feat(file_tools): post-write delta lint on write_file + patch, add JSON/YAML/TOML/Python in-process linters (#20191)

Closes the gap where write_file skipped the post-edit syntax check that
patch already ran, so silent file corruption (bad quote escaping,
truncated writes, etc.) would persist on disk until a later read.

## Changes

tools/file_operations.py:
- Add in-process linters for .py, .json, .yaml, .toml (LINTERS_INPROC).
  Python uses ast.parse, JSON/YAML/TOML use stdlib/PyYAML parsers.
  Zero subprocess overhead; preferred over shell linters when both apply.
- _check_lint() now accepts optional content and routes to in-process
  linter first. Shell linter (py_compile, node --check, tsc, go vet,
  rustfmt) remains the fallback for languages without an in-process
  equivalent.
- New _check_lint_delta() implements the post-first/pre-lazy pattern
  borrowed from Cline and OpenCode: lint post-write state first; only
  if errors are found AND pre-content was captured does it lint the
  pre-state and diff. If the pre-existing file had the SAME errors the
  edit didn't introduce anything new, so the file is reported as 'still
  broken, pre-existing' with success=False but a message explaining the
  errors were pre-existing. If the edit introduced genuinely new errors,
  those are surfaced and pre-existing ones are filtered out.
- WriteResult gains a lint field.
- write_file() captures pre-content for in-process-lintable extensions
  and calls _check_lint_delta after a successful write.
- patch_replace() switches from _check_lint to _check_lint_delta,
  reusing the pre-edit content it already has in scope.

tools/file_tools.py:
- Update write_file schema description to mention the post-write lint.

tests/tools/test_file_operations_edge_cases.py:
- Update existing brace-path tests to use .js (shell linter) now that
  .py is in-process.
- Add TestCheckLintInproc (9 tests) covering Python/JSON/YAML/TOML
  in-process linters.
- Add TestCheckLintDelta (5 tests) covering the post-first/pre-lazy
  short-circuit, new-file path, and the single-error-parser caveat.

## Performance

In-process linters are microseconds per call (ast.parse, json.loads).
The hot path (clean write) runs exactly one lint — matches main's cost
for patch. Pre-state capture is skipped when the file has no applicable
linter. Measured 4.89ms/write average over 100 .py writes including lint.

## Inspiration

- Cline's DiffViewProvider.getNewDiagnosticProblems() — filters pre-write
  diagnostics from post-write diagnostics (src/integrations/editor/DiffViewProvider.ts).
- OpenCode's WriteTool — runs lsp.diagnostics() after write and appends
  errors to tool output (packages/opencode/src/tool/write.ts).
- Claude Code's DiagnosticTrackingService — captures baseline via
  beforeFileEdited() and returns new-diagnostics-only from
  getNewDiagnostics() (src/services/diagnosticTracking.ts).

## Validation

- tests/tools/test_file_operations.py + test_file_operations_edge_cases.py
  + test_file_tools.py + test_file_tools_live.py + test_file_write_safety.py
  + test_write_deny.py + test_patch_parser.py + test_file_ops_cwd_tracking.py:
  228 passed locally.
- Live E2E reproduction of the tips.py corruption incident: broken
  content written; lint field surfaces 'SyntaxError: invalid syntax.
  Perhaps you forgot a comma? (line 6, column 5)' — the exact error
  that would have self-corrected the bug on the next turn.

tests/tools/test_file_operations_edge_cases.py

@@ -82,7 +82,11 @@ class TestIsLikelyBinary:
 
 
 class TestCheckLintBracePaths:
-    """Verify _check_lint handles file paths with curly braces safely."""
+    """Verify _check_lint handles file paths with curly braces safely.
+
+    Uses ``.js`` to exercise the shell-linter path since ``.py`` now goes
+    through the in-process ast.parse linter (see TestCheckLintInproc).
+    """
 
