[josecolella]

Part of #300 (spec v0.9.0). This finishes the hook-input immutability that #273 started; its ClientMetadata freeze landed in #355.

What changes for hooks

• Evaluation context (4.1.4.1). Only before hooks can change it, either by returning an EvaluationContext or by assigning hook_context.evaluation_context. The executor freezes the hook context before after, error and finally run, and at that point it swaps in a frozen copy of the evaluation context. This covers every path into those stages: success, the provider raising, a before hook raising, and short circuit. Neither the caller's context nor a before hook's own object is ever frozen.
• Default value and flag key (4.1.3). Hooks get deep-frozen copies. The default is copied the first time a hook reads it, so hooks that never read it pay nothing.
  • Before this PR, registering any hook made the SDK freeze the caller's own default.
  • If the provider raises, the caller now gets their own default back, not the hooks' copy.
• Hints (4.2.2.1). Hooks get a deep-frozen copy, including nested values. The caller's hash isn't touched.
• Provider metadata (4.2.2.3). Hooks get a frozen copy. Most contrib providers return an unfrozen ProviderMetadata, so before this PR a hook could rename the provider.
• Client metadata (4.2.2.2) was already frozen by fix: freeze ClientMetadata to satisfy spec req 4.2.2.1 #273. This PR adds the spec test.

Why fix!

• Hooks that assign hook_context.evaluation_context or write to its fields in after, error or finally now raise FrozenError. That is handled like any other hook error, so in after the evaluation returns the default.
• HookExecutor#execute now requires default_value:.

Decisions worth a look

• The freeze comes after the provider call, not right after the before hooks. The flagd and go-feature-flag contrib providers write to the context they're given, and a spec covers that case.
• Freezing copies the evaluation context with dup, so EvaluationContext subclasses keep their class.
• Nested values inside the context's fields stay mutable. They're the caller's objects, often from the API- or client-level context. Deep-copying them on every evaluation could be slow, or could fail on arbitrary objects.
• If a hook input can't be deep-copied (a Mutex, Queue, Proc, or a Hash with a default proc inside), hooks get a copy that is frozen only at the top level, and the evaluation still succeeds. Hints, defaults and metadata all use one helper for this, and it's marked as a shortcut in the code.
• Cost. A hooked evaluation with a small flag is about 0.2–0.8µs slower than on main, and evaluations without hooks are unchanged. Unfrozen provider metadata costs about 200ns per hooked evaluation to copy. Contrib providers can avoid that by freezing their metadata, as the built-in providers and flipt already do; that will be a contrib follow-up.

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📥 Commits

Reviewing files that changed from the base of the PR and between b5ac008 and 9d8ddad.

📒 Files selected for processing: 10

• lib/open_feature/sdk/evaluation_context.rb
• lib/open_feature/sdk/hooks/frozen_copy.rb
• lib/open_feature/sdk/hooks/hints.rb
• lib/open_feature/sdk/hooks/hook_context.rb
• sig/open_feature/sdk/evaluation_context.rbs
• sig/open_feature/sdk/hooks/frozen_copy.rbs
• sig/open_feature/sdk/hooks/hook_context.rbs
• spec/open_feature/sdk/hooks/hints_spec.rb
• spec/open_feature/sdk/hooks/hook_context_spec.rb
• spec/specification/hooks_spec.rb

📝 Walkthrough

Hook inputs now expose frozen copies where copying succeeds, and HookExecutor freezes hook context before after, error, and finally hooks. Before hooks retain evaluation-context mutation. The executor receives the original default value for error fallback. Tests and documentation describe these contracts.

Changes: Hook Input Immutability

Layer / File(s)	Summary
Copy hook inputs into frozen snapshots lib/open_feature/sdk/hooks/hook_context.rb, lib/open_feature/sdk/hooks/hints.rb, sig/open_feature/sdk/hooks/hook_context.rbs, spec/open_feature/sdk/hooks/hook_context_spec.rb, spec/open_feature/sdk/hooks/hints_spec.rb, spec/specification/hooks_spec.rb, README.md	Hook context values and hints use frozen copies where copying succeeds. Tests check frozen values and verify that caller-owned values remain unfrozen. The README describes hook input and evaluation-context mutability.
Freeze evaluation context between hook stages lib/open_feature/sdk/evaluation_context.rb, lib/open_feature/sdk/hooks/hook_executor.rb, lib/open_feature/sdk/client.rb, sig/open_feature/sdk/evaluation_context.rbs, sig/open_feature/sdk/hooks/hook_executor.rbs, spec/open_feature/sdk/hooks/hook_executor_spec.rb, spec/specification/hooks_spec.rb	HookExecutor receives default_value and uses it for error fallback. It freezes hook context before after, error, and finally hooks. Tests cover the execution stages and the error and finally paths.

