fix(acp): degrade gracefully when the agent lacks additional directory support - #86
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…y support Contract v2 added a hard failure when the session requests additionalDirectories and the ACP agent does not advertise sessionCapabilities.additionalDirectories. OpenCode has never advertised the capability (its initialize returns only close/fork/list/resume) and ignores the field entirely, so the old driver's unconditional passthrough was a silent no-op. The strict gate therefore turned every published-agent run on the acp-fallback runtime into 'ACP session setup failed: ACP agent does not advertise additional directory support.' in production. Drop the directories instead of failing the session and report them back so the backend can log the degradation (driver.acp.session.additional_directories_dropped).
This was referenced Jul 25, 2026
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Production incident (try.mosoo.ai, 2026-07-25)
Every run on the acp-fallback runtime fails after
run.dispatchedwith:Started when the mosoo pin moved to driver
9d4415d(langgenius/mosoo#399, deployed 07-24 ~14:45Z). Reproduces for all published agents, all models, with/without MCP, restartDriver ineffective — the failure is in capability negotiation, upstream of all of those.Root cause
Contract v2 (#54) added a hard gate in
acp-session-setup.ts: session requestsadditionalDirectories+ agent does not advertisesessionCapabilities.additionalDirectories→ throw. The previous driver passed the field through unconditionally.OpenCode — the acp-fallback agent — has never advertised that capability. Verified against sst/opencode v1.18.4
packages/opencode/src/acp/service.ts: initialize returnssessionCapabilities: { close, fork, list, resume }only, andsession/newdoes not readadditionalDirectoriesat all. The old passthrough was a silent no-op; the new gate converts that long-standing silent degradation into a full outage, because mosoo populatesadditionalDirectoriesfor published agents.Fix
Drop the directories when the capability is missing instead of failing the session, and return
droppedAdditionalDirectoriesso the backend logs the degradation (driver.acp.session.additional_directories_dropped, warn). Behavior with capability-advertising agents is unchanged — covered by a new regression test in both directions (dropped when unsupported, passed through when supported).Tests: 1078 pass locally (1 known macOS-only
/varsymlink case in acp-file-system, green on Linux CI).