claude-sdk-oauth: a content-less user message forces a full-history re-send every time it disappears

[1vivy]

What happened?

sentMessages() hashes every user and toolResult message. A user message whose content array is
empty carries nothing to transmit and is transient — present for one provider call, gone by the next.

Because it is hashed, its removal shifts every later index, commonPrefixLength stops short of
sentCount, and the turn resolves to sent_stream_diverged: the session forks and re-sends the whole
history even though the conversation never changed.

Captured on 2026.8.9-2 from two consecutive provider calls of one session, both 203 messages:

call N      [200] toolResult bash  len=159   sha b518dd86
            [201] user             len=0     sha 12ae32cb   <- content: []
            [202] user             len=8194  sha 5c1ea814

call N+1    [200] toolResult bash  len=159   sha b518dd86
            [201] user             len=8194  sha 5c1ea814   <- shifted up one slot
            [202] toolResult bash  len=3191  sha aec978fe

Index 200 is byte-identical. Only the empty message disappears, and the next decision is
flatten / sent_stream_diverged at 203 messages.

Cost on one affected turn: cacheRead collapsed 582,409 -> 61,594 with cacheWrite 524,339, roughly 8x
its neighbouring turns. That session logged 12 sent_stream_diverged events in 12 minutes.

This is independent of #691 — it reproduces on 5275da1f5, after #751 merged.

Steps to reproduce

#791 carries the regression. To see the failure on unpatched main, check the PR out and revert only
the production change:

gh pr checkout 791
git checkout main -- packages/coding-agent/src/core/extensions/builtin/claude-sdk-oauth/session-sync.ts
cd packages/coding-agent
npx vitest run test/suite/regressions/790-claude-sdk-oauth-empty-user-continuity.test.ts

Observed:

× excludes a content-less user message from the transmitted set
    expected length 2, received 3
× stays a delta when a transient content-less user message disappears
    - "kind": "delta",   "reason": null
    + "kind": "fork",    "reason": "sent_stream_diverged"

Tests  2 failed | 2 passed (4)

Expected: Tests 4 passed (4).

Restore the fix with git checkout - -- packages/coding-agent/src/core/extensions/builtin/claude-sdk-oauth/session-sync.ts.

Expected behavior

A transient message carrying nothing to transmit should not participate in the continuity hash, so an
unchanged conversation stays a delta.

Genuine divergence must keep failing closed: a real rewrite of already-sent history should still report
sent_stream_diverged. The two passing controls in that test file cover both directions.

Version

@code-yeongyu/senpi 2026.8.9-2 (observed) · reproduced on main @ 5275da1f5
@anthropic-ai/claude-agent-sdk 0.3.220
provider claude-sdk-oauth · model claude-opus-5 · thinking high

I have a fix in #791. Written with an LLM; the reproduction, measurements, and verification are mine.

[1vivy]

The dockerfile is something I only posses and made out of a rush ._. I'll figure out whats the expected way to reproduce the issue.