diff --git a/.claude/settings.local.json b/.claude/settings.local.json
index 1bb1854..3b86e28 100644
--- a/.claude/settings.local.json
+++ b/.claude/settings.local.json
@@ -3,7 +3,10 @@
     "allow": [
       "Bash(uv run *)",
       "Bash(uv sync *)",
-      "Bash(git stash *)"
+      "Bash(git stash *)",
+      "Bash(python3 *)",
+      "Bash(curl -s -X POST http://localhost:8000/api/auth/login -d \"username=admin@armament.com&password=changethis\" -H \"Content-Type: application/x-www-form-urlencoded\")",
+      "Bash(PYTHONPATH=. uv run *)"
     ]
   }
 }
diff --git a/alembic/versions/1b99be8a9b1e_add_fits_tables.py b/alembic/versions/1b99be8a9b1e_add_fits_tables.py
new file mode 100644
index 0000000..6005c88
--- /dev/null
+++ b/alembic/versions/1b99be8a9b1e_add_fits_tables.py
@@ -0,0 +1,72 @@
+"""add_fits_tables
+
+Revision ID: 1b99be8a9b1e
+Revises: adf4a5b9493e
+Create Date: 2026-07-14 15:00:00.000000
+"""
+
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+# revision identifiers, used by Alembic.
+revision: str = "1b99be8a9b1e"
+down_revision: Union[str, None] = "adf4a5b9493e"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "fits",
+        sa.Column("id", sa.Integer(), nullable=False),
+        sa.Column("attachment_point_a_id", sa.Integer(), nullable=False),
+        sa.Column("attachment_point_b_id", sa.Integer(), nullable=False),
+        sa.Column("augmented_part_a_id", sa.Integer(), nullable=False),
+        sa.Column("augmented_part_b_id", sa.Integer(), nullable=False),
+        sa.Column(
+            "label",
+            sa.Enum("possible", "exact", "none", name="fitlabel"),
+            nullable=False,
+        ),
+        sa.Column(
+            "created_at", sa.DateTime(timezone=True), server_default=sa.func.now(), nullable=False
+        ),
+        sa.Column(
+            "updated_at", sa.DateTime(timezone=True), server_default=sa.func.now(), nullable=False
+        ),
+        sa.ForeignKeyConstraint(
+            ["attachment_point_a_id"], ["attachment_points.id"], ondelete="CASCADE"
+        ),
+        sa.ForeignKeyConstraint(
+            ["attachment_point_b_id"], ["attachment_points.id"], ondelete="CASCADE"
+        ),
+        sa.ForeignKeyConstraint(
+            ["augmented_part_a_id"], ["augmented_parts.id"], ondelete="CASCADE"
+        ),
+        sa.ForeignKeyConstraint(
+            ["augmented_part_b_id"], ["augmented_parts.id"], ondelete="CASCADE"
+        ),
+        sa.PrimaryKeyConstraint("id"),
+        sa.UniqueConstraint("attachment_point_a_id", "attachment_point_b_id"),
+    )
+    op.create_table(
+        "fit_notes",
+        sa.Column("id", sa.Integer(), nullable=False),
+        sa.Column("fit_id", sa.Integer(), nullable=False),
+        sa.Column("user_id", sa.Integer(), nullable=True),
+        sa.Column("username", sa.String(length=255), nullable=False),
+        sa.Column("body", sa.Text(), nullable=False),
+        sa.Column(
+            "created_at", sa.DateTime(timezone=True), server_default=sa.func.now(), nullable=False
+        ),
+        sa.ForeignKeyConstraint(["fit_id"], ["fits.id"], ondelete="CASCADE"),
+        sa.ForeignKeyConstraint(["user_id"], ["users.id"], ondelete="SET NULL"),
+        sa.PrimaryKeyConstraint("id"),
+    )
+
+
+def downgrade() -> None:
+    op.drop_table("fit_notes")
+    op.drop_table("fits")
diff --git a/app/api/fits.py b/app/api/fits.py
new file mode 100644
index 0000000..545f19f
--- /dev/null
+++ b/app/api/fits.py
@@ -0,0 +1,171 @@
+from fastapi import APIRouter, Depends, HTTPException, status
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.core.database import get_db
+from app.crud import attachment_point as attachment_point_crud
+from app.crud import audit as audit_crud
+from app.crud import fit as fit_crud
+from app.crud import fit_note as fit_note_crud
+from app.deps import require_permission
+from app.models.fit import FitLabel
+from app.models.user import User
+from app.schemas.fit import (
+    FitCreate,
+    FitNoteCreate,
+    FitNoteRead,
+    FitRead,
+    FitRecomputeResult,
+    FitUpdate,
+)
+from app.services import fit_matching
+
+router = APIRouter()
+
+
+@router.get("", response_model=list[FitRead])
+async def list_fits(
+    augmented_part_id: int | None = None,
+    label: FitLabel | None = None,
+    skip: int = 0,
+    limit: int = 100,
+    db: AsyncSession = Depends(get_db),
+    _: User = Depends(require_permission("fits:read")),
+):
+    if augmented_part_id is not None:
+        fits = await fit_crud.list_for_part(db, augmented_part_id)
+        if label is not None:
+            fits = [f for f in fits if f.label == label]
+        return fits[skip : skip + limit]
+    return await fit_crud.list_all(db, skip=skip, limit=limit, label=label)
+
+
+@router.get("/{fit_id}", response_model=FitRead)
+async def get_fit(
+    fit_id: int,
+    db: AsyncSession = Depends(get_db),
+    _: User = Depends(require_permission("fits:read")),
+):
+    fit = await fit_crud.get(db, fit_id)
+    if fit is None:
+        raise HTTPException(status_code=404, detail="Fit not found")
+    return fit
+
+
+@router.post("", response_model=FitRead, status_code=status.HTTP_201_CREATED)
+async def create_fit(
+    payload: FitCreate,
+    db: AsyncSession = Depends(get_db),
+    actor: User = Depends(require_permission("fits:write")),
+):
+    """Manually declare a fit between two attachment points; recorded as Exact."""
+    if payload.attachment_point_a_id == payload.attachment_point_b_id:
+        raise HTTPException(
+            status_code=422, detail="attachment points must be different"
+        )
+    ap_a = await attachment_point_crud.get(db, payload.attachment_point_a_id)
+    ap_b = await attachment_point_crud.get(db, payload.attachment_point_b_id)
+    if ap_a is None or ap_b is None:
+        raise HTTPException(status_code=404, detail="Attachment point not found")
+    if ap_a.augmented_part_id == ap_b.augmented_part_id:
+        raise HTTPException(
+            status_code=422, detail="a part cannot fit itself"
+        )
+    existing = await fit_crud.get_by_attachment_points(db, ap_a, ap_b)
+    if existing is not None:
+        raise HTTPException(status_code=409, detail="Fit already exists")
+
+    fit = await fit_crud.create(db, ap_a=ap_a, ap_b=ap_b, label=FitLabel.EXACT)
+    await audit_crud.log(
+        db,
+        actor=actor,
+        action="create",
+        resource_type="Fit",
+        resource_id=fit.id,
+        description=audit_crud.describe_create(
+            "Fit", fit.id, f"attachment points {ap_a.id}↔{ap_b.id}"
+        ),
+    )
+    return fit
+
+
+@router.patch("/{fit_id}", response_model=FitRead)
+async def update_fit_label(
+    fit_id: int,
+    payload: FitUpdate,
+    db: AsyncSession = Depends(get_db),
+    actor: User = Depends(require_permission("fits:write")),
+):
+    """Promote a Possible fit to Exact, or mark a pairing as None (impossible)."""
