diff --git a/.changeset/record-flow-surface.md b/.changeset/record-flow-surface.md
new file mode 100644
index 0000000000..31781e8baf
--- /dev/null
+++ b/.changeset/record-flow-surface.md
@@ -0,0 +1,9 @@
+---
+'@objectstack/spec': minor
+---
+
+feat(spec): `deriveRecordFlowSurface(def, flow, opts)` — flow-aware record-surface derivation (#2604, extends #2578's `deriveRecordSurface`, ADR-0085 §5 one-shared-derivation).
+
+Decides the default surface per record FLOW: `view` keeps the shipped behavior verbatim (field-heavy → `route`/page, light → drawer overlay); the task flows (`create` / `edit` / `child-create` / `child-edit`) are ALWAYS overlays — never routes — with the derived `'page'` mapped to a full-screen modal (`size: 'full'`) and light objects staying a drawer. `child-*` flows take the CHILD object's def (the overlay sizes to the record being edited; the return target is always the parent detail). Mobile task flows are full-screen modals.
+
+Rationale: viewing a record is shareable state (deep-link belongs there); making/changing one is a transient task whose URL is a false promise (refresh loses the draft) and whose invariant is lossless return to the origin. Renderers treat the result as the DEFAULT only — explicit `navigation.mode`/`size`, `FormView.type`/`modalSize`, or an assigned page still win. No new authorable key (ADR-0085 §2). Additive, no breaking changes.
diff --git a/packages/spec/api-surface.json b/packages/spec/api-surface.json
index de297ad342..03f092895a 100644
--- a/packages/spec/api-surface.json
+++ b/packages/spec/api-surface.json
@@ -367,6 +367,9 @@
     "RangeOperatorSchema (const)",
     "Reaction (type)",
     "ReactionSchema (const)",
+    "RecordFlow (type)",
+    "RecordFlowContainer (type)",
+    "RecordFlowSurface (interface)",
     "RecordSubscription (type)",
     "RecordSubscriptionSchema (const)",
     "RecordSurface (type)",
@@ -462,6 +465,7 @@
     "defineObjectExtension (function)",
     "defineSeed (function)",
     "deriveFieldGroupLayout (function)",
+    "deriveRecordFlowSurface (function)",
     "deriveRecordSurface (function)",
     "fieldForm (const)",
     "hasDynamicTokens (function)",
diff --git a/packages/spec/src/data/record-surface.test.ts b/packages/spec/src/data/record-surface.test.ts
index 9e985a38ad..7f919a1f15 100644
--- a/packages/spec/src/data/record-surface.test.ts
+++ b/packages/spec/src/data/record-surface.test.ts
@@ -3,8 +3,10 @@
 import { describe, it, expect } from 'vitest';
 import {
   deriveRecordSurface,
+  deriveRecordFlowSurface,
   countAuthorableFields,
   RECORD_SURFACE_PAGE_THRESHOLD,
+  type RecordFlow,
 } from './record-surface';
 
 /** Build an object def with `n` plain text fields named f0..f(n-1). */
@@ -54,6 +56,65 @@ describe('deriveRecordSurface (ADR-0085 §5)', () => {
   });
 });
 
+describe('deriveRecordFlowSurface (#2604)', () => {
+  const TASK_FLOWS: RecordFlow[] = ['create', 'edit', 'child-create', 'child-edit'];
+  const heavy = objWithFields(RECORD_SURFACE_PAGE_THRESHOLD);
+  const light = objWithFields(RECORD_SURFACE_PAGE_THRESHOLD - 1);
+
+  it("view keeps the #2578 behavior verbatim: heavy → route('page'), light → overlay('drawer')", () => {
+    expect(deriveRecordFlowSurface(heavy, 'view')).toEqual({
+      container: 'route', surface: 'page', size: 'auto',
+    });
+    expect(deriveRecordFlowSurface(light, 'view')).toEqual({
+      container: 'overlay', surface: 'drawer', size: 'auto',
+    });
+  });
+
+  it('task flows never route: heavy → full-screen modal overlay', () => {
+    for (const flow of TASK_FLOWS) {
+      expect(deriveRecordFlowSurface(heavy, flow)).toEqual({
+        container: 'overlay', surface: 'modal', size: 'full',
+      });
+    }
+  });
+
+  it('task flows on a light object stay a drawer overlay', () => {
+    for (const flow of TASK_FLOWS) {
+      expect(deriveRecordFlowSurface(light, flow)).toEqual({
+        container: 'overlay', surface: 'drawer', size: 'auto',
+      });
+    }
+  });
+
+  it('mobile: view routes to a page; task flows become a full-screen modal', () => {
+    expect(deriveRecordFlowSurface(light, 'view', { viewport: 'mobile' })).toEqual({
+      container: 'route', surface: 'page', size: 'auto',
+    });
+    for (const flow of TASK_FLOWS) {
+      expect(deriveRecordFlowSurface(light, flow, { viewport: 'mobile' })).toEqual({
+        container: 'overlay', surface: 'modal', size: 'full',
+      });
+    }
+  });
+
+  it('child-* flows size to the def they are given (the child), independent of any parent', () => {
+    // A thin child stays a drawer even though its parent (not passed) is heavy.
