Show HN: Understanding your Claude Code with trajectory segmentation

A new Claude Code plugin called trajectory segmentation adds a side pane that breaks long autonomous agent sessions into short, non-overlapping phases, listing each decision the agent made and the reason behind it. The plugin runs a small model every N steps (default 6) to label each chunk as NEW, AMEND, or SKIP, and offers a backfill button to reconstruct phases over up to the whole conversation plus a scoped 'btw' side-question feature that the agent never sees. The tool's stated purpose is to make delegation observable, since an agent can now run for tens of minutes and hundreds of tool calls on a single prompt.

Trajectory segmentation for long Claude Code sessions: a pane on the right that keeps a running, honest account of what the agent is doing, why, and every decision it makes, broken into short phases you can skim. You can delegate intelligence. You cannot delegate understanding. An agent now runs for tens of minutes and hundreds of tool calls on a single prompt. It reads files, edits them, runs commands, changes direction, recovers from a failed test, and by the time it stops you have a result but no picture of how it got there. The transcript holds every step, yet nobody scrolls back through four hundred lines to reconstruct the plot. So the understanding is simply lost, and you are left trusting an outcome you did not watch being made. That gap matters more as the models get better, not less. The more capable the agent, the more you hand it, and the more of the actual work happens while you are not looking. Capability you can buy. Legibility you have to build. This plugin spends a little intelligence to buy back the understanding: a small model watches the trajectory and keeps a running outline of the phases the agent moved through, and for each one the decisions it made and the reason behind them, so at any moment you can read the story instead of the log. It is deliberately cheap to run and worth more than it costs. Paying for a few extra tokens so that a long autonomous run stays explainable is a good trade, and it will look like an obviously good trade as runs get longer. The point of the panel is not to summarise text. It is to make delegation observable, and decisions are the part of a trajectory most worth seeing. - Short, non-overlapping phases. Each card is one thing the agent did: a step range, a one-line title, and a one-sentence summary. The model is biased to open a new phase whenever the action, target or goal shifts, so the outline stays fine-grained instead of collapsing into one long block. - Decisions, separated from the narration. Under each phase, every real choice the agent made is listed as the choice and, beneath it, the why. A decision is a choice with alternatives: an approach taken, an option rejected, a fix chosen after a failure, a tool or command picked for a reason. - Backfill. Turned the plugin on late, or want a cleaner pass with a stronger model? The backfill button asks how far back to go up to the whole conversation and which model and interval to use, then reconstructs the phases over that history. - btw: a side question about one phase. Like Claude Code's own /btw , but scoped: press btw on a card and ask anything about that phase. A model of your choice answers from the phase's steps and decisions, with the outline of every other phase for context, and the agent never sees the exchange. The thread stays on the card. A step is one action of the trajectory: a prompt you typed, an assistant message, or a single tool call with the start of what it returned. Every N steps default 6 the plugin runs a look. A look walks the new steps in N-sized chunks, and for each chunk the model reads the phases it has already written with their decisions and the recent steps, then answers one of three ways: - NEW — the action, target or goal shifted. A new phase card is pushed. This is the usual answer, which is what keeps the phases short. - AMEND — the newest steps are the direct continuation and result of the same action already in the current phase. Its summary is rewritten and any new decision is added to it. - SKIP — the steps did nothing worth recording. Nothing changes. Walking the backlog in chunks is deliberate: it stops one look from swallowing a whole multi-step turn into a single coarse block, so each chunk is roughly one action and becomes its own phase. Each card is a one-line title. Click it to expand the one-sentence summary, the decisions, and three buttons: transcript scrolls the conversation to where the phase starts, steps opens the exact steps it covers in a second pane, and btw asks a side question about it. ✕ dismisses a card. Built on Claude Code function hooks "Claude Mods" , in early access: it needs the environment variable the quick start sets, and the API can change between releases. - Claude Code 2.1.269 or later, with CLAUDE CODE ENABLE FUNCTION HOOKS=1 set. - An interactive terminal session. For the pane to dock on the right: the fullscreen layout the default outside tmux and at least 110 columns. Narrower, the pane sits inline above the prompt. - The model that writes the segments runs through your session's own credentials. Each chunk is one short completion, haiku by default. 