diff --git a/packages/mosaic/src/fleet/v1-v2-migration.spec.ts b/packages/mosaic/src/fleet/v1-v2-migration.spec.ts
index f23656f..75c24e7 100644
--- a/packages/mosaic/src/fleet/v1-v2-migration.spec.ts
+++ b/packages/mosaic/src/fleet/v1-v2-migration.spec.ts
@@ -7,6 +7,7 @@ import { afterEach, describe, expect, it, vi } from 'vitest';
 import { loadFleetRoster } from '../commands/fleet.js';
 import { executeFleetReconcile } from './fleet-reconciler.js';
 import { compareCodePoints } from './deterministic-order.js';
+import type { FleetRoster } from './fleet-roster-v1.js';
 import { SHIPPED_FLEET_ARTIFACT_DISPOSITIONS } from './example-profile-dispositions.js';
 import {
   collectShippedFleetMigrationDispositions,
@@ -17,6 +18,21 @@ import {
 } from './v1-v2-migration.js';
 
 const temporaryDirectories: string[] = [];
+
+type V1StringParityCase = Readonly<{
+  field: string;
+  source: () => string;
+  productionValue?: (roster: FleetRoster) => unknown;
+  expectedProduction?: unknown;
+  productionError?: string;
+  expectedStatus: 'ready' | 'blocked';
+  blockerCode?: string;
+  blockerPath?: string;
+  migrationDecisions?: () => V1ToV2MigrationDecisions;
+  candidateValue?: (preview: V1ToV2MigrationPreview) => unknown;
+  expectedCandidate?: unknown;
+}>;
+
 const frameworkFleet = resolve(
   dirname(fileURLToPath(import.meta.url)),
   '..',
@@ -413,6 +429,267 @@ describe('v1-to-v2 migration preview', (): void => {
     expect(preview.evidence).not.toHaveProperty('candidate');
   });
 
+  it.each([
+    {
+      field: 'runtime',
+      source: (): string => v1Source.replace('runtime: pi', 'runtime: " pi "'),
+      productionValue: (roster: FleetRoster): unknown => roster.agents[0]?.runtime,
+      expectedProduction: ' pi ',
+      expectedStatus: 'blocked',
+      blockerCode: 'unsupported-runtime',
+      blockerPath: 'agents[0].runtime',
+    },
+    {
+      field: 'class',
+      source: (): string => v1Source.replace('class: implementer', 'class: " implementer "'),
+      productionValue: (roster: FleetRoster): unknown => roster.agents[0]?.className,
+      expectedProduction: 'code',
+      expectedStatus: 'ready',
+      candidateValue: (preview: V1ToV2MigrationPreview): unknown =>
+        preview.canonicalRoster?.agents[0]?.className,
+      expectedCandidate: 'code',
+    },
+    {
+      field: 'custom class',
+      source: (): string => v1Source.replace('class: implementer', 'class: " custom-domain "'),
+      productionValue: (roster: FleetRoster): unknown => roster.agents[0]?.className,
+      expectedProduction: 'custom-domain',
+      expectedStatus: 'blocked',
+      blockerCode: 'class-disposition-required',
+      blockerPath: 'agents[0].class',
+    },
+    {
+      field: 'empty class with preserve disposition',
+      source: (): string => v1Source.replace('class: implementer', 'class: ""'),
+      productionValue: (roster: FleetRoster): unknown => roster.agents[0]?.className,
+      expectedProduction: '',
+      expectedStatus: 'blocked',
+      blockerCode: 'empty-v1-field-disposition-required',
+      blockerPath: 'agents[0].class',
+      migrationDecisions: (): V1ToV2MigrationDecisions => ({
+        ...decisions(),
+        agents: {
+          ...decisions().agents,
+          coder0: {
+            ...decisions().agents.coder0!,
+            classDisposition: { action: 'preserve' },
+          },
+        },
+      }),
+    },
+    {
+      field: 'empty tool policy with preserve disposition',
+      source: (): string => v1Source.replace('tool_policy: implementer', 'tool_policy: ""'),
+      productionValue: (roster: FleetRoster): unknown => roster.agents[0]?.toolPolicy,
+      expectedProduction: '',
+      expectedStatus: 'blocked',
+      blockerCode: 'empty-v1-field-disposition-required',
+      blockerPath: 'agents[0].tool_policy',
+      migrationDecisions: (): V1ToV2MigrationDecisions => ({
+        ...decisions(),
+        agents: {
