diff --git a/NucleicRemote/NucleicRemote/Views/Transcript/IncrementalTranscriptProjection.swift b/NucleicRemote/NucleicRemote/Views/Transcript/IncrementalTranscriptProjection.swift
new file mode 100644
index 0000000..54a2ec6
--- /dev/null
+++ b/NucleicRemote/NucleicRemote/Views/Transcript/IncrementalTranscriptProjection.swift
@@ -0,0 +1,417 @@
+import Foundation
+import NucleicProtocol
+
+/// Stable-prefix incremental projector (docs/TRANSCRIPT_INCREMENTAL_PROJECTION.md).
+///
+/// `TranscriptProjection.build` folds the whole stream on every read, so a streaming turn of K
+/// deltas over an N-event transcript costs O(N·K) ≈ O(N²) per session. But the stream is
+/// immutable except at the tail: everything before the live turn is frozen — its `messageID`s and
+/// `toolCallID`s never recur. So this projector **seals** the longest provably-stable prefix of
+/// the folded item list once, and re-folds only the unstable suffix per delta: O(live-tail)
+/// instead of O(N).
+///
+/// The seal point (an index into the raw event stream) is chosen so that
+/// `fold(prefix) ++ fold(tail)` is byte-identical to `fold(whole)`:
+///
+/// 1. **No coalescing key crosses the seam.** Text/thinking/tool items coalesce on
+///    `messageID`/`toolCallID` (and a subagent's events name their parent Task). An exact
+///    interval check over the current stream forbids any seam inside an id's first→last
+///    reference span, so no item can straddle it.
+/// 2. **No id can recur after the seam.** Future arrivals are fenced by closure rules read off
+///    the stream itself: a tool is closed once its result arrived; a message/thinking block once
+///    a different message has started in its scope; everything, once its turn completed. These
+///    are the premises of the design doc ("the stream is immutable except at the tail"); if one
+///    is ever violated — a tail event referencing a sealed id — it is *detected* and the
+///    projector resets and re-folds from scratch, so correctness never rests on them.
+/// 3. **No tool run is split.** `coalesceToolRuns` merges adjacent `.tool` items, so the seam
+///    only falls where the last folded item is a hard separator — a visible non-tool row that a
+///    future tool call can't merge across. (Empty redacted-thinking rows are transparent to runs
+///    and therefore to this rule too.)
+/// 4. **Lock notes fold exactly, even across the seam.** A lock's `released` note lands when the
+///    file lands in the parent — potentially many turns after the edit it brackets — so sealed
+///    edit cards stay reachable through a registry (`TranscriptProjection.PriorEdit`): a tail
+///    note that path-matches a sealed edit patches that card, exactly where a whole-stream fold
+///    would put it. No lag heuristic, no divergence.
+/// 5. **`worktreeRoot` is a carried constant.** Lock-path normalization needs the seq-0
+///    `sessionStarted` cwd; nothing seals until it is known, and it never changes once found.
+///
+/// The equivalence test (`IncrementalProjectionEquivalenceTests`) replays streams and asserts
+/// `incremental(prefix) == build(prefix)` for **every** prefix — the whole correctness argument,
+/// checked mechanically.
+///
+/// A class held in `@State` so reads/updates from `body` don't invalidate the view; not
+/// thread-safe (main-actor use only, like the `ProjectionCache` it replaces).
+final class IncrementalTranscriptProjection {
+
+    // MARK: - Sealed state
+
+    /// Folded output of the stable prefix — appended to at each seal, never re-walked.
+    private var sealedItems: [TranscriptItem] = []
+    /// Watermark into the raw stream: `events[0..<sealedEventCount]` produced `sealedItems`.
+    private var sealedEventCount = 0
+    /// `events[sealedEventCount - 1].seq` at seal time — detects a rewritten/merged prefix
+    /// (a reconnect backfill slotting events in by seq) that shifts history under the watermark.
+    private var sealedLastSeq: UInt64 = 0
+    /// The stream's first seq — detects a session switch / stream reset.
+    private var streamFirstSeq: UInt64?
+    /// Carried constant from the first non-empty `sessionStarted.cwd`. Nothing seals while nil
+    /// (a root arriving later would retroactively change sealed lock folds).
