feat(router): rubber-duck idea capture + feed

The rubber-duck path emits a fenced {"ideas":[…]} block; Ideas.parse strips it
from the chat turn and onUserInput records an IdeaCapturedEvent each (CHAT only).
IdeaReader folds eventStore.allEvents() into the active cross-session board, and
DefaultRouterContextBuilder injects a capped "## Ideas" entry — router-only, so
workflow stages never see it. Extended SYSTEM_PROMPT option (d) to capture ideas.

core/router/src/main/kotlin/com/correx/core/router/IdeaReader.kt

@@ -0,0 +1,37 @@
+package com.correx.core.router
+
+import com.correx.core.events.events.IdeaCapturedEvent
+import com.correx.core.events.events.IdeaDiscardedEvent
+import com.correx.core.events.stores.EventStore
+import com.correx.core.events.types.SessionId
+import com.correx.core.router.model.Idea
+
+/**
+ * Rebuilds the operator's idea board from the event log. Cross-session by design: it folds over
+ * [EventStore.allEvents] rather than a single session's replay, so an idea captured in one session
+ * shows on the board (and feeds the router) in every session. A captured idea is dropped once a
+ * matching [IdeaDiscardedEvent] tombstones it (invariant #1 — the capture stays in the log).
+ */
+class IdeaReader(private val eventStore: EventStore) {
+
+    /** Active ideas (captured, not later discarded) across all sessions, newest first. */
+    fun activeIdeas(): List<Idea> {
+        val discarded = mutableSetOf<String>()
+        val captured = mutableListOf<Idea>()
+        eventStore.allEvents().forEach { stored ->
+            when (val payload = stored.payload) {
+                is IdeaDiscardedEvent -> discarded += payload.ideaId
+                is IdeaCapturedEvent -> captured += Idea(payload.ideaId, payload.text, payload.timestampMs)
+                else -> Unit
+            }
+        }
+        return captured.filterNot { it.id in discarded }.sortedByDescending { it.capturedAtMs }
+    }
+
+    /** The session that captured [ideaId], so a discard tombstone lands alongside its capture. */
+    fun sessionOf(ideaId: String): SessionId? =
+        eventStore.allEvents()
+            .mapNotNull { it.payload as? IdeaCapturedEvent }
+            .firstOrNull { it.ideaId == ideaId }
+            ?.sessionId
+}

core/router/src/main/kotlin/com/correx/core/router/Ideas.kt

@@ -0,0 +1,39 @@
+package com.correx.core.router
+
+import kotlinx.serialization.json.Json
+import kotlinx.serialization.json.JsonObject
+import kotlinx.serialization.json.JsonPrimitive
+import kotlinx.serialization.json.contentOrNull
+import kotlinx.serialization.json.jsonArray
+import kotlinx.serialization.json.jsonObject
+import kotlinx.serialization.json.jsonPrimitive
+
+/**
+ * Parses an optional `ideas` block out of a router chat reply: the rubber-duck path may end its prose
+ * with a fenced ```` ```json {"ideas":["…","…"]} ``` ```` block to save ideas that crystallized.
+ * Lenient and fence-tolerant — scans every fenced block for the one carrying an `ideas` array, accepts
+ * string items or `{text|idea}` objects, and any parse failure or empty array yields no ideas. Returns
+ * the reply with that block stripped (so the operator reads only prose), mirroring [WorkflowProposals].
+ */
+object Ideas {
+
+    data class Parsed(val cleanText: String, val ideas: List<String>)
+
+    private val json = Json { ignoreUnknownKeys = true; isLenient = true }
+    private val fence = Regex("```(?:json)?\\s*(.*?)```", RegexOption.DOT_MATCHES_ALL)
+
+    fun parse(content: String): Parsed = runCatching {
+        fence.findAll(content).firstNotNullOfOrNull { match ->
+            val obj = runCatching { json.parseToJsonElement(match.groupValues[1].trim()).jsonObject }.getOrNull()
+            val array = obj?.get("ideas")?.jsonArray ?: return@firstNotNullOfOrNull null
+            val ideas = array.mapNotNull { element ->
+                when (element) {
+                    is JsonObject -> (element["text"] ?: element["idea"])?.jsonPrimitive?.contentOrNull
+                    is JsonPrimitive -> element.contentOrNull
+                    else -> null
+                }?.trim()?.ifBlank { null }
+            }
+            ideas.takeIf { it.isNotEmpty() }?.let { Parsed(content.removeRange(match.range).trim(), it) }
+        } ?: Parsed(cleanText = content, ideas = emptyList())
+    }.getOrElse { Parsed(cleanText = content, ideas = emptyList()) }
+}

