diff --git a/docs/user-visible-errors.csv b/docs/user-visible-errors.csv
index 14e495a9b5..0af71f3d08 100644
--- a/docs/user-visible-errors.csv
+++ b/docs/user-visible-errors.csv
@@ -12,7 +12,7 @@ harm_rank,location,trigger,current_copy,user_risk,proposed_replacement,implement
 11,src/domain/errors.ts:ArtifactOperationError,Artifact inventory or extraction fails,"Artifact {operation} failed: {reason} at {logicalPath} (declared_sha256=...)",Exposes paths hashes and internal reason,"Reason-specific plain message with safe recovery step.",implemented,focused assertions passed; full `npm run check` passed
 12,src/domain/errors.ts:EvidenceFileError,Evidence file operation fails,"Evidence bundle {operation} failed: {reason}",Internal reason token gives weak recovery,"Reason-specific plain message with safe recovery step.",implemented,focused assertions passed; full `npm run check` passed
 13,src/domain/errors.ts:EvidenceIntegrityError,Evidence validation fails,Raw constructor message,May expose validation internals and lacks recovery,"Evidence is invalid or has changed. Recreate or re-import it, then try again.",implemented,focused assertions passed; full `npm run check` passed
-14,src/domain/errors.ts:UnknownRegistryError,Evidence-state mutation fails,"Residual unknown registry mutation failed: {reason}",Exposes internal subsystem and state token,"Evidence state changed before the update completed. Refresh the current state and try again.",implemented,focused assertions passed; full `npm run check` passed
+14,src/domain/errors.ts:UnknownRegistryError,Evidence-state mutation fails,"Residual unknown registry mutation failed: {reason}",Exposes internal subsystem and state token,"Missing IDs identify the session and retry; concurrent revision conflicts refresh current state.",implemented,focused assertions passed; full `npm run check` passed
 15,src/domain/errors.ts:ProcessCaptureError,Process capture fails,Raw constructor message,May expose process and cleanup internals,"Process capture could not complete. Check command access and limits, then try again.",implemented,focused assertions passed; full `npm run check` passed
 16,src/domain/errors.ts:AnalysisTimeoutError,Analysis deadline expires,"Analysis operation timed out after {timeoutMs}ms: {operation}",Exposes internal deadline and operation; weak recovery,"Analysis took too long. Try a smaller request, then run `rea doctor` if it continues.",implemented,focused assertions passed; full `npm run check` passed
 17,src/domain/errors.ts:HopperTimeoutError,Provider deadline expires,"Hopper did not respond within {timeoutMs}ms",Exposes provider and internal deadline,"Analysis took too long. Try a smaller request, then run `rea doctor` if it continues.",implemented,focused assertions passed; full `npm run check` passed
diff --git a/src/application/ArtifactInventory/scanCanonical.ts b/src/application/ArtifactInventory/scanCanonical.ts
index 78e47e400b..9e4bbe2fc5 100644
--- a/src/application/ArtifactInventory/scanCanonical.ts
+++ b/src/application/ArtifactInventory/scanCanonical.ts
@@ -49,7 +49,7 @@ export const scanCanonicalArtifactInventory = async (
     (!integrity.approved || !integrity.enabled)
   )
     throw new ArtifactReaderFailure(
-      "unavailable",
+      "policy",
       "Integrity continuation requires explicit approval and operator policy",
     );
   const metadata = await lstat(path);
diff --git a/src/artifacts/ArtifactReader.ts b/src/artifacts/ArtifactReader.ts
index ff85c614fc..07fddeb829 100644
--- a/src/artifacts/ArtifactReader.ts
+++ b/src/artifacts/ArtifactReader.ts
@@ -50,6 +50,7 @@ export class ArtifactReaderFailure extends Error {
       | "integrity"
       | "limit"
       | "path"
+      | "policy"
       | "unavailable",
     message: string,
     options?: ErrorOptions,
diff --git a/src/domain/analysisErrorPresentation.ts b/src/domain/analysisErrorPresentation.ts
index 824aba1f28..4da6004a68 100644
--- a/src/domain/analysisErrorPresentation.ts
+++ b/src/domain/analysisErrorPresentation.ts
@@ -24,6 +24,8 @@ export const analysisErrorRemediationAction = (
 ): string => {
   if (error instanceof AnalysisInputError)
     return "Correct the listed arguments and retry.";
+  if (error instanceof UnknownRegistryError && error.reason === "not-found")
