diff --git a/README.md b/README.md
index 774a898..91bd35f 100644
--- a/README.md
+++ b/README.md
@@ -62,7 +62,22 @@ PCTR routes to the consequence. TTP binds the authority for the exact execution
 
 ## Under the hood: the Trust Transfer Protocol
 
-Identity proves *who* is acting. Authorization defines *what* was assigned. **TTP establishes whether the actor is trustworthy enough for this action, right now** — before any downstream authority or execution decision is made.
+**TTP is the open protocol for machine chain trust:** proving whether the chain behind an AI agent, copilot, workflow, pipeline, API, service account, or non-human identity is trustworthy enough to be relied on, before downstream authority and execution decisions occur.
+
+Identity proves *who* is acting. Authorization defines *what* was assigned. **TTP establishes whether the chain behind this action deserves reliance, right now** — before any downstream authority or execution decision is made.
+
+An *isnad* is a chain of transmission: who received what from whom, and whether each link is sound. An autonomous action has the same shape — a model acted on a prompt, handed to a tool, through a workflow, carrying authority delegated by a person no longer in the room. TTP evaluates that chain, and grades it rather than returning a boolean. See [Chain Trust Model](SPECIFICATION.md#chain-trust-model).
+
+| Layer | Responsibility |
+| --- | --- |
+| **TTP** | Machine isnad / chain trust |
+| **SCIM-RE** | Runtime identity and authority schema |
+| **RAP** | Authority decision engine |
+| **Execution Exchange** | Downstream enforcement / control plane |
+| **CortexTrace** | Evidence and trace capture |
+| **ExecutionReceipts** | Cryptographic proof objects |
+
+TTP establishes whether a chain deserves reliance. Downstream authority systems decide whether execution may proceed.
 
 OAuth standardized delegated access. SCIM standardized identity provisioning. **TTP standardizes trust-before-execution for autonomous systems.** It is a platform-agnostic trust protocol — bring any agent, IdP, gateway, or workflow.
 
diff --git a/SPECIFICATION.md b/SPECIFICATION.md
index a8b94b1..5b44481 100644
--- a/SPECIFICATION.md
+++ b/SPECIFICATION.md
@@ -79,6 +79,137 @@ Blocks in the MVP:
 | `authority_context` | Defines action/resource context requiring a proof. |
 | `delegation` | Defines bounded transfer of trust or authority context. |
 
