diff --git a/lib/roast/cogs/agent/providers/pi/messages/tool_call_message.rb b/lib/roast/cogs/agent/providers/pi/messages/tool_call_message.rb
index 0e0aaad6..3f3561ea 100644
--- a/lib/roast/cogs/agent/providers/pi/messages/tool_call_message.rb
+++ b/lib/roast/cogs/agent/providers/pi/messages/tool_call_message.rb
@@ -94,6 +94,30 @@ def format_read
                 details ? "#{label} (#{details})" : label
               end
 
+              # Formats a write tool call.
+              #
+              # Input fields:
+              #   :path    (String) – file to write   [required]
+              #   :content (String) – file contents   [required]
+              #
+              # Output: 'WRITE <path> "<preview>" (+<n> <line|lines>)' – <preview> is
+              # the first line of :content (stripped, truncated to TRUNCATE_LIMIT
+              # chars) and <n> is the number of lines written. The count is always
+              # shown.
+              #
+              # Examples:
+              #   WRITE lib/roast.rb "class Roast" (+10 lines)
+              #   WRITE config/app.yml "enabled: true" (+1 line)
+              #
+              #: () -> String
+              def format_write
+                path, content = arguments.values_at(:path, :content)
+                lines = content.to_s.lines
+                preview = truncate(lines.first.to_s.strip)
+                count = lines.length
+                "WRITE #{path} \"#{preview}\" (+#{count} #{"line".pluralize(count)})"
+              end
+
               # Formats a tool call for which Roast has no dedicated formatter.
               #
               # Output: "<NAME> <key>: <value>, ..." – the upcased tool name, then each
diff --git a/lib/roast/cogs/agent/providers/pi/messages/tool_result_message.rb b/lib/roast/cogs/agent/providers/pi/messages/tool_result_message.rb
index adac0e26..ee62324d 100644
--- a/lib/roast/cogs/agent/providers/pi/messages/tool_result_message.rb
+++ b/lib/roast/cogs/agent/providers/pi/messages/tool_result_message.rb
@@ -91,6 +91,22 @@ def format_read
                 ok_line("#{count} #{"line".pluralize(count)}")
               end
 
+              # Formats a write tool result.
+              #
+              # Input: :path – the path that was written, from the originating call.
+              #
+              # Output: "WRITE OK <path>" – the file path, omitted when the call
+              # had none.
+              #
+              # Examples:
+              #   WRITE OK lib/roast/version.rb
+              #   WRITE OK
+              #
+              #: () -> String
+              def format_write
+                ok_line(@input[:path])
+              end
+
               # Formats a result for which Roast has no dedicated formatter.
               #
               # Content: the tool's output text.
diff --git a/test/roast/cogs/agent/providers/pi/messages/tool_call_message_test.rb b/test/roast/cogs/agent/providers/pi/messages/tool_call_message_test.rb
index 584fe0d0..ac6b1f15 100644
--- a/test/roast/cogs/agent/providers/pi/messages/tool_call_message_test.rb
+++ b/test/roast/cogs/agent/providers/pi/messages/tool_call_message_test.rb
@@ -56,6 +56,33 @@ class ToolCallMessageTest < ActiveSupport::TestCase
           assert_equal "READ lib/roast.rb (from line 30)", msg.format
         end
 
+        test "format renders WRITE with the path, preview, and line count" do
+          msg = ToolCallMessage.new(
+            id: "1",
+            name: "write",
+            arguments: { path: "lib/roast.rb", content: "line one\nline two\nline three" },
+          )
+          assert_equal 'WRITE lib/roast.rb "line one" (+3 lines)', msg.format
+        end
+
+        test "format renders a singular line count for single-line write content" do
+          msg = ToolCallMessage.new(
+            id: "1",
+            name: "write",
+            arguments: { path: "config/app.yml", content: "enabled: true" },
+          )
+          assert_equal 'WRITE config/app.yml "enabled: true" (+1 line)', msg.format
+        end
+
+        test "format truncates a long write preview" do
+          msg = ToolCallMessage.new(
+            id: "1",
+            name: "write",
+            arguments: { path: "lib/roast.rb", content: "x" * 100 },
+          )
+          assert_equal "WRITE lib/roast.rb \"#{"x" * (ToolCallMessage::TRUNCATE_LIMIT - 3)}...\" (+1 line)", msg.format
+        end
+
         test "format renders an unhandled tool as NAME key: value, ..." do
           msg = ToolCallMessage.new(
             id: "1",
diff --git a/test/roast/cogs/agent/providers/pi/messages/tool_result_message_test.rb b/test/roast/cogs/agent/providers/pi/messages/tool_result_message_test.rb
index 24c92d0b..df84bf42 100644
--- a/test/roast/cogs/agent/providers/pi/messages/tool_result_message_test.rb
+++ b/test/roast/cogs/agent/providers/pi/messages/tool_result_message_test.rb
@@ -76,6 +76,31 @@ def setup
           assert_equal "READ OK 0 lines", msg.format(@context)
         end
 
+        test "format renders WRITE OK with the path from the originating call" do
+          @context.add_tool_call(
+            ToolCallMessage.new(id: "1", name: "write", arguments: { path: "lib/roast/version.rb" }),
+          )
+          msg = ToolResultMessage.new(
+            tool_call_id: "1",
+            tool_name: "write",
+            content: "File written successfully",
+            is_error: false,
+          )
+
+          assert_equal "WRITE OK lib/roast/version.rb", msg.format(@context)
+        end
+
+        test "format renders a bare WRITE OK when the path is unavailable" do
+          msg = ToolResultMessage.new(
+            tool_call_id: "1",
+            tool_name: "write",
+            content: "File written successfully",
+            is_error: false,
+          )
+
+          assert_equal "WRITE OK", msg.format(@context)
+        end
+
         test "format renders NAME ERROR with the message for an error result" do
           msg = ToolResultMessage.new(
             tool_call_id: "1",