diff --git a/src/proxy/transformers/request-transformer.ts b/src/proxy/transformers/request-transformer.ts
index 6b294dac..c1ebd5d7 100644
--- a/src/proxy/transformers/request-transformer.ts
+++ b/src/proxy/transformers/request-transformer.ts
@@ -1,5 +1,5 @@
 interface AnthropicThinking {
-  type?: 'enabled' | 'disabled' | string;
+  type?: 'enabled' | 'disabled' | 'adaptive' | string;
   budget_tokens?: number;
 }
 
@@ -14,6 +14,7 @@ interface AnthropicImageBlock {
     type?: string;
     media_type?: string;
     data?: string;
+    url?: string;
   };
 }
 
@@ -28,6 +29,7 @@ interface AnthropicToolResultBlock {
   type: 'tool_result';
   tool_use_id?: string;
   content?: unknown;
+  is_error?: boolean;
 }
 
 type AnthropicContentBlock =
@@ -42,6 +44,16 @@ interface AnthropicMessage {
   content?: string | AnthropicContentBlock[];
 }
 
+interface AnthropicOutputConfig {
+  effort?: 'low' | 'medium' | 'high' | 'max' | string;
+}
+
+interface AnthropicToolChoice {
+  type?: 'auto' | 'any' | 'tool' | 'none' | string;
+  name?: string;
+  disable_parallel_tool_use?: boolean;
+}
+
 interface AnthropicProxyRequestShape {
   model?: unknown;
   system?: unknown;
@@ -52,8 +64,10 @@ interface AnthropicProxyRequestShape {
   stop_sequences?: unknown;
   metadata?: unknown;
   tools?: unknown;
+  tool_choice?: AnthropicToolChoice;
   stream?: unknown;
   thinking?: AnthropicThinking;
+  output_config?: AnthropicOutputConfig;
 }
 
 interface OpenAITextPart {
@@ -100,6 +114,17 @@ export interface ProxyOpenAIRequest {
       parameters: Record<string, unknown>;
     };
   }>;
+  tool_choice?:
+    | 'auto'
+    | 'none'
+    | 'required'
+    | {
+        type: 'function';
+        function: {
+          name: string;
+        };
+      };
+  parallel_tool_calls?: boolean;
   messages: OpenAIMessage[];
   max_tokens?: number;
   temperature?: number;
@@ -108,7 +133,6 @@ export interface ProxyOpenAIRequest {
   metadata?: Record<string, unknown>;
 }
 
-const TOOL_RESULT_SERIALIZATION_FALLBACK = '[unserializable content]';
 const TOOL_USE_ARGUMENTS_FALLBACK = '{}';
 
 function assertObject(value: unknown, label: string): Record<string, unknown> {
@@ -168,17 +192,49 @@ function flattenTextContent(content: unknown, label: string): string {
     .join('\n');
 }
 
-function toToolResultContent(content: unknown, label: string): string {
+/**
+ * Convert tool_result content to OpenAI-compatible format.
+ * Handles strings, arrays with text/image blocks, and error prefixing.
+ * Ported from openclaude's convertToolResultContent.
+ */
+function convertToolResultContent(content: unknown, isError: boolean, label: string): string {
   if (content === undefined) {
     return '';
   }
   if (typeof content === 'string') {
-    return content;
+    return isError ? `Error: ${content}` : content;
   }
-  if (Array.isArray(content)) {
-    return flattenTextContent(content, label);
+  if (!Array.isArray(content)) {
+    const text = safeJsonStringify(content, '[unserializable content]');
+    return isError ? `Error: ${text}` : text;
   }
-  return safeJsonStringify(content, TOOL_RESULT_SERIALIZATION_FALLBACK);
+
+  const parts: string[] = [];
+  for (const [index, block] of content.entries()) {
+    const parsed = assertObject(block, `${label}[${index}]`);
+
+    if (parsed.type === 'text' && typeof parsed.text === 'string') {
+      parts.push(parsed.text);
+      continue;
+    }
+
+    if (parsed.type === 'image') {
+      throw new Error(`${label}[${index}].type "image" is not supported in tool_result content`);
+    }
+
+    if (typeof parsed.text === 'string') {
+      parts.push(parsed.text);
+      continue;
+    }
+
+    throw new Error(`${label}[${index}].type "${String(parsed.type)}" is not supported`);
+  }
+
+  const text = parts.join('\n');
+  if (!text) {
+    return isError ? 'Error:' : '';
+  }
+  return isError ? `Error: ${text}` : text;
 }
 
 function createFallbackToolId(messageIndex: number, blockIndex: number): string {
@@ -187,16 +243,27 @@ function createFallbackToolId(messageIndex: number, blockIndex: number): string
 
 function toImagePart(block: AnthropicImageBlock, label: string): OpenAIImagePart {
   const source = block.source;
-  if (!source || source.type !== 'base64' || !source.media_type || !source.data) {
-    throw new Error(`${label}.source must be a base64 image payload`);
+  if (!source) {
+    throw new Error(`${label}.source is missing`);
   }
 