+## Chain Trust Model
+
+TTP is the open protocol for **machine chain trust**: proving whether the chain behind an
+AI agent, copilot, workflow, pipeline, API, service account, or non-human identity is
+trustworthy enough to be relied on, before downstream authority and execution decisions
+occur.
+
+The name is deliberate. An *isnad* is a chain of transmission — who received what from
+whom, and whether each link in that chain is sound. An autonomous action has the same
+shape: a model acted on a prompt, handed to a tool, invoked through a workflow, carrying
+authority delegated from a person who is no longer in the room. TTP evaluates that chain.
+
+### Layering
+
+```text
+TTP                = machine isnad / chain-trust protocol
+SCIM-RE            = runtime identity + authority schema
+RAP                = authority decision engine
+Execution Exchange = downstream enforcement / customer control plane
+CortexTrace        = evidence + trace capture
+ExecutionReceipts  = cryptographic proof objects
+```
+
+TTP establishes **chain trust**. SCIM-RE structures runtime identity, authority grants,
+attestations and receipts. RAP makes runtime authority decisions. Execution Exchange
+enforces them. CortexTrace captures execution evidence. ExecutionReceipts preserve
+cryptographic proof.
+
+### Chain Primitives
+
+| Primitive | Meaning |
+| --- | --- |
+| `TrustChain` | Verifiable chain behind an autonomous action. |
+| `ChainActor` | Human, agent, model, tool, workflow, pipeline, service account, API, or workload in the chain. |
+| `TransmissionLink` | Handoff between actors, prompts, tools, workflows, systems, or authority contexts. |
+| `ReliabilityProfile` | Historical and current reliability state for a chain actor. |
+| `EvidenceReference` | Pointer to telemetry, receipts, approvals, runtime state, token state, code state, or external evidence. |
+| `Attestation` | Verifiable proof-of-state for an actor, workflow, token, code artifact, model, tool, or environment. |
+| `TrustVerifier` | Verifies signatures, issuers, proof modes, freshness, and evidence integrity. |
+| `TrustValidator` | Applies decay, thresholds, scope, reliability, delegation, chain continuity, and constraints. |
+| `CorroborationSet` | Multiple evidence routes supporting or contradicting the chain. |
+| `LatentDefect` | Hidden defect: stale authority, concealed delegation, prompt or tool drift, policy bypass, approval bypass, missing link. |
+| `TrustClassification` | Graded trust result. Not binary trust. |
+| `TrustTransfer` | Bounded transfer of trust or authority context. |
+| `TrustReceipt` | TTP-level proof that chain trust was evaluated. |
+
+`verify_isnad_chain` in the reference implementation evaluates a `TrustChain`: continuity
+(every link's issuer is the prior link's subject), rooting at a trusted authority, per-link
+validity, and attenuation — trust MUST NOT amplify along a chain.
+
+### Trust Roles
+
+| Role | Purpose |
+| --- | --- |
+| `EvidenceObserver` | Captures raw evidence from tools, workflows, runtimes, identity systems, APIs, pipelines, or agents. |
+| `AttestationIssuer` | Converts evidence into signed, verifiable attestations. |
+| `TrustVerifier` | Verifies attestations, signatures, issuers, proof mode, evidence integrity, and freshness. |
+| `TrustValidator` | Applies trust decay, reliability history, thresholds, scope, chain continuity, delegation, and constraints. |
+| `CorroborationEngine` | Compares evidence paths and detects agreement, contradiction, missing links, or latent defects. |
+| `RuntimeAuthority` | Downstream role that converts validated trust into allow, deny, step-up, escalate, throttle, or constrain. |
+| `ReceiptNotary` | Signs and chain-links trust validation results, runtime decisions, and execution outcomes. |
+| `GovernanceReviewer` | Reviews chains, receipts, defects, and corroboration for audit, compliance, incident response, or oversight. |
+
+The separation is normative, and it is the point of the protocol:
+
+> The `TrustVerifier` MUST NOT make final execution decisions unless it is explicitly also
+> acting as a downstream `RuntimeAuthority`.
+>
+> The `TrustValidator` MUST NOT execute protected actions. It determines whether trust
+> conditions are satisfied.
+>
+> TTP establishes whether a chain deserves reliance. Downstream authority systems decide
+> whether execution may proceed.
+
+### Verification Levels
+
+A deployment states the level it operates at. Higher levels are not merely "more secure" —
+they describe how many independent parties must agree before a chain is relied upon.
+
+| Level | Name | Description |
+| --- | --- | --- |
+| `L0_LOCAL` | Local evaluator | Cleartext local parsing and evaluation, for development. |
+| `L1_SINGLE_VERIFIER` | Single verifier | One verifier validates attestation, proof, issuer and freshness. |
+| `L2_VERIFIER_VALIDATOR` | Verifier + validator | Verifier checks evidence; validator applies decay, thresholds, scope, reliability and policy constraints. |
+| `L3_CORROBORATED_CHAIN` | Corroborated chain | Evidence references, receipts, attestations or approval records are checked for agreement and contradiction. |
+| `L4_VERIFIER_SET` | Distributed verifier set | Multiple verifiers independently verify evidence and produce a quorum or weighted result. |
+| `L5_VALIDATOR_QUORUM` | Validator quorum + runtime authority | Multiple validators produce a quorum-backed classification consumed by runtime authority. |
+| `L6_PRIVACY_PRESERVING` | ZK / selective disclosure | Trust conditions proven without exposing raw scores, sensitive evidence, internal policies or customer data. |
+
+### Trust Classification
+
+Chain trust is graded, not binary. Each classification implies a downstream posture, which
+a `RuntimeAuthority` converts into an execution response.
+
+| Classification | Downstream posture |
+| --- | --- |
+| `TRUST_ACCEPTED` | allow |
+| `TRUST_ACCEPTED_WITH_CONTROLS` | constrain or throttle |
+| `TRUST_REVIEW_RECOMMENDED` | step_up |
+| `TRUST_CONTRADICTED` | escalate |
+| `TRUST_DEFECTIVE` | escalate or deny |
+| `TRUST_REJECTED` | deny |
+| `TRUST_UNKNOWN` | deny or escalate |
+
+`TRUST_UNKNOWN` is not a neutral result. A chain nobody has evaluated is not a trustworthy
+chain, and it MUST NOT be treated as one.
+
+### Chain Continuity
+
+| State | Meaning |
+| --- | --- |
+| `CHAIN_CONTINUOUS` | Every link is present and each link's issuer is the prior link's subject. |
+| `CHAIN_MISSING_LINK` | One handoff in the chain has no evidence behind it. |
+| `CHAIN_MULTI_MISSING_LINK` | More than one handoff is unevidenced. |
+| `CHAIN_SUSPENDED` | A link was valid and has been suspended or revoked. |
+| `CHAIN_AMBIGUOUS` | Evidence supports more than one reading of the chain. |
+| `CHAIN_INFERRED` | Continuity is inferred from context rather than evidenced. |
+| `CHAIN_UNKNOWN` | Continuity has not been established. |
+
+### Latent Defects
+
+A latent defect is a fault the chain does not surface on its own. Detecting one does not
+by itself deny an action; it changes the classification, and the downstream authority
+decides.
+
+`prompt_injection_suspected` · `approval_bypass` · `stale_authority` ·
+`unowned_identity` · `unexpected_tool_use` · `context_loss` ·
+`policy_version_mismatch` · `scope_inflation` · `dependency_substitution` ·
+`token_origin_unclear` · `chain_link_unproven`
+
+
 ## Trust Object Model
 
