diff --git a/python/sglang/srt/debug_utils/source_patcher/__init__.py b/python/sglang/srt/debug_utils/source_patcher/__init__.py
new file mode 100644
index 000000000..c853fad17
--- /dev/null
+++ b/python/sglang/srt/debug_utils/source_patcher/__init__.py
@@ -0,0 +1,12 @@
+from sglang.srt.debug_utils.source_patcher.code_patcher import (
+    CodePatcher,
+    apply_patches_from_config,
+    patch_function,
+)
+from sglang.srt.debug_utils.source_patcher.types import (
+    EditSpec,
+    PatchApplicationError,
+    PatchConfig,
+    PatchSpec,
+    PatchState,
+)
diff --git a/python/sglang/srt/debug_utils/source_patcher/code_patcher.py b/python/sglang/srt/debug_utils/source_patcher/code_patcher.py
new file mode 100644
index 000000000..421f2ccd2
--- /dev/null
+++ b/python/sglang/srt/debug_utils/source_patcher/code_patcher.py
@@ -0,0 +1,188 @@
+import importlib
+import inspect
+import textwrap
+import types
+from collections.abc import Callable
+from typing import Any, Optional
+
+import yaml
+
+from sglang.srt.debug_utils.source_patcher.source_editor import apply_edits
+from sglang.srt.debug_utils.source_patcher.types import (
+    EditSpec,
+    PatchConfig,
+    PatchSpec,
+    PatchState,
+)
+
+
+def apply_patches_from_config(
+    yaml_content: str,
+    *,
+    extra_imports: Optional[list[str]] = None,
+) -> list[PatchState]:
+    """Parse a YAML config string and apply all patches.
+
+    Args:
+        yaml_content: YAML string with patch specifications.
+        extra_imports: Import lines inserted once at the top of each patched
+            function body (e.g. ["from pkg import foo"]).  The caller (dumper)
+            uses this so users don't have to write boilerplate in YAML.
+    """
+    raw: dict[str, Any] = yaml.safe_load(yaml_content)
+    config: PatchConfig = PatchConfig(**raw)
+
+    if extra_imports:
+        config = _inject_preamble(config=config, extra_imports=extra_imports)
+
+    return _apply_specs(config.patches)
+
+
+class CodePatcher:
+    """Context manager that patches functions on enter and restores on exit."""
+
+    def __init__(self, *, patches: list[PatchSpec]) -> None:
+        self._patches = patches
+        self._states: list[PatchState] = []
+
+    def __enter__(self) -> "CodePatcher":
+        self._states = _apply_specs(self._patches)
+        return self
+
+    def __exit__(
+        self,
+        exc_type: Optional[type],
+        exc_val: Optional[BaseException],
+        exc_tb: Optional[Any],
+    ) -> None:
+        for state in reversed(self._states):
+            state.restore()
+        self._states.clear()
+
+
+def patch_function(
+    *,
+    target: Callable[..., Any],
+    edits: list[EditSpec],
+    preamble: str = "",
+) -> PatchState:
+    """Patch a function by modifying its source and replacing __code__.
+
+    1. inspect.getsource -> get original source
+    2. apply_edits -> modify source text
+    3. optionally prepend preamble (e.g. import lines) inside the function body
+    4. compile + exec -> get new code object
+    5. replace target.__code__
+
+    Returns PatchState that can restore the original code.
+    """
+    original_code: types.CodeType = target.__code__
+
+    source: str = inspect.getsource(target)
+    modified_source: str = apply_edits(source=source, edits=edits)
+    modified_source = textwrap.dedent(modified_source)
+
+    if preamble.strip():
+        modified_source = _insert_preamble(source=modified_source, preamble=preamble)
+
+    code: types.CodeType = compile(modified_source, inspect.getfile(target), "exec")
+    temp_namespace: dict[str, Any] = {}
+    exec(code, target.__globals__, temp_namespace)
+
+    new_fn: Any = temp_namespace[target.__name__]
+    target.__code__ = new_fn.__code__
+
+    return PatchState(target_fn=target, original_code=original_code)
+
+
+# --------------------------------- private ---------------------------------
+
+
+def _apply_specs(specs: list[PatchSpec]) -> list[PatchState]:
+    states: list[PatchState] = []
+    for spec in specs:
+        target_fn: Callable[..., Any] = _resolve_target(spec.target)
+        print(f"[source_patcher] patching {spec.target}")
+        state: PatchState = patch_function(
+            target=target_fn, edits=spec.edits, preamble=spec.preamble
+        )
+        states.append(state)
+    return states
+
+
+def _inject_preamble(*, config: PatchConfig, extra_imports: list[str]) -> PatchConfig:
+    """Set preamble on every PatchSpec so imports are inserted once at function top."""
