# Agent Note: Trajectory inspection ledger

Status: implemented

English | [中文](2026-07-27-trajectory-inspection-ledger.zh.md)

## Problem

Trajectory has to make prose, machine payloads, token usage, timing, and nested tool activity readable in the same viewport. The earlier stacked Turn and Step cards preserved hierarchy but spent too much vertical space on repeated chrome, while a completely flat table would erase the causal structure that makes a trajectory useful. Role colors also risked borrowing success and warning semantics, which made visual decoration indistinguishable from runtime state.

## Decision

**Render a compact, turn-aware event ledger with a local record inspector, using the existing DeepSeek design system.**

- The ledger keeps session events in sequence within rewind-delimited branches. Turn boundaries use a slightly heavier rule, the raw Turn id, and a continuous left rail; Request boundaries appear as small points integrated into that structure and use one chronological numbering space across ordinary and compaction requests.
- Event kind and content form the two stable columns. Role tags align toward the content, nested subtools receive a small indentation, and CSS truncation preserves the available preview width. Token usage and duration stay in the inspector.
- Product prose uses the existing sans stack. Turn ids, token counts, durations, tool calls, raw payloads, and other machine data use the existing code stack.
- Existing theme tokens own both light and dark rendering. Neutral borders and surfaces form the structure; distinct low-emphasis role hues support scanning without carrying success or failure meaning, while business blue identifies selection, links, and focus.
- The client runtime exposes a read-only history source independent from Session and SessionManager. Each activated source owns its raw entries, paging, live gap repair, and reconnect rebuild; the ordinary conversation snapshot remains the folded Chat projection. Trajectory opens the source's tail while mounted and requests one older page when the user reaches the loaded range's top, then lazily derives event order, context lineage, schema index, and Requests instead of imposing those structures on every conversation consumer.
- Ordinary generation and compaction calls form one chronological Request projection, distinguished by purpose rather than separate collections. Effective prompt state and its change ride the Request that introduced them; compaction and prompt changes are not independent inspection entities. Request numbering and cumulative usage cover the loaded history window and expand as older pages arrive.
- Call schemas come from the active recorded Request header. Keyless snapshot fixtures deliberately replace that catalog with the non-array `{{tools}}` token, which the durable inspection boundary treats as unavailable instead of attempting to project or fabricate schemas.
- Selecting a record or Request opens an inspector inside Trajectory. Tabs and Summary sections follow the selected entity: Markdown messages expose rendered, source, provenance, and hierarchy views; tools add JSON payload/result and schema views; Requests add options, usage, timing, and result navigation. Scrollable Summary regions keep their scrollbar thumbs transparent until hover or `focus-within`, while retaining the scrollbar reservation and scroll behavior. Images render as media rather than serialized data.
- Turn folding removes all rows after its first record and replaces them with a compact step/tool-call count; Assistant folding applies the same interaction to its tool-call descendants. Global controls fold or expand both levels.
- A long ledger initially positions the loaded tail at the bottom and mounts only the viewport's row window plus bounded overscan. Request-only separators join the next measurable virtual item, with a terminal separator retaining its own fixed clearance, so the virtualizer never owns a zero-height item. Semantic DOM-safe row keys and ARIA indexes expose identity independently from mount position. A tail with known older history virtualizes immediately even when its loaded projection is below the ordinary row threshold. Stable-key virtualizer anchoring preserves the visible item across prepends and appends; the manual scroll-height fallback applies only when completing pagination disables virtualization. Selection, timeline focus, folding, search, and bottom following address records by stable event or tool-call identity rather than requiring their DOM rows to exist. An explicit loading row covers records until initial positioning finishes and while an older page is pending. The raw window base sequence detects a prepend even when a page adds no surface-visible node.
- The separate Waterfall tab is removed. A fixed Overview above the ledger projects every loaded record with known `startedAt` onto three semantic timing lanes using its own duration. While an older prefix remains unloaded and the viewport includes the loaded domain's start, a neutral ellipsis control covers the truncated edge and loads one earlier page without assigning unknown history a fabricated duration; hovering that control suppresses the ordinary timeline cursor. Finalized Assistant spans divide the recorded interval at the first non-empty token delta, so distinct TTFT and decoding colors retain their actual ratio; incomplete timing falls back to one Assistant color. Hovering for 500 ms exposes exact start/end, total duration, TTFT, and decoding time without relying on the browser's native tooltip delay. Dragging left or right commits an inclusive interval filter: any record whose active interval overlaps either boundary remains visible, records without known timing leave the focused ledger, and clearing the selection restores the full branch. Wheel gestures zoom the time domain. A right-button click clears the interval selection; dragging instead pans an already zoomed viewport without mutating it. The Overview keeps the full time domain while focused so the selection can be resized or cleared without losing orientation.
- Live history updates retain the ledger's bottom position only while the user is already following its tail. Scrolling upward clears that follow state, so streamed chunks and newly appended records do not interrupt inspection of earlier rows. Tail following and virtualizer measurement react to row keys and heights rather than content identity, so text-only stream frames neither discard the measurement cache nor repeat a DOM scroll write.
- Token streaming reuses the finalized history inspection, layout, Request numbering, Overview projection, and search results. A frame appends only the current partial Assistant cells and searches that partial when a query is active; text and reasoning deltas do not re-fold or rescan the loaded prefix, while message completion, tool lifecycle, compaction, rewrites, and other structural events rebuild the affected projections. Before those rebuilds, the inspection ledger drops completed-step token payloads that no projection reads while retaining the first visible token for timing, every usage chunk for accounting, and every chunk from unfinished or interrupted steps; the independent history source retains the raw entries.
- History folding rebases only the loaded surface events into a compact contiguous input for the canonical surface manager, then maps its nodes back to absolute session sequences. Structural events therefore retain canonical replacement validation without replaying token chunks or materializing synthetic events for unloaded sequences.
- Trajectory opts into a conversation-owned composer overlay through `data-conversation-composer-overlay`. `ConversationRoot` positions the composer seat and publishes its live height; Trajectory keeps the ledger at full height and reserves that height plus 16 px inside its vertical table and inspector scrollers. Those panes adapt to the available width instead of exposing horizontal scrollbars beneath the overlay.
- This local inspector remains independent from the conversation-wide Chat details column. At narrow widths it overlays the ledger and remains dismissible by keyboard or pointer.

