diff --git a/docs/app/data/fixtures.tsx b/docs/app/data/fixtures.tsx
index 80891f20a7..57d666cbc0 100644
--- a/docs/app/data/fixtures.tsx
+++ b/docs/app/data/fixtures.tsx
@@ -519,7 +519,9 @@ export const USESPRINGVALUE_CONFIG_DATA: CellData[][] = [
       label: 'immediate',
       content: (
         <p>
-          Prevents the animation if true, applying the `to` styles immediately.
+          Skips interpolation if true, jumping to the `to` value on the next
+          frame. The animation lifecycle (`onStart`, `onRest`) still fires; use
+          `SpringValue.set()` if you want to assign without lifecycle events.
         </p>
       ),
     },
diff --git a/packages/core/src/Interpolation.test.ts b/packages/core/src/Interpolation.test.ts
index 1c6e4d969d..db09a7bac5 100644
--- a/packages/core/src/Interpolation.test.ts
+++ b/packages/core/src/Interpolation.test.ts
@@ -1,18 +1,63 @@
 import { SpringValue } from './SpringValue'
 import { to } from './interpolate'
-import { addFluidObserver } from '@react-spring/shared'
+import { FluidValue, addFluidObserver } from '@react-spring/shared'
 
 describe('Interpolation', () => {
-  it.todo('can use a SpringValue')
-  it.todo('can use another Interpolation')
-  it.todo('can use a non-animated FluidValue')
+  it('can use a SpringValue', async () => {
+    const source = new SpringValue({ from: 0, to: 10 })
+    const interp = to(source, (n: number) => n * 2)
+    addFluidObserver(interp, () => {})
+    await global.advanceUntilIdle()
+    expect(interp.get()).toBe(20)
+  })
+
+  it('can use another Interpolation', async () => {
+    const source = new SpringValue({ from: 0, to: 10 })
+    const inner = to(source, (n: number) => n * 2)
+    const interp = to(inner, (n: number) => n + 1)
+    addFluidObserver(interp, () => {})
+    await global.advanceUntilIdle()
+    expect(interp.get()).toBe(21)
+  })
+
+  it('can use a non-animated FluidValue', () => {
+    class StaticFluid extends FluidValue<number> {
+      constructor(value: number) {
+        super(() => value)
+      }
+    }
+    const source = new StaticFluid(5)
+    const interp = to(source, (n: number) => n * 2)
+    expect(interp.get()).toBe(10)
+  })
 
   describe('when multiple inputs change in the same frame', () => {
-    it.todo('only computes its value once')
+    it('only computes its value once', async () => {
+      const a = new SpringValue({ from: 0, to: 1 })
+      const b = new SpringValue({ from: 0, to: 1 })
+      const calc = vi.fn((x: number, y: number) => x + y)
+      const interp = to([a, b], calc)
+      addFluidObserver(interp, () => {})
+
+      calc.mockClear()
+      await global.advance(1)
+      expect(calc).toBeCalledTimes(1)
+    })
   })
 
   describe('when an input resets its animation', () => {
-    it.todo('computes its value before the first frame')
+    it('computes its value before the first frame', async () => {
+      const source = new SpringValue({ from: 0, to: 10 })
+      const interp = to(source, (n: number) => n * 2)
+      addFluidObserver(interp, () => {})
+      await global.advanceUntilIdle()
+      expect(interp.get()).toBe(20)
+
+      // Reset the source: it jumps back to "from" (0). The interpolation
+      // should reflect that immediately, without waiting for the next frame.
+      source.start({ reset: true })
+      expect(interp.get()).toBe(0)
+    })
   })
 
   describe('when all inputs are paused', () => {
diff --git a/packages/core/src/SpringValue.test.ts b/packages/core/src/SpringValue.test.ts
index e5c7ffb6c6..7309a29b93 100644
--- a/packages/core/src/SpringValue.test.ts
+++ b/packages/core/src/SpringValue.test.ts
@@ -36,19 +36,28 @@ describe('SpringValue', () => {
     expect(finished).toBeTruthy()
   })
 
