diff --git a/command/build.go b/command/build.go
index 410f1889d..72c3e3512 100644
--- a/command/build.go
+++ b/command/build.go
@@ -188,31 +188,6 @@ func (c *BuildCommand) RunContext(buildCtx context.Context, cla *BuildArgs) int
 	log.Printf("Force build: %v", cla.Force)
 	log.Printf("On error: %v", cla.OnError)
 
-	// Set the debug and force mode and prepare all the builds
-	// This is only affects json templates, because HCL2
-	// templates have already been prepared in GetBuilds() above.
-	for i := range builds {
-		b := builds[i]
-		log.Printf("Preparing build: %s", b.Name())
-		b.SetDebug(cla.Debug)
-		b.SetForce(cla.Force)
-		b.SetOnError(cla.OnError)
-
-		warnings, err := b.Prepare()
-		if err != nil {
-			c.Ui.Error(err.Error())
-			return 1
-		}
-		if len(warnings) > 0 {
-			ui := buildUis[b]
-			ui.Say(fmt.Sprintf("Warnings for build '%s':\n", b.Name()))
-			for _, warning := range warnings {
-				ui.Say(fmt.Sprintf("* %s", warning))
-			}
-			ui.Say("")
-		}
-	}
-
 	// Run all the builds in parallel and wait for them to complete
 	var wg sync.WaitGroup
 	var artifacts = struct {
diff --git a/hcl2template/types.packer_config.go b/hcl2template/types.packer_config.go
index 41580d3af..a5dd4e3ee 100644
--- a/hcl2template/types.packer_config.go
+++ b/hcl2template/types.packer_config.go
@@ -359,6 +359,19 @@ func (cfg *PackerConfig) GetBuilds(opts packer.GetBuildsOptions) ([]packer.Build
 				PostProcessors: pps,
 				Prepared:       true,
 			}
+			// Prepare just sets the "prepareCalled" flag on CoreBuild, since
+			// we did all the prep here.
+			_, err := pcb.Prepare()
+			if err != nil {
+				diags = append(diags, &hcl.Diagnostic{
+					Severity: hcl.DiagError,
+					Summary:  fmt.Sprintf("Preparing packer core build %s failed", src.Ref().String()),
+					Detail:   err.Error(),
+					Subject:  build.HCL2Ref.DefRange.Ptr(),
+				})
+				continue
+			}
+
 			res = append(res, pcb)
 		}
 	}
diff --git a/helper/config/decode.go b/helper/config/decode.go
index ba022e7e3..327241619 100644
--- a/helper/config/decode.go
+++ b/helper/config/decode.go
@@ -144,7 +144,7 @@ func Decode(target interface{}, config *DecodeOpts, raws ...interface{}) error {
 		for _, unused := range md.Unused {
 			if unused != "type" && !strings.HasPrefix(unused, "packer_") {
 				err = multierror.Append(err, fmt.Errorf(
-					"unknown configuration key: %q; raws is %#v \n\n and ctx data is %#v", unused, raws, ctxData))
+					"unknown configuration key: %q", unused))
 			}
 		}
 		if err != nil {
diff --git a/packer/build.go b/packer/build.go
index 007c6ed3b..53c90490f 100644
--- a/packer/build.go
+++ b/packer/build.go
@@ -135,8 +135,8 @@ func (b *CoreBuild) Name() string {
 // and any hooks. This _must_ be called prior to Run. The parameter is the
 // overrides for the variables within the template (if any).
 func (b *CoreBuild) Prepare() (warn []string, err error) {
-	// For HCL2 templates, the builder and hooks are initialized when the template is parsed.
-	// Calling Prepare(...) is not necessary
+	// For HCL2 templates, the builder and hooks are initialized when the
+	// template is parsed. Calling Prepare(...) is not necessary
 	if b.Prepared {
 		b.prepareCalled = true
 		return
@@ -149,6 +149,8 @@ func (b *CoreBuild) Prepare() (warn []string, err error) {
 		panic("prepare already called")
 	}
 
+	// Templates loaded from HCL2 will never get here. TODO: move this code into
+	// a custom json area instead of just aborting early for HCL.
 	b.prepareCalled = true
 
 	packerConfig := map[string]interface{}{
diff --git a/packer/core.go b/packer/core.go
index fcf72a6c5..011a21a96 100644
--- a/packer/core.go
+++ b/packer/core.go
@@ -216,6 +216,32 @@ func (c *Core) GetBuilds(opts GetBuildsOptions) ([]Build, hcl.Diagnostics) {
 			continue
 		}
 		builds = append(builds, b)
+
+		// Now that build plugin has been launched, call Prepare()
+		log.Printf("Preparing build: %s", b.Name())
+		b.SetDebug(opts.Debug)
+		b.SetForce(opts.Force)
+		b.SetOnError(opts.OnError)
+
+		warnings, err := b.Prepare()
+		if err != nil {
+			diags = append(diags, &hcl.Diagnostic{
+				Severity: hcl.DiagError,
+				Summary:  fmt.Sprintf("Failed to prepare build: %q", n),
+				Detail:   err.Error(),
+			})
+			continue
+		}
+
+		if len(warnings) > 0 {
+			for _, warning := range warnings {
+				diags = append(diags, &hcl.Diagnostic{
+					Severity: hcl.DiagWarning,
+					Summary:  fmt.Sprintf("Warning when preparing build: %q", n),
+					Detail:   warning,
+				})
+			}
+		}
 	}
 	return builds, diags
 }