     @pytest.fixture()
     def ops(self):
@@ -95,12 +99,12 @@ class TestCheckLintBracePaths:
         with patch.object(ops, "_has_command", return_value=True), \
              patch.object(ops, "_exec") as mock_exec:
             mock_exec.return_value = MagicMock(exit_code=0, stdout="")
-            result = ops._check_lint("/tmp/test_file.py")
+            result = ops._check_lint("/tmp/test_file.js")
 
         assert result.success is True
         # Verify the command was built correctly
         cmd_arg = mock_exec.call_args[0][0]
-        assert "'/tmp/test_file.py'" in cmd_arg
+        assert "'/tmp/test_file.js'" in cmd_arg
 
     def test_path_with_curly_braces(self, ops):
         """Path containing ``{`` and ``}`` must not raise KeyError/ValueError."""
@@ -108,7 +112,7 @@ class TestCheckLintBracePaths:
              patch.object(ops, "_exec") as mock_exec:
             mock_exec.return_value = MagicMock(exit_code=0, stdout="")
             # This would raise KeyError with .format() but works with .replace()
-            result = ops._check_lint("/tmp/{test}_file.py")
+            result = ops._check_lint("/tmp/{test}_file.js")
 
         assert result.success is True
         cmd_arg = mock_exec.call_args[0][0]
@@ -119,7 +123,7 @@ class TestCheckLintBracePaths:
         with patch.object(ops, "_has_command", return_value=True), \
              patch.object(ops, "_exec") as mock_exec:
             mock_exec.return_value = MagicMock(exit_code=0, stdout="")
-            result = ops._check_lint("/tmp/{{var}}.py")
+            result = ops._check_lint("/tmp/{{var}}.js")
 
         assert result.success is True
 
@@ -131,7 +135,7 @@ class TestCheckLintBracePaths:
     def test_missing_linter_skipped(self, ops):
         """When the linter binary is not installed, skip gracefully."""
         with patch.object(ops, "_has_command", return_value=False):
-            result = ops._check_lint("/tmp/test.py")
+            result = ops._check_lint("/tmp/test.js")
         assert result.skipped is True
 
     def test_lint_failure_returns_output(self, ops):
@@ -142,12 +146,122 @@ class TestCheckLintBracePaths:
                 exit_code=1,
                 stdout="SyntaxError: invalid syntax",
             )
-            result = ops._check_lint("/tmp/bad.py")
+            result = ops._check_lint("/tmp/bad.js")
 
         assert result.success is False
         assert "SyntaxError" in result.output
 
 
+class TestCheckLintInproc:
+    """Verify in-process linters (.py via ast.parse, .json, .yaml, .toml).
+
+    These bypass the shell linter table entirely and parse content
+    directly in Python — no subprocess, no toolchain dependency.
+    """
+
+    @pytest.fixture()
+    def ops(self):
+        obj = ShellFileOperations.__new__(ShellFileOperations)
+        obj._command_cache = {}
+        return obj
+
+    def test_python_inproc_clean(self, ops):
+        """Valid Python content passes in-process ast.parse."""
+        result = ops._check_lint("/tmp/ok.py", content="x = 1\n")
+        assert result.success is True
+        assert not result.skipped
+        assert result.output == ""
+
+    def test_python_inproc_syntax_error(self, ops):
+        """Invalid Python content fails with SyntaxError + line info."""
+        result = ops._check_lint("/tmp/bad.py", content="def foo(:\n    pass\n")
+        assert result.success is False
+        assert "SyntaxError" in result.output
+        assert "line" in result.output.lower()
+
+    def test_python_inproc_content_explicit(self, ops):
+        """When content is passed explicitly, the file is not re-read."""
+        with patch.object(ops, "_exec") as mock_exec:
+            result = ops._check_lint("/tmp/explicit.py", content="y = 2\n")
+            # _exec must not have been called — content was supplied
+            mock_exec.assert_not_called()
+        assert result.success is True
+
+    def test_json_inproc_clean(self, ops):
+        result = ops._check_lint("/tmp/a.json", content='{"a": 1}')
+        assert result.success is True
+
+    def test_json_inproc_error(self, ops):
+        result = ops._check_lint("/tmp/b.json", content='{"a": 1')
+        assert result.success is False
+        assert "JSONDecodeError" in result.output
+
+    def test_yaml_inproc_clean(self, ops):
+        result = ops._check_lint("/tmp/a.yaml", content="a: 1\nb: 2\n")
+        assert result.success is True
+
+    def test_yaml_inproc_error(self, ops):
+        result = ops._check_lint("/tmp/b.yaml", content='key: "unclosed\n')
+        assert result.success is False
+        assert "YAMLError" in result.output
+
+    def test_toml_inproc_clean(self, ops):
+        result = ops._check_lint("/tmp/a.toml", content='[section]\nk = "v"\n')
+        assert result.success is True
+
+    def test_toml_inproc_error(self, ops):
+        result = ops._check_lint("/tmp/b.toml", content='[section\nk = "v"')
+        assert result.success is False
+        assert "TOMLDecodeError" in result.output
+
+
+class TestCheckLintDelta:
+    """Verify _check_lint_delta() filters pre-existing errors from post-edit output."""
+
+    @pytest.fixture()
+    def ops(self):
+        obj = ShellFileOperations.__new__(ShellFileOperations)
+        obj._command_cache = {}
+        return obj
+
+    def test_clean_post_no_pre_lint(self, ops):
+        """Hot path: post-write is clean, pre-lint should be skipped entirely."""
+        with patch.object(ops, "_check_lint", wraps=ops._check_lint) as wrapped:
+            r = ops._check_lint_delta("/tmp/a.py", pre_content="x = 0\n", post_content="x = 1\n")
+            # Post-lint called exactly once (clean), pre-lint never called.
+            assert wrapped.call_count == 1
+        assert r.success is True
+
+    def test_new_file_reports_all_errors(self, ops):
+        """No pre-content means no delta refinement — all post errors surface."""
+        r = ops._check_lint_delta("/tmp/new.py", pre_content=None, post_content="def x(:\n")
+        assert r.success is False
+        assert "SyntaxError" in r.output
+
+    def test_broken_file_becomes_good(self, ops):
+        """Post-clean short-circuits without any delta refinement."""
+        r = ops._check_lint_delta("/tmp/fix.py", pre_content="def x(:\n", post_content="def x():\n    pass\n")
+        assert r.success is True
+
+    def test_introduces_new_error_filters_pre(self, ops):
+        """Delta filter drops pre-existing errors, surfaces only new ones."""
+        pre = 'def a(:\n    pass\n'  # line 1 broken
+        post = 'def a():\n    pass\n\ndef b(:\n    pass\n'  # line 1 fixed, line 4 broken
+        r = ops._check_lint_delta("/tmp/d.py", pre_content=pre, post_content=post)
+        assert r.success is False
+        assert "New lint errors" in r.output or "line 4" in r.output
+
+    def test_pre_existing_remains_flagged_but_not_new(self, ops):
+        """Single-error parsers (ast) may miss that post is OK — be cautious."""
+        # Pre has line-1 error, post keeps it (and doesn't add anything new)
+        pre = 'def a(:\n    pass\n'
+        post = 'def a(:\n    pass\n\nprint(42)\n'  # still line 1 broken
+        r = ops._check_lint_delta("/tmp/d.py", pre_content=pre, post_content=post)
+        # File is still broken — don't lie and claim success — but flag it as pre-existing
+        assert r.success is False
+        assert "pre-existing" in (r.message or "").lower()
+
+
 # =========================================================================
 # Pagination bounds
 # =========================================================================

