diff --git a/src/agentkit/core/react.py b/src/agentkit/core/react.py
index 836c5ee..31f0f87 100644
--- a/src/agentkit/core/react.py
+++ b/src/agentkit/core/react.py
@@ -153,9 +153,12 @@ class ReActEngine:
     # Default core tools that always get full descriptions injected into the
     # prompt. ``tool_search`` is included so its full description is always
     # available to the LLM when tiered injection is active.
+    # U1: replaced the broken `write_file` placeholder (no real implementation —
+    # only `_FakeTool` stubs) with `str_replace_editor` (workspace-root confined
+    # create/str_replace/insert_at_line/view — see tools/str_replace_editor.py).
     _DEFAULT_CORE_TOOLS: tuple[str, ...] = (
         "read_file",
-        "write_file",
+        "str_replace_editor",
         "bash",
         "search",
         "tool_search",
diff --git a/src/agentkit/tools/__init__.py b/src/agentkit/tools/__init__.py
index ea54dcf..b81018f 100644
--- a/src/agentkit/tools/__init__.py
+++ b/src/agentkit/tools/__init__.py
@@ -19,6 +19,7 @@ from agentkit.tools.web_search import WebSearchTool
 from agentkit.tools.builtin import RunTestsTool, ToolSearchTool
 from agentkit.tools.search import ToolSearchIndex
 from agentkit.tools.file_read import ReadFileTool
+from agentkit.tools.str_replace_editor import StrReplaceEditorTool
 from agentkit.tools.advance_phase import AdvancePhaseTool
 
 # Conditional import: HeadroomRetrieveTool requires HeadroomCompressor
@@ -55,5 +56,6 @@ __all__ = [
     "ParsedOutput",
     "ErrorType",
     "ReadFileTool",
+    "StrReplaceEditorTool",
     "AdvancePhaseTool",
 ]
diff --git a/src/agentkit/tools/str_replace_editor.py b/src/agentkit/tools/str_replace_editor.py
new file mode 100644
index 0000000..0ff8210
--- /dev/null
+++ b/src/agentkit/tools/str_replace_editor.py
@@ -0,0 +1,400 @@
+"""StrReplaceEditorTool — structured file editing with workspace-root security (U1, R1).
+
+Replaces the broken `write_file` placeholder (which had no real implementation —
+only `_FakeTool` stubs in `cli/benchmark.py`). Provides four commands:
+
+  - `create`         write a new file (errors if it already exists — data-loss guard)
+  - `str_replace`    exact-match anchor replace (anchor must be unique in the file)
+  - `insert_at_line` insert text at a 1-based line number (0 = prepend, > EOF = append)
+  - `view`           read file with line numbers (needed so `str_replace` anchors
+                     and `insert_at_line` targets can be discovered)
+
+Security model (file-system analog of the 6-layer terminal security paradigm in
+`server/auth/terminal_security.py` — reject-by-default + prefix match):
+
+  1. Reject absolute paths (force relative interpretation against workspace root).
+  2. Reject any ``..`` path component (path traversal).
+  3. ``Path.resolve()`` follows symlinks, then ``relative_to(workspace_root)``
+     rejects symlink escape and any residual traversal.
+
+Filesystem I/O is wrapped in ``asyncio.to_thread`` to avoid blocking the event loop.
+"""
+
+from __future__ import annotations
+
+import asyncio
+import logging
+from pathlib import Path
+
+from agentkit.tools.base import Tool
+
+logger = logging.getLogger(__name__)
+
+
+class StrReplaceEditorTool(Tool):
+    """Structured file editor with four commands and workspace-root confinement.
+
+    Tool name ``str_replace_editor`` is registered in
+    ``core/react.py:_DEFAULT_CORE_TOOLS`` so its full description is always
+    injected into the LLM prompt (tiered description injection).
+    """
+
+    def __init__(
+        self,
+        workspace_root: str | Path | None = None,
+        name: str = "str_replace_editor",
+        description: str | None = None,
+        input_schema: dict[str, object] | None = None,
+        output_schema: dict[str, object] | None = None,
+        version: str = "1.0.0",
+        tags: list[str] | None = None,
+    ):
+        # Resolve once so later prefix checks compare against a stable, real
+        # directory (no symlink in the workspace root itself).