+    import_block: str = "\n".join(extra_imports)
+    new_patches: list[PatchSpec] = []
+
+    for spec in config.patches:
+        existing: str = spec.preamble
+        combined: str = (
+            existing + "\n" + import_block if existing.strip() else import_block
+        )
+        new_patches.append(
+            PatchSpec(target=spec.target, edits=spec.edits, preamble=combined)
+        )
+
+    return PatchConfig(patches=new_patches)
+
+
+def _insert_preamble(*, source: str, preamble: str) -> str:
+    """Insert preamble lines right after the function signature (and optional docstring)."""
+    lines: list[str] = source.splitlines()
+
+    signature_end: int = _find_signature_end(lines)
+
+    body_start: int = signature_end + 1
+    body_indent: str = ""
+    for i in range(body_start, len(lines)):
+        if lines[i].strip():
+            body_indent = " " * (len(lines[i]) - len(lines[i].lstrip()))
+            body_start = i
+            break
+
+    preamble_lines: list[str] = [
+        body_indent + pl for pl in preamble.strip().splitlines()
+    ]
+    return "\n".join(lines[:body_start] + preamble_lines + lines[body_start:])
+
+
+def _find_signature_end(lines: list[str]) -> int:
+    """Find the line index where the function signature ends (the line with trailing colon)."""
+    for i, line in enumerate(lines):
+        if line.rstrip().endswith(":"):
+            return i
+    return 0
+
+
+def _resolve_target(qualified_name: str) -> Callable[..., Any]:
+    """Resolve 'pkg.mod.Class.method' to the actual function object.
+
+    Tries progressively shorter module paths from right to left,
+    then uses getattr for the remaining attribute chain.
+    """
+    parts: list[str] = qualified_name.split(".")
+
+    target: Any = None
+    for split_idx in range(len(parts), 0, -1):
+        module_path: str = ".".join(parts[:split_idx])
+        try:
+            target = importlib.import_module(module_path)
+            attr_parts: list[str] = parts[split_idx:]
+            break
+        except ImportError:
+            continue
+    else:
+        raise ImportError(f"could not import any module prefix of '{qualified_name}'")
+
+    for attr_name in attr_parts:
+        target = getattr(target, attr_name)
+
+    if isinstance(target, classmethod):
+        target = target.__func__
+    if not callable(target):
+        raise TypeError(
+            f"resolved target '{qualified_name}' is not callable: {type(target)}"
+        )
+
+    return target
diff --git a/python/sglang/srt/debug_utils/source_patcher/source_editor.py b/python/sglang/srt/debug_utils/source_patcher/source_editor.py
new file mode 100644
index 000000000..0f4b0805a
--- /dev/null
+++ b/python/sglang/srt/debug_utils/source_patcher/source_editor.py
@@ -0,0 +1,108 @@
+from sglang.srt.debug_utils.source_patcher.types import EditSpec, PatchApplicationError
+
+
+def apply_edits(*, source: str, edits: list[EditSpec]) -> str:
+    """Apply a sequence of match/replacement edits to source text.
+
+    Each edit is applied sequentially so later edits see the result of earlier ones.
+    """
+    result: str = source
+    for edit in edits:
+        result = _apply_single_edit(source=result, edit=edit)
+    return result
+
+
+def _apply_single_edit(*, source: str, edit: EditSpec) -> str:
+    """Apply a single match/replacement edit to the source text."""
+    match_text: str = edit.match.strip()
+    if not match_text:
+        raise PatchApplicationError("empty match text")
+
+    source_lines: list[str] = source.splitlines()
+    match_lines: list[str] = match_text.splitlines()
+
+    start_idx: int = _find_match(source_lines=source_lines, match_lines=match_lines)
+    match_len: int = len(match_lines)
+
+    original_indent: int = _leading_spaces(source_lines[start_idx])
+
+    effective_replacement: str = _resolve_replacement(edit=edit, match_text=match_text)
+    replacement_lines: list[str] = (
+        effective_replacement.splitlines() if effective_replacement else []
+    )
+    aligned: list[str] = _realign_replacement(
+        replacement_lines=replacement_lines, original_indent=original_indent
+    )
+    new_lines: list[str] = (
+        source_lines[:start_idx] + aligned + source_lines[start_idx + match_len :]
+    )
+
+    trailing_newline: str = "\n" if source.endswith("\n") else ""
+    return "\n".join(new_lines) + trailing_newline
+
+
+def _resolve_replacement(*, edit: EditSpec, match_text: str) -> str:
+    """Return the effective replacement text, handling replacement, prepend, and append modes."""
