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fix(agent): recover interrupted local tool streams - #3726

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fix/incomplete-parallel-tool-stop
Aug 15, 2026
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fix/incomplete-parallel-tool-stop

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Summary

  • recover one wholly uncommitted local tool step when the step budget permits
  • fail closed after any local input was exposed or any provider-executed sibling appeared, preventing duplicate side effects and provider billing
  • preserve interrupted calls and model-visible errors in history during the bounded recovery
  • keep recovered, uncommitted fragments out of the public tool-call collection
  • preserve the last substantive assistant text when the recovery step has no replacement text
  • suppress only exact duplicate text from the bounded recovery step after text was already delivered
  • deliver distinct terminal recovery text through SSE, chunk callbacks, and persisted history
  • suppress shorter replay prefixes without shortening the completed response
  • terminalize incomplete local calls when recovery is unavailable or already exhausted
  • keep every terminal tool result paired with its preserved assistant tool-call part

Why

A provider stream can finish with finishReason: stop while a local tool call is still receiving arguments. The continuation gate previously stopped immediately, so a lone interrupted call could be silently dropped and persisted as completed.

Recovery is bounded to one attempt and only starts before a tool input reaches the public onToolCall callback. If a parallel sibling was already exposed, a retry could repeat a client-side mutation under a new tool-call ID, so that step remains fail closed and the terminalization path reports the interruption instead.

Red-green TDD

The original regression failed on main because a lone truncated call made one model request and then ended without execution or continuation. It now makes one bounded recovery request, executes the reconstructed call exactly once, and produces the final response.

Follow-up regression tests prove that recovery does not run after a sibling input was exposed, after a local result became final, alongside unresolved or completed provider work, after the step budget is exhausted, or after the recovery latch has been used.

The exhausted-recovery test also proves that a placeholder beside substantive text remains in history before its terminal error result, preventing an orphan result on the next provider turn.

The duplicate-text regression failed because the recovery step emitted and persisted the same assistant sentence twice. Recovery now buffers the ordered recovery output and removes text events and chunk callbacks only when the completed text exactly repeats the prior answer.

A distinct-text regression failed because the previous suppression also hid a legitimate replacement answer. Distinct terminal text now reaches SSE, chunk callbacks, the final response, and persisted history. A provider-result regression also proves that the unsuppressed stream state controls continuation, so a terminal recovery does not trigger an extra model step.

A prefix-extension regression failed because a recovery could replay the complete prior sentence before appending new text. Recovery now removes the delivered prefix across SSE deltas and chunk callbacks, persists only the new suffix, and retains the complete recovered answer in the final response.

A shorter-prefix regression failed because a truncated recovery replay was emitted and replaced the complete final text. Recovery now treats text in either prefix direction as already delivered, suppresses the replay, and retains the longer prior answer.

Public-response regressions failed because both a truncated input fragment and an empty-object placeholder appeared as failed calls beside the successfully reconstructed tool calls. Recovery still persists the model-visible error result, but omits the uncommitted fragment from AgentResponse.toolCalls.

Verification

  • focused agent runtime refresh suite: 35 steps passed
  • 9-file agent runtime and hosted-finalization suite: 249 steps passed
  • deno task fmt:check
  • deno task lint
  • deno task typecheck
  • deno task docs:api-reference:check: 44 files current
  • deno task test:unit: 3815 tests and 28526 steps passed, 1 test and 5 steps ignored
  • cwd suite: 10 tests and 193 steps passed
  • cwd-exclusion suite: 2 tests and 2 steps passed

The extra scripts/hooks/pre-push quick-verification stage is blocked by four inherited lint:cli-boundary findings that reproduce unchanged on exact origin/main a2356cab03e4f518c726ead52801439e53de0fc5: three imports in cli/commands/generate/command.ts and one import in cli/commands/build/error-handler.ts. The #3726 diff does not touch those files. Exact-head GitHub CI must be green before this PR is marked ready.