+        self._workspace_root: Path = Path(workspace_root or Path.cwd()).resolve()
+        super().__init__(
+            name=name,
+            description=description or self._default_description(),
+            input_schema=input_schema or self._default_input_schema(),
+            output_schema=output_schema or self._default_output_schema(),
+            version=version,
+            tags=tags or ["io", "file", "edit"],
+        )
+
+    @staticmethod
+    def _default_description() -> str:
+        return (
+            "Edit a file with structured commands. Paths are relative to the "
+            "workspace root (absolute paths and `..` traversal are rejected; "
+            "symlink escape is blocked). Commands: `create` (write a new file — "
+            "errors if it exists), `str_replace` (replace a unique exact-match "
+            "anchor), `insert_at_line` (insert text at a 1-based line; 0=prepend, "
+            ">EOF=append), `view` (read file with line numbers). Always `view` a "
+            "file first to get exact anchors and line numbers."
+        )
+
+    @staticmethod
+    def _default_input_schema() -> dict[str, object]:
+        return {
+            "type": "object",
+            "properties": {
+                "command": {
+                    "type": "string",
+                    "enum": ["create", "str_replace", "insert_at_line", "view"],
+                    "description": "The editing command to execute.",
+                },
+                "path": {
+                    "type": "string",
+                    "description": (
+                        "Relative path to the file within the workspace root. "
+                        "Absolute paths and `..` components are rejected."
+                    ),
+                },
+                "file_text": {
+                    "type": "string",
+                    "description": "Required for `create`: full content of the new file.",
+                },
+                "old_str": {
+                    "type": "string",
+                    "description": (
+                        "Required for `str_replace`: exact text to find (whitespace "
+                        "and indentation must match). Must occur exactly once."
+                    ),
+                },
+                "new_str": {
+                    "type": "string",
+                    "description": (
+                        "Required for `str_replace` and `insert_at_line`: the "
+                        "replacement / insertion text (may be multi-line)."
+                    ),
+                },
+                "insert_line": {
+                    "type": "integer",
+                    "minimum": 0,
+                    "description": (
+                        "Required for `insert_at_line`: 1-based line number to insert "
+                        "BEFORE. 0 = prepend before line 1; greater than the file's "
+                        "line count = append at end."
+                    ),
+                },
+                "start_line": {
+                    "type": "integer",
+                    "minimum": 1,
+                    "description": "Optional for `view`: 1-based start line (inclusive).",
+                },
+                "end_line": {
+                    "type": "integer",
+                    "minimum": 1,
+                    "description": "Optional for `view`: 1-based end line (inclusive).",
+                },
+            },
+            "required": ["command", "path"],
+            "additionalProperties": False,
+        }
+
+    @staticmethod
+    def _default_output_schema() -> dict[str, object]:
+        return {
+            "type": "object",
+            "properties": {
+                "command": {"type": "string"},
+                "path": {"type": "string"},
+                "content": {"type": "string"},
+                "start_line": {"type": "integer"},
+                "end_line": {"type": "integer"},
+                "total_lines": {"type": "integer"},
+                "is_error": {"type": "boolean"},
+                "error": {"type": "string"},
+                "note": {"type": "string"},
+            },
+        }
+
+    # ── path security ─────────────────────────────────────────────────
+
+    def _resolve_within_workspace(self, raw_path: str) -> Path | None:
+        """Resolve ``raw_path`` and verify it stays within the workspace root.
+
+        Returns the resolved absolute Path on success, or ``None`` if the path
+        is absolute, contains a ``..`` component, or resolves outside the
+        workspace root (path traversal or symlink escape). ``Path.resolve()``
+        follows symlinks, so a symlink pointing outside the workspace resolves
+        to an outside path and fails the ``relative_to`` check.