tools/file_operations.py

@@ -119,9 +119,10 @@ class WriteResult:
     """Result from writing a file."""
     bytes_written: int = 0
     dirs_created: bool = False
+    lint: Optional[Dict[str, Any]] = None
     error: Optional[str] = None
     warning: Optional[str] = None
-    
+
     def to_dict(self) -> dict:
         return {k: v for k, v in self.__dict__.items() if v is not None}
 
@@ -202,10 +203,10 @@ class LintResult:
     def to_dict(self) -> dict:
         if self.skipped:
             return {"status": "skipped", "message": self.message}
-        return {
-            "status": "ok" if self.success else "error",
-            "output": self.output
-        }
+        result = {"status": "ok" if self.success else "error", "output": self.output}
+        if self.message:
+            result["message"] = self.message
+        return result
 
 
 @dataclass
@@ -303,7 +304,9 @@ class FileOperations(ABC):
 # Image extensions (subset of binary that we can return as base64)
 IMAGE_EXTENSIONS = {'.png', '.jpg', '.jpeg', '.gif', '.webp', '.bmp', '.ico'}
 
-# Linters by file extension
+# Shell-based linters by file extension.  Invoked via _exec() with the
+# filesystem path.  Cover languages where a compile/type check needs an
+# external toolchain (py_compile, node, tsc, go vet, rustfmt).
 LINTERS = {
     '.py': 'python -m py_compile {file} 2>&1',
     '.js': 'node --check {file} 2>&1',
@@ -312,6 +315,86 @@ LINTERS = {
     '.rs': 'rustfmt --check {file} 2>&1',
 }
 