core/router/src/main/kotlin/com/correx/core/router/RouterContextBuilder.kt

@@ -11,6 +11,7 @@ import com.correx.core.events.types.ContextPackId
 import com.correx.core.events.types.SessionId
 import com.correx.core.events.types.StageId
 import com.correx.core.inference.Tokenizer
+import com.correx.core.router.model.Idea
 import com.correx.core.router.model.NarrationTrigger
 import com.correx.core.router.model.RouterConfig
 import com.correx.core.router.model.RouterL2Entry
@@ -44,6 +45,10 @@ interface RouterContextBuilder {
 class DefaultRouterContextBuilder(
     private val config: RouterConfig,
     private val tokenizer: Tokenizer? = null,
+    // Cross-session ideas to surface to the router (router-only context). Read fresh each build so
+    // newly-captured ideas appear on the next turn; injected via the constructor so the build()
+    // interface — and its many test doubles — stay unchanged.
+    private val ideaProvider: () -> List<Idea> = { emptyList() },
 ) : RouterContextBuilder {
 
     companion object {
@@ -73,6 +78,15 @@ class DefaultRouterContextBuilder(
                 point at the relevant conventions or commands;
             (c) if recalled cross-session memory shows similar prior work, say so and reference it;
             (d) otherwise offer to think the problem through together in chat.
+
+            Rubber-duck capture: while thinking a problem through in chat, when a concrete idea or
+            decision crystallizes, capture it by ending your reply with a fenced json block:
+            ```json
+            {"ideas":["<one idea per item, in the operator's framing>"]}
+            ```
+            These are saved to the operator's idea board (also shown back to you under "## Ideas").
+            Capture only genuinely new, concrete ideas — omit the block when nothing crystallized,
+            and never re-capture an idea already under "## Ideas".
             """.trimIndent()
 
         private val NARRATION_SYSTEM_PROMPT =
@@ -86,6 +100,9 @@ class DefaultRouterContextBuilder(
             """.trimIndent()
 
         private const val RECALLED_MEMORY_PREFIX = "[recalled memory]"
+
+        /** Cap on ideas folded into router context, newest-first, so the board can't balloon L0. */
+        private const val MAX_IDEAS_IN_CONTEXT = 10
     }
 
     override suspend fun build(
@@ -131,10 +148,22 @@ class DefaultRouterContextBuilder(
                     role = EntryRole.SYSTEM,
                 )
             }
+        val ideasEntry: ContextEntry? = ideaProvider()
+            .takeIf { it.isNotEmpty() }
+            ?.let {
+                buildContextEntry(
+                    sourceType = "ideas",
+                    sourceId = "operator-ideas",
+                    content = buildIdeasContent(it),
+                    layer = ContextLayer.L0,
+                    role = EntryRole.SYSTEM,
+                )
+            }
 
         // Compute remaining budget after protected frame.
         val protectedTokens = systemPrompt.tokenEstimate + workflowStatusEntry.tokenEstimate +
-            (availableWorkflowsEntry?.tokenEstimate ?: 0) + (projectProfileEntry?.tokenEstimate ?: 0)
+            (availableWorkflowsEntry?.tokenEstimate ?: 0) + (projectProfileEntry?.tokenEstimate ?: 0) +
+            (ideasEntry?.tokenEstimate ?: 0)
         var remainingBudget = (budget.limit - protectedTokens).coerceAtLeast(0)
 
         val allEntries = mutableListOf<ContextEntry>()
@@ -142,6 +171,7 @@ class DefaultRouterContextBuilder(
         allEntries += workflowStatusEntry
         availableWorkflowsEntry?.let { allEntries += it }
         projectProfileEntry?.let { allEntries += it }
+        ideasEntry?.let { allEntries += it }
 
         var droppedCount = 0
         val truncatedLayers = mutableSetOf<ContextLayer>()
@@ -316,6 +346,14 @@ class DefaultRouterContextBuilder(
             }
         }.trimEnd()
 
+    private fun buildIdeasContent(ideas: List<Idea>): String =
+        buildString {
+            append("## Ideas\n")
+            append("Ideas the operator captured while thinking out loud. Reference them when relevant; ")
+            append("they are notes, not instructions.\n")
+            ideas.take(MAX_IDEAS_IN_CONTEXT).forEach { append("- ${it.text}\n") }
+        }.trimEnd()
+
     private suspend fun buildContextEntry(
         sourceType: String,
         sourceId: String,

