chunk.delta?.type === "signature_delta") {
+          delta = {
+            type: "signature_delta",
+            signature: chunk.delta.signature || "",
+          };
         } else if (chunk.delta?.type === "input_json_delta") {
           delta = {
             type: "input_json_delta",
diff --git a/backend/src/adapters/types.ts b/backend/src/adapters/types.ts
index 0d13fa4..b79bbe1 100644
--- a/backend/src/adapters/types.ts
+++ b/backend/src/adapters/types.ts
@@ -23,6 +23,8 @@ export interface TextContentBlock {
 export interface ThinkingContentBlock {
   type: "thinking";
   thinking: string;
+  /** Signature for thinking blocks (required when replaying in multi-turn) */
+  signature?: string;
 }
 
 /**
@@ -121,6 +123,8 @@ export interface InternalToolDefinition {
   name: string;
   description?: string;
   inputSchema: JsonSchema;
+  /** Anthropic cache control for prompt caching */
+  cacheControl?: { type: "ephemeral" };
 }
 
 // =============================================================================
@@ -230,9 +234,10 @@ export interface InternalStreamChunk {
   contentBlock?: InternalContentBlock;
   /** Delta content (for content_block_delta) */
   delta?: {
-    type: "text_delta" | "thinking_delta" | "input_json_delta";
+    type: "text_delta" | "thinking_delta" | "signature_delta" | "input_json_delta";
     text?: string;
     thinking?: string;
+    signature?: string;
     partialJson?: string;
   };
   /** Message delta (for message_delta) */
diff --git a/backend/src/adapters/upstream/anthropic.ts b/backend/src/adapters/upstream/anthropic.ts
index ac7469c..c236592 100644
--- a/backend/src/adapters/upstream/anthropic.ts
+++ b/backend/src/adapters/upstream/anthropic.ts
@@ -3,6 +3,7 @@
  * Handles communication with Anthropic Claude API
  */
 
+import { parseJsonResponse } from "@/utils/json";
 import type {
   InternalContentBlock,
   InternalMessage,
@@ -17,7 +18,6 @@ import type {
   ToolUseContentBlock,
   UpstreamAdapter,
 } from "../types";
-import { parseJsonResponse } from "@/utils/json";
 
 // =============================================================================
 // Anthropic Request/Response Types
@@ -40,6 +40,7 @@ interface AnthropicContentBlock {
   content?: string | AnthropicContentBlock[];
   is_error?: boolean;
   cache_control?: { type: "ephemeral" };
+  signature?: string;
 }
 
 interface AnthropicMessage {
@@ -96,6 +97,7 @@ interface AnthropicStreamEvent {
     text?: string;
     thinking?: string;
     partial_json?: string;
+    signature?: string;
     stop_reason?: string;
     stop_sequence?: string | null;
   };
@@ -162,7 +164,11 @@ function convertMessage(msg: InternalMessage): AnthropicMessage | null {
         if (block.type === "text") {
           content.push({ type: "text", text: block.text });
         } else if (block.type === "thinking") {
-          content.push({ type: "thinking", thinking: block.thinking });
+          content.push({
+            type: "thinking",
+            thinking: block.thinking,
+            signature: block.signature,
+          });
         }
       }
     }
@@ -245,6 +251,7 @@ function convertTools(
     name: tool.name,
     description: tool.description,
     input_schema: tool.inputSchema,
+    cache_control: tool.cacheControl,
   }));
 }
 
@@ -304,6 +311,7 @@ function convertContentBlock(
       return {
         type: "thinking",
         thinking: block.thinking || "",
+        signature: block.signature,
       } as ThinkingContentBlock;
     case "tool_use":
       return {
@@ -458,11 +466,15 @@ export const anthropicUpstreamAdapter: UpstreamAdapter = {
       ...request.extraParams,
     };
 
-    // Build URL
-    const baseUrl = provider.baseUrl.endsWith("/")
-      ? provider.baseUrl.slice(0, -1)
-      : provider.baseUrl;
-    const url = `${baseUrl}/messages`;
+    // Build URL — strip trailing slash and /v1 suffix to normalize,
+    // then always append /v1/messages. This handles both
+    // "https://api.anthropic.com" and "https://api.anthropic.com/v1".
+    // Note: Any path ending with /v1 will have it stripped (e.g., /custom/v1 → /custom).
+    let baseUrl = provider.baseUrl.replace(/\/+$/, "");
+    if (baseUrl.endsWith("/v1")) {
+      baseUrl = baseUrl.slice(0, -3);
+    }
+    const url = `${baseUrl}/v1/messages`;
 
