[Radu028]

ACP agents such as GitHub Copilot keep their own conversation history, but the ACP adapter appended T3's runtime notes (<runtime_info> plus the <pull_request_linking> policy, about 1,300 characters) to every prompt. A Copilot thread had 20 identical copies after 20 messages. Claude and Codex get this text once.

Fix: The adapter now remembers, per ACP session, the runtime notes it last delivered. It sends them again only when the text changes (for example after a model switch), after a compaction T3 observes, or in a new or replacement session. A rollback replacement clears the record.

Resending after compaction uses the existing compaction markers, which also resend the larger orchestration instruction block. T3 marks a root session compacted when it sees one of these signals:

• An ACP compaction_update with status completed, including one that arrives while no turn is active or after the turn settled. Before this PR only a compaction inside an active turn set the marker, so after an idle compaction the next prompt carried neither block. The marker is set after the flavor routes the update, so a Devin subagent's compaction, which its normalizer routes to the child session, does not resend the root instructions.
• A successful native /compact, including one with preservation instructions after the command such as /compact keep the plan. Before this PR only a bare /compact counted.
• A successful Kiro compaction. Kiro answers a /compact prompt as chat, so T3 sends its own _kiro/session/compact request instead, and that sends no prompt.
• Copilot's raw session.compaction_complete event with success: true, outcome: "applied" or no outcome (older Copilot builds omit it), and no agentId. Copilot CLI 1.0.105 does not send compaction_update. Its runtime contains the ACP update name agent_message_chunk but not that one. It reports compaction in the github.com/copilot/sessionEvent feed that T3 already subscribes to for subagents. On a real Windows thread, Copilot compacted twice between turns and removed 108 of 116 messages, including the first prompt with T3's instructions, and T3 recorded no compaction. The Copilot routing now subscribes to that event and calls a new markSessionCompacted extension hook. The hook marks the session it is given. The Copilot routing passes the root session and skips events with an agentId, which come from a subagent's own compaction.

T3 now advertises ACP v1 session.compaction support at initialize, next to notices. The v1 schema says agents send compaction_update only to clients that advertise it, so without it a conforming v1 agent's automatic compactions stayed invisible and the notes never came back. The adapter also reads the compaction marker after all prompt preparation (text rewriting, attachment paths, image reads and Antigravity's attachment blocks), so a compaction that completes while a prompt is being prepared is not missed.

Advertising the complete contract also promises its patch semantics, so compaction updates now follow field presence for summary, error and _meta: an omitted field keeps the stored value, null (or an empty summary) clears it, and a value replaces it. v1 and v2 updates share that parser.

A runtime that T3 restarts after a hidden prompt failure, or replaces for a rollback, starts with no delivery record, so its first prompt carries the notes. The adapter records delivery when session/prompt returns a response, the same rule as the existing orchestration instruction state. It records it only while that prompt's runtime is still current. Otherwise a prompt from a replaced runtime that settles late could mark the notes as delivered to the replacement, which reuses the session id. The orchestration instruction state had the same race on main, so the same check now covers it. If the RPC fails with an ACP error or a broken connection, the adapter records nothing and the next prompt sends the notes again. A cancelled, refusal or error stop reason does not count as delivered, so the next prompt resends the notes. Grok produces cancelled locally, possibly before session/prompt was written. If the agent did see that prompt, this costs one redundant copy.

Two limitations remain. An agent that compacts without any signal T3 recognizes can still drop the notes, as it can already drop the orchestration instructions today. Compaction items are still projected only inside the turn that started them, as on main for ACP v2 agents. A compaction patch that arrives after that turn settles updates the resend marker but not the displayed item, and now applies to v1 agents too.

The approach follows the root cause and fix branch that Mariusz Mazur (mmazur) posted on https://git.cafe/pingdotgg/t3code/issues/24185. I adapted it to current main, extended the tests, and kept their co-author trailer on the first commit.

