diff --git a/common/web/types/src/util/compiler-interfaces.ts b/common/web/types/src/util/compiler-interfaces.ts
index 92fc1f65ec..3283fb2598 100644
--- a/common/web/types/src/util/compiler-interfaces.ts
+++ b/common/web/types/src/util/compiler-interfaces.ts
@@ -6,6 +6,11 @@ export interface CompilerEvent {
   line?: number;
   code: number;
   message: string;
+  /**
+   * an internal error occurred that should be captured with a stack trace
+   * e.g. to the Keyman sentry instance by kmc
+   */
+  exceptionVar?: any;
 };
 
 export enum CompilerErrorSeverity {
@@ -399,7 +404,13 @@ export const defaultCompilerOptions: CompilerOptions = {
  * @param message
  * @returns
  */
-export const CompilerMessageSpec = (code: number, message: string) : CompilerEvent => { return { code, message } };
+export const CompilerMessageSpec = (code: number, message: string, exceptionVar?: any) : CompilerEvent => ({
+  code,
+  message: exceptionVar
+    ? (message ?? `Unexpected exception`) + `: ${exceptionVar.toString()}\n\nCall stack:\n${(exceptionVar instanceof Error ? exceptionVar.stack : (new Error()).stack)}` :
+    message,
+  exceptionVar
+});
 
 /**
  * @deprecated use `CompilerError.exceptionToString` instead
diff --git a/developer/src/kmc-analyze/src/messages.ts b/developer/src/kmc-analyze/src/messages.ts
index 2512fbdc19..a800fb944a 100644
--- a/developer/src/kmc-analyze/src/messages.ts
+++ b/developer/src/kmc-analyze/src/messages.ts
@@ -1,4 +1,4 @@
-import { CompilerErrorNamespace, CompilerErrorSeverity, CompilerMessageSpec as m, compilerExceptionToString as exc } from "@keymanapp/common-types";
+import { CompilerErrorNamespace, CompilerErrorSeverity, CompilerMessageSpec as m } from "@keymanapp/common-types";
 
 const Namespace = CompilerErrorNamespace.Analyzer;
 const SevInfo = CompilerErrorSeverity.Info | Namespace;
@@ -8,7 +8,7 @@ const SevInfo = CompilerErrorSeverity.Info | Namespace;
 const SevFatal = CompilerErrorSeverity.Fatal | Namespace;
 
 export class AnalyzerMessages {
-  static Fatal_UnexpectedException = (o:{e: any}) => m(this.FATAL_UnexpectedException, `Unexpected exception: ${exc(o.e)}`);
+  static Fatal_UnexpectedException = (o:{e: any}) => m(this.FATAL_UnexpectedException, null, o.e ?? 'unknown error');
   static FATAL_UnexpectedException = SevFatal | 0x0001;
 
   static Info_ScanningFile = (o:{type: string, name: string}) => m(this.INFO_ScanningFile,
diff --git a/developer/src/kmc-keyboard-info/src/messages.ts b/developer/src/kmc-keyboard-info/src/messages.ts
index 06db26815a..e72fd2f0e9 100644
--- a/developer/src/kmc-keyboard-info/src/messages.ts
+++ b/developer/src/kmc-keyboard-info/src/messages.ts
@@ -8,8 +8,7 @@ const SevError = CompilerErrorSeverity.Error | Namespace;
 const SevFatal = CompilerErrorSeverity.Fatal | Namespace;
 
 export class KeyboardInfoCompilerMessages {
-  static Fatal_UnexpectedException = (o:{e: any}) => m(this.FATAL_UnexpectedException,
-    `Unexpected exception: ${(o.e ?? 'unknown error').toString()}\n\nCall stack:\n${(o.e instanceof Error ? o.e.stack : (new Error()).stack)}`);
+  static Fatal_UnexpectedException = (o:{e: any}) => m(this.FATAL_UnexpectedException, null, o.e ?? 'unknown error');
   static FATAL_UnexpectedException = SevFatal | 0x0001;
 