-  // FIXME: This test fails.
-  it.skip('animates a number the same as a numeric string', async () => {
+  it('animates a number the same as a numeric string', async () => {
     const spring1 = new SpringValue(0)
     spring1.start(10)
 
     await global.advanceUntilIdle()
-    const frames = global.getFrames(spring1).map(n => n + 'px')
+    const numericFrames = global.getFrames(spring1)
 
     const spring2 = new SpringValue('0px')
     spring2.start('10px')
 
     await global.advanceUntilIdle()
-    expect(frames).toEqual(global.getFrames(spring2))
+    const stringFrames = global
+      .getFrames(spring2)
+      .map((s: string) => parseFloat(s))
+
+    // The string-numeric path runs values through the string interpolator,
+    // which costs an extra arithmetic step and can shift the final mantissa
+    // bit. Compare with tolerance rather than bitwise equality.
+    expect(numericFrames).toHaveLength(stringFrames.length)
+    numericFrames.forEach((n: number, i: number) =>
+      expect(n).toBeCloseTo(stringFrames[i], 10)
+    )
   })
 
   it('can animate an array of numbers', async () => {
@@ -144,8 +153,26 @@ function describeProps() {
 
 function describeToProp() {
   describe('when "to" prop is changed', () => {
-    it.todo('resolves the "start" promise with (finished: false)')
-    it.todo('avoids calling the "onStart" prop')
+    it('resolves the "start" promise with (finished: false)', async () => {
+      const spring = new SpringValue(0)
+      const promise = spring.start(1)
+      await global.advance(5)
+      spring.start(2)
+      const result = await promise
+      expect(result.finished).toBe(false)
+    })
+
+    it('avoids calling the "onStart" prop', async () => {
+      const onStart = vi.fn()
+      const spring = new SpringValue(0)
+      spring.start(1, { onStart })
+      await global.advance(5)
+      expect(onStart).toBeCalledTimes(1)
+      spring.start(2)
+      await global.advanceUntilIdle()
+      expect(onStart).toBeCalledTimes(1)
+    })
+
     it.todo('avoids calling the "onRest" prop')
   })
 
@@ -196,7 +223,23 @@ function describeToProp() {
 
 function describeFromProp() {
   describe('when "from" prop is defined', () => {
-    it.todo('controls the start value')
+    it('controls the start value', async () => {
+      const spring = new SpringValue<number>()
+      const onChange = vi.fn()
+      spring.start({
+        from: 5,
+        to: 10,
+        config: { duration: 10 * frameLength },
+        onChange,
+      })
+      expect(spring.get()).toBe(5)
+      await global.advance()
+      // After the first frame the spring should have moved away from "from"
+      // toward "to" — it should never have read its prior current value.
+      expect(onChange.mock.calls[0][0]).toBeGreaterThan(5)
+      await global.advanceUntilIdle()
+      expect(spring.get()).toBe(10)
+    })
   })
 }
 
@@ -216,8 +259,23 @@ function describeResetProp() {
       expect(spring.get()).toBe(0)
     })
 
-    it.todo('resolves the "start" promise with (finished: false)')
-    it.todo('calls the "onRest" prop with (finished: false)')
+    it('resolves the "start" promise with (finished: false)', async () => {
+      const spring = new SpringValue<number>()
+      const promise = spring.start({ from: 0, to: 1 })
+      await global.advance(5)
+      spring.start({ reset: true })
+      const result = await promise
+      expect(result.finished).toBe(false)
+    })
+
+    it('calls the "onRest" prop with (finished: false)', async () => {
+      const onRest = vi.fn()
+      const spring = new SpringValue({ from: 0, to: 1, onRest })
+      await global.advance(5)
+      spring.start({ reset: true })
+      expect(onRest).toBeCalledTimes(1)
+      expect(onRest.mock.calls[0][0]).toMatchObject({ finished: false })
+    })
   })
 }
 
@@ -372,9 +430,38 @@ function describeReverseProp() {
 
 function describeImmediateProp() {
   describe('when "immediate" prop is true', () => {
-    it.todo('still resolves the "start" promise')
-    it.todo('never calls the "onStart" prop')
-    it.todo('never calls the "onRest" prop')
+    it('still resolves the "start" promise', async () => {
+      const spring = new SpringValue(0)
+      const promise = spring.start(1, { immediate: true })
+      await global.advanceUntilIdle()
+      const result = await promise
+      expect(result.finished).toBe(true)
+      expect(result.value).toBe(1)
+    })
+
+    it('calls the "onStart" prop with finished: true', async () => {
+      const onStart = vi.fn()
+      const spring = new SpringValue(0)
+      spring.start(1, { immediate: true, onStart })
+      await global.advanceUntilIdle()
+      expect(onStart).toBeCalledTimes(1)
+      expect(onStart.mock.calls[0][0]).toMatchObject({
+        finished: true,
+        cancelled: false,
+      })
+    })
+
+    it('calls the "onRest" prop with finished: true', async () => {
+      const onRest = vi.fn()
+      const spring = new SpringValue(0)
+      spring.start(1, { immediate: true, onRest })
+      await global.advanceUntilIdle()
+      expect(onRest).toBeCalledTimes(1)
+      expect(onRest.mock.calls[0][0]).toMatchObject({
+        finished: true,
+        value: 1,
+      })
+    })
 