+    expect(deriveRecordFlowSurface(objWithFields(3), 'child-create').surface).toBe('drawer');
+    // A fat child gets the full-screen modal.
+    expect(deriveRecordFlowSurface(objWithFields(40), 'child-edit')).toEqual({
+      container: 'overlay', surface: 'modal', size: 'full',
+    });
+  });
+
+  it('honours pageThreshold and tolerates bare/malformed input', () => {
+    expect(deriveRecordFlowSurface(objWithFields(5), 'create', { pageThreshold: 4 }).size).toBe('full');
+    expect(deriveRecordFlowSurface(null, 'create')).toEqual({
+      container: 'overlay', surface: 'drawer', size: 'auto',
+    });
+    expect(deriveRecordFlowSurface({ fields: 'nope' } as unknown, 'view').surface).toBe('drawer');
+  });
+});
+
 describe('countAuthorableFields', () => {
   it('counts visible non-system fields only', () => {
     expect(countAuthorableFields(objWithFields(5))).toBe(5);
diff --git a/packages/spec/src/data/record-surface.ts b/packages/spec/src/data/record-surface.ts
index dcb789fe92..d35bca646c 100644
--- a/packages/spec/src/data/record-surface.ts
+++ b/packages/spec/src/data/record-surface.ts
@@ -91,3 +91,62 @@ export function deriveRecordSurface(def: unknown, opts: RecordSurfaceOptions = {
   if (countAuthorableFields(def) >= threshold) return 'page';
   return 'drawer';
 }
+
+/**
+ * The record flow being opened. `view` shows state; the other four perform a
+ * task (create/change a record). For `child-*` flows — a subtable / related-
+ * list child created or edited from its PARENT's detail — pass the CHILD
+ * object's def: the overlay sizes to the record being edited, while the
+ * return target is always the parent (#2604 D3).
+ */
+export type RecordFlow = 'view' | 'create' | 'edit' | 'child-create' | 'child-edit';
+
+/** How the surface is mounted: a navigated route, or an overlay over the origin. */
+export type RecordFlowContainer = 'route' | 'overlay';
+
+export interface RecordFlowSurface {
+  /**
+   * `'route'` only ever for flow `'view'` (a record is shareable state —
+   * deep-linkable, refresh-safe). Every task flow is an `'overlay'`: close
+   * returns to the origin with its context (scroll / filters / tab) intact,
+   * which is the #2604 return-flow invariant — and a create/edit URL would be
+   * a false promise anyway (refresh loses the draft).
+   */
+  container: RecordFlowContainer;
+  surface: RecordSurface;
+  /** Maps onto `navigation.size` / `FormView.modalSize`; routes ignore it. */
+  size: 'auto' | 'full';
+}
+
+/**
+ * Derive the DEFAULT surface for a record FLOW (#2604; extends
+ * {@link deriveRecordSurface}, ADR-0085 §5 "one shared derivation").
+ *
+ * Rule — the two axes are independent:
+ *   - how BIG (field count, via {@link deriveRecordSurface}) is unchanged;
+ *   - whether it ROUTES is decided by what the flow *is*: viewing a record is
+ *     state → route-capable; making/changing one is a task → always overlay.
+ *
+ * So `view` keeps the #2578 behavior verbatim (`'page'` → route), while the
+ * task flows map the derived `'page'` to a FULL-SCREEN MODAL — same big
+ * canvas, overlay return semantics. This mapping is why `'modal'` exists in
+ * {@link RecordSurface} without the base heuristic ever emitting it.
+ *
+ * Like the base derivation this is a DEFAULT only: explicit `navigation.mode`
+ * / `navigation.size`, `FormView.type` / `modalSize`, or an assigned page win
+ * (the sanctioned per-object overrides — no new authorable key, ADR-0085 §2).
+ */
+export function deriveRecordFlowSurface(
+  def: unknown,
+  flow: RecordFlow,
+  opts: RecordSurfaceOptions = {},
+): RecordFlowSurface {
+  const surface = deriveRecordSurface(def, opts);
+  if (flow === 'view') {
+    return { container: surface === 'page' ? 'route' : 'overlay', surface, size: 'auto' };
+  }
+  // Task flows (create / edit / child-*): never a route. Field-heavy (or
+  // mobile, where the base derivation says 'page') → full-screen modal.
+  if (surface === 'page') return { container: 'overlay', surface: 'modal', size: 'full' };
+  return { container: 'overlay', surface, size: 'auto' };
+}