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SandboxApplyPatchTool splits patch lines with str.splitlines(), so a form feed or 
 in a line breaks Add/Update File

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@abhishekacharya200 is already working on this.

Since Oct 11, 2026.

Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
78/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
python
Domain
tooling

Research direction

Start with _parse_apply_patch_input in src/agents/sandbox/capabilities/tools/apply_patch_tool.py and compare its line splitting with the Lark grammar, which ends lines at LF. Then inspect agents/apply_diff.py, which also splits on "\n". Verify that Add File content preserves the listed characters and that the Update File repro changes const n = 1 to const n = 2.

Written by the indexing model from the issue text.

Description

Please read this first
  • Have you read the docs? Agents SDK docs
  • Have you searched for related issues? Others may have faced similar issues.
Describe the bug

SandboxApplyPatchTool parses the freeform patch with raw_input.splitlines() (_parse_apply_patch_input in src/agents/sandbox/capabilities/tools/apply_patch_tool.py). str.splitlines() also breaks at \v, \f, \x1c-\x1e, \x85, \u2028, \u2029 and a bare \r. The tool's own Lark grammar only ends a line at LF: add_line: "+" /(.*)/ LF and change_line: ("+" | "-" | " ") /(.*)/ LF. agents/apply_diff.py also splits on "\n".

So a patch the grammar accepts is rejected when any line contains one of those characters. For example, a form feed between sections of a Python or C file, or a \u2028 inside a JS string or pasted text:

  • *** Add File: fails with Invalid Add File line: <rest of the line>, and the file is not created.
  • *** Update File: turns the rest of the line into an unprefixed diff line, and apply_diff fails with Invalid Line: <rest of the line>. This happens even when the character is only in a context line.

Splitting the envelope on "\n", as the grammar and apply_diff do, makes both cases work. In the repro below, the same hunk applies cleanly once it is split that way.

Debug information
  • Agents SDK version: main at 125efa0 (0.23.1)
  • Python version: 3.12.3
  • Operating system: Linux
  • Model and model provider: n/a (the parser is called directly)
  • Does the issue reproduce with the latest Agents SDK release? Yes. v0.23.1 has the same splitlines() call and grammar; I ran the repro on main (125efa0).
  • Does the issue occur consistently or intermittently? Consistently
'+page one\x0cpage two' -> Invalid Add File line: page two
'+const s = "a\u2028b";' -> Invalid Add File line: b";
'+x\x85y' -> Invalid Add File line: y
parsed diff: '@@\n const s = "a\nb";\n-const n = 1;\n+const n = 2;\n'
apply_diff -> Invalid Line: b";
'const s = "a\u2028b";\nconst n = 2;\n'
Repro steps
from agents.apply_diff import apply_diff
from agents.sandbox.capabilities.tools import SandboxApplyPatchTool
from agents.testing import scripted_sandbox_session

tool = SandboxApplyPatchTool(session=scripted_sandbox_session())

# 1. Add File: the grammar's add_line is "+" /(.*)/ LF, so these lines are valid input.
for line in ["+page one\fpage two", '+const s = "a\u2028b";', "+x\x85y"]:
    patch = f"*** Begin Patch\n*** Add File: notes.txt\n{line}\n*** End Patch\n"
    try:
        tool.parse_custom_input(patch)
        print(ascii(line), "-> parsed")
    except ValueError as e:
        print(ascii(line), "->", e)

# 2. Update File whose context line contains U+2028.
original = 'const s = "a\u2028b";\nconst n = 1;\n'
patch = (
    "*** Begin Patch\n*** Update File: app.js\n@@\n"
    ' const s = "a\u2028b";\n-const n = 1;\n+const n = 2;\n*** End Patch\n'
)
(op,) = tool.parse_custom_input(patch)
print("parsed diff:", ascii(op.diff))
try:
    apply_diff(original, op.diff)
except ValueError as e:
    print("apply_diff ->", e)

# The same hunk, split on "\n" only, applies cleanly.
print(ascii(apply_diff(original, '@@\n const s = "a\u2028b";\n-const n = 1;\n+const n = 2;\n')))
Expected behavior

The three Add File patches create files whose content keeps the form feed, \u2028 and \x85. The Update File patch changes const n = 1; to const n = 2;, which is what apply_diff produces when the hunk is split on "\n" only.

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