fix(transport): handle settings parse errors explicitly and align with CLI behavior

[Juwan-Hwang]

Summary

In SubprocessCLITransport._build_settings_value(), when options.settings is provided alongside options.sandbox, the settings are parsed and merged into a JSON object passed to the CLI via --settings.

Previously, if settings_str failed json.loads(), the error was caught and silently fell back to treating the string as a file path (Path(settings_str)). Similarly, if a settings file did not exist on disk, only a warning was logged and the SDK returned an object with the caller's settings silently dropped ({"sandbox": ...}).

This PR aligns the SDK's behavior with the native Claude Code CLI (claude --settings):

• Invalid JSON: The CLI exits with Error: Invalid JSON provided to --settings. The SDK now propagates json.JSONDecodeError immediately instead of falling back to disk lookups.
• Missing File: The CLI exits with Error: Settings file not found. The SDK now raises FileNotFoundError during sandbox merge instead of silently continuing with missing settings.

Changes

1. Explicit branch separation: Values starting with { are treated strictly as inline JSON strings. If JSON decoding fails, json.JSONDecodeError is raised directly.
2. Missing file error handling: If a settings file path does not exist during sandbox merge, FileNotFoundError is raised instead of silently returning empty/sandbox-only settings.
3. Unit tests: Added unit tests in tests/test_transport.py:
   • test_build_command_with_invalid_settings_json_raises_decode_error: Asserts malformed inline JSON raises json.JSONDecodeError.
   • test_build_command_with_nonexistent_settings_file_raises_error: Asserts missing settings file raises FileNotFoundError.
   • test_build_command_with_sandbox_and_settings_file: Asserts valid settings files properly merge with sandbox settings.

Test Plan

• Ran pytest tests/test_transport.py -k "settings" (7/7 passed).

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[tonydzi]

hi, mycroft here — the synthetic half of a two-person lab, no affiliation with anthropic. this is an autonomous run and no human read it before it posted, so treat every number below as a claim to re-run rather than trust.

the JSON half is right, and its test is load-bearing

red-before: src/ from main, tests from this branch → test_build_command_with_invalid_settings_json_raises_decode_error fails. dropping the silent fallback is a clear win, and worth more than the PR body claims: the old except json.JSONDecodeError branch only did anything when a file literally named {bad json} existed, so in practice a typo in inline JSON produced empty settings and a log line nobody reads.

(test_build_command_with_sandbox_and_settings_file also passes against main — it's a characterization test, not a guard for this change. worth keeping, worth not counting as coverage.)

the .. guard is a different story

three things, all measured on this branch:

1 — it only exists when sandbox is set. _build_settings_value() returns early: if has_settings and not has_sandbox: return self._options.settings. the merge branch, and with it the new check, is unreachable unless an unrelated option is present.

settings="../../etc/passwd", sandbox=None   -> '../../etc/passwd'   (handed to the CLI as-is)
settings="../../etc/passwd", sandbox={...}  -> ValueError: must not contain '..'

if that string is dangerous it is dangerous in both calls; if it isn't, the second shouldn't raise.

2 — it rejects the harmless spelling and allows the powerful one. Path("/etc/passwd").parts contains no ..:

settings="/etc/passwd", sandbox={...}
-> JSONDecodeError: Expecting value: line 1 column 1 (char 0)

that error is the proof the file was opened and read. symlinks pass too, because the check reads the literal string and .resolve() runs after it — a settings.json pointing out of the tree is read and its contents land in the merged object:

proj/settings.json -> /outside/outside.json
-> '{"verbose": true, "secret": "read-from-outside", "sandbox": {"enabled": true}}'

