diff --git a/open-sse/providers/registry/deepseek.js b/open-sse/providers/registry/deepseek.js
index bb8015b0..2c3a3e73 100644
--- a/open-sse/providers/registry/deepseek.js
+++ b/open-sse/providers/registry/deepseek.js
@@ -25,6 +25,21 @@ export default {
     reasoningInject: {
       scope: "all",
     },
+    quirks: {
+      // DeepSeek's Anthropic-compatible endpoint
+      // (https://api.deepseek.com/anthropic/v1/messages) accepts ONLY the
+      // built-in web_search_* tools and rejects client-defined `custom` tools
+      // (MCP / Read / Bash / etc.) with HTTP 400
+      //   "tools[0]: unknown variant `custom`, expected
+      //    `web_search_20250305` or `web_search_20260209`".
+      //
+      // Declaring this whitelist makes prepareClaudeRequest() forward only
+      // web_search_* tools and strip everything else before sending, so MCP /
+      // function tools are dropped instead of failing the whole request.
+      // DeepSeek's OpenAI-compatible transport is unaffected (targetFormat
+      // there is "openai", not "claude", so prepareClaudeRequest is not run).
+      claudeSupportedToolTypes: ["web_search_20250305", "web_search_20260209"],
+    },
   },
   // Multi-endpoint: pick the transport matching client sourceFormat to skip translation.
   transports: [
diff --git a/open-sse/translator/formats/claude.js b/open-sse/translator/formats/claude.js
index 62a0531a..930c761a 100644
--- a/open-sse/translator/formats/claude.js
+++ b/open-sse/translator/formats/claude.js
@@ -461,8 +461,21 @@ export function prepareClaudeRequest(body, provider = null, apiKey = null, conne
     // Strip built-in tools (e.g. web_search_20250305) and normalize to Anthropic-native shape
     // (drop `type` field, fold `function.{name,description,parameters}`) for non-Anthropic providers
     if (provider !== "claude") {
+      // Provider-specific whitelist of Anthropic tool `type` values that the
+      // upstream actually accepts. When the provider declares it
+      // (e.g. DeepSeek — only web_search_*), keep only listed types; otherwise
+      // keep the prior behaviour of dropping every non-function tool, which is
+      // correct for OpenAI-compatible targets reached through this Claude-format
+      // pass (their tools get normalized below to function-style).
+      const supportedTypes = PROVIDERS[provider]?.quirks?.claudeSupportedToolTypes;
+      const hasWhitelist = Array.isArray(supportedTypes);
       body.tools = body.tools
-        .filter(tool => !tool.type || tool.type === "function")
+        .filter(tool => {
+          const t = tool?.type;
+          if (!t || t === "function") return true;
+          if (hasWhitelist) return supportedTypes.includes(t);
+          return false;
+        })
         .map(tool => {
           if (tool.function) {
             return {
@@ -471,6 +484,13 @@ export function prepareClaudeRequest(body, provider = null, apiKey = null, conne
               input_schema: tool.function.parameters,
             };
           }
+          // When the provider declared a supportedToolTypes whitelist, keep
+          // the surviving tools' `type` field intact — the upstream
+          // Anthropic-compatible endpoint (e.g. DeepSeek) requires it to
+          // route built-ins like web_search_* correctly. Without a
+          // whitelist, preserve prior behaviour and strip `type` so the
+          // tool is normalized to plain Anthropic shape.
+          if (hasWhitelist) return tool;
           const { type, ...rest } = tool;
           return rest;
         });
diff --git a/tests/unit/deepseek-claude-tools.test.js b/tests/unit/deepseek-claude-tools.test.js
new file mode 100644
index 00000000..bd05e208
--- /dev/null
+++ b/tests/unit/deepseek-claude-tools.test.js
@@ -0,0 +1,145 @@
+/**
+ * Regression test: prepareClaudeRequest() must strip client-defined `custom`
+ * tools when forwarding to a provider whose Anthropic-compatible endpoint
+ * does not accept them (DeepSeek — accepts only web_search_*).
+ *
+ * Background:
+ *   When Claude Code talks to a DeepSeek route via /v1/messages, 9router
+ *   forwards the request body as Claude-format to
+ *   https://api.deepseek.com/anthropic/v1/messages. MCP / function tools
+ *   arrive with `type: "custom"`. DeepSeek rejects them with HTTP 400
+ *   "tools[0]: unknown variant `custom`, expected `web_search_20250305`
+ *   or `web_search_20260209`". The previous generic filter dropped them
+ *   but also stripped the web_search_* tools that DeepSeek actually
+ *   accepts. DeepSeek now exposes a `quirks.claudeSupportedToolTypes`
+ *   whitelist and prepareClaudeRequest honours it.