+    if edit.prepend.strip():
+        return edit.prepend.strip() + "\n" + match_text
+    if edit.append.strip():
+        return match_text + "\n" + edit.append.strip()
+    return edit.replacement.strip()
+
+
+def _find_match(*, source_lines: list[str], match_lines: list[str]) -> int:
+    """Find the start index of match_lines in source_lines (strip-compared).
+
+    Returns the index of the first matching line.
+    Raises PatchApplicationError if not found or found multiple times.
+    """
+    stripped_source: list[str] = [line.strip() for line in source_lines]
+    stripped_match: list[str] = [line.strip() for line in match_lines]
+    match_len: int = len(stripped_match)
+
+    found_indices: list[int] = [
+        i
+        for i in range(len(stripped_source) - match_len + 1)
+        if stripped_source[i : i + match_len] == stripped_match
+    ]
+
+    if len(found_indices) == 0:
+        preview: str = "\n".join(match_lines)
+        raise PatchApplicationError(f"match text not found in source:\n{preview}")
+    if len(found_indices) > 1:
+        preview = "\n".join(match_lines)
+        raise PatchApplicationError(
+            f"match text found multiple times ({len(found_indices)} occurrences) in source:\n{preview}"
+        )
+
+    return found_indices[0]
+
+
+def _realign_replacement(
+    *, replacement_lines: list[str], original_indent: int
+) -> list[str]:
+    """Realign replacement lines to the original indentation level.
+
+    Strategy:
+    - Take the leading spaces of the first non-empty replacement line as base_indent
+    - For each replacement line: remove base_indent, add original_indent
+    """
+    non_empty: list[str] = [line for line in replacement_lines if line.strip()]
+    if not non_empty:
+        return []
+
+    base_indent: int = _leading_spaces(non_empty[0])
+    result: list[str] = []
+
+    for line in replacement_lines:
+        if not line.strip():
+            result.append("")
+        else:
+            stripped = line[min(base_indent, len(line) - len(line.lstrip())) :]
+            result.append(" " * original_indent + stripped)
+
+    return result
+
+
+def _leading_spaces(line: str) -> int:
+    return len(line) - len(line.lstrip(" "))
diff --git a/python/sglang/srt/debug_utils/source_patcher/types.py b/python/sglang/srt/debug_utils/source_patcher/types.py
new file mode 100644
index 000000000..9ff44cba6
--- /dev/null
+++ b/python/sglang/srt/debug_utils/source_patcher/types.py
@@ -0,0 +1,63 @@
+import types
+from collections.abc import Callable
+from typing import Any
+
+from pydantic import BaseModel, ConfigDict, model_validator
+
+
+class PatchApplicationError(Exception):
+    """match text not found or not unique in source."""
+
+
+class _StrictBase(BaseModel):
+    model_config = ConfigDict(extra="forbid")
+
+
+class EditSpec(_StrictBase):
+    """Specify one edit: replace, prepend before, or append after the matched text.
+
+    Use ``replacement`` to substitute the matched text (empty string = delete).
+    Use ``prepend`` to keep the matched text and add lines before it.
+    Use ``append`` to keep the matched text and add lines after it.
+    Only one of ``replacement``, ``prepend``, and ``append`` may be set.