+    fit = await fit_crud.get(db, fit_id)
+    if fit is None:
+        raise HTTPException(status_code=404, detail="Fit not found")
+    fit = await fit_crud.set_label(db, fit, payload.label)
+    await audit_crud.log(
+        db,
+        actor=actor,
+        action="update",
+        resource_type="Fit",
+        resource_id=fit.id,
+        description=audit_crud.describe_update(
+            "Fit", fit.id, {"label": payload.label.value}
+        ),
+    )
+    return fit
+
+
+@router.delete("/{fit_id}", status_code=status.HTTP_204_NO_CONTENT)
+async def delete_fit(
+    fit_id: int,
+    db: AsyncSession = Depends(get_db),
+    actor: User = Depends(require_permission("fits:delete")),
+):
+    fit = await fit_crud.get(db, fit_id)
+    if fit is None:
+        raise HTTPException(status_code=404, detail="Fit not found")
+    label = f"attachment points {fit.attachment_point_a_id}↔{fit.attachment_point_b_id}"
+    await fit_crud.delete(db, fit)
+    await audit_crud.log(
+        db,
+        actor=actor,
+        action="delete",
+        resource_type="Fit",
+        resource_id=fit_id,
+        description=audit_crud.describe_delete("Fit", fit_id, label),
+    )
+
+
+@router.post("/recompute", response_model=FitRecomputeResult)
+async def recompute_fits(
+    augmented_part_id: int | None = None,
+    db: AsyncSession = Depends(get_db),
+    actor: User = Depends(require_permission("fits:write")),
+):
+    """Re-run the matching algorithm, scoped to one part or across all parts."""
+    if augmented_part_id is not None:
+        result = await fit_matching.recompute_for_part(db, augmented_part_id)
+    else:
+        result = await fit_matching.recompute_all(db)
+    await audit_crud.log(
+        db,
+        actor=actor,
+        action="update",
+        resource_type="Fit",
+        resource_id=augmented_part_id,
+        description=f"Recomputed fits: {result.created} created, {result.removed} removed",
+    )
+    return FitRecomputeResult(created=result.created, removed=result.removed)
+
+
+@router.post(
+    "/{fit_id}/notes", response_model=FitNoteRead, status_code=status.HTTP_201_CREATED
+)
+async def create_fit_note(
+    fit_id: int,
+    payload: FitNoteCreate,
+    db: AsyncSession = Depends(get_db),
+    actor: User = Depends(require_permission("fits:write")),
+):
+    fit = await fit_crud.get(db, fit_id)
+    if fit is None:
+        raise HTTPException(status_code=404, detail="Fit not found")
+    return await fit_note_crud.create(db, fit=fit, author=actor, body=payload.body)
diff --git a/app/api/router.py b/app/api/router.py
index 61fa1d4..bc5a2af 100644
--- a/app/api/router.py
+++ b/app/api/router.py
@@ -7,6 +7,7 @@ from app.api import (
     augmented_parts,
     auth,
     clamp_connections,
+    fits,
     imported_parts,
     iso_threads,
     manufacturers,
@@ -64,3 +65,4 @@ api_router.include_router(
     augmented_parts.router, prefix="/augmented-parts", tags=["augmented-parts"]
 )
 api_router.include_router(audit_log.router, prefix="/audit-log", tags=["audit-log"])
+api_router.include_router(fits.router, prefix="/fits", tags=["fits"])
diff --git a/app/crud/fit.py b/app/crud/fit.py
new file mode 100644
index 0000000..2402514
--- /dev/null
+++ b/app/crud/fit.py
@@ -0,0 +1,98 @@
+from sqlalchemy import or_, select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.models.attachment_point import AttachmentPoint
+from app.models.fit import Fit, FitLabel
+
+# "updated_at" is included because it's a server-side onupdate default: after an
+# UPDATE, SQLAlchemy doesn't know its new value unless it's named explicitly here
+# too, and access to an unrefreshed attribute would otherwise trigger a lazy load
+# outside of an awaited context (MissingGreenlet) during response serialization.
+_RELATIONSHIPS = ["attachment_point_a", "attachment_point_b", "notes", "updated_at"]
+
+
+def _ordered(
+    ap_a: AttachmentPoint, ap_b: AttachmentPoint
+) -> tuple[AttachmentPoint, AttachmentPoint]:
+    """Canonicalize so attachment_point_a.id < attachment_point_b.id.
+
+    Keeps a single row per unordered pair of attachment points.