+          ...decisions().agents,
+          coder0: {
+            ...decisions().agents.coder0!,
+            toolPolicyDisposition: { action: 'preserve' },
+          },
+        },
+      }),
+    },
+    {
+      field: 'tool policy',
+      source: (): string =>
+        v1Source.replace('tool_policy: implementer', 'tool_policy: " implementer "'),
+      productionValue: (roster: FleetRoster): unknown => roster.agents[0]?.toolPolicy,
+      expectedProduction: ' implementer ',
+      expectedStatus: 'blocked',
+      blockerCode: 'lossy-v1-string-normalization',
+      blockerPath: 'agents[0].tool_policy',
+    },
+    {
+      field: 'camel-case tool policy',
+      source: (): string =>
+        v1Source.replace('tool_policy: implementer', 'toolPolicy: " implementer "'),
+      productionValue: (roster: FleetRoster): unknown => roster.agents[0]?.toolPolicy,
+      expectedProduction: ' implementer ',
+      expectedStatus: 'blocked',
+      blockerCode: 'lossy-v1-string-normalization',
+      blockerPath: 'agents[0].toolPolicy',
+    },
+    {
+      field: 'alias',
+      source: (): string => v1Source.replace('alias: Coder Zero', 'alias: " Coder Zero "'),
+      productionValue: (roster: FleetRoster): unknown => roster.agents[0]?.alias,
+      expectedProduction: ' Coder Zero ',
+      expectedStatus: 'blocked',
+      blockerCode: 'lossy-v1-string-normalization',
+      blockerPath: 'agents[0].alias',
+    },
+    {
+      field: 'agent working directory',
+      source: (): string =>
+        v1Source.replace('working_directory: /srv/coder0', 'working_directory: " /srv/coder0 "'),
+      productionValue: (roster: FleetRoster): unknown => roster.agents[0]?.workingDirectory,
+      expectedProduction: ' /srv/coder0 ',
+      expectedStatus: 'blocked',
+      blockerCode: 'lossy-v1-string-normalization',
+      blockerPath: 'agents[0].working_directory',
+    },
+    {
+      field: 'camel-case agent working directory',
+      source: (): string =>
+        v1Source.replace('working_directory: /srv/coder0', 'workingDirectory: " /srv/coder0 "'),
+      productionValue: (roster: FleetRoster): unknown => roster.agents[0]?.workingDirectory,
+      expectedProduction: ' /srv/coder0 ',
+      expectedStatus: 'blocked',
+      blockerCode: 'lossy-v1-string-normalization',
+      blockerPath: 'agents[0].workingDirectory',
+    },
+    {
+      field: 'default working directory',
+      source: (): string =>
+        v1Source
+          .replace('working_directory: /srv/fleet', 'working_directory: " /srv/fleet "')
+          .replace('    working_directory: /srv/coder0\n', ''),
+      productionValue: (roster: FleetRoster): unknown => roster.defaults.workingDirectory,
+      expectedProduction: ' /srv/fleet ',
+      expectedStatus: 'blocked',
+      blockerCode: 'lossy-v1-string-normalization',
+      blockerPath: 'defaults.working_directory',
+    },
+    {
+      field: 'camel-case default working directory',
+      source: (): string =>
+        v1Source
+          .replace('working_directory: /srv/fleet', 'workingDirectory: " /srv/fleet "')
+          .replace('    working_directory: /srv/coder0\n', ''),
+      productionValue: (roster: FleetRoster): unknown => roster.defaults.workingDirectory,
+      expectedProduction: ' /srv/fleet ',
+      expectedStatus: 'blocked',
+      blockerCode: 'lossy-v1-string-normalization',
+      blockerPath: 'defaults.workingDirectory',
+    },
+    {
+      field: 'tmux holder session',
+      source: (): string =>
+        v1Source.replace('holder_session: _holder', 'holder_session: " _holder "'),
+      productionValue: (roster: FleetRoster): unknown => roster.tmux.holderSession,
+      expectedProduction: ' _holder ',
+      expectedStatus: 'blocked',
+      blockerCode: 'lossy-v1-string-normalization',
+      blockerPath: 'tmux.holder_session',
+    },
+    {
+      field: 'camel-case tmux holder session',
+      source: (): string =>