 ### Subject
diff --git a/packages/pctr/src/decisions.mjs b/packages/pctr/src/decisions.mjs
index cabfe16..4ba903b 100644
--- a/packages/pctr/src/decisions.mjs
+++ b/packages/pctr/src/decisions.mjs
@@ -27,6 +27,46 @@ export const PROCEEDS = {
   STEP_UP: false, ESCALATE: false, SUSPEND: false, DENY: false, REVOKE: false
 };
 
+// TTP grades chain trust rather than returning a boolean, and each grade implies a
+// downstream posture (SPECIFICATION.md, Trust Classification). This is the mapping a
+// RuntimeAuthority applies to turn a classification into one of the responses above.
+// TRUST_UNKNOWN is not neutral: a chain nobody evaluated is not a trustworthy chain.
+export const CLASSIFICATION_POSTURE = {
+  TRUST_ACCEPTED: ['KEEP', 'REROUTE'],
+  TRUST_ACCEPTED_WITH_CONTROLS: ['CONSTRAIN', 'THROTTLE'],
+  TRUST_REVIEW_RECOMMENDED: ['STEP_UP'],
+  TRUST_CONTRADICTED: ['ESCALATE'],
+  TRUST_DEFECTIVE: ['ESCALATE', 'DENY'],
+  TRUST_REJECTED: ['DENY'],
+  TRUST_UNKNOWN: ['DENY', 'ESCALATE']
+};
+
+export const CHAIN_CONTINUITY = [
+  'CHAIN_CONTINUOUS', 'CHAIN_MISSING_LINK', 'CHAIN_MULTI_MISSING_LINK',
+  'CHAIN_SUSPENDED', 'CHAIN_AMBIGUOUS', 'CHAIN_INFERRED', 'CHAIN_UNKNOWN'
+];
+
+export const LATENT_DEFECTS = [
+  'prompt_injection_suspected', 'approval_bypass', 'stale_authority', 'unowned_identity',
+  'unexpected_tool_use', 'context_loss', 'policy_version_mismatch', 'scope_inflation',
+  'dependency_substitution', 'token_origin_unclear', 'chain_link_unproven'
+];
+
+/** The response a classification permits, narrowed by what the situation allows. */
+export function postureFor(classification) {
+  const allowed = CLASSIFICATION_POSTURE[classification] ?? CLASSIFICATION_POSTURE.TRUST_UNKNOWN;
+  return { classification, allowed, proceeds: allowed.some((r) => PROCEEDS[r]) };
+}
+
+/**
+ * A response must sit inside the posture its classification permits. Answering
+ * TRUST_REJECTED with a reroute is how a graded protocol degrades into a boolean one
+ * that always says yes.
+ */
+export function withinPosture(response, classification) {
+  return (CLASSIFICATION_POSTURE[classification] ?? CLASSIFICATION_POSTURE.TRUST_UNKNOWN).includes(response);
+}
+
 /**
  * Decide how to respond when a route is reevaluated. Checks run most-restrictive first,
  * so a revoked credential is never answered with a reroute.
diff --git a/packages/pctr/tests/decisions.test.mjs b/packages/pctr/tests/decisions.test.mjs
index 833a483..44c3ae0 100644
--- a/packages/pctr/tests/decisions.test.mjs
+++ b/packages/pctr/tests/decisions.test.mjs
@@ -1,6 +1,9 @@
 import test from 'node:test';
 import assert from 'node:assert/strict';
-import { respondToChange, reconcile, RESPONSES, isMoreRestrictive, PROCEEDS } from '../src/decisions.mjs';
+import {
+  respondToChange, reconcile, RESPONSES, isMoreRestrictive, PROCEEDS,
+  postureFor, withinPosture, CHAIN_CONTINUITY, LATENT_DEFECTS
+} from '../src/decisions.mjs';
 
