diff --git a/design/prose-tests.md b/design/prose-tests.md
index a2b324fde..7307f07ce 100644
--- a/design/prose-tests.md
+++ b/design/prose-tests.md
@@ -84,6 +84,15 @@ testing.
   and the stubs, and returns a transcript. The asserter sees the
   transcript, the expected path, and the world delta — never the
   reverse.
+- **P3a — three shapes of end state.** A walk either produces a world a
+  recipe can build (`assertion-state.cjs`, byte-compared), leaves the
+  fixture untouched (no assertion state — most entry-skill cases), or
+  mutates into a shape only prose can describe (`world: "claims"`). The
+  third exists because parts of some worlds are authored by the model:
+  a discussion's subtopic names are derived from context and differ run
+  to run. There the delta is taken against the fixture and judged
+  against the case's claims. It is the weakest of the three and is
+  reached for last: a world a recipe can build must be built.
 - **P4b — the asserter is told which substitutions were armed.** The
   recorded actions show a stub as the walker writing a file an agent
   would have produced; without knowing the stub exists, an asserter
diff --git a/tests/prose/README.md b/tests/prose/README.md
index 575586ea6..7071ef7a6 100644
--- a/tests/prose/README.md
+++ b/tests/prose/README.md
@@ -29,6 +29,14 @@ blob code merely relays lives in markdown.
 walker/asserter boundary — the walker must never see the expected path,
 and a file boundary enforces that structurally rather than by convention.
 
+A case whose end state only prose can describe sets `"world": "claims"`
+in `case.json` and carries no `assertion-state.cjs`. The delta is then
+computed against the **fixture** — expected to be non-empty — and judged
+against the case's stated claims. Reach for it only where the
+variability is genuinely the model's to decide: a discussion's subtopic
+names are derived from context and differ run to run, so no recipe can
+pin them. A world a recipe *can* build must be built.
+
 `fixture-state.cjs` and `assertion-state.cjs` are separate because they
 change for different reasons: the fixture when the precondition changes,
 the assertion when the prose's *behaviour* changes. The assertion state
diff --git a/tests/prose/cases/discussion-session-substituted/act.md b/tests/prose/cases/discussion-session-substituted/act.md
new file mode 100644
index 000000000..09ae57f9a
--- /dev/null
+++ b/tests/prose/cases/discussion-session-substituted/act.md
@@ -0,0 +1,5 @@
+Execute skills/workflow-discussion-process/SKILL.md from its first step,
+as the entry skill has just invoked it for work unit `pay`, topic `pay`.
+Follow it through initialisation. Stop once the session loop is behind
+you and the prose moves on to reviewing the discussion — do not begin
+the review, dispatch any agent, or conclude the phase.
diff --git a/tests/prose/cases/discussion-session-substituted/assert.md b/tests/prose/cases/discussion-session-substituted/assert.md
new file mode 100644
index 000000000..a45e704a3
--- /dev/null
+++ b/tests/prose/cases/discussion-session-substituted/assert.md
@@ -0,0 +1,25 @@
+The prose should have taken this path:
+
+1. resume detection finds no discussion file and treats this as a fresh
+   discussion — no resume choice is offered
+2. initialisation registers the discussion in the manifest through the
+   engine before any map command, since the map requires the item to
+   exist
+3. it creates the discussion file from the template, including the
+   terminal Triage section seeded as `(none)`
+4. it derives initial subtopics from the context available and records
+   each on the map through the engine, each landing `pending`
+5. it commits the initialised discussion
+6. the session loop is where the conversation would happen, and the walk
+   resumes after it to reach the review step, which it does not enter
+
+Further claims:
+
+- the subtopic names are derived by the walker from the seed and context,
+  so which names appear and how many is expected to vary between runs.
+  What matters is that at least one was recorded through the engine, that
+  each began `pending`, and that by the end none remain `pending`.
