[Thanatos9404]

Related issue

Refs #7398

Summary

Root cause. On main, a callable guardrail by itself already checkpoints. Since #5638 it is written as null, and the UserWarning quoted in the issue is emitted. The Unable to serialize unknown type: <class 'function'> error comes from Tool.func instead. An @tool function or Tool(func=...) stores the raw function, which has no JSON serializer. So state.model_dump(mode="json") in _do_checkpoint raises, and no checkpoint is written. The issue's exact log (the guardrail warning followed by the error) appears when a guardrail task is serialized before a task whose agent has a tool. With a single task, main fails before it reaches the guardrail, so there is no warning, but it fails with the same PydanticSerializationError.

This was reproduced with a stub BaseLLM and on_events=["task_completed"]. A callable guardrail alone writes checkpoints. An @tool function on any agent or task makes every checkpoint fail.

Fix (in tools/base_tool.py):

• A JSON-only field_serializer writes Tool.func as module.qualname. It uses the existing callable_to_string and _is_non_roundtrippable helpers from crewai.types.callback.
• Some callables can't be restored from a path: lambdas, closures, partials, bound methods and callable objects. These are written as null with a UserWarning that names the tool, and the rest of the checkpoint is still written. This matches what fix: serialize guardrail callables as null for JSON checkpointing #5638 does for guardrails.
• A before validator resolves the path on restore. @tool rebinds the function's name to the Tool it builds, so when the path resolves to a Tool, the validator uses that tool's func. This includes an @tool defined in a class body, whose path is module.Class.name.
• Restoring func requires CREWAI_DESERIALIZE_CALLBACKS=1 because the function is called with arguments the model chooses. This is the same opt-in crewAI already requires for task and crew callbacks. Without it, restore fails with the existing "Refusing to resolve callback path ..." error.

Python-mode dumps and normal Tool construction are unchanged. Event to_json() (tracing) is also unchanged: it dumps with serialize_as_any=True, which doesn't use this serializer, and its output is the same as on main.

Verification

• Tests added or updated for the changed behavior
• Relevant tests and quality checks pass locally

New tests:

• tests/test_checkpoint.py::TestCheckpointFunctionTools
  • test_kickoff_auto_checkpoints_guardrail_task_then_tool_task: runs a real crew.kickoff() with a stub LLM and auto-checkpointing on task_completed, using the issue's layout. Task 1 has a callable guardrail and an agent without tools. Task 2's agent has an @tool function. The test asserts that there is no CheckpointFailedEvent and no "Auto-checkpoint failed" log, and that two checkpoint files are written.
  • test_checkpoint_stores_tool_func_as_dotted_path, test_restore_resolves_tool_func and test_restore_refuses_tool_func_without_opt_in: _do_checkpoint stores func as a dotted path and the guardrail as null. Restoring with the opt-in returns the original function, and the tool runs. Restoring without the opt-in is refused.
• tests/tools/test_base_tool.py::TestToolFuncJsonSerialization: a Tool(func=...) wrapping a module-level function round-trips through JSON. A closure-based @tool dumps func as null with a warning, and Python-mode dumps keep the callable. An @tool defined in a class body round-trips with the opt-in and is refused without it.

Results on Windows 11 with Python 3.13.2, pydantic 2.12.5 and pytest 9.0.3. --allowed-hosts=127.0.0.1 is needed only on Windows, because --block-network blocks asyncio's loopback self-pipe there.

• python -m pytest "lib/crewai/tests/test_checkpoint.py::TestCheckpointFunctionTools" "lib/crewai/tests/tools/test_base_tool.py::TestToolFuncJsonSerialization" -n 0 --allowed-hosts=127.0.0.1 -p no:randomly -q -rA -W "ignore::DeprecationWarning"
  • With this change: 8 passed.
  • With base_tool.py reverted to main: 8 failed. Seven raise PydanticSerializationError: Unable to serialize unknown type: <class 'function'>. The end-to-end test records two CheckpointFailedEvents with that error and logs Auto-checkpoint failed for event task_completed twice.
• python -m pytest lib/crewai/tests/test_checkpoint.py lib/crewai/tests/test_guardrail_serialization.py lib/crewai/tests/tools/ --ignore=lib/crewai/tests/tools/agent_tools/test_read_file_tool.py -n 4 --allowed-hosts=127.0.0.1 -q -rf -W "ignore::DeprecationWarning"
  • With this change: 314 passed, 3 failed.
  • On main: 306 passed, and the same 3 failed.
  • All 3 failures are Windows-only. Two TestJsonProviderFork tests assert /-separated paths, and test_every_reason_is_actually_produced reads a source file with the cp1252 codec.
  • test_read_file_tool.py is excluded because on this machine it crashes the interpreter with a native access violation in crewai_files content detection. It crashes the same way on main.
• ruff check and ruff format --check on base_tool.py pass. mypy --config-file pyproject.toml lib/crewai/src/crewai/tools/base_tool.py (strict) reports no issues.