core/router/src/main/kotlin/com/correx/core/router/RouterFacade.kt

@@ -4,6 +4,7 @@ import com.correx.core.events.events.ChatTurnEvent
 import com.correx.core.events.events.ChatTurnRole
 import com.correx.core.events.events.ContextTruncatedEvent
 import com.correx.core.events.events.EventMetadata
+import com.correx.core.events.events.IdeaCapturedEvent
 import com.correx.core.events.events.L3MemoryRetrievedEvent
 import com.correx.core.events.events.L3RetrievedHit
 import com.correx.core.events.events.NewEvent
@@ -156,12 +157,14 @@ class DefaultRouterFacade(
         )
         val inferenceResponse = provider.infer(inferenceRequest)
         val rawContent = inferenceResponse.text
-        // The triage path may append a structured workflow proposal; strip it from the visible turn
+        // The triage path may append a structured workflow proposal, and the rubber-duck path an
-        // so the operator reads prose, then record the candidates as a WorkflowProposedEvent below.
+        // `ideas` block; strip both from the visible turn so the operator reads only prose, then
+        // record them as events below.
         val proposal = WorkflowProposals.parse(rawContent)
+        val ideas = Ideas.parse(proposal.cleanText)
         // A length-finish with no visible text means the model burned the whole completion
         // budget on hidden reasoning — surface that instead of rendering a silent blank turn.
-        val content = proposal.cleanText.ifBlank {
+        val content = ideas.cleanText.ifBlank {
             if (inferenceResponse.finishReason is FinishReason.Length) {
                 "The model used its entire ${config.generationConfig.maxTokens}-token response budget " +
                     "without producing an answer (likely spent on hidden reasoning). " +
@@ -180,6 +183,7 @@ class DefaultRouterFacade(
 
         if (mode == ChatMode.CHAT) {
             emitWorkflowProposalIfAny(sessionId, input, proposal)
+            emitIdeasCaptured(sessionId, ideas.ideas)
         }
 
         val steeringEmitted = mode == ChatMode.STEERING && rawContent.isNotBlank()
@@ -329,6 +333,31 @@ class DefaultRouterFacade(
         )
     }
 
+    /** Records one [IdeaCapturedEvent] per idea the operator surfaced while rubber-ducking. */
+    private suspend fun emitIdeasCaptured(sessionId: SessionId, ideas: List<String>) {
+        ideas.forEach { text ->
+            val now = Clock.System.now()
+            eventStore.append(
+                NewEvent(
+                    metadata = EventMetadata(
+                        eventId = EventId(UUID.randomUUID().toString()),
+                        sessionId = sessionId,
+                        timestamp = now,
+                        schemaVersion = 1,
+                        causationId = null,
+                        correlationId = null,
+                    ),
+                    payload = IdeaCapturedEvent(
+                        ideaId = UUID.randomUUID().toString(),
+                        sessionId = sessionId,
+                        text = text,
+                        timestampMs = now.toEpochMilliseconds(),
+                    ),
+                ),
+            )
+        }
+    }
+
     private suspend fun emitSteeringNote(sessionId: SessionId, content: String, effectiveStageId: StageId) {
         val now = Clock.System.now()
         eventStore.append(

core/router/src/main/kotlin/com/correx/core/router/model/Idea.kt

@@ -0,0 +1,8 @@
+package com.correx.core.router.model
+
+/** An active idea on the operator's cross-session scratchpad (a captured, not-yet-discarded note). */
+data class Idea(
+    val id: String,
+    val text: String,
+    val capturedAtMs: Long,
+)

core/router/src/test/kotlin/com/correx/core/router/IdeasTest.kt

@@ -0,0 +1,55 @@
+package com.correx.core.router
+
+import org.junit.jupiter.api.Assertions.assertEquals
+import org.junit.jupiter.api.Assertions.assertTrue
+import org.junit.jupiter.api.Test
+
+class IdeasTest {
+
+    @Test
+    fun `parses a fenced ideas block and strips it from the prose`() {
+        val content = """
+            Good point — two things worth remembering.
+
+            ```json
+            {"ideas":["cache the repo map across sessions","add a --dry-run flag"]}
+            ```
+        """.trimIndent()
+
+        val parsed = Ideas.parse(content)
+
+        assertEquals(listOf("cache the repo map across sessions", "add a --dry-run flag"), parsed.ideas)
+        assertTrue(parsed.cleanText.startsWith("Good point"))
+        assertTrue("```" !in parsed.cleanText, "fence must be stripped from the visible turn")
+    }
+
+    @Test
+    fun `accepts object items with a text field`() {
+        val content = "prose\n```json\n{\"ideas\":[{\"text\":\"idea one\"},{\"idea\":\"idea two\"}]}\n```"
+        val parsed = Ideas.parse(content)
+        assertEquals(listOf("idea one", "idea two"), parsed.ideas)
+    }
+
+    @Test
+    fun `finds the ideas block even when it is not the first fence`() {
+        val content = "```json\n{\"propose_workflows\":[{\"id\":\"x\"}]}\n```\n" +
+            "and\n```json\n{\"ideas\":[\"keep this\"]}\n```"
+        val parsed = Ideas.parse(content)
+        assertEquals(listOf("keep this"), parsed.ideas)
+    }
+
+    @Test
+    fun `no block yields no ideas and untouched text`() {
+        val content = "Let's just talk it through."
+        val parsed = Ideas.parse(content)
+        assertTrue(parsed.ideas.isEmpty())
+        assertEquals(content, parsed.cleanText)
+    }
+
+    @Test
+    fun `blank and empty items are dropped`() {
+        val content = "x\n```json\n{\"ideas\":[\"  \",\"real\",\"\"]}\n```"
+        val parsed = Ideas.parse(content)
+        assertEquals(listOf("real"), parsed.ideas)
+    }
+}