   static Error_FileDoesNotExist = (o:{filename: string}) => m(this.ERROR_FileDoesNotExist, `File ${o.filename} does not exist.`);
diff --git a/developer/src/kmc-kmn/src/compiler/messages.ts b/developer/src/kmc-kmn/src/compiler/messages.ts
index 6b0c3ce879..1f6bf7c327 100644
--- a/developer/src/kmc-kmn/src/compiler/messages.ts
+++ b/developer/src/kmc-kmn/src/compiler/messages.ts
@@ -1,4 +1,4 @@
-import { CompilerErrorNamespace, CompilerErrorSeverity, CompilerEvent, CompilerMessageSpec as m, compilerExceptionToString as exc } from "@keymanapp/common-types";
+import { CompilerErrorNamespace, CompilerErrorSeverity, CompilerEvent, CompilerMessageSpec as m } from "@keymanapp/common-types";
 
 const Namespace = CompilerErrorNamespace.KmnCompiler;
 const SevInfo = CompilerErrorSeverity.Info | Namespace;
@@ -46,20 +46,21 @@ export const enum KmnCompilerMessageRanges {
   are reserved for kmcmplib messages.
 */
 export class CompilerMessages {
-  static Fatal_UnexpectedException = (o:{e: any}) => m(this.FATAL_UnexpectedException, `Unexpected exception: ${exc(o.e)}`);
+  static Fatal_UnexpectedException = (o:{e: any}) => m(this.FATAL_UnexpectedException, null, o.e ?? 'unknown error');
   static FATAL_UnexpectedException = SevFatal | 0x900;
 
-  static Fatal_MissingWasmModule = (o:{e?: any}) => m(this.FATAL_MissingWasmModule, `Could not instantiate WASM compiler module or initialization failed: ${exc(o.e)}`);
+  static Fatal_MissingWasmModule = (o:{e?: any}) => m(this.FATAL_MissingWasmModule,
+    `Could not instantiate WASM compiler module or initialization failed`, o.e ?? 'unknown error');
   static FATAL_MissingWasmModule = SevFatal | 0x901;
 
   // TODO: Is this now deprecated?
-  static Fatal_UnableToSetCompilerOptions = () => m(this.FATAL_UnableToSetCompilerOptions, `Unable to set compiler options`);
+  static Fatal_UnableToSetCompilerOptions = () => m(this.FATAL_UnableToSetCompilerOptions, null, `Unable to set compiler options`);
   static FATAL_UnableToSetCompilerOptions = SevFatal | 0x902;
 
-  static Fatal_CallbacksNotSet = () => m(this.FATAL_CallbacksNotSet, `Callbacks were not set with init`);
+  static Fatal_CallbacksNotSet = () => m(this.FATAL_CallbacksNotSet, null, `Callbacks were not set with init`);
   static FATAL_CallbacksNotSet = SevFatal | 0x903;
 
-  static Fatal_UnicodeSetOutOfRange = () => m(this.FATAL_UnicodeSetOutOfRange, `UnicodeSet buffer was too small`);
+  static Fatal_UnicodeSetOutOfRange = () => m(this.FATAL_UnicodeSetOutOfRange, null, `UnicodeSet buffer was too small`);
   static FATAL_UnicodeSetOutOfRange = SevFatal | 0x904;
 
   static Error_UnicodeSetHasStrings = () => m(this.ERROR_UnicodeSetHasStrings, `UnicodeSet contains strings, not allowed`);
@@ -72,7 +73,7 @@ export class CompilerMessages {
   static ERROR_UnicodeSetSyntaxError = SevError | 0x907;
 
   static Error_InvalidKvksFile = (o:{filename: string, e: any}) => m(this.ERROR_InvalidKvksFile,
-    `Error encountered parsing ${o.filename}: ${o.e}`);
+    `Error encountered parsing ${o.filename}: ${o.e ?? 'unknown error'}`); // Note, not fatal, not reporting to Sentry
   static ERROR_InvalidKvksFile = SevError | 0x908;
 
   static Warn_InvalidVkeyInKvksFile = (o:{filename: string, invalidVkey: string}) => m(this.WARN_InvalidVkeyInKvksFile,
@@ -80,11 +81,11 @@ export class CompilerMessages {
   static WARN_InvalidVkeyInKvksFile = SevWarn | 0x909;
 
