diff --git a/docs/adr-004-pin-only-mutation-testing-contract.md b/docs/adr-004-pin-only-mutation-testing-contract.md
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+# Architectural decision record (ADR) 004: pin-only mutation-testing contract
+
+## Status
+
+Accepted (2026-07-22): Keep Whitaker's mutation-testing adoption pin-only and
+contract-test its declared configuration rather than claiming parity with the
+continuous integration (CI) test baseline.
+
+## Date
+
+2026-07-22.
+
+## Context and problem statement
+
+Whitaker delegates mutation testing to the shared
+`leynos/shared-actions/.github/workflows/mutation-cargo.yml` workflow. The
+caller supplies workspace paths, exclusion globs, and `--all-features`, but the
+shared workflow cannot reproduce the per-invocation `RUSTFLAGS` used by the
+repository's `test` and `typecheck` Makefile targets.
+
+Those flags are required by the `whitaker`, `function_attrs_follow_docs`,
+`module_max_lines`, and `no_expect_outside_tests` crates when the
+`dylint-driver` feature enables `feature(rustc_private)`. Applying the flags
+workspace-wide is not viable because, as documented in `.cargo/config.toml`,
+they break `cargo install` for `whitaker-installer`.
+
+The decision is whether the mutation workflow should claim a workspace baseline
+equivalent to `make test`, or expose the closest safe mutation scope and test
+only that declared configuration.
+
+## Decision drivers
+
+- Keep the mutation workflow usable without breaking installer builds.
+- Describe the mutation scope honestly rather than imply CI parity.
+- Detect accidental caller drift while permitting automated commit-pin updates.
+- Keep mutation testing informational and independent of pull-request gates.
+
+## Options considered
+
+| Option                                                  | Shared-workflow feasibility                                                                         | Mutation coverage                                                               | Operational risk                                                                   | CI-baseline claim / verification                                                                 | Rationale                                                                               |
+| ------------------------------------------------------- | --------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------------- | ---------------------------------------------------------------------------------- | ------------------------------------------------------------------------------------------------ | --------------------------------------------------------------------------------------- |
+| A: require full CI-baseline parity                      | Unsafe and unimplementable: the current shared workflow cannot inject the required flags per crate. | Aims for full workspace coverage equivalent to `make test`.                     | Applying the flags workspace-wide breaks `cargo install` for `whitaker-installer`. | Would claim parity, but cannot verify it through the shared workflow.                            | Missing per-crate flags make this option unsafe and unimplementable.                    |
+| B: exclude every crate that needs dynamic-linking flags | Runs reliably with the shared workflow by removing the affected crates.                             | Omits important production code, making the reported scope less useful.         | Lower run risk, but accepts the risk of untested affected crates.                  | Makes no full-baseline claim; verification covers only the reduced scope.                        | Reliable execution deliberately omits important affected crates.                        |
+| C: adopt a pin-only declared-configuration contract     | Closest safe approximation supported by the shared workflow.                                        | Retains useful informational coverage without claiming full workspace coverage. | Keeps mutation testing informational and independent of pull-request gates.        | Verifies the caller's declared security and configuration shape, not a full workspace assertion. | A declared-shape contract is the closest safe approximation, not full workspace parity. |
+
+_Table 1: Comparison of mutation-testing options._
+
+## Decision outcome / proposed direction
+
+Adopt Option C.
+
+In the context of shared mutation testing, facing incompatible per-crate
+compiler-flag requirements, Whitaker chooses a pin-only declared-configuration
+contract over either claimed CI parity or excluding every affected crate, to
+retain useful informational mutation coverage while accepting that the run
+cannot reproduce `make test` crate-for-crate.
+
+The caller remains pinned to a full commit SHA. Its contract test validates the
+pin shape, permissions, concurrency, triggers, and declared inputs. It does not
+hard-code the pin value or assert successful full-workspace mutation testing.
+
+## Known risks and limitations
+
+- Mutation results do not certify the same crate scope as `make test`.
+- Crates needing the dynamic-linking flags may fail or provide incomplete
+  mutation results under the shared workflow.
+- The decision must be revisited if the shared workflow gains a safe per-crate
+  mechanism for supplying the required flags.
diff --git a/docs/contents.md b/docs/contents.md
index 446be541..5f3cb3b8 100644
--- a/docs/contents.md
+++ b/docs/contents.md
@@ -79,6 +79,10 @@
   pipeline](adr-003-formal-proof-strategy-for-clone-detector-pipeline.md)
   records the formal verification direction for the clone detector pipeline and
   its proof boundaries.
+- [Architectural decision record (ADR) 004: pin-only mutation-testing
+  contract](adr-004-pin-only-mutation-testing-contract.md) records why the
+  shared mutation workflow tests its declared configuration without claiming
+  parity with the continuous integration test baseline.
