diff --git a/README.md b/README.md
index e5ac3d81dd..2d3e6fc183 100644
--- a/README.md
+++ b/README.md
@@ -2525,6 +2525,9 @@ Ranged `read_file` calls count lines the same way on every sandbox provider: a l
 Concurrent reads and writes to the same file share a gate across synchronous and asynchronous tool calls. Async callers waiting for that gate do not occupy worker threads needed to finish the current read or write. Cancelling a waiting call leaves the current operation running; a call that already started file inspection still waits for that work to finish before releasing its gate.
 
 The built-in `grep` tool searches either one text file or all matching text files below a directory, so an agent can search an uploaded document directly without first broadening the request to the entire uploads directory.
+On the local sandbox, `grep(literal=True)` also searches long lines in minified text or single-line JSON, within the existing file-size limit. Case-insensitive literal matching avoids repeated-prefix rescans while preserving Unicode case behavior. Regex searches retain their long-line safety guard.
+
+Across sandbox providers, returned previews show only the beginning of each matched line (200 characters by default), so a match later in a long line may not appear in the preview. Use the reported path and line number to inspect the source. The `truncated` flag concerns match-count limits, not preview length.
 E2B's `glob` filter preserves spaces, quotes, and dollar signs in filename patterns, while wildcard matching and root-relative directory scoping remain unchanged.
 E2B `grep` also preserves colons in file and directory names when reporting matching paths and line numbers.
 `glob` and `grep` preserve non-LF separator characters, such as vertical tabs and Unicode line separators, inside returned paths on E2B, BoxLite, Tenki, and OpenSandbox; `grep` also preserves them inside matched text.
diff --git a/backend/packages/harness/deerflow/sandbox/AGENTS.md b/backend/packages/harness/deerflow/sandbox/AGENTS.md
index 7d4266b0e4..b95ffd7353 100644
--- a/backend/packages/harness/deerflow/sandbox/AGENTS.md
+++ b/backend/packages/harness/deerflow/sandbox/AGENTS.md
@@ -113,6 +113,9 @@ Fork lookups reject invalidated clients; cancelled acquires leave borrowers a pe
 
 **Sandbox Tools** (in `packages/harness/deerflow/sandbox/tools.py`):
 
+Case-insensitive literal grep uses KMP + single-character `re.IGNORECASE` for
+linear work and Unicode parity. Regression: `tests/test_local_grep_long_lines.py`.
+
 Remote `list_dir` prints the root, then prunes ignored descendants before the
 500-entry cap. Preserve root literals, host case policy, status markers and the
 subshell. Keep parser filtering as a backstop; test ignored roots, metacharacters,
diff --git a/backend/packages/harness/deerflow/sandbox/search.py b/backend/packages/harness/deerflow/sandbox/search.py
index 0129df04a9..e5f7808b9d 100644
--- a/backend/packages/harness/deerflow/sandbox/search.py
+++ b/backend/packages/harness/deerflow/sandbox/search.py
@@ -3,6 +3,7 @@ import fnmatch
 import os
 import re
 import stat
+from collections.abc import Callable
 from dataclasses import dataclass
 from pathlib import Path, PurePosixPath
 
@@ -196,6 +197,40 @@ def find_glob_matches(root: Path, pattern: str, *, include_dirs: bool = False, m
     return matches, truncated
 
 
+def _make_literal_matcher(pattern: str, *, case_sensitive: bool) -> Callable[[str], bool]:
+    if case_sensitive:
+        return lambda line: pattern in line
+    if not pattern:
+        return lambda _line: True
+
+    # KMP bounds comparisons by pattern + text length. Matching one character
+    # at a time retains re.IGNORECASE's Unicode equivalences without rescanning
+    # a long escaped prefix. Full-string casefold would change cases such as ß.
+    by_character = {character: re.compile(re.escape(character), re.IGNORECASE).fullmatch for character in set(pattern)}
+    character_matches = [by_character[character] for character in pattern]
+    failure = [0] * len(pattern)
+    prefix_length = 0
+    for index in range(1, len(pattern)):
+        while prefix_length and not character_matches[prefix_length](pattern[index]):
+            prefix_length = failure[prefix_length - 1]
+        if character_matches[prefix_length](pattern[index]):
+            prefix_length += 1
+        failure[index] = prefix_length
+
+    def matches_line(line: str) -> bool:
+        matched = 0
+        for character in line:
+            while matched and not character_matches[matched](character):
+                matched = failure[matched - 1]
+            if character_matches[matched](character):
+                matched += 1
+                if matched == len(pattern):
+                    return True
+        return False
+
+    return matches_line
+
+
 def find_grep_matches(
     root: Path,
     pattern: str,
@@ -217,11 +252,14 @@ def find_grep_matches(
     if not root_is_file and not root.is_dir():
         raise NotADirectoryError(root)
 
-    regex_source = re.escape(pattern) if literal else pattern
-    flags = 0 if case_sensitive else re.IGNORECASE
-    regex = re.compile(regex_source, flags)
+    if literal:
+        matches_line = _make_literal_matcher(pattern, case_sensitive=case_sensitive)
+    else:
+        regex = re.compile(pattern, 0 if case_sensitive else re.IGNORECASE)
+        matches_line = regex.search
 