+        """
+        if not isinstance(raw_path, str) or not raw_path:
+            return None
+        p = Path(raw_path)
+        if p.is_absolute():
+            return None  # layer 1: force relative interpretation
+        if ".." in p.parts:
+            return None  # layer 2: reject path traversal
+        resolved = (self._workspace_root / raw_path).resolve()
+        try:
+            resolved.relative_to(self._workspace_root)  # layer 3: symlink escape
+        except ValueError:
+            return None
+        return resolved
+
+    # ── execute ────────────────────────────────────────────────────────
+
+    async def execute(self, **kwargs) -> dict[str, object]:
+        command = kwargs.get("command")
+        raw_path = kwargs.get("path")
+        if command not in ("create", "str_replace", "insert_at_line", "view"):
+            return self._error(
+                f"Unknown command {command!r}; expected one of "
+                "create/str_replace/insert_at_line/view",
+                path=raw_path if isinstance(raw_path, str) else None,
+            )
+        if not isinstance(raw_path, str) or not raw_path:
+            return self._error("`path` is required and must be a non-empty string")
+
+        path = self._resolve_within_workspace(raw_path)
+        if path is None:
+            return self._error(
+                f"Path {raw_path!r} is rejected: absolute paths, `..` traversal, "
+                f"and symlink escape outside the workspace root "
+                f"({self._workspace_root}) are not allowed.",
+                path=raw_path,
+            )
+
+        if command == "create":
+            return await self._cmd_create(path, kwargs)
+        if command == "str_replace":
+            return await self._cmd_str_replace(path, kwargs)
+        if command == "insert_at_line":
+            return await self._cmd_insert_at_line(path, kwargs)
+        return await self._cmd_view(path, kwargs)
+
+    # ── commands ───────────────────────────────────────────────────────
+
+    async def _cmd_create(self, path: Path, kwargs: dict[str, object]) -> dict[str, object]:
+        file_text = kwargs.get("file_text")
+        if not isinstance(file_text, str):
+            return self._error("`file_text` is required for `create`", path=str(path))
+        if path.exists():
+            # Data-loss guard: refuse to overwrite. Use str_replace to edit an
+            # existing file, or delete it first via the shell tool.
+            return self._error(
+                f"File already exists (create refuses to overwrite): {path}. "
+                f"Use str_replace to edit it.",
+                path=str(path),
+            )
+        return await asyncio.to_thread(self._write_file, path, file_text, "create")
+
+    async def _cmd_str_replace(self, path: Path, kwargs: dict[str, object]) -> dict[str, object]:
+        old_str = kwargs.get("old_str")
+        new_str = kwargs.get("new_str")
+        if not isinstance(old_str, str) or old_str == "":
+            return self._error(
+                "`old_str` is required for `str_replace` and must be non-empty",
+                path=str(path),
+            )
+        if not isinstance(new_str, str):
+            return self._error("`new_str` is required for `str_replace`", path=str(path))
+
+        read_result = await asyncio.to_thread(self._read_file, path)
+        if read_result["is_error"]:
+            return read_result
+        content = read_result["content"]
+        count = content.count(old_str)
+        if count == 0:
+            return self._error(
+                f"`old_str` anchor not found in {path}. Use `view` to inspect the "
+                f"exact text (whitespace and indentation must match).",
+                path=str(path),
+            )
+        if count > 1:
+            return self._error(
+                f"`old_str` anchor is not unique: found {count} matches in {path}. "
+                f"Include more surrounding context so the anchor matches once.",
+                path=str(path),
+            )
+        new_content = content.replace(old_str, new_str, 1)
+        return await asyncio.to_thread(self._write_file, path, new_content, "str_replace")
+
+    async def _cmd_insert_at_line(self, path: Path, kwargs: dict[str, object]) -> dict[str, object]:
+        new_str = kwargs.get("new_str")
+        if not isinstance(new_str, str):
+            return self._error("`new_str` is required for `insert_at_line`", path=str(path))
+        insert_line = kwargs.get("insert_line")
+        # bool is a subclass of int — exclude it explicitly.
+        if isinstance(insert_line, bool) or not isinstance(insert_line, int):
+            return self._error(
+                f"`insert_line` is required for `insert_at_line` and must be a "
+                f"non-negative integer, got {insert_line!r}",
+                path=str(path),
+            )
+        if insert_line < 0:
+            return self._error(f"`insert_line` must be >= 0, got {insert_line}", path=str(path))
+
+        read_result = await asyncio.to_thread(self._read_file, path)
+        if read_result["is_error"]:
+            return read_result
+        content = read_result["content"]
+        lines = content.splitlines()
+        # 1-based line N → insert before it (index N-1). 0 → prepend (index 0).
+        # Beyond EOF → append (index len). splitlines drops a trailing newline,
+        # so EOF here means the last logical line.
+        idx = 0 if insert_line == 0 else insert_line - 1
+        idx = max(0, min(idx, len(lines)))
+        new_lines = new_str.splitlines() if new_str != "" else []
+        result_lines = lines[:idx] + new_lines + lines[idx:]
+        new_content = "\n".join(result_lines)
+        # Preserve a trailing newline that existed in the original (splitlines
+        # dropped it). ponytail: only the final newline is restored; rare
+        # double-trailing-newline files collapse to one on insert — acceptable
+        # for an editor on an LF-normalized repo.