Additional context

• Scope: The issue's response_format variant (Unable to serialize unknown type: <class 'pydantic._internal._model_construction.ModelMetaclass'>) happens when an agent's LLM has response_format=<BaseModel subclass>. That is in the LLM serialization path, so it is out of scope here and remains open. That's why this PR references Auto-checkpoint raises PydanticSerializationError when a Task has a Python-callable guardrail #7398 instead of closing it.
• Compatibility: Only Tool's JSON-mode output changes. func used to raise; now it is a dotted path or null. Checkpoints that include tools couldn't be written before, so no existing checkpoint data is affected.
• Restoring: Restoring a checkpoint that contains @tool tools needs CREWAI_DESERIALIZE_CALLBACKS=1, and that includes crewai checkpoint resume. Tools built from closures or lambdas can't be restored. A warning says so at save time and suggests using a module-level function.
• Prior work: Unlike fix(tools): support JSON checkpoint serialization for function tools #7410 (by @Tyagiquamar, auto-closed by the first-time-contributor gate), restore keeps the opt-in and unwraps @tool paths.
• AI disclosure: This PR was written by an AI agent (Claude Opus 5.5 via Claude Code). I directed this work and authorized submitting it. The agent ran the reproduction, tests and checks listed above, and a separate AI review pass checked the change. CONTRIBUTING.md asks for the llm-generated label, which I can't add as an outside contributor — could a maintainer add it?

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Note

Medium Risk
Changes checkpoint JSON shape and restore semantics for tool callables, gated by the same opt-in as other deserialized callbacks; incorrect path resolution could run unintended code when the env flag is enabled.

Overview
Fixes #7398, where crews with @tool function tools could not auto-checkpoint because JSON serialization of Tool.func raised PydanticSerializationError.

Tool.func checkpoint serialization: In JSON mode, func is now written as a module-qualified dotted path (same helpers as guardrails/callbacks). Non-importable callables (closures, lambdas, etc.) serialize as null with a UserWarning suggesting a module-level function, while the rest of the checkpoint still saves.

Restore behavior: A before-validator resolves string paths back to callables when CREWAI_DESERIALIZE_CALLBACKS=1; without it, restore fails with the existing “Refusing to resolve” error. @tool rebinding is handled by unwrapping when the path resolves to a Tool instance (including class-body tools).

Tests: New coverage in test_checkpoint.py and test_base_tool.py for dotted-path storage, round-trip restore, opt-in gating, and end-to-end kickoff with guardrail + tool tasks writing two checkpoints.

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Review summary

Reviewed PR #8104, which adds JSON round-tripping for Tool.func so checkpoints that include function tools can serialize and restore. The change is a field_serializer (_serialize_func) that writes func as an __module__.__qualname__ dotted path (or null for non-roundtrippable callables) and a field_validator (_resolve_func_path → _resolve_tool_func) that resolves the path back to a callable on restore. The deserialization path performs importlib.import_module, but it is fully gated behind the existing CREWAI_DESERIALIZE_CALLBACKS=1 opt-in flag (_trusted_deserialize()), and any non-opt-in restore falls through to string_to_callable, which rejects with "Refusing to resolve callback path". The serialized path is derived from the developer-owned function object itself rather than any attacker-controlled input, and the gating mirrors the framework's existing SerializableCallable/_resolve_dotted_path pattern. No security vulnerability was identified; the four known issues for this target all belong to a different repository and do not relate to this change.

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• lib/crewai/src/crewai/tools/base_tool.py
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📝 Walkthrough

Walkthrough

Function-backed tools now serialize callable references as dotted paths when they can round-trip, and resolve those paths during validation. Non-round-trippable callables serialize as None with a warning. Tests cover callback deserialization, guardrail omission, and automatic checkpoint completion.

Changes

Function Tool Checkpoints

Layer / File(s)	Summary
Tool function serialization and resolution lib/crewai/src/crewai/tools/base_tool.py	Tool.func resolves string paths and serializes callable references as dotted paths when they can round-trip. If serialization cannot produce a path, it emits a warning and JSON serialization returns None.
Checkpoint restoration tests lib/crewai/tests/test_checkpoint.py, lib/crewai/tests/tools/test_base_tool.py	Tests cover function-tool serialization and restoration with callback deserialization enabled or disabled. They also check guardrail omission, automatic checkpoint completion, and warning behavior for local functions.

Suggested reviewers: joaomdmoura

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Priority: ➖ Normal

Merge Risk: 🟡 Moderate · up to 14db0

Checkpoints containing functions defined in a script may fail to restore from another entrypoint. Address or explicitly accept that limitation before merging.

Pre-merge checks | 4 | 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 37.50% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 24 functions across 3 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check		The title clearly and concisely describes the primary change: serializing function tool callables as dotted paths.
Description check		The description includes the related issue, detailed summary, verification results, test coverage, compatibility notes, security context, and additional context. It satisfies the required template.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 2

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Inline comments:
Review comments at @lib/crewai/src/crewai/tools/base_tool.py:
- Line 568: Update the restorable-path selection in the function containing this
assignment so callables defined in `__main__` are treated as non-restorable,
unless a stable import path is available. Preserve the existing
`_is_non_roundtrippable` and `callable_to_string` behavior for other callables.
- Around line 97-105: Update the callable resolution flow around
string_to_callable to support class-qualified @tool paths such as
module.Class.echo. Resolve the longest importable module prefix, traverse the
remaining attributes, and return Tool.func when the final object is a Tool
before falling back to callable resolution.

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[Thanatos9404]

@strix-security review

The restore path changed in 5f776ba (class-qualified @tool paths), so a fresh look at the latest commit would be useful.