+
+def _lint_json_inproc(content: str) -> tuple[bool, str]:
+    """In-process JSON syntax check.  Returns (ok, error_message)."""
+    import json as _json
+    try:
+        _json.loads(content)
+        return True, ""
+    except _json.JSONDecodeError as e:
+        return False, f"JSONDecodeError: {e.msg} (line {e.lineno}, column {e.colno})"
+    except Exception as e:  # noqa: BLE001 — any parse failure is a lint failure
+        return False, f"{type(e).__name__}: {e}"
+
+
+def _lint_yaml_inproc(content: str) -> tuple[bool, str]:
+    """In-process YAML syntax check.  Returns (ok, error_message).
+
+    Skipped gracefully if PyYAML isn't installed — YAML parsing is optional.
+    """
+    try:
+        import yaml as _yaml
+    except ImportError:
+        # PyYAML not available — skip silently, caller treats as no linter.
+        return True, "__SKIP__"
+    try:
+        _yaml.safe_load(content)
+        return True, ""
+    except _yaml.YAMLError as e:
+        return False, f"YAMLError: {e}"
+    except Exception as e:  # noqa: BLE001
+        return False, f"{type(e).__name__}: {e}"
+
+
+def _lint_toml_inproc(content: str) -> tuple[bool, str]:
+    """In-process TOML syntax check (stdlib tomllib, Python 3.11+)."""
+    try:
+        import tomllib as _toml
+    except ImportError:
+        # Pre-3.11 fallback via tomli, if installed.
+        try:
+            import tomli as _toml  # type: ignore[no-redef]
+        except ImportError:
+            return True, "__SKIP__"
+    try:
+        _toml.loads(content)
+        return True, ""
+    except Exception as e:  # tomllib raises TOMLDecodeError, a ValueError subclass
+        return False, f"{type(e).__name__}: {e}"
+
+
+def _lint_python_inproc(content: str) -> tuple[bool, str]:
+    """In-process Python syntax check via ast.parse.
+
+    Catches SyntaxError, IndentationError, and everything else the
+    ast module rejects — matching py_compile's scope but with no
+    subprocess overhead and no dependency on a ``python`` in PATH.
+    """
+    import ast as _ast
+    try:
+        _ast.parse(content)
+        return True, ""
+    except SyntaxError as e:
+        loc = f" (line {e.lineno}, column {e.offset})" if e.lineno else ""
+        return False, f"{type(e).__name__}: {e.msg}{loc}"
+    except Exception as e:  # noqa: BLE001
+        return False, f"{type(e).__name__}: {e}"
+
+
+# In-process linters by file extension.  Preferred over shell linters when
+# present — no subprocess overhead, microseconds per call.  Each callable
+# takes file content (str) and returns (ok: bool, error: str).  An error
+# string of ``"__SKIP__"`` signals the linter isn't available (missing
+# dependency) and should be treated as "no linter".
+LINTERS_INPROC = {
+    '.py': _lint_python_inproc,
+    '.json': _lint_json_inproc,
+    '.yaml': _lint_yaml_inproc,
+    '.yml': _lint_yaml_inproc,
+    '.toml': _lint_toml_inproc,
+}
+
 # Max limits for read operations
 MAX_LINES = 2000
 MAX_LINE_LENGTH = 2000
@@ -745,12 +828,19 @@ class ShellFileOperations(FileOperations):
         files. The content never appears in the shell command string —
         only the file path does.
 
+        After the write, runs a post-first / pre-lazy lint check via
+        ``_check_lint_delta()``.  If the new content is clean, the lint
+        call is O(one parse).  If the new content has errors, the pre-write
+        content is linted too and only errors newly introduced by this
+        write are surfaced — pre-existing problems are filtered out so
+        the agent isn't distracted chasing them.
+
         Args:
             path: File path to write
             content: Content to write
 
         Returns:
-            WriteResult with bytes written or error
+            WriteResult with bytes written, lint summary, or error.
         """
         # Expand ~ and other shell paths
         path = self._expand_path(path)
@@ -759,36 +849,58 @@ class ShellFileOperations(FileOperations):
         if _is_write_denied(path):
             return WriteResult(error=f"Write denied: '{path}' is a protected system/credential file.")
 