-    # Skip lines longer than this to prevent ReDoS on minified / no-newline files.
+    # Only regex searches need the long-line work guard. Literal matching
+    # avoids repeated-prefix rescans, including case-insensitive searches.
     _max_line_chars = line_summary_length * 10
 
     def handle_search_error(error: OSError) -> None:
@@ -265,9 +303,9 @@ def find_grep_matches(
                 for line_number, line in enumerate(handle, start=1):
                     if line.endswith("\r\n"):
                         line = line[:-2] + "\n"
-                    if len(line) > _max_line_chars:
+                    if not literal and len(line) > _max_line_chars:
                         continue
-                    if regex.search(line):
+                    if matches_line(line):
                         matches.append(
                             GrepMatch(
                                 path=str(file_path),
diff --git a/backend/tests/test_local_grep_long_lines.py b/backend/tests/test_local_grep_long_lines.py
new file mode 100644
index 0000000000..acde6a6e09
--- /dev/null
+++ b/backend/tests/test_local_grep_long_lines.py
@@ -0,0 +1,157 @@
+"""Fixed-string local searches must not inherit the regex long-line guard."""
+
+import os
+import re
+import subprocess
+import sys
+from pathlib import Path
+
+import pytest
+
+from deerflow.sandbox.local.local_sandbox import LocalSandbox, PathMapping
+from deerflow.sandbox.search import GrepMatch, find_grep_matches, truncate_line
+
+
+@pytest.mark.parametrize("single_file", [False, True])
+@pytest.mark.parametrize("case_sensitive", [False, True])
+@pytest.mark.parametrize("ending", ["\n", "\r\n", ""])
+def test_literal_grep_finds_text_beyond_long_line_guard(tmp_path: Path, single_file: bool, case_sensitive: bool, ending: str) -> None:
+    target = tmp_path / "data.json"
+    line = "x" * 2500 + "Needle[0].value"
+    target.write_text("header\n" + line + ending, encoding="utf-8", newline="")
+    root = target if single_file else tmp_path
+    pattern = "Needle[0].value" if case_sensitive else "needle[0].VALUE"
+
+    matches, truncated = find_grep_matches(root, pattern, literal=True, case_sensitive=case_sensitive)
+
+    assert matches == [GrepMatch(path=str(target.resolve()), line_number=2, line=truncate_line(line))]
+    assert len(matches[0].line) == 200
+    assert truncated is False
+
+
+def test_regex_grep_keeps_long_line_guard(tmp_path: Path) -> None:
+    target = tmp_path / "data.txt"
+    target.write_text("x" * 2500 + "needle\nshort needle\n", encoding="utf-8", newline="")
+
+    matches, truncated = find_grep_matches(target, "needle")
+
+    assert matches == [GrepMatch(path=str(target.resolve()), line_number=2, line="short needle")]
+    assert truncated is False
+
+
+def test_literal_grep_keeps_file_size_limit(tmp_path: Path) -> None:
+    target = tmp_path / "data.txt"
+    target.write_text("x" * 2500 + "needle", encoding="utf-8")
+
+    matches, truncated = find_grep_matches(target, "needle", literal=True, max_file_size=2500)
+
+    assert matches == []
+    assert truncated is False
+
+
+def test_literal_grep_keeps_glob_filter(tmp_path: Path) -> None:
+    target = tmp_path / "data.json"
+    target.write_text("x" * 2500 + "needle", encoding="utf-8")
+
+    matches, truncated = find_grep_matches(tmp_path, "needle", literal=True, glob_pattern="*.txt")
+
+    assert matches == []
+    assert truncated is False
+
+
+def test_literal_grep_keeps_result_limit(tmp_path: Path) -> None:
+    target = tmp_path / "data.txt"
+    line = "x" * 2500 + "needle\n"
+    target.write_text(line * 3, encoding="utf-8", newline="")
+
+    matches, truncated = find_grep_matches(target, "needle", literal=True, max_results=2)
+
+    assert [match.line_number for match in matches] == [1, 2]
+    assert all(len(match.line) == 200 for match in matches)
+    assert truncated is True
+
+
+def test_literal_grep_keeps_binary_file_exclusion(tmp_path: Path) -> None:
+    target = tmp_path / "data.bin"
+    target.write_bytes(b"\0" + b"x" * 2500 + b"needle")
+
+    matches, truncated = find_grep_matches(target, "needle", literal=True)
+
+    assert matches == []
+    assert truncated is False
+
+
+@pytest.mark.parametrize("single_file", [False, True])
+def test_local_sandbox_literal_grep_keeps_virtual_paths(tmp_path: Path, single_file: bool) -> None:
+    target = tmp_path / "data.json"
+    line = "x" * 2500 + "needle"
+    target.write_text(line, encoding="utf-8")
+    virtual_root = "/mnt/user-data/workspace"
+    sandbox = LocalSandbox(id="local:test", path_mappings=[PathMapping(container_path=virtual_root, local_path=str(tmp_path))])
+    root = f"{virtual_root}/data.json" if single_file else virtual_root
+
+    matches, truncated = sandbox.grep(root, "needle", literal=True)
+
+    assert matches == [GrepMatch(path=f"{virtual_root}/data.json", line_number=1, line=truncate_line(line))]
+    assert truncated is False
+
+
+@pytest.mark.parametrize("case_sensitive", [False, True])
+@pytest.mark.parametrize(
+    ("pattern", "body"),
+    [
+        ("aaaaab", "a" * 30 + "b"),
+        ("aAaB", "a" * 30 + "AAab"),
+        ("i", "İ"),
+        ("i", "ı"),
+        ("s", "ſ"),
+        ("k", "K"),
+        ("iıiX", "İİİİX"),
+        ("ΣσςX", "ςΣσςX"),
+        ("β", "ϐ"),
+        ("μ", "µ"),
+        ("φ", "ϕ"),
+        ("ss", "ß"),
+        ("ß", "ẞ"),
+        ("i\u0307", "İ"),
+        ("Needle[0].value", "needle[0].VALUE"),
+        ("", "no needle"),
+        ("a" * 60, "a" * 30),
+    ],
+)
+def test_literal_long_line_preserves_regex_case_semantics(tmp_path: Path, pattern: str, body: str, case_sensitive: bool) -> None:
+    target = tmp_path / "unicode.txt"
+    line = "x" * 2500 + body
+    target.write_text(line, encoding="utf-8")
+    expected = re.search(re.escape(pattern), line, 0 if case_sensitive else re.IGNORECASE) is not None
+
+    matches, truncated = find_grep_matches(target, pattern, literal=True, case_sensitive=case_sensitive)
+
+    assert bool(matches) is expected
+    assert truncated is False
+
+
+def test_case_insensitive_literal_repeated_prefix_finishes_within_deadline(tmp_path: Path) -> None:
+    missing = tmp_path / "missing.txt"
+    present = tmp_path / "present.txt"
+    missing.write_text("a" * 900_000, encoding="utf-8")
+    present.write_text("a" * 900_000 + "b", encoding="utf-8")
+    env = os.environ.copy()
+    harness = Path(__file__).resolve().parents[1] / "packages" / "harness"
+    env["PYTHONPATH"] = os.pathsep.join(filter(None, (str(harness), env.get("PYTHONPATH", ""))))
+    script = """
+import sys
+from pathlib import Path
+from deerflow.sandbox.search import find_grep_matches
+pattern = "a" * 10_000 + "b"
+assert find_grep_matches(Path(sys.argv[1]), pattern, literal=True) == ([], False)
+matches, truncated = find_grep_matches(Path(sys.argv[2]), pattern, literal=True)
+assert len(matches) == 1 and matches[0].line_number == 1
+assert truncated is False
+"""
+
+    # A subprocess deadline kills and reaps a regressing regex search rather
+    # than leaving an uninterruptible worker running after this test fails.
+    result = subprocess.run([sys.executable, "-c", script, str(missing), str(present)], env=env, capture_output=True, text=True, timeout=10, check=False)
+
+    assert result.returncode == 0, result.stderr
diff --git a/backend/tests/test_sandbox_search_contract.py b/backend/tests/test_sandbox_search_contract.py
index a46146709b..dcc1783d42 100644
--- a/backend/tests/test_sandbox_search_contract.py
+++ b/backend/tests/test_sandbox_search_contract.py
@@ -261,6 +261,24 @@ def test_grep_literal_matches_regex_metacharacters(provider, tmp_path, pattern,
     assert truncated is False
 