+        v1Source.replace('holder_session: _holder', 'holderSession: " _holder "'),
+      productionValue: (roster: FleetRoster): unknown => roster.tmux.holderSession,
+      expectedProduction: ' _holder ',
+      expectedStatus: 'blocked',
+      blockerCode: 'lossy-v1-string-normalization',
+      blockerPath: 'tmux.holderSession',
+    },
+    {
+      field: 'runtime reset command',
+      source: (): string => v1Source.replace('reset_command: /new', 'reset_command: " /new "'),
+      productionValue: (roster: FleetRoster): unknown => roster.runtimes.pi?.resetCommand,
+      expectedProduction: ' /new ',
+      expectedStatus: 'blocked',
+      blockerCode: 'lossy-v1-string-normalization',
+      blockerPath: 'runtimes.pi.reset_command',
+    },
+    {
+      field: 'camel-case runtime reset command',
+      source: (): string => v1Source.replace('reset_command: /new', 'resetCommand: " /new "'),
+      productionValue: (roster: FleetRoster): unknown => roster.runtimes.pi?.resetCommand,
+      expectedProduction: ' /new ',
+      expectedStatus: 'blocked',
+      blockerCode: 'lossy-v1-string-normalization',
+      blockerPath: 'runtimes.pi.resetCommand',
+    },
+    {
+      field: 'remote inventory alias',
+      source: (): string =>
+        v1Source.replace(
+          '  - name: remote0\n    runtime: pi',
+          '  - name: remote0\n    alias: " Remote Zero "\n    runtime: pi',
+        ),
+      productionValue: (roster: FleetRoster): unknown => roster.agents[1]?.alias,
+      expectedProduction: ' Remote Zero ',
+      expectedStatus: 'ready',
+      candidateValue: (preview: V1ToV2MigrationPreview): unknown =>
+        preview.inventoryOnlyAgents.find(({ name }) => name === 'remote0')?.disposition,
+      expectedCandidate: 'inventory-only',
+    },
+    {
+      field: 'host',
+      source: (): string =>
+        v1Source.replace('host: build.example.test', 'host: " build.example.test "'),
+      productionError: 'unsupported targeting characters',
+      expectedStatus: 'blocked',
+      blockerCode: 'invalid-v1-targeting-characters',
+      blockerPath: 'agents[1].host',
+    },
+    {
+      field: 'SSH target',
+      source: (): string =>
+        v1Source.replace(
+          'ssh: operator@build.example.test',
+          'ssh: " operator@build.example.test "',
+        ),
+      productionError: 'unsupported targeting characters',
+      expectedStatus: 'blocked',
+      blockerCode: 'invalid-v1-targeting-characters',
+      blockerPath: 'agents[1].ssh',
+    },
+  ] satisfies readonly V1StringParityCase[])(
+    'matches production v1 semantics for padded $field source',
+    async ({
+      source: buildSource,
+      productionValue,
+      expectedProduction,
+      productionError,
+      expectedStatus,
+      blockerCode,
+      blockerPath,
+      migrationDecisions,
+      candidateValue,
+      expectedCandidate,
+    }: V1StringParityCase) => {
+      const source = buildSource();
+      const rosterPath = join(await mkdtemp(join(tmpdir(), 'v1-string-parity-')), 'roster.yaml');
+      temporaryDirectories.push(dirname(rosterPath));
+      await writeFile(rosterPath, source);
+
+      if (productionError) {
+        await expect(loadFleetRoster(rosterPath)).rejects.toThrow(productionError);
+      } else {
+        const productionRoster = await loadFleetRoster(rosterPath);
+        expect(productionValue?.(productionRoster)).toBe(expectedProduction);
+      }
+
+      const preview = await previewV1ToV2Migration({
+        source,
+        decisions: migrationDecisions?.() ?? decisions(),
+        observations: observations(),
+        personaDirs: await semanticDirs(),
+      });
+
+      expect(preview.status).toBe(expectedStatus);
+      if (expectedStatus === 'blocked') {
+        expect(preview.blockers).toContainEqual(
+          expect.objectContaining({ code: blockerCode, path: blockerPath }),
+        );
