jevcore

TypeSafe Jev for DeepSeek Harness and any other MCP host.

Jev is not a chat model. It answers typed questions — noul (yes/no), choice, score — and returns calibrated probabilities. It does not write prose, and asking it to is a category error. This project gives an agent exactly that surface, and nothing more.

Offline by default. Egress disclosed. Nothing default-on.

Three packages, one decision layer

The adapters are thin on purpose. packages/dsh is four files: it declares tool schemas and translates hook payloads. Everything decision-shaped — the primitives, the providers, the egress contract, the policy, the gates — lives in core, so a new adapter cannot drift from the guarantees the others make. One runtime requirement differs across the three: jevcore-dsh tracks the harness and needs Node ^22.19.0 || >=24.0.0, while jevcore and jevcore-mcp need >=20.

Why this exists

Between 2026-09-17 and 09-20, nineteen plugins appeared that wire Jev into DSH. Auditing their source found a consistent pattern: the module labelled guard, gate, or warden was also the module shipping prompts, tool arguments, and file contents to a third party, and the README generally did not say so. Several were enabled by default. One gate could be reconfigured by the model it was guarding.

This project is the same idea with those failure modes designed out:

The egress contract

This is the part worth reading before installing.

Transmission is decided per feature, and every feature defaults to off. The plugin prints its own contract at load:

[jevcore] provider=mock  endpoint=none  egress=OFF  (no network calls will be made; every answer is synthetic)
[jevcore] ready - provider=mock - gates: safety=off context=off

With provider: live and every feature enabled, the same report becomes explicit about what leaves:

[jevcore] provider=live  endpoint=https://api.typesafe.ai  egress=ON
[jevcore]   SENDS  tool:jev_ask  { state<=16000c questions<=4000c }
[jevcore]   SENDS  tool:jev_rank  { state<=16000c questions<=4000c }
[jevcore]   SENDS  tool:jev_check  { state<=16000c questions<=4000c }
[jevcore]   SENDS  gate:safety  { state<=8000c questions<=2000c }
[jevcore]   SENDS  gate:context  { state<=6000c questions<=2000c }
[jevcore]   redaction is best-effort; it removes named fields and known secret shapes, and cannot
             recognise an unrecognised secret in free text

The tools transmit only when the model chooses to call them, which is visible in the transcript. The gates would run on every matching tool call, which is not, so they are opt-in.

Redaction, and its honest limit

Before anything is sent, src/redact.ts runs two passes: values under sensitive field names (password, token, apiKey, authorization, …) are replaced wholesale, and secret-shaped strings that survive into free text are pattern-matched (Bearer …, sk-…, ts_live_…, AWS/GitHub/Google key shapes, JWTs, private-key headers, credentials in connection strings).

This is a mitigation, not a permission. A secret that sits under an unrecognised key name and does not match a known shape will pass through. If that possibility is unacceptable for your workload, do not enable the live provider.

Install

As a DeepSeek Harness plugin

dsh plugin --profile <profile> add jevcore-dsh

Or from a checkout:

dsh plugin --profile <profile> add /absolute/path/to/jevcore/packages/dsh

packages/dsh imports jevcore by name, so a checkout also needs core resolvable from the profile (add /absolute/path/to/jevcore/packages/core).

Then confirm the row activated — the plugin list should show jev as active, not failed — and check the startup report in the log.

As an MCP server

For a host that speaks MCP, the same three tools are available over stdio:

npx -y jevcore-mcp

To wire it into DeepSeek Harness specifically, install a configuration-only bundle whose patch inserts the harness's own MCP client:

- insert:
    - id: jev-mcp
      name: '@deepseek-ai/dsh-mcp-client'
      config:
        serverName: jev
        transport: stdio
        command: npx
        args: ['-y', 'jevcore-mcp']
        env:
          TYPESAFE_API_KEY: '<the key>'
        failOnStartupError: true

The env map is not optional: DSH strips every credential-shaped name — anything containing KEY, PASSWORD, SECRET or TOKEN, in any case — from the environment it hands a spawned server, then merges this map back in afterwards. A key exported in your shell never arrives, and the server stays on the offline mock without reporting an error.

The provider is chosen from the environment:

With neither key it stays on the offline mock. Unlike the plugin, the MCP server resolves its credential once at startup, so a missing key with JEV_PROVIDER=live is a startup error rather than a per-call surprise.

Its egress report goes to stderr, never stdout — on a stdio transport stdout is the protocol channel, and a stray line there would corrupt the stream.

Going live

There are two routes to Jev. Both call the same models and both return the same typed answers; they differ in who holds your credential and whose servers see your state.