Proof:

• refreshes ACP prompt instructions when the interaction mode changes requires the second prompt in a session to be the bare request, which failed on main because the runtime block was appended. A model switch must send the notes again.
• reinjects ACP instructions after protocol compaction, during reinjection or while idle checks that reinjected prompts carry the notes and that the prompt after them is one bare block. Between turns, it now delivers an idle compaction_update, a subagent's session.compaction_complete (with agentId) and a root session.compaction_complete through the handlers the adapter registers on the mock runtime. Each new assertion fails with its source change reverted:
  • Without the idle marking in adapter.ts, Continue after idle compaction. goes out as a bare prompt.
  • Without the session.compaction_complete case in copilotAcp.ts, Continue after Copilot compaction. goes out as a bare prompt.
  • Without the agentId check, Continue after subagent compaction. carries both blocks again.
  • Requiring outcome: "applied", a legacy report without outcome leaves Continue after legacy Copilot compaction. bare.
  • Marking before routing, a Devin child compaction_update makes Continue after child compaction. carry both blocks.
• The compaction case now also delivers a compaction_update while a prompt is being prepared. The prompt after it must carry both blocks, which failed before the marker was read after preparation.
• advertises ACP session notices and shows them as system notices asserts the exact initialize session capabilities, { notices: {}, compaction: {} }. 15 Fx and Grok replay fixtures record that frame, so their initialize line changed the same way feat(acp): ACP session notices show as notices, not reply text #25704 changed it for notices.
• The compaction case also reports a prompt as cancelled, standing in for Grok's local cancel. On a fresh session and after an idle compaction, the next prompt must still carry both blocks. The Grok and registry message_steering replay fixtures recorded a bare follow-up after a cancelled first prompt; they now expect the resend.
• projects ACP v2 compaction lifecycle and prompt usage in runtimeModel.test.ts runs a sequence of compaction updates through the parser and merge, covering omit, clear and replace for summary, error and _meta.
• The native /compact case also runs /compact keep the plan; the next prompt must carry both blocks, which failed while only a bare /compact counted.
• The native /compact case checks that the prompt after a failed /compact is one bare block. Both compaction cases failed on main.
• The rollback case checks that the first prompt in the replacement session carries the notes. The mock agent reuses its session id, so this exercises the reset.
• resends T3 notes after rollback when the replaced runtime's prompt settles late holds the first runtime's prompt open, ends the turn with an idle report, rolls back, then settles the old prompt as cancelled. The next prompt must carry both blocks. The test waits on a new testHooks.afterPromptCallback before starting the replacement turn, so it does not depend on scheduling. The stale prompt settles with end_turn, since a cancelled one never records delivery. Without the generation check it fails with expected 'test prompt' to include 'T3 Code interaction mode: Default'.
• The replay helper materializeReplayTranscriptRuntimeInstructions follows the adapter's rule: native slash commands go out without the notes, and the notes are expected until a prompt that carried them succeeds. In grok_prompt_error, the retry after a failed prompt carries the notes again.

From apps/server:

vp test run src/orchestration-v2/Adapters/AcpAdapterV2.test.ts \
  src/orchestration-v2/Adapters/AcpRegistryAdapterV2.test.ts \
  src/orchestration-v2/testkit/*ReplayFixtures.integration.test.ts

All 308 tests in those 12 files pass, including every provider replay suite that uses the replay helper. ReplayRuntimeInstructions.test.ts covers the helper itself. vp run check:changed passes.

This logical transcript comes from the idle part of the compaction test, with long text blocks shortened. -> is a frame T3 sends to the mock agent. <- is an update the test delivers between turns through the handler the adapter registered, as the mock agent would.

-> session/prompt {"sessionId":"mock-session-1","prompt":["Finish work."]}
<- session/update {"sessionId":"mock-session-1","update":{"sessionUpdate":"compaction_update","compactionId":"compact-idle","status":"completed"}}
-> session/prompt {"sessionId":"mock-session-1","prompt":["<t3_code_instructions> ## T3 Cod... (4814 chars)","<runtime_info>In case you're ask... (1277 chars)"]}
<- github.com/copilot/sessionEvent {"sessionId":"mock-session-1","type":"session.compaction_complete","agentId":"agent-a","data":{"success":true,"outcome":"applied","trigger":"threshold"}}
-> session/prompt {"sessionId":"mock-session-1","prompt":["Continue after subagent compaction."]}
<- github.com/copilot/sessionEvent {"sessionId":"mock-session-1","type":"session.compaction_complete","data":{"success":true,"outcome":"applied","trigger":"threshold"}}
-> session/prompt {"sessionId":"mock-session-1","prompt":["<t3_code_instructions> ## T3 Cod... (4817 chars)","<runtime_info>In case you're ask... (1277 chars)"]}

A live run with Copilot CLI 1.0.105 on Windows, compared with main, is in this comment.

