diff --git a/src/mitm/handlers/claudeCode.ts b/src/mitm/handlers/claudeCode.ts
index fe07dcb96c..5e693995de 100644
--- a/src/mitm/handlers/claudeCode.ts
+++ b/src/mitm/handlers/claudeCode.ts
@@ -17,7 +17,7 @@ export class ClaudeCodeHandler extends MitmHandlerBase {
     req: IncomingMessage,
     res: ServerResponse,
     body: Buffer,
-    mappedModel: string,
+    mappedModel: string
   ): Promise<void> {
     const startedAt = this.now();
     const intercepted = await this.hookBufferStart(req, body, mappedModel);
@@ -26,6 +26,23 @@ export class ClaudeCodeHandler extends MitmHandlerBase {
       const payload = JSON.parse(body.toString());
       payload.model = mappedModel;
 
+      // Strip trailing assistant prefill to prevent "This model does not support assistant
+      // message prefill" upstream error. Loop over ALL consecutive trailing assistant turns
+      // (not just one) — mirrors the pop-loop already used for Copilot
+      // (open-sse/executors/github.ts::dropTrailingAssistantPrefill) and Antigravity/Vertex
+      // Claude (open-sse/executors/antigravity.ts::stripTrailingAntigravityAssistantTurn).
+      // Guard: never strip messages down to empty — an empty array is itself an invalid
+      // request, so at least one entry (even a lone trailing assistant turn) is always
+      // preserved.
+      if (Array.isArray(payload.messages) && payload.messages.length > 0) {
+        while (
+          payload.messages.length > 1 &&
+          payload.messages[payload.messages.length - 1]?.role === "assistant"
+        ) {
+          payload.messages.pop();
+        }
+      }
+
       const upstreamStart = this.now();
       const upstream = await this.fetchRouter(payload, "/v1/messages", req.headers);
 
diff --git a/tests/unit/mitm-handler-claudeCode.test.ts b/tests/unit/mitm-handler-claudeCode.test.ts
index e02fe8c99b..918e6726bb 100644
--- a/tests/unit/mitm-handler-claudeCode.test.ts
+++ b/tests/unit/mitm-handler-claudeCode.test.ts
@@ -15,3 +15,37 @@ test("claude-code handler — happy path forwards to /v1/messages", async () =>
   const sent = JSON.parse(r.fetchBody);
   assert.equal(sent.model, "claude-opus-4.5");
 });
+
+test("claude-code handler — strips ALL consecutive trailing assistant turns, not just one", async () => {
+  const r = await runHandler(
+    new ClaudeCodeHandler(),
+    {
+      model: "claude-3.5-sonnet",
+      messages: [
+        { role: "user", content: "hi" },
+        { role: "assistant", content: "partial reply 1" },
+        { role: "assistant", content: "partial reply 2" },
+      ],
+    },
+    "claude-opus-4.5"
+  );
+  assert.ok(r.fetchCalled);
+  const sent = JSON.parse(r.fetchBody);
+  assert.equal(sent.messages.length, 1);
+  assert.equal(sent.messages[0].role, "user");
+});
+
+test("claude-code handler — never collapses messages to empty when the ENTIRE history is trailing assistant turns", async () => {
+  const r = await runHandler(
+    new ClaudeCodeHandler(),
+    {
+      model: "claude-3.5-sonnet",
+      messages: [{ role: "assistant", content: "lone assistant turn" }],
+    },
+    "claude-opus-4.5"
+  );
+  assert.ok(r.fetchCalled);
+  const sent = JSON.parse(r.fetchBody);
+  assert.equal(sent.messages.length, 1);
+  assert.equal(sent.messages[0].content, "lone assistant turn");
+});