+    """
+
+    match: str
+    replacement: str = ""
+    prepend: str = ""
+    append: str = ""
+
+    @model_validator(mode="after")
+    def _check_modes_mutually_exclusive(self) -> "EditSpec":
+        active: list[str] = [
+            name
+            for name in ("replacement", "prepend", "append")
+            if getattr(self, name).strip()
+        ]
+        if len(active) > 1:
+            raise ValueError(
+                f"only one of 'replacement', 'prepend', 'append' may be set, "
+                f"got: {', '.join(active)}"
+            )
+        return self
+
+
+class PatchSpec(_StrictBase):
+    target: str
+    edits: list[EditSpec]
+    preamble: str = ""
+
+
+class PatchConfig(_StrictBase):
+    patches: list[PatchSpec]
+
+
+class PatchState:
+    def __init__(
+        self, *, target_fn: Callable[..., Any], original_code: types.CodeType
+    ) -> None:
+        self.target_fn = target_fn
+        self.original_code = original_code
+
+    def restore(self) -> None:
+        self.target_fn.__code__ = self.original_code
diff --git a/test/registered/debug_utils/source_patcher/conftest.py b/test/registered/debug_utils/source_patcher/conftest.py
new file mode 100644
index 000000000..f585c5838
--- /dev/null
+++ b/test/registered/debug_utils/source_patcher/conftest.py
@@ -0,0 +1,66 @@
+"""Shared fixtures for source_patcher tests.
+
+The sample module is defined as an inline string and written to a temp file
+at test time, avoiding CI complaints about fixture files without test registration.
+"""
+
+import importlib.util
+import sys
+import tempfile
+from pathlib import Path
+from types import ModuleType
+
+import pytest
+
+SAMPLE_MODULE_NAME = "_source_patcher_test_fixtures.sample_module"
+
+SAMPLE_MODULE_SOURCE = '''\
+GLOBAL_VAR = "global_value"
+
+
+class HelperClass:
+    """Utility class referenced by SampleClass to test cross-class calls."""
+
+    @staticmethod
+    def format_value(value: str) -> str:
+        return f"[{value}]"
+
+
+class SampleClass:
+    def greet(self, name: str) -> str:
+        greeting = f"hello {name}"
+        return greeting
+
+    def compute(self, x: int) -> int:
+        result = x * 2 + 1
+        return result
+
+    def uses_global(self) -> str:
+        return f"value={GLOBAL_VAR}"
+
+    def uses_helper(self, value: str) -> str:
+        return HelperClass.format_value(value)
+
+
+def standalone_function(a: int, b: int) -> int:
+    return a + b
+'''
+
+
+@pytest.fixture(scope="session")
+def sample_module() -> ModuleType:
+    """Load the sample module from a temp file and register it in sys.modules."""
+    if SAMPLE_MODULE_NAME in sys.modules:
+        return sys.modules[SAMPLE_MODULE_NAME]
+
+    tmpdir = tempfile.mkdtemp(prefix="source_patcher_fixtures_")
+    module_path = Path(tmpdir) / "sample_module.py"
+    module_path.write_text(SAMPLE_MODULE_SOURCE)
+
+    spec = importlib.util.spec_from_file_location(SAMPLE_MODULE_NAME, module_path)
+    assert spec is not None
+    assert spec.loader is not None
+    module = importlib.util.module_from_spec(spec)
+    sys.modules[SAMPLE_MODULE_NAME] = module
+    spec.loader.exec_module(module)
+    return module
diff --git a/test/registered/debug_utils/source_patcher/test_code_patcher.py b/test/registered/debug_utils/source_patcher/test_code_patcher.py
new file mode 100644
index 000000000..56e0ab721
--- /dev/null
+++ b/test/registered/debug_utils/source_patcher/test_code_patcher.py
@@ -0,0 +1,253 @@
+from types import ModuleType
+
+import pytest
+
+from sglang.srt.debug_utils.source_patcher.code_patcher import (
+    CodePatcher,
+    _resolve_target,
+    patch_function,
+)
+from sglang.srt.debug_utils.source_patcher.types import EditSpec, PatchSpec
+
+SAMPLE_MODULE_NAME = "_source_patcher_test_fixtures.sample_module"
+
+
+class TestPatchFunction:
+    def test_basic_patch_changes_behavior(self, sample_module: ModuleType) -> None:
+        cls = sample_module.SampleClass
+        obj = cls()
+        assert obj.greet("world") == "hello world"
+
+        state = patch_function(
+            target=cls.greet,
+            edits=[
+                EditSpec(
+                    match='greeting = f"hello {name}"',
+                    replacement='greeting = f"patched {name}"',
+                )
+            ],
+        )
+        try:
+            assert obj.greet("world") == "patched world"
+        finally:
+            state.restore()
+
+        assert obj.greet("world") == "hello world"
+