+- the discussion file's working sections hold the substituted content,
+  and its Triage section reads `(none)`
+- the discussion item exists in the manifest with its subtopics; no other
+  phase has been touched
diff --git a/tests/prose/cases/discussion-session-substituted/case.json b/tests/prose/cases/discussion-session-substituted/case.json
new file mode 100644
index 000000000..c024e03f7
--- /dev/null
+++ b/tests/prose/cases/discussion-session-substituted/case.json
@@ -0,0 +1,12 @@
+{
+  "origin": "framework — the mid-flow substitution: an unscripted conversation stood in for, so the steps around it can be walked",
+  "world": "claims",
+  "files": [
+    "skills/workflow-discussion-process/SKILL.md",
+    "skills/workflow-discussion-process/references/initialize-discussion.md",
+    "skills/workflow-discussion-process/references/template.md"
+  ],
+  "stubs": {
+    "discussion-pay-decided": "once the discussion file has been created from the template, the map seeded, and the initialisation commit made — the moment the prose would enter the session loop"
+  }
+}
diff --git a/tests/prose/cases/discussion-session-substituted/fixture-state.cjs b/tests/prose/cases/discussion-session-substituted/fixture-state.cjs
new file mode 100644
index 000000000..546391b87
--- /dev/null
+++ b/tests/prose/cases/discussion-session-substituted/fixture-state.cjs
@@ -0,0 +1,13 @@
+'use strict';
+
+// A feature routed to discussion with nothing started — the discussion
+// file does not exist, so initialisation runs for real.
+
+const m = require('../../mainlines/feature.cjs');
+
+module.exports = {
+  build(h) {
+    m.init(h);
+    m.create(h);
+  },
+};
diff --git a/tests/prose/cases/discussion-session-substituted/fixture.md b/tests/prose/cases/discussion-session-substituted/fixture.md
new file mode 100644
index 000000000..9f3a2c546
--- /dev/null
+++ b/tests/prose/cases/discussion-session-substituted/fixture.md
@@ -0,0 +1,3 @@
+The feature exists and no phase has started. Discovery shaped it and
+routed it to discussion; the discussion entry skill has already run and
+handed over, so the processing skill is starting from its first step.
diff --git a/tests/prose/cases/discussion-session-substituted/fixture/.claude/settings.json b/tests/prose/cases/discussion-session-substituted/fixture/.claude/settings.json
new file mode 100644
index 000000000..ae019ede5
--- /dev/null
+++ b/tests/prose/cases/discussion-session-substituted/fixture/.claude/settings.json
@@ -0,0 +1,9 @@
+{
+  "showClearContextOnPlanAccept": true,
+  "permissions": {
+    "allow": [
+      "Edit(.workflows/**)",
+      "Bash(mv .workflows/:*)"
+    ]
+  }
+}
diff --git a/tests/prose/cases/discussion-session-substituted/fixture/.recipe-hash b/tests/prose/cases/discussion-session-substituted/fixture/.recipe-hash
new file mode 100644
index 000000000..af934cf8a
--- /dev/null
+++ b/tests/prose/cases/discussion-session-substituted/fixture/.recipe-hash
@@ -0,0 +1 @@
+9b48d4c996d773219dc60a44fcc014b1f534adc4d22537741a96575da66d2e46
diff --git a/tests/prose/cases/discussion-session-substituted/fixture/.workflows/.state/migrations b/tests/prose/cases/discussion-session-substituted/fixture/.workflows/.state/migrations
new file mode 100644
index 000000000..3c7162acc
--- /dev/null
+++ b/tests/prose/cases/discussion-session-substituted/fixture/.workflows/.state/migrations
@@ -0,0 +1,52 @@
+001
+002
+003
+004
+005
+006
+007
+008
+009
+010
+011
+012
+013
+014
+015
+016
+017
+018
+019
+020
+021
+022
+023
+024
+025
+026
+027
+028
+029
+030
+031
+032
+033
+034
+035
+036
+037
+038
+039
+040
+041
+042
+043
+044
+045
+046
+047
+048
+049
+050
+051
+052
diff --git a/tests/prose/cases/discussion-session-substituted/fixture/.workflows/_gitignore.fixture b/tests/prose/cases/discussion-session-substituted/fixture/.workflows/_gitignore.fixture
new file mode 100644
index 000000000..779cbf28a
--- /dev/null
+++ b/tests/prose/cases/discussion-session-substituted/fixture/.workflows/_gitignore.fixture
@@ -0,0 +1,2 @@
+.cache/
+.manifest.json.*.tmp
diff --git a/tests/prose/cases/discussion-session-substituted/fixture/.workflows/manifest.json b/tests/prose/cases/discussion-session-substituted/fixture/.workflows/manifest.json
new file mode 100644
index 000000000..987f6c236
--- /dev/null
+++ b/tests/prose/cases/discussion-session-substituted/fixture/.workflows/manifest.json
@@ -0,0 +1,7 @@
+{
+  "work_units": {
+    "pay": {
+      "work_type": "feature"
+    }
+  }
+}
diff --git a/tests/prose/cases/discussion-session-substituted/fixture/.workflows/pay/discovery/sessions/session-001.md b/tests/prose/cases/discussion-session-substituted/fixture/.workflows/pay/discovery/sessions/session-001.md
new file mode 100644
index 000000000..2aab96961
--- /dev/null
+++ b/tests/prose/cases/discussion-session-substituted/fixture/.workflows/pay/discovery/sessions/session-001.md
@@ -0,0 +1,39 @@
+# Discovery Session 001
+
+Date: 2026-01-01
+Work unit: pay
+
+## Description (as of session)
+
+Accept card payments at checkout.