   static Error_InvalidDisplayMapFile = (o:{filename: string, e: any}) => m(this.ERROR_InvalidDisplayMapFile,
-    `Error encountered parsing display map ${o.filename}: ${o.e}`);
+    `Error encountered parsing display map ${o.filename}: ${o.e ?? 'unknown error'}`); // Note, not fatal, not reporting to Sentry
   static ERROR_InvalidDisplayMapFile = SevError | 0x90A;
 
   static Error_InvalidKvkFile = (o:{filename: string, e: any}) => m(this.ERROR_InvalidKvkFile,
-    `Error encountered loading ${o.filename}: ${o.e}`);
+    `Error encountered loading ${o.filename}: ${o.e ?? 'unknown error'}`); // Note, not fatal, not reporting to Sentry
   static ERROR_InvalidKvkFile = SevError | 0x90B;
 };
 
diff --git a/developer/src/kmc-ldml/src/compiler/messages.ts b/developer/src/kmc-ldml/src/compiler/messages.ts
index e3ec8bcbf2..4a2686c1a7 100644
--- a/developer/src/kmc-ldml/src/compiler/messages.ts
+++ b/developer/src/kmc-ldml/src/compiler/messages.ts
@@ -60,7 +60,7 @@ export class CompilerMessages {
   static ERROR_MustBeAtLeastOneLayerElement = SevError | 0x000E;
 
   static Fatal_SectionCompilerFailed = (o:{sect: string}) =>
-    m(this.FATAL_SectionCompilerFailed, `The compiler for '${o.sect}' failed unexpectedly.`);
+    m(this.FATAL_SectionCompilerFailed, null, `The compiler for '${o.sect}' failed unexpectedly.`);
   static FATAL_SectionCompilerFailed = SevFatal | 0x000F;
 
   static Error_DisplayIsRepeated = (o:{to: string}) =>
diff --git a/developer/src/kmc-model-info/src/messages.ts b/developer/src/kmc-model-info/src/messages.ts
index 26ba8f315c..a4d61e8b23 100644
--- a/developer/src/kmc-model-info/src/messages.ts
+++ b/developer/src/kmc-model-info/src/messages.ts
@@ -8,8 +8,7 @@ const SevError = CompilerErrorSeverity.Error | Namespace;
 const SevFatal = CompilerErrorSeverity.Fatal | Namespace;
 
 export class ModelInfoCompilerMessages {
-  static Fatal_UnexpectedException = (o:{e: any}) => m(this.FATAL_UnexpectedException,
-    `Unexpected exception: ${(o.e ?? 'unknown error').toString()}\n\nCall stack:\n${(o.e instanceof Error ? o.e.stack : (new Error()).stack)}`);
+  static Fatal_UnexpectedException = (o:{e: any}) => m(this.FATAL_UnexpectedException, null, o.e ?? 'unknown error');
   static FATAL_UnexpectedException = SevFatal | 0x0001;
 
   static Error_FileDoesNotExist = (o:{filename: string}) => m(this.ERROR_FileDoesNotExist, `File ${o.filename} does not exist.`);
diff --git a/developer/src/kmc-model/src/model-compiler-errors.ts b/developer/src/kmc-model/src/model-compiler-errors.ts
index f7062da44e..beb3398caa 100644
--- a/developer/src/kmc-model/src/model-compiler-errors.ts
+++ b/developer/src/kmc-model/src/model-compiler-errors.ts
@@ -1,4 +1,4 @@
-import { CompilerErrorNamespace, CompilerErrorSeverity, CompilerEvent } from "@keymanapp/common-types";
+import { CompilerErrorNamespace, CompilerErrorSeverity, CompilerEvent, CompilerMessageSpec } from "@keymanapp/common-types";
 
 const Namespace = CompilerErrorNamespace.ModelCompiler;
 // const SevInfo = CompilerErrorSeverity.Info | Namespace;
@@ -7,12 +7,11 @@ const SevWarn = CompilerErrorSeverity.Warn | Namespace;
 const SevError = CompilerErrorSeverity.Error | Namespace;
 const SevFatal = CompilerErrorSeverity.Fatal | Namespace;
 