Overlap with https://git.cafe/pingdotgg/t3code/pulls/24740

https://git.cafe/pingdotgg/t3code/pulls/24740 persists a digest of the orchestration instructions so a reopened conversation does not receive them again. This change leaves that state alone. It adds an in-memory map next to promptInstructionStates and reuses the same compaction markers. https://git.cafe/pingdotgg/t3code/pulls/24740 also marks a completed root compaction_update outside turn projection and invalidates its receipt there, so the two conflict around compaction marking, resolvePromptParts and the prompt success taps. Whichever lands second should invalidate the receipt inside markSessionCompacted, so Copilot compactions clear it too. After a server restart or reopen, T3 sends the runtime notes once more, which is what main does for the orchestration instructions today. https://git.cafe/pingdotgg/t3code/pulls/24740's receipt could cover them later.

The Cursor and Muse adapters also append the runtime notes to every turn. I left them out of this PR.

Fixes https://git.cafe/pingdotgg/t3code/issues/24185

Written by Claude Opus 5.5 in Claude Code via T3 Code.

🤖 Generated with Claude Code

Fixes #24185

[macroscopeapp]

Approvability

Verdict: Approved at eb7f45e

Macroscope's review found this PR approvable — ACP sessions retain runtime notes instead of receiving duplicate copies on every message, with reinjection after compaction, instruction changes, or failed delivery. The change is confined to in-memory delivery tracking and replay expectations, with no persistent-data or product-default changes.

^{You can add or adjust custom eligibility rules. Learn more.}

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Walkthrough

The ACP adapter now tracks runtime-instruction delivery and completed compactions per session. It adds runtime instructions to eligible prompts when instructions change or compaction requires reinjection. Capability declarations, Copilot event handling, replay behavior, and tests were updated.

Changes: ACP Instruction and Compaction Handling

Layer / File(s)	Summary
Compaction capability declarations packages/effect-acp/src/compat.ts, packages/provider-acp/src/server/AcpSessionRuntime.ts, apps/server/src/orchestration-v2/testkit/fixtures/*/*_transcript.ndjson, packages/effect-acp/src/rpc.ts, packages/effect-acp/src/client.test.ts	The ACP client capability type and runtime initialization now include session.compaction. Transcript fixtures expect this capability alongside session.notices. The compatible prompt response accepts a response containing only stopReason.
Compaction event tracking packages/provider-acp/src/server/adapter.ts, packages/provider-acp-registry/src/server/copilotAcp.ts, apps/server/scripts/acp-mock-agent.ts, packages/provider-acp/src/server/runtimeModel.ts, packages/provider-acp/src/server/runtimeModel.test.ts	The adapter marks completed compactions and deduplicates reports by session and compaction ID. Copilot marks sessions compacted for qualifying completion events. The mock agent can emit an in-progress update for selected prompt ordinals. Compaction updates preserve omitted fields and support explicit clearing of summary and metadata values.
Prompt instruction delivery and replay packages/provider-acp/src/server/adapter.ts, apps/server/src/orchestration-v2/testkit/ReplayRuntimeInstructions.ts, apps/server/src/orchestration-v2/testkit/fixtures/*/*_transcript.ndjson, apps/server/src/orchestration-v2/Adapters/AntigravityAdapterV2.ts	The adapter builds prompts after attachment processing and records instruction delivery after eligible prompt results. Replay handling tracks successful prompt responses and skips instruction appending for already-instructed sessions and native slash commands. Antigravity uses the renamed attachment-prompt callback and conditionally omits its first generated block.
Prompt and compaction validation apps/server/src/orchestration-v2/Adapters/AcpAdapterV2.test.ts, apps/server/src/orchestration-v2/Adapters/AcpRegistryAdapterV2.test.ts	Tests cover instruction changes, model selection, ACP and Copilot compaction events, event timing, failed compaction, and rollback prompts. Registry adapter tests check runtime instructions in the first prompt and their omission from later prompts.