+
+## Seed
+
+(none)
+
+## Imports
+
+(none)
+
+## Map State at Start
+
+(n/a — single-topic work)
+
+## Exploration
+
+Shaped as a single feature: accept card payments at checkout using
+the existing gateway account. Card-only for v1 came up early and was
+softly agreed; wallet support was noted as a likely deferral. No
+research need surfaced — routed straight to discussion.
+
+## Edits
+
+(none)
+
+## Topics Identified
+
+(none)
+
+## Conclusion
+
+(none)
diff --git a/tests/prose/cases/discussion-session-substituted/fixture/.workflows/pay/manifest.json b/tests/prose/cases/discussion-session-substituted/fixture/.workflows/pay/manifest.json
new file mode 100644
index 000000000..1cf8afb0e
--- /dev/null
+++ b/tests/prose/cases/discussion-session-substituted/fixture/.workflows/pay/manifest.json
@@ -0,0 +1,8 @@
+{
+  "name": "pay",
+  "work_type": "feature",
+  "status": "in-progress",
+  "created": "2026-01-01",
+  "description": "Accept card payments at checkout",
+  "phases": {}
+}
diff --git a/tests/prose/lib/cases.cjs b/tests/prose/lib/cases.cjs
index 006534a5d..a4b3d79df 100644
--- a/tests/prose/lib/cases.cjs
+++ b/tests/prose/lib/cases.cjs
@@ -8,7 +8,8 @@
 // modules.
 //
 //   case.json            the values code branches on:
-//                          { origin, files[], answers[], stubs{name: trigger} }
+//                          { origin, files[], answers[], stubs{name: trigger},
+//                            world: "claims" (optional) }
 //   fixture.md           optional. Prose describing the starting world —
 //                        what has already happened, where the session
 //                        stands. Given to the walker.
@@ -20,10 +21,19 @@
 //                        further claims. Given ONLY to the asserter.
 //   assertion-state.cjs  optional. exports build(h): the world the walk
 //                        should produce. Absent means "unchanged" — the
-//                        walk should leave the fixture state untouched.
+//                        walk should leave the fixture state untouched,
+//                        unless case.json sets world: "claims".
 //   fixture/             generated snapshot of the starting world
 //   assertion/           generated snapshot of the expected world
 //
+// `world: "claims"` is the third case of a walk's end state: it mutates,
+// but into a shape no recipe can pin because prose the model authors is
+// part of it — derived subtopic names, written artifact text. The delta
+// is then computed against the fixture and judged against the case's
+// stated claims rather than byte-compared. Reach for it only when the
+// variability is genuinely the model's to decide; a world a recipe can
+// build must be built.
+//
 // act.md and assert.md are separate files because that is the P4
 // boundary: the walker must never see the expected trace, and a file
 // boundary enforces it structurally rather than by convention.
@@ -93,6 +103,7 @@ function loadCase(id) {
         : { path: spec.slice(0, hash).trim(), anchor: spec.slice(hash + 1).trim() };
     }),
     answers: meta.answers || [],
+    worldMode: meta.world || null,
     stubs: Object.entries(meta.stubs || {}).map(([name, trigger]) => ({ name, trigger })),
     situation: readIf(dir, FILES.situation),
     act: readIf(dir, FILES.act),
@@ -168,6 +179,16 @@ function validateCorpus(cases) {
     if (!c.act) errors.push(`${at}: no ${FILES.act}`);
     if (!c.assert) errors.push(`${at}: no ${FILES.assert} — the expected trace is what catches a silent repair`);
 
+    if (c.worldMode && c.worldMode !== 'claims') {
+      errors.push(`${at}: ${FILES.meta} world must be "claims" when set, not "${c.worldMode}"`);
+    }
+    if (c.worldMode === 'claims' && c.hasAssertionState) {
+      errors.push(`${at}: world "claims" and ${FILES.assertionState} are exclusive — `
+        + 'a world a recipe can build must be built');
+    }
+    if (c.worldMode === 'claims' && !c.hasFixtureState) {
+      errors.push(`${at}: world "claims" needs a world to mutate`);
+    }
     if (c.hasAssertionState && !c.hasFixtureState) {
       errors.push(`${at}: ${FILES.assertionState} without ${FILES.fixtureState} — nothing to act on`);
     }
diff --git a/tests/prose/lib/worlds.cjs b/tests/prose/lib/worlds.cjs
index 566bb03eb..000f5e990 100644
--- a/tests/prose/lib/worlds.cjs
+++ b/tests/prose/lib/worlds.cjs
@@ -246,8 +246,8 @@ function unifiedDiff(label, expectedBuf, actualBuf) {
  * volatile values surface as ordinary differences and the agent rules on
  * them. A case with no assertion-state expects its fixture back unchanged.