## Alternatives considered

**Copy Vite DevTools fonts, colors, glass surfaces, or component shapes.** Rejected: those choices express a different product identity. The implementation only adopts the transferable method: neutral structure, semantic accents, machine-data typography, dense scanning, and shadows reserved for floating layers.

**Keep one card per Turn and Step.** Rejected: repeated card chrome reduced the number of visible records and made cross-step comparison slower.

**Mount every projected record in the table.** Rejected: record projection remains useful for search, timing, and navigation, but keeping every row and its descendants in the DOM makes browser rendering scale with the complete session instead of the visible viewport.

**Exhaust every history page when Trajectory mounts.** Rejected: complete session metrics would be immediately available, but transporting and repeatedly projecting old chunk-heavy pages delays inspection of the current tail. On-demand backward paging makes that cost follow the user's navigation.

**Rebuild the loaded ledger for every streamed token chunk.** Rejected: virtual rows bound DOM work but do not make repeated history folding cheap. Keeping finalized projections stable makes ordinary deltas proportional to the current partial, while structural events remain the explicit full-rebuild boundary.

**Flatten every record without Turn or Request boundaries.** Rejected: a trajectory is not merely a log stream; those boundaries preserve the causal structure without consuming dedicated rows.

**Reuse the global Chat details column.** Rejected: it would couple local inspection to conversation navigation and make a row click unexpectedly change another view's state.

**Override the composer seat from Trajectory CSS.** Rejected: a cross-package selector would depend on generated class specificity and stylesheet order. An explicit view marker keeps seat geometry and active-phase precedence in `ConversationRoot`, while Trajectory owns only its internal clearance.

**Keep timing in a separate Waterfall tab.** Rejected: the placeholder summarized node counts rather than record timing and forced users to switch away from the rows they wanted to focus. A full-domain Overview keeps timing and filtered records in one visual context.

**Change global theme tokens to match the reference.** Rejected: the existing theme already provides paired light and dark semantic layers, and a local redesign does not justify changing unrelated surfaces.

## Consequences

Trajectory shows more useful records per viewport while retaining Turn and Request orientation. Context rewrites and compactions remain inline with their surrounding history, while a rewind begins a successor branch that inherits only the retained prefix. The floating composer leaves the ledger visible to the viewport edge without covering its final rows or hiding horizontal controls. The main ledger omits token usage and duration so content receives the available width; the local inspector exposes those facts together with full payloads, provenance, schemas, and request timing. The Overview uses recorded start/duration and token-boundary facts without fabricating live elapsed time, and its inclusive focus behavior matches the interaction users already know from Chrome DevTools Network. Tail-first paging bounds initial transport and projection work, virtualization bounds mounted row elements, incremental partial projection removes loaded-history length from ordinary token-frame work, and completed-step chunk compaction makes structural rebuilds proportional to inspection-relevant entries rather than the raw token count. Focused component tests pin tail-first paging, prepend anchoring and identity retention, the virtual window, tail following, content-only streaming without repeated scroll writes, streaming structural sharing, high-sequence window folding, timing projection, delayed detail disclosure, folding, record and interval selection, entity-specific tabs, and running/error semantics. A real-browser long-ledger contract pins stable prepend geometry, bounded mounting, top/middle/bottom reachability, and bounded scroll writes across a paced stream; the assembled Web snapshot pins the ledger, Overview timing details, composer overlay geometry, and inspector through the real client composition.