+    """
+    return (ap_a, ap_b) if ap_a.id < ap_b.id else (ap_b, ap_a)
+
+
+async def get(db: AsyncSession, fit_id: int) -> Fit | None:
+    return await db.get(Fit, fit_id)
+
+
+async def get_by_attachment_points(
+    db: AsyncSession, ap_a: AttachmentPoint, ap_b: AttachmentPoint
+) -> Fit | None:
+    first, second = _ordered(ap_a, ap_b)
+    result = await db.execute(
+        select(Fit).where(
+            Fit.attachment_point_a_id == first.id,
+            Fit.attachment_point_b_id == second.id,
+        )
+    )
+    return result.scalar_one_or_none()
+
+
+async def list_for_part(db: AsyncSession, augmented_part_id: int) -> list[Fit]:
+    result = await db.execute(
+        select(Fit)
+        .where(
+            or_(
+                Fit.augmented_part_a_id == augmented_part_id,
+                Fit.augmented_part_b_id == augmented_part_id,
+            )
+        )
+        .order_by(Fit.id)
+    )
+    return list(result.scalars().all())
+
+
+async def list_all(
+    db: AsyncSession,
+    skip: int = 0,
+    limit: int = 100,
+    label: FitLabel | None = None,
+) -> list[Fit]:
+    q = select(Fit)
+    if label is not None:
+        q = q.where(Fit.label == label)
+    result = await db.execute(q.order_by(Fit.id).offset(skip).limit(limit))
+    return list(result.scalars().all())
+
+
+async def create(
+    db: AsyncSession,
+    *,
+    ap_a: AttachmentPoint,
+    ap_b: AttachmentPoint,
+    label: FitLabel,
+) -> Fit:
+    first, second = _ordered(ap_a, ap_b)
+    fit = Fit(
+        attachment_point_a_id=first.id,
+        attachment_point_b_id=second.id,
+        augmented_part_a_id=first.augmented_part_id,
+        augmented_part_b_id=second.augmented_part_id,
+        label=label,
+    )
+    db.add(fit)
+    await db.commit()
+    await db.refresh(fit, attribute_names=_RELATIONSHIPS)
+    return fit
+
+
+async def set_label(db: AsyncSession, fit: Fit, label: FitLabel) -> Fit:
+    fit.label = label
+    await db.commit()
+    await db.refresh(fit, attribute_names=_RELATIONSHIPS)
+    return fit
+
+
+async def delete(db: AsyncSession, fit: Fit) -> None:
+    await db.delete(fit)
+    await db.commit()
diff --git a/app/crud/fit_note.py b/app/crud/fit_note.py
new file mode 100644
index 0000000..ccbeec1
--- /dev/null
+++ b/app/crud/fit_note.py
@@ -0,0 +1,17 @@
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.models.fit import Fit, FitNote
+from app.models.user import User
+
+
+async def create(db: AsyncSession, *, fit: Fit, author: User, body: str) -> FitNote:
+    note = FitNote(
+        fit_id=fit.id,
+        user_id=author.id,
+        username=author.full_name or author.email,
+        body=body,
+    )
+    db.add(note)
+    await db.commit()
+    await db.refresh(note)
+    return note
diff --git a/app/models/__init__.py b/app/models/__init__.py
index e5d5c71..9870033 100644
--- a/app/models/__init__.py
+++ b/app/models/__init__.py
@@ -10,6 +10,7 @@ from app.models.attachment_point_name import AttachmentPointName
 from app.models.audit_log import AuditLog
 from app.models.augmented_part import AugmentedPart
 from app.models.clamp_connection import ClampConnection
+from app.models.fit import Fit, FitLabel, FitNote
 from app.models.imported_part import ImportedPart
 from app.models.imported_part_config import ImportedPartConfig
 from app.models.iso_thread import IsoThread
@@ -31,6 +32,9 @@ __all__ = [
     "AuditLog",
     "AugmentedPart",
     "ClampConnection",
+    "Fit",
+    "FitLabel",
+    "FitNote",
     "ImportedPart",
     "ImportedPartConfig",
     "IsoThread",
diff --git a/app/models/fit.py b/app/models/fit.py
new file mode 100644
index 0000000..ff37d9e
--- /dev/null
+++ b/app/models/fit.py
@@ -0,0 +1,89 @@
+import enum
+from datetime import datetime
+
+from sqlalchemy import DateTime, Enum, ForeignKey, String, Text, UniqueConstraint, func
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from app.core.database import Base
+
+
+class FitLabel(str, enum.Enum):
+    POSSIBLE = "possible"
+    EXACT = "exact"
+    NONE = "none"
+
+
+class Fit(Base):
+    """A candidate or confirmed fit between two attachment points on different parts."""
+
+    __tablename__ = "fits"
+    __table_args__ = (
+        UniqueConstraint("attachment_point_a_id", "attachment_point_b_id"),
+    )
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+
+    # Canonical ordering (attachment_point_a_id < attachment_point_b_id) is enforced
+    # by app.crud.fit so an unordered pair only ever has one row.
+    attachment_point_a_id: Mapped[int] = mapped_column(
+        ForeignKey("attachment_points.id", ondelete="CASCADE"), nullable=False
+    )
+    attachment_point_b_id: Mapped[int] = mapped_column(
+        ForeignKey("attachment_points.id", ondelete="CASCADE"), nullable=False
+    )
+
+    # Denormalized from the attachment points' augmented_part_id, for querying
+    # "fits for this part" without joining through attachment_points.
+    augmented_part_a_id: Mapped[int] = mapped_column(
+        ForeignKey("augmented_parts.id", ondelete="CASCADE"), nullable=False
+    )
+    augmented_part_b_id: Mapped[int] = mapped_column(
+        ForeignKey("augmented_parts.id", ondelete="CASCADE"), nullable=False
+    )
+
+    label: Mapped[FitLabel] = mapped_column(
+        Enum(FitLabel, values_callable=lambda enum_cls: [e.value for e in enum_cls]),
+        nullable=False,
+        default=FitLabel.POSSIBLE,
+    )
+
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now()
+    )
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now(), onupdate=func.now()
+    )
+
+    attachment_point_a = relationship(
+        "AttachmentPoint", foreign_keys=[attachment_point_a_id], lazy="selectin"
+    )
+    attachment_point_b = relationship(
+        "AttachmentPoint", foreign_keys=[attachment_point_b_id], lazy="selectin"
+    )
+    notes: Mapped[list["FitNote"]] = relationship(
+        back_populates="fit",
+        lazy="selectin",
+        order_by="FitNote.created_at",
+        cascade="all, delete-orphan",
+    )
+
+
+class FitNote(Base):
+    __tablename__ = "fit_notes"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    fit_id: Mapped[int] = mapped_column(
+        ForeignKey("fits.id", ondelete="CASCADE"), nullable=False
+    )
+    user_id: Mapped[int | None] = mapped_column(
+        ForeignKey("users.id", ondelete="SET NULL"), nullable=True
+    )
+    # Denormalized snapshot of who wrote the note (mirrors AuditLog.user_email),
+    # so the note stays attributable even if the user is later deleted.
+    username: Mapped[str] = mapped_column(String(255), nullable=False)
+    body: Mapped[str] = mapped_column(Text, nullable=False)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now()
+    )
+
+    fit: Mapped[Fit] = relationship(back_populates="notes")
diff --git a/app/schemas/fit.py b/app/schemas/fit.py
new file mode 100644
index 0000000..7ae5128
--- /dev/null
+++ b/app/schemas/fit.py
@@ -0,0 +1,60 @@
+from datetime import datetime
+
+from pydantic import BaseModel, ConfigDict, field_validator
+
+from app.models.fit import FitLabel
+from app.schemas.attachment_point import AttachmentPointRead
+
+
+class FitNoteCreate(BaseModel):
+    body: str
+
+    @field_validator("body")
+    @classmethod
+    def validate_body(cls, v: str) -> str:
+        if not v.strip():
+            raise ValueError("body must not be blank")
+        return v.strip()
+
+
+class FitNoteRead(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+
+    id: int
+    fit_id: int
+    user_id: int | None
+    username: str
+    body: str
+    created_at: datetime
+
+
+class FitCreate(BaseModel):
+    """Manually-declared fit; recorded as Exact regardless of the match algorithm."""
+
+    attachment_point_a_id: int
+    attachment_point_b_id: int
+
+
+class FitUpdate(BaseModel):
+    label: FitLabel
+
+
+class FitRead(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+
+    id: int
+    augmented_part_a_id: int
+    augmented_part_b_id: int
+    attachment_point_a_id: int
+    attachment_point_b_id: int
+    attachment_point_a: AttachmentPointRead
+    attachment_point_b: AttachmentPointRead
+    label: FitLabel
+    created_at: datetime
+    updated_at: datetime
+    notes: list[FitNoteRead] = []
+
+
+class FitRecomputeResult(BaseModel):
+    created: int
+    removed: int
diff --git a/app/services/fit_matching.py b/app/services/fit_matching.py
new file mode 100644
index 0000000..7d56775
--- /dev/null
+++ b/app/services/fit_matching.py
@@ -0,0 +1,201 @@
+"""Detect "Possible" fits between AugmentedParts.