Closes veryfront/veryfront-issue-inbox#466

Summary by CodeRabbit

  • Bug Fixes

    • Improved recovery when streaming is interrupted during local tool calls.
    • Prevented duplicated assistant text from appearing in streamed responses.
    • Preserved partial tool input and accurately recorded interrupted tool-call errors.
    • Prevented unsafe retries when completed or provider-executed tool calls are present.
    • Suppressed incomplete placeholder tool calls from responses by default.
  • Documentation

    • Corrected the source link in the Agent Runtime API reference.
  • Tests

    • Added extensive coverage for interrupted streams, retries, recovery behavior, preserved text, and error handling.

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github-actions Bot commented Aug 14, 2026 •

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📦 Client bundle boundary

Entrypoint Modules Source size Server leaks
src/index.client.ts 454 3065 KiB ⚠️ 39 known

A server module in a client graph aborts hydration in the browser. New leaks fail CI; known leaks are tracked in scripts/lint/client-bundle-baseline.json to burn down.

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📒 Files selected for processing (6)
  • docs/api-reference/veryfront/agent.md
  • docs/api-reference/veryfront/chat.md
  • src/agent/runtime/index.ts
  • src/agent/runtime/refresh.test.ts
  • src/chat/chat-ui-message-helpers.test.ts
  • src/chat/chat-ui-message-helpers.ts

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  • src/agent/runtime/refresh.test.ts

📝 Walkthrough

Walkthrough

Streaming runtime execution now recovers once from interrupted local tool-call batches. It skips incomplete calls, records tool errors, preserves partial input and placeholders, and retries when another agent step remains. Provider-executed calls remain on their existing path.

Changes

Interrupted local tool-call recovery

Layer / File(s) Summary
Continuation recovery contract
src/agent/runtime/tool-result-continuation.ts, src/agent/runtime/tool-result-continuation.test.ts
shouldContinueAfterStreamStep accepts opt-in recovery settings. It continues for eligible incomplete local calls and rejects finalized or provider-executed calls.
Streaming runtime recovery
src/agent/runtime/index.ts, src/agent/runtime/streamed-assistant-message.ts
The runtime allows one recovery retry, skips interrupted local calls, preserves partial input and placeholders, removes replayed text, records tool errors, and updates substantive assistant text.
Recovery behavior validation
src/agent/runtime/refresh.test.ts
Tests cover interrupted batches, bounded retries, placeholder recovery, skipped execution, emitted errors, replayed text suppression, and preserved final text.
API reference synchronization
docs/api-reference/veryfront/agent.md
The AgentRuntime source link references the updated line.

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

Merge Risk: 🟡 Moderate · up to 27239

This change adds bounded recovery for interrupted local tool calls, but merge readiness remains uncertain because one regression expectation may disagree with the resulting terminal error behavior and part of the recovery path may be unreachable, which could leave interrupted calls mishandled in production. These issues should be resolved or explicitly accepted before merging.

Sequence Diagram(s)

sequenceDiagram
  participant AgentRuntime
  participant shouldContinueAfterStreamStep
  participant LocalToolExecutor
  participant SSEStream
  AgentRuntime->>shouldContinueAfterStreamStep: Check recovery and remaining steps
  shouldContinueAfterStreamStep-->>AgentRuntime: Continue for interrupted local calls
  AgentRuntime->>LocalToolExecutor: Skip interrupted calls and record errors
  AgentRuntime->>SSEStream: Emit deduplicated recovery output
  AgentRuntime->>AgentRuntime: Retry the interrupted batch once
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Possibly related PRs

Suggested reviewers: kwakayama, copilot

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name Status Explanation Resolution
Docstring Coverage ⚠️ Warning Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Write docstrings for the functions missing them to satisfy the coverage threshold.
✅ Passed checks (4 passed)
Check name Status Explanation
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Out of Scope Changes check ✅ Passed Check skipped because no linked issues were found for this pull request.
Description Check ✅ Passed Check skipped - CodeRabbit’s high-level summary is enabled.
Title check ✅ Passed The title clearly and concisely describes the main change: recovering interrupted local tool streams in the agent runtime.
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Comment thread src/agent/runtime/tool-result-continuation.ts Outdated

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Actionable comments posted: 1

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Inline comments:
In `@src/agent/runtime/refresh.test.ts`:
- Around line 1702-1705: Update the tool-output-error count assertion in the
interrupted-batch recovery test to expect three events, while preserving the
existing call-count, mutation-order, and recovery-message assertions.
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  • src/agent/runtime/tool-result-continuation.ts

Comment thread src/agent/runtime/refresh.test.ts
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kojiwakayama marked this pull request as draft August 14, 2026 19:41
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Recovery now treats every unfinished local call, including zero-delta placeholders, as the same bounded batch interruption. A terminal local streamed result closes the recovery path because re-calling the model could repeat an already-applied mutation.