+        if content.endswith("\n") and not new_content.endswith("\n"):
+            new_content += "\n"
+        return await asyncio.to_thread(self._write_file, path, new_content, "insert_at_line")
+
+    async def _cmd_view(self, path: Path, kwargs: dict[str, object]) -> dict[str, object]:
+        start_line = kwargs.get("start_line")
+        end_line = kwargs.get("end_line")
+        if start_line is not None and (
+            not isinstance(start_line, int) or isinstance(start_line, bool) or start_line < 1
+        ):
+            return self._error(
+                f"`start_line` must be a positive integer, got {start_line!r}",
+                path=str(path),
+            )
+        if end_line is not None and (
+            not isinstance(end_line, int) or isinstance(end_line, bool) or end_line < 1
+        ):
+            return self._error(
+                f"`end_line` must be a positive integer, got {end_line!r}",
+                path=str(path),
+            )
+        if start_line is not None and end_line is not None and end_line < start_line:
+            return self._error(
+                f"`end_line` ({end_line}) must be >= `start_line` ({start_line})",
+                path=str(path),
+            )
+
+        read_result = await asyncio.to_thread(self._read_file, path)
+        if read_result["is_error"]:
+            return read_result
+        content = read_result["content"]
+        lines = content.splitlines()
+        total = len(lines)
+        if total == 0:
+            return {
+                "command": "view",
+                "path": str(path),
+                "content": "",
+                "start_line": 0,
+                "end_line": 0,
+                "total_lines": 0,
+                "is_error": False,
+                "note": "empty file",
+            }
+        s = max(1, start_line or 1)
+        e = min(total, end_line or total)
+        if s > total:
+            numbered = ""
+            note = f"range starts beyond EOF (file has {total} lines)"
+        else:
+            sliced = lines[s - 1 : e]
+            # cat -n style: right-aligned 1-based number + tab. ASCII only.
+            numbered = "\n".join(f"{i:>6}\t{line}" for i, line in enumerate(sliced, start=s))
+            note = None
+        result: dict[str, object] = {
+            "command": "view",
+            "path": str(path),
+            "content": numbered,
+            "start_line": s,
+            "end_line": e,
+            "total_lines": total,
+            "is_error": False,
+        }
+        if note:
+            result["note"] = note
+        return result
+
+    # ── blocking filesystem helpers (run via to_thread) ────────────────
+
+    def _read_file(self, path: Path) -> dict[str, object]:
+        if not path.exists():
+            return self._error(f"File not found: {path}", path=str(path))
+        if path.is_dir():
+            return self._error(f"Path is a directory, not a file: {path}", path=str(path))
+        try:
+            content = path.read_text(encoding="utf-8", errors="replace")
+        except PermissionError as e:
+            return self._error(f"Permission denied: {path}", path=str(path), detail=str(e))
+        except OSError as e:
+            return self._error(f"Failed to read {path}: {e}", path=str(path))
+        return {"content": content, "is_error": False}
+
+    def _write_file(self, path: Path, content: str, command: str) -> dict[str, object]:
+        try:
+            path.parent.mkdir(parents=True, exist_ok=True)
+            path.write_text(content, encoding="utf-8")
+        except PermissionError as e:
+            return self._error(f"Permission denied: {path}", path=str(path), detail=str(e))
+        except OSError as e:
+            return self._error(f"Failed to write {path}: {e}", path=str(path))
+        # ponytail: splitlines is O(n) per write; fine for editor-scale files
+        # (<1 MB). For VFS-scale writes, pass len(lines) from the caller instead.
+        return {
+            "command": command,
+            "path": str(path),
+            "content": content,
+            "total_lines": len(content.splitlines()),
+            "is_error": False,
+            "note": f"{command} succeeded",
+        }
+
+    @staticmethod
+    def _error(
+        message: str,
+        *,
+        path: str | None = None,
+        detail: str | None = None,
+    ) -> dict[str, object]:
+        result: dict[str, object] = {"is_error": True, "error": message}
+        if path is not None:
+            result["path"] = path
+        if detail is not None:
+            result["detail"] = detail
+        return result
diff --git a/tests/unit/test_str_replace_editor.py b/tests/unit/test_str_replace_editor.py
new file mode 100644
index 0000000..16c7a79
--- /dev/null
+++ b/tests/unit/test_str_replace_editor.py
@@ -0,0 +1,421 @@
+"""Unit tests for StrReplaceEditorTool (U1, R1).