+    return "Check that the unknown_id belongs to this session, then retry.";
   if (error instanceof ReplayPlanStaleError)
     return "Review the rebuilt replay plan and explicitly approve its new digest.";
   if (error instanceof PermissionRequiredError)
@@ -99,7 +101,7 @@ const artifactErrorCategory = (
   if (reason === "integrity") return "integrity_mismatch";
   if (reason === "limit") return "truncated";
   if (reason === "cancelled") return "cancelled";
-  if (reason === "unavailable") return "unavailable";
+  if (reason === "policy" || reason === "unavailable") return "unavailable";
   return "execution_failure";
 };
 
@@ -139,6 +141,8 @@ export const analysisErrorUserMessage = (error: AnalysisError): string => {
     return evidenceFileMessage(error.reason);
   if (error instanceof InvestigationWorkspaceError)
     return investigationWorkspaceMessage(error.reason);
+  if (error instanceof UnknownRegistryError && error.reason === "not-found")
+    return "The requested residual unknown does not exist in this session. Check the unknown_id and try again.";
   if (error instanceof UnknownRegistryError)
     return "Evidence state changed before the update completed. Refresh the current state and try again.";
   if (error instanceof ConfigurationError)
@@ -203,8 +207,10 @@ const artifactMessage = (reason: ArtifactOperationError["reason"]): string => {
     return "Artifact is too large to process safely. Narrow the requested path or use a smaller artifact.";
   if (reason === "path")
     return "Artifact path is not allowed. Choose a path inside the artifact and try again.";
+  if (reason === "policy")
+    return "Artifact integrity continuation is disabled by policy. Configure REA_ARTIFACT_INTEGRITY_CONTINUE_ENABLED=true and retry only if continuing after mismatches is approved.";
   if (reason === "unavailable")
-    return "Artifact format is not available on this system. Choose another artifact or supported environment.";
+    return "Artifact processing is unavailable for the current target or policy. Check artifact support and required approvals.";
   if (reason === "format" || reason === "integrity")
     return "Artifact is invalid or has changed. Get a fresh copy and try again.";
   return "Artifact could not be read or written. Check file access and try again.";
diff --git a/src/domain/analysisErrorProjection.ts b/src/domain/analysisErrorProjection.ts
index a84cb6a8f3..5fcb7c0ed5 100644
--- a/src/domain/analysisErrorProjection.ts
+++ b/src/domain/analysisErrorProjection.ts
@@ -220,6 +220,7 @@ const requestErrorDetails = (
       issues: error.issues.map((issue) => ({
         path: [...issue.path],
         reason: issue.reason,
+        ...(issue.message === undefined ? {} : { message: issue.message }),
         ...(issue.expected === undefined ? {} : { expected: issue.expected }),
         ...(issue.minimum === undefined ? {} : { minimum: issue.minimum }),
         ...(issue.maximum === undefined ? {} : { maximum: issue.maximum }),
diff --git a/src/domain/errors.ts b/src/domain/errors.ts
index cafdb2479b..80a2cb5266 100644
--- a/src/domain/errors.ts
+++ b/src/domain/errors.ts
@@ -97,6 +97,8 @@ export interface AnalysisInputIssue {
     | "out_of_range"
     | "invalid_value"
     | "invalid_format";
+  /** Schema-authored correction guidance for cross-field or custom checks. */
+  readonly message?: string;
   readonly expected?: JsonValue;
   readonly minimum?: number;
   readonly maximum?: number;
@@ -293,6 +295,7 @@ export class ArtifactOperationError extends AnalysisError {
       | "integrity"
       | "limit"
       | "path"
+      | "policy"
       | "unavailable"
       | "io",
     readonly artifactDetails?: Readonly<{
diff --git a/src/domain/inputIssueProjection.ts b/src/domain/inputIssueProjection.ts
index af384ce696..23e00223b5 100644
--- a/src/domain/inputIssueProjection.ts
+++ b/src/domain/inputIssueProjection.ts
@@ -46,7 +46,13 @@ const projectIssue = (
         expected: issue.values.filter(isSafeExpected),
       },
     ];
-  return [{ path, reason: "invalid_value" }];
+  return [
+    {
+      path,
+      reason: "invalid_value",
+      ...(issue.message.length > 0 ? { message: issue.message } : {}),
