diff --git a/mini_agent/streaming.py b/mini_agent/streaming.py
new file mode 100644
index 00000000..3ae45144
--- /dev/null
+++ b/mini_agent/streaming.py
@@ -0,0 +1,168 @@
+"""Streaming output renderer with stable prefix and unstable suffix pattern.
+
+This module provides incremental rendering for long-running tasks like
+npm test, pytest, etc., reducing visual flicker and improving UX.
+"""
+
+import sys
+from dataclasses import dataclass, field
+
+
+@dataclass
+class StreamingRenderer:
+    """Renders streaming output with stable prefix and unstable suffix.
+
+    Lines that end with a newline are considered "stable" and will not
+    be re-rendered. The last line (without a trailing newline) is
+    considered "unstable" and will be updated on each render.
+
+    This follows the Claude Code pattern for handling long-running
+    command output with minimal visual disruption.
+    """
+
+    stable_prefix: str = field(default="")
+    unstable_content: str = field(default="")
+    last_line_count: int = 0
+
+    def update(self, new_content: str) -> str:
+        """Process new content and return the portion that needs re-rendering.
+
+        Args:
+            new_content: The complete accumulated content
+
+        Returns:
+            The portion that should be re-rendered (unstable lines)
+        """
+        if not new_content:
+            return ""
+
+        # Split into lines
+        lines = new_content.split("\n")
+
+        # Count complete lines (lines ending with \n are stable)
+        # Note: split("\n") adds empty string after trailing \n, so adjust accordingly
+        if new_content.endswith("\n"):
+            complete_lines = len(lines) - 1  # Exclude empty string after trailing \n
+        else:
+            complete_lines = len(lines) - 1  # Last line is incomplete
+
+        # If we have fewer complete lines than before, content was reset
+        if complete_lines < self.last_line_count:
+            self.stable_prefix = ""
+            self.unstable_content = ""
+            self.last_line_count = 0
+            lines = new_content.split("\n")
+            if new_content.endswith("\n"):
+                complete_lines = len(lines) - 1
+            else:
+                complete_lines = len(lines) - 1
+            # Handle the reset content directly and return
+            if complete_lines > 0:
+                self.stable_prefix = "\n".join(lines[:complete_lines])
+                if self.stable_prefix:
+                    self.stable_prefix += "\n"
+            if not new_content.endswith("\n"):
+                self.unstable_content = lines[-1] if lines else ""
+            else:
+                self.unstable_content = ""
+            self.last_line_count = complete_lines
+            return self.render()
+
+        # Stable content: all complete lines
+        if complete_lines > 0:
+            # All lines except the last (potentially incomplete) one
+            self.stable_prefix = "\n".join(lines[:complete_lines])
+            if complete_lines > 0 and self.stable_prefix:
+                self.stable_prefix += "\n"
+
+        # Unstable content: the last line (may be growing or empty)
+        if not new_content.endswith("\n"):
+            # Last line is incomplete (still growing)
+            self.unstable_content = lines[-1] if lines else ""
+        else:
+            # All content is complete
+            self.unstable_content = ""
+
+        self.last_line_count = complete_lines
+
+        # Return what needs to be re-rendered
+        return self.render()
+
+    def render(self) -> str:
+        """Return the complete rendered string.
+
+        Returns:
+            Full rendered output with stable prefix and current unstable content
+        """
+        result = self.stable_prefix
+        if self.unstable_content:
+            result += self.unstable_content
+        return result
+
+    def clear(self) -> None:
+        """Clear all accumulated content."""
+        self.stable_prefix = ""
+        self.unstable_content = ""
+        self.last_line_count = 0
+
+
+def render_streaming(
+    content: str,
+    renderer: StreamingRenderer | None = None,
+    clear_on_newline: bool = False,
+) -> tuple[str, StreamingRenderer]:
+    """Helper function to render streaming content.