+
+A part fits another part if:
+  - They are different PartTypes
+  - They have two attachment points that:
+      - Have different labels (AttachmentPointName)
+      - either:
+          - Are an ISO or UNC thread that exactly match in type and dimensions
+          - Are a Static Connection paired with a Clamp Connection where the
+            static dimension falls within the range of the clamp connection
+
+Matches found here are recorded as Fit rows labeled Possible. Exact (manually
+entered, or promoted from Possible) and None (explicitly marked impossible)
+labels are user-asserted and are never created or removed by this algorithm.
+"""
+
+from collections import defaultdict
+from dataclasses import dataclass
+
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.crud import fit as fit_crud
+from app.models.attachment_point import AttachmentPoint
+from app.models.augmented_part import AugmentedPart
+from app.models.clamp_connection import ClampConnection
+from app.models.fit import FitLabel
+from app.models.static_connection import StaticConnection
+
+MM_PER_INCH = 25.4
+
+
+def _iso_match(ap_a: AttachmentPoint, ap_b: AttachmentPoint) -> bool:
+    a, b = ap_a.iso_thread, ap_b.iso_thread
+    if a is None or b is None:
+        return False
+    return a.diameter_mm == b.diameter_mm and a.pitch_mm == b.pitch_mm
+
+
+def _unc_match(ap_a: AttachmentPoint, ap_b: AttachmentPoint) -> bool:
+    a, b = ap_a.unc_thread, ap_b.unc_thread
+    if a is None or b is None:
+        return False
+    return (
+        a.diameter_fraction == b.diameter_fraction
+        and a.tpi == b.tpi
+        and a.series == b.series
+    )
+
+
+def _clamp_range_mm(clamp: ClampConnection) -> tuple[float | None, float | None]:
+    if clamp.min_mm is not None or clamp.max_mm is not None:
+        return clamp.min_mm, clamp.max_mm
+    min_mm = clamp.min_in * MM_PER_INCH if clamp.min_in is not None else None
+    max_mm = clamp.max_in * MM_PER_INCH if clamp.max_in is not None else None
+    return min_mm, max_mm
+
+
+def _static_in_clamp_range(static: StaticConnection, clamp: ClampConnection) -> bool:
+    min_mm, max_mm = _clamp_range_mm(clamp)
+    if min_mm is None and max_mm is None:
+        return False
+    if min_mm is not None and static.od_mm < min_mm:
+        return False
+    if max_mm is not None and static.od_mm > max_mm:
+        return False
+    return True
+
+
+def _static_clamp_match(ap_a: AttachmentPoint, ap_b: AttachmentPoint) -> bool:
+    if ap_a.static_connection is not None and ap_b.clamp_connection is not None:
+        return _static_in_clamp_range(ap_a.static_connection, ap_b.clamp_connection)
+    if ap_b.static_connection is not None and ap_a.clamp_connection is not None:
+        return _static_in_clamp_range(ap_b.static_connection, ap_a.clamp_connection)
+    return False
+
+
+def attachment_points_match(ap_a: AttachmentPoint, ap_b: AttachmentPoint) -> bool:
+    """True if these two attachment points satisfy the fit-matching algorithm."""
+    if ap_a.attachment_point_name_id == ap_b.attachment_point_name_id:
+        return False
+    return (
+        _iso_match(ap_a, ap_b)
+        or _unc_match(ap_a, ap_b)
+        or _static_clamp_match(ap_a, ap_b)
+    )
+
+
+def parts_could_fit(part_a: AugmentedPart, part_b: AugmentedPart) -> bool:
+    return part_a.id != part_b.id and part_a.part_type_id != part_b.part_type_id
+
+
+@dataclass
+class RecomputeResult:
+    created: int = 0
+    removed: int = 0
+
+    def __iadd__(self, other: "RecomputeResult") -> "RecomputeResult":
+        self.created += other.created
+        self.removed += other.removed
+        return self
+
+
+async def _all_parts_by_id(db: AsyncSession) -> dict[int, AugmentedPart]:
+    result = await db.execute(select(AugmentedPart))
+    return {p.id: p for p in result.scalars().all()}
+
+
+async def _attachment_points_by_part(
+    db: AsyncSession,
+) -> dict[int, list[AttachmentPoint]]:
+    result = await db.execute(select(AttachmentPoint))
+    by_part: dict[int, list[AttachmentPoint]] = defaultdict(list)
+    for ap in result.scalars().all():
+        by_part[ap.augmented_part_id].append(ap)
+    return by_part
+
+
+def _canonical_pair(a_id: int, b_id: int) -> tuple[int, int]:
+    return (a_id, b_id) if a_id < b_id else (b_id, a_id)
+
+
+async def _reconcile(
+    db: AsyncSession,
+    *,
+    scoped_part_ids: set[int],
+    parts_by_id: dict[int, AugmentedPart],
+    points_by_part: dict[int, list[AttachmentPoint]],
+) -> RecomputeResult:
+    """Reconcile Possible fits for the given parts against every other part."""
+    candidate_pairs: set[tuple[int, int]] = set()
+    for part_id in scoped_part_ids:
+        part = parts_by_id.get(part_id)
+        if part is None:
+            continue
+        own_points = points_by_part.get(part_id, [])
+        if not own_points:
+            continue
+        for other_id, other in parts_by_id.items():
+            if not parts_could_fit(part, other):
+                continue
+            for own_ap in own_points:
+                for other_ap in points_by_part.get(other_id, []):
+                    if attachment_points_match(own_ap, other_ap):
+                        candidate_pairs.add(
+                            _canonical_pair(own_ap.id, other_ap.id)
+                        )
+
+    result = RecomputeResult()
+
+    existing_fits = []
+    for part_id in scoped_part_ids:
+        existing_fits.extend(await fit_crud.list_for_part(db, part_id))
+    existing_by_pair = {
+        (f.attachment_point_a_id, f.attachment_point_b_id): f for f in existing_fits
+    }
+
+    for pair, existing in existing_by_pair.items():
+        if existing.label == FitLabel.POSSIBLE and pair not in candidate_pairs:
+            await fit_crud.delete(db, existing)
+            result.removed += 1
+
+    for ap_a_id, ap_b_id in candidate_pairs:
+        if (ap_a_id, ap_b_id) in existing_by_pair:
+            continue
+        ap_a = await db.get(AttachmentPoint, ap_a_id)
+        ap_b = await db.get(AttachmentPoint, ap_b_id)
+        await fit_crud.create(db, ap_a=ap_a, ap_b=ap_b, label=FitLabel.POSSIBLE)
+        result.created += 1
+
+    return result
+
+
+async def recompute_for_part(db: AsyncSession, augmented_part_id: int) -> RecomputeResult:
+    """Reconcile Possible fits involving this part against the current algorithm.
+
+    Creates Possible fits for newly-matching attachment point pairs, and removes
+    previously-algorithmic Possible fits that no longer match. Exact/None labels
+    are user-asserted and are never touched.