+        expect(preview).not.toHaveProperty('canonicalRoster');
+        expect(preview).not.toHaveProperty('canonicalYaml');
+        expect(preview.evidence).not.toHaveProperty('candidate');
+      } else {
+        expect(preview.blockers).toEqual([]);
+        expect(candidateValue?.(preview)).toBe(expectedCandidate);
+      }
+    },
+  );
+
   it.each([
     ['socket_name', "  socket_name: ''\n  holder_session: _holder"],
     ['socketName', "  socketName: ''\n  holder_session: _holder"],
diff --git a/packages/mosaic/src/fleet/v1-v2-migration.ts b/packages/mosaic/src/fleet/v1-v2-migration.ts
index 4c3cb9e..69961d2 100644
--- a/packages/mosaic/src/fleet/v1-v2-migration.ts
+++ b/packages/mosaic/src/fleet/v1-v2-migration.ts
@@ -348,7 +348,8 @@ export async function previewV1ToV2Migration(
 
   assertV1Root(raw, blockers);
   validateV1FieldCatalog(raw, blockers);
-  blockPresentEmptyV1Fields(raw, blockers);
+  blockPresentEmptyV1Fields(raw, decisions.fleetHost, blockers);
+  blockLossyV1StringNormalization(raw, decisions.fleetHost, blockers);
   const tmux = record(raw.tmux);
   const defaults = record(raw.defaults);
   const runtimes = normalizeRuntimes(record(raw.runtimes), blockers);
@@ -439,13 +440,13 @@ export async function previewV1ToV2Migration(
       continue;
     }
 
-    const runtime = stringValue(rawAgent.runtime);
+    const runtime = exactStringValue(rawAgent.runtime);
     if (!isRuntime(runtime)) {
       blockers.push(
         blocker('unsupported-runtime', `${path}.runtime`, 'Runtime must be supported.', name),
       );
     }
-    const requestedClass = stringValue(rawAgent.class) || 'worker';
+    const requestedClass = exactStringValue(rawAgent.class) ?? 'worker';
     const className = resolveClass(
       requestedClass,
       decision.classDisposition,
@@ -454,7 +455,7 @@ export async function previewV1ToV2Migration(
       blockers,
     );
     const requestedToolPolicy =
-      stringValue(rawAgent.tool_policy) || stringValue(rawAgent.toolPolicy);
+      exactStringValue(rawAgent.tool_policy) ?? exactStringValue(rawAgent.toolPolicy);
     const toolPolicy = resolveToolPolicy(
       requestedToolPolicy,
       decision.toolPolicyDisposition,
@@ -511,7 +512,7 @@ export async function previewV1ToV2Migration(
     if (!runtime || !className || !toolPolicy || !desiredState) continue;
     candidateAgents.push({
       name,
-      alias: stringValue(rawAgent.alias) || name,
+      alias: exactStringValue(rawAgent.alias) ?? name,
       class: className,
       runtime,
       provider: decision.provider,
@@ -519,10 +520,10 @@ export async function previewV1ToV2Migration(
       reasoning: decision.reasoning,
       tool_policy: toolPolicy,
       working_directory:
-        stringValue(rawAgent.working_directory) ||
-        stringValue(rawAgent.workingDirectory) ||
-        stringValue(defaults.working_directory) ||
-        stringValue(defaults.workingDirectory) ||
+        exactStringValue(rawAgent.working_directory) ??
+        exactStringValue(rawAgent.workingDirectory) ??
+        exactStringValue(defaults.working_directory) ??
+        exactStringValue(defaults.workingDirectory) ??
         '~/src',
       persistent_persona: persistentPersona,
       reset_between_tasks:
@@ -596,12 +597,14 @@ export async function previewV1ToV2Migration(
           tmux: {
             socket_name: socketName,
             holder_session:
-              stringValue(tmux.holder_session) || stringValue(tmux.holderSession) || '_holder',
+              exactStringValue(tmux.holder_session) ??
+              exactStringValue(tmux.holderSession) ??
+              '_holder',
           },
           defaults: {
             working_directory:
-              stringValue(defaults.working_directory) ||
-              stringValue(defaults.workingDirectory) ||
+              exactStringValue(defaults.working_directory) ??
+              exactStringValue(defaults.workingDirectory) ??
