diff --git a/esphome/espidf/runner.py b/esphome/espidf/runner.py
index 7c568db7be7..9e1f24d5eda 100644
--- a/esphome/espidf/runner.py
+++ b/esphome/espidf/runner.py
@@ -187,20 +187,25 @@ def main() -> int:
 
             if self._filter_pattern is None:
                 self._stream.write(data)
-                return len(data)
+            else:
+                self._line_buffer += data
+                for line in self._line_buffer.splitlines(keepends=True):
+                    if "\n" not in line and "\r" not in line:
+                        # Incomplete — hold until we see a terminator.
+                        self._line_buffer = line
+                        break
+                    self._line_buffer = ""
 
-            self._line_buffer += data
-            for line in self._line_buffer.splitlines(keepends=True):
-                if "\n" not in line and "\r" not in line:
-                    # Incomplete — hold until we see a terminator.
-                    self._line_buffer = line
-                    break
-                self._line_buffer = ""
+                    stripped = ansi_escape.sub("", line).rstrip()
+                    if self._filter_pattern.match(stripped) is not None:
+                        continue
+                    self._stream.write(line)
 
-                stripped = ansi_escape.sub("", line).rstrip()
-                if self._filter_pattern.match(stripped) is not None:
-                    continue
-                self._stream.write(line)
+            # We tell idf.py it is talking to a terminal, so it sends progress
+            # bars and cursor moves. Our own stdout is usually a pipe, which is
+            # block buffered, so without this the build looks frozen until
+            # 8 KiB of output piles up.
+            self._stream.flush()
             return len(data)
 