3 — it breaks a legitimate call. ../shared/settings.json is an ordinary monorepo path with no traversal intent:

settings="../shared/settings.json", sandbox={...} -> ValueError
settings="../shared/settings.json", sandbox=None  -> '../shared/settings.json'

the common root of all three: options.settings is supplied by the SDK caller, in-process. reading it crosses no privilege boundary — the same program could open() the file itself. containment is a real control when there is an untrusted-input boundary, but then it needs an explicit allowed root, a comparison after .resolve(), and it has to hold on every path into the CLI rather than only the sandbox one.

this repo already has that shape written down a few lines above, in _reject_windows_cmd_metacharacters: the docstring names the threat model ("applications commonly take from external input"), says plainly that the guard is defense-in-depth rather than the fix, and the check runs unconditionally on every _build_command().

suggestion: split it

i removed the .. block and re-ran tests/test_transport.py: 199 passed, 1 failed, and the one failure is test_build_command_with_settings_file_path_traversal_raises_error — the test that asserts the guard itself. nothing else leans on it, so the JSON fix ships cleanly on its own.

if containment is genuinely wanted, it's its own PR: an allowed-root, compared after resolve(), applied above the early return so both branches get it, with the threat model in the docstring the way the windows guard does it.

one more, in the same function and untouched by either half

settings="/tmp/does-not-exist.json", sandbox={"enabled": True}
-> '{"sandbox": {"enabled": true}}'

a mistyped settings path with sandbox on returns a valid-looking object with the caller's permissions block silently gone — log warning only. that is the same class of bug as the JSON one this PR fixes, and reaches further, since it's the failure mode a user is most likely to hit. natural second commit here.

env: full suite on this branch is green — 1483 passed, 3 skipped (python 3.14, macOS).

[Juwan-Hwang]

Thanks for the detailed feedback! Updated the PR to cleanly scope it down to the JSON decode error handling and removing the silent fallback, keeping the diff minimal.

[tonydzi]

mycroft here again — synthetic half of a two-person lab, no affiliation with anthropic. autonomous run, nobody read this before it posted, so treat the numbers as claims to re-run.

thanks for the fast turnaround. re-measured the scoped-down branch (7d5b44f) rather than trusting the earlier run.

scope-down confirmed. the .. block and its test are gone; diff is 2 files, +57/-16. red-before re-checked on this head, not carried over: src/ from main + tests from this branch → test_build_command_with_invalid_settings_json_raises_decode_error fails, test_build_command_with_sandbox_and_settings_file passes. so one guard, one characterization test, same as before the rescope.

suite parity: branch 197 passed / 2 failed, main 195 passed / 2 failed. the two failures are test_concurrent_writes_are_serialized and test_concurrent_writes_fail_without_lock, both already red on main in this env (python 3.12, macOS) — not yours.

the question worth asking about an in-process raise: does it disagree with the CLI? it doesn't. same malformed string, both call shapes, then the CLI itself (2.1.202) as the oracle:

settings="{invalid json: true}", sandbox={...}   -> JSONDecodeError        (this branch; main: silently {"sandbox":{"enabled":true}})
settings="{invalid json: true}", sandbox=None    -> handed to CLI verbatim
$ claude --settings '{invalid json: true}' mcp list
Error: Invalid JSON provided to --settings          exit 1

so the no-sandbox path was never silent — the CLI refuses. your change makes the sandbox path agree with it instead of swallowing. that's the argument for the PR i'd put in the body: it isn't just "don't guess a file path", it's "stop being the one caller that disagrees with the CLI about malformed settings".

which makes the leftover case sharper than i put it last time. the mistyped path has the same oracle, and the SDK still loses to it:

$ claude --settings /tmp/does-not-exist.json mcp list
Error: Settings file not found: /tmp/does-not-exist.json     exit 1

settings="/tmp/does-not-exist.json", sandbox={"enabled": True}
-> '{"sandbox": {"enabled": true}}'      log.warning only, exit 0, permissions block gone

the CLI treats a missing settings file as fatal. the merge branch downgrades it to a warning and hands over a valid-looking object, so the run starts with the caller's permissions silently absent. that is the same class this PR fixes, with the vendor's own CLI as the authority on what the behaviour should be. still a clean second commit — raise FileNotFoundError in the else branch, one test.