 
+@pytest.mark.parametrize("single_file", [False, True], ids=["directory", "file"])
+@pytest.mark.parametrize("case_sensitive", [False, True], ids=["ignore-case", "match-case"])
+def test_grep_literal_finds_long_lines_with_bounded_prefix_summaries(provider, tmp_path, single_file, case_sensitive):
+    root = tmp_path / "long-lines"
+    root.mkdir()
+    target = root / "data.json"
+    target.write_text("header\n" + "x" * 2500 + "Needle[0].value\nNeedle0Xvalue\n", encoding="utf-8")
+    search_root = target if single_file else root
+    pattern = "Needle[0].value" if case_sensitive else "needle[0].VALUE"
+
+    matches, truncated = provider.sandbox.grep(str(search_root), pattern, literal=True, case_sensitive=case_sensitive)
+
+    # Search the full eligible line even when the fixed-length preview ends
+    # before the needle. `truncated` describes result limits, not preview text.
+    assert [(match.path, match.line_number, match.line) for match in matches] == [(str(target), 2, "x" * 197 + "...")]
+    assert truncated is False
+
+
 def test_grep_glob_filter_is_root_relative(provider, tree):
     matches, truncated = provider.sandbox.grep(str(tree), "needle", glob="nested/*.txt")
     assert [(m.path, m.line_number) for m in matches] == [(str(tree / "nested" / "result.txt"), 1)]