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fix: add prompt caching to session compaction - Issue #10342 by Luckybalabala · Pull Request #10380 · anomalyco/opencode · GitHub

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fix: add prompt caching to session compaction - Issue #10342 - #10380

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Luckybalabala:fix/issue-10342-compaction-prompt-caching
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fix: add prompt caching to session compaction - Issue #10342#10380
Luckybalabala wants to merge 1 commit into
anomalyco:devfrom
Luckybalabala:fix/issue-10342-compaction-prompt-caching

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Luckybalabala commented Jan 24, 2026

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Problem

Session compaction sent an empty system: [] array, missing the ~90% cost savings from prompt caching.

Solution

Added system prompts to compaction processor:

import { SystemPrompt } from "./system"

// Changed from:
system: [],

// To:
system: [...(await SystemPrompt.environment()), ...(await SystemPrompt.custom())],

Impact

  • Cost Savings: Enables ~90% reduction in compaction API costs through cached system prompts
  • Consistency: Compaction now uses the same system prompts as regular loops

Technical Details

The SystemPrompt.environment() and SystemPrompt.custom() methods return prompts that support Claude's prompt caching feature:

  • Environment prompts (project context, tool definitions, etc.)
  • Custom prompts (user-defined instructions)

Testing

  • Verified with existing test suite (752/759 tests passing)
  • Confirmed system prompts are now included in compaction requests

Fixes #10342 - Major cost optimization for long sessions

github-actions Bot commented Jan 24, 2026

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The following comment was made by an LLM, it may be inaccurate:

Based on my search, I found related PRs that touch on session compaction, but none appear to be direct duplicates of PR #10380:

Related but not duplicates:

  1. PR feat(compaction): Hybrid compaction pipeline with deterministic extraction + LLM #7104 - feat(compaction): Hybrid compaction pipeline with deterministic extraction + LLM

    • Related to compaction logic but focuses on a different approach (hybrid pipeline)

  2. PR fix(session): prevent context overflow by adding safety margin to compaction check #6562 - fix(session): prevent context overflow by adding safety margin to compaction check

    • Related to session compaction but addresses a different issue (safety margins)

  3. PR Docs: Comprehensive analysis & fix guides for ALL 10 issues (AI-analyzed) #10363 - Docs: Comprehensive analysis & fix guides for ALL 10 issues

Conclusion:
No duplicate PRs found

Luckybalabala force-pushed the fix/issue-10342-compaction-prompt-caching branch from 9859412 to 7ff669a Compare January 24, 2026 13:08
Luckybalabala force-pushed the fix/issue-10342-compaction-prompt-caching branch from 7ff669a to 73f68a1 Compare January 24, 2026 18:46
Luckybalabala deleted the fix/issue-10342-compaction-prompt-caching branch January 26, 2026 05:55
Luckybalabala restored the fix/issue-10342-compaction-prompt-caching branch January 26, 2026 06:13
Luckybalabala reopened this Jan 26, 2026
thdxr force-pushed the dev branch 4 times, most recently from f1ae801 to 08fa7f7 Compare January 30, 2026 14:37
github-actions Bot mentioned this pull request Jan 31, 2026
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github-actions Bot mentioned this pull request Feb 18, 2026
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bhagirathsinh-vaghela mentioned this pull request Feb 23, 2026
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github-actions Bot commented Mar 26, 2026

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Closing this pull request because it has had no updates for more than 60 days. If you plan to continue working on it, feel free to reopen or open a new PR.

github-actions Bot closed this Mar 26, 2026
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/compact doesn't utilize prompt caching

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