1. Turn function hooks on in ~/.claude/settings.json merge the env key into what is there : { "env": { "CLAUDE CODE ENABLE FUNCTION HOOKS": "1" } } 2. Load it for one session, from a clone: git clone https://github.com/lucastononro/cc-traj-seg cd cc-traj-seg claude --plugin-dir . or install it as its own marketplace: claude plugin marketplace add lucastononro/cc-traj-seg then claude plugin install cc-traj-seg@cc-traj-seg . 3. It is off by default : installing it costs nothing until you ask. Run /traj in a session to turn the looks on and open the pane. /traj off turns them off again and closes the pane; the phases are kept. The switch persists across sessions, so once on it stays on until you say off. If the plugin was not running from the start of a session, or you want to re-segment the history with a different model, press backfill in the pane or run /traj backfill . A dialog asks three things and then rebuilds the phases: 1. How far — the whole conversation, or the last N steps. 2. Which model — the current one, haiku , sonnet or opus a full id goes through /traj model NAME . 3. Every how many steps — the chunk size for the reconstruction. The answers also become the ongoing settings, and live segmentation continues from the present once the backfill finishes. Claude Code's /btw lets you ask a side question about the conversation without it entering the agent's context. This is the same idea aimed at one phase. Press btw on an expanded card, or run /traj btw 3 why did it retry? . A dialog offers three stock questions why did it do this, what did it try that did not work, what was left undone ; any other question goes through /traj btw N <question . The answering model reads the outline of every phase for context, then the focused phase in full: its summary, its decisions, the steps it covers, and any earlier questions about it. It is told to ground the answer in that phase and to say when something is not in the record rather than guess. Answers open in a btw N pane, newest first, with an ask another button; Esc closes it. The thread is saved with the phase, and the card's meta line counts it. It has its own model setting, /traj btw model NAME , sonnet by default: answering a pointed question is worth a slightly stronger model than naming phases is. Press settings in the pane, or run /traj settings , for a frame with everything the plugin runs on: the on/off switch, the phase model, the interval, the window, the btw model, the tokens this session has used, and the four prompts it sends. Each setting has a change or edit button. The prompts are yours to rewrite. There are four: the segmentation system prompt what a phase is, the SKIP/AMEND/NEW rule, the TITLE/SUMMARY/DECISIONS format , the segmentation prompt template the user turn for each look , the btw system prompt , and the btw prompt template . The templates are mustache-style: the plugin substitutes {{variable}} placeholders, and an unknown name is left in place so the mistake is visible instead of silently blank. The two that matter most: | variable | in the segmentation template | in the btw template | |---|---|---| | {{long-horizon-context}} | every phase so far, oldest first: title, summary, decisions. The model's own memory of the run. | the same outline, with the focused phase marked IN FOCUS | | {{short-horizon-context}} | the last {{window}} steps as n kind lines, with a --- NEW STEPS --- marker before the ones since the last phase | the focused phase in full: title, summary, decisions, its steps, earlier questions about it | The segmentation template also gets {{steps-shown}} , {{step-count}} , {{new-count}} and {{window}} ; the btw template gets {{question}} and {{phase}} . The frame lists the legend. Editing happens in the pane. edit on a prompt row opens it in an editor pane in place of the frame: click in the text and type. Enter breaks a line, Backspace and Delete edit, the arrows, Home, End, PageUp and PageDown move, Tab indents, a paste lands whole, and {{variables}} are highlighted as you type. ctrl+s or the save button stores it; the header shows the character count and whether the text is default, custom and saved, or unsaved. reset to default puts the built-in back. Esc gives the keyboard back to the prompt and keeps your draft; closing the editor returns to the frame. Below the text sits the legend for that prompt's variables, and a warning appears live for a {{name}} the plugin does not know, or for a segmentation template with no {{short-horizon-context}} , since the model would then never see the steps. For an external editor there is a file round-trip too: export to file writes all four prompts and the legend to ~/.claude/cc-traj-seg/prompts.md , one section each; load from file reads them back, and a section left at its default or emptied means default. /traj prompts export , load and reset do the same from the keyboard. Overrides persist across sessions and apply to the next look. The settings frame has a tokens · this session section, and /traj tokens prints the same lines. It has two halves, and they are not equally precise: - agent · as the API reported it. Every completed turn carries the usage the API returned, so this is exact: per model, the number of turns and the input, output, cache-read and cache-write tokens. Under it, the session's live context tokens used of the window, and the percentage , the cost so far in dollars as /cost totals it, and the rate-limit windows the last response reported, with when they reset. - cc-traj-seg · estimated. A plugin's completion