Estimated code review effort: 4 (Complex) | ~45 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant Copilot
  participant AcpAdapter
  participant AcpRuntime
  Copilot->>AcpAdapter: Send qualifying compaction-complete event
  AcpAdapter->>AcpAdapter: Mark session compacted
  AcpAdapter->>AcpRuntime: Submit eligible prompt with runtime instructions
  AcpRuntime-->>AcpAdapter: Return prompt result
  AcpAdapter->>AcpAdapter: Record instruction delivery after eligible result

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Possibly related issues

• pingdotgg/t3code#24465 — This change tracks instruction delivery per session but does not restore that state from the durable native conversation.
• pingdotgg/t3code#24838 — This change affects when runtime instructions are appended, but does not reconcile injected content in ACP user_message echoes.
• pingdotgg/t3code#16663 — The change recognizes /compact commands with trailing arguments and marks successful compaction turns, which relates to the reported ACP Registry compaction dispatch behavior.

Suggested reviewers: maria-rcks, juliusmarminge

Merge Risk: Minimal · previous review 67ad2

The identified compaction-delivery risks are addressed at the reviewed head. No actionable merge-blocking issue remains after normal checks.

Pre-merge checks: 3 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Description check		The description clearly explains the problem, change, and verification. However, the Scope and approval section does not include a maintainer comment that explicitly approves the direction and scope. …	Add a link to the triaged issue or approval discussion and include the maintainer comment that explicitly approves this change’s direction and scope. If the change qualifies as a very small, focused fix for an obvious bug, explain why it qu…

Passed checks: 3 passed

Check name	Status	Explanation
Title check		The title clearly and concisely describes the main change: runtime notes are sent once per ACP session instead of on every message.
Linked Issues check		Issue #24185 requires removal of repeated instructions in Copilot ACP conversations. The PR adds per-session runtime-note delivery state. Unchanged prompts omit the notes after successful delivery. Mo…
Out of Scope Changes check		The adapter state, ACP capability advertisement, Copilot compaction events, v1 and v2 patch handling, runtime model tests, replay helper, fixtures, and test changes support issue #24185. They prevent …

Full details: Linked Issues check

Explanation

Issue #24185 requires removal of repeated instructions in Copilot ACP conversations. The PR adds per-session runtime-note delivery state. Unchanged prompts omit the notes after successful delivery. Model changes, recognized compaction, cancellation, failure, and session replacement trigger the required resend. ACP compaction capability and Copilot event handling cover recognized compaction paths. Focused tests cover the delivery and retry behavior.

Full details: Out of Scope Changes check

Explanation

The adapter state, ACP capability advertisement, Copilot compaction events, v1 and v2 patch handling, runtime model tests, replay helper, fixtures, and test changes support issue #24185. They prevent duplicate notes while preserving delivery after compaction or failed prompts. No unrelated product change is demonstrated. Cursor and Muse adapters remain unchanged and are documented as out of scope.

Full details: Description check

Explanation

The description clearly explains the problem, change, and verification. However, the Scope and approval section does not include a maintainer comment that explicitly approves the direction and scope. The issue link and reference to another proposed fix do not provide that approval.

Resolution

Add a link to the triaged issue or approval discussion and include the maintainer comment that explicitly approves this change’s direction and scope. If the change qualifies as a very small, focused fix for an obvious bug, explain why it qualifies without prior approval.

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Nitpick comments: 1

apps/server/src/orchestration-v2/Adapters/AcpAdapterV2.test.ts: 1

3546-3663: Trivial · Functional Correctness · Quick win

Join the first prompt callback before starting the replacement turn.

firstPromptGate releases the forked prompt, but it does not wait for runRuntimeCallbackAtGeneration(...) to finish. The replacement turn can read delivery state before the stale callback runs. The test can then pass even if the generation guard is removed.

Add a test-only hook at the outer callback completion boundary and await it before runtime.startTurn(...). The existing promptWireSettled signal is too early, and afterPromptSettledWithBackgroundWork applies only to deferred background work.