-  return {
-    type: 'image_url',
-    image_url: {
-      url: `data:${source.media_type};base64,${source.data}`,
-    },
-  };
+  if (source.type === 'url' && source.url) {
+    return {
+      type: 'image_url',
+      image_url: { url: source.url },
+    };
+  }
+
+  if (source.type === 'base64' && source.media_type && source.data) {
+    return {
+      type: 'image_url',
+      image_url: {
+        url: `data:${source.media_type};base64,${source.data}`,
+      },
+    };
+  }
+
+  throw new Error(`${label}.source must be a base64 or url image payload`);
 }
 
 function isImageBlock(block: AnthropicContentBlock): block is AnthropicImageBlock {
@@ -234,30 +301,85 @@ function transformTools(value: unknown): ProxyOpenAIRequest['tools'] {
       (entry): entry is { name?: unknown; description?: unknown; input_schema?: unknown } =>
         typeof entry === 'object' && entry !== null
     )
-    .map((entry) => ({
-      type: 'function' as const,
-      function: {
-        name: typeof entry.name === 'string' ? entry.name : 'tool',
-        ...(typeof entry.description === 'string' ? { description: entry.description } : {}),
-        parameters:
-          typeof entry.input_schema === 'object' && entry.input_schema !== null
-            ? (entry.input_schema as Record<string, unknown>)
-            : { type: 'object', properties: {} },
-      },
-    }));
+    .map((entry) => {
+      const rawSchema =
+        typeof entry.input_schema === 'object' && entry.input_schema !== null
+          ? (entry.input_schema as Record<string, unknown>)
+          : { type: 'object', properties: {} };
+
+      return {
+        type: 'function' as const,
+        function: {
+          name: typeof entry.name === 'string' ? entry.name : 'tool',
+          ...(typeof entry.description === 'string' ? { description: entry.description } : {}),
+          parameters: rawSchema,
+        },
+      };
+    });
 
   return tools.length > 0 ? tools : undefined;
 }
 
+function transformToolChoice(
+  value: AnthropicToolChoice | undefined,
+  hasTools: boolean
+): Pick<ProxyOpenAIRequest, 'tool_choice' | 'parallel_tool_calls'> {
+  if (!value) {
+    return hasTools ? { tool_choice: 'auto' } : {};
+  }
+
+  if (!hasTools) {
+    throw new Error('tool_choice requires tools');
+  }
+
+  const parallelToolCalls =
+    value.disable_parallel_tool_use === true ? { parallel_tool_calls: false } : {};
+
+  switch (value.type) {
+    case undefined:
+    case 'auto':
+      return { tool_choice: 'auto', ...parallelToolCalls };
+    case 'none':
+      return { tool_choice: 'none' };
+    case 'any':
+      return { tool_choice: 'required', ...parallelToolCalls };
+    case 'tool':
+      if (typeof value.name !== 'string' || value.name.trim().length === 0) {
+        throw new Error('tool_choice.name must be a non-empty string when type is "tool"');
+      }
+      return {
+        tool_choice: {
+          type: 'function',
+          function: { name: value.name.trim() },
+        },
+        ...parallelToolCalls,
+      };
+    default:
+      throw new Error('tool_choice.type must be "auto", "any", "tool", or "none"');
+  }
+}
+
 function mapThinkingToReasoning(
-  thinking: AnthropicThinking | undefined
+  thinking: AnthropicThinking | undefined,
+  outputConfig: AnthropicOutputConfig | undefined
 ): Pick<ProxyOpenAIRequest, 'reasoning' | 'reasoning_effort'> {
   if (!thinking || thinking.type === 'disabled') {
     return {};
   }
 
+  if (thinking.type === 'adaptive') {
+    const effort = toOpenAIEffort(resolveOutputConfigEffort(outputConfig) ?? 'high');
+    return {
+      reasoning_effort: effort,
+      reasoning: {
+        enabled: true,
+        effort,
+      },
+    };
+  }
+
   if (thinking.type !== 'enabled') {
-    throw new Error('thinking.type must be "enabled" or "disabled"');
+    throw new Error('thinking.type must be "enabled", "adaptive", or "disabled"');
   }
 