+
+Covers happy path, edge cases, error/failure paths, path-security rejection,
+and the integration contract that the tool is registered as a default core
+tool in ReActEngine and exported from the tools package.
+"""
+
+from __future__ import annotations
+
+import os
+import sys
+from pathlib import Path
+
+import pytest
+
+from agentkit.tools.str_replace_editor import StrReplaceEditorTool
+
+
+# ── fixtures ──────────────────────────────────────────────────────────
+
+
+@pytest.fixture
+def workspace(tmp_path: Path) -> Path:
+    """A clean workspace root directory for each test."""
+    return tmp_path
+
+
+@pytest.fixture
+def tool(workspace: Path) -> StrReplaceEditorTool:
+    return StrReplaceEditorTool(workspace_root=workspace)
+
+
+# ── happy path ────────────────────────────────────────────────────────
+
+
+async def test_create_writes_new_file(tool: StrReplaceEditorTool, workspace: Path) -> None:
+    result = await tool.execute(command="create", path="hello.py", file_text="print('hi')\n")
+    assert result["is_error"] is False
+    assert result["command"] == "create"
+    assert result["total_lines"] == 1
+    assert (workspace / "hello.py").read_text() == "print('hi')\n"
+
+
+async def test_view_returns_content_with_line_numbers(
+    tool: StrReplaceEditorTool, workspace: Path
+) -> None:
+    (workspace / "a.txt").write_text("alpha\nbeta\ngamma\n")
+    result = await tool.execute(command="view", path="a.txt")
+    assert result["is_error"] is False
+    assert result["total_lines"] == 3
+    assert result["start_line"] == 1
+    assert result["end_line"] == 3
+    # cat -n style: right-aligned number + tab.
+    assert result["content"] == "     1\talpha\n     2\tbeta\n     3\tgamma"
+
+
+async def test_str_replace_replaces_unique_anchor(
+    tool: StrReplaceEditorTool, workspace: Path
+) -> None:
+    (workspace / "f.txt").write_text("def foo():\n    return 1\n")
+    result = await tool.execute(
+        command="str_replace",
+        path="f.txt",
+        old_str="return 1",
+        new_str="return 2",
+    )
+    assert result["is_error"] is False
+    assert (workspace / "f.txt").read_text() == "def foo():\n    return 2\n"
+
+
+async def test_insert_at_line_inserts_in_middle(
+    tool: StrReplaceEditorTool, workspace: Path
+) -> None:
+    (workspace / "f.txt").write_text("line1\nline2\nline3\n")
+    result = await tool.execute(
+        command="insert_at_line", path="f.txt", insert_line=2, new_str="INSERTED"
+    )
+    assert result["is_error"] is False
+    assert (workspace / "f.txt").read_text() == "line1\nINSERTED\nline2\nline3\n"
+
+
+# ── edge cases ────────────────────────────────────────────────────────
+
+
+async def test_create_empty_file(tool: StrReplaceEditorTool, workspace: Path) -> None:
+    result = await tool.execute(command="create", path="empty.txt", file_text="")
+    assert result["is_error"] is False
+    assert result["total_lines"] == 0
+    assert (workspace / "empty.txt").read_text() == ""
+    # view of an empty file reports total_lines=0 with a note.
+    view = await tool.execute(command="view", path="empty.txt")
+    assert view["is_error"] is False
+    assert view["total_lines"] == 0
+    assert view["content"] == ""
+    assert view["note"] == "empty file"
+
+
+async def test_str_replace_multiple_matches_is_error(
+    tool: StrReplaceEditorTool, workspace: Path
+) -> None:
+    (workspace / "f.txt").write_text("x\nx\n")
+    result = await tool.execute(command="str_replace", path="f.txt", old_str="x", new_str="y")
+    assert result["is_error"] is True
+    assert "not unique" in result["error"]
+    # File is untouched on error.