infrastructure/src/main/kotlin/com/correx/infrastructure/InfrastructureModule.kt

@@ -24,6 +24,7 @@ import com.correx.core.inference.NoopEmbedder
 import com.correx.core.inference.Tokenizer
 import com.correx.core.router.DefaultRouterContextBuilder
 import com.correx.core.router.DefaultRouterFacade
+import com.correx.core.router.IdeaReader
 import com.correx.core.router.DefaultRouterReducer
 import com.correx.core.router.DefaultRouterRepository
 import com.correx.core.router.RouterFacade
@@ -300,7 +301,8 @@ object InfrastructureModule {
         val projector = RouterProjector(reducer)
         val replayer = DefaultEventReplayer(eventStore, projector)
         val repository = DefaultRouterRepository(replayer)
-        val contextBuilder = DefaultRouterContextBuilder(config, tokenizer)
+        val ideaReader = IdeaReader(eventStore)
+        val contextBuilder = DefaultRouterContextBuilder(config, tokenizer, ideaProvider = { ideaReader.activeIdeas() })
         return DefaultRouterFacade(repository, contextBuilder, inferenceRouter, eventStore, config, null, embedder, l3MemoryStore, workflowSummaryProvider, sessionProfileProvider)
     }
 }

testing/deterministic/src/test/kotlin/RouterFacadeTest.kt

@@ -33,6 +33,7 @@ import com.correx.core.router.ChatMode
 import com.correx.core.router.DefaultRouterContextBuilder
 import com.correx.core.router.DefaultRouterFacade
 import com.correx.core.router.DefaultRouterReducer
+import com.correx.core.router.IdeaReader
 import com.correx.core.router.RouterContextBuilder
 import com.correx.core.router.RouterFacade
 import com.correx.core.router.RouterProjector
@@ -664,6 +665,50 @@ class RouterFacadeTest {
         assertTrue(store.appendedEvents.map { it.payload }.filterIsInstance<WorkflowProposedEvent>().isEmpty())
     }
 
+    @Test
+    fun `rubber-duck ideas are captured, readable cross-session, and removed by a discard`(): Unit = runBlocking {
+        val store = mockEventStore()
+        val reply = "Right — worth keeping.\n```json\n{\"ideas\":[\"cache the repo map\",\"add --dry-run\"]}\n```"
+        val facade = facadeProposing(store, reply, emptyList())
+
+        facade.onUserInput(SessionId("chat-1"), "how do we speed this up?")
+
+        val reader = IdeaReader(store)
+        val ideas = reader.activeIdeas()
+        assertEquals(2, ideas.size)
+        assertTrue(ideas.any { it.text == "cache the repo map" } && ideas.any { it.text == "add --dry-run" })
+
+        // The visible chat turn must not leak the json block.
+        val routerTurn = store.appendedEvents.map { it.payload }
+            .filterIsInstance<ChatTurnEvent>().first { it.role == ChatTurnRole.ROUTER }
+        assertFalse(routerTurn.content.contains("```"))
+
+        // Discard one (from a *different* session) → it drops off the board, the other stays.
+        val drop = ideas.first { it.text == "add --dry-run" }
+        store.append(
+            NewEvent(
+                metadata = EventMetadata(
+                    eventId = EventId("disc-1"), sessionId = SessionId("chat-2"),
+                    timestamp = Instant.parse("2026-01-01T00:00:00Z"), schemaVersion = 1, causationId = null, correlationId = null,
+                ),
+                payload = com.correx.core.events.events.IdeaDiscardedEvent(ideaId = drop.id, sessionId = SessionId("chat-2")),
+            ),
+        )
+        val after = reader.activeIdeas()
+        assertEquals(listOf("cache the repo map"), after.map { it.text })
+    }
+
+    @Test
+    fun `a steering turn never captures ideas`(): Unit = runBlocking {
+        val store = mockEventStore()
+        val reply = "x\n```json\n{\"ideas\":[\"nope\"]}\n```"
+        val facade = facadeProposing(store, reply, emptyList())
+
+        facade.onUserInput(SessionId("s"), "steer", ChatMode.STEERING)
+
+        assertTrue(IdeaReader(store).activeIdeas().isEmpty())
+    }
+
     // --------------------------------------------------------------------------
     // Helpers
     // --------------------------------------------------------------------------