diff --git a/docs/current-handoff.md b/docs/current-handoff.md
index 56426f5..ef3d0f9 100644
--- a/docs/current-handoff.md
+++ b/docs/current-handoff.md
@@ -1265,6 +1265,124 @@ However, it appears as an *assumption* (a hypothesised state of affairs) rather
 
 ---
 
+## v0.61 Experiment 4 — Intervention-Fit Accessibility in User-Facing UI
+
+**Status: D — NOT USER-EXPOSED**
+
+### Execution
+
+| Item | Value |
+|---|---|
+| Branch | `feature/initial-decomposition-v0.61` |
+| Starting HEAD | `42da768 docs(confidence-engine): record v0.61 intervention-fit experiment` |
+| Source trace completed: YES — bounded to reasoning-workspace.jsx lines 1850–2216 |
+| Browser navigation: Two persisted Investigations examined (IDs from Portfolio list) |
+| Model/API calls: ZERO |
+| State mutations: ZERO |
+| Execution errors: NONE |
+
+### Experiment 3 Investigation ID
+
+The handoff documentation does NOT record the exact Investigation ID for Experiment 3. Two manufacturing-scenario Investigations exist in the portfolio with identical scenario text. Neither was definitively identified as Experiment 3's persisted state. However, both Investigations were examined and show structurally identical rendering behaviour regarding assumption nodes.
+
+### SOURCE TRACE
+
+**Provisional assumptions rendered:** YES — in TWO locations in reasoning-workspace.jsx:
+1. Lines 2029–2048: Possible Interpretations section within `initial-proposed-findings` div, conditional on `possibleInterpretations.length > 0`
+2. Lines 2187–2216: Persistent "Possible Interpretations" in workspace grid (row 4, full-width), conditional on `postAnalyseStatus !== "success"` AND `interpretationNodes.length > 0`
+
+Filter for both sections: `n.kind === "assumption" && n.status !== "resolved" && !resolvedIds.has(n.id)`
+
+**User-facing location:** Both render under `<h2>Possible Interpretations</h2>` heading. Cards styled with blue-100 border, blue-50/40 background, labeled "Plausible interpretation" in blue text.
+
+**Assumptions interactive:** NO — cards are plain `<div>` elements (not buttons). The source comment at line 1865 and 2029 explicitly states "(informational, not investigable)". No onClick handler exists on assumption cards. Contrast with Open Question rendering: lines 1899–1910 use `<button>` elements with `startFocused(n.id)` for focused investigation.
+
+**Open Questions source:** `(graph?.nodes || []).filter((n) => n.kind === "unknown" && !resolvedIds.has(n.id))` — unknown-kind nodes, not resolved. Rendered as clickable buttons that invoke `startFocused()` to begin investigation of that node.
+
+**Existing assumption-to-investigation route:** NO. No code path converts an assumption node into a selectable investigative path without another semantic/model update. The filter separates assumptions (`kind === "assumption"`) from unknowns (`kind === "unknown"`) with different rendering roles and no cross-conversion mechanism.
+
+### VISIBLE INVESTIGATION (two persisted Investigations examined)
+
+**Investigation 1** (`53862863-...`):
+- Current Understanding: "A manufacturing business owner is weighing a £120k automated quality inspection investment against the need to analyze complaint trends amid concurrent changes in production scale, supplier, shift scheduling, and tracking methodology."
+- Open Questions (2): (a) complaint rate per unit denominator uncertainty; (b) defect rates by new supplier/shift vs baseline
+
+**Investigation 2** (`d1b44afb-...`):
+- Current Understanding: "A manufacturing business observes a 35% rise in complaints alongside a 40% production increase, coinciding with CRM, supplier, and scheduling changes, prompting a decision on a £120k quality inspection investment."
+- Open Questions (7): CRM tagging threshold; defect rates by supplier/shift; complaint rate per unit overall; complaint rate by customer segments; complaint rate by production shift; complaint rate by process stages; complaint rate by critical defect categories
+
+**Current Understanding intervention-fit meaning:** NO. Neither Current Understanding contains the intervention-fit distinction (i.e., "if complaints are primarily caused by late-delivery logistics rather than manufacturing defects, automated quality inspection may address the wrong problem"). The CU text describes what is being evaluated but never questions whether the proposed action fits the actual problem mechanism.
+
+**Visible Open Questions:** 2 and 7 respectively. None directly address whether automated quality inspection matches the problem type (delivery vs defects).
+
+**Intervention fit explicit Open Question:** NO — neither Investigation has an Open Question equivalent to "does this £120k investment address the actual problem, or is the problem primarily logistical/delivery-driven?"