-const m = (code: number, message: string) : CompilerEvent => { return {
+const m = (code: number, message: string, exceptionVar?: any) : CompilerEvent => ({
+  ...CompilerMessageSpec(code, message, exceptionVar),
   line: ModelCompilerMessageContext.line,
   filename: ModelCompilerMessageContext.filename,
-  code,
-  message
-} };
+});
 
 export class ModelCompilerMessageContext {
   // Context added to all messages
@@ -22,8 +21,7 @@ export class ModelCompilerMessageContext {
 
 export class ModelCompilerMessages {
 
-  static Fatal_UnexpectedException = (o:{e: any}) => m(this.FATAL_UnexpectedException,
-    `Unexpected exception: ${(o.e ?? 'unknown error').toString()}\n\nCall stack:\n${(o.e instanceof Error ? o.e.stack : (new Error()).stack)}`);
+  static Fatal_UnexpectedException = (o:{e: any}) => m(this.FATAL_UnexpectedException, null, o.e ?? 'unknown error');
   static FATAL_UnexpectedException = SevFatal | 0x0001;
 
   static Warn_MixedNormalizationForms = (o:{wordform: string}) => m(this.WARN_MixedNormalizationForms,
diff --git a/developer/src/kmc-package/src/compiler/messages.ts b/developer/src/kmc-package/src/compiler/messages.ts
index 0843d4f921..c230c69772 100644
--- a/developer/src/kmc-package/src/compiler/messages.ts
+++ b/developer/src/kmc-package/src/compiler/messages.ts
@@ -8,8 +8,7 @@ const SevError = CompilerErrorSeverity.Error | Namespace;
 const SevFatal = CompilerErrorSeverity.Fatal | Namespace;
 
 export class CompilerMessages {
-  static Fatal_UnexpectedException = (o:{e: any}) => m(this.FATAL_UnexpectedException,
-    `Unexpected exception: ${(o.e ?? 'unknown error').toString()}\n\nCall stack:\n${(o.e instanceof Error ? o.e.stack : (new Error()).stack)}`);
+  static Fatal_UnexpectedException = (o:{e: any}) => m(this.FATAL_UnexpectedException, null, o.e ?? 'unknown error');
   static FATAL_UnexpectedException = SevFatal | 0x0001;
 
   static Warn_AbsolutePath = (o:{filename: string}) => m(this.WARN_AbsolutePath, `File ${o.filename} has an absolute path, which is not portable.`);
diff --git a/developer/src/kmc/src/commands/analyze.ts b/developer/src/kmc/src/commands/analyze.ts
index 7a2cec2843..eceb29f7bf 100644
--- a/developer/src/kmc/src/commands/analyze.ts
+++ b/developer/src/kmc/src/commands/analyze.ts
@@ -2,7 +2,7 @@ import * as fs from 'fs';
 import * as path from 'path';
 import { Command, Option } from 'commander';
 import { NodeCompilerCallbacks } from '../util/NodeCompilerCallbacks.js';
-import { InfrastructureMessages } from '../messages/messages.js';
+import { InfrastructureMessages } from '../messages/infrastructureMessages.js';
 import { CompilerCallbacks, CompilerLogLevel } from '@keymanapp/common-types';
 import { AnalyzeOskCharacterUse, AnalyzeOskRewritePua } from '@keymanapp/kmc-analyze';
 import { BaseOptions } from '../util/baseOptions.js';
diff --git a/developer/src/kmc/src/commands/build.ts b/developer/src/kmc/src/commands/build.ts
index 04c390e59d..1047dd8208 100644
--- a/developer/src/kmc/src/commands/build.ts
+++ b/developer/src/kmc/src/commands/build.ts
@@ -4,7 +4,7 @@ import { Command } from 'commander';
 import { buildActivities } from './buildClasses/buildActivities.js';
 import { BuildProject } from './buildClasses/BuildProject.js';
 import { NodeCompilerCallbacks } from '../util/NodeCompilerCallbacks.js';
-import { InfrastructureMessages } from '../messages/messages.js';
+import { InfrastructureMessages } from '../messages/infrastructureMessages.js';
 import { CompilerFileCallbacks, CompilerOptions, KeymanFileTypes } from '@keymanapp/common-types';
 import { BaseOptions } from '../util/baseOptions.js';
 import { expandFileLists } from '../util/fileLists.js';
@@ -80,13 +80,18 @@ If no input file is supplied, kmc will build the current folder.`)
 }
 