+    """
+    parts_by_id = await _all_parts_by_id(db)
+    if augmented_part_id not in parts_by_id:
+        return RecomputeResult()
+    points_by_part = await _attachment_points_by_part(db)
+    return await _reconcile(
+        db,
+        scoped_part_ids={augmented_part_id},
+        parts_by_id=parts_by_id,
+        points_by_part=points_by_part,
+    )
+
+
+async def recompute_all(db: AsyncSession) -> RecomputeResult:
+    parts_by_id = await _all_parts_by_id(db)
+    points_by_part = await _attachment_points_by_part(db)
+    return await _reconcile(
+        db,
+        scoped_part_ids=set(parts_by_id),
+        parts_by_id=parts_by_id,
+        points_by_part=points_by_part,
+    )
diff --git a/app/static/css/style.css b/app/static/css/style.css
index bbb7d12..fb41736 100644
--- a/app/static/css/style.css
+++ b/app/static/css/style.css
@@ -460,6 +460,46 @@ textarea:focus {
 .ap-item-conn { flex: 1; }
 .ap-item .row-actions { margin-left: auto; }
 
+.fit-item {
+  padding: 0.5rem 0;
+  border-bottom: 1px solid var(--border);
+  font-size: 0.9rem;
+}
+
+.fit-item:last-of-type { border-bottom: none; }
+
+.fit-item-main {
+  display: flex;
+  align-items: center;
+  gap: 0.5rem;
+  flex-wrap: wrap;
+  margin-bottom: 0.3rem;
+}
+
+.fit-item-arrow { color: var(--muted); }
+
+.fit-badge {
+  font-size: 0.7rem;
+  text-transform: uppercase;
+  letter-spacing: 0.03em;
+  padding: 0.1rem 0.5rem;
+  border-radius: 999px;
+  font-weight: 600;
+}
+
+.fit-badge-possible { background: var(--surface-2); color: var(--muted); }
+.fit-badge-exact { background: #16a34a26; color: #16a34a; }
+.fit-badge-none { background: #dc262626; color: #dc2626; }
+
+.fit-notes { margin-top: 0.4rem; padding-left: 0.25rem; }
+
+.fit-note { font-size: 0.85rem; margin-bottom: 0.3rem; }
+.fit-note-meta { color: var(--muted); font-size: 0.75rem; }
+.fit-note-body { margin: 0.1rem 0 0; }
+
+.fit-note-form { margin-top: 0.3rem; }
+.fit-note-form input[type="text"] { flex: 1; min-width: 12rem; }
+
 /* Pagination */
 .pagination {
   display: flex;
diff --git a/app/templates/admin/_augmented_part_fit_row.html b/app/templates/admin/_augmented_part_fit_row.html
new file mode 100644
index 0000000..15f014f
--- /dev/null
+++ b/app/templates/admin/_augmented_part_fit_row.html
@@ -0,0 +1,62 @@
+{% macro conn_desc(ap) -%}
+  {%- if ap.iso_thread -%}
+    ISO M{{ ap.iso_thread.diameter_mm }}×{{ ap.iso_thread.pitch_mm }}
+  {%- elif ap.unc_thread -%}
+    UNC {{ ap.unc_thread.diameter_fraction }}-{{ ap.unc_thread.tpi }} {{ ap.unc_thread.series.value }}
+  {%- elif ap.clamp_connection -%}
+    Clamp {{ ap.clamp_connection.min_mm if ap.clamp_connection.min_mm is not none else ap.clamp_connection.min_in }}–{{ ap.clamp_connection.max_mm if ap.clamp_connection.max_mm is not none else ap.clamp_connection.max_in }}
+  {%- elif ap.static_connection -%}
+    Static {{ ap.static_connection.od_mm }} mm OD
+  {%- else -%}
+    —
+  {%- endif -%}
+{%- endmacro %}
+
+<div id="fit-{{ fit.id }}" class="fit-item">
+  <div class="fit-item-main">
+    <span class="fit-badge fit-badge-{{ fit.label.value }}">{{ fit.label.value | capitalize }}</span>
+    <span class="fit-item-side"><strong>{{ own_ap.attachment_point_name.name }}</strong> ({{ conn_desc(own_ap) }})</span>
+    <span class="fit-item-arrow">↔</span>
+    <span class="fit-item-side">{{ other_part.description if other_part else "Unknown part" }} — <strong>{{ other_ap.attachment_point_name.name }}</strong> ({{ conn_desc(other_ap) }})</span>
+  </div>
+  <div class="row-actions">
+    {% if fit.label.value != "exact" %}
+      <button class="btn btn-ghost"
+              hx-post="/admin/augmented-parts/{{ augmented_part_id }}/fits/{{ fit.id }}/label"
+              hx-vals='{"label": "exact"}'
+              hx-target="#fit-{{ fit.id }}" hx-swap="outerHTML">Mark Exact</button>
+    {% endif %}
+    {% if fit.label.value != "possible" %}
+      <button class="btn btn-ghost"
+              hx-post="/admin/augmented-parts/{{ augmented_part_id }}/fits/{{ fit.id }}/label"
+              hx-vals='{"label": "possible"}'
+              hx-target="#fit-{{ fit.id }}" hx-swap="outerHTML">Mark Possible</button>
+    {% endif %}
+    {% if fit.label.value != "none" %}
+      <button class="btn btn-ghost"
+              hx-post="/admin/augmented-parts/{{ augmented_part_id }}/fits/{{ fit.id }}/label"
+              hx-vals='{"label": "none"}'
+              hx-target="#fit-{{ fit.id }}" hx-swap="outerHTML">Mark Impossible</button>
+    {% endif %}
+    <button class="btn btn-danger"
+            hx-delete="/admin/augmented-parts/{{ augmented_part_id }}/fits/{{ fit.id }}"
+            hx-target="#ap-{{ augmented_part_id }}-fits"
+            hx-swap="outerHTML"
+            hx-confirm="Delete this fit?">Delete</button>
+  </div>
+
+  <div class="fit-notes">
+    {% for note in fit.notes %}
+      <div class="fit-note">
+        <span class="fit-note-meta">{{ note.username }} · {{ note.created_at.strftime("%Y-%m-%d %H:%M") }}</span>
+        <p class="fit-note-body">{{ note.body }}</p>
+      </div>
+    {% endfor %}
+    <form hx-post="/admin/augmented-parts/{{ augmented_part_id }}/fits/{{ fit.id }}/notes"
+          hx-target="#fit-{{ fit.id }}" hx-swap="outerHTML"
+          class="inline-edit fit-note-form">
+      <input type="text" name="body" placeholder="Add a note…" required>
+      <button type="submit" class="btn btn-ghost">Add note</button>
+    </form>
+  </div>
+</div>
diff --git a/app/templates/admin/_augmented_part_fits.html b/app/templates/admin/_augmented_part_fits.html
new file mode 100644
index 0000000..cf24b45
--- /dev/null
+++ b/app/templates/admin/_augmented_part_fits.html
@@ -0,0 +1,49 @@
+<div id="ap-{{ augmented_part_id }}-fits" class="ap-inline-section">
+  <h3>Fits</h3>
+
+  {% if message %}<p class="alert alert-success">{{ message }}</p>{% endif %}
+  {% if error %}<p class="alert alert-error">{{ error }}</p>{% endif %}
+
+  {% for view in fit_views %}
+    {% with fit=view.fit, own_ap=view.own_ap, other_ap=view.other_ap, other_part=view.other_part %}
+      {% include "admin/_augmented_part_fit_row.html" %}
+    {% endwith %}
+  {% else %}
+    <p class="muted" style="font-size:0.85rem;margin:0 0 0.5rem">No fits recorded yet.</p>
+  {% endfor %}
+
+  <div class="row-actions" style="margin:0.5rem 0">
+    <form hx-post="/admin/augmented-parts/{{ augmented_part_id }}/fits/recompute"
+          hx-target="#ap-{{ augmented_part_id }}-fits" hx-swap="outerHTML">