+ */
+
+import { describe, it, expect } from "vitest";
+import { prepareClaudeRequest } from "../../open-sse/translator/formats/claude.js";
+import { PROVIDERS } from "../../open-sse/providers/index.js";
+
+function makeBody(tools) {
+  return {
+    model: "deepseek-v4-pro",
+    max_tokens: 1024,
+    messages: [{ role: "user", content: "hello" }],
+    tools,
+  };
+}
+
+describe("prepareClaudeRequest — provider: deepseek", () => {
+  it("declares the supportedTypes quirk on the provider transport", () => {
+    expect(PROVIDERS.deepseek).toBeDefined();
+    expect(PROVIDERS.deepseek.quirks).toBeDefined();
+    expect(PROVIDERS.deepseek.quirks.claudeSupportedToolTypes).toEqual([
+      "web_search_20250305",
+      "web_search_20260209",
+    ]);
+  });
+
+  it("strips MCP / custom tools (the regression) before forwarding", () => {
+    const body = makeBody([
+      { type: "custom", name: "Bash", input_schema: { type: "object" } },
+      { type: "custom", name: "Read", input_schema: { type: "object" } },
+      { type: "custom", name: "Glob", input_schema: { type: "object" } },
+    ]);
+
+    const out = prepareClaudeRequest(body, "deepseek");
+
+    expect(out.tools).toBeUndefined();
+    expect(out.tool_choice).toBeUndefined();
+  });
+
+  it("keeps web_search_20250305 and web_search_20260209", () => {
+    const out = prepareClaudeRequest(
+      makeBody([
+        { type: "web_search_20250305", name: "web_search" },
+        { type: "web_search_20260209", name: "web_search" },
+      ]),
+      "deepseek"
+    );
+
+    expect(Array.isArray(out.tools)).toBe(true);
+    expect(out.tools).toHaveLength(2);
+    const types = out.tools.map(t => t.type).sort();
+    expect(types).toEqual(["web_search_20250305", "web_search_20260209"]);
+  });
+
+  it("preserves the `type` field on web_search_* tools (DeepSeek requires it)", () => {
+    // The .map below the filter must NOT strip `type` when the provider
+    // declared a whitelist — DeepSeek would reject a tool object missing
+    // its discriminator field with the same unknown-variant error.
+    const out = prepareClaudeRequest(
+      makeBody([{ type: "web_search_20250305", name: "web_search" }]),
+      "deepseek"
+    );
+
+    expect(out.tools[0].type).toBe("web_search_20250305");
+    expect(out.tools[0].name).toBe("web_search");
+  });
+
+  it("drops `custom` but keeps `web_search_*` when both are present", () => {
+    const out = prepareClaudeRequest(
+      makeBody([
+        { type: "custom", name: "Bash", input_schema: { type: "object" } },
+        { type: "web_search_20250305", name: "web_search" },
+      ]),
+      "deepseek"
+    );
+
+    expect(Array.isArray(out.tools)).toBe(true);
+    expect(out.tools).toHaveLength(1);
+    expect(out.tools[0].type).toBe("web_search_20250305");
+    expect(out.tools[0].name).toBe("web_search");
+  });
+
+  it("survives when body has no tools", () => {
+    const out = prepareClaudeRequest(makeBody(undefined), "deepseek");
+    expect(out.tools).toBeUndefined();
+  });
+
+  it("rejects future / unknown tool types instead of forwarding them", () => {
+    const out = prepareClaudeRequest(
+      makeBody([{ type: "future_tool_2099", name: "x" }]),
+      "deepseek"
+    );
+    expect(out.tools).toBeUndefined();
+  });
+});
+
+describe("prepareClaudeRequest — backward compat: providers without the quirk", () => {
+  // Pick any non-Claude provider that has a Claude-format transport and has
+  // NOT been migrated to the new quirk. This protects GLM / Kimi / future
+  // Anthropic-compatible providers from unintended changes.
+  it("keeps prior behaviour (drop custom + web_search_*, normalize no-type tools)", () => {
+    const candidate = Object.entries(PROVIDERS).find(
+      ([id, p]) =>
+        id !== "claude" &&
+        p?.transports?.some(t => t.format === "claude") &&
+        !p?.quirks?.claudeSupportedToolTypes
+    );
+
+    if (!candidate) {
+      // Every Claude-format provider has been migrated — nothing to verify.
+      return;
+    }
+
+    const [providerId] = candidate;
+
+    const out = prepareClaudeRequest(
+      makeBody([
+        { type: "custom", name: "Bash", input_schema: { type: "object" } },
+        { type: "web_search_20250305", name: "web_search" },
+        { name: "no_type_tool", input_schema: { type: "object" } },
+      ]),
+      providerId
+    );
+
+    if (out.tools !== undefined) {
+      for (const t of out.tools) {
+        expect(t.type).toBeUndefined();
+      }
+    }
+  });
+});
\ No newline at end of file