+    private var worktreeRoot: String?
+    /// Every `messageID`/`toolCallID` referenced by a sealed event. A later event referencing one
+    /// would mutate sealed output — detected here, answered with a full reset (self-healing).
+    private var sealedIDs: Set<String> = []
+    /// Sealed edit-class calls in item order, for cross-seam lock-note folding (rule 4).
+    private var sealedEdits: [TranscriptProjection.PriorEdit] = []
+    /// Where each sealed edit's card sits in `sealedItems` (it may live inside a `.toolBlock`).
+    private var sealedEditIndex: [String: Int] = [:]
+    /// Display toggles are fold inputs; flipping either resets.
+    private var showRaw = false
+    private var showLockEvents = true
+
+    // MARK: - Read memo
+
+    /// `body` re-evaluates far more often than the stream changes (settling layout, scroll
+    /// geometry); this collapses those redundant reads to a cached return, as the previous
+    /// `ProjectionCache` did. The stream is append-only with strictly increasing seq, so
+    /// `(count, firstSeq, lastSeq)` pins it.
+    private struct MemoKey: Equatable {
+        var count: Int
+        var firstSeq: UInt64
+        var lastSeq: UInt64
+        var showRaw: Bool
+        var showLockEvents: Bool
+    }
+    private var memoKey: MemoKey?
+    private var memoValue: [TranscriptItem] = []
+
+    /// Hold-back from the stream edge: never seal into the newest few events. Decoders emit
+    /// tightly-coupled events in one batch (a `toolResult` and its inferred `fileChange`; a
+    /// whole-message's final chunks) that sync may deliver one at a time — holding the edge back
+    /// keeps a mid-batch read from sealing an entity whose trailing batch-mates are still in
+    /// flight. Cheap insurance on top of the closure rules; violations would only cost a reset.
+    private let edgeLag = 4
+
+    /// Test hook: how far the watermark has advanced (the equivalence suite also asserts sealing
+    /// actually happens, so a regression to "never seal" can't pass silently).
+    var sealedEventCountForTesting: Int { sealedEventCount }
+
+    // MARK: - Read
+
+    func items(for events: [AgentEvent], showRaw: Bool, showLockEvents: Bool) -> [TranscriptItem] {
+        let key = MemoKey(count: events.count, firstSeq: events.first?.seq ?? 0,
+                          lastSeq: events.last?.seq ?? 0, showRaw: showRaw, showLockEvents: showLockEvents)
+        if memoKey == key { return memoValue }
+
+        if needsReset(events, showRaw: showRaw, showLockEvents: showLockEvents) { reset() }
+        self.showRaw = showRaw
+        self.showLockEvents = showLockEvents
+        streamFirstSeq = events.first?.seq
+        if worktreeRoot == nil {
+            // Only the unsealed region needs scanning: sealing requires the root, so a sealed
+            // region can only exist after it was found.
+            worktreeRoot = TranscriptProjection.worktreeRoot(in: events[sealedEventCount...])
+        }
+
+        var watermark = chooseWatermark(events)
+        if watermark == nil {
+            // A tail event referenced a sealed id — a closure premise was violated (late file
+            // change, resumed message, post-result subagent child). Refold from scratch; with no
+            // sealed ids the second pass cannot be violated.
+            reset()
+            streamFirstSeq = events.first?.seq
+            worktreeRoot = TranscriptProjection.worktreeRoot(in: events[...])
