diff --git a/open-sse/translator/request/openai-to-claude.js b/open-sse/translator/request/openai-to-claude.js
index f1a9baae..9988c769 100644
--- a/open-sse/translator/request/openai-to-claude.js
+++ b/open-sse/translator/request/openai-to-claude.js
@@ -291,15 +291,35 @@ function getContentBlocksFromMessage(msg, toolNameMap = new Map()) {
   return blocks;
 }
 
-// Convert OpenAI tool choice to Claude format
+// Convert OpenAI tool choice to Claude format.
+// Claude only accepts tool_choice.type of "auto" | "any" | "tool" | "none";
+// anything else (e.g. OpenAI's "function") triggers a 400, so we never pass an
+// unrecognized type through.
+const CLAUDE_TOOL_CHOICE_TYPES = new Set(["auto", "any", "tool", "none"]);
+
 function convertOpenAIToolChoice(choice) {
   if (!choice) return { type: "auto" };
-  if (typeof choice === "object" && choice.type) return choice;
-  if (choice === "auto" || choice === "none") return { type: "auto" };
-  if (choice === "required") return { type: "any" };
-  if (typeof choice === "object" && choice.function) {
-    return { type: "tool", name: choice.function.name };
+
+  // OpenAI string forms: "auto" | "none" | "required"
+  if (typeof choice === "string") {
+    if (choice === "required") return { type: "any" };
+    return { type: "auto" }; // "auto", "none", or anything unexpected
   }
+
+  if (typeof choice === "object") {
+    // OpenAI forced tool: { type: "function", function: { name } }.
+    // Checked before the native pass-through below, because the OpenAI shape
+    // also carries a `.type` ("function") that Claude rejects.
+    if (choice.function?.name) {
+      return { type: "tool", name: choice.function.name };
+    }
+    // Already Claude-native — only pass through types Claude actually accepts,
+    // so a malformed or unknown type can never leak into the upstream request.
+    if (CLAUDE_TOOL_CHOICE_TYPES.has(choice.type)) {
+      return choice;
+    }
+  }
+
   return { type: "auto" };
 }
 
diff --git a/open-sse/utils/claudeCloaking.js b/open-sse/utils/claudeCloaking.js
index 2c9d1f26..688573ab 100644
--- a/open-sse/utils/claudeCloaking.js
+++ b/open-sse/utils/claudeCloaking.js
@@ -35,13 +35,16 @@ export function cloakClaudeTools(body) {
   const tools = body.tools;
   if (!tools || tools.length === 0) return { body, toolNameMap: null };
 
+  const suffix = (name) => `${name}${CLAUDE_TOOL_SUFFIX}`;
   const toolNameMap = new Map();
+  const clientToolNames = new Set();
   const clientDeclarations = [];
 
   // All client tools get renamed with suffix
   for (const tool of tools) {
-    const suffixed = `${tool.name}${CLAUDE_TOOL_SUFFIX}`;
+    const suffixed = suffix(tool.name);
     toolNameMap.set(suffixed, tool.name);
+    clientToolNames.add(tool.name);
     clientDeclarations.push({ ...tool, name: suffixed });
   }
 
@@ -51,17 +54,27 @@ export function cloakClaudeTools(body) {
   // Rename tool_use in message history (all client tools get suffix)
   const renamedMessages = body.messages?.map(msg => {
     if (!Array.isArray(msg.content)) return msg;
-    const renamedContent = msg.content.map(block => {
-      if (block.type === "tool_use") {
-        return { ...block, name: `${block.name}${CLAUDE_TOOL_SUFFIX}` };
-      }
-      return block;
-    });
+    const renamedContent = msg.content.map(block =>
+      block.type === "tool_use" ? { ...block, name: suffix(block.name) } : block
+    );
     return { ...msg, content: renamedContent };
   });
 