  */
-function diffWorld(caseId, worldDir) {
-  const which = fs.existsSync(snapshotDir(caseId, 'assertion')) ? 'assertion' : 'fixture';
+function diffWorld(caseId, worldDir, claimsMode = false) {
+  const which = !claimsMode && fs.existsSync(snapshotDir(caseId, 'assertion')) ? 'assertion' : 'fixture';
   const expected = readSnapshot(caseId, which);
   if (expected === null) throw new Error(`case "${caseId}" has no committed ${which} snapshot`);
   const actual = collectTree(worldDir);
@@ -262,7 +262,12 @@ function diffWorld(caseId, worldDir) {
   for (const rel of expected.keys()) if (!actual.has(rel)) removed.push(rel);
 
   return {
-    expecting: which === 'assertion' ? 'the assertion state' : 'the fixture state, unchanged',
+    expecting: which === 'assertion'
+      ? 'the assertion state'
+      : (claimsMode
+        ? 'no fixed world — the delta below is against the STARTING state, is expected to be '
+          + "non-empty, and must be judged against the case's stated claims"
+        : 'the fixture state, unchanged'),
     added,
     removed,
     changed,
diff --git a/tests/prose/run.cjs b/tests/prose/run.cjs
index 8464a9d33..66b4322a3 100644
--- a/tests/prose/run.cjs
+++ b/tests/prose/run.cjs
@@ -146,7 +146,7 @@ function cmdAssert(argv) {
   let actions = null;
   if (c.hasFixtureState) {
     const dir = requireWorld(argv, c);
-    const delta = worlds.diffWorld(c.id, dir);
+    const delta = worlds.diffWorld(c.id, dir, c.worldMode === 'claims');
     world = {
       expecting: delta.expecting,
       delta: [
diff --git a/tests/prose/stubs/discussion-pay-decided.md b/tests/prose/stubs/discussion-pay-decided.md
new file mode 100644
index 000000000..5a5575d28
--- /dev/null
+++ b/tests/prose/stubs/discussion-pay-decided.md
@@ -0,0 +1,44 @@
+# stub: discussion-pay-decided
+
+The outcome of a discussion session that this framework does not
+simulate — the organic conversation between user and assistant. Stands in
+for the whole session loop:
+
+1. Write the content below over the discussion file the prose created at
+   `.workflows/pay/discussion/pay.md`, replacing the working sections
+   while leaving the file's structure as the template made it.
+2. Move every subtopic the map holds to a settled state through the
+   engine, so the map reflects a finished conversation:
+   `discussion-map set pay pay {subtopic} decided` for each, except any
+   named for wallets or deferral, which take `deferred`.
+3. Resume the prose at the step that follows the session loop.
+
+---
+
+# Discussion — pay
+
+## Context
+
+Accept card payments at checkout using the existing gateway account.
+Shaped in discovery as a single feature; card-only for v1.
+
+## Decisions
+
+- Use the existing gateway account — no new provider onboarding, and no
+  second set of credentials to hold.
+- Card-only for v1. The checkout collects card details directly; wallet
+  flows are a separate surface.
+- Capture is confirmed by gateway webhook. The checkout never polls: a
+  poll would either be slow or hammer the gateway, and the webhook is
+  already guaranteed.
+- An order with no shippable items still needs a tax context. Billing
+  address stands in where there is no shipping address.
+
+## Deferred
+
+- Wallet support (Apple and Google Pay) — revisit after v1 ships and we
+  know the card flow holds.
+
+## Triage
+
+(none)