Suggested fix

@@
   readonly testHooks?: {
     readonly onDeferredFinalizeScheduled?: (debounce: Duration.Input) => Effect.Effect<void>;
     readonly afterPromptSettledWithBackgroundWork?: () => Effect.Effect<void>;
+    readonly afterPromptCallback?: () => Effect.Effect<void>;
     readonly onNativeResponseLifecycle?: (event: {
@@
                 runRuntimeCallbackAtGeneration(
                   promptGeneration,
@@
-                ).pipe(Effect.asVoid),
+                ).pipe(
+                  Effect.andThen(options.testHooks?.afterPromptCallback?.() ?? Effect.void),
+                  Effect.asVoid,
+                ),
@@
       const firstPromptGate = yield* Deferred.make<EffectAcpSchema.PromptResponse>();
+      const firstPromptCallbackSettled = yield* Deferred.make<void>();
       const replacementPromptText = yield* Deferred.make<string>();
@@
       const adapter = yield* makeAcpAdapterV2({
         instanceId,
+        testHooks: {
+          afterPromptCallback: () =>
+            Deferred.succeed(firstPromptCallbackSettled, undefined).pipe(Effect.asVoid),
+        },
         flavor: {
@@
       yield* Deferred.succeed(firstPromptGate, { stopReason: "cancelled" });
+      yield* Deferred.await(firstPromptCallbackSettled);
       yield* runtime.startTurn(

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Review comments at
@apps/server/src/orchestration-v2/Adapters/AcpAdapterV2.test.ts:
- Around line 3546-3663: Update the prompt callback flow in makeAcpAdapterV2 to
expose a test-only signal after runRuntimeCallbackAtGeneration completes, then
await that signal in the late-prompt rollback test before the replacement
runtime.startTurn call. Do not use promptWireSettled or
afterPromptSettledWithBackgroundWork, since neither confirms the outer callback
has finished.

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Reviewing files that changed from the base of the PR and between 3c00ca3 and a6966ef.

Files selected for processing: 2

• apps/server/src/orchestration-v2/Adapters/AcpAdapterV2.test.ts
• packages/provider-acp/src/server/adapter.ts

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[Radu028]

Addressed the nitpick in #26959 (review) in f3bdcb6: the late-prompt rollback test now waits on a new testHooks.afterPromptCallback before starting the replacement turn. With the generation check swapped for an unchecked run of the same body, the test fails.

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[Radu028]

Merged current main (47f2c8d29e) into this branch as 67ad2a4677, without rebasing. Main extended the ACP prompt builder with resolveAttachmentPath, agentVersion and currentModel, and made the PR-linking notes depend on the flavor. The merge keeps this PR's attachment-only builder, which finishes before delivery state is read, passes it main's new inputs, and builds the runtime notes with main's sessionCarriesPullRequestLinking choice. vp run check:changed passes (14 package typechecks, 217 tests).

Live check with GitHub Copilot on Windows

Windows 11, Copilot CLI 1.0.105 over ACP, model claude-haiku-5.5. Each build ran as its own T3 server with an empty home and a throwaway git repo, driven by t3 thread start and t3 thread send --wait. A small wrapper between T3 and copilot --acp recorded only the shape of each session/prompt (block lengths and which T3 markers each block contains) and Copilot's compaction event. It recorded no text.

The same five messages on both builds:

#	Message	main 47f2c8d29e	this PR 67ad2a4677
1	Reply with exactly OK. Remember the code word ORCHID-26959.	instructions (4836) + runtime notes (1325)	instructions (4836) + runtime notes (1325)
2	Reply with exactly OK.	request + runtime notes (1325)	request only (22)
3	/compact Keep the code word ORCHID-26959.	command only	command only, then Copilot sends session.compaction_complete
4	What is the code word? Reply with just the word.	request + runtime notes, no instructions	instructions (4825) + runtime notes (1325)
5	Reply with exactly OK.	request + runtime notes (1325)	request only (22)

Numbers are characters per text block. Copilot completed every turn on both builds and answered ORCHID-26959 after the compaction.

This shows two things:

• With this PR, the notes go out once per session and come back after a real Copilot compaction, which T3 now sees through the github.com/copilot/sessionEvent feed. This was the part not verified live before.
• On main, the prompt after a Copilot compaction carries the runtime notes, as every prompt does, but not T3's orchestration instructions, because main never sees that Copilot compacted. With this PR both come back.