+      <button type="submit" class="btn btn-ghost">Recompute possible fits</button>
+    </form>
+  </div>
+
+  <form hx-post="/admin/augmented-parts/{{ augmented_part_id }}/fits"
+        hx-target="#ap-{{ augmented_part_id }}-fits"
+        hx-swap="outerHTML"
+        class="inline-edit" style="margin-top:0.5rem">
+    <label>This part's point
+      <select name="own_attachment_point_id" required>
+        <option value="">— select —</option>
+        {% for p in own_points %}
+          <option value="{{ p.id }}">{{ p.attachment_point_name.name }}</option>
+        {% endfor %}
+      </select>
+    </label>
+    <label>Fits with part
+      <select name="other_augmented_part_id" required
+              hx-get="/admin/augmented-parts/{{ augmented_part_id }}/fits/other-points"
+              hx-target="#ap-{{ augmented_part_id }}-fit-other-points"
+              hx-trigger="change"
+              hx-include="[name='other_augmented_part_id']">
+        <option value="">— select —</option>
+        {% for p in other_parts %}
+          <option value="{{ p.id }}">{{ p.description | truncate(40) }}</option>
+        {% endfor %}
+      </select>
+    </label>
+    <div id="ap-{{ augmented_part_id }}-fit-other-points"></div>
+    <button type="submit" class="btn btn-primary">Add fit</button>
+  </form>
+</div>
diff --git a/app/templates/admin/_augmented_part_row_edit.html b/app/templates/admin/_augmented_part_row_edit.html
index 773900b..8332e24 100644
--- a/app/templates/admin/_augmented_part_row_edit.html
+++ b/app/templates/admin/_augmented_part_row_edit.html
@@ -58,5 +58,10 @@
          hx-trigger="load"
          hx-swap="outerHTML">
     </div>
+
+    <div hx-get="/admin/augmented-parts/{{ ap.id }}/fits"
+         hx-trigger="load"
+         hx-swap="outerHTML">
+    </div>
   </td>
 </tr>
diff --git a/app/templates/admin/_fit_other_points_select.html b/app/templates/admin/_fit_other_points_select.html
new file mode 100644
index 0000000..352e232
--- /dev/null
+++ b/app/templates/admin/_fit_other_points_select.html
@@ -0,0 +1,6 @@
+<select name="other_attachment_point_id" required>
+  <option value="">— select —</option>
+  {% for p in points %}
+    <option value="{{ p.id }}">{{ p.attachment_point_name.name }}</option>
+  {% endfor %}
+</select>
diff --git a/app/templates/admin/augmented_parts.html b/app/templates/admin/augmented_parts.html
index a6a6140..ebe9fea 100644
--- a/app/templates/admin/augmented_parts.html
+++ b/app/templates/admin/augmented_parts.html
@@ -65,6 +65,17 @@
     </div>
   </div>
 
+  <div class="card">
+    <h2>Recompute Fits</h2>
+    <p class="muted">Re-runs the fit-matching algorithm across every part, adding newly Possible fits and pruning stale ones. Exact and Impossible labels are never changed.</p>
+    <div class="row-actions">
+      <form hx-post="/admin/augmented-parts/fits/recompute" hx-target="#fits-recompute-result" hx-swap="innerHTML">
+        <button type="submit" class="btn btn-primary">Recompute all fits</button>
+      </form>
+    </div>
+    <p id="fits-recompute-result" class="muted"></p>
+  </div>
+
   {% include "admin/_augmented_parts_table.html" %}
 </main>
 {% endblock %}
diff --git a/app/web/admin.py b/app/web/admin.py
index ef9de53..fc8b2eb 100644
--- a/app/web/admin.py
+++ b/app/web/admin.py
@@ -21,14 +21,18 @@ from app.crud import scope_model as scope_model_crud
 from app.crud import attachment_point as attachment_point_crud
 from app.crud import attachment_point_name as apn_crud
 from app.crud import clamp_connection as clamp_connection_crud
+from app.crud import fit as fit_crud
+from app.crud import fit_note as fit_note_crud
 from app.crud import static_connection as static_connection_crud
 from app.crud import sub_part_type as sub_part_type_crud
 from app.crud import unc_thread as unc_thread_crud
 from app.crud import user as user_crud
 from app.deps import get_current_web_user
+from app.models.fit import FitLabel
 from app.models.unc_thread import UncSeries
 from app.models.user import User
 from app.services import augmented_part_sync as augmented_part_sync_service
+from app.services import fit_matching as fit_matching_service
 from app.services import odata as odata_service
 from app.web.templates import templates
 
@@ -1978,6 +1982,258 @@ async def delete_augmented_part_attachment_point(
     )
 
 
+# --- Fits ----------------------------------------------------------------
+
+
+async def _fit_view(db: AsyncSession, fit, part_id: int) -> dict:
+    if fit.augmented_part_a_id == part_id:
+        own_ap, other_ap, other_part_id = (
+            fit.attachment_point_a,
+            fit.attachment_point_b,
+            fit.augmented_part_b_id,
+        )
+    else:
+        own_ap, other_ap, other_part_id = (
+            fit.attachment_point_b,
+            fit.attachment_point_a,
+            fit.augmented_part_a_id,
+        )
+    other_part = await augmented_part_crud.get(db, other_part_id)
+    return {"fit": fit, "own_ap": own_ap, "other_ap": other_ap, "other_part": other_part}
+
+
+async def _augmented_part_fits_ctx(
+    db: AsyncSession,
+    augmented_part_id: int,
+    *,
+    message: str | None = None,
+    error: str | None = None,
+) -> dict:
+    fits = await fit_crud.list_for_part(db, augmented_part_id)
+    other_parts = [
+        p
+        for p in await augmented_part_crud.list_all(db, limit=500)
+        if p.id != augmented_part_id
+    ]
+    return {
+        "augmented_part_id": augmented_part_id,
+        "fit_views": [await _fit_view(db, f, augmented_part_id) for f in fits],
+        "own_points": await attachment_point_crud.list_for_augmented_part(
+            db, augmented_part_id
+        ),
+        "other_parts": other_parts,
+        "message": message,
+        "error": error,
+    }
+
+
+@router.get("/augmented-parts/{augmented_part_id}/fits", response_class=HTMLResponse)
+async def augmented_part_fits(
+    request: Request,
+    augmented_part_id: int,
+    db: AsyncSession = Depends(get_db),
+    _: User = Depends(web_require("fits:read")),
+):
+    ctx = await _augmented_part_fits_ctx(db, augmented_part_id)
+    return templates.TemplateResponse(request, "admin/_augmented_part_fits.html", ctx)
+
+
+@router.post(
+    "/augmented-parts/{augmented_part_id}/fits/recompute", response_class=HTMLResponse
+)
+async def recompute_augmented_part_fits(
+    request: Request,
+    augmented_part_id: int,
+    db: AsyncSession = Depends(get_db),
+    actor: User = Depends(web_require("fits:write")),
+):