+    def test_globals_preserved_after_patch(self, sample_module: ModuleType) -> None:
+        cls = sample_module.SampleClass
+        obj = cls()
+        assert obj.uses_global() == "value=global_value"
+
+        state = patch_function(
+            target=cls.uses_global,
+            edits=[
+                EditSpec(
+                    match='return f"value={GLOBAL_VAR}"',
+                    replacement='return f"patched_value={GLOBAL_VAR}"',
+                )
+            ],
+        )
+        try:
+            assert obj.uses_global() == "patched_value=global_value"
+        finally:
+            state.restore()
+
+    def test_function_identity_preserved(self, sample_module: ModuleType) -> None:
+        cls = sample_module.SampleClass
+        fn_id_before = id(cls.greet)
+
+        state = patch_function(
+            target=cls.greet,
+            edits=[
+                EditSpec(
+                    match='greeting = f"hello {name}"',
+                    replacement='greeting = f"patched {name}"',
+                )
+            ],
+        )
+        try:
+            assert id(cls.greet) == fn_id_before
+        finally:
+            state.restore()
+
+    def test_patch_standalone_function(self, sample_module: ModuleType) -> None:
+        fn = sample_module.standalone_function
+        assert fn(2, 3) == 5
+
+        state = patch_function(
+            target=fn,
+            edits=[
+                EditSpec(
+                    match="return a + b",
+                    replacement="return a * b",
+                )
+            ],
+        )
+        try:
+            assert fn(2, 3) == 6
+        finally:
+            state.restore()
+
+        assert fn(2, 3) == 5
+
+    def test_patched_code_can_reference_global_variable(
+        self, sample_module: ModuleType
+    ) -> None:
+        """Replacement code that references a module-level global should work."""
+        cls = sample_module.SampleClass
+        obj = cls()
+
+        state = patch_function(
+            target=cls.greet,
+            edits=[
+                EditSpec(
+                    match='greeting = f"hello {name}"',
+                    replacement='greeting = f"{GLOBAL_VAR} {name}"',
+                )
+            ],
+        )
+        try:
+            assert obj.greet("world") == "global_value world"
+        finally:
+            state.restore()
+
+    def test_patched_code_can_call_another_class_method(
+        self, sample_module: ModuleType
+    ) -> None:
+        """Replacement code that calls HelperClass.format_value should work."""
+        cls = sample_module.SampleClass
+        obj = cls()
+
+        state = patch_function(
+            target=cls.greet,
+            edits=[
+                EditSpec(
+                    match='greeting = f"hello {name}"',
+                    replacement="greeting = HelperClass.format_value(name)",
+                )
+            ],
+        )
+        try:
+            assert obj.greet("world") == "[world]"
+        finally:
+            state.restore()
+
+    def test_patched_code_uses_helper_via_existing_method(
+        self, sample_module: ModuleType
+    ) -> None:
+        """The uses_helper method already calls HelperClass; verify it survives patching."""
+        cls = sample_module.SampleClass
+        obj = cls()
+        assert obj.uses_helper("test") == "[test]"
+
+        state = patch_function(
+            target=cls.uses_helper,
+            edits=[
+                EditSpec(
+                    match="return HelperClass.format_value(value)",
+                    replacement='return HelperClass.format_value("patched_" + value)',
+                )
+            ],
+        )
+        try:
+            assert obj.uses_helper("test") == "[patched_test]"
+        finally:
+            state.restore()
+
+        assert obj.uses_helper("test") == "[test]"
+
+
+class TestResolveTarget:
+    def test_resolve_class_method(self, sample_module: ModuleType) -> None:
+        target = _resolve_target(f"{SAMPLE_MODULE_NAME}.SampleClass.greet")
+        assert target is sample_module.SampleClass.greet
+
+    def test_resolve_standalone_function(self, sample_module: ModuleType) -> None:
+        target = _resolve_target(f"{SAMPLE_MODULE_NAME}.standalone_function")
+        assert target is sample_module.standalone_function
+
+    def test_resolve_nonexistent_raises(self, sample_module: ModuleType) -> None:
+        with pytest.raises((ImportError, AttributeError)):
