YAML front matter with dates fails in Ruby with DisallowedClass

Agent diagnostic brief

Exact symptom

Tried to load unspecified class: Date (Psych::DisallowedClass)

Where it has been observed

macOS with Homebrew Python 3.14 (PEP 668 marker) and Ruby 4.0 (Psych); Short-lived validation shells without a project virtual environment.

Likely distinct causes

An unquoted date scalar deserializes as Date, which safe loading does not permit.
The YAML is malformed: a syntax error would be reported instead.
Python lacks PyYAML: a separate ModuleNotFoundError.

How to distinguish them

The exception class is Psych::DisallowedClass and it names Date.
Quoting the date or permitting Date makes the same text load.
pip reports externally-managed-environment for the system interpreter.

Current approaches

Permit Date explicitly or validate YAML in an isolated env (solution-psych-disallowed-class-date, revision 1).

Known obsolete approaches

Installing into the system Python with pip.

Versions and freshness

Observed 2026-08-12..31; reproduced with Ruby 4.0.6 and Python 3.14.7 on 2026-09-11
Editorial and primary-source review date: 2026-09-11.

What remains unknown

How front-matter parsers in other languages treat unquoted dates.
Independent reproduction by a different operator has not been established.

Deeper evidence

Problem

YAML front matter with dates fails in Ruby with DisallowedClass

Observed symptom

Agents validating YAML front matter in fresh shells find no PyYAML in the system Python, cannot pip-install into the externally managed interpreter, and fall back to Ruby, where front matter containing an unquoted date fails to load.
This historical observation is same-operator evidence and does not establish prevalence.

Operator goal

Validate YAML front matter from a fresh agent shell.

Current understanding

Psych 4 and later load YAML safely by default and reject Date unless permitted_classes includes it.
Homebrew Python is marked externally managed (PEP 668), so pip refuses to install packages into it.

Known limitations

Other tagged types such as Time or Symbol need their own permission.

Known negative results

pip install pyyaml into the system interpreter. Result: Refused with externally-managed-environment. Why it misleads: PEP 668 protects the managed interpreter; use a virtual environment or tool-managed environment instead of overriding it.
Plain YAML.load in Ruby. Result: Failed on unquoted dates with Psych::DisallowedClass. Why it misleads: Psych 4+ load is safe by default and does not permit Date unless allowed.
No same-operator execution in this release counts as an independent reproduction.

Primary and recurrence sources

PEP 668: externally managed environments
Primary · Primary specification · Date not supplied · Reviewed 2026-09-11 · Rights: allowed_to_summarize
Current primary material reviewed for claude-historical-corpus-1: PEP 668: externally managed environments.
Ruby Psych documentation
Primary · Primary documentation · Date not supplied · Reviewed 2026-09-11 · Rights: allowed_to_summarize
Current primary material reviewed for claude-historical-corpus-1: Ruby Psych documentation.

Rights and provenance

Origin

Based on a real operator execution; identifying project details removed.

Rights

State: allowed_to_summarize
Review basis: Owner-authorized factual synthesis of the operator's own Claude-assisted execution, independently written from reviewed current primary sources; no transcript expression is published.

Editorial review date

2026-09-11

Known approaches

solution · Revision 1

Permit Date explicitly or validate YAML in an isolated env

Production corpus importer · 2026-09-11T20:00:00.000Z
Operator Knowledge for Agents editorial · Historical operator record · Digital source: trainedAlgorithmicMedia · Rights: owned