     // Build headers (Anthropic uses x-api-key instead of Authorization Bearer)
     const headers: Record<string, string> = {
@@ -566,6 +578,15 @@ export const anthropicUpstreamAdapter: UpstreamAdapter = {
               index: event.index,
               delta: { type: "thinking_delta", thinking: delta.thinking },
             };
+          } else if (delta?.type === "signature_delta") {
+            yield {
+              type: "content_block_delta",
+              index: event.index,
+              delta: {
+                type: "signature_delta",
+                signature: delta.signature,
+              },
+            };
           } else if (delta?.type === "input_json_delta") {
             yield {
               type: "content_block_delta",
diff --git a/backend/src/api/v1/messages.ts b/backend/src/api/v1/messages.ts
index bd36dc5..a9bdae7 100644
--- a/backend/src/api/v1/messages.ts
+++ b/backend/src/api/v1/messages.ts
@@ -3,14 +3,15 @@
  * Provides Anthropic-compatible API format for clients
  */
 
+import type { TProperties } from "@sinclair/typebox";
 import { Elysia, t } from "elysia";
 import type { ModelWithProvider } from "@/adapters/types";
+import type { CachedResponseType } from "@/db/schema";
 import {
   getRequestAdapter,
   getResponseAdapter,
   getUpstreamAdapter,
 } from "@/adapters";
-import { createLogger } from "@/utils/logger";
 import { getModelsWithProviderBySystemName } from "@/db";
 import { apiKeyPlugin, type ApiKey } from "@/plugins/apiKeyPlugin";
 import {
@@ -18,6 +19,11 @@ import {
   consumeTokens,
 } from "@/plugins/apiKeyRateLimitPlugin";
 import { rateLimitPlugin } from "@/plugins/rateLimitPlugin";
+import {
+  executeWithFailover,
+  selectMultipleCandidates,
+  type FailoverConfig,
+} from "@/services/failover";
 import {
   extractUpstreamHeaders,
   filterCandidates,
@@ -27,12 +33,8 @@ import {
   PROVIDER_HEADER,
 } from "@/utils/api-helpers";
 import { addCompletions, type Completion } from "@/utils/completions";
-import { StreamingContext } from "@/utils/streaming-context";
-import {
-  executeWithFailover,
-  selectMultipleCandidates,
-  type FailoverConfig,
-} from "@/services/failover";
+import { safeParseToolArgs } from "@/utils/json";
+import { createLogger } from "@/utils/logger";
 import {
   checkReqId,
   finalizeReqId,
@@ -42,8 +44,7 @@ import {
   type ApiFormat,
   type ReqIdContext,
 } from "@/utils/reqIdHandler";
-import type { CachedResponseType } from "@/db/schema";
-import { safeParseToolArgs } from "@/utils/json";
+import { StreamingContext } from "@/utils/streaming-context";
 
 const logger = createLogger("messagesApi");
 
@@ -51,15 +52,19 @@ const logger = createLogger("messagesApi");
 // Request Schema
 // =============================================================================
 
+// Helper: t.Object with additionalProperties: true for proxy transparency
+const tLooseObject = <T extends TProperties>(properties: T) =>
+  t.Object(properties, { additionalProperties: true });
+
 // Anthropic content block types
-const tAnthropicTextBlock = t.Object({
+const tAnthropicTextBlock = tLooseObject({
   type: t.Literal("text"),
   text: t.String(),
 });
 
-const tAnthropicImageBlock = t.Object({
+const tAnthropicImageBlock = tLooseObject({
   type: t.Literal("image"),
-  source: t.Object({
+  source: tLooseObject({
     type: t.String(),
     media_type: t.Optional(t.String()),
     data: t.Optional(t.String()),
@@ -67,24 +72,24 @@ const tAnthropicImageBlock = t.Object({
   }),
 });
 
-const tAnthropicToolUseBlock = t.Object({
+const tAnthropicToolUseBlock = tLooseObject({
   type: t.Literal("tool_use"),
   id: t.String(),
   name: t.String(),
   input: t.Record(t.String(), t.Unknown()),
 });
 