+    },
+  ];
 };
 
 const valueAtPath = (
diff --git a/tests/errors.test.ts b/tests/errors.test.ts
index f90b138a94..ea26e66107 100644
--- a/tests/errors.test.ts
+++ b/tests/errors.test.ts
@@ -242,6 +242,61 @@ describe("analysis error projection", () => {
     ).toEqual([]);
   });
 
+  it("gives missing residual unknowns a lookup-specific remediation", () => {
+    expect(
+      projectAnalysisError(new UnknownRegistryError("not-found")),
+    ).toMatchObject({
+      code: "execution_failure",
+      message:
+        "The requested residual unknown does not exist in this session. Check the unknown_id and try again.",
+      remediation: {
+        action:
+          "Check that the unknown_id belongs to this session, then retry.",
+      },
+      details: { reason: "not-found" },
+    });
+  });
+
+  it("retains custom input guidance in the projected error details", () => {
+    expect(
+      projectAnalysisError(
+        new AnalysisInputError("find_changed_behavior", undefined, [
+          {
+            path: [],
+            reason: "invalid_value",
+            message:
+              "Supply either existing comparisons or one investigation_run",
+          },
+        ]),
+      ),
+    ).toMatchObject({
+      details: {
+        issues: [
+          {
+            path: [],
+            reason: "invalid_value",
+            message:
+              "Supply either existing comparisons or one investigation_run",
+          },
+        ],
+      },
+    });
+  });
+
+  it("distinguishes disabled integrity continuation from format support", () => {
+    expect(
+      projectAnalysisError(
+        new ArtifactOperationError("inventory_artifact", "policy"),
+      ),
+    ).toMatchObject({
+      code: "artifact_operation_failed",
+      category: "unavailable",
+      message: expect.stringContaining(
+        "REA_ARTIFACT_INTEGRITY_CONTINUE_ENABLED=true",
+      ),
+    });
+  });
+
   it("validates every closed error-reason variant against the generated contract", () => {
     const variants: AnalysisError[] = [
       ...(
@@ -251,6 +306,7 @@ describe("analysis error projection", () => {
           "integrity",
           "limit",
           "path",
+          "policy",
           "unavailable",
           "io",
         ] as const
@@ -313,7 +369,7 @@ describe("analysis error projection", () => {
       ).map((diagnostic) => new HopperRemoteError(9, "safe", diagnostic)),
     ];
 
-    expect(variants).toHaveLength(38);
+    expect(variants).toHaveLength(39);
     for (const variant of variants) {
       const projected = projectAnalysisError(variant);
       const parsed = analysisErrorProjectionSchema.safeParse(projected);
diff --git a/tests/inputIssueProjection.test.ts b/tests/inputIssueProjection.test.ts
new file mode 100644
index 0000000000..7a6727615f
--- /dev/null
+++ b/tests/inputIssueProjection.test.ts
@@ -0,0 +1,40 @@
+import { describe, expect, it } from "vitest";
+import { z } from "zod";
+
+import { artifactExtractionInputSchema } from "../src/contracts/artifactToolContracts.js";
+import { projectInputIssues } from "../src/domain/inputIssueProjection.js";
+
+describe("input issue projection", () => {
+  it("preserves schema-authored custom correction guidance", () => {
+    const schema = z
+      .object({ left: z.string().optional(), right: z.string().optional() })
+      .superRefine((value, context) => {
+        if (value.left === undefined && value.right === undefined)
+          context.addIssue({
+            code: "custom",
+            path: [],
+            message: "Supply either left or right",
+          });
+      });
+    const parsed = schema.safeParse({});
+    if (parsed.success) throw new Error("expected invalid input");
+
+    expect(projectInputIssues(parsed.error.issues, {})).toEqual([
+      {
+        path: [],
+        reason: "invalid_value",
+        message: "Supply either left or right",
+      },
+    ]);
+  });
+
+  it("rejects relative extraction destinations at the caller boundary", () => {
+    expect(
+      artifactExtractionInputSchema.safeParse({
+        approved: true,
+        output_root: "relative/output",
+        occurrence_ids: [`occ_${"0".repeat(64)}`],
+      }).success,
+    ).toBe(false);
+  });
+});