## Candidate action - Permit Date explicitly or validate YAML in an isolated env. Use an isolated environment for Python YAML (virtual environment or a tool-managed one-off environment) or, with Ruby, pass permitted_classes: [Date]; do not break system packages. This is a candidate procedure supported by same-operator executions within the stated version boundary, not a universal fix. ## Applicability - Use when the observed signature is: Tried to load unspecified class: Date (Psych::DisallowedClass). - Observed scope: macOS with Homebrew Python 3.14 (PEP 668 marker) and Ruby 4.0 (Psych); Short-lived validation shells without a project virtual environment. - Stop if the first failing stage or product boundary differs. ## Procedure - In Ruby, use YAML.safe_load(text, permitted_classes: [Date]). - In Python, use a virtual environment or a tool-managed one-off environment with PyYAML. - Do not pass --break-system-packages to a managed interpreter. - Re-run the validation and check each field. ## Limitations - Other tagged types such as Time or Symbol need their own permission. ## Obsolete approaches - Installing into the system Python with pip. ## Negative results - pip install pyyaml into the system interpreter. Result: Refused with externally-managed-environment. Why it misleads: PEP 668 protects the managed interpreter; use a virtual environment or tool-managed environment instead of overriding it. - Plain YAML.load in Ruby. Result: Failed on unquoted dates with Psych::DisallowedClass. Why it misleads: Psych 4+ load is safe by default and does not permit Date unless allowed. - No external or same-operator report was promoted to independent reproduction credit. ## Evidence boundary - Grounded in current primary source records src-ruby-psych-current, src-pep-668. - Grade A same-operator observation (2026-08-12..2026-08-31): Front matter with unquoted dates that failed with Psych::DisallowedClass loaded successfully with permitted_classes: [Date], and the per-field validation reported success. - Only immutable manifest executions count as Knowledge for Agents Attempts or Outcomes. ## What remains unknown - How front-matter parsers in other languages treat unquoted dates. - Whether the approach works outside the stated environment remains unknown.

Candidate id

claude-psych-disallowed-class-date

Historical period

2026-08-12..2026-08-31

Historical date

2026-08-31

Verification grade

Operator relationship

same_operator

Independent reproduction

false

History source

same-operator Claude agent session history

Provenance disclosure

Based on a real operator execution; identifying project details removed.

Applicability

State

partial

Text

Applies when the failed stage matches: Tried to load unspecified class: Date (Psych::DisallowedClass).

Facts

Component: YAML safe loading
Operation: validate YAML front matter from a fresh shell
Protocol: YAML 1.1 timestamps

Pack

Candidate action: Permit Date explicitly or validate YAML in an isolated env. Use an isolated environment for Python YAML (virtual environment or a tool-managed one-off environment) or, with Ruby, pass permitted_classes: [Date]; do not break system packages. This is a candidate procedure supported by same-operator executions within the stated version boundary, not a universal fix.
Applicability: Use when the observed signature is: Tried to load unspecified class: Date (Psych::DisallowedClass).
Observed scope: macOS with Homebrew Python 3.14 (PEP 668 marker) and Ruby 4.0 (Psych); Short-lived validation shells without a project virtual environment.
Stop if the first failing stage or product boundary differs.
Steps: In Ruby, use YAML.safe_load(text, permitted_classes: [Date]).
In Python, use a virtual environment or a tool-managed one-off environment with PyYAML.
Do not pass --break-system-packages to a managed interpreter.
Re-run the validation and check each field.
Limitations: Other tagged types such as Time or Symbol need their own permission.
Obsolete approaches: Installing into the system Python with pip.
Negative results: pip install pyyaml into the system interpreter. Result: Refused with externally-managed-environment. Why it misleads: PEP 668 protects the managed interpreter; use a virtual environment or tool-managed environment instead of overriding it.
Plain YAML.load in Ruby. Result: Failed on unquoted dates with Psych::DisallowedClass. Why it misleads: Psych 4+ load is safe by default and does not permit Date unless allowed.
No external or same-operator report was promoted to independent reproduction credit.
Evidence boundary: Grounded in current primary source records src-ruby-psych-current, src-pep-668.
Grade A same-operator observation (2026-08-12..2026-08-31): Front matter with unquoted dates that failed with Psych::DisallowedClass loaded successfully with permitted_classes: [Date], and the per-field validation reported success.
Only immutable manifest executions count as Knowledge for Agents Attempts or Outcomes.
What remains unknown: How front-matter parsers in other languages treat unquoted dates.
Whether the approach works outside the stated environment remains unknown.

Rights

State: allowed_to_summarize
Review basis: Original diagnostic procedure synthesized from owner-authorized execution facts and linked current primary sources; no private source code, logs, or transcript expression is published.

Source ids

src-ruby-psych-current
src-pep-668

Editorial review date

2026-09-11

Seo metadata

Meta title: Permit Date explicitly or validate YAML in an isolated env | Knowledge for Agents
Meta description: Candidate procedure for yaml front matter with dates fails in ruby with disallowedclass: applicability, steps, current sources, limitations, negative results, and unknowns.

Historical execution

Disclosure: Based on a real operator execution; identifying project details removed.
Operator relationship: same_operator
Independent reproduction: false

Problem id

problem-psych-disallowed-class-date

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Sources and related records

No source relations recorded.