fix(core): invalid model error handling

It looks like there was a change in opencode 1.0 that changed how
invalid model errors are surfaced so this change fixes that.

Also add unit test coverage for send_message changing
state.user_message_count (and fix a bug with it not being updated in the
catch)

Hopefully fixes #131

Author: cameronr

lua/opencode/core.lua

@@ -147,31 +147,35 @@ function M.send_message(prompt, opts)
   params.parts = context.format_message(prompt, opts.context)
   M.before_run(opts)
 
-  -- Capture the session ID to ensure we track the message count for the correct session
   local session_id = state.active_session.id
-  local sent_message_count = vim.deepcopy(state.user_message_count)
-  sent_message_count[session_id] = (sent_message_count[session_id] or 0) + 1
-  state.user_message_count = sent_message_count
+
+  ---Helper to update state.user_message_count. Have to deepcopy since it's a table to make
+  ---sure notification events fire. Prevents negative values (in case of an untracked code path)
+  local function update_sent_message_count(num)
+    local sent_message_count = vim.deepcopy(state.user_message_count)
+    local new_value = (sent_message_count[session_id] or 0) + num
+    sent_message_count[session_id] = new_value >= 0 and new_value or 0
+    state.user_message_count = sent_message_count
+  end
+
+  update_sent_message_count(1)
 
   state.api_client
     :create_message(session_id, params)
     :and_then(function(response)
+      update_sent_message_count(-1)
+
       if not response or not response.info or not response.parts then
-        -- fall back to full render. incremental render is handled
-        -- event manager
-        ui.render_output()
+        vim.notify('Invalid response from opencode: ' .. vim.inspect(response), vim.log.levels.ERROR)
+        M.cancel()
+        return
       end
 
-      local received_message_count = vim.deepcopy(state.user_message_count)
-      received_message_count[response.info.sessionID] = (received_message_count[response.info.sessionID] ~= nil)
-          and (received_message_count[response.info.sessionID] - 1)
-        or 0
-      state.user_message_count = received_message_count
-
       M.after_run(prompt)
     end)
     :catch(function(err)
       vim.notify('Error sending message to session: ' .. vim.inspect(err), vim.log.levels.ERROR)
+      update_sent_message_count(-1)
       M.cancel()
     end)
 end

tests/unit/core_spec.lua

@@ -271,6 +271,81 @@ describe('opencode.core', function()
       assert.equal(state.current_model, 'test/model')
       state.api_client.create_message = orig
     end)
+
+    it('increments and decrements user_message_count correctly', function()
+      state.windows = { mock = 'windows' }
+      state.active_session = { id = 'sess1' }
+      state.user_message_count = {}
+
+      -- Capture the count at different stages
+      local count_before = state.user_message_count['sess1'] or 0
+      local count_during = nil
+      local count_after = nil
+
+      local orig = state.api_client.create_message
+      state.api_client.create_message = function(_, sid, params)
+        -- Capture count while message is in flight
+        count_during = state.user_message_count['sess1']
+        return Promise.new():resolve({
+          id = 'm1',
+          info = { id = 'm1' },
+          parts = {},
+        })
+      end
+
+      core.send_message('hello world')
+
+      -- Wait for promise to resolve
+      vim.wait(50, function()
+        count_after = state.user_message_count['sess1'] or 0
+        return count_after == 0
+      end)
+
+      -- Verify: starts at 0, increments to 1, then back to 0
+      assert.equal(0, count_before)
+      assert.equal(1, count_during)
+      assert.equal(0, count_after)
+
+      state.api_client.create_message = orig
+    end)
+
+    it('decrements user_message_count on error', function()
+      state.windows = { mock = 'windows' }
+      state.active_session = { id = 'sess1' }
+      state.user_message_count = {}
+
+      -- Capture the count at different stages
+      local count_before = state.user_message_count['sess1'] or 0
+      local count_during = nil
+      local count_after = nil
+
+      local orig = state.api_client.create_message
+      state.api_client.create_message = function(_, sid, params)
+        -- Capture count while message is in flight
+        count_during = state.user_message_count['sess1']
+        return Promise.new():reject('Test error')
+      end
+
+      -- Stub cancel to prevent it from trying to abort the session
+      local orig_cancel = core.cancel
+      stub(core, 'cancel')
+
+      core.send_message('hello world')
+
+      -- Wait for promise to reject
+      vim.wait(50, function()
+        count_after = state.user_message_count['sess1'] or 0
+        return count_after == 0
+      end)
+
+      -- Verify: starts at 0, increments to 1, then back to 0 even on error
+      assert.equal(0, count_before)
+      assert.equal(1, count_during)
+      assert.equal(0, count_after)
+
+      state.api_client.create_message = orig
+      core.cancel = orig_cancel
+    end)
   end)
 
   describe('opencode_ok (version checks)', function()