diff --git a/AGENTS.md b/AGENTS.md
index e9e96adffe..6e6fb12d79 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -35,4 +35,5 @@ Detailed guidance lives in `docs/contributing/` and topic-specific guides under
 | [Design Decisions](docs/contributing/design-decisions.md) | Understanding architectural principles and key decisions |
 | [Vouch System](docs/contributing/vouch-system.md) | Working with the contributor vouch gate or PR workflows |
 | [Tier Conventions](docs/contributing/tier-conventions.md) | Using the term "tier" in code or docs — covers the three distinct tier contexts |
+| [Harness Composition](docs/contributing/harness-composition.md) | Changing merge/compose or diff functions in `internal/harness/` — covers the bidirectional invariant between compose and diff |
 | [CI Workflows](docs/contributing/ci-workflows.md) | Adding or modifying GitHub Actions workflows under `.github/workflows/` |
diff --git a/docs/contributing/harness-composition.md b/docs/contributing/harness-composition.md
new file mode 100644
index 0000000000..e792368049
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+++ b/docs/contributing/harness-composition.md
@@ -0,0 +1,88 @@
+# Harness Composition
+
+When changing merge or compose functions in `internal/harness/`, be aware that
+these functions have an architectural counterpart: diff functions that extract
+the delta between a composed result and its base. The diff functions were
+removed with the scaffold agent extraction (see
+[ADR 0045](../ADRs/0045-forge-portable-harness-schema.md), Consequences →
+"Bidirectional composition"), but the bidirectional invariant remains
+architecturally important — any future re-addition of diff-based operations
+must mirror the current merge semantics.
+
+## Why this matters
+
+The `fullsend agent migrate-customizations` command
+([cli-internals](../guides/dev/cli-internals.md)) converts deprecated
+`customized/` directory overlays into config-driven agents with `base:`
+composition. The current implementation moves files from `customized/` to
+regular directories via `rewriteCustomizedPaths` and registers the agent in
+config — it does not use diff functions today. However,
+[ADR 0064](../ADRs/0064-deprecate-customized-directory-overlay.md) envisions a
+diff-based migration flow where overrides are extracted by diffing a customized
+harness against its upstream base. If diff functions are re-added and do not
+mirror the merge logic, overrides would be lost or duplicated — silently
+corrupting harness customizations.
+
+This constraint is documented in
+[ADR 0045](../ADRs/0045-forge-portable-harness-schema.md) (Consequences →
+"Bidirectional composition") but is not visible during normal documentation
+reads because ADRs are subject to immutability policy.
+
+## Compose/merge functions and their expected diff counterparts
+
+The merge functions below live in `internal/harness/compose.go` (for `base:`
+composition) and `internal/harness/forge.go` (for runtime forge-config
+application). The diff counterparts listed in the right column **do not
+currently exist** — they were removed with the scaffold agent extraction. The
+names shown are the expected counterparts based on ADR 0045; if diff functions
+are re-added, they should follow this naming and scope.
+
+| Compose / Merge (compose.go, forge.go) | Expected diff counterpart (not currently present) | Scope |
+|---|---|---|
+| `mergeBaseIntoChild` | `DiffHarness` | Top-level harness fields |
+| `mergeForgeConfigInto` | `diffForgeConfig` | Per-platform forge config in `base:` composition |
+| `mergeForgeConfig` | `diffForgeConfig` | Forge config applied at runtime |
+| `mergeForgeBlocks` | (per-key iteration over `diffForgeConfig`) | Map of platform → ForgeConfig |
+| Field-level helpers (e.g. `mergeSkills`, `mergeHostFiles`) | Corresponding diff helpers | Individual collection fields |
+
+## Checklist for changes
+
+When modifying merge or compose logic:
+
+1. **Check whether diff functions exist.** Look for `internal/harness/diff.go`
+   or functions matching `Diff*`/`diff*` in the harness package. If they do not
+   exist, note in your commit message that the diff side is absent and skip
+   steps 2–4.
+2. **Identify the paired function.** Use the table above or search for the
+   counterpart by name pattern (`merge*` ↔ `diff*`, `Diff*`).
+3. **Mirror the change.** If you add a field to a merge function, add the
+   corresponding extraction to the diff function. If you change merge
+   semantics (e.g. from whole-replace to field-level merge), update the
+   diff to produce the correct delta under the new semantics.
+4. **Test the round-trip.** Verify that `compose(base, diff(composed, base))`
+   produces the original `composed` result. Existing tests in
+   `compose_test.go` cover the merge side; diff tests should follow the
+   same patterns.
+5. **Check field-level helpers.** Changes to helpers like `mergeSkills` or
+   `mergeHostFiles` may also need corresponding diff helpers.
+
+## When the diff side is unaffected
+
+Not every compose change requires a diff change. If the modification is purely
+internal (e.g. performance optimization that preserves semantics) or affects
+only fields that use whole-replace semantics in both directions, the diff side
+may already be correct. In that case, document in your commit message why the
+diff side is unaffected.
+
+## Historical context
+
+[ADR 0045](../ADRs/0045-forge-portable-harness-schema.md) introduced `base:`
+composition and its inverse. The ADR's Consequences section states that
+`DiffHarness` was "removed with the scaffold agent extraction." The diff
+functions do not currently exist in this repository. The bidirectional
+constraint remains architecturally important — any future re-addition of diff
+functions must mirror the current merge semantics.
+
+See also: [Issue #662](https://github.com/guyoron1/fullsend/issues/662)
+tracks a broader harness field integration checklist covering expansion,
+environment construction, composition carry-forward, and security pipelines.