+    assert (workspace / "f.txt").read_text() == "x\nx\n"
+
+
+async def test_insert_at_line_zero_prepends(tool: StrReplaceEditorTool, workspace: Path) -> None:
+    (workspace / "f.txt").write_text("line1\nline2\n")
+    result = await tool.execute(
+        command="insert_at_line", path="f.txt", insert_line=0, new_str="TOP"
+    )
+    assert result["is_error"] is False
+    assert (workspace / "f.txt").read_text() == "TOP\nline1\nline2\n"
+
+
+async def test_insert_at_line_beyond_eof_appends(
+    tool: StrReplaceEditorTool, workspace: Path
+) -> None:
+    (workspace / "f.txt").write_text("line1\nline2\n")
+    result = await tool.execute(
+        command="insert_at_line", path="f.txt", insert_line=99, new_str="BOTTOM"
+    )
+    assert result["is_error"] is False
+    assert (workspace / "f.txt").read_text() == "line1\nline2\nBOTTOM\n"
+
+
+async def test_insert_at_line_multiline_text(tool: StrReplaceEditorTool, workspace: Path) -> None:
+    (workspace / "f.txt").write_text("a\nb\n")
+    result = await tool.execute(
+        command="insert_at_line",
+        path="f.txt",
+        insert_line=2,
+        new_str="x\ny\nz",
+    )
+    assert result["is_error"] is False
+    assert (workspace / "f.txt").read_text() == "a\nx\ny\nz\nb\n"
+
+
+async def test_view_with_line_range(tool: StrReplaceEditorTool, workspace: Path) -> None:
+    (workspace / "f.txt").write_text("one\ntwo\nthree\nfour\nfive\n")
+    result = await tool.execute(command="view", path="f.txt", start_line=2, end_line=4)
+    assert result["is_error"] is False
+    assert result["start_line"] == 2
+    assert result["end_line"] == 4
+    assert result["total_lines"] == 5
+    assert result["content"] == "     2\ttwo\n     3\tthree\n     4\tfour"
+
+
+async def test_view_range_beyond_eof_returns_empty(
+    tool: StrReplaceEditorTool, workspace: Path
+) -> None:
+    (workspace / "f.txt").write_text("only\n")
+    result = await tool.execute(command="view", path="f.txt", start_line=10, end_line=20)
+    assert result["is_error"] is False
+    assert result["content"] == ""
+    assert result["start_line"] == 10
+
+
+# ── error and failure paths ───────────────────────────────────────────
+
+
+async def test_create_refuses_overwrite(tool: StrReplaceEditorTool, workspace: Path) -> None:
+    (workspace / "f.txt").write_text("existing\n")
+    result = await tool.execute(command="create", path="f.txt", file_text="new\n")
+    assert result["is_error"] is True
+    assert "already exists" in result["error"]
+    # Original content preserved (data-loss guard).
+    assert (workspace / "f.txt").read_text() == "existing\n"
+
+
+async def test_str_replace_anchor_not_found(tool: StrReplaceEditorTool, workspace: Path) -> None:
+    (workspace / "f.txt").write_text("hello world\n")
+    result = await tool.execute(
+        command="str_replace", path="f.txt", old_str="goodbye", new_str="hi"
+    )
+    assert result["is_error"] is True
+    assert "not found" in result["error"]
+
+
+async def test_str_replace_empty_old_str_rejected(
+    tool: StrReplaceEditorTool, workspace: Path
+) -> None:
+    (workspace / "f.txt").write_text("x\n")
+    result = await tool.execute(command="str_replace", path="f.txt", old_str="", new_str="y")
+    assert result["is_error"] is True
+    assert "old_str" in result["error"]
+
+
+async def test_str_replace_on_missing_file(tool: StrReplaceEditorTool, workspace: Path) -> None:
+    result = await tool.execute(command="str_replace", path="nope.txt", old_str="a", new_str="b")
+    assert result["is_error"] is True
+    assert "not found" in result["error"].lower()
+
+
+async def test_path_traversal_rejected(tool: StrReplaceEditorTool, workspace: Path) -> None:
+    result = await tool.execute(command="view", path="../../etc/passwd")
+    assert result["is_error"] is True
+    assert "rejected" in result["error"]
+
+
+async def test_path_traversal_create_rejected(
+    tool: StrReplaceEditorTool, workspace: Path, tmp_path: Path
+) -> None:
+    # Even if the target would resolve inside a sibling dir, `..` is rejected.
+    result = await tool.execute(command="create", path="../sibling.txt", file_text="x")
+    assert result["is_error"] is True
+
+
+async def test_absolute_path_rejected(tool: StrReplaceEditorTool, workspace: Path) -> None:
+    # Absolute path to a real file outside the workspace.
