diff --git a/app/api/routers/proposals.py b/app/api/routers/proposals.py
index 4b03742..b3ed736 100644
--- a/app/api/routers/proposals.py
+++ b/app/api/routers/proposals.py
@@ -51,7 +51,7 @@ def _serialize_proposal(db: Session, proposal: Proposal, *, include_essentials:
         "project_id": proposal.project_id,
         "created_by_id": proposal.created_by_id,
         "created_by_username": creator.username if creator else None,
-        "feat_task_id": proposal.feat_task_id,  # DEPRECATED — read-only compat
+        "feat_task_id": proposal.feat_task_id,  # DEPRECATED (BE-PR-010): read-only for legacy rows. Clients should use generated_tasks.
         "created_at": proposal.created_at,
         "updated_at": proposal.updated_at,
     }
@@ -237,7 +237,7 @@ def update_proposal(
         raise HTTPException(status_code=403, detail="Proposal edit permission denied")
 
     data = proposal_in.model_dump(exclude_unset=True)
-    # Never allow client to set feat_task_id
+    # DEPRECATED (BE-PR-010): feat_task_id is read-only; strip from client input
     data.pop("feat_task_id", None)
 
     for key, value in data.items():
@@ -365,7 +365,7 @@ def accept_proposal(
         })
         next_num = task.id + 1  # use real id for next code to stay consistent
 
-    # Update proposal status (do NOT write feat_task_id — deprecated)
+    # Update proposal status — feat_task_id is NOT written (deprecated per BE-PR-010)
     proposal.status = ProposalStatus.ACCEPTED
 
     db.commit()
diff --git a/app/models/proposal.py b/app/models/proposal.py
index 80e6fbf..073fc0b 100644
--- a/app/models/proposal.py
+++ b/app/models/proposal.py
@@ -24,11 +24,21 @@ class Proposal(Base):
                           one Project.
     - ``created_by_id`` — FK to ``users.id``; the user who authored the Proposal.
                           Nullable for legacy rows created before tracking was added.
-    - ``feat_task_id``  — **DEPRECATED**.  Previously stored the single generated
-                          ``story/feature`` task id on accept.  Will be replaced by
-                          the Essential → story-task mapping (see BE-PR-008).
-                          Kept in the DB column for read-only backward compat; new
-                          code MUST NOT write to this field.
+    - ``feat_task_id``  — **DEPRECATED (BE-PR-010)**.  Previously stored the single
+                          generated ``story/feature`` task id on old-style accept.
+                          Superseded by the Essential → story-task mapping via
+                          ``Task.source_proposal_id`` / ``Task.source_essential_id``
+                          (see BE-PR-008).
+
+                          **Compat strategy:**
+                          - DB column is RETAINED for read-only backward compatibility.
+                          - Existing rows that have a value will continue to expose it
+                            via API responses (read-only).
+                          - New code MUST NOT write to this field.
+                          - Clients SHOULD migrate to ``generated_tasks`` on the
+                            Proposal detail endpoint.
+                          - Column will be dropped in a future migration once all
+                            clients have migrated.
     """
 
     __tablename__ = "proposes"  # keep DB table name for compat
@@ -60,11 +70,14 @@ class Proposal(Base):
         comment="Author of the proposal (nullable for legacy rows)",
     )
 
-    # DEPRECATED — see class docstring.  Read-only; will be removed once
-    # Essential-based accept (BE-PR-007 / BE-PR-008) is fully rolled out.
+    # DEPRECATED (BE-PR-010) — see class docstring for full compat strategy.
+    # Read-only; column retained for backward compat with legacy rows.
+    # New accept flow writes Task.source_proposal_id instead.
+    # Will be dropped in a future schema migration.
     feat_task_id = Column(
         String(64), nullable=True,
-        comment="DEPRECATED: id of the single story/feature task generated on old-style accept",
+        comment="DEPRECATED (BE-PR-010): legacy single story/feature task id. "
+                "Superseded by Task.source_proposal_id. Read-only; do not write.",
     )
 