returns only the reply's text, never its usage, so this plugin's own calls are counted from characters at about four per token and shown with ≈ . Per model and per purpose: the phase looks live and backfill and the btw answers, with the call count and the estimated input and output. Both halves are kept per session in the plugin's store, so a resumed session shows its own. | /traj | turn the looks on and open the pane off by default | | /traj now | segment the steps since the last phase, right away | | /traj backfill | segment the history so far asks how far, which model, and N | | /traj btw N question | ask a side question about phase N the newest if omitted ; with no question, a dialog asks | | /traj btw model NAME | which model answers btw questions default sonnet | | /traj off | turn the looks off and close the pane: no model calls at all; the phases are kept, and /traj now and backfill still work | | /traj settings | the settings frame: models, cadence, the four prompts, and tokens | | /traj tokens | tokens this session: the agent per model as the API reported it, context and cost, and this plugin's calls estimated | | /traj prompts export / load / reset | the prompts as a markdown file to edit, read back, or all back to default | | /traj every N | look every N steps default 6 | | /traj window N | how many of the latest steps the model sees per chunk default 40 | | /traj model NAME | which model writes the phases: haiku default , sonnet , opus , or a full id | | /traj clear | drop every phase | | /traj stop | hide the pane; the looks keep running | | /traj help | the list above, and the current settings | In the pane, now , backfill , settings , clear and close mirror the commands; a card's title expands it; transcript scrolls the conversation to the phase's first row; steps opens its steps in a tab that scrolls while it holds the keyboard and closes on Esc; btw asks about it; ✕ dismisses the card. Settings persist across sessions. Phases are kept per session, so claude --resume shows the session's own. - N every . Smaller means more phases, finer-grained, at more model calls; larger is coarser and cheaper. Six is fine-grained by default; raise it if you want fewer, broader phases. - Model. haiku is enough to name a phase and its decisions cheaply. Switch to sonnet for a run you care about, or backfill the whole conversation with sonnet at the end for a clean narrative. - Window sees . How many recent steps the model reads per chunk, on top of its own earlier phases. Raise it if a phase misreads something that happened a little earlier. - hooks/register.tsx is the hooks module. It hooks tool.call and turn.complete to count steps and, when a look is due, walks the backlog in every -sized chunks, calling $.model.complete for each without making the turn wait. ui.render for { component: 'Pane' } draws the cards and a second pane for one phase's steps; the backfill dialog is $.ui.ask . Hooks on UserMessage and AssistantMessage renders remember each transcript row's id for the transcript button; a tool row is addressed by its tool-use id directly. - hooks/editor.tsx is the prompt editor, a surface module with its own keyboard and cursor, over the pure buffer in hooks/edit.ts insert, break, delete, move, soft wrap, click-to-place, variable tokens , which tests/edit.test.ts covers. - hooks/traj.ts is the pure part: the transcript flattened to steps, the context variables and the mustache rendering of both templates, the default system prompts, the SKIP/AMEND/NEW reply protocol, the choice-and-why decision parsing, decision merging, the prompts-file round trip, and the argument and dialog-answer parsers. tests/traj.test.ts covers it. bun test or: npx -y bun@1 test bunx --bun oxlint@1.83.0 hooks tests --deny-warnings claude plugin validate .claude-plugin/plugin.json lists the hooked events and $ calls Type checking needs the early-access types: run /plugin-types in a session in this folder writes the git-ignored .claude/types/ , then bunx -p typescript tsc -p . . Edits hot-reload into a running session; module state resets on a reload, so reopen the pane. Four things the engine taught this plugin: a helper that receives $ must be a top-level function declaration in the module; the engine's own node await next e cannot sit under a Box with a width ; a plugin's $.ui.ask dialog reliably returns only its option labels, since text typed under "Other" is routed through the permission flow and comes back as a denial, which is why the prompt editor is a surface module with its own keyboard rather than a dialog; and a paste reaches a surface module's onKey as one event carrying the whole text. The screenshots and the gif were captured from a real session driven through tmux docs/capture/cast.py turns tmux capture-pane -e frames into an asciicast that agg renders . - One chunk yields at most one phase, so the finest granularity is one phase per N steps; lower N for finer. - The model sees only the recent steps plus its own earlier phases, so a phase can misread something further back. That is the trade for a small, cheap prompt. - transcript moves the conversation only for a row the terminal has drawn in this session; on a resumed session older rows may not be addressable, and the steps pane is the fallback. - Nothing draws in claude -p , the desktop app or mobile. MIT. See LICENSE https://github.com/lucastononro/cc-traj-seg/blob/main/LICENSE .