 async function build(filename: string, parentCallbacks: NodeCompilerCallbacks, options: CompilerOptions): Promise<boolean> {
-
-  if(!fs.existsSync(filename)) {
-    parentCallbacks.reportMessage(InfrastructureMessages.Error_FileDoesNotExist({filename}));
-    return false;
-  }
-
   try {
+    // TEST: allow command-line simulation of infrastructure fatal errors, and
+    // also for unit tests
+    if(process.env.SENTRY_CLIENT_TEST_BUILD_EXCEPTION == '1') {
+      throw new Error('Test exception from SENTRY_CLIENT_TEST_BUILD_EXCEPTION');
+    }
+
+    if(!fs.existsSync(filename)) {
+      parentCallbacks.reportMessage(InfrastructureMessages.Error_FileDoesNotExist({filename}));
+      return false;
+    }
+
     let builder = null;
 
     // If infile is a directory, then we treat that as a project and build it
@@ -136,3 +141,10 @@ async function build(filename: string, parentCallbacks: NodeCompilerCallbacks, o
     return false;
   }
 }
+
+/**
+ * these are exported only for unit tests, do not use
+ */
+export const unitTestEndpoints = {
+  build
+};
diff --git a/developer/src/kmc/src/commands/buildClasses/BuildKeyboardInfo.ts b/developer/src/kmc/src/commands/buildClasses/BuildKeyboardInfo.ts
index 14670c99b4..42bc4e4a95 100644
--- a/developer/src/kmc/src/commands/buildClasses/BuildKeyboardInfo.ts
+++ b/developer/src/kmc/src/commands/buildClasses/BuildKeyboardInfo.ts
@@ -3,7 +3,7 @@ import { BuildActivity } from './BuildActivity.js';
 import { CompilerCallbacks, CompilerOptions, KeymanDeveloperProject, KeymanFileTypes } from '@keymanapp/common-types';
 import { KeyboardInfoCompiler } from '@keymanapp/kmc-keyboard-info';
 import { loadProject } from '../../util/projectLoader.js';
-import { InfrastructureMessages } from '../../messages/messages.js';
+import { InfrastructureMessages } from '../../messages/infrastructureMessages.js';
 import { KmpCompiler } from '@keymanapp/kmc-package';
 import { calculateSourcePath } from '../../util/calculateSourcePath.js';
 
diff --git a/developer/src/kmc/src/commands/buildClasses/BuildModelInfo.ts b/developer/src/kmc/src/commands/buildClasses/BuildModelInfo.ts
index bfb167bdcc..9dd4d476e7 100644
--- a/developer/src/kmc/src/commands/buildClasses/BuildModelInfo.ts
+++ b/developer/src/kmc/src/commands/buildClasses/BuildModelInfo.ts
@@ -4,7 +4,7 @@ import { CompilerCallbacks, CompilerOptions, KeymanFileTypes } from '@keymanapp/
 import { writeMergedModelMetadataFile } from '@keymanapp/kmc-model-info';
 import { KmpCompiler } from '@keymanapp/kmc-package';
 import { loadProject } from '../../util/projectLoader.js';
-import { InfrastructureMessages } from '../../messages/messages.js';
+import { InfrastructureMessages } from '../../messages/infrastructureMessages.js';
 import { calculateSourcePath } from '../../util/calculateSourcePath.js';
 
 export class BuildModelInfo extends BuildActivity {
diff --git a/developer/src/kmc/src/commands/buildClasses/BuildProject.ts b/developer/src/kmc/src/commands/buildClasses/BuildProject.ts
index 44cb990704..460a5af88f 100644
--- a/developer/src/kmc/src/commands/buildClasses/BuildProject.ts
+++ b/developer/src/kmc/src/commands/buildClasses/BuildProject.ts
@@ -3,7 +3,7 @@ import * as fs from 'fs';
 import { CompilerCallbacks, CompilerFileCallbacks, CompilerOptions, KeymanDeveloperProject, KeymanDeveloperProjectFile, KeymanFileTypes } from '@keymanapp/common-types';
 import { BuildActivity } from './BuildActivity.js';
 import { buildActivities } from './buildActivities.js';
-import { InfrastructureMessages } from '../../messages/messages.js';
+import { InfrastructureMessages } from '../../messages/infrastructureMessages.js';
 import { loadProject } from '../../util/projectLoader.js';
 
 export class BuildProject extends BuildActivity {
diff --git a/developer/src/kmc/src/kmc.ts b/developer/src/kmc/src/kmc.ts
index 502c94d3c0..d22fc22348 100644
--- a/developer/src/kmc/src/kmc.ts
+++ b/developer/src/kmc/src/kmc.ts
@@ -17,6 +17,10 @@ try {
   KeymanSentry.captureException(e);
 }
 
