How to Resolve "Failed to Load Weights" Errors with Engine Version Mismatch in Needle

The "failed to load weights" error occurs when a checkpoint's format_version does not match the CHECKPOINT_FORMAT_VERSION constant (currently 2) defined in needle/model/run.py, requiring you to rebuild the checkpoint or align your Needle installation with the checkpoint's creation version.

Needle, the open-source inference engine from Cactus Compute, enforces strict checkpoint compatibility through a format version validation system. When you attempt to load a model checkpoint created with a different engine version, the library immediately rejects it to prevent runtime incompatibilities. Understanding the version check implemented in the source code is essential for resolving these loading errors efficiently.

Understanding the Checkpoint Format Version Check

The root cause of the error lies in the validation logic inside needle/model/run.py. The load_checkpoint function extracts a format_version field from the checkpoint header and compares it against the constant CHECKPOINT_FORMAT_VERSION, defined as 2 on lines 13-15. When these values differ, the code raises a descriptive ValueError:

if version != CHECKPOINT_FORMAT_VERSION:
    raise ValueError(
        f"{path} is not a format‑v{CHECKPOINT_FORMAT_VERSION} checkpoint "
        f"(got format_version={version!r}). Old encoder‑decoder/tool‑calling "
        f"checkpoints are incompatible with this branch."
    )

Source: [needle/model/run.py lines 86-90](https://github.com/cactus-compute/needle/blob/main/needle/model/run.py#L86-L90)

This check ensures that only checkpoints serialized with the current library format can initialize the engine, preventing silent failures from schema mismatches.

Identifying the Version Mismatch

Before applying fixes, confirm the versions involved. Check your installed Needle version programmatically:

import needle
print(needle.__version__)  # Defined in needle/__init__.py lines 85-87

Source: [needle/__init__.py](https://github.com/cactus-compute/needle/blob/main/needle/__init__.py#L85-L87)

When loading fails, the error message explicitly states the expected format version versus the version found in your checkpoint file. For example, if the checkpoint reports format_version=1 while the library expects 2, the mismatch is confirmed.

Three Methods to Resolve the Error

You have three primary approaches to align the checkpoint format with the library's expectations.

Rebuild the Checkpoint with the Current Needle Version

The recommended solution uses the needle build command to regenerate the checkpoint using your current library version. This process re-serializes the weights with the correct CHECKPOINT_FORMAT_VERSION:

# Convert an old or incompatible checkpoint to the current format

needle build --ckpt old_checkpoint.ckpt --out compatible_checkpoint.cact

After rebuilding, load the new checkpoint without errors:

import needle
agent = needle.Needle(weights="compatible_checkpoint.cact")
result = agent.complete("Explain quantum computing")

Match Your Needle Version to the Checkpoint

If you cannot rebuild the checkpoint, install the Needle version that matches its creation format. For legacy checkpoints requiring format version 1, downgrade your installation:

pip install "needle<2.0.0"

Verify compatibility by loading the weights:

import needle

# This will succeed only when versions align

model = needle.Needle(weights="legacy_checkpoint.cact")

Convert Checkpoints Programmatically

For batch conversions or custom pipelines, load the checkpoint with the older compatible library version and immediately re-save it using the newer version:

import needle

# Load with the version that matches the checkpoint's format

legacy_model = needle.Needle(weights="old_format.cact")

# Re-save automatically writes the current CHECKPOINT_FORMAT_VERSION

legacy_model.save("upgraded_format.cact")

The newly saved file contains the updated format version header required by current Needle releases.

Handling Errors in the Playground UI

When using the Needle Playground interface, the same validation occurs through the Engine.load_weights method in playground/server.py (lines 48-55). If you encounter the error in the web UI, apply one of the resolution methods above, then restart the server and reload your weights using the upload button.

Summary

• Needle checkpoints include a format_version field that must match the library's CHECKPOINT_FORMAT_VERSION constant (currently 2).
• The validation occurs in needle/model/run.py within the load_checkpoint function, which raises a ValueError on any mismatch.
• Rebuild checkpoints using needle build to upgrade them to the current format while preserving the underlying weights.
• Downgrade your Needle installation to match legacy checkpoint formats when rebuilding is not feasible.
• Convert programmatically by loading with an older library version and saving with the current one to bridge format differences.

Frequently Asked Questions

What is the current CHECKPOINT_FORMAT_VERSION in Needle?

The current CHECKPOINT_FORMAT_VERSION is defined as 2 in needle/model/run.py (lines 13-15). All checkpoints created with recent Needle versions use this format, while older checkpoints may use version 1 or other legacy formats that are explicitly rejected by the current loader.

Can I load a checkpoint created with Needle 1.x using Needle 2.x?

No. According to the source code in needle/model/run.py (lines 86-90), Needle 2.x explicitly rejects checkpoints with a format_version other than 2. The error message states that "old encoder-decoder/tool-calling checkpoints are incompatible with this branch." You must either rebuild the checkpoint using a current version of the needle build tool or downgrade your Needle installation to the version used to create the checkpoint.

Where does the "failed to load weights" error originate in the codebase?

The error originates in the load_checkpoint function in needle/model/run.py when the checkpoint's extracted version does not match CHECKPOINT_FORMAT_VERSION. If you are using the Playground UI, the error propagates through Engine.load_weights in playground/server.py (lines 48-55), which wraps the underlying checkpoint loading logic.

How do I verify which format version a checkpoint uses?

Attempt to load the checkpoint in a Python session; the resulting ValueError explicitly states the format version found in the file versus the expected version (e.g., "got format_version='1'"). Alternatively, you can inspect the checkpoint file's metadata header if available, though the error message from the Needle loader provides the most reliable identification.

{
  "mcpServers": {
    "instagit": {
      "command": "npx",
      "args": ["-y", "instagit@latest"]
    }
  }
}