[fal.ai] frame_processor: Pipeline setup fails — 'Invalid graph: Graph must have at least one sink node'

[livepeer-tessa]

Summary

The pipeline setup fails with a graph validation error when no sink (output) node is present in the pipeline configuration. Observed on the fal.ai production worker in the last 12h.

Error Message

scope.server.frame_processor - ERROR - [6a6ce6ba] Pipeline setup failed: Invalid graph: Graph must have at least one sink node

Frequency (last 12h, 2026-04-15 06:11–18:11 UTC)

1 occurrence at 15:58:07 UTC

• Job ID: 0dddbac2-6211-4d5a-a893-42a5ac360c59
• Node ID: 91c8de9b-c9ef-f598-2a9d-7ac3d8aa1f60
• Worker: fal-jobs/GPU-H100/0dddbac2-6211-4d5a-a893-42a5ac360c59
• App: github_f1lhgmk5v76a0ev1w0u378by-scope-app--prod (App ID: 5d41a7f8-0588-4893-88cb-e618824195e4)
• Request ID: 6a6ce6ba

Analysis

The graph validation layer requires at least one sink node (a node with no outgoing edges / an output node) in the pipeline. This error fires when the user submits or saves a pipeline graph that has no terminal output node — e.g., all nodes feed into other nodes but nothing terminates the chain.

Root causes:

1. User error: User built a pipeline without connecting an output/sink node (e.g. forgot to add a Preview or Output node)
2. Graph serialization bug: A valid pipeline graph is being deserialized incorrectly, stripping sink nodes
3. Plugin/node type issue: The sink node type exists but isn't recognized as a sink by the validator (e.g. a new output node type not registered)

Impact

• Functional: High — pipeline fails to start entirely, no output rendered
• UX: Poor — error message is technical; user gets no guidance on how to fix their graph

Suggested Fix

1. Better error message to user: Surface a human-readable error in the UI: "Your pipeline has no output node. Add a Preview or Output node to see results."
2. Graph validator: Verify the sink detection logic correctly identifies all output node types (Preview, Output, Syphon out, NDI out, etc.)
3. Proactive validation in UI: Warn the user before they submit if no sink node is detected, rather than failing silently on the server

cc @mjh1 @emranemran

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Filed automatically by Scope Error Monitor — 2026-04-15 18:11 UTC

[livepeer-tessa]

Recurrence — 2026-04-16 06:11–18:11 UTC

2 new occurrences in this 12h window, both on scope-app--prod:

• 2026-04-16T10:23:12 UTC — session 3e996b22
• 2026-04-16T10:26:25 UTC — session 657ad009

Both are in close temporal proximity (~3 min apart) suggesting the same user triggered this twice. No stack trace available — the error is the terminal message from frame_processor.

App: github_f1lhgmk5v76a0ev1w0u378by-scope-app--prod

Updated automatically by Scope Error Monitor — 2026-04-16 18:11 UTC