+// Ensure any messages reported to Sentry have had time to be uploaded before we
+// exit. In most cases, this will be a no-op so should not affect performance.
+await KeymanSentry.close();
+
 async function run() {
   /* Arguments */
 
diff --git a/developer/src/kmc/src/messages/messages.ts b/developer/src/kmc/src/messages/infrastructureMessages.ts
similarity index 96%
rename from developer/src/kmc/src/messages/messages.ts
rename to developer/src/kmc/src/messages/infrastructureMessages.ts
index 95c2e73741..ee828dae75 100644
--- a/developer/src/kmc/src/messages/messages.ts
+++ b/developer/src/kmc/src/messages/infrastructureMessages.ts
@@ -8,8 +8,7 @@ const SevError = CompilerErrorSeverity.Error | Namespace;
 const SevFatal = CompilerErrorSeverity.Fatal | Namespace;
 
 export class InfrastructureMessages {
-  static Fatal_UnexpectedException = (o:{e: any}) => m(this.FATAL_UnexpectedException,
-    `Unexpected exception: ${(o.e ?? 'unknown error').toString()}\n\nCall stack:\n${(o.e instanceof Error ? o.e.stack : (new Error()).stack)}`);
+  static Fatal_UnexpectedException = (o:{e: any}) => m(this.FATAL_UnexpectedException, null, o.e ?? 'unknown error');
   static FATAL_UnexpectedException = SevFatal | 0x0001;
 
   // For this message, we override the filename with the passed-in file. A bit of a hack but does the job
diff --git a/developer/src/kmc/src/util/KeymanSentry.ts b/developer/src/kmc/src/util/KeymanSentry.ts
index a2a466b83d..c7fa7eb7eb 100644
--- a/developer/src/kmc/src/util/KeymanSentry.ts
+++ b/developer/src/kmc/src/util/KeymanSentry.ts
@@ -89,18 +89,34 @@ export class KeymanSentry {
     isInit = true;
   }
 
+  private static writeSentryMessage(eventId: string) {
+    process.stderr.write(`
+    This error has been automatically reported to the Keyman team.
+      Identifier:  ${eventId}
+      Application: Keyman Developer
+      Reported at: https://sentry.io/organizations/keyman/projects/keyman-developer/events/${eventId}/
+    `);
+  }
+
+  static async reportException(e: any, silent: boolean = true) {
+    if(isInit) {
+      const eventId = await Sentry.captureException(e);
+      if(!silent) {
+        this.writeSentryMessage(eventId);
+      }
+      return eventId;
+    }
+    return null;
+  }
+
   static async captureException(e: any) {
     if(isInit) {
       const eventId = Sentry.captureException(e);
       process.stderr.write(`
-Fatal error: ${(e??'').toString()}
-
-This error has been automatically reported to the Keyman team.
-  Identifier:  ${eventId}
-  Application: Keyman Developer
-  Reported at: https://sentry.io/organizations/keyman/projects/keyman-developer/events/${eventId}/
-`);
-      await Sentry.close(2000);
+      Fatal error: ${(e??'').toString()}
+      `);
+      this.writeSentryMessage(eventId);
+      this.close();
 