 const preview = (over = {}) => ({ severity: 'HIGH', recordsAffected: 10, financialExposure: 100, ...over });
 const route = (over = {}) => ({ routeId: 'r1', agents: ['a'], trustStates: [{ agentId: 'a', evidenceStale: false }], ...over });
@@ -94,3 +97,41 @@ test('reconcile refuses to answer a change with a weaker response than it warran
   // A stronger response is allowed to win.
   assert.equal(reconcile(weak, strong).response, 'SUSPEND');
 });
+
+test('every TTP trust classification maps to a permitted downstream posture', () => {
+  // SPECIFICATION.md, Trust Classification. Chain trust is graded, not binary.
+  const expected = {
+    TRUST_ACCEPTED: true, TRUST_ACCEPTED_WITH_CONTROLS: true,
+    TRUST_REVIEW_RECOMMENDED: false, TRUST_CONTRADICTED: false,
+    TRUST_DEFECTIVE: false, TRUST_REJECTED: false, TRUST_UNKNOWN: false
+  };
+  for (const [classification, proceeds] of Object.entries(expected)) {
+    const posture = postureFor(classification);
+    assert.equal(posture.proceeds, proceeds, classification);
+    for (const response of posture.allowed) assert.ok(RESPONSES.includes(response), `${response} is a real response`);
+  }
+});
+
+test('an unevaluated chain is not a trustworthy chain', () => {
+  // TRUST_UNKNOWN must not be a neutral result, and an unrecognised classification
+  // must be treated as unknown rather than waved through.
+  assert.deepEqual(postureFor('TRUST_UNKNOWN').allowed, ['DENY', 'ESCALATE']);
+  assert.equal(postureFor('something-nobody-defined').proceeds, false);
+});
+
+test('a response may not step outside the posture its classification permits', () => {
+  assert.equal(withinPosture('KEEP', 'TRUST_ACCEPTED'), true);
+  assert.equal(withinPosture('CONSTRAIN', 'TRUST_ACCEPTED_WITH_CONTROLS'), true);
+  // Answering a rejected chain with a reroute is how a graded protocol turns into a
+  // boolean one that always says yes.
+  assert.equal(withinPosture('REROUTE', 'TRUST_REJECTED'), false);
+  assert.equal(withinPosture('KEEP', 'TRUST_UNKNOWN'), false);
+});
+
+test('the chain-continuity and latent-defect vocabularies match the specification', () => {
+  assert.equal(CHAIN_CONTINUITY.length, 7);
+  assert.ok(CHAIN_CONTINUITY.includes('CHAIN_MISSING_LINK'));
+  assert.equal(LATENT_DEFECTS.length, 11);
+  assert.ok(LATENT_DEFECTS.includes('approval_bypass'));
+  assert.ok(LATENT_DEFECTS.includes('prompt_injection_suspected'));
+});