               '~/src',
             runtime: defaultRuntime,
           },
@@ -769,8 +772,8 @@ function classifyAgentLocality(
   agent: Record<string, unknown>,
   fleetHost: string | undefined,
 ): AgentLocality {
-  const host = stringValue(agent.host);
-  const ssh = stringValue(agent.ssh);
+  const host = exactStringValue(agent.host);
+  const ssh = exactStringValue(agent.ssh);
   const hasHost = agent.host !== undefined;
   const hasSsh = agent.ssh !== undefined;
   if (!hasHost && !hasSsh) return 'local';
@@ -837,6 +840,7 @@ function inventoryEntry(path: string): MigrationInventoryEntry {
 
 function blockPresentEmptyV1Fields(
   raw: Record<string, unknown>,
+  fleetHost: string | undefined,
   blockers: MigrationBlocker[],
 ): void {
   const tmux = record(raw.tmux);
@@ -852,7 +856,14 @@ function blockPresentEmptyV1Fields(
     );
   }
   for (const [index, agentValue] of (Array.isArray(raw.agents) ? raw.agents : []).entries()) {
-    blockPresentEmptyField(record(agentValue), ['alias'], `agents[${index}]`, blockers);
+    const agent = record(agentValue);
+    if (classifyAgentLocality(agent, fleetHost) === 'remote') continue;
+    blockPresentEmptyField(
+      agent,
+      ['alias', 'class', 'tool_policy', 'toolPolicy'],
+      `agents[${index}]`,
+      blockers,
+    );
     blockPresentEmptyField(
       record(agentValue),
       ['working_directory', 'workingDirectory'],
@@ -881,6 +892,55 @@ function blockPresentEmptyField(
   }
 }
 
+function blockLossyV1StringNormalization(
+  raw: Record<string, unknown>,
+  fleetHost: string | undefined,
+  blockers: MigrationBlocker[],
+): void {
+  const tmux = record(raw.tmux);
+  blockPaddedV1Field(tmux, ['holder_session', 'holderSession'], 'tmux', blockers);
+  const defaults = record(raw.defaults);
+  blockPaddedV1Field(defaults, ['working_directory', 'workingDirectory'], 'defaults', blockers);
+  for (const [runtimeName, runtimeValue] of Object.entries(record(raw.runtimes))) {
+    blockPaddedV1Field(
+      record(runtimeValue),
+      ['reset_command', 'resetCommand'],
+      `runtimes.${runtimeName}`,
+      blockers,
+    );
+  }
+  for (const [index, agentValue] of (Array.isArray(raw.agents) ? raw.agents : []).entries()) {
+    const agent = record(agentValue);
+    if (classifyAgentLocality(agent, fleetHost) === 'remote') continue;
+    blockPaddedV1Field(
+      agent,
+      ['alias', 'tool_policy', 'toolPolicy', 'working_directory', 'workingDirectory'],
+      `agents[${index}]`,
+      blockers,
+    );
+  }
+}
+
+function blockPaddedV1Field(
+  value: Record<string, unknown>,
+  fields: readonly string[],
+  path: string,
+  blockers: MigrationBlocker[],
+): void {
+  for (const field of fields) {
+    const source = exactStringValue(value[field]);
+    if (source !== undefined && source !== source.trim()) {
+      blockers.push(
+        blocker(
+          'lossy-v1-string-normalization',
+          `${path}.${field}`,
+          'A whitespace-padded v1 value cannot be silently normalized by the v2 compiler.',
+        ),
+      );
+    }
+  }
+}
+
 function validateV1FieldCatalog(raw: Record<string, unknown>, blockers: MigrationBlocker[]): void {
   rejectUnknownKeys(raw, ROOT_FIELDS, 'root', blockers);
   requireObjectWhenPresent(raw.tmux, 'tmux', blockers);
@@ -942,7 +1002,7 @@ function validateV1FieldCatalog(raw: Record<string, unknown>, blockers: Migratio
         blocker('invalid-v1-field-type', `${path}.name`, 'A valid agent name is required.'),
       );
     }
-    if (stringValue(agent.runtime) === undefined) {
+    if (exactStringValue(agent.runtime) === undefined) {
       blockers.push(
         blocker('invalid-v1-field-type', `${path}.runtime`, 'Agent runtime is required.'),
       );
@@ -1219,7 +1279,7 @@ function normalizeRuntimes(
     }
     const runtime = record(value);
     const resetCommand =
-      stringValue(runtime.reset_command) || stringValue(runtime.resetCommand) || '/clear';
+      exactStringValue(runtime.reset_command) ?? exactStringValue(runtime.resetCommand) ?? '/clear';
     result[name] = { reset_command: resetCommand };
   }
   return result;
@@ -1233,7 +1293,7 @@ function resolveClass(
   blockers: MigrationBlocker[],
 ): string | undefined {
   const canonical = canonicalizeRoleClass(requested).canonicalClass;
-  if (canonical !== requested || AUTOMATIC_CLASSES.has(canonical)) {
+  if (AUTOMATIC_CLASSES.has(canonical)) {
     if (disposition !== undefined) {
       blockers.push(
         blocker(