+            _resolve_target(f"{SAMPLE_MODULE_NAME}.NonexistentClass.method")
+
+
+class TestCodePatcher:
+    def test_context_manager_patches_and_restores(
+        self, sample_module: ModuleType
+    ) -> None:
+        cls = sample_module.SampleClass
+        obj = cls()
+        assert obj.greet("world") == "hello world"
+
+        patches = [
+            PatchSpec(
+                target=f"{SAMPLE_MODULE_NAME}.SampleClass.greet",
+                edits=[
+                    EditSpec(
+                        match='greeting = f"hello {name}"',
+                        replacement='greeting = f"ctx_patched {name}"',
+                    )
+                ],
+            )
+        ]
+
+        with CodePatcher(patches=patches):
+            assert obj.greet("world") == "ctx_patched world"
+
+        assert obj.greet("world") == "hello world"
+
+    def test_context_manager_multiple_patches(self, sample_module: ModuleType) -> None:
+        cls = sample_module.SampleClass
+        obj = cls()
+
+        patches = [
+            PatchSpec(
+                target=f"{SAMPLE_MODULE_NAME}.SampleClass.greet",
+                edits=[
+                    EditSpec(
+                        match='greeting = f"hello {name}"',
+                        replacement='greeting = f"p1 {name}"',
+                    )
+                ],
+            ),
+            PatchSpec(
+                target=f"{SAMPLE_MODULE_NAME}.SampleClass.compute",
+                edits=[
+                    EditSpec(
+                        match="result = x * 2 + 1",
+                        replacement="result = x * 100",
+                    )
+                ],
+            ),
+        ]
+
+        with CodePatcher(patches=patches):
+            assert obj.greet("world") == "p1 world"
+            assert obj.compute(5) == 500
+
+        assert obj.greet("world") == "hello world"
+        assert obj.compute(5) == 11
+
+    def test_restores_on_exception(self, sample_module: ModuleType) -> None:
+        cls = sample_module.SampleClass
+        obj = cls()
+
+        patches = [
+            PatchSpec(
+                target=f"{SAMPLE_MODULE_NAME}.SampleClass.greet",
+                edits=[
+                    EditSpec(
+                        match='greeting = f"hello {name}"',
+                        replacement='greeting = f"err_patched {name}"',
+                    )
+                ],
+            )
+        ]
+
+        with pytest.raises(RuntimeError):
+            with CodePatcher(patches=patches):
+                assert obj.greet("world") == "err_patched world"
+                raise RuntimeError("test error")
+
+        assert obj.greet("world") == "hello world"
diff --git a/test/registered/debug_utils/source_patcher/test_source_editor.py b/test/registered/debug_utils/source_patcher/test_source_editor.py
new file mode 100644
index 000000000..10510732f
--- /dev/null
+++ b/test/registered/debug_utils/source_patcher/test_source_editor.py
@@ -0,0 +1,289 @@
+import pytest
+from pydantic import ValidationError
+
+from sglang.srt.debug_utils.source_patcher.source_editor import apply_edits
+from sglang.srt.debug_utils.source_patcher.types import EditSpec, PatchApplicationError
+
+
+class TestApplyEdits:
+    """Tests for the apply_edits() source text transformation function."""
+
+    def test_single_line_match_to_multiline_replacement(self) -> None:
+        source = "def foo():\n" "    x = compute()\n" "    return x\n"
+        edits = [
+            EditSpec(
+                match="x = compute()",
+                replacement="x = compute()\nprint(x)",
+            )
+        ]
+        result = apply_edits(source=source, edits=edits)
+        assert result == (
+            "def foo():\n" "    x = compute()\n" "    print(x)\n" "    return x\n"
+        )
+
+    def test_pure_insertion(self) -> None:
+        source = "def foo():\n" "    a = 1\n" "    b = 2\n"
+        edits = [
+            EditSpec(
+                match="a = 1",
+                replacement="a = 1\nprint(a)",
+            )
+        ]
+        result = apply_edits(source=source, edits=edits)
+        assert result == ("def foo():\n" "    a = 1\n" "    print(a)\n" "    b = 2\n")
+
+    def test_pure_deletion_via_empty_replacement(self) -> None:
+        source = "def foo():\n" "    debug_log()\n" "    return 42\n"
+        edits = [
+            EditSpec(
+                match="debug_log()",
+                replacement="",
