feat(core): elide historic middle-pane note snapshots from model context

Every user message sent while a note is open carries a full snapshot of
that note in userMessageContext, so a long chat over one open note
resends N full copies on every model call. The elision decorator now
rewrites prior-turn user messages to keep the pane kind and path but
replace note content above a small floor with a placeholder pointing at
the still-readable file. The current message's snapshot is untouched,
so "summarize this" keeps working; the system prompt already tells the
model later middle-pane context overrides earlier. Config:
elideHistoricMiddlePaneContent, default on.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

apps/x/packages/core/docs/turn-runtime-design.md

@@ -393,18 +393,21 @@ Rules:
 The app wires the resolver through a decorator (`context-elision.ts`) that
 applies transmit-time elision to the materialized prefix: tool results from
 prior turns above a size threshold are replaced with a short placeholder
-telling the model to re-run the tool if it needs the output, and inline
-image parts from prior turns (video-mode webcam and screen-share frames) are
+telling the model to re-run the tool if it needs the output; inline image
+parts from prior turns (video-mode webcam and screen-share frames) are
 replaced with a text part recording how many frames of each kind were
-dropped. The durable log is untouched; only the transmitted bytes change.
+dropped; and middle-pane note snapshots on prior-turn user messages keep
+their kind and path but have their content replaced with a placeholder
+pointing at the still-readable file. The durable log is untouched; only the
+transmitted bytes change.
 Elision is a pure function of each message, so resolved prefixes stay
 byte-stable across calls (provider prefix caches keep working), and the
 current turn's own messages never pass through the resolver, so in-flight
 tool results and just-captured frames are always sent verbatim. Policy lives
 in `config/context.json` (`elideHistoricToolResults`, default true;
 `elideHistoricToolResultsThresholdChars`, default 10000;
-`elideHistoricImages`, default true). The inspect CLI composes through the
-same decorated resolver.
+`elideHistoricImages`, default true; `elideHistoricMiddlePaneContent`,
+default true). The inspect CLI composes through the same decorated resolver.
 
 ### 6.7 Agent snapshot inheritance
 

apps/x/packages/core/src/turns/context-elision.test.ts

@@ -4,6 +4,7 @@ import type { TurnContext, TurnEvent } from "@x/shared/dist/turns.js";
 import {
     ElidingContextResolver,
     elideHistoricImages,
+    elideHistoricMiddlePaneContent,
     elideHistoricToolResults,
 } from "./context-elision.js";
 import { TurnRepoContextResolver } from "./context-resolver.js";
@@ -238,10 +239,60 @@ describe("elideHistoricImages", () => {
     });
 });
 
+function noteMessage(content: string) {
+    return {
+        role: "user" as const,
+        content: "make this punchier",
+        userMessageContext: {
+            currentDateTime: "Thursday, July 2, 2026 at 10:00 AM GMT",
+            middlePane: {
+                kind: "note" as const,
+                path: "knowledge/Notes/Draft.md",
+                content,
+            },
+        },
+    };
+}
+
+describe("elideHistoricMiddlePaneContent", () => {
+    it("replaces large note snapshots, keeping kind and path", () => {
+        const [elided] = elideHistoricMiddlePaneContent([
+            noteMessage("n".repeat(600)),
+        ]);
+        if (elided.role !== "user") throw new Error("expected user message");
+        const middlePane = elided.userMessageContext?.middlePane;
+        if (middlePane?.kind !== "note") throw new Error("expected note pane");
+        expect(middlePane.path).toBe("knowledge/Notes/Draft.md");
+        expect(middlePane.content).toContain("omitted from history");
+        expect(middlePane.content).toContain("600");
+        expect(elided.userMessageContext?.currentDateTime).toBeDefined();
+        expect(elided.content).toBe("make this punchier");
+    });
+
+    it("keeps small notes and non-note panes untouched", () => {
+        const small = noteMessage("short todo list");
+        const browser = {
+            role: "user" as const,
+            content: "what is this page?",
+            userMessageContext: {
+                middlePane: {
+                    kind: "browser" as const,
+                    url: "https://example.com",
+                    title: "Example",
+                },
+            },
+        };
+        const plain = user("no context at all");
+        const messages = [small, browser, plain];
+        expect(elideHistoricMiddlePaneContent(messages)).toEqual(messages);
+    });
+});
+
 const POLICY_OFF = {
     toolResults: false,
     toolResultThresholdChars: 100,
     images: false,
+    middlePaneContent: false,
 };
 