+    result = await fit_matching_service.recompute_for_part(db, augmented_part_id)
+    await audit_crud.log(
+        db,
+        actor=actor,
+        action="update",
+        resource_type="Fit",
+        resource_id=augmented_part_id,
+        description=(
+            f"Recomputed fits for AugmentedPart #{augmented_part_id}: "
+            f"{result.created} created, {result.removed} removed"
+        ),
+    )
+    ctx = await _augmented_part_fits_ctx(
+        db,
+        augmented_part_id,
+        message=f"{result.created} created, {result.removed} removed.",
+    )
+    return templates.TemplateResponse(request, "admin/_augmented_part_fits.html", ctx)
+
+
+@router.get(
+    "/augmented-parts/{augmented_part_id}/fits/other-points",
+    response_class=HTMLResponse,
+)
+async def fit_other_points(
+    request: Request,
+    augmented_part_id: int,
+    other_augmented_part_id: str = "",
+    db: AsyncSession = Depends(get_db),
+    _: User = Depends(web_require("fits:write")),
+):
+    points = (
+        await attachment_point_crud.list_for_augmented_part(
+            db, int(other_augmented_part_id)
+        )
+        if other_augmented_part_id
+        else []
+    )
+    return templates.TemplateResponse(
+        request, "admin/_fit_other_points_select.html", {"points": points}
+    )
+
+
+@router.post("/augmented-parts/{augmented_part_id}/fits", response_class=HTMLResponse)
+async def create_augmented_part_fit(
+    request: Request,
+    augmented_part_id: int,
+    own_attachment_point_id: int = Form(...),
+    other_attachment_point_id: str = Form(""),
+    db: AsyncSession = Depends(get_db),
+    actor: User = Depends(web_require("fits:write")),
+):
+    error = None
+    if not other_attachment_point_id:
+        error = "Select the other part's attachment point."
+    else:
+        ap_a = await attachment_point_crud.get(db, own_attachment_point_id)
+        ap_b = await attachment_point_crud.get(db, int(other_attachment_point_id))
+        if ap_a is None or ap_b is None:
+            error = "Attachment point not found."
+        elif ap_a.augmented_part_id == ap_b.augmented_part_id:
+            error = "A part cannot fit itself."
+        elif await fit_crud.get_by_attachment_points(db, ap_a, ap_b) is not None:
+            error = "That fit already exists."
+        else:
+            fit = await fit_crud.create(db, ap_a=ap_a, ap_b=ap_b, label=FitLabel.EXACT)
+            await audit_crud.log(
+                db,
+                actor=actor,
+                action="create",
+                resource_type="Fit",
+                resource_id=fit.id,
+                description=audit_crud.describe_create(
+                    "Fit", fit.id, f"attachment points {ap_a.id}↔{ap_b.id}"
+                ),
+            )
+    ctx = await _augmented_part_fits_ctx(db, augmented_part_id, error=error)
+    return templates.TemplateResponse(request, "admin/_augmented_part_fits.html", ctx)
+
+
+@router.post(
+    "/augmented-parts/{augmented_part_id}/fits/{fit_id}/label",
+    response_class=HTMLResponse,
+)
+async def update_augmented_part_fit_label(
+    request: Request,
+    augmented_part_id: int,
+    fit_id: int,
+    label: FitLabel = Form(...),
+    db: AsyncSession = Depends(get_db),
+    actor: User = Depends(web_require("fits:write")),
+):
+    fit = await fit_crud.get(db, fit_id)
+    if fit is None:
+        raise HTTPException(status_code=404, detail="Fit not found")
+    fit = await fit_crud.set_label(db, fit, label)
+    await audit_crud.log(
+        db,
+        actor=actor,
+        action="update",
+        resource_type="Fit",
+        resource_id=fit.id,
+        description=audit_crud.describe_update("Fit", fit.id, {"label": label.value}),
+    )
+    view = await _fit_view(db, fit, augmented_part_id)
+    return templates.TemplateResponse(
+        request,
+        "admin/_augmented_part_fit_row.html",
+        {"augmented_part_id": augmented_part_id, **view},
+    )
+
+
+@router.post(
+    "/augmented-parts/{augmented_part_id}/fits/{fit_id}/notes",
+    response_class=HTMLResponse,
+)
+async def create_augmented_part_fit_note(
+    request: Request,
+    augmented_part_id: int,
+    fit_id: int,
+    body: str = Form(...),
+    db: AsyncSession = Depends(get_db),
+    actor: User = Depends(web_require("fits:write")),
+):
+    fit = await fit_crud.get(db, fit_id)
+    if fit is None:
+        raise HTTPException(status_code=404, detail="Fit not found")
+    if body.strip():
+        await fit_note_crud.create(db, fit=fit, author=actor, body=body.strip())
+        await db.refresh(fit, attribute_names=["notes"])
+    view = await _fit_view(db, fit, augmented_part_id)
+    return templates.TemplateResponse(
+        request,
+        "admin/_augmented_part_fit_row.html",
+        {"augmented_part_id": augmented_part_id, **view},
+    )
+
+
+@router.delete(
+    "/augmented-parts/{augmented_part_id}/fits/{fit_id}", response_class=HTMLResponse
+)
+async def delete_augmented_part_fit(
+    request: Request,
+    augmented_part_id: int,
+    fit_id: int,
+    db: AsyncSession = Depends(get_db),
+    actor: User = Depends(web_require("fits:delete")),
+):
+    fit = await fit_crud.get(db, fit_id)
+    if fit is not None:
+        label = f"attachment points {fit.attachment_point_a_id}↔{fit.attachment_point_b_id}"
+        await fit_crud.delete(db, fit)
+        await audit_crud.log(
+            db,
+            actor=actor,
+            action="delete",
+            resource_type="Fit",
+            resource_id=fit_id,
+            description=audit_crud.describe_delete("Fit", fit_id, label),
+        )
+    ctx = await _augmented_part_fits_ctx(db, augmented_part_id)
+    return templates.TemplateResponse(request, "admin/_augmented_part_fits.html", ctx)
+
+
+@router.post("/augmented-parts/fits/recompute", response_class=HTMLResponse)
+async def recompute_all_fits(
+    request: Request,
+    db: AsyncSession = Depends(get_db),
+    actor: User = Depends(web_require("fits:write")),
+):
+    result = await fit_matching_service.recompute_all(db)
+    await audit_crud.log(
+        db,
+        actor=actor,
+        action="update",
+        resource_type="Fit",
+        resource_id=None,
+        description=(
+            f"Recomputed fits for all parts: {result.created} created, "
+            f"{result.removed} removed"
+        ),
+    )
+    return HTMLResponse(
+        f"Recompute complete: {result.created} created, {result.removed} removed."
