diff --git a/docs/current-handoff.md b/docs/current-handoff.md
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--- a/docs/current-handoff.md
+++ b/docs/current-handoff.md
@@ -174,6 +174,15 @@ Experiment 54N tested whether an interpretation disagreement can be judged for m
   - `npx vitest run tests/graph/question-formulator.test.js` → PASS (29/29)
 - **Still unproven live:** the exact 60B.24 regression through the full end-to-end runtime path.
 
+### 60B.28 explicit uncertainty-prefix proposition coverage
+
+- 60B.27 exposed a narrow coverage gap: descriptions starting `Unknown whether...` fell through to nominal label extraction instead of the existing bounded `whether ...` proposition path.
+- `extractMeaning()` now extends the same case-insensitive prefix normalisation used before proposition extraction to also strip explicit uncertainty markers `unknown` and `uncertain`.
+- As a result, `Whether...`, `Unknown whether...`, `Uncertain whether...`, `Uncertainty about whether...`, and `Uncertainty regarding whether...` now all expose `whether X` before existing rationale stripping and evidence framing run.
+- Focused deterministic verification passed:
+  - `npx vitest run tests/graph/question-formulator.test.js` → PASS (37/37)
+- **Still unproven live:** the exact 60B.27 customer-signing case through the full end-to-end runtime path.
+
 ### When This Knowledge-Management Phase Is Complete
 
 Provisional criteria for review (all confirmed met by Experiment 38 cold-start test):
diff --git a/docs/experiment-60b28.md b/docs/experiment-60b28.md
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+# Experiment 60B.28 — Explicit Uncertainty Prefix Proposition Coverage
+
+**Branch:** `feature/proposition-prefix-coverage-v0.31`  
+**Starting HEAD:** `4e66e1ffbf1b8aa9103f72430c14e26d48f7f1fd`  
+**Date:** 2026-08-14  
+**Status:** COMPLETE
+
+## Objective
+
+Close the narrow live coverage gap identified in 60B.27 by extending bounded proposition-prefix normalisation so explicit uncertainty forms beginning:
+
+```text
+Unknown whether...
+Uncertain whether...
+```
+
+enter the same `whether ...` proposition-extraction path already established in 60B.26.
+
+## 60B.27 Coverage Gap
+
+60B.27 confirmed that the live cleanup from 60B.26 was working correctly for:
+
+- material target preservation
+- evidence framing activation
+- explanatory rationale removal from the final question
+- source graph meaning preservation
+
+But the live node description began:
+
+```text
+Unknown whether one prospective enterprise customer will sign if we launch this year, so that the remaining £700k of the expected £1.2M annual revenue is realized.
+```
+
+The existing bounded proposition path recognised:
+
+```text
+Whether...
+Uncertainty about whether...
+Uncertainty regarding whether...
+```
+
+but not:
+
+```text
+Unknown whether...
+Uncertain whether...
+```
+
+So formulation fell back to the nominal label instead of preserving the full unresolved proposition.
+
+## Exact Prefix Normalisation Added
+
+Production change was limited to `lib/graph/question-formulator.js`.
+
+Inside `extractMeaning()`, the description-start normalisation used before the existing `^whether` proposition check now also strips these explicit uncertainty prefixes case-insensitively:
+
+```text
+unknown
+uncertain
+```
+
+This means the following bounded forms are now treated equivalently for proposition extraction:
+
+```text
+Whether X...
+Unknown whether X...
+Uncertain whether X...
+Uncertainty about whether X...
+Uncertainty regarding whether X...
+```
+
+Each now exposes:
+
+```text
+whether X
+```
+
+before the existing rationale-boundary stripping and evidence framing logic runs.
+
+## Focused Regression Behaviour
+
+The exact 60B.27 regression now formulates:
+
+```text
+What evidence would clarify whether one prospective enterprise customer will sign if we launch this year?
+```
+
+The final question excludes:
+
+- `£700k`
+- `£1.2M`
+- `annual revenue`
+
+while the source node description remains unchanged.
+
+Additional focused deterministic coverage also confirms:
+
+- `Uncertain whether...` preserves the proposition with evidence framing
+- bare `Whether...` remains unchanged
+- `Uncertainty about whether...` remains unchanged
+- `Uncertainty regarding whether...` remains unchanged
+- direct interrogatives remain unchanged
+- nominal non-`whether` behaviour remains unchanged
+- formulation-only cleanup does not mutate the source description
+
+## Preserved Existing Paths
+
+This change did **not**:
+
+- redesign question formulation
+- broaden parsing beyond explicit uncertainty markers
+- convert arbitrary `unknown` descriptions into propositions
+- mutate graph descriptions
+- change question templates, routing, provider logic, schema, or proposal application
+
+## Verification
+
+Focused command run exactly as bounded:
+
+```bash
+npx vitest run tests/graph/question-formulator.test.js
+```
+
+Result:
+
+```text
+PASS — 37/37 tests
+```
+
+## What Remains Unproven Until Exact Live 60B.27 Rerun
+
+Deterministic formulation coverage is now proven for the targeted prefix gap, but the exact live end-to-end 60B.27 rerun is still required to reconfirm that the same proposition-preserving output appears through the full runtime path with live model-selected graph updates.