+    result = await tool.execute(command="view", path="/etc/passwd")
+    assert result["is_error"] is True
+    assert "rejected" in result["error"]
+
+
+async def test_absolute_path_inside_workspace_also_rejected(
+    tool: StrReplaceEditorTool, workspace: Path
+) -> None:
+    # Absolute paths are rejected outright (force relative interpretation),
+    # even when the path would resolve inside the workspace.
+    target = workspace / "inside.txt"
+    target.write_text("ok\n")
+    result = await tool.execute(command="view", path=str(target))
+    assert result["is_error"] is True
+    assert "rejected" in result["error"]
+
+
+async def test_symlink_escape_rejected(tmp_path: Path) -> None:
+    # Use a workspace SUBDIR of tmp_path so a file under tmp_path (but not
+    # under the workspace) counts as "outside the workspace".
+    workspace = tmp_path / "ws"
+    workspace.mkdir()
+    tool = StrReplaceEditorTool(workspace_root=workspace)
+    # Real secret file OUTSIDE the workspace (sibling, still under tmp_path).
+    outside = tmp_path / "secret.txt"
+    outside.write_text("top secret\n")
+    # Symlink inside the workspace pointing to the outside file.
+    link = workspace / "escape.txt"
+    os.symlink(outside, link)
+    # view through the symlink must be rejected (symlink escape).
+    result = await tool.execute(command="view", path="escape.txt")
+    assert result["is_error"] is True
+    assert "rejected" in result["error"]
+    # create through a symlink that escapes must also be rejected.
+    result2 = await tool.execute(
+        command="create", path="escape.txt", file_text="overwrite\n"
+    )
+    assert result2["is_error"] is True
+    # The outside file must NOT have been overwritten (data-loss guard).
+    assert outside.read_text() == "top secret\n"
+
+
+async def test_symlink_to_inside_workspace_allowed(
+    tool: StrReplaceEditorTool, workspace: Path
+) -> None:
+    # A symlink whose target is INSIDE the workspace is allowed (no escape).
+    real = workspace / "real.txt"
+    real.write_text("content\n")
+    link = workspace / "link.txt"
+    os.symlink(real, link)
+    result = await tool.execute(command="view", path="link.txt")
+    assert result["is_error"] is False
+    assert "content" in result["content"]
+
+
+async def test_file_outside_workspace_rejected(tool: StrReplaceEditorTool, tmp_path: Path) -> None:
+    # A relative path that climbs out via `..` is rejected by the `..` rule,
+    # but also verify a nested traversal attempt is caught.
+    result = await tool.execute(command="view", path="sub/../../etc/passwd")
+    assert result["is_error"] is True
+
+
+async def test_unknown_command_rejected(tool: StrReplaceEditorTool, workspace: Path) -> None:
+    result = await tool.execute(command="delete", path="f.txt")
+    assert result["is_error"] is True
+    assert "Unknown command" in result["error"]
+
+
+async def test_missing_path_rejected(tool: StrReplaceEditorTool) -> None:
+    result = await tool.execute(command="view", path="")
+    assert result["is_error"] is True
+    assert "path" in result["error"].lower()
+
+
+async def test_missing_file_text_rejected(tool: StrReplaceEditorTool, workspace: Path) -> None:
+    result = await tool.execute(command="create", path="f.txt")
+    assert result["is_error"] is True
+    assert "file_text" in result["error"]
+
+
+async def test_missing_insert_line_rejected(tool: StrReplaceEditorTool, workspace: Path) -> None:
+    (workspace / "f.txt").write_text("a\n")
+    result = await tool.execute(command="insert_at_line", path="f.txt", new_str="b")
+    assert result["is_error"] is True
+    assert "insert_line" in result["error"]
+
+
+async def test_insert_line_negative_rejected(tool: StrReplaceEditorTool, workspace: Path) -> None:
+    (workspace / "f.txt").write_text("a\n")
+    result = await tool.execute(command="insert_at_line", path="f.txt", insert_line=-1, new_str="b")
+    assert result["is_error"] is True
+
+
+async def test_view_directory_rejected(tool: StrReplaceEditorTool, workspace: Path) -> None:
+    (workspace / "subdir").mkdir()
+    result = await tool.execute(command="view", path="subdir")
+    assert result["is_error"] is True
+    assert "directory" in result["error"].lower()
+
+
+async def test_create_in_nested_subdir_creates_parents(
+    tool: StrReplaceEditorTool, workspace: Path
+) -> None:
+    result = await tool.execute(
+        command="create",
+        path="nested/deep/file.txt",
+        file_text="deep\n",
+    )
+    assert result["is_error"] is False
+    assert (workspace / "nested" / "deep" / "file.txt").read_text() == "deep\n"
+
+
+# ── integration contract ──────────────────────────────────────────────
+
+
+def test_str_replace_editor_in_default_core_tools() -> None:
+    """The tool must be a default core tool so its full description is
+    always injected into the LLM prompt (tiered injection)."""