   const effort =
@@ -274,12 +396,37 @@ function mapThinkingToReasoning(
   };
 }
 
+const VALID_EFFORT_LEVELS = new Set(['low', 'medium', 'high', 'max']);
+
+function resolveOutputConfigEffort(
+  outputConfig: AnthropicOutputConfig | undefined
+): string | undefined {
+  if (!outputConfig || typeof outputConfig.effort !== 'string') {
+    return undefined;
+  }
+  const normalized = outputConfig.effort.trim().toLowerCase();
+  return VALID_EFFORT_LEVELS.has(normalized) ? normalized : undefined;
+}
+
+/**
+ * Map Anthropic effort levels to OpenAI-compatible reasoning_effort.
+ * Anthropic's `max` has no standard OpenAI equivalent — most providers
+ * only accept low/medium/high and reject unknown values with a 400.
+ * Ported from openclaude's standardEffortToOpenAI() which maps max -> xhigh
+ * for Codex; for generic OpenAI-compat providers we clamp to high.
+ */
+function toOpenAIEffort(effort: string): string {
+  return effort === 'max' ? 'high' : effort;
+}
+
 function transformMessages(messagesValue: unknown): OpenAIMessage[] {
   if (!Array.isArray(messagesValue)) {
     throw new Error('messages must be an array');
   }
 
   const translatedMessages: OpenAIMessage[] = [];
+  let pendingToolUseIds: Set<string> | null = null;
+  let hasPendingToolUseIds = false;
 
   messagesValue.forEach((message, messageIndex) => {
     const parsedMessage = assertObject(message, `messages[${messageIndex}]`) as AnthropicMessage;
@@ -288,8 +435,19 @@ function transformMessages(messagesValue: unknown): OpenAIMessage[] {
       throw new Error(`messages[${messageIndex}].role must be "user" or "assistant"`);
     }
 
+    if (pendingToolUseIds && pendingToolUseIds.size > 0 && role !== 'user') {
+      throw new Error(
+        `messages[${messageIndex}].role must be "user" with tool_result blocks after assistant tool_use`
+      );
+    }
+
     const content = parsedMessage.content;
     if (typeof content === 'string') {
+      if (pendingToolUseIds && pendingToolUseIds.size > 0) {
+        throw new Error(
+          `messages[${messageIndex}].content must start with tool_result blocks for pending tool_use ids`
+        );
+      }
       translatedMessages.push({ role, content });
       return;
     }
@@ -298,10 +456,119 @@ function transformMessages(messagesValue: unknown): OpenAIMessage[] {
       throw new Error(`messages[${messageIndex}].content must be a string or array`);
     }
 
-    const userParts: OpenAIContentPart[] = [];
+    if (role === 'user') {
+      const userParts: OpenAIContentPart[] = [];
+      let sawToolResult = false;
+      const resolvedToolUseIds = new Set<string>();
+
+      content.forEach((block, blockIndex) => {
+        const parsed = assertObject(
+          block,
+          `messages[${messageIndex}].content[${blockIndex}]`
+        ) as AnthropicContentBlock;
+
+        if (parsed.type === 'thinking' || parsed.type === 'redacted_thinking') {
+          return;
+        }
+
+        if (parsed.type === 'text') {
+          if (sawToolResult) {
+            throw new Error(
+              `messages[${messageIndex}].content[${blockIndex}] text is not allowed after tool_result blocks`
+            );
+          }
+          const text = typeof parsed.text === 'string' ? parsed.text : '';
+          userParts.push({ type: 'text', text });
+          return;
+        }
+
+        if (isImageBlock(parsed)) {
+          if (sawToolResult) {
+            throw new Error(
+              `messages[${messageIndex}].content[${blockIndex}] image is not allowed after tool_result blocks`
+            );
+          }
+          userParts.push(toImagePart(parsed, `messages[${messageIndex}].content[${blockIndex}]`));
+          return;
+        }
+
+        if (isToolResultBlock(parsed)) {
+          if (!pendingToolUseIds || pendingToolUseIds.size === 0) {
+            throw new Error(
+              `messages[${messageIndex}].content[${blockIndex}] tool_result requires a preceding assistant tool_use`
+            );
+          }
+          if (userParts.length > 0) {
+            throw new Error(
+              `messages[${messageIndex}].content[${blockIndex}] tool_result blocks must come before other user content`
+            );
+          }
+          if (typeof parsed.tool_use_id !== 'string' || parsed.tool_use_id.trim().length === 0) {
+            throw new Error(
+              `messages[${messageIndex}].content[${blockIndex}].tool_use_id must be a non-empty string`
+            );
+          }
+          if (!pendingToolUseIds.has(parsed.tool_use_id)) {
+            throw new Error(
+              `messages[${messageIndex}].content[${blockIndex}].tool_use_id "${parsed.tool_use_id}" does not match a pending tool_use`
+            );
+          }
+          if (resolvedToolUseIds.has(parsed.tool_use_id)) {
+            throw new Error(
+              `messages[${messageIndex}].content[${blockIndex}].tool_use_id "${parsed.tool_use_id}" is duplicated`
+            );
+          }
+          sawToolResult = true;
+          resolvedToolUseIds.add(parsed.tool_use_id);
+          translatedMessages.push({
+            role: 'tool',
+            tool_call_id: parsed.tool_use_id,
+            content: convertToolResultContent(
+              parsed.content,
+              parsed.is_error === true,
+              `messages[${messageIndex}].content[${blockIndex}].content`
+            ),
+          });
+          return;
+        }
+
+        if (isToolUseBlock(parsed)) {
+          throw new Error(
+            `messages[${messageIndex}].content[${blockIndex}] tool_use requires assistant role`
+          );
+        }
+
+        throw new Error(
+          `messages[${messageIndex}].content[${blockIndex}].type "${String(parsed.type)}" is not supported`
+        );
+      });
+
+      if (sawToolResult) {
+        if (resolvedToolUseIds.size !== pendingToolUseIds?.size) {
+          throw new Error(
+            `messages[${messageIndex}].content must provide tool_result blocks for all pending tool_use ids`
+          );
+        }
+        pendingToolUseIds = null;
+        hasPendingToolUseIds = false;
+        return;
+      }
+
+      if (pendingToolUseIds && pendingToolUseIds.size > 0) {
+        throw new Error(
+          `messages[${messageIndex}].content must start with tool_result blocks for pending tool_use ids`
+        );
+      }
+
+      if (userParts.length > 0) {
+        flushUserContent(translatedMessages, userParts);
+      }
+      return;
+    }
+
+    // Assistant role
     const assistantTextParts: string[] = [];
     const toolCalls: NonNullable<OpenAIMessage['tool_calls']> = [];
-    let sawToolResult = false;
 