 describe("ElidingContextResolver", () => {
@@ -327,4 +378,23 @@ describe("ElidingContextResolver", () => {
         expect(input.content.some((p) => p.type === "image")).toBe(false);
         expect(JSON.stringify(input.content)).toContain("1 webcam frame");
     });
+
+    it("elides middle-pane note snapshots in the resolved prefix when enabled", async () => {
+        const repo = new InMemoryTurnRepo();
+        const log = toolTurnLog(T1, [], "small").map((event) =>
+            event.type === "turn_created"
+                ? { ...event, input: noteMessage("n".repeat(600)) }
+                : event,
+        );
+        repo.seed(log);
+        const resolved = await resolver(repo, {
+            ...POLICY_OFF,
+            middlePaneContent: true,
+        }).resolve({ previousTurnId: T1 });
+        const input = resolved[0];
+        if (input.role !== "user") throw new Error("expected user message");
+        const middlePane = input.userMessageContext?.middlePane;
+        if (middlePane?.kind !== "note") throw new Error("expected note pane");
+        expect(middlePane.content).toContain("omitted from history");
+    });
 });

apps/x/packages/core/src/turns/context-elision.ts

@@ -25,6 +25,11 @@ import type { ITurnRepo } from "./repo.js";
 //     text carries the takeaway, and fresh frames arrive with every new call
 //     message. Unlike tool results they cannot be re-fetched, so the
 //     placeholder only records what was there.
+//   - Middle-pane note content: every user message sent while a note is
+//     open carries a full snapshot of that note in userMessageContext. Only
+//     the newest snapshot matters (the system prompt already tells the model
+//     later middle-pane context overrides earlier), and the current file is
+//     always re-readable at the recorded path.
 //
 // Elision is a pure function of each message's content, so resolved prefixes
 // stay byte-stable across calls and turns (provider prefix caches keep
@@ -35,18 +40,25 @@ export interface ElisionPolicy {
     toolResults: boolean;
     toolResultThresholdChars: number;
     images: boolean;
+    middlePaneContent: boolean;
 }
 
 export const DEFAULT_ELISION_POLICY: ElisionPolicy = {
     toolResults: true,
     toolResultThresholdChars: 10_000,
     images: true,
+    middlePaneContent: true,
 };
 
+// Notes smaller than this stay verbatim in history: below the floor the
+// placeholder saves nothing and a short note may carry useful context.
+const MIDDLE_PANE_CONTENT_FLOOR_CHARS = 500;
+
 const ContextConfig = z.object({
     elideHistoricToolResults: z.boolean().optional(),
     elideHistoricToolResultsThresholdChars: z.number().int().min(0).optional(),
     elideHistoricImages: z.boolean().optional(),
+    elideHistoricMiddlePaneContent: z.boolean().optional(),
 });
 
 const CONTEXT_CONFIG_PATH = path.join(WorkDir, "config", "context.json");
@@ -70,6 +82,9 @@ export function loadElisionPolicy(): ElisionPolicy {
                 DEFAULT_ELISION_POLICY.toolResultThresholdChars,
             images:
                 parsed.elideHistoricImages ?? DEFAULT_ELISION_POLICY.images,
+            middlePaneContent:
+                parsed.elideHistoricMiddlePaneContent ??
+                DEFAULT_ELISION_POLICY.middlePaneContent,
         };
     } catch {
         return DEFAULT_ELISION_POLICY;
@@ -124,6 +139,34 @@ export function elideHistoricImages(
     });
 }
 
+export function elideHistoricMiddlePaneContent(
+    messages: Array<z.infer<typeof ConversationMessage>>,
+): Array<z.infer<typeof ConversationMessage>> {
+    return messages.map((message) => {
+        if (message.role !== "user") {
+            return message;
+        }
+        const middlePane = message.userMessageContext?.middlePane;
+        if (
+            !middlePane ||
+            middlePane.kind !== "note" ||
+            middlePane.content.length <= MIDDLE_PANE_CONTENT_FLOOR_CHARS
+        ) {
+            return message;
+        }
+        return {
+            ...message,
+            userMessageContext: {
+                ...message.userMessageContext,
+                middlePane: {
+                    ...middlePane,
+                    content: `[Note content (${middlePane.content.length} characters) omitted from history to save context. This snapshot is stale; read the file at the path above if its content is needed now.]`,
+                },
+            },
+        };
+    });
+}
+
 // IContextResolver decorator: applies the elision policy to the materialized
 // cross-turn prefix. Agent snapshot resolution is delegated untouched.
 export class ElidingContextResolver implements IContextResolver {
@@ -155,6 +198,9 @@ export class ElidingContextResolver implements IContextResolver {
         if (policy.images) {
             prefix = elideHistoricImages(prefix);
         }
+        if (policy.middlePaneContent) {
+            prefix = elideHistoricMiddlePaneContent(prefix);
+        }
         return prefix;
     }