Fix progress text metadata negotiation

[rattus128]

The preview of Get Image Size particulars broke due to FE protocol updates.

Example Test Conditions:

Linux, firefox
Load Image -> Get Image Size -> Preview as Text

Before:

After:

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between acd7185 and 9ff4b7a.

📒 Files selected for processing (2)

• comfy_api/feature_flags.py
• server.py

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📝 Walkthrough

Walkthrough

This pull request introduces a new server feature flag supports_progress_text_metadata and modifies the send_progress_text function accordingly. The feature flag is added to SERVER_FEATURE_FLAGS as a default-enabled capability. When enabled, send_progress_text conditionally prepends metadata to progress text messages: a 4-byte length header followed by the current executing prompt ID encoded as UTF-8. The existing node ID and text payload follow this optional header. Additionally, the function now determines the target socket ID from either the provided sid parameter or defaults to self.client_id when enqueueing binary event messages.

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'Fix progress text metadata negotiation' directly aligns with the PR's main objective of addressing progress text metadata negotiation between server and client.
Description check	✅ Passed	The description is related to the changeset, explaining a bug fix for the preview functionality via protocol updates and providing test conditions with before/after screenshots.

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[alexisrolland]

Hi @rattus128, why would we use a feature flag for this?

[guill]

@alexisrolland I believe it's using a feature flag because it's changing an unversioned binary format. Somehow, we need to know whether to send the old version format (which only old clients will be able to parse) or the new version format (which only new clients will be able to parse).
There was a discussion on this in Slack, but my ask was that we move away from this pattern of using unversioned binary formats in general. At the very least, I think we should be using the same pattern as we do for previews where we embed json in the binary so that we're able to add more fields in the future without adding yet another feature flag (or maintaining multiple serialization/deserialization implementations).

While the feature flags system will mostly work here, it does mean that users that are connected to ComfyUI at the time a deployment is rolled out (a common case on Comfy Cloud) may start experiencing parsing errors until they refresh.