     content.forEach((block, blockIndex) => {
       const parsed = assertObject(
@@ -309,32 +576,17 @@ function transformMessages(messagesValue: unknown): OpenAIMessage[] {
         `messages[${messageIndex}].content[${blockIndex}]`
       ) as AnthropicContentBlock;
 
-      if (parsed.type === 'text') {
-        const text = typeof parsed.text === 'string' ? parsed.text : '';
-        if (role === 'user') {
-          userParts.push({ type: 'text', text });
-        } else {
-          assistantTextParts.push(text);
-        }
+      if (parsed.type === 'thinking' || parsed.type === 'redacted_thinking') {
         return;
       }
 
-      if (isImageBlock(parsed)) {
-        if (role !== 'user') {
-          throw new Error(
-            `messages[${messageIndex}].content[${blockIndex}] image requires user role`
-          );
-        }
-        userParts.push(toImagePart(parsed, `messages[${messageIndex}].content[${blockIndex}]`));
+      if (parsed.type === 'text') {
+        const text = typeof parsed.text === 'string' ? parsed.text : '';
+        assistantTextParts.push(text);
         return;
       }
 
       if (isToolUseBlock(parsed)) {
-        if (role !== 'assistant') {
-          throw new Error(
-            `messages[${messageIndex}].content[${blockIndex}] tool_use requires assistant role`
-          );
-        }
         toolCalls.push({
           id:
             typeof parsed.id === 'string' && parsed.id.length > 0
@@ -349,28 +601,16 @@ function transformMessages(messagesValue: unknown): OpenAIMessage[] {
         return;
       }
 
+      if (isImageBlock(parsed)) {
+        throw new Error(
+          `messages[${messageIndex}].content[${blockIndex}] image requires user role`
+        );
+      }
+
       if (isToolResultBlock(parsed)) {
-        if (role !== 'user') {
-          throw new Error(
-            `messages[${messageIndex}].content[${blockIndex}] tool_result requires user role`
-          );
-        }
-        if (typeof parsed.tool_use_id !== 'string' || parsed.tool_use_id.trim().length === 0) {
-          throw new Error(
-            `messages[${messageIndex}].content[${blockIndex}].tool_use_id must be a non-empty string`
-          );
-        }
-        sawToolResult = true;
-        flushUserContent(translatedMessages, userParts);
-        translatedMessages.push({
-          role: 'tool',
-          tool_call_id: parsed.tool_use_id,
-          content: toToolResultContent(
-            parsed.content,
-            `messages[${messageIndex}].content[${blockIndex}].content`
-          ),
-        });
-        return;
+        throw new Error(
+          `messages[${messageIndex}].content[${blockIndex}] tool_result requires user role`
+        );
       }
 
       throw new Error(
@@ -378,26 +618,72 @@ function transformMessages(messagesValue: unknown): OpenAIMessage[] {
       );
     });
 
