diff --git a/open-sse/executors/base.ts b/open-sse/executors/base.ts
index 8bda0a1107..f940e630d6 100644
--- a/open-sse/executors/base.ts
+++ b/open-sse/executors/base.ts
@@ -317,6 +317,27 @@ export function sanitizeReasoningEffortForProvider(
   return body;
 }
 
+/**
+ * Strip the OmniRoute provider prefix from versioned built-in tool model
+ * fields (e.g. `cc/claude-opus-4-8` → `claude-opus-4-8`). Versioned built-in
+ * tool types carry an 8-digit date suffix (`advisor_20260301`, `bash_20250124`);
+ * the real Claude CLI sends a bare model id there, never a prefixed one, so a
+ * leaked OmniRoute prefix makes Anthropic reject the request. Mutates in place.
+ */
+export function stripVersionedToolModelPrefix(tools: unknown): void {
+  if (!Array.isArray(tools)) return;
+  for (const t of tools as Array<Record<string, unknown>>) {
+    if (
+      typeof t.type === "string" &&
+      /^[a-z][a-z0-9_]*_\d{8}$/.test(t.type) &&
+      typeof t.model === "string" &&
+      t.model.includes("/")
+    ) {
+      t.model = t.model.split("/").pop();
+    }
+  }
+}
+
 /**
  * BaseExecutor - Base class for provider executors.
  * Implements the Strategy pattern: subclasses override specific methods
@@ -824,6 +845,9 @@ export class BaseExecutor {
             for (const t of tb.tools as Array<Record<string, unknown>>) {
               delete t.cache_control;
             }
+            // Also strip OmniRoute provider prefix from versioned built-in tool
+            // model fields (e.g. cc/claude-opus-4-8 → claude-opus-4-8).
+            stripVersionedToolModelPrefix(tb.tools);
           }
 
           // Per-request behavior overrides via custom client headers.
diff --git a/tests/unit/claude-oauth-tool-cloak.test.ts b/tests/unit/claude-oauth-tool-cloak.test.ts
index cdf1b25420..5d8caadd1f 100644
--- a/tests/unit/claude-oauth-tool-cloak.test.ts
+++ b/tests/unit/claude-oauth-tool-cloak.test.ts
@@ -18,6 +18,7 @@ import {
   sanitizeClaudeToolSchema,
   sanitizeClaudeToolSchemas,
 } from "../../open-sse/translator/helpers/schemaCoercion.ts";
+import { stripVersionedToolModelPrefix } from "../../open-sse/executors/base.ts";
 
 type AnyRecord = Record<string, unknown>;
 const schemaOf = (tools: unknown, i = 0): AnyRecord =>
@@ -270,3 +271,50 @@ describe("review fixes — established aliases + kill-switch", () => {
     }
   });
 });
+
+describe("native claude OAuth path — versioned built-in tool model prefix stripping", () => {
+  // Exercises the REAL production helper from base.ts (not a re-implementation),
+  // so reverting the fix in base.ts breaks these assertions.
+  it("strips cc/ prefix from advisor_20260301 model field", () => {
+    const tools: AnyRecord[] = [
+      { type: "advisor_20260301", name: "advisor", model: "cc/claude-opus-4-8" },
+      { type: "bash_20250124", name: "Bash", model: "kiro/claude-opus-4.8" },
+      { name: "Read", input_schema: { type: "object", properties: {} } },
+    ];
+    stripVersionedToolModelPrefix(tools);
+    assert.equal(tools[0].model, "claude-opus-4-8", "cc/ prefix stripped from advisor model");
+    assert.equal(tools[1].model, "claude-opus-4.8", "kiro/ prefix stripped from bash model");
+    assert.equal("model" in tools[2], false, "regular tool untouched");
+  });
+
+  it("strips multi-segment prefix (claude/claude-sonnet-4-6) from versioned tool model", () => {
+    const tools: AnyRecord[] = [
+      { type: "bash_20250124", name: "Bash", model: "claude/claude-sonnet-4-6" },
+    ];
+    stripVersionedToolModelPrefix(tools);
+    assert.equal(tools[0].model, "claude-sonnet-4-6");
+  });
+
+  it("leaves bare model on versioned tool unchanged", () => {
+    const tools: AnyRecord[] = [
+      { type: "advisor_20260301", name: "advisor", model: "claude-opus-4-8" },
+    ];
+    stripVersionedToolModelPrefix(tools);
+    assert.equal(tools[0].model, "claude-opus-4-8");
+  });
+
+  it("leaves non-versioned tool types untouched even with a prefixed model", () => {
+    const tools: AnyRecord[] = [
+      { type: "custom", name: "x", model: "cc/claude-opus-4-8" },
+      { type: "advisor_2026", name: "y", model: "cc/claude-opus-4-8" }, // not 8 digits
+    ];
+    stripVersionedToolModelPrefix(tools);
+    assert.equal(tools[0].model, "cc/claude-opus-4-8", "non-versioned type untouched");
+    assert.equal(tools[1].model, "cc/claude-opus-4-8", "short date suffix untouched");
+  });
+
+  it("is a no-op for non-array input", () => {
+    assert.doesNotThrow(() => stripVersionedToolModelPrefix(undefined));
+    assert.doesNotThrow(() => stripVersionedToolModelPrefix({ tools: [] }));
+  });
+});