+            )
+        ]
+        result = apply_edits(source=source, edits=edits)
+        assert result == ("def foo():\n" "    return 42\n")
+
+    def test_deletion_fewer_lines(self) -> None:
+        source = "def foo():\n" "    a = 1\n" "    b = 2\n" "    c = 3\n"
+        edits = [
+            EditSpec(
+                match="a = 1\nb = 2",
+                replacement="ab = 3",
+            )
+        ]
+        result = apply_edits(source=source, edits=edits)
+        assert result == ("def foo():\n" "    ab = 3\n" "    c = 3\n")
+
+    def test_multiline_match_to_multiline_replacement(self) -> None:
+        source = (
+            "def foo():\n"
+            "    result = self.attn(\n"
+            "        q=q,\n"
+            "        k=k,\n"
+            "    )\n"
+            "    return result\n"
+        )
+        edits = [
+            EditSpec(
+                match="result = self.attn(\n    q=q,\n    k=k,\n)",
+                replacement="result = self.attn(\n    q=q,\n    k=k,\n    v=v,\n)",
+            )
+        ]
+        result = apply_edits(source=source, edits=edits)
+        assert result == (
+            "def foo():\n"
+            "    result = self.attn(\n"
+            "        q=q,\n"
+            "        k=k,\n"
+            "        v=v,\n"
+            "    )\n"
+            "    return result\n"
+        )
+
+    def test_indent_alignment_deep_nesting(self) -> None:
+        source = (
+            "class Foo:\n"
+            "    class Bar:\n"
+            "        def method(self):\n"
+            "            x = compute()\n"
+            "            return x\n"
+        )
+        edits = [
+            EditSpec(
+                match="x = compute()",
+                replacement="x = compute()\nprint(x)",
+            )
+        ]
+        result = apply_edits(source=source, edits=edits)
+        assert result == (
+            "class Foo:\n"
+            "    class Bar:\n"
+            "        def method(self):\n"
+            "            x = compute()\n"
+            "            print(x)\n"
+            "            return x\n"
+        )
+
+    def test_match_not_found_raises(self) -> None:
+        source = "def foo():\n    return 1\n"
+        edits = [EditSpec(match="nonexistent_call()", replacement="replaced()")]
+        with pytest.raises(PatchApplicationError, match="not found"):
+            apply_edits(source=source, edits=edits)
+
+    def test_match_found_multiple_times_raises(self) -> None:
+        source = "def foo():\n" "    print(1)\n" "    print(1)\n"
+        edits = [EditSpec(match="print(1)", replacement="print(2)")]
+        with pytest.raises(PatchApplicationError, match="multiple"):
+            apply_edits(source=source, edits=edits)
+
+    def test_multiple_edits_applied_sequentially(self) -> None:
+        source = "def foo():\n" "    a = 1\n" "    b = 2\n" "    return a + b\n"
+        edits = [
+            EditSpec(match="a = 1", replacement="a = 10"),
+            EditSpec(match="b = 2", replacement="b = 20"),
+        ]
+        result = apply_edits(source=source, edits=edits)
+        assert result == (
+            "def foo():\n" "    a = 10\n" "    b = 20\n" "    return a + b\n"
+        )
+
+    def test_strip_matching_ignores_leading_trailing_whitespace(self) -> None:
+        source = "def foo():\n" "    x = compute()\n" "    return x\n"
+        edits = [
+            EditSpec(
+                match="  x = compute()  ",
+                replacement="x = replaced()",
+            )
+        ]
+        result = apply_edits(source=source, edits=edits)
+        assert result == ("def foo():\n" "    x = replaced()\n" "    return x\n")
+
+    def test_replacement_indented_text_realigned(self) -> None:
+        """replacement text with its own indentation gets realigned to match source."""
+        source = "def foo():\n" "        x = compute()\n" "        return x\n"
+        edits = [
+            EditSpec(
+                match="x = compute()",
+                replacement="x = compute()\nprint(x)",
+            )
+        ]
+        result = apply_edits(source=source, edits=edits)
+        assert result == (
+            "def foo():\n"
+            "        x = compute()\n"
+            "        print(x)\n"
+            "        return x\n"
+        )
+
+    def test_replacement_with_existing_indent_realigned(self) -> None:
+        """replacement text already has indentation that should be rebased."""