-    if (role === 'assistant') {
-      translatedMessages.push({
-        role: 'assistant',
-        content: assistantTextParts.join('\n'),
-        tool_calls: toolCalls.length > 0 ? toolCalls : undefined,
-      });
+    if (assistantTextParts.length === 0 && toolCalls.length === 0) {
       return;
     }
 
-    if (userParts.length > 0 || !sawToolResult) {
-      flushUserContent(translatedMessages, userParts);
-    }
+    pendingToolUseIds =
+      toolCalls.length > 0 ? new Set(toolCalls.map((toolCall) => toolCall.id)) : null;
+    hasPendingToolUseIds = toolCalls.length > 0;
+
+    translatedMessages.push({
+      role: 'assistant',
+      content: assistantTextParts.join('\n'),
+      tool_calls: toolCalls.length > 0 ? toolCalls : undefined,
+    });
   });
 
+  if (hasPendingToolUseIds) {
+    throw new Error('messages must provide tool_result blocks for the latest assistant tool_use');
+  }
+
   return translatedMessages;
 }
 
+/**
+ * Coalesce consecutive messages of the same role.
+ * OpenAI/vLLM/Ollama/Mistral require strict user<->assistant alternation.
+ * Multiple consecutive tool messages are allowed (assistant -> tool* -> user).
+ * Ported from openclaude's coalescing pass.
+ */
+function coalesceMessages(messages: OpenAIMessage[]): OpenAIMessage[] {
+  const coalesced: OpenAIMessage[] = [];
+
+  for (const msg of messages) {
+    const prev = coalesced[coalesced.length - 1];
+
+    if (prev && prev.role === msg.role && msg.role !== 'tool' && msg.role !== 'system') {
+      const prevContent = prev.content;
+      const curContent = msg.content;
+
+      if (typeof prevContent === 'string' && typeof curContent === 'string') {
+        prev.content = prevContent + (prevContent && curContent ? '\n' : '') + curContent;
+      } else {
+        const toArray = (
+          c: string | OpenAIContentPart[] | null | undefined
+        ): OpenAIContentPart[] => {
+          if (!c) return [];
+          if (typeof c === 'string') return c ? [{ type: 'text', text: c }] : [];
+          return c;
+        };
+        prev.content = [...toArray(prevContent), ...toArray(curContent)];
+      }
+
+      if (msg.tool_calls?.length) {
+        prev.tool_calls = [...(prev.tool_calls ?? []), ...msg.tool_calls];
+      }
+    } else {
+      coalesced.push({ ...msg });
+    }
+  }
+
+  return coalesced;
+}
+
 export class ProxyRequestTransformer {
   transform(raw: unknown): ProxyOpenAIRequest {
     const source = assertObject(raw || {}, 'request') as AnthropicProxyRequestShape;
+    const tools = transformTools(source.tools);
     const messages = transformMessages(source.messages);
     const system = source.system;
     const allMessages =
@@ -414,14 +700,15 @@ export class ProxyRequestTransformer {
           ? source.model.trim()
           : undefined,
       stream: source.stream === true,
-      messages: allMessages,
+      messages: coalesceMessages(allMessages),
       max_tokens: asNumber(source.max_tokens),
       temperature: asNumber(source.temperature),
       top_p: asNumber(source.top_p),
       stop: asStringArray(source.stop_sequences),
       metadata: asMetadata(source.metadata),
-      tools: transformTools(source.tools),
-      ...mapThinkingToReasoning(source.thinking),
+      tools,
+      ...transformToolChoice(source.tool_choice, tools !== undefined),
+      ...mapThinkingToReasoning(source.thinking, source.output_config),
     };
   }
 }
diff --git a/tests/integration/proxy/messages-endpoint.test.ts b/tests/integration/proxy/messages-endpoint.test.ts
index d6ce7469..0c3fe0d0 100644
--- a/tests/integration/proxy/messages-endpoint.test.ts
+++ b/tests/integration/proxy/messages-endpoint.test.ts
@@ -116,13 +116,63 @@ describe('openai proxy messages endpoint', () => {
 
     const parsedUpstream = upstreamBody as {
       messages?: Array<{ role: string; content: string }>;
+      tool_choice?: unknown;
       tools?: Array<{ type: string; function: { name: string } }>;
     };
     expect(parsedUpstream.messages?.[0]).toEqual({ role: 'user', content: 'Find docs' });
+    expect(parsedUpstream.tool_choice).toBe('auto');
     expect(parsedUpstream.tools?.[0]?.type).toBe('function');
     expect(parsedUpstream.tools?.[0]?.function.name).toBe('search');
   });
 
