No valid checkpoint to restore + Max length reached on multi-turn tool calls (Qwen3.6-MoE, NPU)

Summary

On multi-turn tool-call conversations, flm serve fails with No valid checkpoint to restore followed by Max length reached, stopping prefilling.... The model never produces a final answer — the second turn (after the tool result is returned) is rejected.

Environment

FLM version	1.0.5
Model	qwen3.6-moe:35b-a3b (NPU2)
NPU firmware	1.1.2.64
Platform	Linux, AMD Ryzen AI 9 HX 470 (Strix Halo)
Client	OpenCode 1.18.31 (OpenAI-compatible API)
Endpoint	POST /v1/chat/completions

Reproduction

1. Start flm serve qwen3.6-moe:35b-a3b --ctx-len 262144 --pmode performance.
2. Send a request with tools defined. The model correctly emits a <tool_call> and finish_reason = tool_calls.
3. Send a follow-up request with the same conversation plus the assistant (tool_calls) and tool (result) messages.
4. FLM fails — see log below.

Log

[FLM]  Use cached prompt!
[FLM]  Matched 2 out of 4 messages (2 new to prefill).
[FLM]  Start prefill...
[FLM]  Total images: 0
[FLM]  No valid checkpoint to restore
[WARNING]  Max length reached, stopping prefilling...
[FLM]  Creating checkpoint at context length 0
[🔵 ]  NPU Lock Released!

The server returns:

{"error":{"message":"Max length reached!","type":"model_error","code":400}}

Root cause

Qwen3_6_MOE::insert() calls qwen3_6_moe_npu::restore() when meta_info.restore_allowed == true. On the second turn, restore() returns a negative value (no valid checkpoint), but the method then unconditionally assigns:

this->total_tokens = restore_idx;   // becomes -1
this->token_history = checkpoint_his;

checkpoint_his still holds the previous turn's token history (~16k tokens). total_tokens is now desynchronized from the actual KV-cache size. _shared_insert() then checks:

if (this->total_tokens + tokens.size() >= this->MAX_L) {
    header_print("WARNING", "Max length reached, stopping prefilling...");
    return false;
}

Because total_tokens was never reset on restore failure, the check fires even though the actual context is well below MAX_L (32 768). clear_context() is never called, so the KV cache is left in an inconsistent state.

Proposed fix

In common/AutoModel/modeling_qwen3_6_moe.cpp, inside Qwen3_6_MOE::insert():

1. Handle restore() failure explicitly — reset state instead of assigning a negative restore_idx:

     if (meta_info.restore_allowed) {
         restore_idx = qwen3_6_moe_engine->restore();
-        this->total_tokens = restore_idx;
-        this->token_history = checkpoint_his;
+        if (restore_idx < 0) {
+            this->lm_engine->clear_context();
+            this->total_tokens = 0;
+            this->token_history.clear();
+            this->checkpoint_his.clear();
+        } else {
+            this->total_tokens = restore_idx;
+            this->token_history = checkpoint_his;
+        }
     }

2. Re-sync total_tokens with token_history after _shared_insert() — guarantees the counter reflects the real KV-cache size regardless of what happened in restore() / clear_context() / _shared_insert():

     bool success = has_images
         ? this->_shared_insert(meta_info, tokens, is_cancelled, &image_payload, last_image_token_index)
         : this->_shared_insert(meta_info, tokens, is_cancelled, nullptr);
 
+    if (this->total_tokens != (int)this->token_history.size()) {
+        this->total_tokens = this->token_history.size();
+    }
+
     checkpoint_his = token_history;
     int checkpoint_idx = qwen3_6_moe_engine->checkpoint();
     return success;
 }

Verification

After applying the patch and rebuilding FLM, the same multi-turn tool call now completes successfully. Full log:

[FLM]  Use cached prompt!
[FLM]  Matched 2 out of 4 messages (2 new to prefill).
[FLM]  Start prefill...
[FLM]  Total images: 0
[FLM]  No valid checkpoint to restore
[FLM]  Prefill chunk 1/5 with 4096 tokens
[FLM]  Prefill chunk 2/5 with 4096 tokens
[FLM]  Prefill chunk 3/5 with 4096 tokens
[FLM]  Prefill chunk 4/5 with 4096 tokens
[FLM]  Prefill chunk 5/5 with 303 tokens
[FLM]  Creating checkpoint at context length 16687
[FLM]  Start generating...
[FLM]  Model RAW Output:
Вот список проиндексированных проектов:
... (correct answer)

The model now returns the expected final answer instead of a 400 error.

Impact

Tool-calling agents (OpenCode, Claude Code, Cline, and any OpenAI-compatible client that performs multi-step tool calls) are unusable with flm serve on qwen3.6-moe without this fix. The bug affects all multi-turn tool interactions, not just a specific client.

---

Start in common/AutoModel/modeling_qwen3_6_moe.cpp at Qwen3_6_MOE::insert(), then trace restore(), _shared_insert(), clear_context(), and the token-history state. Rebuild FLM and repeat the documented multi-turn tool-call request; done means the second turn completes with a final answer instead of the max-length 400 error.