+        # Capture pre-write content for lint-delta computation.  Only do this
+        # when an in-process OR shell linter exists for this extension — no
+        # point paying for the read otherwise.  For in-process linters we
+        # pass the content directly; for shell linters the pre-state isn't
+        # useful (we'd have to re-write-read to lint the old version, which
+        # defeats the purpose), so we skip the capture and accept the naive
+        # "all errors" report.
+        ext = os.path.splitext(path)[1].lower()
+        pre_content: Optional[str] = None
+        if ext in LINTERS_INPROC:
+            # Best-effort read; failure (file missing, permission) leaves
+            # pre_content as None which makes the delta step degrade
+            # gracefully to "report all errors".
+            read_cmd = f"cat {self._escape_shell_arg(path)} 2>/dev/null"
+            read_result = self._exec(read_cmd)
+            if read_result.exit_code == 0 and read_result.stdout:
+                pre_content = read_result.stdout
+
         # Create parent directories
         parent = os.path.dirname(path)
         dirs_created = False
-        
+
         if parent:
             mkdir_cmd = f"mkdir -p {self._escape_shell_arg(parent)}"
             mkdir_result = self._exec(mkdir_cmd)
             if mkdir_result.exit_code == 0:
                 dirs_created = True
-        
+
         # Write via stdin pipe — content bypasses shell arg parsing entirely,
         # so there's no ARG_MAX limit regardless of file size.
         write_cmd = f"cat > {self._escape_shell_arg(path)}"
         write_result = self._exec(write_cmd, stdin_data=content)
-        
+
         if write_result.exit_code != 0:
             return WriteResult(error=f"Failed to write file: {write_result.stdout}")
-        
+
         # Get bytes written (wc -c is POSIX, works on Linux + macOS)
         stat_cmd = f"wc -c < {self._escape_shell_arg(path)} 2>/dev/null"
         stat_result = self._exec(stat_cmd)
-        
+
         try:
             bytes_written = int(stat_result.stdout.strip())
         except ValueError:
             bytes_written = len(content.encode('utf-8'))
-        
+
+        # Post-write lint with delta refinement.
+        lint_result = self._check_lint_delta(path, pre_content=pre_content, post_content=content)
+
         return WriteResult(
             bytes_written=bytes_written,
-            dirs_created=dirs_created
+            dirs_created=dirs_created,
+            lint=lint_result.to_dict() if lint_result else None,
         )
     
     # =========================================================================
@@ -864,10 +976,12 @@ class ShellFileOperations(FileOperations):
 
         # Generate diff
         diff = self._unified_diff(content, new_content, path)
-        
-        # Auto-lint
-        lint_result = self._check_lint(path)
-        
+
+        # Auto-lint with delta refinement: only surface errors introduced
+        # by this patch, filtering out pre-existing lint failures so the
+        # agent isn't distracted by problems that were already there.
+        lint_result = self._check_lint_delta(path, pre_content=content, post_content=new_content)
+
         return PatchResult(
             success=True,
             diff=diff,
@@ -905,37 +1019,143 @@ class ShellFileOperations(FileOperations):
         result = apply_v4a_operations(operations, self)
         return result
     