+  it('preserves tool schemas and forwards explicit tool_choice semantics upstream', async () => {
+    const response = await requestProxy({
+      model: 'hf-model',
+      messages: [{ role: 'user', content: [{ type: 'text', text: 'Search docs' }] }],
+      tools: [
+        {
+          name: 'search',
+          description: 'Search docs',
+          input_schema: {
+            type: 'object',
+            properties: {
+              q: { type: 'string', pattern: '^[a-z]+$' },
+            },
+            required: ['q'],
+            additionalProperties: true,
+          },
+        },
+      ],
+      tool_choice: {
+        type: 'tool',
+        name: 'search',
+        disable_parallel_tool_use: true,
+      },
+    });
+
+    expect(response.status).toBe(200);
+
+    const parsedUpstream = upstreamBody as {
+      tool_choice?: unknown;
+      parallel_tool_calls?: boolean;
+      tools?: Array<{ type: string; function: { parameters: Record<string, unknown> } }>;
+    };
+
+    expect(parsedUpstream.tool_choice).toEqual({
+      type: 'function',
+      function: { name: 'search' },
+    });
+    expect(parsedUpstream.parallel_tool_calls).toBe(false);
+    expect(parsedUpstream.tools?.[0]?.function.parameters).toEqual({
+      type: 'object',
+      properties: {
+        q: { type: 'string', pattern: '^[a-z]+$' },
+      },
+      required: ['q'],
+      additionalProperties: true,
+    });
+  });
+
   it('falls back to Anthropic JSON for non-streaming requests', async () => {
     const response = await requestProxy({
       model: 'hf-model',
@@ -155,6 +205,24 @@ describe('openai proxy messages endpoint', () => {
     expect(body.error?.message).toContain('Invalid JSON');
   });
 