Not run live: the resend after a model switch, the retry after a cancelled or refused prompt, and rollback replacement. The focused tests cover those.

[Radu028]

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[Rika-xie]

Confirming this on another ACP provider, plus one datapoint the change detection should probably cover.

Environment: T3 Code 0.1.5 (macOS 26.5), provider acpRegistry, agent = Hermes Agent over ACP, model deepseek-flash.

Both blocks are appended to every ordinary prompt in a normally progressing thread: <runtime_info> and the full <pull_request_linking> policy, with no mode change, no model switch and no compaction between turns. Checked against T3's own message store (orchestration_v2_projection_messages): the user copy is stored as typed (e.g. 34 characters) while the provider-side copy carries <t3_code_instructions> and the runtime/PR suffix.

One extra case worth handling: in the same thread, the larger <t3_code_instructions> block also comes back after the thread is settled and reopened across days, even though that block is already gated on interaction mode / MCP / device-tool state. So "deliver once per session" will only hold if a reopen counts as a new delivery point — otherwise the state-discard path from #24465 keeps re-injecting it no matter what the runtime-notes memo does.

Happy to test the branch against this environment (acpRegistry + macOS + multi-day reopen) if that helps.

[Radu028]

Merged current main (c6ca605630) into this branch, without rebasing, plus six follow-up commits.

Main added Kiro, which compacts through its own _kiro/session/compact request and sends no prompt. For that:

• A successful native compaction now marks the session compacted, so the next prompt carries T3's instruction block and runtime notes again. It does not count as a delivery.
• The compaction check keeps both paths: /compact with or without preservation instructions for agents that compact through the prompt (Grok), and a bare /compact for Kiro's native call.
• The 18 Kiro replay fixtures and the Antigravity driver test now expect the compaction: {} client capability in initialize, like the Fx and Grok fixtures earlier in this PR. Before that, kiro_unknown_model failed with ACP replay frame mismatch on initialize.

Fixed while reviewing the merge:

• e085954c59: a runtime restarted after a hidden prompt failure (an end_turn the outcome check rejects, as with Antigravity) kept the delivery records, so its first prompt carried neither block. The restart now clears them like a replacement does. The existing hidden-outcome restart test now checks that the replacement prompt carries both.
• The replay helper materializeReplayTranscriptRuntimeInstructions now follows the adapter's delivery rule more closely, and has its own test file. It tracks the notes each session received for its current model, and resets them after a completed compaction_update (once per compaction id, including one that completes while a prompt runs or is still open when a turn succeeds), a successful /compact on Grok, a Kiro compaction answered with success: true, and a new initialize or session/close. Before this, kiro_compaction and provider_thread_resume timed out waiting for a prompt without the notes. It does not mirror Copilot compaction events, compactions routed to a child session, or ACP v2 prompts that settle through an idle state_update, since no recording has them yet.

All 18 replay fixture suites pass (254 tests), plus AcpAdapterV2, AcpRegistryAdapterV2, KiroAdapterV2 and AntigravityDriver, and the effect-acp, provider-acp, provider-acp-registry and provider-kiro packages. vp run check:changed passes.

[juliusmarminge]

Evidence

Real T3 servers with fresh synthetic state, real Copilot CLI 1.0.110, and Claude Haiku 4.5. Send the ORCHID memory prompt, two short follow-ups, /compact Keep the code word ORCHID-26959., the code-word question, and one final follow-up, waiting after every step. Evidence is captured server/CLI output and Copilot's native history.

Before	After
f70e349eca	7f1b9ae941
Native history: each runtime block occurs on prompts 1,2,3,4,5.	Native history: each runtime block occurs only on prompts 1,4, with prompt 4 immediately after compaction.
Successful native compaction; larger T3 instructions absent afterward.	Successful native compaction; larger T3 instructions restored afterward.
Command output	Command output

Both sessions returned ORCHID-26959 after compaction. Shared driver · History exporter.

Not shown:

• Configuration-change and replacement-session reinjection were not exercised; this run covered uninterrupted prompts and recognized real Copilot compaction.
• Failed-delivery retry behavior was not exercised because all prompts completed successfully.
• Browser interaction and video were not captured; this server behavior was driven through T3's CLI and verified in Copilot's native history.