     if len(sys.argv) < 2:
diff --git a/tests/unit_tests/fixtures/espidf/filtering_probe.py b/tests/unit_tests/fixtures/espidf/filtering_probe.py
new file mode 100644
index 00000000000..04c2b2ed8cc
--- /dev/null
+++ b/tests/unit_tests/fixtures/espidf/filtering_probe.py
@@ -0,0 +1,15 @@
+"""Write a mix of noisy and useful build lines, without flushing.
+
+Run through ``esphome/espidf/runner.py`` by test_espidf_runner.py. The
+runner's shim owns both the filtering and the flushing, so this script
+only writes.
+"""
+
+import sys
+
+sys.stdout.write("Project build complete.\n")
+sys.stdout.write("Compiling main.cpp\n")
+sys.stdout.write("-- Component paths: /a /b /c\n")
+sys.stdout.write("[2/9] Building C object\n")
+# No terminator, so the shim has to hold this one back.
+sys.stdout.write("still going")
diff --git a/tests/unit_tests/fixtures/espidf/streaming_probe.py b/tests/unit_tests/fixtures/espidf/streaming_probe.py
new file mode 100644
index 00000000000..c05741e3117
--- /dev/null
+++ b/tests/unit_tests/fixtures/espidf/streaming_probe.py
@@ -0,0 +1,14 @@
+"""Print one line, then stay alive so the caller can prove it streamed.
+
+Run through ``esphome/espidf/runner.py`` by test_espidf_runner.py. The
+runner wraps stdout in its filtering shim, so this script deliberately
+does not flush: the shim has to do it. The long sleep keeps the process
+running, so anything the caller reads must have arrived while the build
+was still going rather than at exit.
+"""
+
+import sys
+import time
+
+sys.stdout.write("Compiling main.cpp\n")
+time.sleep(60)
diff --git a/tests/unit_tests/test_espidf_runner.py b/tests/unit_tests/test_espidf_runner.py
new file mode 100644
index 00000000000..831c8d1cc85
--- /dev/null
+++ b/tests/unit_tests/test_espidf_runner.py
@@ -0,0 +1,127 @@
+"""Tests for esphome.espidf.runner."""
+
+from __future__ import annotations
+
+import io
+import os
+from pathlib import Path
+import subprocess
+import sys
+import threading
+
+import pytest
+
+from esphome.espidf import runner
+
+# A flushing runner delivers the first line in well under a second; this is
+# only ever waited out when the shim has gone back to buffering, so keep it
+# just long enough to cover interpreter startup on a loaded CI machine.
+FIRST_LINE_TIMEOUT = 10.0
+
+
+def _run_main(
+    monkeypatch: pytest.MonkeyPatch, probe: Path, *args: str
+) -> tuple[io.BytesIO, io.TextIOWrapper]:
+    """Run ``runner.main()`` in-process against a buffered fake stdout.
+
+    ``main`` rewrites ``sys.path``, ``sys.argv``, both std streams and
+    ``os.get_terminal_size``; every one of those is monkeypatched so it is
+    put back afterwards. The fake stdout is block buffered like a pipe, so
+    the caller can tell whether the shim flushed. The wrapper comes back with
+    the buffer because dropping it would close the buffer underneath us.
+    """
+    buf = io.BytesIO()
+    stream = io.TextIOWrapper(buf, encoding="utf-8", newline="\n", line_buffering=False)
+
+    monkeypatch.setattr(sys, "path", list(sys.path))
+    monkeypatch.setattr(sys, "argv", ["runner.py", str(probe), *args])
+    monkeypatch.setattr(sys, "stdout", stream)
+    monkeypatch.setattr(sys, "stderr", stream)
+    monkeypatch.setattr(os, "get_terminal_size", os.get_terminal_size)
+
+    assert runner.main() == 0
+    return buf, stream
+
+
+def test_main_filters_noise_and_flushes_each_write(
+    monkeypatch: pytest.MonkeyPatch, fixture_path: Path
+) -> None:
+    """Useful lines reach the stream right away; noisy ones are dropped."""
+    buf, _stream = _run_main(
+        monkeypatch, fixture_path / "espidf" / "filtering_probe.py"
+    )
+
+    # Read before any flush of our own: the shim has to have flushed.
+    output = buf.getvalue().decode("utf-8")
+
+    assert "Compiling main.cpp\n" in output
+    assert "[2/9] Building C object\n" in output
+    # Matched by FILTER_IDF_LINES, so they never leave the runner.
+    assert "Project build complete." not in output
+    assert "-- Component paths:" not in output
+    # Held back because no terminator arrived.
+    assert "still going" not in output
+
+
+def test_main_keeps_everything_in_verbose_mode(
+    monkeypatch: pytest.MonkeyPatch, fixture_path: Path
+) -> None:
+    """``-v`` turns the filter off so the noisy lines survive."""
+    buf, _stream = _run_main(
+        monkeypatch, fixture_path / "espidf" / "filtering_probe.py", "-v"
+    )
+
+    output = buf.getvalue().decode("utf-8")
+
+    assert "Project build complete.\n" in output
+    assert "-- Component paths: /a /b /c\n" in output
+    # With no filter there is no line buffering, so the partial line goes
+    # straight through as well.
+    assert output.endswith("still going")
+
+
+def test_runner_streams_output_before_the_build_finishes(
+    fixture_path: Path, probe_env: dict[str, str]
+) -> None:
+    """The runner must flush, or a dashboard build looks frozen.
+
+    ``toolchain.py`` spawns the runner as a plain script with no ``-u``, and
+    hands it a pipe when esphome itself is running under the dashboard. A
+    pipe is block buffered, so without a flush in the shim's ``write()`` the
+    output sits in the child until 8 KiB piles up or the build ends.
+    """
+    runner_py = Path(runner.__file__)
+    probe = fixture_path / "espidf" / "streaming_probe.py"
+
+    with subprocess.Popen(
+        [sys.executable, str(runner_py), str(probe)],
+        stdout=subprocess.PIPE,
+        # Keep stderr: if the runner dies on startup, its traceback is the
+        # only clue about why no line showed up.
+        stderr=subprocess.PIPE,
+        env=probe_env,
+        text=True,
+    ) as proc:
+        assert proc.stdout is not None
+        assert proc.stderr is not None
+        first_line: list[str] = []
+        reader = threading.Thread(
+            target=lambda: first_line.append(proc.stdout.readline()), daemon=True
+        )
+        try:
+            reader.start()
+            reader.join(FIRST_LINE_TIMEOUT)
+            still_running = proc.poll() is None
+
+            # The probe sleeps for a minute after writing, so reaching us at
+            # all means the line was flushed rather than released at exit.
+            assert first_line == ["Compiling main.cpp\n"], (
+                f"runner stderr: {'' if still_running else proc.stderr.read()}"
+            )
+            assert still_running
+        finally:
+            proc.kill()
+            proc.wait()
+            # Join before leaving the block, so the reader is done rather than
+            # racing ``Popen`` closing the pipe under it.
+            reader.join(1.0)