+  it('returns invalid_request_error for orphan tool_result blocks', async () => {
+    const response = await requestProxy({
+      model: 'hf-model',
+      messages: [
+        {
+          role: 'user',
+          content: [{ type: 'tool_result', tool_use_id: 'toolu_orphan', content: 'orphan' }],
+        },
+      ],
+    });
+
+    const body = (await response.json()) as { error?: { type?: string; message?: string } };
+
+    expect(response.status).toBe(400);
+    expect(body.error?.type).toBe('invalid_request_error');
+    expect(body.error?.message).toContain('tool_result requires a preceding assistant tool_use');
+  });
+
   it('rejects requests without the local proxy auth token', async () => {
     const response = await fetch(`http://127.0.0.1:${proxyPort}/v1/messages`, {
       method: 'POST',
diff --git a/tests/integration/proxy/request-routing.test.ts b/tests/integration/proxy/request-routing.test.ts
index d5a8ce0d..b9f7afe8 100644
--- a/tests/integration/proxy/request-routing.test.ts
+++ b/tests/integration/proxy/request-routing.test.ts
@@ -214,4 +214,75 @@ describe('openai proxy request routing', () => {
     expect(hits).toEqual(['thinker']);
     expect(bodies[0]?.body).toMatchObject({ model: 'deepseek-reasoner' });
   });
+
+  it('routes adaptive thinking requests through the configured think scenario', async () => {
+    const primaryPort = await getPort();
+    const thinkPort = await getPort();
+    const hits: string[] = [];
+    const bodies: Array<{ label: string; body: unknown }> = [];
+    await startMockUpstream(primaryPort, 'primary', hits, bodies);
+    await startMockUpstream(thinkPort, 'thinker', hits, bodies);
+
+    const primarySettings = writeSettings('hf', {
+      ANTHROPIC_BASE_URL: `http://127.0.0.1:${primaryPort}`,
+      ANTHROPIC_AUTH_TOKEN: 'hf_token',
+      ANTHROPIC_MODEL: 'hf-default',
+      CCS_DROID_PROVIDER: 'generic-chat-completion-api',
+    });
+    const thinkSettings = writeSettings('thinker', {
+      ANTHROPIC_BASE_URL: `http://127.0.0.1:${thinkPort}`,
+      ANTHROPIC_AUTH_TOKEN: 'think_token',
+      ANTHROPIC_MODEL: 'deepseek-reasoner',
+      CCS_DROID_PROVIDER: 'generic-chat-completion-api',
+    });
+
+    fs.writeFileSync(
+      path.join(tempDir, '.ccs', 'config.json'),
+      JSON.stringify(
+        {
+          profiles: { hf: primarySettings, thinker: thinkSettings },
+          proxy: {
+            routing: {
+              think: 'thinker:deepseek-reasoner',
+            },
+          },
+        },
+        null,
+        2
+      ),
+      'utf8'
+    );
+
+    const profile: OpenAICompatProfileConfig = {
+      profileName: 'hf',
+      settingsPath: primarySettings,
+      baseUrl: `http://127.0.0.1:${primaryPort}`,
+      apiKey: 'hf_token',
+      provider: 'generic-chat-completion-api',
+      model: 'hf-default',
+    };
+    proxyServer = startOpenAICompatProxyServer({
+      profile,
+      port: proxyPort,
+      authToken: 'test-proxy-token',
+    });
+
+    const response = await requestProxy({
+      model: 'hf-default',
+      thinking: { type: 'adaptive' },
+      output_config: { effort: 'max' },
+      messages: [{ role: 'user', content: 'think adaptively' }],
+    });
+
+    expect(response.status).toBe(200);
+    expect(await response.json()).toMatchObject({
+      content: [{ type: 'text', text: 'Reply from thinker' }],
+    });
+    expect(hits).toEqual(['thinker']);
+    expect(bodies[0]?.body).toMatchObject({
+      model: 'deepseek-reasoner',
+      reasoning_effort: 'high',
+      reasoning: { enabled: true, effort: 'high' },
+    });
+  });
 });
diff --git a/tests/unit/proxy/transformers/request-transformer-regressions.test.ts b/tests/unit/proxy/transformers/request-transformer-regressions.test.ts
new file mode 100644
index 00000000..e9a05c18
--- /dev/null
+++ b/tests/unit/proxy/transformers/request-transformer-regressions.test.ts
@@ -0,0 +1,255 @@
+import { describe, expect, it } from 'bun:test';
+
+import { ProxyRequestTransformer } from '../../../../src/proxy/transformers/request-transformer';
+
+describe('ProxyRequestTransformer regressions', () => {
+  it('drops assistant messages that only contain stripped thinking blocks', () => {
+    const result = new ProxyRequestTransformer().transform({
+      messages: [
+        {
+          role: 'assistant',
+          content: [
+            { type: 'thinking', text: 'internal' },
+            { type: 'redacted_thinking', text: 'hidden' },
+          ],
+        },
+      ],
+    });
+
+    expect(result.messages).toEqual([]);
+  });
+
+  it('maps adaptive thinking through output_config effort for OpenAI-compatible upstreams', () => {
+    const result = new ProxyRequestTransformer().transform({
+      messages: [{ role: 'user', content: 'hello' }],
+      thinking: { type: 'adaptive' },
+      output_config: { effort: 'max' },
+    });
+
+    expect(result.reasoning_effort).toBe('high');
+    expect(result.reasoning).toEqual({ enabled: true, effort: 'high' });
+  });
+
+  it('rejects unsupported thinking types instead of silently dropping them', () => {
+    expect(() =>
+      new ProxyRequestTransformer().transform({
+        messages: [{ role: 'user', content: 'hello' }],
+        thinking: { type: 'typo' },
+      })
+    ).toThrow('thinking.type must be "enabled", "adaptive", or "disabled"');
+  });
+
+  it('keeps Anthropic role validation for tool_use, image, and tool_result blocks', () => {
+    expect(() =>
+      new ProxyRequestTransformer().transform({
+        messages: [{ role: 'user', content: [{ type: 'tool_use', name: 'search', input: {} }] }],
+      })
+    ).toThrow('tool_use requires assistant role');
+
+    expect(() =>
+      new ProxyRequestTransformer().transform({
+        messages: [
+          {
+            role: 'assistant',
+            content: [
+              {
+                type: 'image',
+                source: { type: 'url', url: 'https://example.com/image.png' },
+              },
+            ],
+          },
+        ],