Priority: ⚪ Pending latest changes

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant Client
  participant HookExecutor
  participant BeforeHook
  participant Provider
  participant LaterHooks
  Client->>HookExecutor: Execute with default_value
  HookExecutor->>BeforeHook: Run before hook
  BeforeHook->>HookExecutor: Update or replace evaluation context
  HookExecutor->>Provider: Evaluate flag
  Provider->>HookExecutor: Return evaluation result
  HookExecutor->>LaterHooks: Run after, error, or finally hooks with frozen context

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Possibly related issues

• open-feature/ruby-sdk#300 tracks the immutable hook-input requirements implemented by this PR.

Possibly related PRs

• open-feature/ruby-sdk#356: Both PRs address the client metadata immutability contract.
• open-feature/ruby-sdk#347: This PR passes the original default_value to HookExecutor to preserve the default returned when evaluation raises.

Suggested reviewers: mschoenlaub

Merge Risk: 🔵 Low · up to b5ac0

Hooks receive frozen copies of their inputs, with one narrow exception. A default value containing a Mutex, Queue or Proc is passed through uncopied, so a before hook could still modify it. This is low risk and can be fixed or documented as a follow-up.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 38.10% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 21 functions across 9 files. (4 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Title check		The title clearly and concisely describes the main change: making hook inputs immutable. The fix! marker also indicates a breaking change.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.
Description check		The pull request description clearly explains the hook input immutability changes, evaluation context behavior, default value handling, metadata copying, and the required HookExecutor API change. Thes…

Full details: Description check

Explanation

The pull request description clearly explains the hook input immutability changes, evaluation context behavior, default value handling, metadata copying, and the required HookExecutor API change. These topics match the changeset.

Full details: Docstring Coverage

Explanation

Docstring coverage is 38.10% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 21 functions across 9 files. (4 skipped: 4 unsupported.)

Resolution

Write docstrings for the functions missing them to satisfy the coverage threshold.

✨ Finishing Touches: 💡 1

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.68%. Comparing base (e0ea1f7) to head (9d8ddad).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #357   +/-   ##
=======================================
  Coverage   99.68%   99.68%           
=======================================
  Files          33       34    +1     
  Lines         943      967   +24     
=======================================
+ Hits          940      964   +24     
  Misses          3        3           

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🟡 Changes recommended

Shared context strings and the copy-failure fallback still allow hooks to mutate caller-owned data.

2 open issues

• Snapshot shares mutable scalar targeting data · New
• Fallback exposes caller-owned containers in object defaults · New

What changed in this PR

This PR advances OpenFeature v0.9.0 compliance by isolating hook inputs from caller-owned objects and restricting evaluation-context changes to before hooks.

Changes:

• Adds frozen copies of hook inputs and freezes evaluation-context snapshots before later hook stages.
• Preserves the caller’s original default when evaluation raises.
• Updates signatures, documentation, and tests for the immutability behavior.

File	Description
spec/​specification/​hooks_spec.rb	Adds specification coverage for immutable hook inputs.
spec/​open_feature/​sdk/​hooks/​hook_executor_spec.rb	Updates executor calls and tests short-circuit freezing.
spec/​open_feature/​sdk/​hooks/​hook_context_spec.rb	Tests lazy copying, context freezing, and copy-failure fallback.
spec/​open_feature/​sdk/​hooks/​hints_spec.rb	Tests nested hint immutability and rejected values.
sig/​open_feature/​sdk/​hooks/​hook_executor.rbs	Adds the required default-value argument.
sig/​open_feature/​sdk/​hooks/​hook_context.rbs	Declares copying and freezing methods.
sig/​open_feature/​sdk/​evaluation_context.rbs	Declares the freeze override.
README.md	Documents hook-input mutation restrictions.
lib/​open_feature/​sdk/​hooks/​hook_executor.rb	Freezes later-stage inputs and preserves error defaults.
lib/​open_feature/​sdk/​hooks/​hook_context.rb	Copies hook inputs and snapshots evaluation context.
lib/​open_feature/​sdk/​hooks/​hints.rb	Deep-copies and freezes hints.
lib/​open_feature/​sdk/​evaluation_context.rb	Freezes the fields hash with its context.
lib/​open_feature/​sdk/​client.rb	Passes the original default to the executor.

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Actionable comments posted: 1

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Inline comments:
Review comments at @lib/open_feature/sdk/hooks/hook_context.rb:
- Around line 52-53: Update frozen_copy so defaults that cannot be deep-copied
are rejected before hooks run, or provide hooks an isolated immutable copy;
never return the caller-owned mutable default as a fallback after hook
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• README.md
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• lib/open_feature/sdk/evaluation_context.rb
• lib/open_feature/sdk/hooks/hints.rb
• lib/open_feature/sdk/hooks/hook_context.rb
• lib/open_feature/sdk/hooks/hook_executor.rb
• sig/open_feature/sdk/evaluation_context.rbs
• sig/open_feature/sdk/hooks/hook_context.rbs
• sig/open_feature/sdk/hooks/hook_executor.rbs
• spec/open_feature/sdk/hooks/hints_spec.rb
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• spec/open_feature/sdk/hooks/hook_executor_spec.rb
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