+
+    Args:
+        content: New content to render
+        renderer: Existing renderer to update (creates new if None)
+        clear_on_newline: If True, clears screen and re-renders on newline
+
+    Returns:
+        Tuple of (rendered_output, renderer_instance)
+    """
+    if renderer is None:
+        renderer = StreamingRenderer()
+
+    rendered = renderer.update(content)
+
+    # Optionally handle terminal clear for newline detection
+    if clear_on_newline and "\n" in content:
+        # Move cursor to beginning of line and clear
+        sys.stdout.write("\r\033[K")
+        sys.stdout.flush()
+
+    return rendered, renderer
+
+
+def erase_lines(n: int) -> str:
+    """Generate ANSI escape sequence to erase n lines.
+
+    Args:
+        n: Number of lines to erase
+
+    Returns:
+        ANSI escape sequence
+    """
+    return "\033[{}A\033[K".format(n)
+
+
+def move_cursor_up(n: int) -> str:
+    """Generate ANSI escape sequence to move cursor up n lines.
+
+    Args:
+        n: Number of lines to move up
+
+    Returns:
+        ANSI escape sequence
+    """
+    return "\033[{}A".format(n)
+
+
+def clear_line() -> str:
+    """Generate ANSI escape sequence to clear current line.
+
+    Returns:
+        ANSI escape sequence
+    """
+    return "\r\033[K"
diff --git a/tests/test_streaming.py b/tests/test_streaming.py
new file mode 100644
index 00000000..e4d04b36
--- /dev/null
+++ b/tests/test_streaming.py
@@ -0,0 +1,209 @@
+"""Test cases for Streaming Renderer."""
+
+import pytest
+
+from mini_agent.streaming import (
+    StreamingRenderer,
+    clear_line,
+    erase_lines,
+    move_cursor_up,
+    render_streaming,
+)
+
+
+class TestStreamingRenderer:
+    """Tests for StreamingRenderer class."""
+
+    def test_empty_content(self):
+        """Test handling of empty content."""
+        renderer = StreamingRenderer()
+        result = renderer.update("")
+        assert result == ""
+        assert renderer.stable_prefix == ""
+        assert renderer.unstable_content == ""
+
+    def test_single_incomplete_line(self):
+        """Test a single line without newline (unstable)."""
+        renderer = StreamingRenderer()
+        result = renderer.update("test output")
+        assert result == "test output"
+        assert renderer.stable_prefix == ""
+        assert renderer.unstable_content == "test output"
+
+    def test_single_complete_line(self):
+        """Test a single line with newline (stable)."""
+        renderer = StreamingRenderer()
+        result = renderer.update("test output\n")
+        assert result == "test output\n"
+        assert renderer.stable_prefix == "test output\n"
+        assert renderer.unstable_content == ""
+
+    def test_multiple_lines_last_incomplete(self):
+        """Test multiple lines with last one incomplete."""
+        renderer = StreamingRenderer()
+        result = renderer.update("line1\nline2\nline3")
+        assert result == "line1\nline2\nline3"
+        assert renderer.stable_prefix == "line1\nline2\n"
+        assert renderer.unstable_content == "line3"
+
+    def test_multiple_lines_all_complete(self):
+        """Test multiple lines, all complete."""
+        renderer = StreamingRenderer()
+        result = renderer.update("line1\nline2\nline3\n")
+        assert result == "line1\nline2\nline3\n"
+        assert renderer.stable_prefix == "line1\nline2\nline3\n"
+        assert renderer.unstable_content == ""
+
+    def test_incremental_update(self):
+        """Test incremental updates to growing output."""
+        renderer = StreamingRenderer()
+
+        # Initial partial line
+        result1 = renderer.update("test")
+        assert result1 == "test"
+        assert renderer.unstable_content == "test"
+        assert renderer.stable_prefix == ""
+
+        # Continue the line
+        result2 = renderer.update("test output")
+        assert result2 == "test output"
+        assert renderer.unstable_content == "test output"
+
+        # Complete the line
+        result3 = renderer.update("test output\n")
+        assert result3 == "test output\n"
+        assert renderer.stable_prefix == "test output\n"
+        assert renderer.unstable_content == ""
+
+    def test_incremental_with_multiple_lines(self):
+        """Test incremental updates with multiple lines."""