+            watermark = chooseWatermark(events)
+        }
+        seal(events, upTo: watermark ?? sealedEventCount)
+
+        let result = render(events)
+        memoKey = key
+        memoValue = result
+        return result
+    }
+
+    // MARK: - Reset / identity
+
+    private func needsReset(_ events: [AgentEvent], showRaw: Bool, showLockEvents: Bool) -> Bool {
+        if showRaw != self.showRaw || showLockEvents != self.showLockEvents { return true }
+        if events.count < sealedEventCount { return true }
+        if sealedEventCount > 0 {
+            if events.first?.seq != streamFirstSeq { return true }
+            if events[sealedEventCount - 1].seq != sealedLastSeq { return true }
+        } else if streamFirstSeq != nil, events.first?.seq != streamFirstSeq {
+            return true  // nothing sealed, but the carried worktreeRoot belongs to the old stream
+        }
+        return false
+    }
+
+    private func reset() {
+        sealedItems = []
+        sealedEventCount = 0
+        sealedLastSeq = 0
+        streamFirstSeq = nil
+        worktreeRoot = nil
+        sealedIDs = []
+        sealedEdits = []
+        sealedEditIndex = [:]
+        memoKey = nil
+        memoValue = []
+    }
+
+    // MARK: - Watermark selection
+
+    /// The coalescing ids an event mentions: its `messageID` or `toolCallID`, plus the parent
+    /// Task id for subagent-owned events. Two events sharing an id must land on the same side of
+    /// the seam; the parent link chains a subagent's whole scope (and, transitively, deeper
+    /// descendants) to its spawn.
+    private static func refs(of event: AgentEvent) -> [String] {
+        switch event.kind {
+        case .userText(let c), .assistantText(let c), .thinking(let c):
+            if let parent = c.parentToolCallID { return [c.messageID, parent] }
+            return [c.messageID]
+        case .toolCallStarted(let c), .toolCallCompleted(let c):
+            if let parent = c.parentToolCallID { return [c.toolCallID, parent] }
+            return [c.toolCallID]
+        case .toolCallInputDelta(let d): return [d.toolCallID]
+        case .toolResult(let r): return [r.toolCallID]
+        case .fileChange(let f): return f.toolCallID.map { [$0] } ?? []
+        default: return []
+        }
+    }
+
+    /// One id's life within the unsealed region.
+    private struct IDSpan {
+        var firstRef: Int
+        var lastRef: Int
+        /// Result arrived → the tool (or Task, with its children) is done.
+        var resultSeen = false
+        /// A later chunk with a different messageID in the same scope → this message is done
+        /// (its authoritative non-partial text can only arrive before the next message starts).
+        var closedByChunk = false
+        /// For message/thinking ids: the owning subagent scope ("" = top level). The owner
+        /// Task's result closes everything inside it.
+        var chunkScope: String?
+    }
+
+    /// What an event *creates* in the folded item list, for the run-split rule (3).
+    private enum Creation {
+        /// A visible, never-dropped, non-tool row — a safe last-item for a seam.
+        case separator
+        /// A `.tool` row a future adjacent call could merge with.
+        case tool
+        /// A thinking row: a separator iff its final text is non-empty (an empty redacted block
+        /// is transparent to run coalescing, so it must be transparent to the seam rule too).
+        case thinking(String)
+        /// A lock note that may fold away (dropping it can fuse the runs around it), so it
+        /// counts as nothing — the seam just waits for the next hard separator.
+        case transparent
+    }
+
+    /// The furthest event index the stream can be sealed to right now, or nil when a region event
+    /// references an already-sealed id (premise violation → caller resets).
+    private func chooseWatermark(_ events: [AgentEvent]) -> Int? {
+        let start = sealedEventCount
+        let n = events.count - start
+        // Everything below needs the worktree root (rule 5); without it, just verify no sealed-id
+        // violation … but nothing is sealed if no root was ever found, so there is nothing to do.
+        guard worktreeRoot != nil else { return start }
+        guard n > edgeLag else {
+            // Too little unsealed to advance, but tail refs must still be validated against
+            // sealed ids so a violation triggers the reset path.