+    )
+
+
 # --- Part Colours ------------------------------------------------------------
 
 
diff --git a/scripts/seed.py b/scripts/seed.py
index 1b2d256..de3f125 100644
--- a/scripts/seed.py
+++ b/scripts/seed.py
@@ -71,6 +71,9 @@ PERMISSIONS = [
     ("augmented_parts:read", "View augmented parts"),
     ("augmented_parts:write", "Create and update augmented parts"),
     ("augmented_parts:delete", "Delete augmented parts"),
+    ("fits:read", "View part fits"),
+    ("fits:write", "Create, promote and recompute part fits"),
+    ("fits:delete", "Delete part fits"),
     ("audit_log:read", "View audit log"),
 ]
 
@@ -92,6 +95,7 @@ ROLES = {
         "attachment_point_names:read",
         "attachment_points:read",
         "augmented_parts:read",
+        "fits:read",
     ],
 }
 
diff --git a/tests/test_fit_matching.py b/tests/test_fit_matching.py
new file mode 100644
index 0000000..ef862a6
--- /dev/null
+++ b/tests/test_fit_matching.py
@@ -0,0 +1,102 @@
+from app.models.attachment_point import AttachmentPoint
+from app.models.augmented_part import AugmentedPart
+from app.models.clamp_connection import ClampConnection
+from app.models.iso_thread import IsoThread
+from app.models.static_connection import StaticConnection
+from app.models.unc_thread import UncSeries, UncThread
+from app.services.fit_matching import attachment_points_match, parts_could_fit
+
+
+def _ap(id_, name_id, **conn):
+    ap = AttachmentPoint(attachment_point_name_id=name_id, augmented_part_id=1)
+    ap.id = id_
+    for field, value in conn.items():
+        setattr(ap, field, value)
+    return ap
+
+
+def test_iso_thread_exact_match():
+    a = _ap(1, 1, iso_thread=IsoThread(diameter_mm=8, pitch_mm=1.25))
+    b = _ap(2, 2, iso_thread=IsoThread(diameter_mm=8, pitch_mm=1.25))
+    assert attachment_points_match(a, b)
+
+
+def test_iso_thread_mismatched_dimensions():
+    a = _ap(1, 1, iso_thread=IsoThread(diameter_mm=8, pitch_mm=1.25))
+    b = _ap(2, 2, iso_thread=IsoThread(diameter_mm=8, pitch_mm=1.0))
+    assert not attachment_points_match(a, b)
+
+
+def test_same_label_never_matches_even_with_identical_threads():
+    a = _ap(1, 1, iso_thread=IsoThread(diameter_mm=8, pitch_mm=1.25))
+    b = _ap(2, 1, iso_thread=IsoThread(diameter_mm=8, pitch_mm=1.25))
+    assert not attachment_points_match(a, b)
+
+
+def test_unc_thread_exact_match():
+    a = _ap(1, 1, unc_thread=UncThread(diameter_fraction="1/4", tpi=20, series=UncSeries.UNC))
+    b = _ap(2, 2, unc_thread=UncThread(diameter_fraction="1/4", tpi=20, series=UncSeries.UNC))
+    assert attachment_points_match(a, b)
+
+
+def test_unc_thread_series_mismatch():
+    a = _ap(1, 1, unc_thread=UncThread(diameter_fraction="1/4", tpi=20, series=UncSeries.UNC))
+    b = _ap(2, 2, unc_thread=UncThread(diameter_fraction="1/4", tpi=20, series=UncSeries.UNF))
+    assert not attachment_points_match(a, b)
+
+
+def test_iso_and_unc_never_match_each_other():
+    a = _ap(1, 1, iso_thread=IsoThread(diameter_mm=8, pitch_mm=1.25))
+    b = _ap(2, 2, unc_thread=UncThread(diameter_fraction="1/4", tpi=20, series=UncSeries.UNC))
+    assert not attachment_points_match(a, b)
+
+
+def test_static_within_clamp_range_mm():
+    a = _ap(1, 1, static_connection=StaticConnection(od_mm=30, od_in=1.18))
+    b = _ap(2, 2, clamp_connection=ClampConnection(min_mm=25, max_mm=35))
+    assert attachment_points_match(a, b)
+
+
+def test_static_outside_clamp_range_mm():
+    a = _ap(1, 1, static_connection=StaticConnection(od_mm=40, od_in=1.57))
+    b = _ap(2, 2, clamp_connection=ClampConnection(min_mm=25, max_mm=35))
+    assert not attachment_points_match(a, b)
+
+
+def test_static_matches_clamp_defined_only_in_inches():
+    a = _ap(1, 1, static_connection=StaticConnection(od_mm=30, od_in=1.1811))
+    b = _ap(2, 2, clamp_connection=ClampConnection(min_in=1.0, max_in=1.5))
+    assert attachment_points_match(a, b)
+
+
+def test_static_matches_clamp_with_only_a_minimum():
+    a = _ap(1, 1, static_connection=StaticConnection(od_mm=40, od_in=1.57))
+    b = _ap(2, 2, clamp_connection=ClampConnection(min_mm=25))
+    assert attachment_points_match(a, b)
+
+
+def test_clamp_with_no_bounds_never_matches():
+    a = _ap(1, 1, static_connection=StaticConnection(od_mm=30, od_in=1.18))
+    b = _ap(2, 2, clamp_connection=ClampConnection())
+    assert not attachment_points_match(a, b)
+
+
+def test_order_of_static_and_clamp_does_not_matter():
+    a = _ap(1, 1, clamp_connection=ClampConnection(min_mm=25, max_mm=35))
+    b = _ap(2, 2, static_connection=StaticConnection(od_mm=30, od_in=1.18))
+    assert attachment_points_match(a, b)
+
+
+def test_two_bare_attachment_points_never_match():
+    a = _ap(1, 1)
+    b = _ap(2, 2)
+    assert not attachment_points_match(a, b)
+
+
+def test_parts_could_fit_requires_different_types_and_identity():
+    a = AugmentedPart(id=1, part_type_id=1, description="a", colour_id=1, manufacturer_id=1)
+    b = AugmentedPart(id=2, part_type_id=1, description="b", colour_id=1, manufacturer_id=1)
+    c = AugmentedPart(id=3, part_type_id=2, description="c", colour_id=1, manufacturer_id=1)
+    assert not parts_could_fit(a, b)
+    assert parts_could_fit(a, c)
+    assert not parts_could_fit(a, a)