+    from agentkit.core.react import ReActEngine
+
+    assert "str_replace_editor" in ReActEngine._DEFAULT_CORE_TOOLS
+    # The broken write_file placeholder must be gone.
+    assert "write_file" not in ReActEngine._DEFAULT_CORE_TOOLS
+
+
+def test_tool_exported_from_tools_package() -> None:
+    from agentkit.tools import StrReplaceEditorTool as Exported
+
+    assert Exported is StrReplaceEditorTool
+
+
+def test_tool_name_and_schema(tool: StrReplaceEditorTool) -> None:
+    assert tool.name == "str_replace_editor"
+    assert tool.input_schema is not None
+    props = tool.input_schema["properties"]
+    assert "command" in props
+    assert set(props["command"]["enum"]) == {
+        "create",
+        "str_replace",
+        "insert_at_line",
+        "view",
+    }
+    # Description mentions all four commands so the LLM knows what it can do.
+    assert "create" in tool.description
+    assert "str_replace" in tool.description
+    assert "insert_at_line" in tool.description
+    assert "view" in tool.description
+
+
+def test_tool_appears_in_prompt_when_registered() -> None:
+    """When a StrReplaceEditorTool is in the tool list and is a default core
+    tool, its full description (name + parameters) must appear in the
+    ReActEngine tool-use prompt (tiered injection contract)."""
+    from unittest.mock import MagicMock
+
+    from agentkit.core.react import ReActEngine
+
+    engine = ReActEngine(llm_gateway=MagicMock(), max_steps=1)
+    prompt = engine._build_tool_use_prompt([StrReplaceEditorTool()])
+    # Full description injected (core tool).
+    assert "str_replace_editor" in prompt
+    assert "create" in prompt and "str_replace" in prompt
+    assert "insert_at_line" in prompt and "view" in prompt
+
+
+# ── end-to-end workflow ───────────────────────────────────────────────
+
+
+async def test_create_view_str_replace_insert_workflow(
+    tool: StrReplaceEditorTool, workspace: Path
+) -> None:
+    # 1. create
+    created = await tool.execute(
+        command="create",
+        path="app.py",
+        file_text="def main():\n    pass\n",
+    )
+    assert created["is_error"] is False
+
+    # 2. view (get exact anchors / line numbers)
+    viewed = await tool.execute(command="view", path="app.py")
+    assert viewed["is_error"] is False
+    assert "     1\tdef main():" in viewed["content"]
+
+    # 3. str_replace
+    replaced = await tool.execute(
+        command="str_replace",
+        path="app.py",
+        old_str="    pass",
+        new_str="    return 42",
+    )
+    assert replaced["is_error"] is False
+
+    # 4. insert_at_line (add a docstring at the top)
+    inserted = await tool.execute(
+        command="insert_at_line",
+        path="app.py",
+        insert_line=0,
+        new_str='"""Module doc."""',
+    )
+    assert inserted["is_error"] is False
+
+    final = (workspace / "app.py").read_text()
+    assert final == '"""Module doc."""\ndef main():\n    return 42\n'
+
+
+if __name__ == "__main__":
+    # Allow direct execution for a quick smoke check without pytest.
+    sys.exit(pytest.main([__file__, "-x", "-q"]))
diff --git a/tests/unit/tools/test_tool_search.py b/tests/unit/tools/test_tool_search.py
index 43b65ec..077c651 100644
--- a/tests/unit/tools/test_tool_search.py
+++ b/tests/unit/tools/test_tool_search.py
@@ -382,7 +382,7 @@ class TestReActTieredInjection:
         engine = self._make_engine()
         tools = [
             FakeTool(name="read_file", description="Read a file."),
-            FakeTool(name="write_file", description="Write a file."),
+            FakeTool(name="str_replace_editor", description="Edit a file."),
         ]
         result = engine._maybe_add_tool_search(tools)
         assert len(result) == 2