TypeSafe directly — use this if you have a key from console.typesafe.ai:

- insert:
    - id: jev
      name: 'jevcore-dsh'
      config:
        provider: live
        apiKeyRef: TYPESAFE_API_KEY   # a reference, never the key
        model: jev-latest

Through OpenRouter — use this if a TypeSafe key is impractical and you already have an OpenRouter key. OpenRouter serves the System One models at the same POST /v1/systemone path TypeSafe does, one level below its own API root, so this is the documented route to Jev rather than an approximation of it:

- insert:
    - id: jev
      name: 'jevcore-dsh'
      config:
        provider: openrouter
        openRouterApiKeyRef: OPENROUTER_API_KEY
        model: jev-latest              # a bare `jev-*` id, or `typesafe/jev-1.13`

The route is reached by pointing the official @typesafe-ai/sdk at https://openrouter.ai/api with your OpenRouter key — OpenRouter's own documented integration, so there is no second client to keep in step.

Two things to know about the OpenRouter route:

• Your state goes to OpenRouter, not to TypeSafe. A different third party with different retention and logging. The startup report names the endpoint for exactly this reason — read it rather than inferring the destination from the provider's name.
• It returns a cost, which TypeSafe's own route does not, so usage.costUsd is populated here and absent there.

The model id must be a System One one. jev-latest bare is the default and needs no prefix; typesafe/ is accepted on a versioned id such as typesafe/jev-1.13, but not on the moving tag — typesafe/jev-latest is not accepted by the route. Any other id would be routed to a chat model, which answers with prose this plugin cannot interpret as a decision, so it is refused before the call rather than misread after it.

Either way the credential is resolved through DSH's credential service first, then the environment variable of that name. It is read per call, so a key added while the process is running is picked up. It is never logged, never returned from a tool, and never written to configuration. Each route has its own reference (apiKeyRef and openRouterApiKeyRef) so the two cannot accidentally share a key.

@typesafe-ai/sdk is the only optional dependency; the plugin loads and runs offline without either, and only needs the one for the route you choose.

Configuration

The declared state caps are 16,000 characters for the three tools and 8,000 / 6,000 for the safety and context gates. maxStateChars replaces all of them, and the startup report shows the effective value rather than the declared one, so what it prints is what is enforced.

Gates accept a bare boolean (safety: false) or an object with onUndecided: ask (default), allow, or deny.

ask is a question, so it needs something to ask. A deployment that composes no approval service cannot escalate to a human, and DeepSeek Harness then refuses the call instead of running it: every tool the safety gate matches is refused, which an operator experiences as the plugin having broken shell commands. docs/approval.md has the mechanism, and the plugin README covers the deployment view.

An unknown value is rejected at load with a message naming the key, rather than being silently ignored — a config typo should not quietly change the privacy posture.

Using it

From another plugin, with no model in the loop

The service is the primary surface. This is the point of a decision model: a routing or gating decision should not cost a model round-trip.

const jev = ctx.get('jev')
const result = await jev.ask({
  feature: 'tool:jev_ask',
  state: { ticket: 'I was charged twice.' },
  questions: {
    urgent: { type: 'noul', instructions: 'Does this convey urgency?' },
    team: {
      type: 'choice',
      instructions: 'Which team should handle this?',
      criteria: { billing: 'Payments, invoices, refunds', technical: 'Bugs, outages' },
    },
  },
})

result.answers carries probabilities and confidence. Deciding what to do with them is your code's job — see src/policy.ts for a worked example with an explicit confidence floor, where an unsure answer produces ask rather than allow.

From the model

Three tools, deliberately few and orthogonal:

• jev_ask — a batch of typed questions over one state.
• jev_rank — score and sort candidates against one criterion, one question per candidate in a single round-trip. The probabilities are independent per-candidate judgments, not a distribution.
• jev_check — does this evidence support this claim? Returns supported, contradicted, conflicted, insufficient, undecided, or unknown. Contradiction outranks support, because evidence that both supports and refutes is a conflict, not a weak yes.

A bundled skill

The plugin registers a skill, typesafe-ai-dsh, teaching an agent when a Jev judgment is the right tool and when it is a category error. It is registered through the skill registry rather than shipped as a directory for a provider to scan, so it needs no dependency on where a profile keeps its skills and it disappears cleanly when the plugin is removed.

The body lives in skills/typesafe-ai-dsh/SKILL.md and is read at load rather than embedded, so the file a human edits is the file that ships. A test asserts the two cannot drift.

The registry is declared as an optional dependency: a profile that does not compose the skill subsystem still gets the service and the three tools, with a warning that the skill was skipped.

The mock provider

With provider: mock, every answer is derived from a hash of the question and state, so tests can assert exact values and no socket is opened. Synthetic results are labelled as such in three places: provider: "mock", a mock/jev-synthetic model name, and a warning field on the result. A mock that could be mistaken for a real judgment would be worse than no mock at all.

Status

Honest accounting of what has and has not been verified.