+            for r in 0..<n where Self.refs(of: events[start + r]).contains(where: sealedIDs.contains) {
+                return nil
+            }
+            return start
+        }
+
+        var spans: [String: IDSpan] = [:]
+        var creations: [Creation?] = Array(repeating: nil, count: n)
+        var thinkingText: [String: String] = [:]
+        var seenMessageItem = Set<String>()
+        var seenThinkingItem = Set<String>()
+        var seenToolItem = Set<String>()
+        var lastChunkInScope: [String: String] = [:]
+        var lastBoundary = -1  // region index of the latest turnCompleted/runFinished
+
+        for r in 0..<n {
+            let event = events[start + r]
+            for id in Self.refs(of: event) {
+                if sealedIDs.contains(id) { return nil }
+                if var span = spans[id] {
+                    span.lastRef = r
+                    spans[id] = span
+                } else {
+                    spans[id] = IDSpan(firstRef: r, lastRef: r)
+                }
+            }
+            switch event.kind {
+            case .userText(let c), .assistantText(let c):
+                trackChunk(c, in: &spans, lastChunkInScope: &lastChunkInScope)
+                if seenMessageItem.insert(c.messageID).inserted { creations[r] = .separator }
+            case .thinking(let c):
+                trackChunk(c, in: &spans, lastChunkInScope: &lastChunkInScope)
+                let existing = thinkingText[c.messageID] ?? ""
+                thinkingText[c.messageID] = c.isPartial ? existing + c.text : c.text
+                if seenThinkingItem.insert(c.messageID).inserted { creations[r] = .thinking(c.messageID) }
+            case .toolCallStarted(let c), .toolCallCompleted(let c):
+                if seenToolItem.insert(c.toolCallID).inserted { creations[r] = .tool }
+            case .toolResult(let result):
+                spans[result.toolCallID]?.resultSeen = true
+            case .toolCallInputDelta, .fileChange, .approvalResolved:
+                break  // refs (if any) tracked above; creates nothing
+            case .turnCompleted, .runFinished:
+                creations[r] = .separator
+                lastBoundary = r
+            case .sessionStarted, .usage, .rateLimit, .approvalRequested, .error:
+                creations[r] = .separator
+            case .note(let note):
+                if note.lockEvent && !showLockEvents { break }
+                if let lock = note.lock, !lock.paths.isEmpty { creations[r] = .transparent }
+                else { creations[r] = .separator }
+            case .raw:
+                if showRaw { creations[r] = .separator }
+            }
+        }
+
+        // Future-proofing (rule 2): an id wholly before the seam must be *closed* — provably done
+        // taking new events. An open id caps the seam at its first reference (it stays whole in
+        // the tail).
+        var cap = n - edgeLag
+        for (_, span) in spans where !isClosed(span, spans: spans, lastBoundary: lastBoundary) {
+            cap = min(cap, span.firstRef)
+        }
+
+        // Interval isolation (rule 1): no seam inside any id's [firstRef, lastRef] span.
+        // maxLastFromBefore[w] = the furthest lastRef among ids first referenced before w; a seam
+        // at w is isolation-safe iff that never reaches w.
+        var maxLastAtFirst = [Int](repeating: -1, count: n)
+        for span in spans.values {
+            maxLastAtFirst[span.firstRef] = max(maxLastAtFirst[span.firstRef], span.lastRef)
+        }
+
+        // Run-split rule (3): replay creations in item order; a seam is placeable after event r
+        // only while the last solid (visible, surviving) item is a hard separator. The replay
+        // resolves each thinking row against its *final* region text, which is exactly what the
+        // sealed fold will contain (open ids were already excluded by `cap`).
+        var separatorOK = [Bool](repeating: false, count: n)
+        var lastSolidIsSeparator = true  // sealed prefix is empty or ends with a separator (invariant)
+        for r in 0..<n {
+            switch creations[r] {
+            case .separator: lastSolidIsSeparator = true
+            case .tool: lastSolidIsSeparator = false
+            case .thinking(let id):
+                let text = thinkingText[id] ?? ""
+                if !text.trimmingCharacters(in: .whitespacesAndNewlines).isEmpty {
+                    lastSolidIsSeparator = true
+                }
+            case .transparent, nil: break
+            }
+            separatorOK[r] = lastSolidIsSeparator
+        }
+
+        var runningMaxLast = -1
+        var best = 0
+        for w in 1...min(cap, n - edgeLag) where w >= 1 {
+            runningMaxLast = max(runningMaxLast, maxLastAtFirst[w - 1])
+            if separatorOK[w - 1] && runningMaxLast < w { best = w }
+        }
+        return start + best
+    }
+
+    /// Track a text/thinking chunk for message-closure: a new messageID in a scope closes the
+    /// previous one (chunks of one message never resume after the next begins — decoder order).