Constraint: Provider-executed calls and exhausted step budgets remain fail-closed
Rejected: Keep placeholder recovery outside the retry latch | repeated placeholders could consume every remaining step
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Do not retry an interrupted local batch after any matching final local streamed result
Tested: 9 focused suites, 242 steps; repository fmt, lint, typecheck; generated API reference check
Not-tested: Live provider stream truncation

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Comment thread src/agent/runtime/index.ts Outdated
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kojiwakayama marked this pull request as draft August 14, 2026 19:58
A finalized local call emits tool-input-available before the stream step is evaluated. Recovery now stops when any local sibling reached that boundary, even if its result never arrived, because a client callback may already have applied the mutation.

Constraint: Safe recovery is limited to batches where every local call remains uncommitted
Rejected: Retry finalized local siblings without results | public client callbacks can execute them before server recovery
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Buffer local input-available events before widening recovery to mixed finalized and interrupted batches
Tested: 9 focused suites, 242 steps; repository fmt, lint, typecheck; generated API reference check
Not-tested: Live browser callback side effects under provider truncation
@kojiwakayama kojiwakayama changed the title fix(agent): retry interrupted local tool batches fix(agent): recover interrupted local tool streams Aug 14, 2026
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Comment thread src/agent/runtime/tool-result-continuation.ts Outdated
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Comment thread src/chat/chat-ui-message-helpers.ts Outdated
Comment thread src/agent/runtime/index.ts
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Comment thread src/agent/runtime/index.ts
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kojiwakayama marked this pull request as draft August 14, 2026 23:46
…el-tool-stop

# Conflicts:
#	docs/api-reference/veryfront/agent.md
Interrupted recovery now ends replay-prefix comparison at each non-text stream boundary. A suppressed prefix cannot be re-emitted after a provider tool or reasoning event when a later text segment diverges.

Constraint: Provider and reasoning events must remain live while replay text is withheld.

Rejected: Compare the full recovery text across event boundaries | it reorders a retained prefix after an already-delivered tool event.

Confidence: high

Scope-risk: narrow

Directive: Treat each text segment around non-text events as an independent replay candidate.

Tested: 4 focused suites, 184 steps; fmt, lint, check; full pre-push 3815 tests and 28535 steps plus cwd suites.

Not-tested: Live browser rendering with a provider-native tool.
…llel-tool-stop' into fix/incomplete-parallel-tool-stop
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Comment thread src/agent/runtime/index.ts
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Merged via the queue into main with commit 9a0d477 Aug 15, 2026
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kwakayama added a commit that referenced this pull request Aug 15, 2026
Recovery declined after exposed reasoning terminated the run with an
interrupted local tool call whose tool-input-start was still buffered.
recordToolError gates its tool-output-error on inputAnnounced, which is
never true for such a call, so the stream ended after the reasoning block
with no text, no tool call and no error. Flush the buffered
tool-input-start on that path so the failure renders as a tool card
matching what the assistant message already persists.

Log the declined recovery with step, tool name and persisted reasoning
part count. The check re-asks shouldContinueAfterStreamStep with the gate
lifted, so it fires only when a batch would actually have been replayed.

Move the "was this reasoning persisted?" predicate into
isPersistedReasoningPart, exported from streamed-assistant-message.ts and
used by both the message builder's skip check and the recovery gate. It
was stated twice as exact logical negations; a new StreamingReasoningPart
field could have updated one and silently re-permitted replay.

Reasoning is default-on across the hosted catalog, so the gate makes
#3726 inert on most hosted paths. Track the reconciliation protocol that
would lift it in #3736 and reference it from the gate.
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