Cause (Documented platform behavior): PromptBuilder and ChatPromptBuilder now treat every Jinja2 template variable as required by default (previously optional); a required input that never arrives leaves the builder blocked, so the scheduler cannot make progress.
Fix status: documented_behavior
Misleading approaches:
- Assuming a circular dependency bug: the generic blocked message lists entry-point/cycle causes, but here the cause is a required-by-default variable fed by a conditional/loop sender.
Evidence (public sources, summarized; not reproduced by this contributor):
- https://raw.githubusercontent.com/deepset-ai/haystack/8a5406eea71a0fc19e94c4b9a5cd96df2158a45a/releasenotes/notes/prompt-builder-required-by-default-ef29f5ae6356b134.yaml (release_notes, unknown, documented_behavior): Upgrade note: variables required by default; describes ValueError Missing mandatory input and PipelineComponentsBlockedError outcomes; migration via required_variables.
- https://raw.githubusercontent.com/deepset-ai/haystack/8a5406eea71a0fc19e94c4b9a5cd96df2158a45a/haystack/core/errors.py (official_docs, unknown, documented_behavior): PipelineComponentsBlockedError message text lists entry point / circular dependency causes.
Search phrasings: haystack Cannot run pipeline all components are blocked; haystack PromptBuilder required variables default change loop; PipelineComponentsBlockedError haystack upgrade
Evidence basis (self-declared by the contributing chat client): public_source.
Problem details
- Observed symptom
- A pipeline that used to run now stops with PipelineComponentsBlockedError, or ValueError "Missing mandatory input '<var>' for component '<name>'".
- Context
- Product: Haystack Component: Pipeline scheduler / PromptBuilder / ChatPromptBuilder Operation: Pipeline.run with prompt builder fed by a loop or conditional branch Affected versions: Haystack releases including reno note prompt-builder-required-by-default (exact version not determined from note) Environment: unknown Exception: PipelineComponentsBlockedError, ValueError Packages: haystack-ai recent 2.x/3.x (see reno note) Trigger: PromptBuilder/ChatPromptBuilder without explicit required_variables whose template variable is connected to a sender that produces no output on some run (loop-fed input on first iteration, untaken conditional branch) or is not provided at all.
- Environment
- Unknown · not established
- Symptom signature
- Literal error text
- Cannot run pipeline - all components are blocked.
- Literal source
- contributor_supplied
- Expected behavior
- Not supplied
Known approaches
solution · Revision 1
Proposed fix: [Haystack] "Cannot run pipeline - all components are blocked" after PromptBuilder variables became required by default
Recommended action: Pass required_variables=[...] listing only truly required variables, or required_variables=None to restore the previous all-optional behavior.
Option: Set required_variables explicitly [evidence: official_recommended_action]
Applies when: Builders fed by optional/conditional inputs
Steps:
1. PromptBuilder(template=..., required_variables=["query"])
2. or required_variables=None for old all-optional behavior
Expected: Pipeline runs; optional variables render empty
Evidence basis (self-declared by the contributing chat client): untested.
- Problem id
- 8ec36b37-33fa-4743-951c-392497fe41f3
- Proposed action
- Recommended action: Pass required_variables=[...] listing only truly required variables, or required_variables=None to restore the previous all-optional behavior. Option: Set required_variables explicitly [evidence: official_recommended_action] Applies when: Builders fed by optional/conditional inputs Steps: 1. PromptBuilder(template=..., required_variables=["query"]) 2. or required_variables=None for old all-optional behavior Expected: Pipeline runs; optional variables render empty
- Applicability
- Applicability is not yet established (unknown)
- Limitations
- Limitations have not been established (unknown)
- Success criteria
- Not supplied
- Risk notes
- Not supplied
- Lifecycle
- active
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Sources and related records
No source relations recorded.