+    private func trackChunk(
+        _ chunk: TextChunk, in spans: inout [String: IDSpan], lastChunkInScope: inout [String: String]
+    ) {
+        let scope = chunk.parentToolCallID ?? ""
+        spans[chunk.messageID]?.chunkScope = scope
+        if let previous = lastChunkInScope[scope], previous != chunk.messageID {
+            spans[previous]?.closedByChunk = true
+        }
+        lastChunkInScope[scope] = chunk.messageID
+    }
+
+    private func isClosed(_ span: IDSpan, spans: [String: IDSpan], lastBoundary: Int) -> Bool {
+        if span.lastRef < lastBoundary { return true }  // its turn completed; ids don't cross turns
+        if span.resultSeen { return true }
+        if span.closedByChunk { return true }
+        if let scope = span.chunkScope, !scope.isEmpty, spans[scope]?.resultSeen == true {
+            return true  // the owning subagent returned; its inner stream is done
+        }
+        return false
+    }
+
+    // MARK: - Sealing
+
+    private func seal(_ events: [AgentEvent], upTo watermark: Int) {
+        guard watermark > sealedEventCount else { return }
+        let slice = events[sealedEventCount..<watermark]
+        let segment = TranscriptProjection.buildSegment(
+            slice, worktreeRoot: worktreeRoot, priorEdits: sealedEdits,
+            showRaw: showRaw, showLockEvents: showLockEvents)
+
+        // Lock notes in this segment that folded onto edits sealed earlier: bake them in — the
+        // note is now sealed too, so the fold is final.
+        for patch in segment.priorLockPatches {
+            Self.applyLock(patch.lock, to: patch.toolCallID, at: sealedEditIndex, in: &sealedItems)
+        }
+        let editIDs = Set(segment.edits.map(\.toolCallID))
+        for item in segment.items {
+            let index = sealedItems.count
+            switch item.kind {
+            case .tool(let group) where editIDs.contains(group.toolCallID):
+                sealedEditIndex[group.toolCallID] = index
+            case .toolBlock(let groups):
+                for group in groups where editIDs.contains(group.toolCallID) {
+                    sealedEditIndex[group.toolCallID] = index
+                }
+            default:
+                break
+            }
+            sealedItems.append(item)
+        }
+        sealedEdits.append(contentsOf: segment.edits)
+        for event in slice {
+            for id in Self.refs(of: event) { sealedIDs.insert(id) }
+        }
+        sealedEventCount = watermark
+        sealedLastSeq = events[watermark - 1].seq
+    }
+
+    // MARK: - Rendering
+
+    private func render(_ events: [AgentEvent]) -> [TranscriptItem] {
+        let tailSlice = events[sealedEventCount...]
+        guard !tailSlice.isEmpty else { return sealedItems }
+        let tail = TranscriptProjection.buildSegment(
+            tailSlice, worktreeRoot: worktreeRoot, priorEdits: sealedEdits,
+            showRaw: showRaw, showLockEvents: showLockEvents)
+        if tail.priorLockPatches.isEmpty { return sealedItems + tail.items }
+        // A live (unsealed) lock note folded onto a sealed edit card: patch a copy per read —
+        // the note may still be re-evaluated until it seals, so the base stays unpatched.
+        var patched = sealedItems
+        for patch in tail.priorLockPatches {
+            Self.applyLock(patch.lock, to: patch.toolCallID, at: sealedEditIndex, in: &patched)
+        }
+        return patched + tail.items
+    }
+
+    /// Append a folded lock line to a sealed edit's card, whether it renders alone or inside a
+    /// coalesced `.toolBlock`.
+    private static func applyLock(
+        _ lock: NoteLock, to toolCallID: String, at index: [String: Int],
+        in items: inout [TranscriptItem]
+    ) {
+        guard let i = index[toolCallID] else { return }
+        switch items[i].kind {
+        case .tool(var group) where group.toolCallID == toolCallID:
+            group.lockLines.append(lock)
+            items[i].kind = .tool(group)
+        case .toolBlock(var groups):
+            guard let k = groups.firstIndex(where: { $0.toolCallID == toolCallID }) else { return }
+            groups[k].lockLines.append(lock)
+            items[i].kind = .toolBlock(groups)
+        default:
+            break
+        }
+    }
+}