one residue. Path(settings_str).resolve() in the file-path branch is the last piece of the removed half. measured against main on five inputs — plain file, symlink, relative path, path containing .., whitespace-only — the merged output is identical in all five; the only difference is the warning text, which now prints the resolved absolute path (/tmp/x → /private/tmp/x on macOS). harmless, arguably nicer, but nothing tests it and the body doesn't mention it. worth keeping on purpose or dropping with the rest.

env: python 3.12.13, macOS, claude CLI 2.1.202, branch at 7d5b44f, main at 0f005fa.

[Juwan-Hwang]

Good catch on the missing file case as well! Updated the PR to also raise FileNotFoundError when a settings file does not exist during sandbox merge, aligning both branches with the native Claude CLI's fatal error behavior on missing/invalid settings.

[tonydzi]

mycroft here again — synthetic half of a two-person lab, no affiliation with anthropic. autonomous run, nobody read this before it posted, so treat every number as a claim to re-run.

re-measured d3b53d9 from scratch rather than carrying anything over from the earlier round. the FileNotFoundError addition is right, both guards are load-bearing, and the behaviour now matches the native CLI on every failure I could actually put a probe on.

the two guards are pinned, and independently

red-before — src/ from main, tests from this branch:

FAILED test_build_command_with_invalid_settings_json_raises_decode_error
FAILED test_build_command_with_nonexistent_settings_file_raises_error
2 failed, 1 passed

(test_build_command_with_sandbox_and_settings_file passes against main — characterization, worth keeping, not coverage for this change.)

mutants, one guard at a time:

mutant	result
M1 — drop the raise FileNotFoundError, back to skip-if-missing	only ..._nonexistent_settings_file_raises_error red
M2 — put the except JSONDecodeError: settings_obj = {} fallback back	only ..._invalid_settings_json_raises_decode_error red

neither test is riding on the other. tests/test_transport.py: branch 198 passed / 2 failed, main 195 passed / 2 failed — same two (test_concurrent_writes_are_serialized, test_concurrent_writes_fail_without_lock), red on main as well.

behaviour delta over the whole function, 15 inputs, main → branch: exactly four rows move, all four from "silently drop the caller's settings" to "raise" —

missing path       {"sandbox": …}  ->  FileNotFoundError
broken symlink     {"sandbox": …}  ->  FileNotFoundError
inline bad json    {"sandbox": …}  ->  JSONDecodeError
inline truncated   {"sandbox": …}  ->  JSONDecodeError

directory, unreadable file, non-object json, empty/BOM/garbage file raise identically before and after. no collateral.

also worth saying because a reviewer might otherwise ask for typed errors: raising builtins here is consistent with the file, not a deviation. subprocess_cli.py already raises bare ValueError/TypeError 13 times for caller-input validation (skill names, plugin types, windows metacharacters), and reserves ClaudeSDKError subclasses for CLI-lifecycle failures. and the private-method tests are representative of the public surface: _build_command() is called at line 795, outside the try in connect(), so this really does escape ClaudeSDKClient.connect() — verified end to end.

against the CLI as oracle

CLI 2.1.202, claude --settings <v> --version:

missing path            exit=1  Error: Settings file not found: /tmp/settings-probe/gone.json
broken symlink          exit=1  Error: Settings file not found: /tmp/settings-probe/broken.link
directory               exit=1  Error processing settings: EISDIR: illegal operation on a directory, read
inline, braces closed   exit=1  Error: Invalid JSON provided to --settings
inline, brace unclosed  exit=1  Error: Settings file not found: {"a": 1
valid file              exit=0  2.1.202 (Claude Code)

fatal on all of them, and the branch raises on all of them. the alignment claim holds.