-const tAnthropicToolResultBlock = t.Object({
+const tAnthropicToolResultBlock = tLooseObject({
   type: t.Literal("tool_result"),
   tool_use_id: t.String(),
   content: t.Optional(t.Union([t.String(), t.Array(t.Unknown())])),
   is_error: t.Optional(t.Boolean()),
 });
 
-const tAnthropicThinkingBlock = t.Object({
+const tAnthropicThinkingBlock = tLooseObject({
   type: t.Literal("thinking"),
   thinking: t.String(),
-  signature: t.String(),
+  signature: t.Optional(t.String()),
 });
 
 const tAnthropicContentBlock = t.Union([
@@ -96,7 +101,7 @@ const tAnthropicContentBlock = t.Union([
 ]);
 
 // Anthropic tool definition
-const tAnthropicTool = t.Object({
+const tAnthropicTool = tLooseObject({
   name: t.String(),
   description: t.Optional(t.String()),
   input_schema: t.Record(t.String(), t.Unknown()),
@@ -104,42 +109,36 @@ const tAnthropicTool = t.Object({
 
 // Anthropic tool choice
 const tAnthropicToolChoice = t.Union([
-  t.Object({ type: t.Literal("auto") }),
-  t.Object({ type: t.Literal("any") }),
-  t.Object({ type: t.Literal("tool"), name: t.String() }),
+  tLooseObject({ type: t.Literal("auto") }),
+  tLooseObject({ type: t.Literal("any") }),
+  tLooseObject({ type: t.Literal("tool"), name: t.String() }),
 ]);
 
 // Anthropic metadata
-const tAnthropicMetadata = t.Object({
+const tAnthropicMetadata = tLooseObject({
   user_id: t.Optional(t.String()),
 });
 
 // Anthropic Messages API request schema
-const tAnthropicMessageCreate = t.Object(
-  {
-    model: t.String(),
-    messages: t.Array(
-      t.Object(
-        {
-          role: t.String(),
-          content: t.Union([t.String(), t.Array(tAnthropicContentBlock)]),
-        },
-        { additionalProperties: true },
-      ),
-    ),
-    max_tokens: t.Number(),
-    system: t.Optional(t.Union([t.String(), t.Array(tAnthropicTextBlock)])),
-    stream: t.Optional(t.Boolean()),
-    temperature: t.Optional(t.Number()),
-    top_p: t.Optional(t.Number()),
-    top_k: t.Optional(t.Number()),
-    stop_sequences: t.Optional(t.Array(t.String())),
-    tools: t.Optional(t.Array(tAnthropicTool)),
-    tool_choice: t.Optional(tAnthropicToolChoice),
-    metadata: t.Optional(tAnthropicMetadata),
-  },
-  { additionalProperties: true },
-);
+const tAnthropicMessageCreate = tLooseObject({
+  model: t.String(),
+  messages: t.Array(
+    tLooseObject({
+      role: t.String(),
+      content: t.Union([t.String(), t.Array(tAnthropicContentBlock)]),
+    }),
+  ),
+  max_tokens: t.Number(),
+  system: t.Optional(t.Union([t.String(), t.Array(tAnthropicTextBlock)])),
+  stream: t.Optional(t.Boolean()),
+  temperature: t.Optional(t.Number()),
+  top_p: t.Optional(t.Number()),
+  top_k: t.Optional(t.Number()),
+  stop_sequences: t.Optional(t.Array(t.String())),
+  tools: t.Optional(t.Array(tAnthropicTool)),
+  tool_choice: t.Optional(tAnthropicToolChoice),
+  metadata: t.Optional(tAnthropicMetadata),
+});
 
 /**
  * Build completion record for logging
diff --git a/backend/src/utils/api-helpers.ts b/backend/src/utils/api-helpers.ts
index 4b8217f..0d7b330 100644
--- a/backend/src/utils/api-helpers.ts
+++ b/backend/src/utils/api-helpers.ts
@@ -3,9 +3,9 @@
  * Extracted from completions.ts, messages.ts, and responses.ts to reduce code duplication
  */
 
-import { createLogger } from "@/utils/logger";
 import type { InternalResponse, ModelWithProvider } from "@/adapters/types";
 import type { ToolCallType } from "@/db/schema";
+import { createLogger } from "@/utils/logger";
 
 const logger = createLogger("api-helpers");
 
@@ -31,7 +31,6 @@ export const EXCLUDED_HEADERS = new Set([
   "accept",
   "accept-encoding",
   "accept-language",
-  "user-agent",
   "origin",
   "referer",
   "cookie",