fix(core): send only changed entries in instruction updates - #57626

Issue for this PR

Related to #55529

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Problem

When a skill, MCP server instruction or project reference changed, the update message restated the whole list. Additions and removals were already small deltas, but a single edited description resent every entry, which wastes context in long sessions.

Change

Skill, MCP and reference updates now send only the entries that changed. Each changed entry is rendered whole with the existing entry renderer, under a line saying it replaces the previously listed entry with the same ID or name. This is not a field-level diff. For MCP that framing matters: a server that turns off Code Mode loses its "Use tools from this server through execute..." line, and the new entry replaces the old one.

When one update has additions, changes and removals, they go in one text in a fixed order: added, changed, removed. The full list is still restated only when no difference is detected.

Before and after, when one skill's description changes:

-The available skills have changed. This list supersedes the previous available skills list.
-Skills provide specialized instructions and workflows for specific tasks.
-Use the skill tool to load a skill when a task matches its description.
-The user may also invoke a skill directly. ...
-<available_skills>
-  <skill>
-    <id>debugging</id>
-    <name>Debugging</name>
-    <description>Diagnose hard bugs</description>
-  </skill>
-  <skill>
-    <id>effect</id>
-    <name>Effect</name>
-    <description>Build applications with Effect v4</description>
-  </skill>
-  ... every other skill ...
-</available_skills>
+The following skills have changed. Each entry replaces the previously listed skill with the same ID:
+  <skill>
+    <id>effect</id>
+    <name>Effect</name>
+    <description>Build applications with Effect v4</description>
+  </skill>

Update texts are rendered once and stored in the durable event, so update messages already in existing sessions stay byte-identical. I did not change the baseline (initial) text, which is re-rendered on every request, so existing sessions keep their prompt cache.

Out of scope

The Code Mode catalog part of #55529 is not addressed here.

How did you verify your code works?

From packages/core:

• bun typecheck: passes
• bun run script/test.ts test/skill/instructions.test.ts test/mcp-instructions.test.ts test/reference-instructions.test.ts: 17 pass. I updated the tests that asserted the full-list restatement and added a reference test that covers added, changed and removed entries in one update.
• bun run script/test.ts test/session-runner.test.ts test/session-compaction.test.ts: 249 pass

Screenshots / recordings

Not a UI change.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

Added by the opencode bot (AI-generated).

Related: #55529