-    def _check_lint(self, path: str) -> LintResult:
+    def _check_lint(self, path: str, content: Optional[str] = None) -> LintResult:
         """
         Run syntax check on a file after editing.
-        
+
+        Prefers the in-process linter for structured formats (JSON, YAML,
+        TOML) when possible — those parse via the Python stdlib in
+        microseconds and don't require a subprocess.  Falls back to the
+        shell linter table for compiled/type-checked languages
+        (py_compile, node --check, tsc, go vet, rustfmt).
+
         Args:
-            path: File path to lint
-        
+            path: File path (used to select the linter + for shell invocation).
+            content: Optional file content.  If provided AND an in-process
+                     linter matches the extension, we lint the content
+                     directly without re-reading the file from disk.  Ignored
+                     for shell linters.
+
         Returns:
-            LintResult with status and any errors
+            LintResult with status and any errors.
         """
         ext = os.path.splitext(path)[1].lower()
-        
+
+        # Prefer in-process linter when available.
+        inproc = LINTERS_INPROC.get(ext)
+        if inproc is not None:
+            # Need content — either passed in or read from disk.
+            if content is None:
+                read_cmd = f"cat {self._escape_shell_arg(path)} 2>/dev/null"
+                read_result = self._exec(read_cmd)
+                if read_result.exit_code != 0:
+                    return LintResult(skipped=True, message=f"Failed to read {path} for lint")
+                content = read_result.stdout
+            ok, err = inproc(content)
+            if err == "__SKIP__":
+                return LintResult(skipped=True, message=f"No linter available for {ext} (missing dependency)")
+            return LintResult(success=ok, output="" if ok else err)
+
+        # Fall back to shell linter.
         if ext not in LINTERS:
             return LintResult(skipped=True, message=f"No linter for {ext} files")
-        
-        # Check if linter command is available
+
         linter_cmd = LINTERS[ext]
         # Extract the base command (first word)
         base_cmd = linter_cmd.split()[0]
-        
+
         if not self._has_command(base_cmd):
             return LintResult(skipped=True, message=f"{base_cmd} not available")
-        
+
         # Run linter
         cmd = linter_cmd.replace("{file}", self._escape_shell_arg(path))
         result = self._exec(cmd, timeout=30)
-        
+
         return LintResult(
             success=result.exit_code == 0,
             output=result.stdout.strip() if result.stdout.strip() else ""
         )
+
+    def _check_lint_delta(self, path: str, pre_content: Optional[str],
+                          post_content: Optional[str] = None) -> LintResult:
+        """
+        Run post-write lint with pre-write baseline comparison.
+
+        Strategy (post-first, pre-lazy):
+        1. Lint the post-write state.  If clean → return clean immediately.
+           This is the hot path and matches _check_lint() in cost.
+        2. If post-lint found errors AND we have pre-write content, lint
+           that too.  If the pre-write file was already broken, return only
+           the *new* errors introduced by this edit — errors that existed
+           before aren't the agent's problem to chase right now.
+        3. If pre_content is None (new file or unavailable), skip the delta
+           step and return all post-write errors.
+
+        This mirrors Cline's and OpenCode's post-edit LSP pattern: surface
+        only the errors this specific edit introduced, so the agent doesn't
+        get distracted by pre-existing problems.
+
+        Args:
+            path: File path (for linter selection).
+            pre_content: File content BEFORE the write.  Pass None for new
+                         files or when the pre-state isn't available — the
+                         delta refinement is skipped and all post errors
+                         are returned.
+            post_content: File content AFTER the write.  Optional; if None,
+                          the shell linter reads from disk (same as
+                          _check_lint).
+
+        Returns:
+            LintResult.  ``output`` contains either the full post-lint
+            errors (no pre-state) or just the new-error lines (delta
+            refinement applied).
+        """
+        post = self._check_lint(path, content=post_content)
+
+        # Hot path: clean post-write, no pre-lint needed.
+        if post.success or post.skipped:
+            return post
+
+        # Post-write has errors.  If we have pre-content, run the delta
+        # refinement to filter out pre-existing errors.
+        if pre_content is None:
+            return post
+
+        pre = self._check_lint(path, content=pre_content)
+        if pre.success or pre.skipped or not pre.output:
+            # Pre-write was clean (or we couldn't lint it) — post errors
+            # are all new.  Return the full post output.
+            return post
+
+        # Both pre- and post-write had errors.  Compute the set-difference
+        # on non-empty stripped lines.  Caveat: single-error parsers
+        # (ast.parse, json.loads) stop at the first error and don't report
+        # later ones — if the pre-existing error blocks parsing before
+        # reaching the edit region, we can't prove the edit is clean.  So
+        # if every post error also appeared pre-edit, we report the file
+        # as still broken but annotate that this edit introduced nothing
+        # new on top — the agent knows it's inherited state, not fresh
+        # damage, without silently dropping the error.
+        pre_lines = {ln.strip() for ln in pre.output.splitlines() if ln.strip()}
+        post_lines = [ln for ln in post.output.splitlines() if ln.strip() and ln.strip() not in pre_lines]
+
+        if not post_lines:
+            # Every error in post was also in pre — this edit didn't make
+            # anything obviously worse, but the file remains broken and
+            # the agent should know.
+            return LintResult(
+                success=False,
+                output=post.output,
+                message="Pre-existing lint errors — this edit didn't introduce new ones but the file is still broken.",
+            )
+
+        return LintResult(
+            success=False,
+            output=(
+                "New lint errors introduced by this edit "
+                "(pre-existing errors filtered out):\n" + "\n".join(post_lines)
+            )
+        )
     
     # =========================================================================
     # SEARCH Implementation

tools/file_tools.py

@@ -1042,7 +1042,7 @@ READ_FILE_SCHEMA = {
 
 WRITE_FILE_SCHEMA = {
     "name": "write_file",
-    "description": "Write content to a file, completely replacing existing content. Use this instead of echo/cat heredoc in terminal. Creates parent directories automatically. OVERWRITES the entire file — use 'patch' for targeted edits.",
+    "description": "Write content to a file, completely replacing existing content. Use this instead of echo/cat heredoc in terminal. Creates parent directories automatically. OVERWRITES the entire file — use 'patch' for targeted edits. Auto-runs syntax checks on .py/.json/.yaml/.toml and other linted languages; only NEW errors introduced by this write are surfaced (pre-existing errors are filtered out).",
     "parameters": {
         "type": "object",
         "properties": {