     created_at = Column(DateTime(timezone=True), server_default=func.now())
diff --git a/app/schemas/schemas.py b/app/schemas/schemas.py
index 8835322..d1fb556 100644
--- a/app/schemas/schemas.py
+++ b/app/schemas/schemas.py
@@ -297,7 +297,7 @@ class ProposalResponse(ProposalBase):
     project_id: int
     created_by_id: Optional[int] = None
     created_by_username: Optional[str] = None
-    feat_task_id: Optional[str] = None   # DEPRECATED — will be removed after BE-PR-008
+    feat_task_id: Optional[str] = None   # DEPRECATED (BE-PR-010): legacy field, read-only. Use generated_tasks instead.
     created_at: datetime
     updated_at: Optional[datetime] = None
 
diff --git a/docs/BE-PR-010-feat-task-id-deprecation.md b/docs/BE-PR-010-feat-task-id-deprecation.md
new file mode 100644
index 0000000..46d2a5b
--- /dev/null
+++ b/docs/BE-PR-010-feat-task-id-deprecation.md
@@ -0,0 +1,62 @@
+# BE-PR-010: `feat_task_id` Deprecation & Compatibility Strategy
+
+> Date: 2026-03-30
+
+## Background
+
+The `feat_task_id` column on the `proposes` table was used by the **old** Proposal
+Accept flow to store the ID of the single `story/feature` task generated when a
+Proposal was accepted.
+
+With the new Essential-based Accept flow (BE-PR-007 / BE-PR-008), accepting a
+Proposal now generates **multiple** story tasks (one per Essential), tracked via:
+- `Task.source_proposal_id` → FK back to the Proposal
+- `Task.source_essential_id` → FK back to the specific Essential
+
+This makes `feat_task_id` obsolete.
+
+## Decision: Retain Column, Deprecate Semantics
+
+| Aspect | Decision |
+|--------|----------|
+| DB column | **Retained** — no schema migration required now |
+| Existing data | Legacy rows with a non-NULL `feat_task_id` continue to expose the value via API |
+| New writes | **Prohibited** — new accept flow does NOT write `feat_task_id` |
+| API response | Field still present in `ProposalResponse` for backward compat |
+| Client guidance | Use `generated_tasks` on the Proposal detail endpoint instead |
+| Future removal | Column will be dropped in a future migration once all clients have migrated |
+
+## Read Compatibility
+
+- `GET /projects/{id}/proposals` — returns `feat_task_id` (may be `null`)
+- `GET /projects/{id}/proposals/{id}` — returns `feat_task_id` + `generated_tasks[]`
+- `PATCH /projects/{id}/proposals/{id}` — `feat_task_id` in request body is silently ignored
+
+## Migration Path for Clients
+
+### Backend consumers
+Use `Proposal.generated_tasks` relationship (or query `Task` by `source_proposal_id`).
+
+### Frontend
+Replace `propose.feat_task_id` references with the `generated_tasks` array from
+the detail endpoint. The detail page should list all generated tasks, not just one.
+
+### CLI
+CLI does not reference `feat_task_id`. No changes needed.
+
+## Files Changed
+
+| File | Change |
+|------|--------|
+| `app/models/proposal.py` | Updated docstring & column comment with deprecation notice |
+| `app/schemas/schemas.py` | Marked `feat_task_id` field as deprecated |
+| `app/api/routers/proposals.py` | Updated comments; field still serialized read-only |
+| `tests/test_propose.py` | Updated accept tests to assert `feat_task_id is None` |
+
+## Frontend References (to be updated in FE-PR-002+)
+
+- `src/types/index.ts:139` — `feat_task_id: string | null`
+- `src/pages/ProposeDetailPage.tsx:145,180-181` — displays feat_task_id
+- `src/pages/ProposesPage.tsx:83` — displays feat_task_id in list
+
+These will be addressed when the frontend Proposal/Essential tasks are implemented.