+      })
+    ).toThrow('image requires user role');
+
+    expect(() =>
+      new ProxyRequestTransformer().transform({
+        messages: [
+          {
+            role: 'assistant',
+            content: [{ type: 'tool_result', tool_use_id: 'toolu_1', content: 'nope' }],
+          },
+        ],
+      })
+    ).toThrow('tool_result requires user role');
+  });
+
+  it('rejects orphaned, incomplete, or mixed-order tool_result blocks', () => {
+    expect(() =>
+      new ProxyRequestTransformer().transform({
+        messages: [
+          {
+            role: 'user',
+            content: [{ type: 'tool_result', tool_use_id: 'toolu_1', content: 'orphan' }],
+          },
+        ],
+      })
+    ).toThrow('tool_result requires a preceding assistant tool_use');
+
+    expect(() =>
+      new ProxyRequestTransformer().transform({
+        messages: [
+          {
+            role: 'assistant',
+            content: [
+              { type: 'tool_use', id: 'toolu_1', name: 'search', input: { q: 'docs' } },
+              { type: 'tool_use', id: 'toolu_2', name: 'open', input: { url: 'https://example.com' } },
+            ],
+          },
+          {
+            role: 'user',
+            content: [{ type: 'tool_result', tool_use_id: 'toolu_1', content: 'partial' }],
+          },
+        ],
+      })
+    ).toThrow('must provide tool_result blocks for all pending tool_use ids');
+
+    expect(() =>
+      new ProxyRequestTransformer().transform({
+        messages: [
+          {
+            role: 'assistant',
+            content: [{ type: 'tool_use', id: 'toolu_1', name: 'vision', input: { detail: 'high' } }],
+          },
+          {
+            role: 'user',
+            content: [
+              { type: 'text', text: 'Here you go' },
+              { type: 'tool_result', tool_use_id: 'toolu_1', content: 'result' },
+            ],
+          },
+        ],
+      })
+    ).toThrow('tool_result blocks must come before other user content');
+
+    expect(() =>
+      new ProxyRequestTransformer().transform({
+        messages: [
+          {
+            role: 'assistant',
+            content: [{ type: 'tool_use', id: 'toolu_1', name: 'vision', input: { detail: 'high' } }],
+          },
+          {
+            role: 'user',
+            content: 'plain follow-up',
+          },
+        ],
+      })
+    ).toThrow('must start with tool_result blocks for pending tool_use ids');
+  });
+
+  it('rejects tool_result content that cannot be represented as OpenAI tool text', () => {
+    expect(() =>
+      new ProxyRequestTransformer().transform({
+        messages: [
+          {
+            role: 'assistant',
+            content: [{ type: 'tool_use', id: 'toolu_1', name: 'vision', input: { detail: 'high' } }],
+          },
+          {
+            role: 'user',
+            content: [
+              {
+                type: 'tool_result',
+                tool_use_id: 'toolu_1',
+                content: [{ type: 'image', source: { type: 'url', url: 'https://example.com/error.png' } }],
+              },
+            ],
+          },
+        ],
+      })
+    ).toThrow('type "image" is not supported in tool_result content');
+  });
+
+  it('rejects unsupported assistant blocks instead of silently dropping them', () => {
+    expect(() =>
+      new ProxyRequestTransformer().transform({
+        messages: [
+          {
+            role: 'assistant',
+            content: [{ type: 'server_tool_use', id: 'srv_1' }],
+          },
+        ],
+      })
+    ).toThrow('type "server_tool_use" is not supported');
+  });
+
+  it('translates url images and tool_choice while coalescing repeated turns', () => {
+    const result = new ProxyRequestTransformer().transform({
+      tool_choice: {
+        type: 'tool',
+        name: 'vision',
+        disable_parallel_tool_use: true,
+      },
+      tools: [{ name: 'vision', description: 'Inspect image', input_schema: { type: 'object' } }],
+      messages: [
+        {
+          role: 'user',
+          content: [{ type: 'image', source: { type: 'url', url: 'https://example.com/cat.png' } }],
+        },
+        { role: 'user', content: [{ type: 'text', text: 'Describe it' }] },
+        {
+          role: 'assistant',
+          content: [{ type: 'text', text: 'Checking' }],
+        },
+        {
+          role: 'assistant',
+          content: [{ type: 'tool_use', id: 'toolu_1', name: 'vision', input: { detail: 'high' } }],
+        },
+        {
+          role: 'user',
+          content: [
+            {
+              type: 'tool_result',
+              tool_use_id: 'toolu_1',
+              is_error: true,
+              content: [{ type: 'text', text: 'fetch failed' }],
+            },
+          ],
+        },
+      ],
+    });
+
+    expect(result.tool_choice).toEqual({
+      type: 'function',
+      function: { name: 'vision' },
+    });
+    expect(result.parallel_tool_calls).toBe(false);
+
+    expect(result.messages[0]).toEqual({
+      role: 'user',
+      content: [
+        { type: 'image_url', image_url: { url: 'https://example.com/cat.png' } },
+        { type: 'text', text: 'Describe it' },
+      ],
+    });
+    expect(result.messages[1]).toEqual({
+      role: 'assistant',
+      content: 'Checking',
+      tool_calls: [
+        {
+          id: 'toolu_1',
+          type: 'function',
+          function: {
+            name: 'vision',
+            arguments: '{"detail":"high"}',
+          },
+        },
+      ],
+    });
+    expect(result.messages[2]).toEqual({
+      role: 'tool',
+      tool_call_id: 'toolu_1',
+      content: 'Error: fetch failed',
+    });
+  });
+
+  it('defaults tools to auto tool_choice when none is specified', () => {
+    const result = new ProxyRequestTransformer().transform({
+      messages: [{ role: 'user', content: 'hello' }],
+      tools: [{ name: 'search', description: 'Search docs', input_schema: { type: 'object' } }],
+    });
+
+    expect(result.tool_choice).toBe('auto');
+  });
+});