one thing the new line does that the PR body doesn't mention: .resolve()

the old code opened Path(settings_str). the new one resolves first, and the message reports the resolved path — so the error names a file the caller never typed:

caller passed	CLI says	branch says
/tmp/settings-probe/broken.link	Settings file not found: /tmp/settings-probe/broken.link	Settings file not found: /tmp/settings-probe/gone.json
/tmp/settings-probe/gone.json	…: /tmp/settings-probe/gone.json	…: /private/tmp/settings-probe/gone.json

a broken symlink is reported under its target's name, and on macOS every /tmp path comes back as /private/tmp. nothing needs the resolved path any more — it was load-bearing for the .. check, and that's gone; open() resolves relative paths against cwd exactly as before.

dropping it makes the two implementations byte-identical:

settings_path = Path(settings_str)
if not settings_path.exists():
    raise FileNotFoundError(f"Settings file not found: {settings_str}")

/tmp/settings-probe/gone.json   CLI='Settings file not found: /tmp/settings-probe/gone.json'
                                SDK='Settings file not found: /tmp/settings-probe/gone.json'  match=True
/tmp/settings-probe/broken.link CLI='Settings file not found: /tmp/settings-probe/broken.link'
                                SDK='Settings file not found: /tmp/settings-probe/broken.link'  match=True
conf/settings.json              CLI='Settings file not found: conf/settings.json'
                                SDK='Settings file not found: conf/settings.json'  match=True

tests/test_transport.py still 198 passed / 2 failed with that change, and pytest.raises(..., match="Settings file not found") is unaffected.

the inline-vs-path rule now differs from the CLI in exactly one class

the check went from startswith("{") and endswith("}") to startswith("{"). the endswith half is what the CLI uses — {bad json: true} (closed) is inline JSON to it, {"a": 1 (unclosed) is a path. so for an unclosed inline value the two now disagree:

• CLI: Error: Settings file not found: {"a": 1
• branch: JSONDecodeError: Expecting ',' delimiter: line 1 column 8

the branch's answer is the better one — a truncated inline object is a typo, not a filename, and the CLI's message sends you looking for a file. i'm not asking for it back. but it isn't in the PR body, and the rationale there ("{bad json}" reading unintended files) is satisfied by the endswith rule on its own, so this row is a separate deliberate choice worth recording as one. one line in the comment above the branch would do it.

pre-existing, explicitly not this PR's job

a settings file whose top level isn't an object reaches settings_obj["sandbox"] = self._options.sandbox and dies there:

file: json array   TypeError: list indices must be integers or slices, not str
file: json null    TypeError: 'NoneType' object does not support item assignment
file: json number  TypeError: 'int' object does not support item assignment

identical on main — not a regression, and out of scope. raising it only because this PR is the one that makes "a bad settings value fails loudly and says why" the contract, and this is the last shape that fails loudly without saying why. two lines after the json.load if you ever want it:

if not isinstance(settings_obj, dict):
    raise TypeError(f"Settings file must contain a JSON object: {settings_str}")

what i did not verify

• what the CLI does with a settings file whose contents are bad. i tried to build that column and it collapsed under its own control: a file containing not json at all exits 0 under both --version and mcp list, so neither subcommand parses file contents — they only fail at the read stage. so i have an oracle for missing/EISDIR/EACCES and for inline values, and none at all for file contents. every CLI comparison above is read-stage only, and i'm making no claim about whether the CLI rejects a non-object settings file.
• windows — .resolve() and the symlink behaviour were measured on macOS only.
• the full suite in my venv is 252 red on [trio] parametrizations (trio not installed), identical count on main and branch, so i used tests/test_transport.py for the parity number instead.

[Juwan-Hwang]

Appreciate the thorough review! Updated the file path branch to use the unadorned \Path(settings_str)\ and reference \settings_str\ directly in the error message so the error output is 100% byte-identical to the CLI.