feat: add tool call support to chat completions endpoint

- Accept tools and tool_choice fields in CompletionRequest
- Convert OpenAI tools to Gemini format and store in MitmStore
- Detect MITM-captured function calls in streaming poll loop
- Emit tool_calls delta chunks in OpenAI streaming format
- Finish with 'tool_calls' reason instead of 'stop' when tools used
- Only clear tools when request has none (prevents stale state leak)

src/api/completions.rs

@@ -78,10 +78,22 @@ pub(crate) async fn handle_completions(
         }
     };
 
-    // Clear any stale tool definitions from other endpoints (e.g. /v1/responses)
-    // to prevent them leaking into completions requests. The completions endpoint
-    // does not support our custom tool call flow, so tools must never be injected.
+    // Store client tools from this request (or clear stale ones from other endpoints)
+    if let Some(ref tools) = body.tools {
+        let gemini_tools = crate::mitm::modify::openai_tools_to_gemini(tools);
+        if !gemini_tools.is_empty() {
+            state.mitm_store.set_tools(gemini_tools).await;
+            if let Some(ref choice) = body.tool_choice {
+                let gemini_config = crate::mitm::modify::openai_tool_choice_to_gemini(choice);
+                state.mitm_store.set_tool_config(gemini_config).await;
+            }
+            info!(count = tools.len(), "Completions: stored client tools for MITM injection");
+        } else {
             state.mitm_store.clear_tools().await;
+        }
+    } else {
+        state.mitm_store.clear_tools().await;
+    }
     state.mitm_store.clear_active_function_call();
 
     let token = state.backend.oauth_token().await;
@@ -225,6 +237,57 @@ async fn chat_completions_stream(
                             }
                         }
 
+                        // Check for MITM-captured function calls (tool use)
+                        let captured = state.mitm_store.take_any_function_calls().await;
+                        if let Some(ref calls) = captured {
+                            if !calls.is_empty() {
+                                // Emit tool_calls in OpenAI streaming format
+                                let mut tool_calls = Vec::new();
+                                for (i, fc) in calls.iter().enumerate() {
+                                    let call_id = format!(
+                                        "call_{}",
+                                        uuid::Uuid::new_v4().to_string().replace('-', "")[..24].to_string()
+                                    );
+                                    let arguments = serde_json::to_string(&fc.args).unwrap_or_default();
+                                    tool_calls.push(serde_json::json!({
+                                        "index": i,
+                                        "id": call_id,
+                                        "type": "function",
+                                        "function": {
+                                            "name": fc.name,
+                                            "arguments": arguments,
+                                        },
+                                    }));
+                                }
+                                yield Ok(Event::default().data(serde_json::to_string(&serde_json::json!({
+                                    "id": completion_id,
+                                    "object": "chat.completion.chunk",
+                                    "created": now_unix(),
+                                    "model": model_name,
+                                    "choices": [{
+                                        "index": 0,
+                                        "delta": {"tool_calls": tool_calls},
+                                        "finish_reason": serde_json::Value::Null,
+                                    }],
+                                })).unwrap_or_default()));
+
+                                // Finish with tool_calls reason
+                                yield Ok(Event::default().data(serde_json::to_string(&serde_json::json!({
+                                    "id": completion_id,
+                                    "object": "chat.completion.chunk",
+                                    "created": now_unix(),
+                                    "model": model_name,
+                                    "choices": [{
+                                        "index": 0,
+                                        "delta": {},
+                                        "finish_reason": "tool_calls",
+                                    }],
+                                })).unwrap_or_default()));
+                                yield Ok(Event::default().data("[DONE]"));
+                                return;
+                            }
+                        }
+
                         // Done check: need DONE status AND non-empty text
                         if is_response_done(steps) && !last_text.is_empty() {
                             debug!("Completions stream done, text length={}", last_text.len());

src/api/types.rs

@@ -49,6 +49,10 @@ pub(crate) struct CompletionRequest {
     pub stream: bool,
     #[serde(default = "default_timeout")]
     pub timeout: u64,
+    /// OpenAI-format tool definitions
+    pub tools: Option<Vec<serde_json::Value>>,
+    /// Tool choice: "auto", "none", "required", or {"type":"function","function":{"name":"..."}}
+    pub tool_choice: Option<serde_json::Value>,
 }
 
 #[derive(Deserialize)]