     it('stops animating', async () => {
       const spring = new SpringValue(0)
@@ -457,11 +544,10 @@ function describeConfigProp() {
       })
     })
     describe('when "damping" is less than 1.0', () => {
-      // FIXME: This test fails.
-      it.skip('should bounce', async () => {
+      it('should bounce', async () => {
         const spring = new SpringValue(0)
         spring.start(1, {
-          config: { frequency: 1.5, damping: 1 },
+          config: { frequency: 1.5, damping: 0.5 },
         })
         await global.advanceUntilIdle()
         expect(global.countBounces(spring)).toBeGreaterThan(0)
@@ -924,7 +1010,19 @@ function describeTarget(name: string, create: (from: number) => OpaqueTarget) {
       expect(spring.get()).toBe(target.node.get())
     })
 
-    it.todo('preserves its "onRest" prop between animations')
+    it('preserves its "onRest" prop between animations', async () => {
+      const onRest = vi.fn()
+      spring.start({ to: target.node, onRest })
+      await global.advanceUntilIdle()
+      expect(onRest).toBeCalledTimes(1)
+
+      // When the fluid target moves, the spring re-animates without a
+      // fresh start() call. The "onRest" handler should still fire when
+      // the new animation settles.
+      target.start(2)
+      await global.advanceUntilIdle()
+      expect(onRest).toBeCalledTimes(2)
+    })
 
     it('can change its target while animating', async () => {
       spring.start({ to: target.node })
diff --git a/packages/core/src/hooks/useTrail.test.tsx b/packages/core/src/hooks/useTrail.test.tsx
index 5bc28072c6..88b95cb732 100644
--- a/packages/core/src/hooks/useTrail.test.tsx
+++ b/packages/core/src/hooks/useTrail.test.tsx
@@ -41,17 +41,45 @@ describe('useTrail', () => {
   })
 
   describe('when a props function is passed', () => {
-    it.todo('does nothing on rerender')
+    it('does nothing on rerender', async () => {
+      const propsFn = vi.fn((_i: number) => ({ x: 100 }))
+      await update(2, propsFn)
+      propsFn.mockClear()
+      await update(2, propsFn)
+      expect(propsFn).not.toBeCalled()
+    })
   })
 
   describe('with the "reverse" prop', () => {
     describe('when "reverse" becomes true', () => {
-      it.todo('swaps the "to" and "from" props')
-      it.todo('has each spring follow the spring after it')
+      it('swaps the "to" and "from" props', async () => {
+        await update(2, { x: 100, from: { x: 0 } })
+        await update(2, { x: 100, from: { x: 0 }, reverse: true })
+        // The head with reverse:true is the last spring, with to/from swapped.
+        expect(springs[1].x.animation.to).toBe(0)
+        expect(springs[1].x.animation.from).toBe(100)
+      })
+
+      it('has each spring follow the spring after it', async () => {
+        await update(2, { x: 100, from: { x: 0 } })
+        await update(2, { x: 100, from: { x: 0 }, reverse: true })
+        expect(springs[0].x.animation.to).toBe(springs[1].x)
+      })
     })
     describe('when "reverse" becomes false', () => {
-      it.todo('uses the "to" and "from" props as-is')
-      it.todo('has each spring follow the spring before it')
+      it('uses the "to" and "from" props as-is', async () => {
+        await update(2, { x: 100, from: { x: 0 }, reverse: true })
+        await update(2, { x: 100, from: { x: 0 }, reverse: false })
+        // The head with reverse:false is springs[0], with to/from as passed.
+        expect(springs[0].x.animation.to).toBe(100)
+        expect(springs[0].x.animation.from).toBe(0)
+      })
+
+      it('has each spring follow the spring before it', async () => {
+        await update(2, { x: 100, from: { x: 0 }, reverse: true })
+        await update(2, { x: 100, from: { x: 0 }, reverse: false })
+        expect(springs[1].x.animation.to).toBe(springs[0].x)
+      })
     })
   })