 
 ## Requests for comments
 
diff --git a/docs/developers-guide.md b/docs/developers-guide.md
index fb6add6d..92b1938d 100644
--- a/docs/developers-guide.md
+++ b/docs/developers-guide.md
@@ -291,6 +291,91 @@ make check-fmt  # Verify formatting
 make fmt        # Apply formatting
 ```
 
+## Mutation-testing workflow contract tests
+
+Whitaker runs scheduled, informational mutation testing through a thin caller
+workflow, [`.github/workflows/mutation-testing.yml`](../.github/workflows/mutation-testing.yml),
+which delegates to the shared reusable workflow
+`leynos/shared-actions/.github/workflows/mutation-cargo.yml`. The heavy
+lifting — running `cargo-mutants`, sharding, and summarizing survivors — lives
+in `shared-actions`; this repository carries only declarative configuration.
+The run is **informational only**: it never gates a pull request. Survivors
+are reported through the job summary and downloadable artefacts so they can be
+triaged into tests, not enforced as a blocking check.
+
+The workflow runs in two modes. A **daily schedule** (04:50 UTC) fires a
+change-scoped run that mutates only the source files touched within the
+detection window, so quiet days are cheap no-ops. A **manual dispatch** (the
+Actions "Run workflow" control) mutates the whole workspace; select a branch
+in that control to exercise a feature branch.
+
+The caller passes a small set of configuration inputs, each carrying intent:
+
+- `paths` — `src/,common/,crates/,installer/,suite/`, the workspace member
+  prefixes rooted at the repository root; there are no top-level `examples/`
+  or `benches/` directories to add.
+- `exclude-globs` — scaffolding whose surviving mutants would be noise rather
+  than genuine test gaps: the `rustc_*` proxy crates and the `clippy_utils`
+  stub (both re-export compiler internals), each lint's `ui/` and `examples/`
+  dylint fixtures, and the shared test infrastructure in `src/testing` and
+  `common/src/test_support`.
+- `extra-args` — `--all-features`, matching the feature baseline the
+  Makefile's `CARGO_FLAGS` uses for `make test`, so feature-gated code is not
+  reported as untested.
+
+Unlike the excludes applied by `make test`'s `TEST_EXCLUDES`, the mutation
+caller does **not** exclude the `whitaker` root crate or the
+`function_attrs_follow_docs`, `module_max_lines`, and `no_expect_outside_tests`
+lint crates from mutation scope (only their `ui/`/`examples/` fixtures are
+excluded). Those crates enable `feature(rustc_private)` under the
+`dylint-driver` feature and need the dynamic-linking `RUSTFLAGS`
+(`-C prefer-dynamic -Z force-unstable-if-unmarked`) that the `test` and
+`typecheck` Makefile targets inject per invocation — see the note in
+[`.cargo/config.toml`](../.cargo/config.toml), which deliberately keeps that
+flag out of workspace-wide configuration because it would break `cargo
+install` for `whitaker-installer`. The shared mutation workflow has no
+equivalent per-crate RUSTFLAGS step, so it cannot reproduce
+`TEST_CARGO_FLAGS` faithfully across the whole workspace. Consequently this
+adoption is **pin-only**: the caller declares the best approximation of the CI
+scope it safely can, rather than a `--test-workspace` run that mirrors
+`make test` crate-for-crate, and the contract test asserts only that this
+declared configuration holds — not that a full workspace mutation baseline
+passes. [ADR 004](adr-004-pin-only-mutation-testing-contract.md) records this
+decision, its alternatives, and the accepted limitations.
+
+The `uses:` reference pins the shared workflow to a full 40-character commit
+SHA rather than a branch or tag, so a force-push upstream cannot silently
+change what runs here. The contract test asserts only that the pin is a full
+commit SHA, not a particular value, so Dependabot bumps it automatically
+without any accompanying test edit.
+
+### Workflow contract tests
+
+Because the caller is configuration rather than code, a contract test suite,
+[`tests/workflow_contracts/mutation_testing_test.py`](../tests/workflow_contracts/mutation_testing_test.py),
+pins the shape it must uphold, failing the pull request when the caller
+drifts — repointing the pin at a branch, widening the token scope, or
+dropping a configuration input — rather than letting the breakage surface
+only in a scheduled run. Run it locally with:
+
+```sh
+make test-workflow-contracts
+```
+
+which wraps `uv run --with 'pytest>=8' --with 'pyyaml>=6' pytest
+tests/workflow_contracts -q`. The suite validates:
+
+- the `uses:` reference targets `mutation-cargo.yml` pinned to a full commit
+  SHA;
+- the `with:` block carries exactly the expected `paths`, `exclude-globs`,
+  and `extra-args` configuration described above;
+- job permissions are least-privilege (`contents: read`, `id-token: write`)
+  and the workflow-level default token scope is empty;
+- `concurrency` serializes runs per ref without cancelling one in progress;
+  and
+- the triggers keep the daily schedule and a plain `workflow_dispatch` with no
+  legacy branch input.
+
 ## Proof workflows
 
 Whitaker now ships repository-managed proof tooling for the formal verification