       // For local development, we don't want to bury the trace; we need the cast to avoid
       // TS2367 (comparison appears to be unintentional)
@@ -112,4 +128,10 @@ This error has been automatically reported to the Keyman team.
       throw e;
     }
   }
+
+  static async close() {
+    if(isInit) {
+      await Sentry.close(2000);
+    }
+  }
 }
\ No newline at end of file
diff --git a/developer/src/kmc/src/util/NodeCompilerCallbacks.ts b/developer/src/kmc/src/util/NodeCompilerCallbacks.ts
index 0b66b301be..58fb355e3d 100644
--- a/developer/src/kmc/src/util/NodeCompilerCallbacks.ts
+++ b/developer/src/kmc/src/util/NodeCompilerCallbacks.ts
@@ -6,9 +6,10 @@ import { CompilerCallbacks, CompilerEvent,
          CompilerError,
          CompilerCallbackOptions,
          CompilerFileCallbacks} from '@keymanapp/common-types';
-import { InfrastructureMessages } from '../messages/messages.js';
+import { InfrastructureMessages } from '../messages/infrastructureMessages.js';
 import chalk from 'chalk';
 import supportsColor from 'supports-color';
+import { KeymanSentry } from './KeymanSentry.js';
 
 const color = chalk.default;
 const severityColors: {[value in CompilerErrorSeverity]: chalk.Chalk} = {
@@ -121,6 +122,14 @@ export class NodeCompilerCallbacks implements CompilerCallbacks {
 
     this.messages.push({...event});
 
+    // report fatal errors to Sentry, but don't display; note, it won't be
+    // reported if user has disabled the Sentry setting
+    if(CompilerError.severity(event.code) == CompilerErrorSeverity.Fatal) {
+      // this is async so returns a Promise, we'll let it resolve
+      // in its own time, and it can print its message then
+      KeymanSentry.reportException(event.exceptionVar, false);
+    }
+
     if(CompilerError.severity(event.code) < compilerLogLevelToSeverity[this.options.logLevel]) {
       // collect messages but don't print to console
       return;
diff --git a/developer/src/kmc/src/util/fileLists.ts b/developer/src/kmc/src/util/fileLists.ts
index 11ce21905e..ecd786d0ae 100644
--- a/developer/src/kmc/src/util/fileLists.ts
+++ b/developer/src/kmc/src/util/fileLists.ts
@@ -1,7 +1,7 @@
 import * as fs from 'fs';
 import * as path from 'path';
 import { CompilerCallbacks } from "@keymanapp/common-types";
-import { InfrastructureMessages } from "../messages/messages.js";
+import { InfrastructureMessages } from "../messages/infrastructureMessages.js";
 
 /**
  * Replaces each entry starting with `@` with the content of the file, with one
diff --git a/developer/src/kmc/src/util/projectLoader.ts b/developer/src/kmc/src/util/projectLoader.ts
index e92a84c399..c7141522a3 100644
--- a/developer/src/kmc/src/util/projectLoader.ts
+++ b/developer/src/kmc/src/util/projectLoader.ts
@@ -2,7 +2,7 @@
 import * as path from 'path';
 import * as fs from 'fs';
 import { CompilerCallbacks, KeymanDeveloperProject, KeymanFileTypes, KPJFileReader } from "@keymanapp/common-types";
-import { InfrastructureMessages } from "../messages/messages.js";
+import { InfrastructureMessages } from "../messages/infrastructureMessages.js";
 
 export const isProject = (filename: string): boolean =>
   fs.existsSync(filename) && (
diff --git a/developer/src/kmc/test/test-messages.ts b/developer/src/kmc/test/test-infrastructureMessages.ts
similarity index 71%
rename from developer/src/kmc/test/test-messages.ts
rename to developer/src/kmc/test/test-infrastructureMessages.ts
index 9fa2c0efbd..a9b55d075d 100644
--- a/developer/src/kmc/test/test-messages.ts
+++ b/developer/src/kmc/test/test-infrastructureMessages.ts
@@ -1,10 +1,11 @@
 import 'mocha';
 import { assert } from 'chai';
-import { InfrastructureMessages } from '../src/messages/messages.js';
+import { InfrastructureMessages } from '../src/messages/infrastructureMessages.js';
 import { verifyCompilerMessagesObject } from '@keymanapp/developer-test-helpers';
 import { makePathToFixture } from './helpers/index.js';
 import { NodeCompilerCallbacks } from '../src/util/NodeCompilerCallbacks.js';
 import { CompilerErrorNamespace } from '@keymanapp/common-types';
+import { unitTestEndpoints } from '../src/commands/build.js';
 
 describe('InfrastructureMessages', function () {
   it('should have a valid InfrastructureMessages object', function() {
@@ -15,34 +16,20 @@ describe('InfrastructureMessages', function () {
   // Message tests
   //
 