+  const cloakedBody = { ...body, tools: allTools, messages: renamedMessages || body.messages };
+
+  // A forced tool_choice ({ type: "tool", name }) must point at the suffixed
+  // tool name, otherwise Claude rejects it: "Tool '<name>' not found in provided tools".
+  // Only rewrite when the choice targets one of the client tools we actually
+  // renamed — never a decoy/built-in name (those are sent unsuffixed).
+  if (
+    body.tool_choice?.type === "tool" &&
+    clientToolNames.has(body.tool_choice.name)
+  ) {
+    cloakedBody.tool_choice = { ...body.tool_choice, name: suffix(body.tool_choice.name) };
+  }
+
   return {
-    body: { ...body, tools: allTools, messages: renamedMessages || body.messages },
+    body: cloakedBody,
     toolNameMap: toolNameMap.size > 0 ? toolNameMap : null
   };
 }
diff --git a/tests/unit/claude-cloaking.test.js b/tests/unit/claude-cloaking.test.js
new file mode 100644
index 00000000..6b6e5dc6
--- /dev/null
+++ b/tests/unit/claude-cloaking.test.js
@@ -0,0 +1,76 @@
+/**
+ * Unit tests for open-sse/utils/claudeCloaking.js
+ *
+ * Tests cover:
+ *  - cloakClaudeTools() - tool renaming and forced tool_choice suffixing
+ */
+
+import { describe, it, expect } from "vitest";
+import { cloakClaudeTools } from "../../open-sse/utils/claudeCloaking.js";
+import { CLAUDE_TOOL_SUFFIX } from "../../open-sse/config/appConstants.js";
+
+describe("cloakClaudeTools", () => {
+  const baseBody = {
+    tools: [{ name: "todo_write", description: "write todos", input_schema: { type: "object", properties: {} } }],
+    messages: [{ role: "user", content: [{ type: "text", text: "add a todo" }] }]
+  };
+
+  it("suffixes client tool names and maps them back", () => {
+    const { body, toolNameMap } = cloakClaudeTools(baseBody);
+    const suffixed = `todo_write${CLAUDE_TOOL_SUFFIX}`;
+    expect(body.tools.find(t => t.name === suffixed)).toBeDefined();
+    expect(toolNameMap.get(suffixed)).toBe("todo_write");
+  });
+
+  it("suffixes a forced tool_choice to match the renamed tool", () => {
+    const { body } = cloakClaudeTools({
+      ...baseBody,
+      tool_choice: { type: "tool", name: "todo_write" }
+    });
+    // Without this, Claude rejects: "Tool 'todo_write' not found in provided tools".
+    expect(body.tool_choice).toEqual({ type: "tool", name: `todo_write${CLAUDE_TOOL_SUFFIX}` });
+  });
+
+  it("suffixes only the chosen tool when several are present", () => {
+    const { body } = cloakClaudeTools({
+      tools: [
+        { name: "search", input_schema: { type: "object", properties: {} } },
+        { name: "todo_write", input_schema: { type: "object", properties: {} } }
+      ],
+      tool_choice: { type: "tool", name: "todo_write" }
+    });
+    expect(body.tool_choice).toEqual({ type: "tool", name: `todo_write${CLAUDE_TOOL_SUFFIX}` });
+  });
+
+  it("leaves non-forced tool_choice untouched", () => {
+    const auto = cloakClaudeTools({ ...baseBody, tool_choice: { type: "auto" } });
+    expect(auto.body.tool_choice).toEqual({ type: "auto" });
+
+    const none = cloakClaudeTools({ ...baseBody });
+    expect(none.body.tool_choice).toBeUndefined();
+  });
+
+  it("does not suffix a forced choice that targets a non-client (decoy/built-in) tool", () => {
+    // "Bash" is an injected decoy sent unsuffixed; forcing it must stay as-is.
+    const { body } = cloakClaudeTools({ ...baseBody, tool_choice: { type: "tool", name: "Bash" } });
+    expect(body.tool_choice).toEqual({ type: "tool", name: "Bash" });
+  });
+
+  it("renames tool_use names in message history", () => {
+    const { body } = cloakClaudeTools({
+      ...baseBody,
+      messages: [
+        { role: "assistant", content: [{ type: "tool_use", id: "t1", name: "todo_write", input: {} }] }
+      ]
+    });
+    const block = body.messages[0].content[0];
+    expect(block.name).toBe(`todo_write${CLAUDE_TOOL_SUFFIX}`);
+  });
+
+  it("returns the body unchanged when there are no tools", () => {
+    const input = { messages: [{ role: "user", content: "hi" }], tool_choice: { type: "tool", name: "x" } };
+    const { body, toolNameMap } = cloakClaudeTools(input);
+    expect(body).toBe(input);
+    expect(toolNameMap).toBeNull();
+  });
+});
diff --git a/tests/unit/openai-to-claude.test.js b/tests/unit/openai-to-claude.test.js
index 136f6d00..45b67fb2 100644
--- a/tests/unit/openai-to-claude.test.js
+++ b/tests/unit/openai-to-claude.test.js
@@ -122,6 +122,50 @@ describe("openaiToClaudeRequest", () => {
       expect(systemText).toContain("You must respond with valid JSON");
     });
   });
+
+  describe("tool_choice handling", () => {
+    const baseBody = {
+      messages: [{ role: "user", content: "add a todo" }],
+      tools: [{
+        type: "function",
+        function: { name: "todo_write", description: "write todos", parameters: { type: "object", properties: {} } }
+      }]
+    };
+
+    const choiceOf = (tc) =>
+      openaiToClaudeRequest("claude-sonnet-4.5", { ...baseBody, tool_choice: tc }, false).tool_choice;
+
+    it("converts OpenAI forced tool ({type:'function'}) to Claude {type:'tool'}", () => {
+      // Must NOT leak the OpenAI "function" type — Claude only accepts auto|any|tool|none.
+      expect(choiceOf({ type: "function", function: { name: "todo_write" } }))
+        .toEqual({ type: "tool", name: "todo_write" });
+    });
+
+    it("maps string tool_choice values", () => {
+      expect(choiceOf("auto")).toEqual({ type: "auto" });
+      expect(choiceOf("none")).toEqual({ type: "auto" });
+      expect(choiceOf("required")).toEqual({ type: "any" });
+    });
+
+    it("passes through Claude-native tool_choice objects unchanged", () => {
+      expect(choiceOf({ type: "tool", name: "todo_write" })).toEqual({ type: "tool", name: "todo_write" });
+      expect(choiceOf({ type: "any" })).toEqual({ type: "any" });
+      expect(choiceOf({ type: "none" })).toEqual({ type: "none" });
+    });
+
+    it("never leaks an invalid type (falls back to auto)", () => {
+      // Malformed forced choice with no tool name, and unknown types, must not
+      // pass an invalid `type` through to Claude.
+      expect(choiceOf({ type: "function", function: {} })).toEqual({ type: "auto" });
+      expect(choiceOf({ type: "function" })).toEqual({ type: "auto" });
+      expect(choiceOf({ type: "bogus" })).toEqual({ type: "auto" });
+    });
+
+    it("omits tool_choice entirely when the request has none", () => {
+      const result = openaiToClaudeRequest("claude-sonnet-4.5", baseBody, false);
+      expect(result.tool_choice).toBeUndefined();
+    });
+  });
 });
 
 describe("openaiToClaudeResponse", () => {