[LinuxDevil]

What changed

• server/author/proposal/save.ts (new): writeProposal — the single write path for model saves and the card's Apply button. It re-validates the stored card's spec with today's agentSpecV2Schema (INVALID_STORED_PROPOSAL), writes exactly that spec inside withAgentLock, captures a source: 'author' version with requestText/authorSessionId, saves new agents disabled with createdFrom: { kind: 'author', ref: sessionId }, links created ids into session.createdAgentIds, publishes agent.changed/version on the workspace bus, and maps thrown errors to WRITE_FAILED. Also requestTextFor — the last non-empty, non-uiEvent user text before the card, cut to 400 chars.
• server/author/proposal/failures.ts: the four new refusal builders (INVALID_STORED_PROPOSAL, BASE_CHANGED, AGENT_GONE, WRITE_FAILED) with the contract's model/person copy.
• server/author/tools/saveTools.ts (new): saveAgent / saveAgentUpdate tools — AA10a blocking gate → stored-card resolution → writeProposal over deps.workspace() (an unresolvable workspace is AGENT_GONE); a success marks the identity saved in proposalTurnState. Registered in AUTHOR_TOOL_FACTORIES (17 tools now; AuthorToolDeps gains sessions).
• POST /assistant/sessions/:id/save in server/author/routes.ts: body { proposalId }, 404/409-BUSY (running reply or save in flight), UNKNOWN_PROPOSAL/STALE_PROPOSAL/ALREADY_SAVED/BASE_CHANGED/AGENT_GONE as 400s (WRITE_FAILED → 500, details.latestProposalId carried), then appends the uiEvent: { kind: 'save' } user message and answers { ok, agentId, version, name, enabled, message }. The workspace resolves from the stored session.workspaceId via deps.workspaceResolver, not the request header.
• RuntimeClient.saveProposal(sessionId, proposalId) returning AuthorSaveResponse; RuntimeApiError now keeps the server's code and details.

How it was tested

• npm run typecheck:server, npm run test:server — 85 files / 1121 tests green (incl. new save.test.ts 11, saveFlow.test.ts 4, authorSave.test.ts 14)
• Studio npm run typecheck, npm test -- --run — 88 files / 968 tests green
• Root npx tsc --noEmit, npm run lint, npm run test:coverage, npm run fallow (0 dead exports, maintainability 89.2), npm run docs:ratchet (at baseline), npm run api:check, npm run docs:llms:check, npm run size-check — all green

Acceptance criteria

• Turn 1 proposes, turn 2 calls saveAgent → agent file exists, meta.enabled === false, createdFrom.kind === 'author', version 1 has source: 'author' and requestText = turn 1's user text
• Propose and save in the same turn → UNANSWERED, nothing written
• Apply route on the newest card → 200, transcript ends with the uiEvent message; a second Apply → 400 ALREADY_SAVED
• Apply on an older card → 400 STALE_PROPOSAL with details.latestProposalId
• Update save after a canvas edit → BASE_CHANGED; nothing written
• The written spec deep-equals the card's spec (no re-sent copy, no model input)
• While a turn runs, Apply → 409 BUSY

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