+        source = "def foo():\n" "    if True:\n" "        x = 1\n" "        return x\n"
+        edits = [
+            EditSpec(
+                match="x = 1",
+                replacement="x = 1\nif x > 0:\n    print(x)",
+            )
+        ]
+        result = apply_edits(source=source, edits=edits)
+        assert result == (
+            "def foo():\n"
+            "    if True:\n"
+            "        x = 1\n"
+            "        if x > 0:\n"
+            "            print(x)\n"
+            "        return x\n"
+        )
+
+    def test_append_keeps_match_and_adds_after(self) -> None:
+        source = "def foo():\n" "    x = compute()\n" "    return x\n"
+        edits = [EditSpec(match="x = compute()", append="print(x)")]
+        result = apply_edits(source=source, edits=edits)
+        assert result == (
+            "def foo():\n" "    x = compute()\n" "    print(x)\n" "    return x\n"
+        )
+
+    def test_append_multiline_match(self) -> None:
+        source = (
+            "def foo():\n"
+            "    result = call(\n"
+            "        a=1,\n"
+            "        b=2,\n"
+            "    )\n"
+            "    return result\n"
+        )
+        edits = [
+            EditSpec(
+                match="result = call(\n    a=1,\n    b=2,\n)",
+                append="dumper.dump('result', result)",
+            )
+        ]
+        result = apply_edits(source=source, edits=edits)
+        assert result == (
+            "def foo():\n"
+            "    result = call(\n"
+            "        a=1,\n"
+            "        b=2,\n"
+            "    )\n"
+            "    dumper.dump('result', result)\n"
+            "    return result\n"
+        )
+
+    def test_prepend_adds_before_match(self) -> None:
+        source = "def foo():\n" "    x = compute()\n" "    return x\n"
+        edits = [EditSpec(match="x = compute()", prepend="print('before')")]
+        result = apply_edits(source=source, edits=edits)
+        assert result == (
+            "def foo():\n"
+            "    print('before')\n"
+            "    x = compute()\n"
+            "    return x\n"
+        )
+
+    def test_prepend_multiline(self) -> None:
+        source = "def foo():\n" "    return x\n"
+        edits = [EditSpec(match="return x", prepend="a = 1\nb = 2")]
+        result = apply_edits(source=source, edits=edits)
+        assert result == ("def foo():\n" "    a = 1\n" "    b = 2\n" "    return x\n")
+
+    def test_prepend_deep_indent(self) -> None:
+        source = (
+            "class Foo:\n"
+            "    class Bar:\n"
+            "        def method(self):\n"
+            "            return x\n"
+        )
+        edits = [EditSpec(match="return x", prepend="dumper.dump('x', x)")]
+        result = apply_edits(source=source, edits=edits)
+        assert result == (
+            "class Foo:\n"
+            "    class Bar:\n"
+            "        def method(self):\n"
+            "            dumper.dump('x', x)\n"
+            "            return x\n"
+        )
+
+    def test_prepend_multiline_match(self) -> None:
+        source = (
+            "def foo():\n"
+            "    result = call(\n"
+            "        a=1,\n"
+            "    )\n"
+            "    return result\n"
+        )
+        edits = [
+            EditSpec(
+                match="result = call(\n    a=1,\n)",
+                prepend="dumper.dump('before', x)",
+            )
+        ]
+        result = apply_edits(source=source, edits=edits)
+        assert result == (
+            "def foo():\n"
+            "    dumper.dump('before', x)\n"
+            "    result = call(\n"
+            "        a=1,\n"
+            "    )\n"
+            "    return result\n"
+        )
+
+    def test_replacement_and_append_mutually_exclusive(self) -> None:
+        with pytest.raises(ValidationError, match="only one of"):
+            EditSpec(match="x = 1", replacement="x = 2", append="print(x)")
+
+    def test_replacement_and_prepend_mutually_exclusive(self) -> None:
+        with pytest.raises(ValidationError, match="only one of"):
+            EditSpec(match="x = 1", replacement="x = 2", prepend="print(x)")
+
+    def test_prepend_and_append_mutually_exclusive(self) -> None:
+        with pytest.raises(ValidationError, match="only one of"):
+            EditSpec(match="x = 1", prepend="a()", append="b()")
+
+    def test_second_edit_sees_result_of_first(self) -> None:
+        """Edits are applied sequentially; second edit matches modified source."""
+        source = "def foo():\n" "    x = 1\n" "    return x\n"
+        edits = [
+            EditSpec(match="x = 1", replacement="x = 1\ny = 2"),
+            EditSpec(match="y = 2", replacement="y = 20"),
+        ]
+        result = apply_edits(source=source, edits=edits)
+        assert result == ("def foo():\n" "    x = 1\n" "    y = 20\n" "    return x\n")