+
+**Directly selectable:** NO.
+
+**Intervention-fit assumption visible:** NO. Zero Possible Interpretations sections rendered on either page.
+
+**Visible section:** N/A — no Possible Interpretations section exists on either surface.
+
+**Assumption selectable:** N/A (no assumptions rendered).
+
+**Other visible investigative route:** NO — no visible control allows the user to investigate whether automated inspection addresses the actual problem mechanism. "Review current understanding" button leads to Report page, which is a derived summary not an investigative surface. Developer details panel shows model/validation metrics but not assumption content.
+
+### THREE-LAYER RESULT
+
+- Represented in canonical reasoning state: YES (Experiment 3 record confirmed intervention-fit as conditional assumption `nyi88el` in graph)
+- Visible to user: NO — zero Possible Interpretations sections on either examined Investigation; Current Understanding does not contain the distinction; Open Questions do not address it
+- Selectable/investigable by user: NO
+
+### ACCESSIBILITY CLASSIFICATION
+
+**D — NOT USER-EXPOSED**
+
+Intervention-fit meaning exists in canonical reasoning state (Experiment 3 graph contained assumption `nyi88el`), but is absent from the normal user-facing Investigation surface. The Possible Interpretations section that would render assumption nodes exists as code but zero assumption-kind nodes meeting the filter criteria appear on either examined Investigation.
+
+### Evidence
+
+1. **Source trace:** reasoning-workspace.jsx lines 2029–2048 and 2187–2216 define Possible Interpretations rendering for assumption nodes — strictly informational, non-interactive `<div>` elements. Contrast with Open Questions at lines 1899–1910 which use clickable `<button>` elements invoking `startFocused()`.
+
+2. **Source trace:** line 1865 comment explicitly states "(informational, not investigable)" — structural separation between unknown-kind (investigable) and assumption-kind (informational).
+
+3. **Source trace:** Open Questions source is exclusively `n.kind === "unknown"` nodes (line 1856). No code exists that converts assumption meaning into an open question or makes assumptions investigable via UI interaction.
+
+4. **Browser observation Investigation 1:** Current Understanding + Situation + 2 Open Questions visible. Zero Possible Interpretations sections.
+
+5. **Browser observation Investigation 2:** Current Understanding + Situation + 7 Open Questions visible. Zero Possible Interpretations sections. Decomposition instability confirmed (7 questions vs 2 in Investigation 1).
+
+6. **"What remains plausible" section** (lines 1983–1986): exists only inside `overviewState` conditional on the Report-generation surface, NOT as a persistent Investigation surface. Only appears when user clicks "Review current understanding."
+
+7. **No cross-conversion path:** No code converts assumption nodes into selectable investigative paths. The kind distinction (assumption vs unknown) is structural, not cosmetic.
+
+### PRODUCT-PRINCIPLE CHECK
+
+- Engine selected what to investigate: NO (no steering in decomposition output)
+- Engine prioritised a question: NO (zero prioritisation language observed)
+- User retained choice among surfaced Open Questions: YES — all visible Open Questions are selectable buttons; user owns which to pursue first. However, the full set of uncertainty was not presented as investigable paths.
+
+### NEXT EVIDENCE QUESTION
+
+> Before deciding whether the fix is semantic (initial decomposition should produce explicit intervention-fit unknowns) or structural (existing assumption-kind meaning must be projected into selectable Open Question surface), what does the persisted graph data in Experiment 3's Investigation actually contain — and do any of those nodes qualify for Possible Interpretations rendering?
+
+This question distinguishes whether the problem is:
+- **Semantic:** decomposition never produced intervention-fit assumptions → no nodes to render
+- **Structural:** assumption-kind nodes exist but are structurally barred from user selection
+
+### GIT
+
+**Documentation commit:** pending
+**Working tree:** pending
+
+### CHANGES
+
+- Production changed: NO
+- Tests changed: NO
+- Prompt/schema changed: NO
+- Canonical Investigation state changed: NO
+
+---
+
 Consult `docs/design-evolution/README.md` for progressive loading of product reasoning and provenance chronology; load the relevant chapter only when a specific historical question requires it.
 
 The current handoff captures all operational facts needed to resume today. For historical decisions, experiment evidence, or methodology evolution — consult the design evolution archive index or task-context packs as appropriate.