fix(teacher): 区分词元预算与传输字节避免短问题被拦截

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+# Task: Fix teacher context budget rejection
+
+## Identity
+
+- Task ID: 20260924-teacher-context-budget-3a9e6c81
+- Mode: Feature
+- Branch: codex/20260924-teacher-context-budget-3a9e6c81-teacher-context-budget
+- Worktree: D:\Datas\OthersProjects\.codex-worktrees\makelore\20260924-teacher-context-budget-3a9e6c81
+- Base commit: eb3dc85a2f0ed8457b7f24684c2da3ab192c2e46
+- Owner: codex
+- Status: Ready for Integration
+
+## Scope
+
+- Diagnose and fix teacher context-budget rejection for a short question, preserving published configuration, student billing, cloud/legacy execution and current read-only tool boundaries. Own client context budgeting, runner integration, focused regressions, README and this record.
+
+## Intent And Constraints
+
+- Official check/start/status passed in the isolated feature worktree above. Read entry, own record, relevant teacher decisions/architecture/domain/evidence/reflection/commitments and 108 peer records. Completed teacher/packaging scopes overlap historically; historical placeholder records remain unknown/read-only with no concrete dependency. Planning Gate Passed; canonical memory, other tasks, main and user project files stay read-only.
+- Apply diagnosing-bugs: establish a red runner test, inspect only relevant local topic metadata, reproduce the actual legacy configuration, then correct the budgeting seam and test real preparation/compilation/submission. No subagents, paid model calls, deployment, installation or automatic topic migration.
+- Confirmed local metadata: current 方法老师 topic has no runtime=yuxi, no requests or discussion, input budget 8000, output 1500, system prompt 2751 UTF-8 bytes. No user conversation text or secret was printed or copied. The screenshot is a legacy local-topic preflight failure, not proof of a cloud model limit.
+
+## Outcome
+
+- Fixed native token budgeting: text, reasoning, tool schemas and calls share an explicit approximate 2 UTF-8 bytes/token estimate; tool-result limits convert back to bytes under the existing 2400-byte cap. Published token ceilings, model limits, student billing and final provider authority remain intact; no configuration or topic migration.
+- Context compaction now measures the selected runner's complete envelope. Cloud compilation uses exact JSON query bytes instead of a fixed 256-byte escaping reserve, so ordinary code quotes/newlines cannot pass compilation then fail submission. Excerpts preserve head/tail and source IDs without mutating original read-tool material.
+- Fixed configuration/current-discussion overflow is distinguished from oversized questions/references. Teacher behavior, current question, explicit references and discussion instructions remain complete. Updated README explains the approximate native versus exact transport boundary.
+- No Renderer workflow changed. Before: a short first question could be rejected with advice to shorten it or start a topic. After: the same 8000-budget legacy shape and cloud long-code case submit successfully; truly oversized fixed inputs identify configuration as the cause.
+
+- Reproduced two reachable failures before changing product code: legacy model preparation plus empty-source first discussion needs 9013 byte-based estimated units but allows only 4456 after tool reserves (synthetic 2700-byte prompt matching observed shape); cloud compilation accepts long ordinary quoted code then JSON serialization exceeds its byte budget.
+
+## Verification
+
+- Green: 9 focused teacher/context/read/model/structured-discussion/Renderer files, 289 tests passed. Includes actual CodingTeacherService + native runner + real temporary project-file read/continuation at the existing 8000 budget with a 2700-byte prompt and long source; actual service + cloud transport compaction; escaped-code cloud envelope and unchanged protocol; negative fixed-config overflow; six read rounds, cancellation, request identity and scope remain covered.
+- pnpm 10.33.4 frozen offline install, standard pnpm run typecheck and scoped ESLint passed. Production Vite build passed for Renderer/Main/Preload/utility. Expanded Main tsc reports 66 diagnostics; a CompilerHost replay substituting exact base versions of all changed Electron files also reports the same 66, with zero new diagnostics. No full-suite or real-model success is claimed.
+- Existing Electron UI fixtures replace the Host API and bypass this budget seam; this Main-only change is covered at real service/runner boundaries instead of treating a mocked UI pass as reproduction evidence. No installed application, live Yuxi instance or user topic was modified.
+
+- Red loops: pnpm exec vitest run tests/unit/coding-teacher-model.test.ts -t 'existing 8000 budget' --maxWorkers=1; pnpm exec vitest run tests/unit/coding-teacher-cloud.test.ts -t 'long code context' --maxWorkers=1. Both fail with the exact screenshot message at their respective real preflight seam. Empty-source cloud question with the discussion protocol passes.
+- Native model limits are tokens; existing implementation equates them to UTF-8 bytes, unlike model tokenization. DeepSeek documentation distinguishes character estimates and actual usage (https://api-docs.deepseek.com/zh-cn/quick_start/token_usage/). Keep native token estimates explicitly approximate and model/usage authoritative; retain the separate exact cloud query byte guard.
+
+## Follow-ups
+
+- Merge, rebuilt installer and installed-app verification remain pending. The observed 方法老师 topic is still a legacy definition; selecting/publishing a Yuxi teacher is a separate Operations action and existing topic versions remain fixed.
+
+## Promotion Candidates
+
+- Target: teacher architecture/domain/current-state at Integration Gate. Proposal: record distinct approximate native token and exact cloud JSON-byte budgeting, with source excerpt fitting and configuration-specific errors. Evidence: red/green runner cases, 289 tests, type/lint/build and unchanged actual-topic metadata. Future impact: new prompt/protocol content must use the runner's measurement at compilation; no silent topic migration or budget override. No semantic conflict or further product-policy approval identified; feature mode does not promote canonical documents.