+        renderer = StreamingRenderer()
+
+        # First line complete
+        renderer.update("line1\n")
+        assert renderer.stable_prefix == "line1\n"
+
+        # Second line incomplete
+        result = renderer.update("line1\nline2")
+        assert result == "line1\nline2"
+        assert renderer.stable_prefix == "line1\n"
+        assert renderer.unstable_content == "line2"
+
+        # Second line completes
+        result = renderer.update("line1\nline2\n")
+        assert result == "line1\nline2\n"
+        assert renderer.stable_prefix == "line1\nline2\n"
+        assert renderer.unstable_content == ""
+
+    def test_clear(self):
+        """Test clearing the renderer."""
+        renderer = StreamingRenderer()
+        renderer.update("line1\nline2\n")
+        assert renderer.stable_prefix == "line1\nline2\n"
+
+        renderer.clear()
+        assert renderer.stable_prefix == ""
+        assert renderer.unstable_content == ""
+        assert renderer.last_line_count == 0
+
+    def test_render_returns_full_content(self):
+        """Test that render() returns complete content."""
+        renderer = StreamingRenderer()
+        renderer.update("line1\nline2\nline3")
+
+        # render() should return same as last update()
+        result = renderer.render()
+        assert result == "line1\nline2\nline3"
+
+    def test_content_reset_detection(self):
+        """Test detection of content reset (fewer lines than before)."""
+        renderer = StreamingRenderer()
+
+        # Start with multiple lines
+        renderer.update("line1\nline2\nline3\n")
+        assert renderer.last_line_count == 3
+
+        # Simulate content reset - new content has fewer lines
+        renderer.update("new1\n")
+        assert renderer.stable_prefix == "new1\n"
+        assert renderer.unstable_content == ""
+
+    def test_preserves_stable_lines(self):
+        """Test that stable lines are never re-rendered."""
+        renderer = StreamingRenderer()
+
+        # Build up stable content
+        renderer.update("stable1\n")
+        assert renderer.stable_prefix == "stable1\n"
+
+        renderer.update("stable1\nstable2\n")
+        assert renderer.stable_prefix == "stable1\nstable2\n"
+
+        # Update unstable line multiple times
+        renderer.update("stable1\nstable2\ngrowing...")
+        renderer.update("stable1\nstable2\ngrowing... more")
+        renderer.update("stable1\nstable2\ngrowing... complete")
+
+        # Stable prefix should be unchanged
+        assert renderer.stable_prefix == "stable1\nstable2\n"
+
+    def test_finalize_with_newline(self):
+        """Test finalizing unstable line with newline."""
+        renderer = StreamingRenderer()
+
+        renderer.update("test")
+        assert renderer.unstable_content == "test"
+
+        renderer.update("test\n")
+        assert renderer.unstable_content == ""
+        assert renderer.stable_prefix == "test\n"
+
+
+class TestHelperFunctions:
+    """Tests for helper functions."""
+
+    def test_clear_line(self):
+        """Test clear_line function."""
+        result = clear_line()
+        assert result == "\r\033[K"
+
+    def test_move_cursor_up(self):
+        """Test move_cursor_up function."""
+        result = move_cursor_up(3)
+        assert result == "\033[3A"
+
+    def test_erase_lines(self):
+        """Test erase_lines function."""
+        result = erase_lines(5)
+        assert result == "\033[5A\033[K"
+
+
+class TestRenderStreamingHelper:
+    """Tests for render_streaming helper function."""
+
+    def test_create_new_renderer(self):
+        """Test creating renderer via helper."""
+        result, renderer = render_streaming("test content\n")
+        assert result == "test content\n"
+        assert isinstance(renderer, StreamingRenderer)
+
+    def test_update_existing_renderer(self):
+        """Test updating existing renderer."""
+        renderer = StreamingRenderer()
+        renderer.update("existing\n")
+
+        result, returned_renderer = render_streaming("existing\nupdate", renderer=renderer)
+        assert returned_renderer is renderer
+        assert result == "existing\nupdate"
+
+    def test_incremental_rendering(self):
+        """Test incremental rendering with helper."""
+        renderer = None
+
+        # Simulate streaming output
+        chunks = ["test", "ing s", "tream", "ing\n", "line2\n", "line3"]
+
+        for chunk in chunks:
+            output, renderer = render_streaming(chunk, renderer=renderer)
+            # Just verify no errors occur
+            assert renderer is not None