add ADR for change in format behavior

adr/2024-11-2-assertion-format.md

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+# [short title of solved problem and solution]
+
+* Status: proposed
+  <!-- will update below to only those who participated in the vote -->
+* Deciders: @gregsdennis @jdesrosiers @jviotti @mwadams @karenetheridge @awwright @benjam @relequestual
+* Date: 2024-11-02
+* Technical Story: https://github.com/json-schema-org/json-schema-spec/issues/1520
+* Voting issue: https://github.com/json-schema-org/TSC/issues/19
+
+## Context and Problem Statement
+
+There's a long and sticky history around format.
+
+1. Going back all the way to Draft 01, format has never required validation.
+2. Whether to support format validation has always been the decision of the implementation.
+3. The extent to which formats are validated has also been the decision of the implementation.
+
+The result of all of this is that implementation support for validation has been spotty at best. Despite the JSON Schema specs referencing very concretely defined formats (by referencing other specs), implementations that do support validation don't all support each format equally. This has been the primary driving force behind keeping format as an opt-in validation.
+
+With 2019-09, we decided that it was time to give the option of format validation to the schema author. They could enable validation by using a meta-schema which listed the Format Vocabulary with a true value, which meant, "format validation is required to process this schema."
+
+In 2020-12, we further refined this by offering two separate vocabularies, one that treats the keyword as an annotation and one that treats it as an assertion. The argument was that the behavior of a keyword shouldn't change based on whether the vocabulary was required or not.
+
+However, the fact remains that our users consistently report (via questions in Slack, GitHub, and StackOverflow) that they expect format to validate. (The most recent case I can think of was only last week, in .Net's effort to build a short-term solution for schema generation from types.)
+
+Due to this consistency in user expectations have decided to:
+
+1. make format an assertion keyword and strictly,
+2. enforce it by moving the appropriate tests into the required section of the Test Suite.
+
+## Decision Drivers
+
+* User expectation
+* Current behavior
+* Historical context
+* Disparity of current implementation support vs the proposed requirements
+
+## Considered Options
+
+### `format` remains an annotation keyword by default
+
+This is the current state. The primary benefit is that we don't need to make a breaking change.
+
+The primary downside is that the current system of (1) configuring the tool or (2) incluing the `format-assertion` vocab[^1] is confusing for many and doesn't align with user expectations.
+
+[^1] The `format-assertion` vocabulary will no longer be an option since we have demoted vocabularies to a proposal for the stable release.  This leaves tool configuration as the only option to enable `format` validation.
+
+### `format` becomes an assertion keyword by default
+
+We change the spec to require `format` validation.  Furthermore:
+
+* Implementations SHOULD support `format` with the defined values
+* Implementations MAY support others, but only by explicit config
+* Implementations MUST refuse to process a schema that contains an unsupported format
+
+## Decision Outcome
+
+The TSC has decided via vote (see voting issue above) that we should change `format` to act as an assertion by default, in line with option (2).
+
+### Positive Consequences <!-- optional -->
+
+* Aligns with user expectations.
+* Users are still able to have purely annotative behavior through use of something like `x-format`.
+* Increased consistency for `format` validation across implementations.
+
+### Negative Consequences <!-- optional -->
+
+* This is a breaking change, which means that we will likely have to re-educate our users.
+* The burden on implementations will be greater since format validation was previously optional.
+
+## Links <!-- optional -->
+
+* [Link type] [Link to ADR] <!-- example: Refined by [ADR-0005](0005-example.md) -->
+* … <!-- numbers of links can vary -->