-  /*
-  TODO:
+  // FATAL_UnexpectedException
 
-  let callbacks = new TestCompilerCallbacks();
-
-  async function testForMessage(context: Mocha.Context, fixture: string[], messageId?: number) {
-    context.timeout(10000);
-
-    callbacks.clear();
-
-    const builder = new BuildKmnKeyboard();
-    const path = makePathToFixture(...fixture);
-    let result = await builder.build(path, callbacks, {
-      compilerVersion: false,
-      compilerWarningsAsErrors: true,
-      debug: false,
-      warnDeprecatedCode: true,
+  it('should generate FATAL_UnexpectedException if an exception is raised', async function() {
+    const ncb = new NodeCompilerCallbacks({logLevel: 'silent'});
+    process.env.SENTRY_CLIENT_TEST_BUILD_EXCEPTION = '1';
+    await unitTestEndpoints.build(null, ncb, {});
+    delete process.env.SENTRY_CLIENT_TEST_BUILD_EXCEPTION;
+    assert.isTrue(ncb.hasMessage(InfrastructureMessages.FATAL_UnexpectedException),
+      `FATAL_UnexpectedException not generated, instead got: `+JSON.stringify(ncb.messages,null,2));
+    assert.lengthOf(ncb.messages, 1);
+    assert.instanceOf<Error>(ncb.messages[0].exceptionVar, Error);
     });
 
-    if(messageId) {
-      assert.isTrue(callbacks.hasMessage(messageId), `messageId ${messageId.toString(16)} not generated, instead got: `+JSON.stringify(callbacks.messages,null,2));
-      assert.lengthOf(callbacks.messages, 1);
-    } else {
-      assert.lengthOf(callbacks.messages, 0, `messages should be empty, but instead got: `+JSON.stringify(callbacks.messages,null,2));
-      assert.isTrue(result);
-    }
-  }
-
+/*
   // ERROR_FileDoesNotExist
 
   it('should generate ERROR_FileDoesNotExist if a file does not exist', async function() {
diff --git a/developer/src/kmc/test/test-project-build.ts b/developer/src/kmc/test/test-project-build.ts
index 85929d7844..088de1e40f 100644
--- a/developer/src/kmc/test/test-project-build.ts
+++ b/developer/src/kmc/test/test-project-build.ts
@@ -3,7 +3,7 @@ import { assert } from 'chai';
 import 'mocha';
 import { BuildProject } from '../src/commands/buildClasses/BuildProject.js';
 import { makePathToFixture } from './helpers/index.js';
-import { InfrastructureMessages } from '../src/messages/messages.js';
+import { InfrastructureMessages } from '../src/messages/infrastructureMessages.js';
 
 const callbacks = new TestCompilerCallbacks();
 
diff --git a/package-lock.json b/package-lock.json
index 49ef815928..405d2154cf 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -35,6 +35,7 @@
         "@keymanapp/ldml-keyboard-constants": "file:core/include/ldml"
       },
       "devDependencies": {
+        "@types/chai": "^4.3.5",
         "@typescript-eslint/eslint-plugin": "^5.59.1",
         "chai": "^4.3.4",
         "esbuild": "^0.15.16",
@@ -3649,9 +3650,10 @@
       }
     },
     "node_modules/@types/chai": {
-      "version": "4.3.0",
-      "dev": true,
-      "license": "MIT"
+      "version": "4.3.5",
+      "resolved": "https://registry.npmjs.org/@types/chai/-/chai-4.3.5.tgz",
+      "integrity": "sha512-mEo1sAde+UCE6b2hxn332f1g1E8WfYRu6p5SvTKr2ZKC1f7gFJXk4h5PyGP9Dt6gCaG8y8XhwnXWC6Iy2cmBng==",
+      "dev": true
     },
     "node_modules/@types/component-emitter": {
       "version": "1.2.11",
diff --git a/package.json b/package.json
index ebf64981ff..0afd9ade6e 100644
--- a/package.json
+++ b/package.json
@@ -2,6 +2,7 @@
   "name": "root",
   "private": true,
   "devDependencies": {
+    "@types/chai": "^4.3.5",
     "@typescript-eslint/eslint-plugin": "^5.59.1",
     "chai": "^4.3.4",
     "esbuild": "^0.15.16",