Verified

• 410 tests pass across three packages (314 core, 66 DSH, 30 MCP), with no network access and no TYPESAFE_API_KEY. CI clears the variable and expects the suite to pass anyway.
• The OpenRouter route is verified against the real API. pnpm --filter jevcore run probe:live drives the provider and pnpm --filter jevcore-mcp run smoke:live drives the whole MCP surface — transport, tool schemas, service and provider — against real System One models (typesafe/jev-1.13-20260917). A three-primitive batch returned a noul at 0.91, a choice at 0.97, and a score of 1.05 on a four-level rubric, with usage reported as { inputTokens: 469, outputTokens: 68, costUsd: 0.000019698 }. jev_rank ordered a credential runbook above a billing guide; jev_check returned contradicted. Both scripts need OPENROUTER_API_KEY and are excluded from CI.
• The payload shapes are checked against the vendors' own type definitions. test/vendor-conformance.test.ts pins this project's question and answer types to both SDKs, so a drift in either direction is a compile error rather than a malformed request on the one route that costs money and transmits data. That check is what found score.criteria being sent as a keyed map when the API requires an ordered array — a defect the stubbed unit tests had passed over throughout. There is also a script that parsed our payloads against OpenRouter's published zod schemas for the alpha/decisions route; both the script and that route are gone, because the OpenRouter provider now uses the documented /v1/systemone path through the same client TypeSafe uses, which the conformance test already covers.
• The plugin activates in a running harness and its tools work. The plugin row reports active; jev_ask returned urgent=true (0.8307) and team=billing (0.5027) against the mock in 1 ms, and jev_check returned verdict="insufficient" with its three probabilities. Every result carried provider: "mock" and zero token usage, so the default path made no network call.
• The bundled skill registers. typesafe-ai-dsh appears in the session skill catalog.
• The default path makes no network call: asserted by spying on globalThis.fetch while mounting the plugin and answering through the service, and again while assembling the MCP runtime.
• A disabled gate registers no event listener, and a denied egress never reaches the provider.
• Config satisfies the Standard Schema protocol Cordis requires before a plugin starts.
• Every DSH API used here (ctx.provide, ctx.effect, tools.register, defineTool, tools/pre-execute, tools/post-execute, credentials.resolve) was checked against the installed runtime before use, and the payload types come from the installed declaration files.

Not verified, or known-broken

• The TypeSafe route is exercised, but only lightly. packages/core/scripts/probe-live.mjs (pnpm --filter jevcore run probe:typesafe) asks the real API three things: ten claim/evidence pairs whose verdict is known in advance, one question repeated six times, and a noul carrying a criteria: {true, false} boundary. Two separate runs agreed — supporting evidence scored 0.95, contradicting evidence 0.10, evidence silent about the claim 0.03, and the repeated question varied by 0.01 or less. The boundary was accepted. What remains untested is everything around the request rather than the request itself: quota, rate-limit and entitlement behaviour on a real account.
• jev_ask's description carries a corrupted dash on the build that is currently running. It reads branches on —?routing where an em dash followed by a space belongs. Cause: a UTF-8 round-trip early in development replaced the third byte of the em dash with ?. It is fixed on disk — four occurrences, zero remaining, confirmed in both source and build output — but the running process loaded its module before the fix and cannot re-read it without a restart. The dash itself is cosmetic, but the staleness is not: the same process also lacks every fix made since it started, including the band field that keeps a 0.51 from being read as a settled yes.
• The MCP server has been driven end to end by a real MCP client over stdio (pnpm --filter jevcore-mcp run smoke): handshake, tool discovery, three successful calls, and an error result for an invalid batch. It has not been driven by any other third-party host.
• Gate behaviour on live traffic. The gates are tested against synthetic answers and real hook payload shapes, but no real tool call has been gated end to end.
• The context gate cannot recover context already spent. It withholds a result from reaching the agent; it does not prune anything retroactively, and the README of any plugin claiming otherwise should be read sceptically.
• No long-running or adversarial testing. Redaction is pattern-based and will miss an unrecognised secret shape.

Design notes

Three decisions that are easy to get wrong and expensive to get wrong:

A judge that cannot answer must not mean "allow". If Jev is unreachable, or answers below the confidence floor, the gates resolve through onUndecided, which defaults to ask. The only way to get fail-open behaviour is to configure it. The context gate is the deliberate exception: it fails open unconditionally, because losing a real tool result to a phantom "irrelevant" verdict is worse than keeping an uninformative one.

Nothing the model can say changes the gate. No tool exposes gate configuration, thresholds, or scope. A guard the guarded process can reconfigure is not a guard.

A probability is not a permission. Jev returns numbers; src/policy.ts turns them into allow/ask/deny against locally configured thresholds. An answer that names a value outside the declared criteria is invalid and denies — the guarantee of a typed decision model is that it cannot return an undeclared value, so a violation means something upstream is wrong.

Development

pnpm install
pnpm run check      # typecheck + tests + build
pnpm test           # tests only

No test requires a credential or a network connection, and CI enforces that by running with TYPESAFE_API_KEY cleared.

License

Apache License 2.0 © 2026 jevcore contributors

TypeSafe, Jev, and System One are trademarks of TypeSafe AI. This project is an independent integration and is not affiliated with or endorsed by TypeSafe AI.